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<a name="proto.users_guide"></a><a class="link" href="users_guide.html" title="Users' Guide">Users' Guide</a>
</h2></div></div></div>
<div class="toc"><dl class="toc">
<dt><span class="section"><a href="users_guide.html#boost_proto.users_guide.getting_started">Getting Started</a></span></dt>
<dt><span class="section"><a href="users_guide.html#boost_proto.users_guide.front_end">Fronts Ends: Defining
      Terminals and Non-Terminals of Your EDSL</a></span></dt>
<dt><span class="section"><a href="users_guide.html#boost_proto.users_guide.intermediate_form">Intermediate
      Form: Understanding and Introspecting Expressions</a></span></dt>
<dt><span class="section"><a href="users_guide.html#boost_proto.users_guide.back_end">Back Ends: Making Expression
      Templates Do Useful Work</a></span></dt>
<dt><span class="section"><a href="users_guide.html#boost_proto.users_guide.examples">Examples</a></span></dt>
<dt><span class="section"><a href="users_guide.html#boost_proto.users_guide.resources">Background and Resources</a></span></dt>
<dt><span class="section"><a href="users_guide.html#boost_proto.users_guide.glossary">Glossary</a></span></dt>
</dl></div>
<h4>
<a name="proto.users_guide.h0"></a>
      <span class="phrase"><a name="proto.users_guide.compilers__compiler_construction_toolkits__and_proto"></a></span><a class="link" href="users_guide.html#proto.users_guide.compilers__compiler_construction_toolkits__and_proto">Compilers,
      Compiler Construction Toolkits, and Proto</a>
    </h4>
<p>
      Most compilers have front ends and back ends. The front end parses the text
      of an input program into some intermediate form like an abstract syntax tree,
      and the back end takes the intermediate form and generates an executable from
      it.
    </p>
<p>
      A library built with Proto is essentially a compiler for an embedded domain-specific
      language (EDSL). It also has a front end, an intermediate form, and a back
      end. The front end is comprised of the symbols (a.k.a., terminals), members,
      operators and functions that make up the user-visible aspects of the EDSL.
      The back end is made of evaluation contexts and transforms that give meaning
      and behavior to the expression templates generated by the front end. In between
      is the intermediate form: the expression template itself, which is an abstract
      syntax tree in a very real sense.
    </p>
<p>
      To build a library with Proto, you will first decide what your interface will
      be; that is, you'll design a programming language for your domain and build
      the front end with tools provided by Proto. Then you'll design the back end
      by writing evaluation contexts and/or transforms that accept expression templates
      and do interesting things with them.
    </p>
<p>
      This users' guide is organized as follows. After a <a class="link" href="users_guide.html#boost_proto.users_guide.getting_started" title="Getting Started">Getting
      Started guide</a>, we'll cover the tools Proto provides for defining and
      manipulating the three major parts of a compiler:
    </p>
<div class="variablelist">
<p class="title"><b></b></p>
<dl class="variablelist">
<dt><span class="term"><a class="link" href="users_guide.html#boost_proto.users_guide.front_end" title="Fronts Ends: Defining Terminals and Non-Terminals of Your EDSL">Front Ends</a></span></dt>
<dd><p>
            How to define the aspects of your EDSL with which your users will interact
            directly.
          </p></dd>
<dt><span class="term"><a class="link" href="users_guide.html#boost_proto.users_guide.intermediate_form" title="Intermediate Form: Understanding and Introspecting Expressions">Intermediate
        Form</a></span></dt>
<dd><p>
            What Proto expression templates look like, how to discover their structure
            and access their constituents.
          </p></dd>
<dt><span class="term"><a class="link" href="users_guide.html#boost_proto.users_guide.back_end" title="Back Ends: Making Expression Templates Do Useful Work">Back Ends</a></span></dt>
<dd><p>
            How to define evaluation contexts and transforms that make expression
            templates do interesting things.
          </p></dd>
</dl>
</div>
<p>
      After that, you may be interested in seeing some <a class="link" href="users_guide.html#boost_proto.users_guide.examples" title="Examples">Examples</a>
      to get a better idea of how the pieces all fit together.
    </p>
<div class="section">
<div class="titlepage"><div><div><h3 class="title">
<a name="boost_proto.users_guide.getting_started"></a><a class="link" href="users_guide.html#boost_proto.users_guide.getting_started" title="Getting Started">Getting Started</a>
</h3></div></div></div>
<div class="toc"><dl class="toc">
<dt><span class="section"><a href="users_guide.html#boost_proto.users_guide.getting_started.installing_proto">Installing
        Proto</a></span></dt>
<dt><span class="section"><a href="users_guide.html#boost_proto.users_guide.getting_started.naming">Naming
        Conventions</a></span></dt>
<dt><span class="section"><a href="users_guide.html#boost_proto.users_guide.getting_started.hello_world">Hello
        World</a></span></dt>
<dt><span class="section"><a href="users_guide.html#boost_proto.users_guide.getting_started.hello_calculator">Hello
        Calculator</a></span></dt>
</dl></div>
<div class="section">
<div class="titlepage"><div><div><h4 class="title">
<a name="boost_proto.users_guide.getting_started.installing_proto"></a><a class="link" href="users_guide.html#boost_proto.users_guide.getting_started.installing_proto" title="Installing Proto">Installing
        Proto</a>
</h4></div></div></div>
<h6>
<a name="boost_proto.users_guide.getting_started.installing_proto.h0"></a>
          <span class="phrase"><a name="boost_proto.users_guide.getting_started.installing_proto.getting_proto"></a></span><a class="link" href="users_guide.html#boost_proto.users_guide.getting_started.installing_proto.getting_proto">Getting
          Proto</a>
        </h6>
<p>
          You can get Proto by downloading Boost (Proto is in version 1.37 and later),
          or by accessing Boost's SVN repository on SourceForge.net. Just go to
          <a href="http://svn.boost.org/trac/boost/wiki/BoostSubversion" target="_top">http://svn.boost.org/trac/boost/wiki/BoostSubversion</a>
          and follow the instructions there for anonymous SVN access.
        </p>
<h6>
<a name="boost_proto.users_guide.getting_started.installing_proto.h1"></a>
          <span class="phrase"><a name="boost_proto.users_guide.getting_started.installing_proto.building_with_proto"></a></span><a class="link" href="users_guide.html#boost_proto.users_guide.getting_started.installing_proto.building_with_proto">Building
          with Proto</a>
        </h6>
<p>
          Proto is a header-only template library, which means you don't need to
          alter your build scripts or link to any separate lib file to use it. All
          you need to do is <code class="computeroutput"><span class="preprocessor">#include</span>
          <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">proto</span><span class="special">/</span><span class="identifier">proto</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span></code>. Or, you might decide to just include
          the core of Proto (<code class="computeroutput"><span class="preprocessor">#include</span>
          <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">proto</span><span class="special">/</span><span class="identifier">core</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span></code>) and whichever contexts and transforms
          you happen to use.
        </p>
<h6>
<a name="boost_proto.users_guide.getting_started.installing_proto.h2"></a>
          <span class="phrase"><a name="boost_proto.users_guide.getting_started.installing_proto.requirements"></a></span><a class="link" href="users_guide.html#boost_proto.users_guide.getting_started.installing_proto.requirements">Requirements</a>
        </h6>
<p>
          Proto depends on Boost. You must use either Boost version 1.34.1 or higher,
          or the version in SVN trunk.
        </p>
<h6>
<a name="boost_proto.users_guide.getting_started.installing_proto.h3"></a>
          <span class="phrase"><a name="boost_proto.users_guide.getting_started.installing_proto.supported_compilers"></a></span><a class="link" href="users_guide.html#boost_proto.users_guide.getting_started.installing_proto.supported_compilers">Supported
          Compilers</a>
        </h6>
<p>
          Currently, Boost.Proto is known to work on the following compilers:
        </p>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
<li class="listitem">
              Visual C++ 8 and higher
            </li>
<li class="listitem">
              GNU C++ 3.4 and higher
            </li>
<li class="listitem">
              Intel on Linux 8.1 and higher
            </li>
<li class="listitem">
              Intel on Windows 9.1 and higher
            </li>
</ul></div>
<div class="note"><table border="0" summary="Note">
<tr>
<td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../doc/src/images/note.png"></td>
<th align="left">Note</th>
</tr>
<tr><td align="left" valign="top"><p>
            Please send any questions, comments and bug reports to eric &lt;at&gt;
            boostpro &lt;dot&gt; com.
          </p></td></tr>
</table></div>
</div>
<div class="section">
<div class="titlepage"><div><div><h4 class="title">
<a name="boost_proto.users_guide.getting_started.naming"></a><a class="link" href="users_guide.html#boost_proto.users_guide.getting_started.naming" title="Naming Conventions">Naming
        Conventions</a>
</h4></div></div></div>
<p>
          Proto is a large library and probably quite unlike any library you've used
          before. Proto uses some consistent naming conventions to make it easier
          to navigate, and they're described below.
        </p>
<h6>
<a name="boost_proto.users_guide.getting_started.naming.h0"></a>
          <span class="phrase"><a name="boost_proto.users_guide.getting_started.naming.functions"></a></span><a class="link" href="users_guide.html#boost_proto.users_guide.getting_started.naming.functions">Functions</a>
        </h6>
<p>
          All of Proto's functions are defined in the <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">proto</span></code>
          namespace. For example, there is a function called <code class="computeroutput"><span class="identifier">value</span><span class="special">()</span></code> defined in <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">proto</span></code>
          that accepts a terminal expression and returns the terminal's value.
        </p>
<h6>
<a name="boost_proto.users_guide.getting_started.naming.h1"></a>
          <span class="phrase"><a name="boost_proto.users_guide.getting_started.naming.metafunctions"></a></span><a class="link" href="users_guide.html#boost_proto.users_guide.getting_started.naming.metafunctions">Metafunctions</a>
        </h6>
<p>
          Proto defines <span class="emphasis"><em>metafunctions</em></span> that correspond to each
          of Proto's free functions. The metafunctions are used to compute the functions'
          return types. All of Proto's metafunctions live in the <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">result_of</span></code>
          namespace and have the same name as the functions to which they correspond.
          For instance, there is a class template <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">result_of</span><span class="special">::</span><span class="identifier">value</span><span class="special">&lt;&gt;</span></code> that you can use to compute the
          return type of the <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">value</span><span class="special">()</span></code> function.
        </p>
<h6>
<a name="boost_proto.users_guide.getting_started.naming.h2"></a>
          <span class="phrase"><a name="boost_proto.users_guide.getting_started.naming.function_objects"></a></span><a class="link" href="users_guide.html#boost_proto.users_guide.getting_started.naming.function_objects">Function
          Objects</a>
        </h6>
<p>
          Proto defines <span class="emphasis"><em>function object</em></span> equivalents of all of
          its free functions. (A function object is an instance of a class type that
          defines an <code class="computeroutput"><span class="keyword">operator</span><span class="special">()</span></code>
          member function.) All of Proto's function object types are defined in the
          <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">functional</span></code> namespace and have the same
          name as their corresponding free functions. For example, <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">functional</span><span class="special">::</span><span class="identifier">value</span></code> is a class that defines a function
          object that does the same thing as the <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">value</span><span class="special">()</span></code> free function.
        </p>
<h6>
<a name="boost_proto.users_guide.getting_started.naming.h3"></a>
          <span class="phrase"><a name="boost_proto.users_guide.getting_started.naming.primitive_transforms"></a></span><a class="link" href="users_guide.html#boost_proto.users_guide.getting_started.naming.primitive_transforms">Primitive
          Transforms</a>
        </h6>
<p>
          Proto also defines <span class="emphasis"><em>primitive transforms</em></span> -- class types
          that can be used to compose larger transforms for manipulating expression
          trees. Many of Proto's free functions have corresponding primitive transforms.
          These live in the <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">proto</span></code>
          namespace and their names have a leading underscore. For instance, the
          transform corresponding to the <code class="computeroutput"><span class="identifier">value</span><span class="special">()</span></code> function is called <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_value</span></code>.
        </p>
<p>
          The following table summarizes the discussion above:
        </p>
<div class="table">
<a name="boost_proto.users_guide.getting_started.naming.t0"></a><p class="title"><b>Table&#160;32.1.&#160;Proto Naming Conventions</b></p>
<div class="table-contents"><table class="table" summary="Proto Naming Conventions">
<colgroup>
<col>
<col>
</colgroup>
<thead><tr>
<th>
                  <p>
                    Entity
                  </p>
                </th>
<th>
                  <p>
                    Example
                  </p>
                </th>
</tr></thead>
<tbody>
<tr>
<td>
                  <p>
                    Free Function
                  </p>
                </td>
<td>
                  <p>
                    <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">value</span><span class="special">()</span></code>
                  </p>
                </td>
</tr>
<tr>
<td>
                  <p>
                    Metafunction
                  </p>
                </td>
<td>
                  <p>
                    <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">result_of</span><span class="special">::</span><span class="identifier">value</span><span class="special">&lt;&gt;</span></code>
                  </p>
                </td>
</tr>
<tr>
<td>
                  <p>
                    Function Object
                  </p>
                </td>
<td>
                  <p>
                    <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">functional</span><span class="special">::</span><span class="identifier">value</span></code>
                  </p>
                </td>
</tr>
<tr>
<td>
                  <p>
                    Transform
                  </p>
                </td>
<td>
                  <p>
                    <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_value</span></code>
                  </p>
                </td>
</tr>
</tbody>
</table></div>
</div>
<br class="table-break">
</div>
<div class="section">
<div class="titlepage"><div><div><h4 class="title">
<a name="boost_proto.users_guide.getting_started.hello_world"></a><a class="link" href="users_guide.html#boost_proto.users_guide.getting_started.hello_world" title="Hello World">Hello
        World</a>
</h4></div></div></div>
<p>
          Below is a very simple program that uses Proto to build an expression template
          and then execute it.
        </p>
<pre class="programlisting"><span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">iostream</span><span class="special">&gt;</span>
<span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">proto</span><span class="special">/</span><span class="identifier">proto</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
<span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">typeof</span><span class="special">/</span><span class="identifier">std</span><span class="special">/</span><span class="identifier">ostream</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
<span class="keyword">using</span> <span class="keyword">namespace</span> <span class="identifier">boost</span><span class="special">;</span>

<span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">ostream</span> <span class="special">&amp;</span> <span class="special">&gt;::</span><span class="identifier">type</span> <span class="identifier">cout_</span> <span class="special">=</span> <span class="special">{</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">};</span>

<span class="keyword">template</span><span class="special">&lt;</span> <span class="keyword">typename</span> <span class="identifier">Expr</span> <span class="special">&gt;</span>
<span class="keyword">void</span> <span class="identifier">evaluate</span><span class="special">(</span> <span class="identifier">Expr</span> <span class="keyword">const</span> <span class="special">&amp;</span> <span class="identifier">expr</span> <span class="special">)</span>
<span class="special">{</span>
    <span class="identifier">proto</span><span class="special">::</span><span class="identifier">default_context</span> <span class="identifier">ctx</span><span class="special">;</span>
    <span class="identifier">proto</span><span class="special">::</span><span class="identifier">eval</span><span class="special">(</span><span class="identifier">expr</span><span class="special">,</span> <span class="identifier">ctx</span><span class="special">);</span>
<span class="special">}</span>

<span class="keyword">int</span> <span class="identifier">main</span><span class="special">()</span>
<span class="special">{</span>
    <span class="identifier">evaluate</span><span class="special">(</span> <span class="identifier">cout_</span> <span class="special">&lt;&lt;</span> <span class="string">"hello"</span> <span class="special">&lt;&lt;</span> <span class="char">','</span> <span class="special">&lt;&lt;</span> <span class="string">" world"</span> <span class="special">);</span>
    <span class="keyword">return</span> <span class="number">0</span><span class="special">;</span>
<span class="special">}</span>
</pre>
<p>
          This program outputs the following:
        </p>
<pre class="programlisting">hello, world
</pre>
<p>
          This program builds an object representing the output operation and passes
          it to an <code class="computeroutput"><span class="identifier">evaluate</span><span class="special">()</span></code>
          function, which then executes it.
        </p>
<p>
          The basic idea of expression templates is to overload all the operators
          so that, rather than evaluating the expression immediately, they build
          a tree-like representation of the expression so that it can be evaluated
          later. For each operator in an expression, at least one operand must be
          Protofied in order for Proto's operator overloads to be found. In the expression
          ...
        </p>
<pre class="programlisting"><span class="identifier">cout_</span> <span class="special">&lt;&lt;</span> <span class="string">"hello"</span> <span class="special">&lt;&lt;</span> <span class="char">','</span> <span class="special">&lt;&lt;</span> <span class="string">" world"</span>
</pre>
<p>
          ... the Protofied sub-expression is <code class="computeroutput"><span class="identifier">cout_</span></code>,
          which is the Proto-ification of <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span></code>.
          The presence of <code class="computeroutput"><span class="identifier">cout_</span></code> "infects"
          the expression, and brings Proto's tree-building operator overloads into
          consideration. Any literals in the expression are then Protofied by wrapping
          them in a Proto terminal before they are combined into larger Proto expressions.
        </p>
<p>
          Once Proto's operator overloads have built the expression tree, the expression
          can be lazily evaluated later by walking the tree. That is what <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">eval</span><span class="special">()</span></code>
          does. It is a general tree-walking expression evaluator, whose behavior
          is customizable via a <span class="emphasis"><em>context</em></span> parameter. The use of
          <code class="computeroutput"><a class="link" href="../boost/proto/context/default_context.html" title="Struct default_context">proto::default_context</a></code>
          assigns the standard meanings to the operators in the expression. (By using
          a different context, you could give the operators in your expressions different
          semantics. By default, Proto makes no assumptions about what operators
          actually <span class="emphasis"><em>mean</em></span>.)
        </p>
<h6>
<a name="boost_proto.users_guide.getting_started.hello_world.h0"></a>
          <span class="phrase"><a name="boost_proto.users_guide.getting_started.hello_world.proto_design_philosophy"></a></span><a class="link" href="users_guide.html#boost_proto.users_guide.getting_started.hello_world.proto_design_philosophy">Proto
          Design Philosophy</a>
        </h6>
<p>
          Before we continue, let's use the above example to illustrate an important
          design principle of Proto's. The expression template created in the <span class="emphasis"><em>hello
          world</em></span> example is totally general and abstract. It is not tied
          in any way to any particular domain or application, nor does it have any
          particular meaning or behavior on its own, until it is evaluated in a
          <span class="emphasis"><em>context</em></span>. Expression templates are really just heterogeneous
          trees, which might mean something in one domain, and something else entirely
          in a different one.
        </p>
<p>
          As we'll see later, there is a way to create Proto expression trees that
          are <span class="emphasis"><em>not</em></span> purely abstract, and that have meaning and
          behaviors independent of any context. There is also a way to control which
          operators are overloaded for your particular domain. But that is not the
          default behavior. We'll see later why the default is often a good thing.
        </p>
</div>
<div class="section">
<div class="titlepage"><div><div><h4 class="title">
<a name="boost_proto.users_guide.getting_started.hello_calculator"></a><a class="link" href="users_guide.html#boost_proto.users_guide.getting_started.hello_calculator" title="Hello Calculator">Hello
        Calculator</a>
</h4></div></div></div>
<p>
          "Hello, world" is nice, but it doesn't get you very far. Let's
          use Proto to build a EDSL (embedded domain-specific language) for a lazily-evaluated
          calculator. We'll see how to define the terminals in your mini-language,
          how to compose them into larger expressions, and how to define an evaluation
          context so that your expressions can do useful work. When we're done, we'll
          have a mini-language that will allow us to declare a lazily-evaluated arithmetic
          expression, such as <code class="computeroutput"><span class="special">(</span><span class="identifier">_2</span>
          <span class="special">-</span> <span class="identifier">_1</span><span class="special">)</span> <span class="special">/</span> <span class="identifier">_2</span>
          <span class="special">*</span> <span class="number">100</span></code>,
          where <code class="computeroutput"><span class="identifier">_1</span></code> and <code class="computeroutput"><span class="identifier">_2</span></code> are placeholders for values to be
          passed in when the expression is evaluated.
        </p>
<h6>
<a name="boost_proto.users_guide.getting_started.hello_calculator.h0"></a>
          <span class="phrase"><a name="boost_proto.users_guide.getting_started.hello_calculator.defining_terminals"></a></span><a class="link" href="users_guide.html#boost_proto.users_guide.getting_started.hello_calculator.defining_terminals">Defining
          Terminals</a>
        </h6>
<p>
          The first order of business is to define the placeholders <code class="computeroutput"><span class="identifier">_1</span></code> and <code class="computeroutput"><span class="identifier">_2</span></code>.
          For that, we'll use the <code class="computeroutput"><a class="link" href="../boost/proto/terminal.html" title="Struct template terminal">proto::terminal&lt;&gt;</a></code>
          metafunction.
        </p>
<pre class="programlisting"><span class="comment">// Define a placeholder type</span>
<span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">int</span> <span class="identifier">I</span><span class="special">&gt;</span>
<span class="keyword">struct</span> <span class="identifier">placeholder</span>
<span class="special">{};</span>

<span class="comment">// Define the Protofied placeholder terminals</span>
<span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span><span class="identifier">placeholder</span><span class="special">&lt;</span><span class="number">0</span><span class="special">&gt;</span> <span class="special">&gt;::</span><span class="identifier">type</span> <span class="keyword">const</span> <span class="identifier">_1</span> <span class="special">=</span> <span class="special">{{}};</span>
<span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span><span class="identifier">placeholder</span><span class="special">&lt;</span><span class="number">1</span><span class="special">&gt;</span> <span class="special">&gt;::</span><span class="identifier">type</span> <span class="keyword">const</span> <span class="identifier">_2</span> <span class="special">=</span> <span class="special">{{}};</span>
</pre>
<p>
          The initialization may look a little odd at first, but there is a good
          reason for doing things this way. The objects <code class="computeroutput"><span class="identifier">_1</span></code>
          and <code class="computeroutput"><span class="identifier">_2</span></code> above do not require
          run-time construction -- they are <span class="emphasis"><em>statically initialized</em></span>,
          which means they are essentially initialized at compile time. See the
          <a class="link" href="appendices.html#boost_proto.appendices.rationale.static_initialization" title="Static Initialization">Static
          Initialization</a> section in the <a class="link" href="appendices.html#boost_proto.appendices.rationale" title="Appendix C: Rationale">Rationale</a>
          appendix for more information.
        </p>
<h6>
<a name="boost_proto.users_guide.getting_started.hello_calculator.h1"></a>
          <span class="phrase"><a name="boost_proto.users_guide.getting_started.hello_calculator.constructing_expression_trees"></a></span><a class="link" href="users_guide.html#boost_proto.users_guide.getting_started.hello_calculator.constructing_expression_trees">Constructing
          Expression Trees</a>
        </h6>
<p>
          Now that we have terminals, we can use Proto's operator overloads to combine
          these terminals into larger expressions. So, for instance, we can immediately
          say things like:
        </p>
<pre class="programlisting"><span class="comment">// This builds an expression template</span>
<span class="special">(</span><span class="identifier">_2</span> <span class="special">-</span> <span class="identifier">_1</span><span class="special">)</span> <span class="special">/</span> <span class="identifier">_2</span> <span class="special">*</span> <span class="number">100</span><span class="special">;</span>
</pre>
<p>
          This creates an expression tree with a node for each operator. The type
          of the resulting object is large and complex, but we are not terribly interested
          in it right now.
        </p>
<p>
          So far, the object is just a tree representing the expression. It has no
          behavior. In particular, it is not yet a calculator. Below we'll see how
          to make it a calculator by defining an evaluation context.
        </p>
<h6>
<a name="boost_proto.users_guide.getting_started.hello_calculator.h2"></a>
          <span class="phrase"><a name="boost_proto.users_guide.getting_started.hello_calculator.evaluating_expression_trees"></a></span><a class="link" href="users_guide.html#boost_proto.users_guide.getting_started.hello_calculator.evaluating_expression_trees">Evaluating
          Expression Trees</a>
        </h6>
<p>
          No doubt you want your expression templates to actually <span class="emphasis"><em>do</em></span>
          something. One approach is to define an <span class="emphasis"><em>evaluation context</em></span>.
          The context is like a function object that associates behaviors with the
          node types in your expression tree. The following example should make it
          clear. It is explained below.
        </p>
<pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">calculator_context</span>
  <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">callable_context</span><span class="special">&lt;</span> <span class="identifier">calculator_context</span> <span class="keyword">const</span> <span class="special">&gt;</span>
<span class="special">{</span>
    <span class="comment">// Values to replace the placeholders</span>
    <span class="identifier">std</span><span class="special">::</span><span class="identifier">vector</span><span class="special">&lt;</span><span class="keyword">double</span><span class="special">&gt;</span> <span class="identifier">args</span><span class="special">;</span>

    <span class="comment">// Define the result type of the calculator.</span>
    <span class="comment">// (This makes the calculator_context "callable".)</span>
    <span class="keyword">typedef</span> <span class="keyword">double</span> <span class="identifier">result_type</span><span class="special">;</span>

    <span class="comment">// Handle the placeholders:</span>
    <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">int</span> <span class="identifier">I</span><span class="special">&gt;</span>
    <span class="keyword">double</span> <span class="keyword">operator</span><span class="special">()(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">,</span> <span class="identifier">placeholder</span><span class="special">&lt;</span><span class="identifier">I</span><span class="special">&gt;)</span> <span class="keyword">const</span>
    <span class="special">{</span>
        <span class="keyword">return</span> <span class="keyword">this</span><span class="special">-&gt;</span><span class="identifier">args</span><span class="special">[</span><span class="identifier">I</span><span class="special">];</span>
    <span class="special">}</span>
<span class="special">};</span>
</pre>
<p>
          In <code class="computeroutput"><span class="identifier">calculator_context</span></code>,
          we specify how Proto should evaluate the placeholder terminals by defining
          the appropriate overloads of the function call operator. For any other
          nodes in the expression tree (e.g., arithmetic operations or non-placeholder
          terminals), Proto will evaluate the expression in the "default"
          way. For example, a binary plus node is evaluated by first evaluating the
          left and right operands and adding the results. Proto's default evaluator
          uses the <a href="../../../libs/typeof/index.html" target="_top">Boost.Typeof</a>
          library to compute return types.
        </p>
<p>
          Now that we have an evaluation context for our calculator, we can use it
          to evaluate our arithmetic expressions, as below:
        </p>
<pre class="programlisting"><span class="identifier">calculator_context</span> <span class="identifier">ctx</span><span class="special">;</span>
<span class="identifier">ctx</span><span class="special">.</span><span class="identifier">args</span><span class="special">.</span><span class="identifier">push_back</span><span class="special">(</span><span class="number">45</span><span class="special">);</span> <span class="comment">// the value of _1 is 45</span>
<span class="identifier">ctx</span><span class="special">.</span><span class="identifier">args</span><span class="special">.</span><span class="identifier">push_back</span><span class="special">(</span><span class="number">50</span><span class="special">);</span> <span class="comment">// the value of _2 is 50</span>

<span class="comment">// Create an arithmetic expression and immediately evaluate it</span>
<span class="keyword">double</span> <span class="identifier">d</span> <span class="special">=</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">eval</span><span class="special">(</span> <span class="special">(</span><span class="identifier">_2</span> <span class="special">-</span> <span class="identifier">_1</span><span class="special">)</span> <span class="special">/</span> <span class="identifier">_2</span> <span class="special">*</span> <span class="number">100</span><span class="special">,</span> <span class="identifier">ctx</span> <span class="special">);</span>

<span class="comment">// This prints "10"</span>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="identifier">d</span> <span class="special">&lt;&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span>
</pre>
<p>
          Later, we'll see how to define more interesting evaluation contexts and
          expression transforms that give you total control over how your expressions
          are evaluated.
        </p>
<h6>
<a name="boost_proto.users_guide.getting_started.hello_calculator.h3"></a>
          <span class="phrase"><a name="boost_proto.users_guide.getting_started.hello_calculator.customizing_expression_trees"></a></span><a class="link" href="users_guide.html#boost_proto.users_guide.getting_started.hello_calculator.customizing_expression_trees">Customizing
          Expression Trees</a>
        </h6>
<p>
          Our calculator EDSL is already pretty useful, and for many EDSL scenarios,
          no more would be needed. But let's keep going. Imagine how much nicer it
          would be if all calculator expressions overloaded <code class="computeroutput"><span class="keyword">operator</span><span class="special">()</span></code> so that they could be used as function
          objects. We can do that by creating a calculator <span class="emphasis"><em>domain</em></span>
          and telling Proto that all expressions in the calculator domain have extra
          members. Here is how to define a calculator domain:
        </p>
<pre class="programlisting"><span class="comment">// Forward-declare an expression wrapper</span>
<span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">Expr</span><span class="special">&gt;</span>
<span class="keyword">struct</span> <span class="identifier">calculator</span><span class="special">;</span>

<span class="comment">// Define a calculator domain. Expression within</span>
<span class="comment">// the calculator domain will be wrapped in the</span>
<span class="comment">// calculator&lt;&gt; expression wrapper.</span>
<span class="keyword">struct</span> <span class="identifier">calculator_domain</span>
  <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">domain</span><span class="special">&lt;</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">generator</span><span class="special">&lt;</span><span class="identifier">calculator</span><span class="special">&gt;</span> <span class="special">&gt;</span>
<span class="special">{};</span>
</pre>
<p>
          The <code class="computeroutput"><span class="identifier">calculator</span><span class="special">&lt;&gt;</span></code>
          type will be an expression wrapper. It will behave just like the expression
          that it wraps, but it will have extra member functions that we will define.
          The <code class="computeroutput"><span class="identifier">calculator_domain</span></code> is
          what informs Proto about our wrapper. It is used below in the definition
          of <code class="computeroutput"><span class="identifier">calculator</span><span class="special">&lt;&gt;</span></code>.
          Read on for a description.
        </p>
<pre class="programlisting"><span class="comment">// Define a calculator expression wrapper. It behaves just like</span>
<span class="comment">// the expression it wraps, but with an extra operator() member</span>
<span class="comment">// function that evaluates the expression.    </span>
<span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">Expr</span><span class="special">&gt;</span>
<span class="keyword">struct</span> <span class="identifier">calculator</span>
  <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">extends</span><span class="special">&lt;</span><span class="identifier">Expr</span><span class="special">,</span> <span class="identifier">calculator</span><span class="special">&lt;</span><span class="identifier">Expr</span><span class="special">&gt;,</span> <span class="identifier">calculator_domain</span><span class="special">&gt;</span>
<span class="special">{</span>
    <span class="keyword">typedef</span>
        <span class="identifier">proto</span><span class="special">::</span><span class="identifier">extends</span><span class="special">&lt;</span><span class="identifier">Expr</span><span class="special">,</span> <span class="identifier">calculator</span><span class="special">&lt;</span><span class="identifier">Expr</span><span class="special">&gt;,</span> <span class="identifier">calculator_domain</span><span class="special">&gt;</span>
    <span class="identifier">base_type</span><span class="special">;</span>

    <span class="identifier">calculator</span><span class="special">(</span><span class="identifier">Expr</span> <span class="keyword">const</span> <span class="special">&amp;</span><span class="identifier">expr</span> <span class="special">=</span> <span class="identifier">Expr</span><span class="special">())</span>
      <span class="special">:</span> <span class="identifier">base_type</span><span class="special">(</span><span class="identifier">expr</span><span class="special">)</span>
    <span class="special">{}</span>

    <span class="keyword">typedef</span> <span class="keyword">double</span> <span class="identifier">result_type</span><span class="special">;</span>

    <span class="comment">// Overload operator() to invoke proto::eval() with</span>
    <span class="comment">// our calculator_context.</span>
    <span class="keyword">double</span> <span class="keyword">operator</span><span class="special">()(</span><span class="keyword">double</span> <span class="identifier">a1</span> <span class="special">=</span> <span class="number">0</span><span class="special">,</span> <span class="keyword">double</span> <span class="identifier">a2</span> <span class="special">=</span> <span class="number">0</span><span class="special">)</span> <span class="keyword">const</span>
    <span class="special">{</span>
        <span class="identifier">calculator_context</span> <span class="identifier">ctx</span><span class="special">;</span>
        <span class="identifier">ctx</span><span class="special">.</span><span class="identifier">args</span><span class="special">.</span><span class="identifier">push_back</span><span class="special">(</span><span class="identifier">a1</span><span class="special">);</span>
        <span class="identifier">ctx</span><span class="special">.</span><span class="identifier">args</span><span class="special">.</span><span class="identifier">push_back</span><span class="special">(</span><span class="identifier">a2</span><span class="special">);</span>

        <span class="keyword">return</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">eval</span><span class="special">(*</span><span class="keyword">this</span><span class="special">,</span> <span class="identifier">ctx</span><span class="special">);</span>
    <span class="special">}</span>
<span class="special">};</span>
</pre>
<p>
          The <code class="computeroutput"><span class="identifier">calculator</span><span class="special">&lt;&gt;</span></code>
          struct is an expression <span class="emphasis"><em>extension</em></span>. It uses <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">extends</span><span class="special">&lt;&gt;</span></code>
          to effectively add additional members to an expression type. When composing
          larger expressions from smaller ones, Proto notes what domain the smaller
          expressions are in. The larger expression is in the same domain and is
          automatically wrapped in the domain's extension wrapper.
        </p>
<p>
          All that remains to be done is to put our placeholders in the calculator
          domain. We do that by wrapping them in our <code class="computeroutput"><span class="identifier">calculator</span><span class="special">&lt;&gt;</span></code> wrapper, as below:
        </p>
<pre class="programlisting"><span class="comment">// Define the Protofied placeholder terminals, in the</span>
<span class="comment">// calculator domain.</span>
<span class="identifier">calculator</span><span class="special">&lt;</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span><span class="identifier">placeholder</span><span class="special">&lt;</span><span class="number">0</span><span class="special">&gt;</span> <span class="special">&gt;::</span><span class="identifier">type</span><span class="special">&gt;</span> <span class="keyword">const</span> <span class="identifier">_1</span><span class="special">;</span>
<span class="identifier">calculator</span><span class="special">&lt;</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span><span class="identifier">placeholder</span><span class="special">&lt;</span><span class="number">1</span><span class="special">&gt;</span> <span class="special">&gt;::</span><span class="identifier">type</span><span class="special">&gt;</span> <span class="keyword">const</span> <span class="identifier">_2</span><span class="special">;</span>
</pre>
<p>
          Any larger expression that contain these placeholders will automatically
          be wrapped in the <code class="computeroutput"><span class="identifier">calculator</span><span class="special">&lt;&gt;</span></code> wrapper and have our <code class="computeroutput"><span class="keyword">operator</span><span class="special">()</span></code>
          overload. That means we can use them as function objects as follows.
        </p>
<pre class="programlisting"><span class="keyword">double</span> <span class="identifier">result</span> <span class="special">=</span> <span class="special">((</span><span class="identifier">_2</span> <span class="special">-</span> <span class="identifier">_1</span><span class="special">)</span> <span class="special">/</span> <span class="identifier">_2</span> <span class="special">*</span> <span class="number">100</span><span class="special">)(</span><span class="number">45.0</span><span class="special">,</span> <span class="number">50.0</span><span class="special">);</span>
<span class="identifier">assert</span><span class="special">(</span><span class="identifier">result</span> <span class="special">==</span> <span class="special">(</span><span class="number">50.0</span> <span class="special">-</span> <span class="number">45.0</span><span class="special">)</span> <span class="special">/</span> <span class="number">50.0</span> <span class="special">*</span> <span class="number">100</span><span class="special">));</span>
</pre>
<p>
          Since calculator expressions are now valid function objects, we can use
          them with standard algorithms, as shown below:
        </p>
<pre class="programlisting"><span class="keyword">double</span> <span class="identifier">a1</span><span class="special">[</span><span class="number">4</span><span class="special">]</span> <span class="special">=</span> <span class="special">{</span> <span class="number">56</span><span class="special">,</span> <span class="number">84</span><span class="special">,</span> <span class="number">37</span><span class="special">,</span> <span class="number">69</span> <span class="special">};</span>
<span class="keyword">double</span> <span class="identifier">a2</span><span class="special">[</span><span class="number">4</span><span class="special">]</span> <span class="special">=</span> <span class="special">{</span> <span class="number">65</span><span class="special">,</span> <span class="number">120</span><span class="special">,</span> <span class="number">60</span><span class="special">,</span> <span class="number">70</span> <span class="special">};</span>
<span class="keyword">double</span> <span class="identifier">a3</span><span class="special">[</span><span class="number">4</span><span class="special">]</span> <span class="special">=</span> <span class="special">{</span> <span class="number">0</span> <span class="special">};</span>

<span class="comment">// Use std::transform() and a calculator expression</span>
<span class="comment">// to calculate percentages given two input sequences:</span>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">transform</span><span class="special">(</span><span class="identifier">a1</span><span class="special">,</span> <span class="identifier">a1</span><span class="special">+</span><span class="number">4</span><span class="special">,</span> <span class="identifier">a2</span><span class="special">,</span> <span class="identifier">a3</span><span class="special">,</span> <span class="special">(</span><span class="identifier">_2</span> <span class="special">-</span> <span class="identifier">_1</span><span class="special">)</span> <span class="special">/</span> <span class="identifier">_2</span> <span class="special">*</span> <span class="number">100</span><span class="special">);</span>
</pre>
<p>
          Now, let's use the calculator example to explore some other useful features
          of Proto.
        </p>
<h6>
<a name="boost_proto.users_guide.getting_started.hello_calculator.h4"></a>
          <span class="phrase"><a name="boost_proto.users_guide.getting_started.hello_calculator.detecting_invalid_expressions"></a></span><a class="link" href="users_guide.html#boost_proto.users_guide.getting_started.hello_calculator.detecting_invalid_expressions">Detecting
          Invalid Expressions</a>
        </h6>
<p>
          You may have noticed that you didn't have to define an overloaded <code class="computeroutput"><span class="keyword">operator</span><span class="special">-()</span></code>
          or <code class="computeroutput"><span class="keyword">operator</span><span class="special">/()</span></code>
          -- Proto defined them for you. In fact, Proto overloads <span class="emphasis"><em>all</em></span>
          the operators for you, even though they may not mean anything in your domain-specific
          language. That means it may be possible to create expressions that are
          invalid in your domain. You can detect invalid expressions with Proto by
          defining the <span class="emphasis"><em>grammar</em></span> of your domain-specific language.
        </p>
<p>
          For simplicity, assume that our calculator EDSL should only allow addition,
          subtraction, multiplication and division. Any expression involving any
          other operator is invalid. Using Proto, we can state this requirement by
          defining the grammar of the calculator EDSL. It looks as follows:
        </p>
<pre class="programlisting"><span class="comment">// Define the grammar of calculator expressions</span>
<span class="keyword">struct</span> <span class="identifier">calculator_grammar</span>
  <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">or_</span><span class="special">&lt;</span>
        <span class="identifier">proto</span><span class="special">::</span><span class="identifier">plus</span><span class="special">&lt;</span> <span class="identifier">calculator_grammar</span><span class="special">,</span> <span class="identifier">calculator_grammar</span> <span class="special">&gt;</span>
      <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">minus</span><span class="special">&lt;</span> <span class="identifier">calculator_grammar</span><span class="special">,</span> <span class="identifier">calculator_grammar</span> <span class="special">&gt;</span>
      <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">multiplies</span><span class="special">&lt;</span> <span class="identifier">calculator_grammar</span><span class="special">,</span> <span class="identifier">calculator_grammar</span> <span class="special">&gt;</span>
      <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">divides</span><span class="special">&lt;</span> <span class="identifier">calculator_grammar</span><span class="special">,</span> <span class="identifier">calculator_grammar</span> <span class="special">&gt;</span>
      <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_</span> <span class="special">&gt;</span>
    <span class="special">&gt;</span>
<span class="special">{};</span>
</pre>
<p>
          You can read the above grammar as follows: an expression tree conforms
          to the calculator grammar if it is a binary plus, minus, multiplies or
          divides node, where both child nodes also conform to the calculator grammar;
          or if it is a terminal. In a Proto grammar, <code class="computeroutput"><a class="link" href="../boost/proto/_.html" title="Struct _">proto::_</a></code> is a wildcard that matches
          any type, so <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span>
          <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_</span> <span class="special">&gt;</span></code>
          matches any terminal, whether it is a placeholder or a literal.
        </p>
<div class="note"><table border="0" summary="Note">
<tr>
<td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../doc/src/images/note.png"></td>
<th align="left">Note</th>
</tr>
<tr><td align="left" valign="top"><p>
            This grammar is actually a little looser than we would like. Only placeholders
            and literals that are convertible to doubles are valid terminals. Later
            on we'll see how to express things like that in Proto grammars.
          </p></td></tr>
</table></div>
<p>
          Once you have defined the grammar of your EDSL, you can use the <code class="computeroutput"><a class="link" href="../boost/proto/matches.html" title="Struct template matches">proto::matches&lt;&gt;</a></code> metafunction to check
          whether a given expression type conforms to the grammar. For instance,
          we might add the following to our <code class="computeroutput"><span class="identifier">calculator</span><span class="special">::</span><span class="keyword">operator</span><span class="special">()</span></code> overload:
        </p>
<pre class="programlisting"><span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">Expr</span><span class="special">&gt;</span>
<span class="keyword">struct</span> <span class="identifier">calculator</span>
  <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">extends</span><span class="special">&lt;</span> <span class="comment">/* ... as before ... */</span> <span class="special">&gt;</span>
<span class="special">{</span>
    <span class="comment">/* ... */</span>
    <span class="keyword">double</span> <span class="keyword">operator</span><span class="special">()(</span><span class="keyword">double</span> <span class="identifier">a1</span> <span class="special">=</span> <span class="number">0</span><span class="special">,</span> <span class="keyword">double</span> <span class="identifier">a2</span> <span class="special">=</span> <span class="number">0</span><span class="special">)</span> <span class="keyword">const</span>
    <span class="special">{</span>
        <span class="comment">// Check here that the expression we are about to</span>
        <span class="comment">// evaluate actually conforms to the calculator grammar.</span>
        <span class="identifier">BOOST_MPL_ASSERT</span><span class="special">((</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">matches</span><span class="special">&lt;</span><span class="identifier">Expr</span><span class="special">,</span> <span class="identifier">calculator_grammar</span><span class="special">&gt;));</span>
        <span class="comment">/* ... */</span>
    <span class="special">}</span>
<span class="special">};</span>
</pre>
<p>
          The addition of the <code class="computeroutput"><span class="identifier">BOOST_MPL_ASSERT</span><span class="special">()</span></code> line enforces at compile time that we
          only evaluate expressions that conform to the calculator EDSL's grammar.
          With Proto grammars, <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">matches</span><span class="special">&lt;&gt;</span></code> and <code class="computeroutput"><span class="identifier">BOOST_MPL_ASSERT</span><span class="special">()</span></code> it is very easy to give the users of
          your EDSL short and readable compile-time errors when they accidentally
          misuse your EDSL.
        </p>
<div class="note"><table border="0" summary="Note">
<tr>
<td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../doc/src/images/note.png"></td>
<th align="left">Note</th>
</tr>
<tr><td align="left" valign="top"><p>
            <code class="computeroutput"><span class="identifier">BOOST_MPL_ASSERT</span><span class="special">()</span></code>
            is part of the Boost Metaprogramming Library. To use it, just <code class="computeroutput"><span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">mpl</span><span class="special">/</span><span class="identifier">assert</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span></code>.
          </p></td></tr>
</table></div>
<h6>
<a name="boost_proto.users_guide.getting_started.hello_calculator.h5"></a>
          <span class="phrase"><a name="boost_proto.users_guide.getting_started.hello_calculator.controlling_operator_overloads"></a></span><a class="link" href="users_guide.html#boost_proto.users_guide.getting_started.hello_calculator.controlling_operator_overloads">Controlling
          Operator Overloads</a>
        </h6>
<p>
          Grammars and <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">matches</span><span class="special">&lt;&gt;</span></code>
          make it possible to detect when a user has created an invalid expression
          and issue a compile-time error. But what if you want to prevent users from
          creating invalid expressions in the first place? By using grammars and
          domains together, you can disable any of Proto's operator overloads that
          would create an invalid expression. It is as simple as specifying the EDSL's
          grammar when you define the domain, as shown below:
        </p>
<pre class="programlisting"><span class="comment">// Define a calculator domain. Expression within</span>
<span class="comment">// the calculator domain will be wrapped in the</span>
<span class="comment">// calculator&lt;&gt; expression wrapper.</span>
<span class="comment">// NEW: Any operator overloads that would create an</span>
<span class="comment">//      expression that does not conform to the</span>
<span class="comment">//      calculator grammar is automatically disabled.</span>
<span class="keyword">struct</span> <span class="identifier">calculator_domain</span>
  <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">domain</span><span class="special">&lt;</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">generator</span><span class="special">&lt;</span><span class="identifier">calculator</span><span class="special">&gt;,</span> <span class="identifier">calculator_grammar</span> <span class="special">&gt;</span>
<span class="special">{};</span>
</pre>
<p>
          The only thing we changed is we added <code class="computeroutput"><span class="identifier">calculator_grammar</span></code>
          as the second template parameter to the <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">domain</span><span class="special">&lt;&gt;</span></code> template when defining <code class="computeroutput"><span class="identifier">calculator_domain</span></code>. With this simple addition,
          we disable any of Proto's operator overloads that would create an invalid
          calculator expression.
        </p>
<h6>
<a name="boost_proto.users_guide.getting_started.hello_calculator.h6"></a>
          <span class="phrase"><a name="boost_proto.users_guide.getting_started.hello_calculator.____and_much_more"></a></span><a class="link" href="users_guide.html#boost_proto.users_guide.getting_started.hello_calculator.____and_much_more">...
          And Much More</a>
        </h6>
<p>
          Hopefully, this gives you an idea of what sorts of things Proto can do
          for you. But this only scratches the surface. The rest of this users' guide
          will describe all these features and others in more detail.
        </p>
<p>
          Happy metaprogramming!
        </p>
</div>
</div>
<div class="section">
<div class="titlepage"><div><div><h3 class="title">
<a name="boost_proto.users_guide.front_end"></a><a class="link" href="users_guide.html#boost_proto.users_guide.front_end" title="Fronts Ends: Defining Terminals and Non-Terminals of Your EDSL">Fronts Ends: Defining
      Terminals and Non-Terminals of Your EDSL</a>
</h3></div></div></div>
<div class="toc"><dl class="toc">
<dt><span class="section"><a href="users_guide.html#boost_proto.users_guide.front_end.making_terminals">Making
        Terminals</a></span></dt>
<dt><span class="section"><a href="users_guide.html#boost_proto.users_guide.front_end.proto_s_operator_overloads">Proto's
        Operator Overloads</a></span></dt>
<dt><span class="section"><a href="users_guide.html#boost_proto.users_guide.front_end.making_lazy_functions">Making
        Lazy Functions</a></span></dt>
<dt><span class="section"><a href="users_guide.html#boost_proto.users_guide.front_end.customizing_expressions_in_your_domain">Customizing
        Expressions in Your Domain</a></span></dt>
<dt><span class="section"><a href="users_guide.html#boost_proto.users_guide.front_end.define_operators">Adapting
        Existing Types to Proto</a></span></dt>
<dt><span class="section"><a href="users_guide.html#boost_proto.users_guide.front_end.code_repetition">Generating
        Repetitive Code with the Preprocessor</a></span></dt>
</dl></div>
<p>
        Here is the fun part: designing your own mini-programming language. In this
        section we'll talk about the nuts and bolts of designing an EDSL interface
        using Proto. We'll cover the definition of terminals and lazy functions that
        the users of your EDSL will get to program with. We'll also talk about Proto's
        expression template-building operator overloads, and about ways to add additional
        members to expressions within your domain.
      </p>
<div class="section">
<div class="titlepage"><div><div><h4 class="title">
<a name="boost_proto.users_guide.front_end.making_terminals"></a><a class="link" href="users_guide.html#boost_proto.users_guide.front_end.making_terminals" title="Making Terminals">Making
        Terminals</a>
</h4></div></div></div>
<p>
          As we saw with the Calculator example from the Introduction, the simplest
          way to get an EDSL up and running is simply to define some terminals, as
          follows.
        </p>
<pre class="programlisting"><span class="comment">// Define a literal integer Proto expression.</span>
<span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">&gt;::</span><span class="identifier">type</span> <span class="identifier">i</span> <span class="special">=</span> <span class="special">{</span><span class="number">0</span><span class="special">};</span>

<span class="comment">// This creates an expression template.</span>
<span class="identifier">i</span> <span class="special">+</span> <span class="number">1</span><span class="special">;</span>
</pre>
<p>
          With some terminals and Proto's operator overloads, you can immediately
          start creating expression templates.
        </p>
<p>
          Defining terminals -- with aggregate initialization -- can be a little
          awkward at times. Proto provides an easier-to-use wrapper for literals
          that can be used to construct Protofied terminal expressions. It's called
          <code class="computeroutput"><a class="link" href="../boost/proto/literal.html" title="Struct template literal">proto::literal&lt;&gt;</a></code>.
        </p>
<pre class="programlisting"><span class="comment">// Define a literal integer Proto expression.</span>
<span class="identifier">proto</span><span class="special">::</span><span class="identifier">literal</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">&gt;</span> <span class="identifier">i</span> <span class="special">=</span> <span class="number">0</span><span class="special">;</span>

<span class="comment">// Proto literals are really just Proto terminal expressions.</span>
<span class="comment">// For example, this builds a Proto expression template:</span>
<span class="identifier">i</span> <span class="special">+</span> <span class="number">1</span><span class="special">;</span>
</pre>
<p>
          There is also a <code class="computeroutput"><a class="link" href="../boost/proto/lit_idp728253920.html" title="Function lit">proto::lit()</a></code> function for constructing
          a <code class="computeroutput"><a class="link" href="../boost/proto/literal.html" title="Struct template literal">proto::literal&lt;&gt;</a></code> in-place. The above
          expression can simply be written as:
        </p>
<pre class="programlisting"><span class="comment">// proto::lit(0) creates an integer terminal expression</span>
<span class="identifier">proto</span><span class="special">::</span><span class="identifier">lit</span><span class="special">(</span><span class="number">0</span><span class="special">)</span> <span class="special">+</span> <span class="number">1</span><span class="special">;</span>
</pre>
</div>
<div class="section">
<div class="titlepage"><div><div><h4 class="title">
<a name="boost_proto.users_guide.front_end.proto_s_operator_overloads"></a><a class="link" href="users_guide.html#boost_proto.users_guide.front_end.proto_s_operator_overloads" title="Proto's Operator Overloads">Proto's
        Operator Overloads</a>
</h4></div></div></div>
<p>
          Once we have some Proto terminals, expressions involving those terminals
          build expression trees for us. Proto defines overloads for each of C++'s
          overloadable operators in the <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">proto</span></code>
          namespace. As long as one operand is a Proto expression, the result of
          the operation is a tree node representing that operation.
        </p>
<div class="note"><table border="0" summary="Note">
<tr>
<td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../doc/src/images/note.png"></td>
<th align="left">Note</th>
</tr>
<tr><td align="left" valign="top"><p>
            Proto's operator overloads live in the <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">proto</span></code>
            namespace and are found via ADL (argument-dependent lookup). That is
            why expressions must be "tainted" with Proto-ness for Proto
            to be able to build trees out of expressions.
          </p></td></tr>
</table></div>
<p>
          As a result of Proto's operator overloads, we can say:
        </p>
<pre class="programlisting"><span class="special">-</span><span class="identifier">_1</span><span class="special">;</span>        <span class="comment">// OK, build a unary-negate tree node</span>
<span class="identifier">_1</span> <span class="special">+</span> <span class="number">42</span><span class="special">;</span>    <span class="comment">// OK, build a binary-plus tree node</span>
</pre>
<p>
          For the most part, this Just Works and you don't need to think about it,
          but a few operators are special and it can be helpful to know how Proto
          handles them.
        </p>
<h6>
<a name="boost_proto.users_guide.front_end.proto_s_operator_overloads.h0"></a>
          <span class="phrase"><a name="boost_proto.users_guide.front_end.proto_s_operator_overloads.assignment__subscript__and_function_call_operators"></a></span><a class="link" href="users_guide.html#boost_proto.users_guide.front_end.proto_s_operator_overloads.assignment__subscript__and_function_call_operators">Assignment,
          Subscript, and Function Call Operators</a>
        </h6>
<p>
          Proto also overloads <code class="computeroutput"><span class="keyword">operator</span><span class="special">=</span></code>, <code class="computeroutput"><span class="keyword">operator</span><span class="special">[]</span></code>, and <code class="computeroutput"><span class="keyword">operator</span><span class="special">()</span></code>, but these operators are member functions
          of the expression template rather than free functions in Proto's namespace.
          The following are valid Proto expressions:
        </p>
<pre class="programlisting"><span class="identifier">_1</span> <span class="special">=</span> <span class="number">5</span><span class="special">;</span>     <span class="comment">// OK, builds a binary assign tree node</span>
<span class="identifier">_1</span><span class="special">[</span><span class="number">6</span><span class="special">];</span>      <span class="comment">// OK, builds a binary subscript tree node</span>
<span class="identifier">_1</span><span class="special">();</span>       <span class="comment">// OK, builds a unary function tree node</span>
<span class="identifier">_1</span><span class="special">(</span><span class="number">7</span><span class="special">);</span>      <span class="comment">// OK, builds a binary function tree node</span>
<span class="identifier">_1</span><span class="special">(</span><span class="number">8</span><span class="special">,</span><span class="number">9</span><span class="special">);</span>    <span class="comment">// OK, builds a ternary function tree node</span>
<span class="comment">// ... etc.</span>
</pre>
<p>
          For the first two lines, assignment and subscript, it should be fairly
          unsurprising that the resulting expression node should be binary. After
          all, there are two operands in each expression. It may be surprising at
          first that what appears to be a function call with no arguments, <code class="computeroutput"><span class="identifier">_1</span><span class="special">()</span></code>,
          actually creates an expression node with one child. The child is <code class="computeroutput"><span class="identifier">_1</span></code> itself. Likewise, the expression
          <code class="computeroutput"><span class="identifier">_1</span><span class="special">(</span><span class="number">7</span><span class="special">)</span></code> has two
          children: <code class="computeroutput"><span class="identifier">_1</span></code> and <code class="computeroutput"><span class="number">7</span></code>.
        </p>
<p>
          Because these operators can only be defined as member functions, the following
          expressions are invalid:
        </p>
<pre class="programlisting"><span class="keyword">int</span> <span class="identifier">i</span><span class="special">;</span>
<span class="identifier">i</span> <span class="special">=</span> <span class="identifier">_1</span><span class="special">;</span>         <span class="comment">// ERROR: cannot assign _1 to an int</span>

<span class="keyword">int</span> <span class="special">*</span><span class="identifier">p</span><span class="special">;</span>
<span class="identifier">p</span><span class="special">[</span><span class="identifier">_1</span><span class="special">];</span>          <span class="comment">// ERROR: cannot use _1 as an index</span>

<span class="identifier">std</span><span class="special">::</span><span class="identifier">sin</span><span class="special">(</span><span class="identifier">_1</span><span class="special">);</span>   <span class="comment">// ERROR: cannot call std::sin() with _1</span>
</pre>
<p>
          Also, C++ has special rules for overloads of <code class="computeroutput"><span class="keyword">operator</span><span class="special">-&gt;</span></code> that make it useless for building
          expression templates, so Proto does not overload it.
        </p>
<h6>
<a name="boost_proto.users_guide.front_end.proto_s_operator_overloads.h1"></a>
          <span class="phrase"><a name="boost_proto.users_guide.front_end.proto_s_operator_overloads.the_address_of_operator"></a></span><a class="link" href="users_guide.html#boost_proto.users_guide.front_end.proto_s_operator_overloads.the_address_of_operator">The
          Address-Of Operator</a>
        </h6>
<p>
          Proto overloads the address-of operator for expression types, so that the
          following code creates a new unary address-of tree node:
        </p>
<pre class="programlisting"><span class="special">&amp;</span><span class="identifier">_1</span><span class="special">;</span>    <span class="comment">// OK, creates a unary address-of tree node</span>
</pre>
<p>
          It does <span class="emphasis"><em>not</em></span> return the address of the <code class="computeroutput"><span class="identifier">_1</span></code> object. However, there is special
          code in Proto such that a unary address-of node is implicitly convertible
          to a pointer to its child. In other words, the following code works and
          does what you might expect, but not in the obvious way:
        </p>
<pre class="programlisting"><span class="keyword">typedef</span>
    <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span> <span class="identifier">placeholder</span><span class="special">&lt;</span><span class="number">0</span><span class="special">&gt;</span> <span class="special">&gt;::</span><span class="identifier">type</span>
<span class="identifier">_1_type</span><span class="special">;</span>

<span class="identifier">_1_type</span> <span class="keyword">const</span> <span class="identifier">_1</span> <span class="special">=</span> <span class="special">{{}};</span>
<span class="identifier">_1_type</span> <span class="keyword">const</span> <span class="special">*</span> <span class="identifier">p</span> <span class="special">=</span> <span class="special">&amp;</span><span class="identifier">_1</span><span class="special">;</span> <span class="comment">// OK, &amp;_1 implicitly converted</span>
</pre>
</div>
<div class="section">
<div class="titlepage"><div><div><h4 class="title">
<a name="boost_proto.users_guide.front_end.making_lazy_functions"></a><a class="link" href="users_guide.html#boost_proto.users_guide.front_end.making_lazy_functions" title="Making Lazy Functions">Making
        Lazy Functions</a>
</h4></div></div></div>
<p>
          If we limited ourselves to nothing but terminals and operator overloads,
          our embedded domain-specific languages wouldn't be very expressive. Imagine
          that we wanted to extend our calculator EDSL with a full suite of math
          functions like <code class="computeroutput"><span class="identifier">sin</span><span class="special">()</span></code>
          and <code class="computeroutput"><span class="identifier">pow</span><span class="special">()</span></code>
          that we could invoke lazily as follows.
        </p>
<pre class="programlisting"><span class="comment">// A calculator expression that takes one argument</span>
<span class="comment">// and takes the sine of it.</span>
<span class="identifier">sin</span><span class="special">(</span><span class="identifier">_1</span><span class="special">);</span>
</pre>
<p>
          We would like the above to create an expression template representing a
          function invocation. When that expression is evaluated, it should cause
          the function to be invoked. (At least, that's the meaning of function invocation
          we'd like the calculator EDSL to have.) You can define <code class="computeroutput"><span class="identifier">sin</span></code>
          quite simply as follows.
        </p>
<pre class="programlisting"><span class="comment">// "sin" is a Proto terminal containing a function pointer</span>
<span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span> <span class="keyword">double</span><span class="special">(*)(</span><span class="keyword">double</span><span class="special">)</span> <span class="special">&gt;::</span><span class="identifier">type</span> <span class="keyword">const</span> <span class="identifier">sin</span> <span class="special">=</span> <span class="special">{&amp;</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">sin</span><span class="special">};</span>
</pre>
<p>
          In the above, we define <code class="computeroutput"><span class="identifier">sin</span></code>
          as a Proto terminal containing a pointer to the <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">sin</span><span class="special">()</span></code> function. Now we can use <code class="computeroutput"><span class="identifier">sin</span></code> as a lazy function. The <code class="computeroutput"><span class="identifier">default_context</span></code> that we saw in the Introduction
          knows how to evaluate lazy functions. Consider the following:
        </p>
<pre class="programlisting"><span class="keyword">double</span> <span class="identifier">pi</span> <span class="special">=</span> <span class="number">3.1415926535</span><span class="special">;</span>
<span class="identifier">proto</span><span class="special">::</span><span class="identifier">default_context</span> <span class="identifier">ctx</span><span class="special">;</span>
<span class="comment">// Create a lazy "sin" invocation and immediately evaluate it</span>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">eval</span><span class="special">(</span> <span class="identifier">sin</span><span class="special">(</span><span class="identifier">pi</span><span class="special">/</span><span class="number">2</span><span class="special">),</span> <span class="identifier">ctx</span> <span class="special">)</span> <span class="special">&lt;&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span>
</pre>
<p>
          The above code prints out:
        </p>
<pre class="programlisting">1</pre>
<p>
          I'm no expert at trigonometry, but that looks right to me.
        </p>
<p>
          We can write <code class="computeroutput"><span class="identifier">sin</span><span class="special">(</span><span class="identifier">pi</span><span class="special">/</span><span class="number">2</span><span class="special">)</span></code> because the <code class="computeroutput"><span class="identifier">sin</span></code>
          object, which is a Proto terminal, has an overloaded <code class="computeroutput"><span class="keyword">operator</span><span class="special">()()</span></code> that builds a node representing a function
          call invocation. The actual type of <code class="computeroutput"><span class="identifier">sin</span><span class="special">(</span><span class="identifier">pi</span><span class="special">/</span><span class="number">2</span><span class="special">)</span></code> is actually
          something like this:
        </p>
<pre class="programlisting"><span class="comment">// The type of the expression sin(pi/2):</span>
<span class="identifier">proto</span><span class="special">::</span><span class="identifier">function</span><span class="special">&lt;</span>
    <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span> <span class="keyword">double</span><span class="special">(*)(</span><span class="keyword">double</span><span class="special">)</span> <span class="special">&gt;::</span><span class="identifier">type</span> <span class="keyword">const</span> <span class="special">&amp;</span>
    <span class="identifier">proto</span><span class="special">::</span><span class="identifier">result_of</span><span class="special">::</span><span class="identifier">as_child</span><span class="special">&lt;</span> <span class="keyword">double</span> <span class="keyword">const</span> <span class="special">&gt;::</span><span class="identifier">type</span>
<span class="special">&gt;::</span><span class="identifier">type</span>
</pre>
<p>
          This type further expands to an unsightly node type with a <span class="emphasis"><em>tag</em></span>
          type of <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">function</span></code> and two children: the first
          representing the function to be invoked, and the second representing the
          argument to the function. (Node tag types describe the operation that created
          the node. The difference between <code class="computeroutput"><span class="identifier">a</span>
          <span class="special">+</span> <span class="identifier">b</span></code>
          and <code class="computeroutput"><span class="identifier">a</span> <span class="special">-</span>
          <span class="identifier">b</span></code> is that the former has tag
          type <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">plus</span></code> and the latter has tag type <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">minus</span></code>. Tag types are pure compile-time
          information.)
        </p>
<div class="note"><table border="0" summary="Note">
<tr>
<td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../doc/src/images/note.png"></td>
<th align="left">Note</th>
</tr>
<tr><td align="left" valign="top"><p>
            In the type computation above, <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">result_of</span><span class="special">::</span><span class="identifier">as_child</span><span class="special">&lt;&gt;</span></code> is a metafunction that ensures
            its argument is a Proto expression type. If it isn't one already, it
            becomes a Proto terminal. We'll learn more about this metafunction, along
            with <code class="computeroutput"><a class="link" href="../boost/proto/as_child_idp731140288.html" title="Function as_child">proto::as_child()</a></code>, its runtime counterpart,
            <a class="link" href="users_guide.html#boost_proto.users_guide.front_end.customizing_expressions_in_your_domain.per_domain_as_child" title="Controlling How Child Expressions Are Captured">later</a>.
            For now, you can forget about it.
          </p></td></tr>
</table></div>
<p>
          It is important to note that there is nothing special about terminals that
          contain function pointers. <span class="emphasis"><em>Any</em></span> Proto expression has
          an overloaded function call operator. Consider:
        </p>
<pre class="programlisting"><span class="comment">// This compiles!</span>
<span class="identifier">proto</span><span class="special">::</span><span class="identifier">lit</span><span class="special">(</span><span class="number">1</span><span class="special">)(</span><span class="number">2</span><span class="special">)(</span><span class="number">3</span><span class="special">,</span><span class="number">4</span><span class="special">)(</span><span class="number">5</span><span class="special">,</span><span class="number">6</span><span class="special">,</span><span class="number">7</span><span class="special">,</span><span class="number">8</span><span class="special">);</span>
</pre>
<p>
          That may look strange at first. It creates an integer terminal with <code class="computeroutput"><a class="link" href="../boost/proto/lit_idp728253920.html" title="Function lit">proto::lit()</a></code>, and then invokes it like
          a function again and again. What does it mean? Who knows?! You get to decide
          when you define an evaluation context or a transform. But more on that
          later.
        </p>
<h6>
<a name="boost_proto.users_guide.front_end.making_lazy_functions.h0"></a>
          <span class="phrase"><a name="boost_proto.users_guide.front_end.making_lazy_functions.making_lazy_functions__continued"></a></span><a class="link" href="users_guide.html#boost_proto.users_guide.front_end.making_lazy_functions.making_lazy_functions__continued">Making
          Lazy Functions, Continued</a>
        </h6>
<p>
          Now, what if we wanted to add a <code class="computeroutput"><span class="identifier">pow</span><span class="special">()</span></code> function to our calculator EDSL that
          users could invoke as follows?
        </p>
<pre class="programlisting"><span class="comment">// A calculator expression that takes one argument</span>
<span class="comment">// and raises it to the 2nd power</span>
<span class="identifier">pow</span><span class="special">&lt;</span> <span class="number">2</span> <span class="special">&gt;(</span><span class="identifier">_1</span><span class="special">);</span>
</pre>
<p>
          The simple technique described above of making <code class="computeroutput"><span class="identifier">pow</span></code>
          a terminal containing a function pointer doesn't work here. If <code class="computeroutput"><span class="identifier">pow</span></code> is an object, then the expression
          <code class="computeroutput"><span class="identifier">pow</span><span class="special">&lt;</span>
          <span class="number">2</span> <span class="special">&gt;(</span><span class="identifier">_1</span><span class="special">)</span></code> is
          not valid C++. (Well, technically it is; it means, <code class="computeroutput"><span class="identifier">pow</span></code>
          less than 2, greater than <code class="computeroutput"><span class="special">(</span><span class="identifier">_1</span><span class="special">)</span></code>,
          which is nothing at all like what we want.) <code class="computeroutput"><span class="identifier">pow</span></code>
          should be a real function template. But it must be an unusual function:
          one that returns an expression template.
        </p>
<p>
          With <code class="computeroutput"><span class="identifier">sin</span></code>, we relied on
          Proto to provide an overloaded <code class="computeroutput"><span class="keyword">operator</span><span class="special">()()</span></code> to build an expression node with tag
          type <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">function</span></code> for us. Now we'll need to do
          so ourselves. As before, the node will have two children: the function
          to invoke and the function's argument.
        </p>
<p>
          With <code class="computeroutput"><span class="identifier">sin</span></code>, the function
          to invoke was a raw function pointer wrapped in a Proto terminal. In the
          case of <code class="computeroutput"><span class="identifier">pow</span></code>, we want it
          to be a terminal containing TR1-style function object. This will allow
          us to parameterize the function on the exponent. Below is the implementation
          of a simple TR1-style wrapper for the <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">pow</span></code>
          function:
        </p>
<pre class="programlisting"><span class="comment">// Define a pow_fun function object</span>
<span class="keyword">template</span><span class="special">&lt;</span> <span class="keyword">int</span> <span class="identifier">Exp</span> <span class="special">&gt;</span>
<span class="keyword">struct</span> <span class="identifier">pow_fun</span>
<span class="special">{</span>
    <span class="keyword">typedef</span> <span class="keyword">double</span> <span class="identifier">result_type</span><span class="special">;</span>

    <span class="keyword">double</span> <span class="keyword">operator</span><span class="special">()(</span><span class="keyword">double</span> <span class="identifier">d</span><span class="special">)</span> <span class="keyword">const</span>
    <span class="special">{</span>
        <span class="keyword">return</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">pow</span><span class="special">(</span><span class="identifier">d</span><span class="special">,</span> <span class="identifier">Exp</span><span class="special">);</span>
    <span class="special">}</span>
<span class="special">};</span>
</pre>
<p>
          Following the <code class="computeroutput"><span class="identifier">sin</span></code> example,
          we want <code class="computeroutput"><span class="identifier">pow</span><span class="special">&lt;</span>
          <span class="number">1</span> <span class="special">&gt;(</span>
          <span class="identifier">pi</span><span class="special">/</span><span class="number">2</span> <span class="special">)</span></code> to have
          a type like this:
        </p>
<pre class="programlisting"><span class="comment">// The type of the expression pow&lt;1&gt;(pi/2):</span>
<span class="identifier">proto</span><span class="special">::</span><span class="identifier">function</span><span class="special">&lt;</span>
    <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span> <span class="identifier">pow_fun</span><span class="special">&lt;</span><span class="number">1</span><span class="special">&gt;</span> <span class="special">&gt;::</span><span class="identifier">type</span>
    <span class="identifier">proto</span><span class="special">::</span><span class="identifier">result_of</span><span class="special">::</span><span class="identifier">as_child</span><span class="special">&lt;</span> <span class="keyword">double</span> <span class="keyword">const</span> <span class="special">&gt;::</span><span class="identifier">type</span>
<span class="special">&gt;::</span><span class="identifier">type</span>
</pre>
<p>
          We could write a <code class="computeroutput"><span class="identifier">pow</span><span class="special">()</span></code>
          function using code like this, but it's verbose and error prone; it's too
          easy to introduce subtle bugs by forgetting to call <code class="computeroutput"><a class="link" href="../boost/proto/as_child_idp731140288.html" title="Function as_child">proto::as_child()</a></code>
          where necessary, resulting in code that seems to work but sometimes doesn't.
          Proto provides a better way to construct expression nodes: <code class="computeroutput"><a class="link" href="../boost/proto/make_expr_idp728389200.html" title="Function make_expr">proto::make_expr()</a></code>.
        </p>
<h6>
<a name="boost_proto.users_guide.front_end.making_lazy_functions.h1"></a>
          <span class="phrase"><a name="boost_proto.users_guide.front_end.making_lazy_functions.lazy_functions_made_simple_with__literal_make_expr____literal_"></a></span><a class="link" href="users_guide.html#boost_proto.users_guide.front_end.making_lazy_functions.lazy_functions_made_simple_with__literal_make_expr____literal_">Lazy
          Functions Made Simple With <code class="literal">make_expr()</code></a>
        </h6>
<p>
          Proto provides a helper for building expression templates called <code class="computeroutput"><a class="link" href="../boost/proto/make_expr_idp728389200.html" title="Function make_expr">proto::make_expr()</a></code>. We can concisely define
          the <code class="computeroutput"><span class="identifier">pow</span><span class="special">()</span></code>
          function with it as below.
        </p>
<pre class="programlisting"><span class="comment">// Define a lazy pow() function for the calculator EDSL.</span>
<span class="comment">// Can be used as: pow&lt; 2 &gt;(_1)</span>
<span class="keyword">template</span><span class="special">&lt;</span> <span class="keyword">int</span> <span class="identifier">Exp</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">Arg</span> <span class="special">&gt;</span>
<span class="keyword">typename</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">result_of</span><span class="special">::</span><span class="identifier">make_expr</span><span class="special">&lt;</span>
    <span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">function</span>  <span class="comment">// Tag type</span>
  <span class="special">,</span> <span class="identifier">pow_fun</span><span class="special">&lt;</span> <span class="identifier">Exp</span> <span class="special">&gt;</span>        <span class="comment">// First child (by value)</span>
  <span class="special">,</span> <span class="identifier">Arg</span> <span class="keyword">const</span> <span class="special">&amp;</span>           <span class="comment">// Second child (by reference)</span>
<span class="special">&gt;::</span><span class="identifier">type</span> <span class="keyword">const</span>
<span class="identifier">pow</span><span class="special">(</span><span class="identifier">Arg</span> <span class="keyword">const</span> <span class="special">&amp;</span><span class="identifier">arg</span><span class="special">)</span>
<span class="special">{</span>
    <span class="keyword">return</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">make_expr</span><span class="special">&lt;</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">function</span><span class="special">&gt;(</span>
        <span class="identifier">pow_fun</span><span class="special">&lt;</span><span class="identifier">Exp</span><span class="special">&gt;()</span>    <span class="comment">// First child (by value)</span>
      <span class="special">,</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">ref</span><span class="special">(</span><span class="identifier">arg</span><span class="special">)</span>   <span class="comment">// Second child (by reference)</span>
    <span class="special">);</span>
<span class="special">}</span>
</pre>
<p>
          There are some things to notice about the above code. We use <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">result_of</span><span class="special">::</span><span class="identifier">make_expr</span><span class="special">&lt;&gt;</span></code>
          to calculate the return type. The first template parameter is the tag type
          for the expression node we're building -- in this case, <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">function</span></code>.
        </p>
<p>
          Subsequent template parameters to <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">result_of</span><span class="special">::</span><span class="identifier">make_expr</span><span class="special">&lt;&gt;</span></code> represent child nodes. If a child
          type is not already a Proto expression, it is automatically made into a
          terminal with <code class="computeroutput"><a class="link" href="../boost/proto/as_child_idp731140288.html" title="Function as_child">proto::as_child()</a></code>.
          A type such as <code class="computeroutput"><span class="identifier">pow_fun</span><span class="special">&lt;</span><span class="identifier">Exp</span><span class="special">&gt;</span></code> results in terminal that is held by
          value, whereas a type like <code class="computeroutput"><span class="identifier">Arg</span>
          <span class="keyword">const</span> <span class="special">&amp;</span></code>
          (note the reference) indicates that the result should be held by reference.
        </p>
<p>
          In the function body is the runtime invocation of <code class="computeroutput"><a class="link" href="../boost/proto/make_expr_idp728389200.html" title="Function make_expr">proto::make_expr()</a></code>.
          It closely mirrors the return type calculation. <code class="computeroutput"><a class="link" href="../boost/proto/make_expr_idp728389200.html" title="Function make_expr">proto::make_expr()</a></code>
          requires you to specify the node's tag type as a template parameter. The
          arguments to the function become the node's children. When a child should
          be stored by value, nothing special needs to be done. When a child should
          be stored by reference, you must use the <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">ref</span><span class="special">()</span></code> function to wrap the argument.
        </p>
<p>
          And that's it! <code class="computeroutput"><a class="link" href="../boost/proto/make_expr_idp728389200.html" title="Function make_expr">proto::make_expr()</a></code>
          is the lazy person's way to make a lazy funtion.
        </p>
</div>
<div class="section">
<div class="titlepage"><div><div><h4 class="title">
<a name="boost_proto.users_guide.front_end.customizing_expressions_in_your_domain"></a><a class="link" href="users_guide.html#boost_proto.users_guide.front_end.customizing_expressions_in_your_domain" title="Customizing Expressions in Your Domain">Customizing
        Expressions in Your Domain</a>
</h4></div></div></div>
<div class="toc"><dl class="toc">
<dt><span class="section"><a href="users_guide.html#boost_proto.users_guide.front_end.customizing_expressions_in_your_domain.domains">Domains</a></span></dt>
<dt><span class="section"><a href="users_guide.html#boost_proto.users_guide.front_end.customizing_expressions_in_your_domain.extends">The
          <code class="literal">extends&lt;&gt;</code> Expression Wrapper</a></span></dt>
<dt><span class="section"><a href="users_guide.html#boost_proto.users_guide.front_end.customizing_expressions_in_your_domain.expression_generators">Expression
          Generators</a></span></dt>
<dt><span class="section"><a href="users_guide.html#boost_proto.users_guide.front_end.customizing_expressions_in_your_domain.inhibiting_overloads">Controlling
          Operator Overloads</a></span></dt>
<dt><span class="section"><a href="users_guide.html#boost_proto.users_guide.front_end.customizing_expressions_in_your_domain.per_domain_as_child">Controlling
          How Child Expressions Are Captured</a></span></dt>
<dt><span class="section"><a href="users_guide.html#boost_proto.users_guide.front_end.customizing_expressions_in_your_domain.subdomains">EDSL
          Interoperatability: Sub-Domains</a></span></dt>
</dl></div>
<p>
          In this section, we'll learn all about <span class="emphasis"><em>domains</em></span>. In
          particular, we'll learn:
        </p>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
<li class="listitem">
              How to associate Proto expressions with a domain,
            </li>
<li class="listitem">
              How to add members to expressions within a domain,
            </li>
<li class="listitem">
              How to use a <span class="emphasis"><em>generator</em></span> to post-process all new
              expressions created in your domain,
            </li>
<li class="listitem">
              How to control which operators are overloaded in a domain,
            </li>
<li class="listitem">
              How to specify capturing policies for child expressions and non-Proto
              objects, and
            </li>
<li class="listitem">
              How to make expressions from separate domains interoperate.
            </li>
</ul></div>
<div class="section">
<div class="titlepage"><div><div><h5 class="title">
<a name="boost_proto.users_guide.front_end.customizing_expressions_in_your_domain.domains"></a><a class="link" href="users_guide.html#boost_proto.users_guide.front_end.customizing_expressions_in_your_domain.domains" title="Domains">Domains</a>
</h5></div></div></div>
<p>
            In the <a class="link" href="users_guide.html#boost_proto.users_guide.getting_started.hello_calculator" title="Hello Calculator">Hello
            Calculator</a> section, we looked into making calculator expressions
            directly usable as lambda expressions in calls to STL algorithms, as
            below:
          </p>
<pre class="programlisting"><span class="keyword">double</span> <span class="identifier">data</span><span class="special">[]</span> <span class="special">=</span> <span class="special">{</span><span class="number">1.</span><span class="special">,</span> <span class="number">2.</span><span class="special">,</span> <span class="number">3.</span><span class="special">,</span> <span class="number">4.</span><span class="special">};</span>

<span class="comment">// Use the calculator EDSL to square each element ... HOW?</span>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">transform</span><span class="special">(</span> <span class="identifier">data</span><span class="special">,</span> <span class="identifier">data</span> <span class="special">+</span> <span class="number">4</span><span class="special">,</span> <span class="identifier">data</span><span class="special">,</span> <span class="identifier">_1</span> <span class="special">*</span> <span class="identifier">_1</span> <span class="special">);</span>
</pre>
<p>
            The difficulty, if you recall, was that by default Proto expressions
            don't have interesting behaviors of their own. They're just trees. In
            particular, the expression <code class="computeroutput"><span class="identifier">_1</span>
            <span class="special">*</span> <span class="identifier">_1</span></code>
            won't have an <code class="computeroutput"><span class="keyword">operator</span><span class="special">()</span></code>
            that takes a double and returns a double like <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">transform</span><span class="special">()</span></code> expects -- unless we give it one. To
            make this work, we needed to define an expression wrapper type that defined
            the <code class="computeroutput"><span class="keyword">operator</span><span class="special">()</span></code>
            member function, and we needed to associate the wrapper with the calculator
            <span class="emphasis"><em>domain</em></span>.
          </p>
<p>
            In Proto, the term <span class="emphasis"><em>domain</em></span> refers to a type that
            associates expressions in that domain to an expression <span class="emphasis"><em>generator</em></span>.
            The generator is just a function object that accepts an expression and
            does something to it, like wrapping it in an expression wrapper.
          </p>
<p>
            You can also use a domain to associate expressions with a grammar. When
            you specify a domain's grammar, Proto ensures that all the expressions
            it generates in that domain conform to the domain's grammar. It does
            that by disabling any operator overloads that would create invalid expressions.
          </p>
</div>
<div class="section">
<div class="titlepage"><div><div><h5 class="title">
<a name="boost_proto.users_guide.front_end.customizing_expressions_in_your_domain.extends"></a><a class="link" href="users_guide.html#boost_proto.users_guide.front_end.customizing_expressions_in_your_domain.extends" title="The extends&lt;&gt; Expression Wrapper">The
          <code class="literal">extends&lt;&gt;</code> Expression Wrapper</a>
</h5></div></div></div>
<p>
            The first step to giving your calculator expressions extra behaviors
            is to define a calculator domain. All expressions within the calculator
            domain will be imbued with calculator-ness, as we'll see.
          </p>
<pre class="programlisting"><span class="comment">// A type to be used as a domain tag (to be defined below)</span>
<span class="keyword">struct</span> <span class="identifier">calculator_domain</span><span class="special">;</span>
</pre>
<p>
            We use this domain type when extending the <code class="computeroutput"><a class="link" href="../boost/proto/expr.html" title="Struct template expr">proto::expr&lt;&gt;</a></code>
            type, which we do with the <code class="computeroutput"><a class="link" href="../boost/proto/extends.html" title="Struct template extends">proto::extends&lt;&gt;</a></code>
            class template. Here is our expression wrapper, which imbues an expression
            with calculator-ness. It is described below.
          </p>
<pre class="programlisting"><span class="comment">// The calculator&lt;&gt; expression wrapper makes expressions</span>
<span class="comment">// function objects.</span>
<span class="keyword">template</span><span class="special">&lt;</span> <span class="keyword">typename</span> <span class="identifier">Expr</span> <span class="special">&gt;</span>
<span class="keyword">struct</span> <span class="identifier">calculator</span>
  <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">extends</span><span class="special">&lt;</span> <span class="identifier">Expr</span><span class="special">,</span> <span class="identifier">calculator</span><span class="special">&lt;</span> <span class="identifier">Expr</span> <span class="special">&gt;,</span> <span class="identifier">calculator_domain</span> <span class="special">&gt;</span>
<span class="special">{</span>
    <span class="keyword">typedef</span>
        <span class="identifier">proto</span><span class="special">::</span><span class="identifier">extends</span><span class="special">&lt;</span> <span class="identifier">Expr</span><span class="special">,</span> <span class="identifier">calculator</span><span class="special">&lt;</span> <span class="identifier">Expr</span> <span class="special">&gt;,</span> <span class="identifier">calculator_domain</span> <span class="special">&gt;</span>
    <span class="identifier">base_type</span><span class="special">;</span>

    <span class="identifier">calculator</span><span class="special">(</span> <span class="identifier">Expr</span> <span class="keyword">const</span> <span class="special">&amp;</span><span class="identifier">expr</span> <span class="special">=</span> <span class="identifier">Expr</span><span class="special">()</span> <span class="special">)</span>
      <span class="special">:</span> <span class="identifier">base_type</span><span class="special">(</span> <span class="identifier">expr</span> <span class="special">)</span>
    <span class="special">{}</span>

    <span class="comment">// This is usually needed because by default, the compiler-</span>
    <span class="comment">// generated assignment operator hides extends&lt;&gt;::operator=</span>
    <span class="identifier">BOOST_PROTO_EXTENDS_USING_ASSIGN</span><span class="special">(</span><span class="identifier">calculator</span><span class="special">)</span>

    <span class="keyword">typedef</span> <span class="keyword">double</span> <span class="identifier">result_type</span><span class="special">;</span>

    <span class="comment">// Hide base_type::operator() by defining our own which</span>
    <span class="comment">// evaluates the calculator expression with a calculator context.</span>
    <span class="identifier">result_type</span> <span class="keyword">operator</span><span class="special">()(</span> <span class="keyword">double</span> <span class="identifier">d1</span> <span class="special">=</span> <span class="number">0.0</span><span class="special">,</span> <span class="keyword">double</span> <span class="identifier">d2</span> <span class="special">=</span> <span class="number">0.0</span> <span class="special">)</span> <span class="keyword">const</span>
    <span class="special">{</span>
        <span class="comment">// As defined in the Hello Calculator section.</span>
        <span class="identifier">calculator_context</span> <span class="identifier">ctx</span><span class="special">;</span>

        <span class="comment">// ctx.args is a vector&lt;double&gt; that holds the values</span>
        <span class="comment">// with which we replace the placeholders (e.g., _1 and _2)</span>
        <span class="comment">// in the expression.</span>
        <span class="identifier">ctx</span><span class="special">.</span><span class="identifier">args</span><span class="special">.</span><span class="identifier">push_back</span><span class="special">(</span> <span class="identifier">d1</span> <span class="special">);</span> <span class="comment">// _1 gets the value of d1</span>
        <span class="identifier">ctx</span><span class="special">.</span><span class="identifier">args</span><span class="special">.</span><span class="identifier">push_back</span><span class="special">(</span> <span class="identifier">d2</span> <span class="special">);</span> <span class="comment">// _2 gets the value of d2</span>

        <span class="keyword">return</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">eval</span><span class="special">(*</span><span class="keyword">this</span><span class="special">,</span> <span class="identifier">ctx</span> <span class="special">);</span> <span class="comment">// evaluate the expression</span>
    <span class="special">}</span>
<span class="special">};</span>
</pre>
<p>
            We want calculator expressions to be function objects, so we have to
            define an <code class="computeroutput"><span class="keyword">operator</span><span class="special">()</span></code>
            that takes and returns doubles. The <code class="computeroutput"><span class="identifier">calculator</span><span class="special">&lt;&gt;</span></code> wrapper above does that with
            the help of the <code class="computeroutput"><a class="link" href="../boost/proto/extends.html" title="Struct template extends">proto::extends&lt;&gt;</a></code>
            template. The first template to <code class="computeroutput"><a class="link" href="../boost/proto/extends.html" title="Struct template extends">proto::extends&lt;&gt;</a></code>
            parameter is the expression type we are extending. The second is the
            type of the wrapped expression. The third parameter is the domain that
            this wrapper is associated with. A wrapper type like <code class="computeroutput"><span class="identifier">calculator</span><span class="special">&lt;&gt;</span></code> that inherits from <code class="computeroutput"><a class="link" href="../boost/proto/extends.html" title="Struct template extends">proto::extends&lt;&gt;</a></code> behaves just like
            the expression type it has extended, with any additional behaviors you
            choose to give it.
          </p>
<div class="note"><table border="0" summary="Note">
<tr>
<td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../doc/src/images/note.png"></td>
<th align="left">Note</th>
</tr>
<tr><td align="left" valign="top">
<p>
              <span class="bold"><strong>Why not just inherit from <code class="literal">proto::expr&lt;&gt;</code>?</strong></span>
            </p>
<p>
              You might be thinking that this expression extension business is unnecessarily
              complicated. After all, isn't this why C++ supports inheritance? Why
              can't <code class="literal">calculator&lt;Expr&gt;</code> just inherit from
              <code class="literal">Expr</code> directly? The reason is because <code class="literal">Expr</code>,
              which presumably is an instantiation of <code class="computeroutput"><a class="link" href="../boost/proto/expr.html" title="Struct template expr">proto::expr&lt;&gt;</a></code>,
              has expression template-building operator overloads that will be incorrect
              for derived types. They will store <code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code> by reference to <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">expr</span><span class="special">&lt;&gt;</span></code>, effectively slicing off any
              derived parts. <code class="computeroutput"><a class="link" href="../boost/proto/extends.html" title="Struct template extends">proto::extends&lt;&gt;</a></code>
              gives your derived types operator overloads that don't slice off your
              additional members.
            </p>
</td></tr>
</table></div>
<p>
            Although not strictly necessary in this case, we bring <code class="computeroutput"><span class="identifier">extends</span><span class="special">&lt;&gt;::</span><span class="keyword">operator</span><span class="special">=</span></code>
            into scope with the <code class="computeroutput"><span class="identifier">BOOST_PROTO_EXTENDS_USING_ASSIGN</span><span class="special">()</span></code> macro. This is really only necessary
            if you want expressions like <code class="computeroutput"><span class="identifier">_1</span>
            <span class="special">=</span> <span class="number">3</span></code>
            to create a lazily evaluated assignment. <code class="computeroutput"><a class="link" href="../boost/proto/extends.html" title="Struct template extends">proto::extends&lt;&gt;</a></code>
            defines the appropriate <code class="computeroutput"><span class="keyword">operator</span><span class="special">=</span></code> for you, but the compiler-generated
            <code class="computeroutput"><span class="identifier">calculator</span><span class="special">&lt;&gt;::</span><span class="keyword">operator</span><span class="special">=</span></code>
            will hide it unless you make it available with the macro.
          </p>
<p>
            Note that in the implementation of <code class="computeroutput"><span class="identifier">calculator</span><span class="special">&lt;&gt;::</span><span class="keyword">operator</span><span class="special">()</span></code>, we evaluate the expression with the
            <code class="computeroutput"><span class="identifier">calculator_context</span></code> we
            defined earlier. As we saw before, the context is what gives the operators
            their meaning. In the case of the calculator, the context is also what
            defines the meaning of the placeholder terminals.
          </p>
<p>
            Now that we have defined the <code class="computeroutput"><span class="identifier">calculator</span><span class="special">&lt;&gt;</span></code> expression wrapper, we need to
            wrap the placeholders to imbue them with calculator-ness:
          </p>
<pre class="programlisting"><span class="identifier">calculator</span><span class="special">&lt;</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span> <span class="identifier">placeholder</span><span class="special">&lt;</span><span class="number">0</span><span class="special">&gt;</span> <span class="special">&gt;::</span><span class="identifier">type</span> <span class="special">&gt;</span> <span class="keyword">const</span> <span class="identifier">_1</span><span class="special">;</span>
<span class="identifier">calculator</span><span class="special">&lt;</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span> <span class="identifier">placeholder</span><span class="special">&lt;</span><span class="number">1</span><span class="special">&gt;</span> <span class="special">&gt;::</span><span class="identifier">type</span> <span class="special">&gt;</span> <span class="keyword">const</span> <span class="identifier">_2</span><span class="special">;</span>
</pre>
<h6>
<a name="boost_proto.users_guide.front_end.customizing_expressions_in_your_domain.extends.h0"></a>
            <span class="phrase"><a name="boost_proto.users_guide.front_end.customizing_expressions_in_your_domain.extends.retaining_pod_ness_with__literal_boost_proto_extends____literal_"></a></span><a class="link" href="users_guide.html#boost_proto.users_guide.front_end.customizing_expressions_in_your_domain.extends.retaining_pod_ness_with__literal_boost_proto_extends____literal_">Retaining
            POD-ness with <code class="literal">BOOST_PROTO_EXTENDS()</code></a>
          </h6>
<p>
            To use <code class="computeroutput"><a class="link" href="../boost/proto/extends.html" title="Struct template extends">proto::extends&lt;&gt;</a></code>, your extension type
            must derive from <code class="computeroutput"><a class="link" href="../boost/proto/extends.html" title="Struct template extends">proto::extends&lt;&gt;</a></code>.
            Unfortunately, that means that your extension type is no longer POD and
            its instances cannot be <span class="emphasis"><em>statically initialized</em></span>.
            (See the <a class="link" href="appendices.html#boost_proto.appendices.rationale.static_initialization" title="Static Initialization">Static
            Initialization</a> section in the <a class="link" href="appendices.html#boost_proto.appendices.rationale" title="Appendix C: Rationale">Rationale</a>
            appendix for why this matters.) In particular, as defined above, the
            global placeholder objects <code class="computeroutput"><span class="identifier">_1</span></code>
            and <code class="computeroutput"><span class="identifier">_2</span></code> will need to be
            initialized at runtime, which could lead to subtle order of initialization
            bugs.
          </p>
<p>
            There is another way to make an expression extension that doesn't sacrifice
            POD-ness : the <code class="literal"><code class="computeroutput"><a class="link" href="../BOOST_PROTO_EXTENDS.html" title="Macro BOOST_PROTO_EXTENDS">BOOST_PROTO_EXTENDS</a></code>()</code>
            macro. You can use it much like you use <code class="computeroutput"><a class="link" href="../boost/proto/extends.html" title="Struct template extends">proto::extends&lt;&gt;</a></code>.
            We can use <code class="literal"><code class="computeroutput"><a class="link" href="../BOOST_PROTO_EXTENDS.html" title="Macro BOOST_PROTO_EXTENDS">BOOST_PROTO_EXTENDS</a></code>()</code>
            to keep <code class="computeroutput"><span class="identifier">calculator</span><span class="special">&lt;&gt;</span></code>
            a POD and our placeholders statically initialized.
          </p>
<pre class="programlisting"><span class="comment">// The calculator&lt;&gt; expression wrapper makes expressions</span>
<span class="comment">// function objects.</span>
<span class="keyword">template</span><span class="special">&lt;</span> <span class="keyword">typename</span> <span class="identifier">Expr</span> <span class="special">&gt;</span>
<span class="keyword">struct</span> <span class="identifier">calculator</span>
<span class="special">{</span>
    <span class="comment">// Use BOOST_PROTO_EXTENDS() instead of proto::extends&lt;&gt; to</span>
    <span class="comment">// make this type a Proto expression extension.</span>
    <span class="identifier">BOOST_PROTO_EXTENDS</span><span class="special">(</span><span class="identifier">Expr</span><span class="special">,</span> <span class="identifier">calculator</span><span class="special">&lt;</span><span class="identifier">Expr</span><span class="special">&gt;,</span> <span class="identifier">calculator_domain</span><span class="special">)</span>

    <span class="keyword">typedef</span> <span class="keyword">double</span> <span class="identifier">result_type</span><span class="special">;</span>

    <span class="identifier">result_type</span> <span class="keyword">operator</span><span class="special">()(</span> <span class="keyword">double</span> <span class="identifier">d1</span> <span class="special">=</span> <span class="number">0.0</span><span class="special">,</span> <span class="keyword">double</span> <span class="identifier">d2</span> <span class="special">=</span> <span class="number">0.0</span> <span class="special">)</span> <span class="keyword">const</span>
    <span class="special">{</span>
        <span class="comment">/* ... as before ... */</span>
    <span class="special">}</span>
<span class="special">};</span>
</pre>
<p>
            With the new <code class="computeroutput"><span class="identifier">calculator</span><span class="special">&lt;&gt;</span></code> type, we can redefine our placeholders
            to be statically initialized:
          </p>
<pre class="programlisting"><span class="identifier">calculator</span><span class="special">&lt;</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span> <span class="identifier">placeholder</span><span class="special">&lt;</span><span class="number">0</span><span class="special">&gt;</span> <span class="special">&gt;::</span><span class="identifier">type</span> <span class="special">&gt;</span> <span class="keyword">const</span> <span class="identifier">_1</span> <span class="special">=</span> <span class="special">{{{}}};</span>
<span class="identifier">calculator</span><span class="special">&lt;</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span> <span class="identifier">placeholder</span><span class="special">&lt;</span><span class="number">1</span><span class="special">&gt;</span> <span class="special">&gt;::</span><span class="identifier">type</span> <span class="special">&gt;</span> <span class="keyword">const</span> <span class="identifier">_2</span> <span class="special">=</span> <span class="special">{{{}}};</span>
</pre>
<p>
            We need to make one additional small change to accommodate the POD-ness
            of our expression extension, which we'll describe below in the section
            on expression generators.
          </p>
<p>
            What does <code class="literal"><code class="computeroutput"><a class="link" href="../BOOST_PROTO_EXTENDS.html" title="Macro BOOST_PROTO_EXTENDS">BOOST_PROTO_EXTENDS</a></code>()</code>
            do? It defines a data member of the expression type being extended; some
            nested typedefs that Proto requires; <code class="computeroutput"><span class="keyword">operator</span><span class="special">=</span></code>, <code class="computeroutput"><span class="keyword">operator</span><span class="special">[]</span></code> and <code class="computeroutput"><span class="keyword">operator</span><span class="special">()</span></code> overloads for building expression templates;
            and a nested <code class="computeroutput"><span class="identifier">result</span><span class="special">&lt;&gt;</span></code>
            template for calculating the return type of <code class="computeroutput"><span class="keyword">operator</span><span class="special">()</span></code>. In this case, however, the <code class="computeroutput"><span class="keyword">operator</span><span class="special">()</span></code>
            overloads and the <code class="computeroutput"><span class="identifier">result</span><span class="special">&lt;&gt;</span></code> template are not needed because
            we are defining our own <code class="computeroutput"><span class="keyword">operator</span><span class="special">()</span></code> in the <code class="computeroutput"><span class="identifier">calculator</span><span class="special">&lt;&gt;</span></code> type. Proto provides additional
            macros for finer control over which member functions are defined. We
            could improve our <code class="computeroutput"><span class="identifier">calculator</span><span class="special">&lt;&gt;</span></code> type as follows:
          </p>
<pre class="programlisting"><span class="comment">// The calculator&lt;&gt; expression wrapper makes expressions</span>
<span class="comment">// function objects.</span>
<span class="keyword">template</span><span class="special">&lt;</span> <span class="keyword">typename</span> <span class="identifier">Expr</span> <span class="special">&gt;</span>
<span class="keyword">struct</span> <span class="identifier">calculator</span>
<span class="special">{</span>
    <span class="comment">// Use BOOST_PROTO_BASIC_EXTENDS() instead of proto::extends&lt;&gt; to</span>
    <span class="comment">// make this type a Proto expression extension:</span>
    <span class="identifier">BOOST_PROTO_BASIC_EXTENDS</span><span class="special">(</span><span class="identifier">Expr</span><span class="special">,</span> <span class="identifier">calculator</span><span class="special">&lt;</span><span class="identifier">Expr</span><span class="special">&gt;,</span> <span class="identifier">calculator_domain</span><span class="special">)</span>

    <span class="comment">// Define operator[] to build expression templates:</span>
    <span class="identifier">BOOST_PROTO_EXTENDS_SUBSCRIPT</span><span class="special">()</span>

    <span class="comment">// Define operator= to build expression templates:</span>
    <span class="identifier">BOOST_PROTO_EXTENDS_ASSIGN</span><span class="special">()</span>

    <span class="keyword">typedef</span> <span class="keyword">double</span> <span class="identifier">result_type</span><span class="special">;</span>

    <span class="identifier">result_type</span> <span class="keyword">operator</span><span class="special">()(</span> <span class="keyword">double</span> <span class="identifier">d1</span> <span class="special">=</span> <span class="number">0.0</span><span class="special">,</span> <span class="keyword">double</span> <span class="identifier">d2</span> <span class="special">=</span> <span class="number">0.0</span> <span class="special">)</span> <span class="keyword">const</span>
    <span class="special">{</span>
        <span class="comment">/* ... as before ... */</span>
    <span class="special">}</span>
<span class="special">};</span>
</pre>
<p>
            Notice that we are now using <code class="literal"><code class="computeroutput"><a class="link" href="../BOOST_PROTO_BASIC_EXTENDS.html" title="Macro BOOST_PROTO_BASIC_EXTENDS">BOOST_PROTO_BASIC_EXTENDS</a></code>()</code>
            instead of <code class="literal"><code class="computeroutput"><a class="link" href="../BOOST_PROTO_EXTENDS.html" title="Macro BOOST_PROTO_EXTENDS">BOOST_PROTO_EXTENDS</a></code>()</code>.
            This just adds the data member and the nested typedefs but not any of
            the overloaded operators. Those are added separately with <code class="literal"><code class="computeroutput"><a class="link" href="../BOOST_PROTO_EXTENDS_ASSIGN.html" title="Macro BOOST_PROTO_EXTENDS_ASSIGN">BOOST_PROTO_EXTENDS_ASSIGN</a></code>()</code>
            and <code class="literal"><code class="computeroutput"><a class="link" href="../BOOST_PROTO_EXTENDS_SUBSCRIPT.html" title="Macro BOOST_PROTO_EXTENDS_SUBSCRIPT">BOOST_PROTO_EXTENDS_SUBSCRIPT</a></code>()</code>.
            We are leaving out the function call operator and the nested <code class="computeroutput"><span class="identifier">result</span><span class="special">&lt;&gt;</span></code>
            template that could have been defined with Proto's <code class="literal"><code class="computeroutput"><a class="link" href="../BOOST_PROTO_EXTENDS_FUNCTION.html" title="Macro BOOST_PROTO_EXTENDS_FUNCTION">BOOST_PROTO_EXTENDS_FUNCTION</a></code>()</code>
            macro.
          </p>
<p>
            In summary, here are the macros you can use to define expression extensions,
            and a brief description of each.
          </p>
<div class="table">
<a name="boost_proto.users_guide.front_end.customizing_expressions_in_your_domain.extends.t0"></a><p class="title"><b>Table&#160;32.2.&#160;Expression Extension Macros</b></p>
<div class="table-contents"><table class="table" summary="Expression Extension Macros">
<colgroup>
<col>
<col>
</colgroup>
<thead><tr>
<th>
                    <p>
                      Macro
                    </p>
                  </th>
<th>
                    <p>
                      Purpose
                    </p>
                  </th>
</tr></thead>
<tbody>
<tr>
<td>
                    <p>
</p>
<pre class="programlisting"><code class="computeroutput"><a class="link" href="../BOOST_PROTO_BASIC_EXTENDS.html" title="Macro BOOST_PROTO_BASIC_EXTENDS">BOOST_PROTO_BASIC_EXTENDS</a></code><span class="special">(</span>
    <em class="replaceable"><code>expression</code></em>
  <span class="special">,</span> <em class="replaceable"><code>extension</code></em>
  <span class="special">,</span> <em class="replaceable"><code>domain</code></em>
<span class="special">)</span></pre>
<p>
                    </p>
                  </td>
<td>
                    <p>
                      Defines a data member of type <code class="computeroutput"><em class="replaceable"><code>expression</code></em></code>
                      and some nested typedefs that Proto requires.
                    </p>
                  </td>
</tr>
<tr>
<td>
                    <p>
                      <code class="literal"><code class="computeroutput"><a class="link" href="../BOOST_PROTO_EXTENDS_ASSIGN.html" title="Macro BOOST_PROTO_EXTENDS_ASSIGN">BOOST_PROTO_EXTENDS_ASSIGN</a></code>()</code>
                    </p>
                  </td>
<td>
                    <p>
                      Defines <code class="computeroutput"><span class="keyword">operator</span><span class="special">=</span></code>. Only valid when preceded
                      by <code class="literal"><code class="computeroutput"><a class="link" href="../BOOST_PROTO_BASIC_EXTENDS.html" title="Macro BOOST_PROTO_BASIC_EXTENDS">BOOST_PROTO_BASIC_EXTENDS</a></code>()</code>.
                    </p>
                  </td>
</tr>
<tr>
<td>
                    <p>
                      <code class="literal"><code class="computeroutput"><a class="link" href="../BOOST_PROTO_EXTENDS_SUBSCRIPT.html" title="Macro BOOST_PROTO_EXTENDS_SUBSCRIPT">BOOST_PROTO_EXTENDS_SUBSCRIPT</a></code>()</code>
                    </p>
                  </td>
<td>
                    <p>
                      Defines <code class="computeroutput"><span class="keyword">operator</span><span class="special">[]</span></code>. Only valid when preceded
                      by <code class="literal"><code class="computeroutput"><a class="link" href="../BOOST_PROTO_BASIC_EXTENDS.html" title="Macro BOOST_PROTO_BASIC_EXTENDS">BOOST_PROTO_BASIC_EXTENDS</a></code>()</code>.
                    </p>
                  </td>
</tr>
<tr>
<td>
                    <p>
                      <code class="literal"><code class="computeroutput"><a class="link" href="../BOOST_PROTO_EXTENDS_FUNCTION.html" title="Macro BOOST_PROTO_EXTENDS_FUNCTION">BOOST_PROTO_EXTENDS_FUNCTION</a></code>()</code>
                    </p>
                  </td>
<td>
                    <p>
                      Defines <code class="computeroutput"><span class="keyword">operator</span><span class="special">()</span></code> and a nested <code class="computeroutput"><span class="identifier">result</span><span class="special">&lt;&gt;</span></code>
                      template for return type calculation. Only valid when preceded
                      by <code class="literal"><code class="computeroutput"><a class="link" href="../BOOST_PROTO_BASIC_EXTENDS.html" title="Macro BOOST_PROTO_BASIC_EXTENDS">BOOST_PROTO_BASIC_EXTENDS</a></code>()</code>.
                    </p>
                  </td>
</tr>
<tr>
<td>
                    <p>
</p>
<pre class="programlisting"><code class="computeroutput"><a class="link" href="../BOOST_PROTO_EXTENDS.html" title="Macro BOOST_PROTO_EXTENDS">BOOST_PROTO_EXTENDS</a></code><span class="special">(</span>
    <em class="replaceable"><code>expression</code></em>
  <span class="special">,</span> <em class="replaceable"><code>extension</code></em>
  <span class="special">,</span> <em class="replaceable"><code>domain</code></em>
<span class="special">)</span></pre>
<p>
                    </p>
                  </td>
<td>
                    <p>
                      Equivalent to:
</p>
<pre class="programlisting"><code class="computeroutput"><a class="link" href="../BOOST_PROTO_BASIC_EXTENDS.html" title="Macro BOOST_PROTO_BASIC_EXTENDS">BOOST_PROTO_BASIC_EXTENDS</a></code><span class="special">(</span><em class="replaceable"><code>expression</code></em><span class="special">,</span> <em class="replaceable"><code>extension</code></em><span class="special">,</span> <em class="replaceable"><code>domain</code></em><span class="special">)</span>

  <code class="literal"><code class="computeroutput"><a class="link" href="../BOOST_PROTO_EXTENDS_ASSIGN.html" title="Macro BOOST_PROTO_EXTENDS_ASSIGN">BOOST_PROTO_EXTENDS_ASSIGN</a></code>()</code>

  <code class="literal"><code class="computeroutput"><a class="link" href="../BOOST_PROTO_EXTENDS_SUBSCRIPT.html" title="Macro BOOST_PROTO_EXTENDS_SUBSCRIPT">BOOST_PROTO_EXTENDS_SUBSCRIPT</a></code>()</code>

  <code class="literal"><code class="computeroutput"><a class="link" href="../BOOST_PROTO_EXTENDS_FUNCTION.html" title="Macro BOOST_PROTO_EXTENDS_FUNCTION">BOOST_PROTO_EXTENDS_FUNCTION</a></code>()</code></pre>
<p>
                    </p>
                  </td>
</tr>
</tbody>
</table></div>
</div>
<br class="table-break"><div class="warning"><table border="0" summary="Warning">
<tr>
<td rowspan="2" align="center" valign="top" width="25"><img alt="[Warning]" src="../../../doc/src/images/warning.png"></td>
<th align="left">Warning</th>
</tr>
<tr><td align="left" valign="top">
<p>
              <span class="bold"><strong>Argument-Dependent Lookup and <code class="literal"><code class="computeroutput"><a class="link" href="../BOOST_PROTO_EXTENDS.html" title="Macro BOOST_PROTO_EXTENDS">BOOST_PROTO_EXTENDS</a></code>()</code></strong></span>
            </p>
<p>
              Proto's operator overloads are defined in the <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">proto</span></code>
              namespace and are found by argument-dependent lookup (ADL). This usually
              just works because expressions are made up of types that live in the
              <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">proto</span></code> namespace. However, sometimes
              when you use <code class="literal"><code class="computeroutput"><a class="link" href="../BOOST_PROTO_EXTENDS.html" title="Macro BOOST_PROTO_EXTENDS">BOOST_PROTO_EXTENDS</a></code>()</code>
              that is not the case. Consider:
            </p>
<p>
</p>
<pre class="programlisting"><span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">&gt;</span>
<span class="keyword">struct</span> <span class="identifier">my_complex</span>
<span class="special">{</span>
    <span class="identifier">BOOST_PROTO_EXTENDS</span><span class="special">(</span>
        <span class="keyword">typename</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">complex</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;</span> <span class="special">&gt;::</span><span class="identifier">type</span>
      <span class="special">,</span> <span class="identifier">my_complex</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;</span>
      <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">default_domain</span>
    <span class="special">)</span>
<span class="special">};</span>

<span class="keyword">int</span> <span class="identifier">main</span><span class="special">()</span>
<span class="special">{</span>
    <span class="identifier">my_complex</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">&gt;</span> <span class="identifier">c0</span><span class="special">,</span> <span class="identifier">c1</span><span class="special">;</span>

    <span class="identifier">c0</span> <span class="special">+</span> <span class="identifier">c1</span><span class="special">;</span> <span class="comment">// ERROR: operator+ not found</span>
<span class="special">}</span>
</pre>
<p>
            </p>
<p>
              The problem has to do with how argument-dependent lookup works. The
              type <code class="computeroutput"><span class="identifier">my_complex</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">&gt;</span></code>
              is not associated in any way with the <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">proto</span></code>
              namespace, so the operators defined there are not considered. (Had
              we inherited from <code class="computeroutput"><a class="link" href="../boost/proto/extends.html" title="Struct template extends">proto::extends&lt;&gt;</a></code>
              instead of used <code class="literal"><code class="computeroutput"><a class="link" href="../BOOST_PROTO_EXTENDS.html" title="Macro BOOST_PROTO_EXTENDS">BOOST_PROTO_EXTENDS</a></code>()</code>,
              we would have avoided the problem because inheriting from a type in
              <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">proto</span></code> namespace is enough to get
              ADL to kick in.)
            </p>
<p>
              So what can we do? By adding an extra dummy template parameter that
              defaults to a type in the <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">proto</span></code>
              namespace, we can trick ADL into finding the right operator overloads.
              The solution looks like this:
            </p>
<p>
</p>
<pre class="programlisting"><span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">,</span> <span class="keyword">class</span> <span class="identifier">Dummy</span> <span class="special">=</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">is_proto_expr</span><span class="special">&gt;</span>
<span class="keyword">struct</span> <span class="identifier">my_complex</span>
<span class="special">{</span>
    <span class="identifier">BOOST_PROTO_EXTENDS</span><span class="special">(</span>
        <span class="keyword">typename</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">complex</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;</span> <span class="special">&gt;::</span><span class="identifier">type</span>
      <span class="special">,</span> <span class="identifier">my_complex</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;</span>
      <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">default_domain</span>
    <span class="special">)</span>
<span class="special">};</span>

<span class="keyword">int</span> <span class="identifier">main</span><span class="special">()</span>
<span class="special">{</span>
    <span class="identifier">my_complex</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">&gt;</span> <span class="identifier">c0</span><span class="special">,</span> <span class="identifier">c1</span><span class="special">;</span>

    <span class="identifier">c0</span> <span class="special">+</span> <span class="identifier">c1</span><span class="special">;</span> <span class="comment">// OK, operator+ found now!</span>
<span class="special">}</span>
</pre>
<p>
            </p>
<p>
              The type <code class="computeroutput"><a class="link" href="../boost/proto/is_proto_expr.html" title="Struct is_proto_expr">proto::is_proto_expr</a></code> is nothing
              but an empty struct, but by making it a template parameter we make
              <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">proto</span></code> an associated namespace of
              <code class="computeroutput"><span class="identifier">my_complex</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">&gt;</span></code>.
              Now ADL can successfully find Proto's operator overloads.
            </p>
</td></tr>
</table></div>
</div>
<div class="section">
<div class="titlepage"><div><div><h5 class="title">
<a name="boost_proto.users_guide.front_end.customizing_expressions_in_your_domain.expression_generators"></a><a class="link" href="users_guide.html#boost_proto.users_guide.front_end.customizing_expressions_in_your_domain.expression_generators" title="Expression Generators">Expression
          Generators</a>
</h5></div></div></div>
<p>
            The last thing that remains to be done is to tell Proto that it needs
            to wrap all of our calculator expressions in our <code class="computeroutput"><span class="identifier">calculator</span><span class="special">&lt;&gt;</span></code> wrapper. We have already wrapped
            the placeholders, but we want <span class="emphasis"><em>all</em></span> expressions that
            involve the calculator placeholders to be calculators. We can do that
            by specifying an expression generator when we define our <code class="computeroutput"><span class="identifier">calculator_domain</span></code>, as follows:
          </p>
<pre class="programlisting"><span class="comment">// Define the calculator_domain we forward-declared above.</span>
<span class="comment">// Specify that all expression in this domain should be wrapped</span>
<span class="comment">// in the calculator&lt;&gt; expression wrapper.</span>
<span class="keyword">struct</span> <span class="identifier">calculator_domain</span>
  <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">domain</span><span class="special">&lt;</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">generator</span><span class="special">&lt;</span> <span class="identifier">calculator</span> <span class="special">&gt;</span> <span class="special">&gt;</span>
<span class="special">{};</span>
</pre>
<p>
            The first template parameter to <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">domain</span><span class="special">&lt;&gt;</span></code> is the generator. "Generator"
            is just a fancy name for a function object that accepts an expression
            and does something to it. <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">generator</span><span class="special">&lt;&gt;</span></code> is a very simple one --- it wraps
            an expression in the wrapper you specify. <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">domain</span><span class="special">&lt;&gt;</span></code> inherits from its generator parameter,
            so all domains are themselves function objects.
          </p>
<p>
            If we used <code class="literal"><code class="computeroutput"><a class="link" href="../BOOST_PROTO_EXTENDS.html" title="Macro BOOST_PROTO_EXTENDS">BOOST_PROTO_EXTENDS</a></code>()</code>
            to keep our expression extension type POD, then we need to use <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">pod_generator</span><span class="special">&lt;&gt;</span></code>
            instead of <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">generator</span><span class="special">&lt;&gt;</span></code>,
            as follows:
          </p>
<pre class="programlisting"><span class="comment">// If calculator&lt;&gt; uses BOOST_PROTO_EXTENDS() instead of </span>
<span class="comment">// use proto::extends&lt;&gt;, use proto::pod_generator&lt;&gt; instead</span>
<span class="comment">// of proto::generator&lt;&gt;.</span>
<span class="keyword">struct</span> <span class="identifier">calculator_domain</span>
  <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">domain</span><span class="special">&lt;</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">pod_generator</span><span class="special">&lt;</span> <span class="identifier">calculator</span> <span class="special">&gt;</span> <span class="special">&gt;</span>
<span class="special">{};</span>
</pre>
<p>
            After Proto has calculated a new expression type, it checks the domains
            of the child expressions. They must match. Assuming they do, Proto creates
            the new expression and passes it to <code class="computeroutput"><em class="replaceable"><code>Domain</code></em><span class="special">::</span><span class="keyword">operator</span><span class="special">()</span></code> for any additional processing. If we
            don't specify a generator, the new expression gets passed through unchanged.
            But since we've specified a generator above, <code class="computeroutput"><span class="identifier">calculator_domain</span><span class="special">::</span><span class="keyword">operator</span><span class="special">()</span></code> returns <code class="computeroutput"><span class="identifier">calculator</span><span class="special">&lt;&gt;</span></code> objects.
          </p>
<p>
            Now we can use calculator expressions as function objects to STL algorithms,
            as follows:
          </p>
<pre class="programlisting"><span class="keyword">double</span> <span class="identifier">data</span><span class="special">[]</span> <span class="special">=</span> <span class="special">{</span><span class="number">1.</span><span class="special">,</span> <span class="number">2.</span><span class="special">,</span> <span class="number">3.</span><span class="special">,</span> <span class="number">4.</span><span class="special">};</span>

<span class="comment">// Use the calculator EDSL to square each element ... WORKS! :-)</span>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">transform</span><span class="special">(</span> <span class="identifier">data</span><span class="special">,</span> <span class="identifier">data</span> <span class="special">+</span> <span class="number">4</span><span class="special">,</span> <span class="identifier">data</span><span class="special">,</span> <span class="identifier">_1</span> <span class="special">*</span> <span class="identifier">_1</span> <span class="special">);</span>
</pre>
</div>
<div class="section">
<div class="titlepage"><div><div><h5 class="title">
<a name="boost_proto.users_guide.front_end.customizing_expressions_in_your_domain.inhibiting_overloads"></a><a class="link" href="users_guide.html#boost_proto.users_guide.front_end.customizing_expressions_in_your_domain.inhibiting_overloads" title="Controlling Operator Overloads">Controlling
          Operator Overloads</a>
</h5></div></div></div>
<p>
            By default, Proto defines every possible operator overload for Protofied
            expressions. This makes it simple to bang together an EDSL. In some cases,
            however, the presence of Proto's promiscuous overloads can lead to confusion
            or worse. When that happens, you'll have to disable some of Proto's overloaded
            operators. That is done by defining the grammar for your domain and specifying
            it as the second parameter of the <code class="computeroutput"><a class="link" href="../boost/proto/domain.html" title="Struct template domain">proto::domain&lt;&gt;</a></code>
            template.
          </p>
<p>
            In the <a class="link" href="users_guide.html#boost_proto.users_guide.getting_started.hello_calculator" title="Hello Calculator">Hello
            Calculator</a> section, we saw an example of a Proto grammar, which
            is repeated here:
          </p>
<pre class="programlisting"><span class="comment">// Define the grammar of calculator expressions</span>
<span class="keyword">struct</span> <span class="identifier">calculator_grammar</span>
  <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">or_</span><span class="special">&lt;</span>
        <span class="identifier">proto</span><span class="special">::</span><span class="identifier">plus</span><span class="special">&lt;</span> <span class="identifier">calculator_grammar</span><span class="special">,</span> <span class="identifier">calculator_grammar</span> <span class="special">&gt;</span>
      <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">minus</span><span class="special">&lt;</span> <span class="identifier">calculator_grammar</span><span class="special">,</span> <span class="identifier">calculator_grammar</span> <span class="special">&gt;</span>
      <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">multiplies</span><span class="special">&lt;</span> <span class="identifier">calculator_grammar</span><span class="special">,</span> <span class="identifier">calculator_grammar</span> <span class="special">&gt;</span>
      <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">divides</span><span class="special">&lt;</span> <span class="identifier">calculator_grammar</span><span class="special">,</span> <span class="identifier">calculator_grammar</span> <span class="special">&gt;</span>
      <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_</span> <span class="special">&gt;</span>
    <span class="special">&gt;</span>
<span class="special">{};</span>
</pre>
<p>
            We'll have much more to say about grammars in subsequent sections, but
            for now, we'll just say that the <code class="computeroutput"><span class="identifier">calculator_grammar</span></code>
            struct describes a subset of all expression types -- the subset that
            comprise valid calculator expressions. We would like to prohibit Proto
            from creating a calculator expression that does not conform to this grammar.
            We do that by changing the definition of the <code class="computeroutput"><span class="identifier">calculator_domain</span></code>
            struct.
          </p>
<pre class="programlisting"><span class="comment">// Define the calculator_domain. Expressions in the calculator</span>
<span class="comment">// domain are wrapped in the calculator&lt;&gt; wrapper, and they must</span>
<span class="comment">// conform to the calculator_grammar:</span>
<span class="keyword">struct</span> <span class="identifier">calculator_domain</span>
  <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">domain</span><span class="special">&lt;</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">generator</span><span class="special">&lt;</span> <span class="identifier">calculator</span> <span class="special">&gt;,</span> <span class="bold"><strong>calculator_grammar</strong></span>  <span class="special">&gt;</span>
<span class="special">{};</span>
</pre>
<p>
            The only new addition is <code class="computeroutput"><span class="identifier">calculator_grammar</span></code>
            as the second template parameter to the <code class="computeroutput"><a class="link" href="../boost/proto/domain.html" title="Struct template domain">proto::domain&lt;&gt;</a></code>
            template. That has the effect of disabling any of Proto's operator overloads
            that would create an invalid calculator expression.
          </p>
<p>
            Another common use for this feature would be to disable Proto's unary
            <code class="computeroutput"><span class="keyword">operator</span><span class="special">&amp;</span></code>
            overload. It may be surprising for users of your EDSL that they cannot
            take the address of their expressions! You can very easily disable Proto's
            unary <code class="computeroutput"><span class="keyword">operator</span><span class="special">&amp;</span></code>
            overload for your domain with a very simple grammar, as below:
          </p>
<pre class="programlisting"><span class="comment">// For expressions in my_domain, disable Proto's</span>
<span class="comment">// unary address-of operator.</span>
<span class="keyword">struct</span> <span class="identifier">my_domain</span>
  <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">domain</span><span class="special">&lt;</span>
        <span class="identifier">proto</span><span class="special">::</span><span class="identifier">generator</span><span class="special">&lt;</span> <span class="identifier">my_wrapper</span> <span class="special">&gt;</span>
        <span class="comment">// A simple grammar that matches any expression that</span>
        <span class="comment">// is not a unary address-of expression.</span>
      <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">not_</span><span class="special">&lt;</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">address_of</span><span class="special">&lt;</span> <span class="identifier">_</span> <span class="special">&gt;</span> <span class="special">&gt;</span>
    <span class="special">&gt;</span>
<span class="special">{};</span>
</pre>
<p>
            The type <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">not_</span><span class="special">&lt;</span>
            <span class="identifier">proto</span><span class="special">::</span><span class="identifier">address_of</span><span class="special">&lt;</span>
            <span class="identifier">_</span> <span class="special">&gt;</span>
            <span class="special">&gt;</span></code> is a very simple grammar
            that matches all expressions except unary address-of expressions. In
            the section describing Proto's intermediate form, we'll have much more
            to say about grammars.
          </p>
</div>
<div class="section">
<div class="titlepage"><div><div><h5 class="title">
<a name="boost_proto.users_guide.front_end.customizing_expressions_in_your_domain.per_domain_as_child"></a><a class="link" href="users_guide.html#boost_proto.users_guide.front_end.customizing_expressions_in_your_domain.per_domain_as_child" title="Controlling How Child Expressions Are Captured">Controlling
          How Child Expressions Are Captured</a>
</h5></div></div></div>
<div class="note"><table border="0" summary="Note">
<tr>
<td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../doc/src/images/note.png"></td>
<th align="left">Note</th>
</tr>
<tr><td align="left" valign="top"><p>
              This is an advanced topic. Feel free to skip this if you're just getting
              started with Proto.
            </p></td></tr>
</table></div>
<p>
            Proto's operator overloads build expressions from sub-expressions. The
            sub-expressions become children of the new expression. By default, the
            children are stored in the parent by reference. This section describes
            how to change that default.
          </p>
<h6>
<a name="boost_proto.users_guide.front_end.customizing_expressions_in_your_domain.per_domain_as_child.h0"></a>
            <span class="phrase"><a name="boost_proto.users_guide.front_end.customizing_expressions_in_your_domain.per_domain_as_child.primer___literal_as_child__literal__vs___literal_as_expr__literal_"></a></span><a class="link" href="users_guide.html#boost_proto.users_guide.front_end.customizing_expressions_in_your_domain.per_domain_as_child.primer___literal_as_child__literal__vs___literal_as_expr__literal_">Primer:
            <code class="literal">as_child</code> vs. <code class="literal">as_expr</code></a>
          </h6>
<p>
            Proto lets you independently customize the behavior of <code class="computeroutput"><a class="link" href="../boost/proto/as_child_idp731140288.html" title="Function as_child">proto::as_child()</a></code> and <code class="computeroutput"><a class="link" href="../boost/proto/as_expr_idp731118320.html" title="Function as_expr">proto::as_expr()</a></code>.
            Both accept an object <code class="literal">x</code> and return a Proto expression
            by turning <code class="literal">x</code> it into a Proto terminal if necessary.
            Although similar, the two functions are used in different situations
            and have subtly different behavior by default. It's important to understand
            the difference so that you know which to customize to achieve the behavior
            you want.
          </p>
<p>
            To wit: <code class="computeroutput"><a class="link" href="../boost/proto/as_expr_idp731118320.html" title="Function as_expr">proto::as_expr()</a></code> is typically used by
            <span class="emphasis"><em>you</em></span> to turn an object into a Proto expression that
            is to be held in a local variable, as so:
          </p>
<pre class="programlisting"><span class="keyword">auto</span> <span class="identifier">l</span> <span class="special">=</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">as_expr</span><span class="special">(</span><span class="identifier">x</span><span class="special">);</span> <span class="comment">// Turn x into a Proto expression, hold the result in a local</span>
</pre>
<p>
            The above works regardless of whether <code class="computeroutput"><span class="identifier">x</span></code>
            is already a Proto expression or not. The object <code class="computeroutput"><span class="identifier">l</span></code>
            is guaranteed to be a valid Proto expression. If <code class="computeroutput"><span class="identifier">x</span></code>
            is a non-Proto object, it is turned into a terminal expression that holds
            <code class="computeroutput"><span class="identifier">x</span></code> <span class="emphasis"><em>by value</em></span>.<a href="#ftn.boost_proto.users_guide.front_end.customizing_expressions_in_your_domain.per_domain_as_child.f0" class="footnote" name="boost_proto.users_guide.front_end.customizing_expressions_in_your_domain.per_domain_as_child.f0"><sup class="footnote">[28]</sup></a> If <code class="computeroutput"><span class="identifier">x</span></code> is a
            Proto object already, <code class="computeroutput"><a class="link" href="../boost/proto/as_expr_idp731118320.html" title="Function as_expr">proto::as_expr()</a></code>
            returns it <span class="emphasis"><em>by value</em></span> unmodified.
          </p>
<p>
            In contrast, <code class="computeroutput"><a class="link" href="../boost/proto/as_child_idp731140288.html" title="Function as_child">proto::as_child()</a></code>
            is used internally by Proto to pre-process objects before making them
            children of another expression. Since it's internal to Proto, you don't
            see it explicitly, but it's there behind the scenes in expressions like
            this:
          </p>
<pre class="programlisting"><span class="identifier">x</span> <span class="special">+</span> <span class="identifier">y</span><span class="special">;</span> <span class="comment">// Consider that y is a Proto expression, but x may or may not be.</span>
</pre>
<p>
            In this case, Proto builds a plus node from the two children. Both are
            pre-processed by passing them to <code class="computeroutput"><a class="link" href="../boost/proto/as_child_idp731140288.html" title="Function as_child">proto::as_child()</a></code>
            before making them children of the new node. If <code class="computeroutput"><span class="identifier">x</span></code>
            is not a Proto expression, it becomes one by being wrapped in a Proto
            terminal that holds it <span class="emphasis"><em>by reference</em></span>. If <code class="computeroutput"><span class="identifier">x</span></code> is already a Proto expression, <code class="computeroutput"><a class="link" href="../boost/proto/as_child_idp731140288.html" title="Function as_child">proto::as_child()</a></code> returns it <span class="emphasis"><em>by
            reference</em></span> unmodified. Contrast this with the above description
            for <code class="computeroutput"><a class="link" href="../boost/proto/as_expr_idp731118320.html" title="Function as_expr">proto::as_expr()</a></code>.
          </p>
<p>
            The table below summarizes the above description.
          </p>
<div class="table">
<a name="boost_proto.users_guide.front_end.customizing_expressions_in_your_domain.per_domain_as_child.t0"></a><p class="title"><b>Table&#160;32.3.&#160;proto::as_expr() vs. proto::as_child()</b></p>
<div class="table-contents"><table class="table" summary="proto::as_expr() vs. proto::as_child()">
<colgroup>
<col>
<col>
<col>
</colgroup>
<thead><tr>
<th>
                    <p>
                      <span class="bold"><strong>Function</strong></span>
                    </p>
                  </th>
<th>
                    <p>
                      <span class="bold"><strong>When <code class="literal">t</code> is not a Proto
                      expr...</strong></span>
                    </p>
                  </th>
<th>
                    <p>
                      <span class="bold"><strong>When <code class="literal">t</code> is a Proto
                      expr...</strong></span>
                    </p>
                  </th>
</tr></thead>
<tbody>
<tr>
<td>
                    <p>
                      <code class="literal">proto::as_expr(t)</code>
                    </p>
                  </td>
<td>
                    <p>
                      Return (by value) a new Proto terminal holding <code class="literal">t</code>
                      by value.
                    </p>
                  </td>
<td>
                    <p>
                      Return <code class="literal">t</code> by value unmodified.
                    </p>
                  </td>
</tr>
<tr>
<td>
                    <p>
                      <code class="literal">proto::as_child(t)</code>
                    </p>
                  </td>
<td>
                    <p>
                      Return (by value) a new Proto terminal holding <code class="literal">t</code>
                      by reference.
                    </p>
                  </td>
<td>
                    <p>
                      Return <code class="literal">t</code> by reference unmodified.
                    </p>
                  </td>
</tr>
</tbody>
</table></div>
</div>
<br class="table-break"><div class="note"><table border="0" summary="Note">
<tr>
<td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../doc/src/images/note.png"></td>
<th align="left">Note</th>
</tr>
<tr><td align="left" valign="top"><p>
              There is one important place where Proto uses both <code class="computeroutput"><span class="identifier">as_expr</span></code>
              <span class="emphasis"><em>and</em></span> <code class="computeroutput"><span class="identifier">as_child</span></code>:
              <code class="computeroutput"><a class="link" href="../boost/proto/make_expr_idp728389200.html" title="Function make_expr">proto::make_expr()</a></code>. The <code class="computeroutput"><a class="link" href="../boost/proto/make_expr_idp728389200.html" title="Function make_expr">proto::make_expr()</a></code> function requires
              you to specify for each child whether it should be held by value or
              by reference. Proto uses <code class="computeroutput"><a class="link" href="../boost/proto/as_expr_idp731118320.html" title="Function as_expr">proto::as_expr()</a></code>
              to pre-process the children to be held by value, and <code class="computeroutput"><a class="link" href="../boost/proto/as_child_idp731140288.html" title="Function as_child">proto::as_child()</a></code> for the ones to be
              held by reference.
            </p></td></tr>
</table></div>
<p>
            Now that you know what <code class="computeroutput"><a class="link" href="../boost/proto/as_child_idp731140288.html" title="Function as_child">proto::as_child()</a></code>
            and <code class="computeroutput"><a class="link" href="../boost/proto/as_expr_idp731118320.html" title="Function as_expr">proto::as_expr()</a></code> are, where they are
            used, and what they do by default, you may decide that one or both of
            these functions should have different behavior for your domain. For instance,
            given the above description of <code class="computeroutput"><a class="link" href="../boost/proto/as_child_idp731140288.html" title="Function as_child">proto::as_child()</a></code>,
            the following code is always wrong:
          </p>
<pre class="programlisting"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">literal</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">&gt;</span> <span class="identifier">i</span><span class="special">(</span><span class="number">0</span><span class="special">);</span>
<span class="keyword">auto</span> <span class="identifier">l</span> <span class="special">=</span> <span class="identifier">i</span> <span class="special">+</span> <span class="number">42</span><span class="special">;</span> <span class="comment">// This is WRONG! Don't do this.</span>
</pre>
<p>
            Why is this wrong? Because <code class="computeroutput"><a class="link" href="../boost/proto/as_child_idp731140288.html" title="Function as_child">proto::as_child()</a></code>
            will turn the integer literal 42 into a Proto terminal that holds a reference
            to a temporary integer initialized with 42. The lifetime of that temporary
            ends at the semicolon, guaranteeing that the local <code class="computeroutput"><span class="identifier">l</span></code>
            is left holding a dangling reference to a deceased integer. What to do?
            One answer is to use <code class="computeroutput"><a class="link" href="../boost/proto/deep_copy_idp727047232.html" title="Function template deep_copy">proto::deep_copy()</a></code>.
            Another is to customize the behavior of <code class="computeroutput"><a class="link" href="../boost/proto/as_child_idp731140288.html" title="Function as_child">proto::as_child()</a></code>
            for your domain. Read on for the details.
          </p>
<h6>
<a name="boost_proto.users_guide.front_end.customizing_expressions_in_your_domain.per_domain_as_child.h1"></a>
            <span class="phrase"><a name="boost_proto.users_guide.front_end.customizing_expressions_in_your_domain.per_domain_as_child.per_domain__literal_as_child__literal_"></a></span><a class="link" href="users_guide.html#boost_proto.users_guide.front_end.customizing_expressions_in_your_domain.per_domain_as_child.per_domain__literal_as_child__literal_">Per-Domain
            <code class="literal">as_child</code></a>
          </h6>
<p>
            To control how Proto builds expressions out of sub-expressions in your
            domain, define your domain as usual, and then define a nested <code class="computeroutput"><span class="identifier">as_child</span><span class="special">&lt;&gt;</span></code>
            class template within it, as follows:
          </p>
<pre class="programlisting"><span class="keyword">class</span> <span class="identifier">my_domain</span>
  <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">domain</span><span class="special">&lt;</span> <span class="identifier">my_generator</span><span class="special">,</span> <span class="identifier">my_grammar</span> <span class="special">&gt;</span>
<span class="special">{</span>
    <span class="comment">// Here is where you define how Proto should handle</span>
    <span class="comment">// sub-expressions that are about to be glommed into</span>
    <span class="comment">// a larger expression.</span>
    <span class="keyword">template</span><span class="special">&lt;</span> <span class="keyword">typename</span> <span class="identifier">T</span> <span class="special">&gt;</span>
    <span class="keyword">struct</span> <span class="identifier">as_child</span>
    <span class="special">{</span>
        <span class="keyword">typedef</span> <span class="emphasis"><em><code class="literal">unspecified-Proto-expr-type</code></em></span> <span class="identifier">result_type</span><span class="special">;</span>

        <span class="identifier">result_type</span> <span class="keyword">operator</span><span class="special">()(</span> <span class="identifier">T</span> <span class="special">&amp;</span> <span class="identifier">t</span> <span class="special">)</span> <span class="keyword">const</span>
        <span class="special">{</span>
            <span class="keyword">return</span> <span class="emphasis"><em><code class="literal">unspecified-Proto-expr-object</code></em></span><span class="special">;</span>
        <span class="special">}</span>
    <span class="special">};</span>
<span class="special">};</span>
</pre>
<p>
            There's one important thing to note: in the above code, the template
            parameter <code class="literal">T</code> may or may not be a Proto expression type,
            but the result <span class="emphasis"><em>must</em></span> be a Proto expression type,
            or a reference to one. That means that most user-defined <code class="literal">as_child&lt;&gt;</code>
            templates will need to check whether <code class="literal">T</code> is an expression
            or not (using <code class="computeroutput"><a class="link" href="../boost/proto/is_expr.html" title="Struct template is_expr">proto::is_expr&lt;&gt;</a></code>), and then turn non-expressions
            into Proto terminals by wrapping them as <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span> <span class="comment">/* ... */</span>
            <span class="special">&gt;::</span><span class="identifier">type</span></code>
            or equivalent.
          </p>
<h6>
<a name="boost_proto.users_guide.front_end.customizing_expressions_in_your_domain.per_domain_as_child.h2"></a>
            <span class="phrase"><a name="boost_proto.users_guide.front_end.customizing_expressions_in_your_domain.per_domain_as_child.per_domain__literal_as_expr__literal_"></a></span><a class="link" href="users_guide.html#boost_proto.users_guide.front_end.customizing_expressions_in_your_domain.per_domain_as_child.per_domain__literal_as_expr__literal_">Per-Domain
            <code class="literal">as_expr</code></a>
          </h6>
<p>
            Although less common, Proto also lets you customize the behavior of
            <code class="computeroutput"><a class="link" href="../boost/proto/as_expr_idp731118320.html" title="Function as_expr">proto::as_expr()</a></code> on a per-domain basis.
            The technique is identical to that for <code class="literal">as_child</code>. See
            below:
          </p>
<pre class="programlisting"><span class="keyword">class</span> <span class="identifier">my_domain</span>
  <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">domain</span><span class="special">&lt;</span> <span class="identifier">my_generator</span><span class="special">,</span> <span class="identifier">my_grammar</span> <span class="special">&gt;</span>
<span class="special">{</span>
    <span class="comment">// Here is where you define how Proto should handle</span>
    <span class="comment">// objects that are to be turned into expressions</span>
    <span class="comment">// fit for storage in local variables.</span>
    <span class="keyword">template</span><span class="special">&lt;</span> <span class="keyword">typename</span> <span class="identifier">T</span> <span class="special">&gt;</span>
    <span class="keyword">struct</span> <span class="identifier">as_expr</span>
    <span class="special">{</span>
        <span class="keyword">typedef</span> <span class="emphasis"><em><code class="literal">unspecified-Proto-expr-type</code></em></span> <span class="identifier">result_type</span><span class="special">;</span>

        <span class="identifier">result_type</span> <span class="keyword">operator</span><span class="special">()(</span> <span class="identifier">T</span> <span class="special">&amp;</span> <span class="identifier">t</span> <span class="special">)</span> <span class="keyword">const</span>
        <span class="special">{</span>
            <span class="keyword">return</span> <span class="emphasis"><em><code class="literal">unspecified-Proto-expr-object</code></em></span><span class="special">;</span>
        <span class="special">}</span>
    <span class="special">};</span>
<span class="special">};</span>
</pre>
<h6>
<a name="boost_proto.users_guide.front_end.customizing_expressions_in_your_domain.per_domain_as_child.h3"></a>
            <span class="phrase"><a name="boost_proto.users_guide.front_end.customizing_expressions_in_your_domain.per_domain_as_child.making_proto_expressions__literal_auto__literal__safe"></a></span><a class="link" href="users_guide.html#boost_proto.users_guide.front_end.customizing_expressions_in_your_domain.per_domain_as_child.making_proto_expressions__literal_auto__literal__safe">Making
            Proto Expressions <code class="literal">auto</code>-safe</a>
          </h6>
<p>
            Let's look again at the problem described above involving the C++11
            <code class="computeroutput"><span class="keyword">auto</span></code> keyword and the default
            behavior of <code class="computeroutput"><a class="link" href="../boost/proto/as_child_idp731140288.html" title="Function as_child">proto::as_child()</a></code>.
          </p>
<pre class="programlisting"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">literal</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">&gt;</span> <span class="identifier">i</span><span class="special">(</span><span class="number">0</span><span class="special">);</span>
<span class="keyword">auto</span> <span class="identifier">l</span> <span class="special">=</span> <span class="identifier">i</span> <span class="special">+</span> <span class="number">42</span><span class="special">;</span> <span class="comment">// This is WRONG! Don't do this.</span>
</pre>
<p>
            Recall that the problem is the lifetime of the temporary integer created
            to hold the value 42. The local <code class="computeroutput"><span class="identifier">l</span></code>
            will be left holding a dangling reference to it after its lifetime is
            over. What if we want Proto to make expressions safe to store this way
            in local variables? We can do so very easily by making <code class="computeroutput"><a class="link" href="../boost/proto/as_child_idp731140288.html" title="Function as_child">proto::as_child()</a></code> behave just like <code class="computeroutput"><a class="link" href="../boost/proto/as_expr_idp731118320.html" title="Function as_expr">proto::as_expr()</a></code>. The following code
            achieves this:
          </p>
<pre class="programlisting"><span class="keyword">template</span><span class="special">&lt;</span> <span class="keyword">typename</span> <span class="identifier">E</span> <span class="special">&gt;</span>
<span class="keyword">struct</span> <span class="identifier">my_expr</span><span class="special">;</span>

<span class="keyword">struct</span> <span class="identifier">my_generator</span>
  <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">pod_generator</span><span class="special">&lt;</span> <span class="identifier">my_expr</span> <span class="special">&gt;</span>
<span class="special">{};</span>

<span class="keyword">struct</span> <span class="identifier">my_domain</span>
  <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">domain</span><span class="special">&lt;</span> <span class="identifier">my_generator</span> <span class="special">&gt;</span>
<span class="special">{</span>
     <span class="comment">// Make as_child() behave like as_expr() in my_domain.</span>
     <span class="comment">// (proto_base_domain is a typedef for proto::domain&lt; my_generator &gt;</span>
     <span class="comment">// that is defined in proto::domain&lt;&gt;.)</span>
     <span class="keyword">template</span><span class="special">&lt;</span> <span class="keyword">typename</span> <span class="identifier">T</span> <span class="special">&gt;</span>
     <span class="keyword">struct</span> <span class="identifier">as_child</span>
       <span class="special">:</span> <span class="identifier">proto_base_domain</span><span class="special">::</span><span class="identifier">as_expr</span><span class="special">&lt;</span> <span class="identifier">T</span> <span class="special">&gt;</span>
     <span class="special">{};</span>
<span class="special">};</span>

<span class="keyword">template</span><span class="special">&lt;</span> <span class="keyword">typename</span> <span class="identifier">E</span> <span class="special">&gt;</span>
<span class="keyword">struct</span> <span class="identifier">my_expr</span>
<span class="special">{</span>
    <span class="identifier">BOOST_PROTO_EXTENDS</span><span class="special">(</span> <span class="identifier">E</span><span class="special">,</span> <span class="identifier">my_expr</span><span class="special">&lt;</span> <span class="identifier">E</span> <span class="special">&gt;,</span> <span class="identifier">my_domain</span> <span class="special">)</span>
<span class="special">};</span>

<span class="comment">/* ... */</span>

<span class="identifier">proto</span><span class="special">::</span><span class="identifier">literal</span><span class="special">&lt;</span> <span class="keyword">int</span><span class="special">,</span> <span class="identifier">my_domain</span> <span class="special">&gt;</span> <span class="identifier">i</span><span class="special">(</span><span class="number">0</span><span class="special">);</span>
<span class="keyword">auto</span> <span class="identifier">l</span> <span class="special">=</span> <span class="identifier">i</span> <span class="special">+</span> <span class="number">42</span><span class="special">;</span> <span class="comment">// OK! Everything is stored by value here.</span>
</pre>
<p>
            Notice that <code class="computeroutput"><span class="identifier">my_domain</span><span class="special">::</span><span class="identifier">as_child</span><span class="special">&lt;&gt;</span></code> simply defers to the default
            implementation of <code class="computeroutput"><span class="identifier">as_expr</span><span class="special">&lt;&gt;</span></code> found in <code class="computeroutput"><a class="link" href="../boost/proto/domain.html" title="Struct template domain">proto::domain&lt;&gt;</a></code>.
            By simply cross-wiring our domain's <code class="computeroutput"><span class="identifier">as_child</span><span class="special">&lt;&gt;</span></code> to <code class="computeroutput"><span class="identifier">as_expr</span><span class="special">&lt;&gt;</span></code>, we guarantee that all terminals
            that can be held by value are, and that all child expressions are also
            held by value. This increases copying and may incur a runtime performance
            cost, but it eliminates any spector of lifetime management issues.
          </p>
<p>
            For another example, see the definition of <code class="computeroutput"><span class="identifier">lldomain</span></code>
            in <code class="literal">libs/proto/example/lambda.hpp</code>. That example is
            a complete reimplementation of the Boost Lambda Library (BLL) on top
            of Boost.Proto. The function objects the BLL generates are safe to be
            stored in local variables. To emulate this with Proto, the <code class="computeroutput"><span class="identifier">lldomain</span></code> cross-wires <code class="computeroutput"><span class="identifier">as_child</span><span class="special">&lt;&gt;</span></code>
            to <code class="computeroutput"><span class="identifier">as_expr</span><span class="special">&lt;&gt;</span></code>
            as above, but with one extra twist: objects with array type are also
            stored by reference. Check it out.
          </p>
</div>
<div class="section">
<div class="titlepage"><div><div><h5 class="title">
<a name="boost_proto.users_guide.front_end.customizing_expressions_in_your_domain.subdomains"></a><a class="link" href="users_guide.html#boost_proto.users_guide.front_end.customizing_expressions_in_your_domain.subdomains" title="EDSL Interoperatability: Sub-Domains">EDSL
          Interoperatability: Sub-Domains</a>
</h5></div></div></div>
<div class="note"><table border="0" summary="Note">
<tr>
<td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../doc/src/images/note.png"></td>
<th align="left">Note</th>
</tr>
<tr><td align="left" valign="top"><p>
              This is an advanced topic. Feel free to skip this if you're just getting
              started with Proto.
            </p></td></tr>
</table></div>
<p>
            The ability to <span class="emphasis"><em>compose</em></span> different EDSLs is one of
            their most exciting features. Consider how you build a parser using yacc.
            You write your grammar rules in yacc's domain-specific language. Then
            you embed semantic actions written in C within your grammar. Boost's
            Spirit parser generator gives you the same ability. You write grammar
            rules using Spirit.Qi and embed semantic actions using the Phoenix library.
            Phoenix and Spirit are both Proto-based domain-specific languages with
            their own distinct syntax and semantics. But you can freely embed Phoenix
            expressions within Spirit expressions. This section describes Proto's
            <span class="emphasis"><em>sub-domain</em></span> feature that lets you define families
            of interoperable domains.
          </p>
<h6>
<a name="boost_proto.users_guide.front_end.customizing_expressions_in_your_domain.subdomains.h0"></a>
            <span class="phrase"><a name="boost_proto.users_guide.front_end.customizing_expressions_in_your_domain.subdomains.dueling_domains"></a></span><a class="link" href="users_guide.html#boost_proto.users_guide.front_end.customizing_expressions_in_your_domain.subdomains.dueling_domains">Dueling
            Domains</a>
          </h6>
<p>
            When you try to create an expression from two sub-expressions in different
            domains, what is the domain of the resulting expression? This is the
            fundamental problem that is addressed by sub-domains. Consider the following
            code:
          </p>
<pre class="programlisting"><span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">proto</span><span class="special">/</span><span class="identifier">proto</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
<span class="keyword">namespace</span> <span class="identifier">proto</span> <span class="special">=</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">proto</span><span class="special">;</span>

<span class="comment">// Forward-declare two expression wrappers</span>
<span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">E</span><span class="special">&gt;</span> <span class="keyword">struct</span> <span class="identifier">spirit_expr</span><span class="special">;</span>
<span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">E</span><span class="special">&gt;</span> <span class="keyword">struct</span> <span class="identifier">phoenix_expr</span><span class="special">;</span>

<span class="comment">// Define two domains</span>
<span class="keyword">struct</span> <span class="identifier">spirit_domain</span>  <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">domain</span><span class="special">&lt;</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">generator</span><span class="special">&lt;</span><span class="identifier">spirit_expr</span><span class="special">&gt;</span> <span class="special">&gt;</span> <span class="special">{};</span>
<span class="keyword">struct</span> <span class="identifier">phoenix_domain</span> <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">domain</span><span class="special">&lt;</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">generator</span><span class="special">&lt;</span><span class="identifier">phoenix_expr</span><span class="special">&gt;</span> <span class="special">&gt;</span> <span class="special">{};</span>

<span class="comment">// Implement the two expression wrappers</span>
<span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">E</span><span class="special">&gt;</span>
<span class="keyword">struct</span> <span class="identifier">spirit_expr</span>
  <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">extends</span><span class="special">&lt;</span><span class="identifier">E</span><span class="special">,</span> <span class="identifier">spirit_expr</span><span class="special">&lt;</span><span class="identifier">E</span><span class="special">&gt;,</span> <span class="identifier">spirit_domain</span><span class="special">&gt;</span>
<span class="special">{</span>
    <span class="identifier">spirit_expr</span><span class="special">(</span><span class="identifier">E</span> <span class="keyword">const</span> <span class="special">&amp;</span><span class="identifier">e</span> <span class="special">=</span> <span class="identifier">E</span><span class="special">())</span> <span class="special">:</span> <span class="identifier">spirit_expr</span><span class="special">::</span><span class="identifier">proto_extends</span><span class="special">(</span><span class="identifier">e</span><span class="special">)</span> <span class="special">{}</span>
<span class="special">};</span>

<span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">E</span><span class="special">&gt;</span>
<span class="keyword">struct</span> <span class="identifier">phoenix_expr</span>
  <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">extends</span><span class="special">&lt;</span><span class="identifier">E</span><span class="special">,</span> <span class="identifier">phoenix_expr</span><span class="special">&lt;</span><span class="identifier">E</span><span class="special">&gt;,</span> <span class="identifier">phoenix_domain</span><span class="special">&gt;</span>
<span class="special">{</span>
    <span class="identifier">phoenix_expr</span><span class="special">(</span><span class="identifier">E</span> <span class="keyword">const</span> <span class="special">&amp;</span><span class="identifier">e</span> <span class="special">=</span> <span class="identifier">E</span><span class="special">())</span> <span class="special">:</span> <span class="identifier">phoenix_expr</span><span class="special">::</span><span class="identifier">proto_extends</span><span class="special">(</span><span class="identifier">e</span><span class="special">)</span> <span class="special">{}</span>
<span class="special">};</span>

<span class="keyword">int</span> <span class="identifier">main</span><span class="special">()</span>
<span class="special">{</span>
    <span class="identifier">proto</span><span class="special">::</span><span class="identifier">literal</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">,</span> <span class="identifier">spirit_domain</span><span class="special">&gt;</span> <span class="identifier">sp</span><span class="special">(</span><span class="number">0</span><span class="special">);</span>
    <span class="identifier">proto</span><span class="special">::</span><span class="identifier">literal</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">,</span> <span class="identifier">phoenix_domain</span><span class="special">&gt;</span> <span class="identifier">phx</span><span class="special">(</span><span class="number">0</span><span class="special">);</span>

    <span class="comment">// Whoops! What does it mean to add two expressions in different domains?</span>
    <span class="identifier">sp</span> <span class="special">+</span> <span class="identifier">phx</span><span class="special">;</span> <span class="comment">// ERROR</span>
<span class="special">}</span>
</pre>
<p>
            Above, we define two domains called <code class="computeroutput"><span class="identifier">spirit_domain</span></code>
            and <code class="computeroutput"><span class="identifier">phoenix_domain</span></code> and
            declare two int literals in each. Then we try to compose them into a
            larger expression using Proto's binary plus operator, and it fails. Proto
            can't figure out whether the resulting expression should be in the Spirit
            domain or the Phoenix domain, and thus whether it should be an instance
            of <code class="computeroutput"><span class="identifier">spirit_expr</span><span class="special">&lt;&gt;</span></code>
            or <code class="computeroutput"><span class="identifier">phoenix_expr</span><span class="special">&lt;&gt;</span></code>.
            We have to tell Proto how to resolve the conflict. We can do that by
            declaring that Phoenix is a sub-domain of Spirit as in the following
            definition of <code class="computeroutput"><span class="identifier">phoenix_domain</span></code>:
          </p>
<pre class="programlisting"><span class="comment">// Declare that phoenix_domain is a sub-domain of spirit_domain</span>
<span class="keyword">struct</span> <span class="identifier">phoenix_domain</span>
  <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">domain</span><span class="special">&lt;</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">generator</span><span class="special">&lt;</span><span class="identifier">phoenix_expr</span><span class="special">&gt;,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_</span><span class="special">,</span> <span class="bold"><strong>spirit_domain</strong></span><span class="special">&gt;</span>
<span class="special">{};</span>
</pre>
<p>
            The third template parameter to <code class="computeroutput"><a class="link" href="../boost/proto/domain.html" title="Struct template domain">proto::domain&lt;&gt;</a></code>
            is the super-domain. By defining <code class="computeroutput"><span class="identifier">phoenix_domain</span></code>
            as above, we are saying that Phoenix expressions can be combined with
            Spirit expressions, and that when that happens, the resulting expression
            should be a Spirit expression.
          </p>
<div class="note"><table border="0" summary="Note">
<tr>
<td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../doc/src/images/note.png"></td>
<th align="left">Note</th>
</tr>
<tr><td align="left" valign="top"><p>
              If you are wondering what the purpose of <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_</span></code>
              is in the definition of <code class="computeroutput"><span class="identifier">phoenix_domain</span></code>
              above, recall that the second template parameter to <code class="computeroutput"><a class="link" href="../boost/proto/domain.html" title="Struct template domain">proto::domain&lt;&gt;</a></code>
              is the domain's grammar. <span class="quote">&#8220;<span class="quote"><code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_</span></code></span>&#8221;</span>
              is the default and signifies that the domain places no restrictions
              on the expressions that are valid within it.
            </p></td></tr>
</table></div>
<h6>
<a name="boost_proto.users_guide.front_end.customizing_expressions_in_your_domain.subdomains.h1"></a>
            <span class="phrase"><a name="boost_proto.users_guide.front_end.customizing_expressions_in_your_domain.subdomains.domain_resolution"></a></span><a class="link" href="users_guide.html#boost_proto.users_guide.front_end.customizing_expressions_in_your_domain.subdomains.domain_resolution">Domain
            Resolution</a>
          </h6>
<p>
            When there are multiple domains in play within a given expression, Proto
            uses some rules to figure out which domain "wins". The rules
            are loosely modeled on the rules for C++ inheritance. <code class="computeroutput"><span class="identifier">Phoenix_domain</span></code>
            is a sub-domain of <code class="computeroutput"><span class="identifier">spirit_domain</span></code>.
            You can liken that to a derived/base relationship that gives Phoenix
            expressions a kind of implicit conversion to Spirit expressions. And
            since Phoenix expressions can be "converted" to Spirit expressions,
            they can be freely combined with Spirit expressions and the result is
            a Spirit expression.
          </p>
<div class="note"><table border="0" summary="Note">
<tr>
<td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../doc/src/images/note.png"></td>
<th align="left">Note</th>
</tr>
<tr><td align="left" valign="top"><p>
              Super- and sub-domains are not actually implemented using inheritance.
              This is only a helpful mental model.
            </p></td></tr>
</table></div>
<p>
            The analogy with inheritance holds even in the case of three domains
            when two are sub-domains of the third. Imagine another domain called
            <code class="computeroutput"><span class="identifier">foobar_domain</span></code> that was
            also a sub-domain of <code class="computeroutput"><span class="identifier">spirit_domain</span></code>.
            Expressions in the <code class="computeroutput"><span class="identifier">foobar_domain</span></code>
            could be combined with expressions in the <code class="computeroutput"><span class="identifier">phoenix_domain</span></code>
            and the resulting expression would be in the <code class="computeroutput"><span class="identifier">spirit_domain</span></code>.
            That's because expressions in the two sub-domains both have "conversions"
            to the super-domain, so the operation is allowed and the super-domain
            wins.
          </p>
<h6>
<a name="boost_proto.users_guide.front_end.customizing_expressions_in_your_domain.subdomains.h2"></a>
            <span class="phrase"><a name="boost_proto.users_guide.front_end.customizing_expressions_in_your_domain.subdomains.the_default_domain"></a></span><a class="link" href="users_guide.html#boost_proto.users_guide.front_end.customizing_expressions_in_your_domain.subdomains.the_default_domain">The
            Default Domain</a>
          </h6>
<p>
            When you don't assign a Proto expression to a particular domain, Proto
            considers it a member of the so-called default domain, <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">default_domain</span></code>. Even non-Proto objects
            are treated as terminals in the default domain. Consider:
          </p>
<pre class="programlisting"><span class="keyword">int</span> <span class="identifier">main</span><span class="special">()</span>
<span class="special">{</span>
    <span class="identifier">proto</span><span class="special">::</span><span class="identifier">literal</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">,</span> <span class="identifier">spirit_domain</span><span class="special">&gt;</span> <span class="identifier">sp</span><span class="special">(</span><span class="number">0</span><span class="special">);</span>

    <span class="comment">// Add 1 to a spirit expression. Result is a spirit expression.</span>
    <span class="identifier">sp</span> <span class="special">+</span> <span class="number">1</span><span class="special">;</span>
<span class="special">}</span>
</pre>
<p>
            Expressions in the default domain (or non-expressions like <code class="literal">1</code>)
            have a kind of implicit conversion to expressions every other domain
            type. What's more, you can define your domain to be a sub-domain of the
            default domain. In so doing, you give expressions in your domain conversions
            to expressions in every other domain. This is like a <span class="quote">&#8220;<span class="quote">free love</span>&#8221;</span>
            domain, because it will freely mix with all other domains.
          </p>
<p>
            Let's think again about the Phoenix EDSL. Since it provides generally
            useful lambda functionality, it's reasonable to assume that lots of other
            EDSLs besides Spirit might want the ability to embed Phoenix expressions.
            In other words, <code class="computeroutput"><span class="identifier">phoenix_domain</span></code>
            should be a sub-domain of <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">default_domain</span></code>,
            not <code class="computeroutput"><span class="identifier">spirit_domain</span></code>:
          </p>
<pre class="programlisting"><span class="comment">// Declare that phoenix_domain is a sub-domain of proto::default_domain</span>
<span class="keyword">struct</span> <span class="identifier">phoenix_domain</span>
  <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">domain</span><span class="special">&lt;</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">generator</span><span class="special">&lt;</span><span class="identifier">phoenix_expr</span><span class="special">&gt;,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_</span><span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">default_domain</span><span class="special">&gt;</span>
<span class="special">{};</span>
</pre>
<p>
            That's much better. Phoenix expressions can now be put anywhere.
          </p>
<h6>
<a name="boost_proto.users_guide.front_end.customizing_expressions_in_your_domain.subdomains.h3"></a>
            <span class="phrase"><a name="boost_proto.users_guide.front_end.customizing_expressions_in_your_domain.subdomains.sub_domain_summary"></a></span><a class="link" href="users_guide.html#boost_proto.users_guide.front_end.customizing_expressions_in_your_domain.subdomains.sub_domain_summary">Sub-Domain
            Summary</a>
          </h6>
<p>
            Use Proto sub-domains to make it possible to mix expressions from multiple
            domains. And when you want expressions in your domain to freely combine
            with <span class="emphasis"><em>all</em></span> expressions, make it a sub-domain of <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">default_domain</span></code>.
          </p>
</div>
</div>
<div class="section">
<div class="titlepage"><div><div><h4 class="title">
<a name="boost_proto.users_guide.front_end.define_operators"></a><a class="link" href="users_guide.html#boost_proto.users_guide.front_end.define_operators" title="Adapting Existing Types to Proto">Adapting
        Existing Types to Proto</a>
</h4></div></div></div>
<p>
          The preceding discussions of defining Proto front ends have all made a
          big assumption: that you have the luxury of defining everything from scratch.
          What happens if you have existing types, say a matrix type and a vector
          type, that you would like to treat as if they were Proto terminals? Proto
          usually trades only in its own expression types, but with <code class="literal"><code class="computeroutput"><a class="link" href="../BOOST_PROTO_DEFINE_OPERATORS.html" title="Macro BOOST_PROTO_DEFINE_OPERATORS">BOOST_PROTO_DEFINE_OPERATORS</a></code>()</code>,
          it can accomodate your custom terminal types, too.
        </p>
<p>
          Let's say, for instance, that you have the following types and that you
          can't modify then to make them <span class="quote">&#8220;<span class="quote">native</span>&#8221;</span> Proto terminal types.
        </p>
<pre class="programlisting"><span class="keyword">namespace</span> <span class="identifier">math</span>
<span class="special">{</span>
    <span class="comment">// A matrix type ...</span>
    <span class="keyword">struct</span> <span class="identifier">matrix</span> <span class="special">{</span> <span class="comment">/*...*/</span> <span class="special">};</span>

    <span class="comment">// A vector type ...</span>
    <span class="keyword">struct</span> <span class="identifier">vector</span> <span class="special">{</span> <span class="comment">/*...*/</span> <span class="special">};</span>
<span class="special">}</span>
</pre>
<p>
          You can non-intrusively make objects of these types Proto terminals by
          defining the proper operator overloads using <code class="literal"><code class="computeroutput"><a class="link" href="../BOOST_PROTO_DEFINE_OPERATORS.html" title="Macro BOOST_PROTO_DEFINE_OPERATORS">BOOST_PROTO_DEFINE_OPERATORS</a></code>()</code>.
          The basic procedure is as follows:
        </p>
<div class="orderedlist"><ol class="orderedlist" type="1">
<li class="listitem">
              Define a trait that returns true for your types and false for all others.
            </li>
<li class="listitem">
              Reopen the namespace of your types and use <code class="literal"><code class="computeroutput"><a class="link" href="../BOOST_PROTO_DEFINE_OPERATORS.html" title="Macro BOOST_PROTO_DEFINE_OPERATORS">BOOST_PROTO_DEFINE_OPERATORS</a></code>()</code>
              to define a set of operator overloads, passing the name of the trait
              as the first macro parameter, and the name of a Proto domain (e.g.,
              <code class="computeroutput"><a class="link" href="../boost/proto/default_domain.html" title="Struct default_domain">proto::default_domain</a></code>)
              as the second.
            </li>
</ol></div>
<p>
          The following code demonstrates how it works.
        </p>
<pre class="programlisting"><span class="keyword">namespace</span> <span class="identifier">math</span>
<span class="special">{</span>
    <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">T</span><span class="special">&gt;</span>
    <span class="keyword">struct</span> <span class="identifier">is_terminal</span>
      <span class="special">:</span> <span class="identifier">mpl</span><span class="special">::</span><span class="identifier">false_</span>
    <span class="special">{};</span>

    <span class="comment">// OK, "matrix" is a custom terminal type</span>
    <span class="keyword">template</span><span class="special">&lt;&gt;</span>
    <span class="keyword">struct</span> <span class="identifier">is_terminal</span><span class="special">&lt;</span><span class="identifier">matrix</span><span class="special">&gt;</span>
      <span class="special">:</span> <span class="identifier">mpl</span><span class="special">::</span><span class="identifier">true_</span>
    <span class="special">{};</span>

    <span class="comment">// OK, "vector" is a custom terminal type</span>
    <span class="keyword">template</span><span class="special">&lt;&gt;</span>
    <span class="keyword">struct</span> <span class="identifier">is_terminal</span><span class="special">&lt;</span><span class="identifier">vector</span><span class="special">&gt;</span>
      <span class="special">:</span> <span class="identifier">mpl</span><span class="special">::</span><span class="identifier">true_</span>
    <span class="special">{};</span>

    <span class="comment">// Define all the operator overloads to construct Proto</span>
    <span class="comment">// expression templates, treating "matrix" and "vector"</span>
    <span class="comment">// objects as if they were Proto terminals.</span>
    <span class="identifier">BOOST_PROTO_DEFINE_OPERATORS</span><span class="special">(</span><span class="identifier">is_terminal</span><span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">default_domain</span><span class="special">)</span>
<span class="special">}</span>
</pre>
<p>
          The invocation of the <code class="literal"><code class="computeroutput"><a class="link" href="../BOOST_PROTO_DEFINE_OPERATORS.html" title="Macro BOOST_PROTO_DEFINE_OPERATORS">BOOST_PROTO_DEFINE_OPERATORS</a></code>()</code>
          macro defines a complete set of operator overloads that treat <code class="computeroutput"><span class="identifier">matrix</span></code> and <code class="computeroutput"><span class="identifier">vector</span></code>
          objects as if they were Proto terminals. And since the operators are defined
          in the same namespace as the <code class="computeroutput"><span class="identifier">matrix</span></code>
          and <code class="computeroutput"><span class="identifier">vector</span></code> types, the operators
          will be found by argument-dependent lookup. With the code above, we can
          now construct expression templates with matrices and vectors, as shown
          below.
        </p>
<pre class="programlisting"><span class="identifier">math</span><span class="special">::</span><span class="identifier">matrix</span> <span class="identifier">m1</span><span class="special">;</span>
<span class="identifier">math</span><span class="special">::</span><span class="identifier">vector</span> <span class="identifier">v1</span><span class="special">;</span>
<span class="identifier">proto</span><span class="special">::</span><span class="identifier">literal</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">&gt;</span> <span class="identifier">i</span><span class="special">(</span><span class="number">0</span><span class="special">);</span>

<span class="identifier">m1</span> <span class="special">*</span> <span class="number">1</span><span class="special">;</span>  <span class="comment">// custom terminal and literals are OK</span>
<span class="identifier">m1</span> <span class="special">*</span> <span class="identifier">i</span><span class="special">;</span>  <span class="comment">// custom terminal and Proto expressions are OK</span>
<span class="identifier">m1</span> <span class="special">*</span> <span class="identifier">v1</span><span class="special">;</span> <span class="comment">// two custom terminals are OK, too.</span>
</pre>
</div>
<div class="section">
<div class="titlepage"><div><div><h4 class="title">
<a name="boost_proto.users_guide.front_end.code_repetition"></a><a class="link" href="users_guide.html#boost_proto.users_guide.front_end.code_repetition" title="Generating Repetitive Code with the Preprocessor">Generating
        Repetitive Code with the Preprocessor</a>
</h4></div></div></div>
<p>
          Sometimes as an EDSL designer, to make the lives of your users easy, you
          have to make your own life hard. Giving your users natural and flexible
          syntax often involves writing large numbers of repetitive function overloads.
          It can be enough to give you repetitive stress injury! Before you hurt
          yourself, check out the macros Proto provides for automating many repetitive
          code-generation chores.
        </p>
<p>
          Imagine that we are writing a lambda EDSL, and we would like to enable
          syntax for constructing temporary objects of any type using the following
          syntax:
        </p>
<pre class="programlisting"><span class="comment">// A lambda expression that takes two arguments and</span>
<span class="comment">// uses them to construct a temporary std::complex&lt;&gt;</span>
<span class="identifier">construct</span><span class="special">&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">complex</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">&gt;</span> <span class="special">&gt;(</span> <span class="identifier">_1</span><span class="special">,</span> <span class="identifier">_2</span> <span class="special">)</span>
</pre>
<p>
          For the sake of the discussion, imagine that we already have a function
          object template <code class="computeroutput"><span class="identifier">construct_impl</span><span class="special">&lt;&gt;</span></code> that accepts arguments and constructs
          new objects from them. We would want the above lambda expression to be
          equivalent to the following:
        </p>
<pre class="programlisting"><span class="comment">// The above lambda expression should be roughly equivalent</span>
<span class="comment">// to the following:</span>
<span class="identifier">proto</span><span class="special">::</span><span class="identifier">make_expr</span><span class="special">&lt;</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">function</span><span class="special">&gt;(</span>
    <span class="identifier">construct_impl</span><span class="special">&lt;</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">complex</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">&gt;</span> <span class="special">&gt;()</span> <span class="comment">// The function to invoke lazily</span>
  <span class="special">,</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">ref</span><span class="special">(</span><span class="identifier">_1</span><span class="special">)</span>                       <span class="comment">// The first argument to the function</span>
  <span class="special">,</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">ref</span><span class="special">(</span><span class="identifier">_2</span><span class="special">)</span>                       <span class="comment">// The second argument to the function</span>
<span class="special">);</span>
</pre>
<p>
          We can define our <code class="computeroutput"><span class="identifier">construct</span><span class="special">()</span></code> function template as follows:
        </p>
<pre class="programlisting"><span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">T</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">A0</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">A1</span><span class="special">&gt;</span>
<span class="keyword">typename</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">result_of</span><span class="special">::</span><span class="identifier">make_expr</span><span class="special">&lt;</span>
    <span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">function</span>
  <span class="special">,</span> <span class="identifier">construct_impl</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;</span>
  <span class="special">,</span> <span class="identifier">A0</span> <span class="keyword">const</span> <span class="special">&amp;</span>
  <span class="special">,</span> <span class="identifier">A1</span> <span class="keyword">const</span> <span class="special">&amp;</span>
<span class="special">&gt;::</span><span class="identifier">type</span> <span class="keyword">const</span>
<span class="identifier">construct</span><span class="special">(</span><span class="identifier">A0</span> <span class="keyword">const</span> <span class="special">&amp;</span><span class="identifier">a0</span><span class="special">,</span> <span class="identifier">A1</span> <span class="keyword">const</span> <span class="special">&amp;</span><span class="identifier">a1</span><span class="special">)</span>
<span class="special">{</span>
    <span class="keyword">return</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">make_expr</span><span class="special">&lt;</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">function</span><span class="special">&gt;(</span>
        <span class="identifier">construct_impl</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;()</span>
      <span class="special">,</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">ref</span><span class="special">(</span><span class="identifier">a0</span><span class="special">)</span>
      <span class="special">,</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">ref</span><span class="special">(</span><span class="identifier">a1</span><span class="special">)</span>
    <span class="special">);</span>
<span class="special">}</span>
</pre>
<p>
          This works for two arguments, but we would like it to work for any number
          of arguments, up to ( <code class="literal"><code class="computeroutput"><a class="link" href="../BOOST_PROTO_MAX_ARITY.html" title="Macro BOOST_PROTO_MAX_ARITY">BOOST_PROTO_MAX_ARITY</a></code></code>
          - 1). (Why "- 1"? Because one child is taken up by the <code class="computeroutput"><span class="identifier">construct_impl</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;()</span></code>
          terminal leaving room for only ( <code class="literal"><code class="computeroutput"><a class="link" href="../BOOST_PROTO_MAX_ARITY.html" title="Macro BOOST_PROTO_MAX_ARITY">BOOST_PROTO_MAX_ARITY</a></code></code>
          - 1) other children.)
        </p>
<p>
          For cases like this, Proto provides the <code class="literal"><code class="computeroutput"><a class="link" href="../BOOST_PROTO_REPEAT.html" title="Macro BOOST_PROTO_REPEAT">BOOST_PROTO_REPEAT</a></code>()</code>
          and <code class="literal"><code class="computeroutput"><a class="link" href="../BOOST_PROTO_REPEAT_FROM_TO.html" title="Macro BOOST_PROTO_REPEAT_FROM_TO">BOOST_PROTO_REPEAT_FROM_TO</a></code>()</code>
          macros. To use it, we turn the function definition above into a macro as
          follows:
        </p>
<pre class="programlisting"><span class="preprocessor">#define</span> <span class="identifier">M0</span><span class="special">(</span><span class="identifier">N</span><span class="special">,</span> <span class="identifier">typename_A</span><span class="special">,</span> <span class="identifier">A_const_ref</span><span class="special">,</span> <span class="identifier">A_const_ref_a</span><span class="special">,</span> <span class="identifier">ref_a</span><span class="special">)</span>  <span class="special">\</span>
<span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">T</span><span class="special">,</span> <span class="identifier">typename_A</span><span class="special">(</span><span class="identifier">N</span><span class="special">)&gt;</span>                           <span class="special">\</span>
<span class="keyword">typename</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">result_of</span><span class="special">::</span><span class="identifier">make_expr</span><span class="special">&lt;</span>                         <span class="special">\</span>
    <span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">function</span>                                      <span class="special">\</span>
  <span class="special">,</span> <span class="identifier">construct_impl</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;</span>                                         <span class="special">\</span>
  <span class="special">,</span> <span class="identifier">A_const_ref</span><span class="special">(</span><span class="identifier">N</span><span class="special">)</span>                                            <span class="special">\</span>
<span class="special">&gt;::</span><span class="identifier">type</span> <span class="keyword">const</span>                                                 <span class="special">\</span>
<span class="identifier">construct</span><span class="special">(</span><span class="identifier">A_const_ref_a</span><span class="special">(</span><span class="identifier">N</span><span class="special">))</span>                                   <span class="special">\</span>
<span class="special">{</span>                                                             <span class="special">\</span>
    <span class="keyword">return</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">make_expr</span><span class="special">&lt;</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">function</span><span class="special">&gt;(</span>            <span class="special">\</span>
        <span class="identifier">construct_impl</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;()</span>                                   <span class="special">\</span>
      <span class="special">,</span> <span class="identifier">ref_a</span><span class="special">(</span><span class="identifier">N</span><span class="special">)</span>                                              <span class="special">\</span>
    <span class="special">);</span>                                                        <span class="special">\</span>
<span class="special">}</span>
</pre>
<p>
          Notice that we turned the function into a macro that takes 5 arguments.
          The first is the current iteration number. The rest are the names of other
          macros that generate different sequences. For instance, Proto passes as
          the second parameter the name of a macro that will expand to <code class="computeroutput"><span class="keyword">typename</span> <span class="identifier">A0</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">A1</span><span class="special">,</span> <span class="special">...</span></code>.
        </p>
<p>
          Now that we have turned our function into a macro, we can pass the macro
          to <code class="literal"><code class="computeroutput"><a class="link" href="../BOOST_PROTO_REPEAT_FROM_TO.html" title="Macro BOOST_PROTO_REPEAT_FROM_TO">BOOST_PROTO_REPEAT_FROM_TO</a></code>()</code>.
          Proto will invoke it iteratively, generating all the function overloads
          for us.
        </p>
<pre class="programlisting"><span class="comment">// Generate overloads of construct() that accept from</span>
<span class="comment">// 1 to BOOST_PROTO_MAX_ARITY-1 arguments:</span>
<span class="identifier">BOOST_PROTO_REPEAT_FROM_TO</span><span class="special">(</span><span class="number">1</span><span class="special">,</span> <span class="identifier">BOOST_PROTO_MAX_ARITY</span><span class="special">,</span> <span class="identifier">M0</span><span class="special">)</span>
<span class="preprocessor">#undef</span> <span class="identifier">M0</span>
</pre>
<h6>
<a name="boost_proto.users_guide.front_end.code_repetition.h0"></a>
          <span class="phrase"><a name="boost_proto.users_guide.front_end.code_repetition.non_default_sequences"></a></span><a class="link" href="users_guide.html#boost_proto.users_guide.front_end.code_repetition.non_default_sequences">Non-Default
          Sequences</a>
        </h6>
<p>
          As mentioned above, Proto passes as the last 4 arguments to your macro
          the names of other macros that generate various sequences. The macros
          <code class="literal"><code class="computeroutput"><a class="link" href="../BOOST_PROTO_REPEAT.html" title="Macro BOOST_PROTO_REPEAT">BOOST_PROTO_REPEAT</a></code>()</code>
          and <code class="literal"><code class="computeroutput"><a class="link" href="../BOOST_PROTO_REPEAT_FROM_TO.html" title="Macro BOOST_PROTO_REPEAT_FROM_TO">BOOST_PROTO_REPEAT_FROM_TO</a></code>()</code>
          select defaults for these parameters. If the defaults do not meet your
          needs, you can use <code class="literal"><code class="computeroutput"><a class="link" href="../BOOST_PROTO_REPEAT_EX.html" title="Macro BOOST_PROTO_REPEAT_EX">BOOST_PROTO_REPEAT_EX</a></code>()</code>
          and <code class="literal"><code class="computeroutput"><a class="link" href="../BOOST_PROTO_REPEAT_FROM_TO_EX.html" title="Macro BOOST_PROTO_REPEAT_FROM_TO_EX">BOOST_PROTO_REPEAT_FROM_TO_EX</a></code>()</code>
          and pass different macros that generate different sequences. Proto defines
          a number of such macros for use as parameters to <code class="literal"><code class="computeroutput"><a class="link" href="../BOOST_PROTO_REPEAT_EX.html" title="Macro BOOST_PROTO_REPEAT_EX">BOOST_PROTO_REPEAT_EX</a></code>()</code>
          and <code class="literal"><code class="computeroutput"><a class="link" href="../BOOST_PROTO_REPEAT_FROM_TO_EX.html" title="Macro BOOST_PROTO_REPEAT_FROM_TO_EX">BOOST_PROTO_REPEAT_FROM_TO_EX</a></code>()</code>.
          Check the reference section for <code class="computeroutput"><a class="link" href="reference.html#header.boost.proto.repeat_hpp" title="Header &lt;boost/proto/repeat.hpp&gt;">boost/proto/repeat.hpp</a></code>
          for all the details.
        </p>
<p>
          Also, check out <code class="literal"><code class="computeroutput"><a class="link" href="../BOOST_PROTO_LOCAL_ITERATE.html" title="Macro BOOST_PROTO_LOCAL_ITERATE">BOOST_PROTO_LOCAL_ITERATE</a></code>()</code>.
          It works similarly to <code class="literal"><code class="computeroutput"><a class="link" href="../BOOST_PROTO_REPEAT.html" title="Macro BOOST_PROTO_REPEAT">BOOST_PROTO_REPEAT</a></code>()</code>
          and friends, but it can be easier to use when you want to change one macro
          argument and accept defaults for the others.
        </p>
</div>
</div>
<div class="section">
<div class="titlepage"><div><div><h3 class="title">
<a name="boost_proto.users_guide.intermediate_form"></a><a class="link" href="users_guide.html#boost_proto.users_guide.intermediate_form" title="Intermediate Form: Understanding and Introspecting Expressions">Intermediate
      Form: Understanding and Introspecting Expressions</a>
</h3></div></div></div>
<div class="toc"><dl class="toc">
<dt><span class="section"><a href="users_guide.html#boost_proto.users_guide.intermediate_form.left_right_child">Accessing
        Parts of an Expression</a></span></dt>
<dt><span class="section"><a href="users_guide.html#boost_proto.users_guide.intermediate_form.deep_copying_expressions">Deep-copying
        Expressions</a></span></dt>
<dt><span class="section"><a href="users_guide.html#boost_proto.users_guide.intermediate_form.debugging_expressions">Debugging
        Expressions</a></span></dt>
<dt><span class="section"><a href="users_guide.html#boost_proto.users_guide.intermediate_form.tags_and_metafunctions">Operator
        Tags and Metafunctions</a></span></dt>
<dt><span class="section"><a href="users_guide.html#boost_proto.users_guide.intermediate_form.expressions_as_fusion_sequences">Expressions
        as Fusion Sequences</a></span></dt>
<dt><span class="section"><a href="users_guide.html#boost_proto.users_guide.intermediate_form.expression_introspection">Expression
        Introspection: Defining a Grammar</a></span></dt>
</dl></div>
<p>
        By now, you know a bit about how to build a front-end for your EDSL "compiler"
        -- you can define terminals and functions that generate expression templates.
        But we haven't said anything about the expression templates themselves. What
        do they look like? What can you do with them? In this section we'll see.
      </p>
<h5>
<a name="boost_proto.users_guide.intermediate_form.h0"></a>
        <span class="phrase"><a name="boost_proto.users_guide.intermediate_form.the__literal_expr_lt__gt___literal__type"></a></span><a class="link" href="users_guide.html#boost_proto.users_guide.intermediate_form.the__literal_expr_lt__gt___literal__type">The
        <code class="literal">expr&lt;&gt;</code> Type</a>
      </h5>
<p>
        All Proto expressions are an instantiation of a template called <code class="computeroutput"><a class="link" href="../boost/proto/expr.html" title="Struct template expr">proto::expr&lt;&gt;</a></code> (or a wrapper around
        such an instantiation). When we define a terminal as below, we are really
        initializing an instance of the <code class="computeroutput"><a class="link" href="../boost/proto/expr.html" title="Struct template expr">proto::expr&lt;&gt;</a></code>
        template.
      </p>
<pre class="programlisting"><span class="comment">// Define a placeholder type</span>
<span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">int</span> <span class="identifier">I</span><span class="special">&gt;</span>
<span class="keyword">struct</span> <span class="identifier">placeholder</span>
<span class="special">{};</span>

<span class="comment">// Define the Protofied placeholder terminal</span>
<span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span> <span class="identifier">placeholder</span><span class="special">&lt;</span><span class="number">0</span><span class="special">&gt;</span> <span class="special">&gt;::</span><span class="identifier">type</span> <span class="keyword">const</span> <span class="identifier">_1</span> <span class="special">=</span> <span class="special">{{}};</span>
</pre>
<p>
        The actual type of <code class="computeroutput"><span class="identifier">_1</span></code> looks
        like this:
      </p>
<pre class="programlisting"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">expr</span><span class="special">&lt;</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">term</span><span class="special">&lt;</span> <span class="identifier">placeholder</span><span class="special">&lt;</span><span class="number">0</span><span class="special">&gt;</span> <span class="special">&gt;,</span> <span class="number">0</span> <span class="special">&gt;</span>
</pre>
<p>
        The <code class="computeroutput"><a class="link" href="../boost/proto/expr.html" title="Struct template expr">proto::expr&lt;&gt;</a></code> template is the most
        important type in Proto. Although you will rarely need to deal with it directly,
        it's always there behind the scenes holding your expression trees together.
        In fact, <code class="computeroutput"><a class="link" href="../boost/proto/expr.html" title="Struct template expr">proto::expr&lt;&gt;</a></code> <span class="emphasis"><em>is</em></span>
        the expression tree -- branches, leaves and all.
      </p>
<p>
        The <code class="computeroutput"><a class="link" href="../boost/proto/expr.html" title="Struct template expr">proto::expr&lt;&gt;</a></code> template makes up the
        nodes in expression trees. The first template parameter is the node type;
        in this case, <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">terminal</span></code>.
        That means that <code class="computeroutput"><span class="identifier">_1</span></code> is a leaf-node
        in the expression tree. The second template parameter is a list of child
        types, or in the case of terminals, the terminal's value type. Terminals
        will always have only one type in the type list. The last parameter is the
        arity of the expression. Terminals have arity 0, unary expressions have arity
        1, etc.
      </p>
<p>
        The <code class="computeroutput"><a class="link" href="../boost/proto/expr.html" title="Struct template expr">proto::expr&lt;&gt;</a></code> struct is defined as
        follows:
      </p>
<pre class="programlisting"><span class="keyword">template</span><span class="special">&lt;</span> <span class="keyword">typename</span> <span class="identifier">Tag</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">Args</span><span class="special">,</span> <span class="keyword">long</span> <span class="identifier">Arity</span> <span class="special">=</span> <span class="identifier">Args</span><span class="special">::</span><span class="identifier">arity</span> <span class="special">&gt;</span>
<span class="keyword">struct</span> <span class="identifier">expr</span><span class="special">;</span>

<span class="keyword">template</span><span class="special">&lt;</span> <span class="keyword">typename</span> <span class="identifier">Tag</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">Args</span> <span class="special">&gt;</span>
<span class="keyword">struct</span> <span class="identifier">expr</span><span class="special">&lt;</span> <span class="identifier">Tag</span><span class="special">,</span> <span class="identifier">Args</span><span class="special">,</span> <span class="number">1</span> <span class="special">&gt;</span>
<span class="special">{</span>
    <span class="keyword">typedef</span> <span class="keyword">typename</span> <span class="identifier">Args</span><span class="special">::</span><span class="identifier">child0</span> <span class="identifier">proto_child0</span><span class="special">;</span>
    <span class="identifier">proto_child0</span> <span class="identifier">child0</span><span class="special">;</span>
    <span class="comment">// ...</span>
<span class="special">};</span>
</pre>
<p>
        The <code class="computeroutput"><a class="link" href="../boost/proto/expr.html" title="Struct template expr">proto::expr&lt;&gt;</a></code> struct does not define
        a constructor, or anything else that would prevent static initialization.
        All <code class="computeroutput"><a class="link" href="../boost/proto/expr.html" title="Struct template expr">proto::expr&lt;&gt;</a></code> objects are initialized
        using <span class="emphasis"><em>aggregate initialization</em></span>, with curly braces. In
        our example, <code class="computeroutput"><span class="identifier">_1</span></code> is initialized
        with the initializer <code class="computeroutput"><span class="special">{{}}</span></code>. The
        outer braces are the initializer for the <code class="computeroutput"><a class="link" href="../boost/proto/expr.html" title="Struct template expr">proto::expr&lt;&gt;</a></code>
        struct, and the inner braces are for the member <code class="computeroutput"><span class="identifier">_1</span><span class="special">.</span><span class="identifier">child0</span></code>
        which is of type <code class="computeroutput"><span class="identifier">placeholder</span><span class="special">&lt;</span><span class="number">0</span><span class="special">&gt;</span></code>.
        Note that we use braces to initialize <code class="computeroutput"><span class="identifier">_1</span><span class="special">.</span><span class="identifier">child0</span></code>
        because <code class="computeroutput"><span class="identifier">placeholder</span><span class="special">&lt;</span><span class="number">0</span><span class="special">&gt;</span></code> is also
        an aggregate.
      </p>
<h5>
<a name="boost_proto.users_guide.intermediate_form.h1"></a>
        <span class="phrase"><a name="boost_proto.users_guide.intermediate_form.building_expression_trees"></a></span><a class="link" href="users_guide.html#boost_proto.users_guide.intermediate_form.building_expression_trees">Building
        Expression Trees</a>
      </h5>
<p>
        The <code class="computeroutput"><span class="identifier">_1</span></code> node is an instantiation
        of <code class="computeroutput"><a class="link" href="../boost/proto/expr.html" title="Struct template expr">proto::expr&lt;&gt;</a></code>, and expressions containing
        <code class="computeroutput"><span class="identifier">_1</span></code> are also instantiations
        of <code class="computeroutput"><a class="link" href="../boost/proto/expr.html" title="Struct template expr">proto::expr&lt;&gt;</a></code>. To use Proto effectively,
        you won't have to bother yourself with the actual types that Proto generates.
        These are details, but you're likely to encounter these types in compiler
        error messages, so it's helpful to be familiar with them. The types look
        like this:
      </p>
<pre class="programlisting"><span class="comment">// The type of the expression -_1</span>
<span class="keyword">typedef</span>
    <span class="identifier">proto</span><span class="special">::</span><span class="identifier">expr</span><span class="special">&lt;</span>
        <span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">negate</span>
      <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">list1</span><span class="special">&lt;</span>
            <span class="identifier">proto</span><span class="special">::</span><span class="identifier">expr</span><span class="special">&lt;</span>
                <span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">terminal</span>
              <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">term</span><span class="special">&lt;</span> <span class="identifier">placeholder</span><span class="special">&lt;</span><span class="number">0</span><span class="special">&gt;</span> <span class="special">&gt;</span>
              <span class="special">,</span> <span class="number">0</span>
            <span class="special">&gt;</span> <span class="keyword">const</span> <span class="special">&amp;</span>
        <span class="special">&gt;</span>
      <span class="special">,</span> <span class="number">1</span>
    <span class="special">&gt;</span>
<span class="identifier">negate_placeholder_type</span><span class="special">;</span>

<span class="identifier">negate_placeholder_type</span> <span class="identifier">x</span> <span class="special">=</span> <span class="special">-</span><span class="identifier">_1</span><span class="special">;</span>

<span class="comment">// The type of the expression _1 + 42</span>
<span class="keyword">typedef</span>
    <span class="identifier">proto</span><span class="special">::</span><span class="identifier">expr</span><span class="special">&lt;</span>
        <span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">plus</span>
      <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">list2</span><span class="special">&lt;</span>
            <span class="identifier">proto</span><span class="special">::</span><span class="identifier">expr</span><span class="special">&lt;</span>
                <span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">terminal</span>
              <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">term</span><span class="special">&lt;</span> <span class="identifier">placeholder</span><span class="special">&lt;</span><span class="number">0</span><span class="special">&gt;</span> <span class="special">&gt;</span>
              <span class="special">,</span> <span class="number">0</span>
            <span class="special">&gt;</span> <span class="keyword">const</span> <span class="special">&amp;</span>
          <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">expr</span><span class="special">&lt;</span>
                <span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">terminal</span>
              <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">term</span><span class="special">&lt;</span> <span class="keyword">int</span> <span class="keyword">const</span> <span class="special">&amp;</span> <span class="special">&gt;</span>
              <span class="special">,</span> <span class="number">0</span>
            <span class="special">&gt;</span>
        <span class="special">&gt;</span>
      <span class="special">,</span> <span class="number">2</span>
    <span class="special">&gt;</span>
<span class="identifier">placeholder_plus_int_type</span><span class="special">;</span>

<span class="identifier">placeholder_plus_int_type</span> <span class="identifier">y</span> <span class="special">=</span> <span class="identifier">_1</span> <span class="special">+</span> <span class="number">42</span><span class="special">;</span>
</pre>
<p>
        There are a few things to note about these types:
      </p>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
<li class="listitem">
            Terminals have arity zero, unary expressions have arity one and binary
            expressions have arity two.
          </li>
<li class="listitem">
            When one Proto expression is made a child node of another Proto expression,
            it is held by reference, <span class="emphasis"><em>even if it is a temporary object</em></span>.
            This last point becomes important later.
          </li>
<li class="listitem">
            Non-Proto expressions, such as the integer literal, are turned into Proto
            expressions by wrapping them in new <code class="computeroutput"><span class="identifier">expr</span><span class="special">&lt;&gt;</span></code> terminal objects. These new wrappers
            are not themselves held by reference, but the object wrapped <span class="emphasis"><em>is</em></span>.
            Notice that the type of the Protofied <code class="computeroutput"><span class="number">42</span></code>
            literal is <code class="computeroutput"><span class="keyword">int</span> <span class="keyword">const</span>
            <span class="special">&amp;</span></code> -- held by reference.
          </li>
</ul></div>
<p>
        The types make it clear: everything in a Proto expression tree is held by
        reference. That means that building an expression tree is exceptionally cheap.
        It involves no copying at all.
      </p>
<div class="note"><table border="0" summary="Note">
<tr>
<td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../doc/src/images/note.png"></td>
<th align="left">Note</th>
</tr>
<tr><td align="left" valign="top"><p>
          An astute reader will notice that the object <code class="computeroutput"><span class="identifier">y</span></code>
          defined above will be left holding a dangling reference to a temporary
          int. In the sorts of high-performance applications Proto addresses, it
          is typical to build and evaluate an expression tree before any temporary
          objects go out of scope, so this dangling reference situation often doesn't
          arise, but it is certainly something to be aware of. Proto provides utilities
          for deep-copying expression trees so they can be passed around as value
          types without concern for dangling references.
        </p></td></tr>
</table></div>
<div class="section">
<div class="titlepage"><div><div><h4 class="title">
<a name="boost_proto.users_guide.intermediate_form.left_right_child"></a><a class="link" href="users_guide.html#boost_proto.users_guide.intermediate_form.left_right_child" title="Accessing Parts of an Expression">Accessing
        Parts of an Expression</a>
</h4></div></div></div>
<p>
          After assembling an expression into a tree, you'll naturally want to be
          able to do the reverse, and access a node's children. You may even want
          to be able to iterate over the children with algorithms from the Boost.Fusion
          library. This section shows how.
        </p>
<h6>
<a name="boost_proto.users_guide.intermediate_form.left_right_child.h0"></a>
          <span class="phrase"><a name="boost_proto.users_guide.intermediate_form.left_right_child.getting_expression_tags_and_arities"></a></span><a class="link" href="users_guide.html#boost_proto.users_guide.intermediate_form.left_right_child.getting_expression_tags_and_arities">Getting
          Expression Tags and Arities</a>
        </h6>
<p>
          Every node in an expression tree has both a <span class="emphasis"><em>tag</em></span> type
          that describes the node, and an <span class="emphasis"><em>arity</em></span> corresponding
          to the number of child nodes it has. You can use the <code class="computeroutput"><a class="link" href="../boost/proto/tag_of.html" title="Struct template tag_of">proto::tag_of&lt;&gt;</a></code>
          and <code class="computeroutput"><a class="link" href="../boost/proto/arity_of.html" title="Struct template arity_of">proto::arity_of&lt;&gt;</a></code> metafunctions to fetch
          them. Consider the following:
        </p>
<pre class="programlisting"><span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">Expr</span><span class="special">&gt;</span>
<span class="keyword">void</span> <span class="identifier">check_plus_node</span><span class="special">(</span><span class="identifier">Expr</span> <span class="keyword">const</span> <span class="special">&amp;)</span>
<span class="special">{</span>
    <span class="comment">// Assert that the tag type is proto::tag::plus</span>
    <span class="identifier">BOOST_STATIC_ASSERT</span><span class="special">((</span>
        <span class="identifier">boost</span><span class="special">::</span><span class="identifier">is_same</span><span class="special">&lt;</span>
            <span class="keyword">typename</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag_of</span><span class="special">&lt;</span><span class="identifier">Expr</span><span class="special">&gt;::</span><span class="identifier">type</span>
          <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">plus</span>
        <span class="special">&gt;::</span><span class="identifier">value</span>
    <span class="special">));</span>

    <span class="comment">// Assert that the arity is 2</span>
    <span class="identifier">BOOST_STATIC_ASSERT</span><span class="special">(</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">arity_of</span><span class="special">&lt;</span><span class="identifier">Expr</span><span class="special">&gt;::</span><span class="identifier">value</span> <span class="special">==</span> <span class="number">2</span> <span class="special">);</span>
<span class="special">}</span>

<span class="comment">// Create a binary plus node and use check_plus_node()</span>
<span class="comment">// to verify its tag type and arity:</span>
<span class="identifier">check_plus_node</span><span class="special">(</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">lit</span><span class="special">(</span><span class="number">1</span><span class="special">)</span> <span class="special">+</span> <span class="number">2</span> <span class="special">);</span>
</pre>
<p>
          For a given type <code class="computeroutput"><span class="identifier">Expr</span></code>,
          you could access the tag and arity directly as <code class="computeroutput"><span class="identifier">Expr</span><span class="special">::</span><span class="identifier">proto_tag</span></code>
          and <code class="computeroutput"><span class="identifier">Expr</span><span class="special">::</span><span class="identifier">proto_arity</span></code>, where <code class="computeroutput"><span class="identifier">Expr</span><span class="special">::</span><span class="identifier">proto_arity</span></code>
          is an MPL Integral Constant.
        </p>
<h6>
<a name="boost_proto.users_guide.intermediate_form.left_right_child.h1"></a>
          <span class="phrase"><a name="boost_proto.users_guide.intermediate_form.left_right_child.getting_terminal_values"></a></span><a class="link" href="users_guide.html#boost_proto.users_guide.intermediate_form.left_right_child.getting_terminal_values">Getting
          Terminal Values</a>
        </h6>
<p>
          There is no simpler expression than a terminal, and no more basic operation
          than extracting its value. As we've already seen, that is what <code class="computeroutput"><a class="link" href="../boost/proto/value_idp731209856.html" title="Function value">proto::value()</a></code> is for.
        </p>
<pre class="programlisting"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">ostream</span> <span class="special">&amp;</span> <span class="special">&gt;::</span><span class="identifier">type</span> <span class="identifier">cout_</span> <span class="special">=</span> <span class="special">{</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span><span class="special">};</span>

<span class="comment">// Get the value of the cout_ terminal:</span>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">ostream</span> <span class="special">&amp;</span> <span class="identifier">sout</span> <span class="special">=</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">value</span><span class="special">(</span> <span class="identifier">cout_</span> <span class="special">);</span>

<span class="comment">// Assert that we got back what we put in:</span>
<span class="identifier">assert</span><span class="special">(</span> <span class="special">&amp;</span><span class="identifier">sout</span> <span class="special">==</span> <span class="special">&amp;</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">);</span>
</pre>
<p>
          To compute the return type of the <code class="computeroutput"><a class="link" href="../boost/proto/value_idp731209856.html" title="Function value">proto::value()</a></code>
          function, you can use <code class="computeroutput"><a class="link" href="../boost/proto/result_of/value.html" title="Struct template value">proto::result_of::value&lt;&gt;</a></code>.
          When the parameter to <code class="computeroutput"><a class="link" href="../boost/proto/result_of/value.html" title="Struct template value">proto::result_of::value&lt;&gt;</a></code>
          is a non-reference type, the result type of the metafunction is the type
          of the value as suitable for storage by value; that is, top-level reference
          and qualifiers are stripped from it. But when instantiated with a reference
          type, the result type has a reference <span class="emphasis"><em>added</em></span> to it,
          yielding a type suitable for storage by reference. If you want to know
          the actual type of the terminal's value including whether it is stored
          by value or reference, you can use <code class="computeroutput"><span class="identifier">fusion</span><span class="special">::</span><span class="identifier">result_of</span><span class="special">::</span><span class="identifier">value_at</span><span class="special">&lt;</span><span class="identifier">Expr</span><span class="special">,</span> <span class="number">0</span><span class="special">&gt;::</span><span class="identifier">type</span></code>.
        </p>
<p>
          The following table summarizes the above paragraph.
        </p>
<div class="table">
<a name="boost_proto.users_guide.intermediate_form.left_right_child.t0"></a><p class="title"><b>Table&#160;32.4.&#160;Accessing Value Types</b></p>
<div class="table-contents"><table class="table" summary="Accessing Value Types">
<colgroup>
<col>
<col>
<col>
</colgroup>
<thead><tr>
<th>
                  <p>
                    Metafunction Invocation
                  </p>
                </th>
<th>
                  <p>
                    When the Value Type Is ...
                  </p>
                </th>
<th>
                  <p>
                    The Result Is ...
                  </p>
                </th>
</tr></thead>
<tbody>
<tr>
<td>
                  <p>
                    <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">result_of</span><span class="special">::</span><span class="identifier">value</span><span class="special">&lt;</span><span class="identifier">Expr</span><span class="special">&gt;::</span><span class="identifier">type</span></code>
                  </p>
                </td>
<td>
                  <p>
                    <code class="computeroutput"><span class="identifier">T</span></code>
                  </p>
                </td>
<td>
                  <p>
</p>
<pre class="programlisting"><span class="keyword">typename</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">remove_const</span><span class="special">&lt;</span>
    <span class="keyword">typename</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">remove_reference</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;::</span><span class="identifier">type</span>
<span class="special">&gt;::</span><span class="identifier">type</span> <a href="#ftn.boost_proto.users_guide.intermediate_form.left_right_child.f0" class="footnote" name="boost_proto.users_guide.intermediate_form.left_right_child.f0"><sup class="footnote">[a]</sup></a></pre>
<p>
                  </p>
                </td>
</tr>
<tr>
<td>
                  <p>
                    <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">result_of</span><span class="special">::</span><span class="identifier">value</span><span class="special">&lt;</span><span class="identifier">Expr</span> <span class="special">&amp;&gt;::</span><span class="identifier">type</span></code>
                  </p>
                </td>
<td>
                  <p>
                    <code class="computeroutput"><span class="identifier">T</span></code>
                  </p>
                </td>
<td>
                  <p>
</p>
<pre class="programlisting"><span class="keyword">typename</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">add_reference</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;::</span><span class="identifier">type</span></pre>
<p>
                  </p>
                </td>
</tr>
<tr>
<td>
                  <p>
                    <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">result_of</span><span class="special">::</span><span class="identifier">value</span><span class="special">&lt;</span><span class="identifier">Expr</span> <span class="keyword">const</span>
                    <span class="special">&amp;&gt;::</span><span class="identifier">type</span></code>
                  </p>
                </td>
<td>
                  <p>
                    <code class="computeroutput"><span class="identifier">T</span></code>
                  </p>
                </td>
<td>
                  <p>
</p>
<pre class="programlisting"><span class="keyword">typename</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">add_reference</span><span class="special">&lt;</span>
    <span class="keyword">typename</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">add_const</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;::</span><span class="identifier">type</span>
<span class="special">&gt;::</span><span class="identifier">type</span></pre>
<p>
                  </p>
                </td>
</tr>
<tr>
<td>
                  <p>
                    <code class="computeroutput"><span class="identifier">fusion</span><span class="special">::</span><span class="identifier">result_of</span><span class="special">::</span><span class="identifier">value_at</span><span class="special">&lt;</span><span class="identifier">Expr</span><span class="special">,</span>
                    <span class="number">0</span><span class="special">&gt;::</span><span class="identifier">type</span></code>
                  </p>
                </td>
<td>
                  <p>
                    <code class="computeroutput"><span class="identifier">T</span></code>
                  </p>
                </td>
<td>
                  <p>
                    <code class="computeroutput"><span class="identifier">T</span></code>
                  </p>
                </td>
</tr>
</tbody>
<tbody class="footnotes"><tr><td colspan="3"><div id="ftn.boost_proto.users_guide.intermediate_form.left_right_child.f0" class="footnote"><p><a href="#boost_proto.users_guide.intermediate_form.left_right_child.f0" class="para"><sup class="para">[a] </sup></a>If <code class="computeroutput"><span class="identifier">T</span></code> is a reference-to-function type, then the result type is simply <code class="computeroutput"><span class="identifier">T</span></code>.</p></div></td></tr></tbody>
</table></div>
</div>
<br class="table-break"><h6>
<a name="boost_proto.users_guide.intermediate_form.left_right_child.h2"></a>
          <span class="phrase"><a name="boost_proto.users_guide.intermediate_form.left_right_child.getting_child_expressions"></a></span><a class="link" href="users_guide.html#boost_proto.users_guide.intermediate_form.left_right_child.getting_child_expressions">Getting
          Child Expressions</a>
        </h6>
<p>
          Each non-terminal node in an expression tree corresponds to an operator
          in an expression, and the children correspond to the operands, or arguments
          of the operator. To access them, you can use the <code class="computeroutput"><a class="link" href="../boost/proto/child_c_idp731190800.html" title="Function child_c">proto::child_c()</a></code>
          function template, as demonstrated below:
        </p>
<pre class="programlisting"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">&gt;::</span><span class="identifier">type</span> <span class="identifier">i</span> <span class="special">=</span> <span class="special">{</span><span class="number">42</span><span class="special">};</span>

<span class="comment">// Get the 0-th operand of an addition operation:</span>
<span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">&gt;::</span><span class="identifier">type</span> <span class="special">&amp;</span><span class="identifier">ri</span> <span class="special">=</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">child_c</span><span class="special">&lt;</span><span class="number">0</span><span class="special">&gt;(</span> <span class="identifier">i</span> <span class="special">+</span> <span class="number">2</span> <span class="special">);</span>

<span class="comment">// Assert that we got back what we put in:</span>
<span class="identifier">assert</span><span class="special">(</span> <span class="special">&amp;</span><span class="identifier">i</span> <span class="special">==</span> <span class="special">&amp;</span><span class="identifier">ri</span> <span class="special">);</span>
</pre>
<p>
          You can use the <code class="computeroutput"><a class="link" href="../boost/proto/result_of/child_c.html" title="Struct template child_c">proto::result_of::child_c&lt;&gt;</a></code>
          metafunction to get the type of the Nth child of an expression node. Usually
          you don't care to know whether a child is stored by value or by reference,
          so when you ask for the type of the Nth child of an expression <code class="computeroutput"><span class="identifier">Expr</span></code> (where <code class="computeroutput"><span class="identifier">Expr</span></code>
          is not a reference type), you get the child's type after references and
          cv-qualifiers have been stripped from it.
        </p>
<pre class="programlisting"><span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">Expr</span><span class="special">&gt;</span>
<span class="keyword">void</span> <span class="identifier">test_result_of_child_c</span><span class="special">(</span><span class="identifier">Expr</span> <span class="keyword">const</span> <span class="special">&amp;</span><span class="identifier">expr</span><span class="special">)</span>
<span class="special">{</span>
    <span class="keyword">typedef</span> <span class="keyword">typename</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">result_of</span><span class="special">::</span><span class="identifier">child_c</span><span class="special">&lt;</span><span class="identifier">Expr</span><span class="special">,</span> <span class="number">0</span><span class="special">&gt;::</span><span class="identifier">type</span> <span class="identifier">type</span><span class="special">;</span>

    <span class="comment">// Since Expr is not a reference type,</span>
    <span class="comment">// result_of::child_c&lt;Expr, 0&gt;::type is a</span>
    <span class="comment">// non-cv qualified, non-reference type:</span>
    <span class="identifier">BOOST_MPL_ASSERT</span><span class="special">((</span>
        <span class="identifier">boost</span><span class="special">::</span><span class="identifier">is_same</span><span class="special">&lt;</span> <span class="identifier">type</span><span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">&gt;::</span><span class="identifier">type</span> <span class="special">&gt;</span>
    <span class="special">));</span>
<span class="special">}</span>

<span class="comment">// ...</span>
<span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">&gt;::</span><span class="identifier">type</span> <span class="identifier">i</span> <span class="special">=</span> <span class="special">{</span><span class="number">42</span><span class="special">};</span>
<span class="identifier">test_result_of_child_c</span><span class="special">(</span> <span class="identifier">i</span> <span class="special">+</span> <span class="number">2</span> <span class="special">);</span>
</pre>
<p>
          However, if you ask for the type of the Nth child of <code class="computeroutput"><span class="identifier">Expr</span>
          <span class="special">&amp;</span></code> or <code class="computeroutput"><span class="identifier">Expr</span>
          <span class="keyword">const</span> <span class="special">&amp;</span></code>
          (note the reference), the result type will be a reference, regardless of
          whether the child is actually stored by reference or not. If you need to
          know exactly how the child is stored in the node, whether by reference
          or by value, you can use <code class="computeroutput"><span class="identifier">fusion</span><span class="special">::</span><span class="identifier">result_of</span><span class="special">::</span><span class="identifier">value_at</span><span class="special">&lt;</span><span class="identifier">Expr</span><span class="special">,</span> <span class="identifier">N</span><span class="special">&gt;::</span><span class="identifier">type</span></code>. The following table summarizes
          the behavior of the <code class="computeroutput"><a class="link" href="../boost/proto/result_of/child_c.html" title="Struct template child_c">proto::result_of::child_c&lt;&gt;</a></code>
          metafunction.
        </p>
<div class="table">
<a name="boost_proto.users_guide.intermediate_form.left_right_child.t1"></a><p class="title"><b>Table&#160;32.5.&#160;Accessing Child Types</b></p>
<div class="table-contents"><table class="table" summary="Accessing Child Types">
<colgroup>
<col>
<col>
<col>
</colgroup>
<thead><tr>
<th>
                  <p>
                    Metafunction Invocation
                  </p>
                </th>
<th>
                  <p>
                    When the Child Is ...
                  </p>
                </th>
<th>
                  <p>
                    The Result Is ...
                  </p>
                </th>
</tr></thead>
<tbody>
<tr>
<td>
                  <p>
                    <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">result_of</span><span class="special">::</span><span class="identifier">child_c</span><span class="special">&lt;</span><span class="identifier">Expr</span><span class="special">,</span>
                    <span class="identifier">N</span><span class="special">&gt;::</span><span class="identifier">type</span></code>
                  </p>
                </td>
<td>
                  <p>
                    <code class="computeroutput"><span class="identifier">T</span></code>
                  </p>
                </td>
<td>
                  <p>
</p>
<pre class="programlisting"><span class="keyword">typename</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">remove_const</span><span class="special">&lt;</span>
    <span class="keyword">typename</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">remove_reference</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;::</span><span class="identifier">type</span>
<span class="special">&gt;::</span><span class="identifier">type</span></pre>
<p>
                  </p>
                </td>
</tr>
<tr>
<td>
                  <p>
                    <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">result_of</span><span class="special">::</span><span class="identifier">child_c</span><span class="special">&lt;</span><span class="identifier">Expr</span> <span class="special">&amp;,</span>
                    <span class="identifier">N</span><span class="special">&gt;::</span><span class="identifier">type</span></code>
                  </p>
                </td>
<td>
                  <p>
                    <code class="computeroutput"><span class="identifier">T</span></code>
                  </p>
                </td>
<td>
                  <p>
</p>
<pre class="programlisting"><span class="keyword">typename</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">add_reference</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;::</span><span class="identifier">type</span></pre>
<p>
                  </p>
                </td>
</tr>
<tr>
<td>
                  <p>
                    <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">result_of</span><span class="special">::</span><span class="identifier">child_c</span><span class="special">&lt;</span><span class="identifier">Expr</span> <span class="keyword">const</span>
                    <span class="special">&amp;,</span> <span class="identifier">N</span><span class="special">&gt;::</span><span class="identifier">type</span></code>
                  </p>
                </td>
<td>
                  <p>
                    <code class="computeroutput"><span class="identifier">T</span></code>
                  </p>
                </td>
<td>
                  <p>
</p>
<pre class="programlisting"><span class="keyword">typename</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">add_reference</span><span class="special">&lt;</span>
    <span class="keyword">typename</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">add_const</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;::</span><span class="identifier">type</span>
<span class="special">&gt;::</span><span class="identifier">type</span></pre>
<p>
                  </p>
                </td>
</tr>
<tr>
<td>
                  <p>
                    <code class="computeroutput"><span class="identifier">fusion</span><span class="special">::</span><span class="identifier">result_of</span><span class="special">::</span><span class="identifier">value_at</span><span class="special">&lt;</span><span class="identifier">Expr</span><span class="special">,</span>
                    <span class="identifier">N</span><span class="special">&gt;::</span><span class="identifier">type</span></code>
                  </p>
                </td>
<td>
                  <p>
                    <code class="computeroutput"><span class="identifier">T</span></code>
                  </p>
                </td>
<td>
                  <p>
                    <code class="computeroutput"><span class="identifier">T</span></code>
                  </p>
                </td>
</tr>
</tbody>
</table></div>
</div>
<br class="table-break"><h6>
<a name="boost_proto.users_guide.intermediate_form.left_right_child.h3"></a>
          <span class="phrase"><a name="boost_proto.users_guide.intermediate_form.left_right_child.common_shortcuts"></a></span><a class="link" href="users_guide.html#boost_proto.users_guide.intermediate_form.left_right_child.common_shortcuts">Common
          Shortcuts</a>
        </h6>
<p>
          Most operators in C++ are unary or binary, so accessing the only operand,
          or the left and right operands, are very common operations. For this reason,
          Proto provides the <code class="computeroutput"><a class="link" href="../boost/proto/child_idp731162304.html" title="Function child">proto::child()</a></code>,
          <code class="computeroutput"><a class="link" href="../boost/proto/left_idp731222320.html" title="Function left">proto::left()</a></code>, and <code class="computeroutput"><a class="link" href="../boost/proto/right_idp731236800.html" title="Function right">proto::right()</a></code>
          functions. <code class="computeroutput"><a class="link" href="../boost/proto/child_idp731162304.html" title="Function child">proto::child()</a></code> and <code class="computeroutput"><a class="link" href="../boost/proto/left_idp731222320.html" title="Function left">proto::left()</a></code>
          are synonymous with <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">child_c</span><span class="special">&lt;</span><span class="number">0</span><span class="special">&gt;()</span></code>,
          and <code class="computeroutput"><a class="link" href="../boost/proto/right_idp731236800.html" title="Function right">proto::right()</a></code> is synonymous with <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">child_c</span><span class="special">&lt;</span><span class="number">1</span><span class="special">&gt;()</span></code>.
        </p>
<p>
          There are also <code class="computeroutput"><a class="link" href="../boost/proto/result_of/child.html" title="Struct template child">proto::result_of::child&lt;&gt;</a></code>,
          <code class="computeroutput"><a class="link" href="../boost/proto/result_of/left.html" title="Struct template left">proto::result_of::left&lt;&gt;</a></code>, and <code class="computeroutput"><a class="link" href="../boost/proto/result_of/right.html" title="Struct template right">proto::result_of::right&lt;&gt;</a></code>
          metafunctions that merely forward to their <code class="computeroutput"><a class="link" href="../boost/proto/result_of/child_c.html" title="Struct template child_c">proto::result_of::child_c&lt;&gt;</a></code>
          counterparts.
        </p>
</div>
<div class="section">
<div class="titlepage"><div><div><h4 class="title">
<a name="boost_proto.users_guide.intermediate_form.deep_copying_expressions"></a><a class="link" href="users_guide.html#boost_proto.users_guide.intermediate_form.deep_copying_expressions" title="Deep-copying Expressions">Deep-copying
        Expressions</a>
</h4></div></div></div>
<p>
          When you build an expression template with Proto, all the intermediate
          child nodes are held <span class="emphasis"><em>by reference</em></span>. The avoids needless
          copies, which is crucial if you want your EDSL to perform well at runtime.
          Naturally, there is a danger if the temporary objects go out of scope before
          you try to evaluate your expression template. This is especially a problem
          in C++0x with the new <code class="computeroutput"><span class="keyword">decltype</span></code>
          and <code class="computeroutput"><span class="keyword">auto</span></code> keywords. Consider:
        </p>
<pre class="programlisting"><span class="comment">// OOPS: "ex" is left holding dangling references</span>
<span class="keyword">auto</span> <span class="identifier">ex</span> <span class="special">=</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">lit</span><span class="special">(</span><span class="number">1</span><span class="special">)</span> <span class="special">+</span> <span class="number">2</span><span class="special">;</span>
</pre>
<p>
          The problem can happen in today's C++ also if you use <code class="computeroutput"><span class="identifier">BOOST_TYPEOF</span><span class="special">()</span></code> or <code class="computeroutput"><span class="identifier">BOOST_AUTO</span><span class="special">()</span></code>, or if you try to pass an expression
          template outside the scope of its constituents.
        </p>
<p>
          In these cases, you want to deep-copy your expression template so that
          all intermediate nodes and the terminals are held <span class="emphasis"><em>by value</em></span>.
          That way, you can safely assign the expression template to a local variable
          or return it from a function without worrying about dangling references.
          You can do this with <code class="computeroutput"><a class="link" href="../boost/proto/deep_copy_idp727047232.html" title="Function template deep_copy">proto::deep_copy()</a></code>
          as fo llows:
        </p>
<pre class="programlisting"><span class="comment">// OK, "ex" has no dangling references</span>
<span class="keyword">auto</span> <span class="identifier">ex</span> <span class="special">=</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">deep_copy</span><span class="special">(</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">lit</span><span class="special">(</span><span class="number">1</span><span class="special">)</span> <span class="special">+</span> <span class="number">2</span> <span class="special">);</span>
</pre>
<p>
          If you are using <a href="../../../libs/typeof/index.html" target="_top">Boost.Typeof</a>,
          it would look like this:
        </p>
<pre class="programlisting"><span class="comment">// OK, use BOOST_AUTO() and proto::deep_copy() to</span>
<span class="comment">// store an expression template in a local variable </span>
<span class="identifier">BOOST_AUTO</span><span class="special">(</span> <span class="identifier">ex</span><span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">deep_copy</span><span class="special">(</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">lit</span><span class="special">(</span><span class="number">1</span><span class="special">)</span> <span class="special">+</span> <span class="number">2</span> <span class="special">)</span> <span class="special">);</span>
</pre>
<p>
          For the above code to work, you must include the <code class="computeroutput"><a class="link" href="reference.html#header.boost.proto.proto_typeof_hpp" title="Header &lt;boost/proto/proto_typeof.hpp&gt;">boost/proto/proto_typeof.hpp</a></code>
          header, which also defines the <code class="literal"><code class="computeroutput"><a class="link" href="../BOOST_PROTO_AUTO.html" title="Macro BOOST_PROTO_AUTO">BOOST_PROTO_AUTO</a></code>()</code>
          macro which automatically deep-copies its argument. With <code class="literal"><code class="computeroutput"><a class="link" href="../BOOST_PROTO_AUTO.html" title="Macro BOOST_PROTO_AUTO">BOOST_PROTO_AUTO</a></code>()</code>, the above
          code can be writen as:
        </p>
<pre class="programlisting"><span class="comment">// OK, BOOST_PROTO_AUTO() automatically deep-copies</span>
<span class="comment">// its argument: </span>
<span class="identifier">BOOST_PROTO_AUTO</span><span class="special">(</span> <span class="identifier">ex</span><span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">lit</span><span class="special">(</span><span class="number">1</span><span class="special">)</span> <span class="special">+</span> <span class="number">2</span> <span class="special">);</span>
</pre>
<p>
          When deep-copying an expression tree, all intermediate nodes and all terminals
          are stored by value. The only exception is terminals that are function
          references, which are left alone.
        </p>
<div class="note"><table border="0" summary="Note">
<tr>
<td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../doc/src/images/note.png"></td>
<th align="left">Note</th>
</tr>
<tr><td align="left" valign="top"><p>
            <code class="computeroutput"><a class="link" href="../boost/proto/deep_copy_idp727047232.html" title="Function template deep_copy">proto::deep_copy()</a></code> makes no exception for
            arrays, which it stores by value. That can potentially cause a large
            amount of data to be copied.
          </p></td></tr>
</table></div>
</div>
<div class="section">
<div class="titlepage"><div><div><h4 class="title">
<a name="boost_proto.users_guide.intermediate_form.debugging_expressions"></a><a class="link" href="users_guide.html#boost_proto.users_guide.intermediate_form.debugging_expressions" title="Debugging Expressions">Debugging
        Expressions</a>
</h4></div></div></div>
<p>
          Proto provides a utility for pretty-printing expression trees that comes
          in very handy when you're trying to debug your EDSL. It's called <code class="computeroutput"><a class="link" href="../boost/proto/display_expr_idp726944752.html" title="Function display_expr">proto::display_expr()</a></code>, and you pass it the expression
          to print and optionally, an <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">ostream</span></code>
          to which to send the output. Consider:
        </p>
<pre class="programlisting"><span class="comment">// Use display_expr() to pretty-print an expression tree</span>
<span class="identifier">proto</span><span class="special">::</span><span class="identifier">display_expr</span><span class="special">(</span>
    <span class="identifier">proto</span><span class="special">::</span><span class="identifier">lit</span><span class="special">(</span><span class="string">"hello"</span><span class="special">)</span> <span class="special">+</span> <span class="number">42</span>
<span class="special">);</span>
</pre>
<p>
          The above code writes this to <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span></code>:
        </p>
<pre class="programlisting">plus(
    terminal(hello)
  , terminal(42)
)</pre>
<p>
          In order to call <code class="computeroutput"><a class="link" href="../boost/proto/display_expr_idp726944752.html" title="Function display_expr">proto::display_expr()</a></code>,
          all the terminals in the expression must be Streamable (that is, they can
          be written to a <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">ostream</span></code>). In addition, the tag types
          must all be Streamable as well. Here is an example that includes a custom
          terminal type and a custom tag:
        </p>
<pre class="programlisting"><span class="comment">// A custom tag type that is Streamable</span>
<span class="keyword">struct</span> <span class="identifier">MyTag</span>
<span class="special">{</span>
    <span class="keyword">friend</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">ostream</span> <span class="special">&amp;</span><span class="keyword">operator</span><span class="special">&lt;&lt;(</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">ostream</span> <span class="special">&amp;</span><span class="identifier">s</span><span class="special">,</span> <span class="identifier">MyTag</span><span class="special">)</span>
    <span class="special">{</span>
        <span class="keyword">return</span> <span class="identifier">s</span> <span class="special">&lt;&lt;</span> <span class="string">"MyTag"</span><span class="special">;</span>
    <span class="special">}</span>
<span class="special">};</span>

<span class="comment">// Some other Streamable type</span>
<span class="keyword">struct</span> <span class="identifier">MyTerminal</span>
<span class="special">{</span>
    <span class="keyword">friend</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">ostream</span> <span class="special">&amp;</span><span class="keyword">operator</span><span class="special">&lt;&lt;(</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">ostream</span> <span class="special">&amp;</span><span class="identifier">s</span><span class="special">,</span> <span class="identifier">MyTerminal</span><span class="special">)</span>
    <span class="special">{</span>
        <span class="keyword">return</span> <span class="identifier">s</span> <span class="special">&lt;&lt;</span> <span class="string">"MyTerminal"</span><span class="special">;</span>
    <span class="special">}</span>
<span class="special">};</span>

<span class="keyword">int</span> <span class="identifier">main</span><span class="special">()</span>
<span class="special">{</span>
    <span class="comment">// Display an expression tree that contains a custom</span>
    <span class="comment">// tag and a user-defined type in a terminal</span>
    <span class="identifier">proto</span><span class="special">::</span><span class="identifier">display_expr</span><span class="special">(</span>
        <span class="identifier">proto</span><span class="special">::</span><span class="identifier">make_expr</span><span class="special">&lt;</span><span class="identifier">MyTag</span><span class="special">&gt;(</span><span class="identifier">MyTerminal</span><span class="special">())</span> <span class="special">+</span> <span class="number">42</span>
    <span class="special">);</span>
<span class="special">}</span>
</pre>
<p>
          The above code prints the following:
        </p>
<pre class="programlisting">plus(
    MyTag(
        terminal(MyTerminal)
    )
  , terminal(42)
)</pre>
</div>
<div class="section">
<div class="titlepage"><div><div><h4 class="title">
<a name="boost_proto.users_guide.intermediate_form.tags_and_metafunctions"></a><a class="link" href="users_guide.html#boost_proto.users_guide.intermediate_form.tags_and_metafunctions" title="Operator Tags and Metafunctions">Operator
        Tags and Metafunctions</a>
</h4></div></div></div>
<p>
          The following table lists the overloadable C++ operators, the Proto tag
          types for each, and the name of the metafunctions for generating the corresponding
          Proto expression types. And as we'll see later, the metafunctions are also
          usable as grammars for matching such nodes, as well as pass-through transforms.
        </p>
<div class="table">
<a name="boost_proto.users_guide.intermediate_form.tags_and_metafunctions.t0"></a><p class="title"><b>Table&#160;32.6.&#160;Operators, Tags and Metafunctions</b></p>
<div class="table-contents"><table class="table" summary="Operators, Tags and Metafunctions">
<colgroup>
<col>
<col>
<col>
</colgroup>
<thead><tr>
<th>
                  <p>
                    Operator
                  </p>
                </th>
<th>
                  <p>
                    Proto Tag
                  </p>
                </th>
<th>
                  <p>
                    Proto Metafunction
                  </p>
                </th>
</tr></thead>
<tbody>
<tr>
<td>
                  <p>
                    unary <code class="computeroutput"><span class="special">+</span></code>
                  </p>
                </td>
<td>
                  <p>
                    <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">unary_plus</span></code>
                  </p>
                </td>
<td>
                  <p>
                    <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">unary_plus</span><span class="special">&lt;&gt;</span></code>
                  </p>
                </td>
</tr>
<tr>
<td>
                  <p>
                    unary <code class="computeroutput"><span class="special">-</span></code>
                  </p>
                </td>
<td>
                  <p>
                    <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">negate</span></code>
                  </p>
                </td>
<td>
                  <p>
                    <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">negate</span><span class="special">&lt;&gt;</span></code>
                  </p>
                </td>
</tr>
<tr>
<td>
                  <p>
                    unary <code class="computeroutput"><span class="special">*</span></code>
                  </p>
                </td>
<td>
                  <p>
                    <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">dereference</span></code>
                  </p>
                </td>
<td>
                  <p>
                    <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">dereference</span><span class="special">&lt;&gt;</span></code>
                  </p>
                </td>
</tr>
<tr>
<td>
                  <p>
                    unary <code class="computeroutput"><span class="special">~</span></code>
                  </p>
                </td>
<td>
                  <p>
                    <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">complement</span></code>
                  </p>
                </td>
<td>
                  <p>
                    <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">complement</span><span class="special">&lt;&gt;</span></code>
                  </p>
                </td>
</tr>
<tr>
<td>
                  <p>
                    unary <code class="computeroutput"><span class="special">&amp;</span></code>
                  </p>
                </td>
<td>
                  <p>
                    <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">address_of</span></code>
                  </p>
                </td>
<td>
                  <p>
                    <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">address_of</span><span class="special">&lt;&gt;</span></code>
                  </p>
                </td>
</tr>
<tr>
<td>
                  <p>
                    unary <code class="computeroutput"><span class="special">!</span></code>
                  </p>
                </td>
<td>
                  <p>
                    <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">logical_not</span></code>
                  </p>
                </td>
<td>
                  <p>
                    <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">logical_not</span><span class="special">&lt;&gt;</span></code>
                  </p>
                </td>
</tr>
<tr>
<td>
                  <p>
                    unary prefix <code class="computeroutput"><span class="special">++</span></code>
                  </p>
                </td>
<td>
                  <p>
                    <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">pre_inc</span></code>
                  </p>
                </td>
<td>
                  <p>
                    <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">pre_inc</span><span class="special">&lt;&gt;</span></code>
                  </p>
                </td>
</tr>
<tr>
<td>
                  <p>
                    unary prefix <code class="computeroutput"><span class="special">--</span></code>
                  </p>
                </td>
<td>
                  <p>
                    <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">pre_dec</span></code>
                  </p>
                </td>
<td>
                  <p>
                    <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">pre_dec</span><span class="special">&lt;&gt;</span></code>
                  </p>
                </td>
</tr>
<tr>
<td>
                  <p>
                    unary postfix <code class="computeroutput"><span class="special">++</span></code>
                  </p>
                </td>
<td>
                  <p>
                    <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">post_inc</span></code>
                  </p>
                </td>
<td>
                  <p>
                    <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">post_inc</span><span class="special">&lt;&gt;</span></code>
                  </p>
                </td>
</tr>
<tr>
<td>
                  <p>
                    unary postfix <code class="computeroutput"><span class="special">--</span></code>
                  </p>
                </td>
<td>
                  <p>
                    <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">post_dec</span></code>
                  </p>
                </td>
<td>
                  <p>
                    <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">post_dec</span><span class="special">&lt;&gt;</span></code>
                  </p>
                </td>
</tr>
<tr>
<td>
                  <p>
                    binary <code class="computeroutput"><span class="special">&lt;&lt;</span></code>
                  </p>
                </td>
<td>
                  <p>
                    <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">shift_left</span></code>
                  </p>
                </td>
<td>
                  <p>
                    <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">shift_left</span><span class="special">&lt;&gt;</span></code>
                  </p>
                </td>
</tr>
<tr>
<td>
                  <p>
                    binary <code class="computeroutput"><span class="special">&gt;&gt;</span></code>
                  </p>
                </td>
<td>
                  <p>
                    <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">shift_right</span></code>
                  </p>
                </td>
<td>
                  <p>
                    <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">shift_right</span><span class="special">&lt;&gt;</span></code>
                  </p>
                </td>
</tr>
<tr>
<td>
                  <p>
                    binary <code class="computeroutput"><span class="special">*</span></code>
                  </p>
                </td>
<td>
                  <p>
                    <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">multiplies</span></code>
                  </p>
                </td>
<td>
                  <p>
                    <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">multiplies</span><span class="special">&lt;&gt;</span></code>
                  </p>
                </td>
</tr>
<tr>
<td>
                  <p>
                    binary <code class="computeroutput"><span class="special">/</span></code>
                  </p>
                </td>
<td>
                  <p>
                    <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">divides</span></code>
                  </p>
                </td>
<td>
                  <p>
                    <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">divides</span><span class="special">&lt;&gt;</span></code>
                  </p>
                </td>
</tr>
<tr>
<td>
                  <p>
                    binary <code class="computeroutput"><span class="special">%</span></code>
                  </p>
                </td>
<td>
                  <p>
                    <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">modulus</span></code>
                  </p>
                </td>
<td>
                  <p>
                    <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">modulus</span><span class="special">&lt;&gt;</span></code>
                  </p>
                </td>
</tr>
<tr>
<td>
                  <p>
                    binary <code class="computeroutput"><span class="special">+</span></code>
                  </p>
                </td>
<td>
                  <p>
                    <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">plus</span></code>
                  </p>
                </td>
<td>
                  <p>
                    <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">plus</span><span class="special">&lt;&gt;</span></code>
                  </p>
                </td>
</tr>
<tr>
<td>
                  <p>
                    binary <code class="computeroutput"><span class="special">-</span></code>
                  </p>
                </td>
<td>
                  <p>
                    <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">minus</span></code>
                  </p>
                </td>
<td>
                  <p>
                    <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">minus</span><span class="special">&lt;&gt;</span></code>
                  </p>
                </td>
</tr>
<tr>
<td>
                  <p>
                    binary <code class="computeroutput"><span class="special">&lt;</span></code>
                  </p>
                </td>
<td>
                  <p>
                    <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">less</span></code>
                  </p>
                </td>
<td>
                  <p>
                    <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">less</span><span class="special">&lt;&gt;</span></code>
                  </p>
                </td>
</tr>
<tr>
<td>
                  <p>
                    binary <code class="computeroutput"><span class="special">&gt;</span></code>
                  </p>
                </td>
<td>
                  <p>
                    <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">greater</span></code>
                  </p>
                </td>
<td>
                  <p>
                    <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">greater</span><span class="special">&lt;&gt;</span></code>
                  </p>
                </td>
</tr>
<tr>
<td>
                  <p>
                    binary <code class="computeroutput"><span class="special">&lt;=</span></code>
                  </p>
                </td>
<td>
                  <p>
                    <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">less_equal</span></code>
                  </p>
                </td>
<td>
                  <p>
                    <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">less_equal</span><span class="special">&lt;&gt;</span></code>
                  </p>
                </td>
</tr>
<tr>
<td>
                  <p>
                    binary <code class="computeroutput"><span class="special">&gt;=</span></code>
                  </p>
                </td>
<td>
                  <p>
                    <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">greater_equal</span></code>
                  </p>
                </td>
<td>
                  <p>
                    <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">greater_equal</span><span class="special">&lt;&gt;</span></code>
                  </p>
                </td>
</tr>
<tr>
<td>
                  <p>
                    binary <code class="computeroutput"><span class="special">==</span></code>
                  </p>
                </td>
<td>
                  <p>
                    <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">equal_to</span></code>
                  </p>
                </td>
<td>
                  <p>
                    <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">equal_to</span><span class="special">&lt;&gt;</span></code>
                  </p>
                </td>
</tr>
<tr>
<td>
                  <p>
                    binary <code class="computeroutput"><span class="special">!=</span></code>
                  </p>
                </td>
<td>
                  <p>
                    <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">not_equal_to</span></code>
                  </p>
                </td>
<td>
                  <p>
                    <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">not_equal_to</span><span class="special">&lt;&gt;</span></code>
                  </p>
                </td>
</tr>
<tr>
<td>
                  <p>
                    binary <code class="computeroutput"><span class="special">||</span></code>
                  </p>
                </td>
<td>
                  <p>
                    <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">logical_or</span></code>
                  </p>
                </td>
<td>
                  <p>
                    <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">logical_or</span><span class="special">&lt;&gt;</span></code>
                  </p>
                </td>
</tr>
<tr>
<td>
                  <p>
                    binary <code class="computeroutput"><span class="special">&amp;&amp;</span></code>
                  </p>
                </td>
<td>
                  <p>
                    <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">logical_and</span></code>
                  </p>
                </td>
<td>
                  <p>
                    <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">logical_and</span><span class="special">&lt;&gt;</span></code>
                  </p>
                </td>
</tr>
<tr>
<td>
                  <p>
                    binary <code class="computeroutput"><span class="special">&amp;</span></code>
                  </p>
                </td>
<td>
                  <p>
                    <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">bitwise_and</span></code>
                  </p>
                </td>
<td>
                  <p>
                    <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">bitwise_and</span><span class="special">&lt;&gt;</span></code>
                  </p>
                </td>
</tr>
<tr>
<td>
                  <p>
                    binary <code class="computeroutput"><span class="special">|</span></code>
                  </p>
                </td>
<td>
                  <p>
                    <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">bitwise_or</span></code>
                  </p>
                </td>
<td>
                  <p>
                    <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">bitwise_or</span><span class="special">&lt;&gt;</span></code>
                  </p>
                </td>
</tr>
<tr>
<td>
                  <p>
                    binary <code class="computeroutput"><span class="special">^</span></code>
                  </p>
                </td>
<td>
                  <p>
                    <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">bitwise_xor</span></code>
                  </p>
                </td>
<td>
                  <p>
                    <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">bitwise_xor</span><span class="special">&lt;&gt;</span></code>
                  </p>
                </td>
</tr>
<tr>
<td>
                  <p>
                    binary <code class="computeroutput"><span class="special">,</span></code>
                  </p>
                </td>
<td>
                  <p>
                    <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">comma</span></code>
                  </p>
                </td>
<td>
                  <p>
                    <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">comma</span><span class="special">&lt;&gt;</span></code>
                  </p>
                </td>
</tr>
<tr>
<td>
                  <p>
                    binary <code class="computeroutput"><span class="special">-&gt;*</span></code>
                  </p>
                </td>
<td>
                  <p>
                    <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">mem_ptr</span></code>
                  </p>
                </td>
<td>
                  <p>
                    <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">mem_ptr</span><span class="special">&lt;&gt;</span></code>
                  </p>
                </td>
</tr>
<tr>
<td>
                  <p>
                    binary <code class="computeroutput"><span class="special">=</span></code>
                  </p>
                </td>
<td>
                  <p>
                    <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">assign</span></code>
                  </p>
                </td>
<td>
                  <p>
                    <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">assign</span><span class="special">&lt;&gt;</span></code>
                  </p>
                </td>
</tr>
<tr>
<td>
                  <p>
                    binary <code class="computeroutput"><span class="special">&lt;&lt;=</span></code>
                  </p>
                </td>
<td>
                  <p>
                    <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">shift_left_assign</span></code>
                  </p>
                </td>
<td>
                  <p>
                    <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">shift_left_assign</span><span class="special">&lt;&gt;</span></code>
                  </p>
                </td>
</tr>
<tr>
<td>
                  <p>
                    binary <code class="computeroutput"><span class="special">&gt;&gt;=</span></code>
                  </p>
                </td>
<td>
                  <p>
                    <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">shift_right_assign</span></code>
                  </p>
                </td>
<td>
                  <p>
                    <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">shift_right_assign</span><span class="special">&lt;&gt;</span></code>
                  </p>
                </td>
</tr>
<tr>
<td>
                  <p>
                    binary <code class="computeroutput"><span class="special">*=</span></code>
                  </p>
                </td>
<td>
                  <p>
                    <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">multiplies_assign</span></code>
                  </p>
                </td>
<td>
                  <p>
                    <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">multiplies_assign</span><span class="special">&lt;&gt;</span></code>
                  </p>
                </td>
</tr>
<tr>
<td>
                  <p>
                    binary <code class="computeroutput"><span class="special">/=</span></code>
                  </p>
                </td>
<td>
                  <p>
                    <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">divides_assign</span></code>
                  </p>
                </td>
<td>
                  <p>
                    <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">divides_assign</span><span class="special">&lt;&gt;</span></code>
                  </p>
                </td>
</tr>
<tr>
<td>
                  <p>
                    binary <code class="computeroutput"><span class="special">%=</span></code>
                  </p>
                </td>
<td>
                  <p>
                    <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">modulus_assign</span></code>
                  </p>
                </td>
<td>
                  <p>
                    <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">modulus_assign</span><span class="special">&lt;&gt;</span></code>
                  </p>
                </td>
</tr>
<tr>
<td>
                  <p>
                    binary <code class="computeroutput"><span class="special">+=</span></code>
                  </p>
                </td>
<td>
                  <p>
                    <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">plus_assign</span></code>
                  </p>
                </td>
<td>
                  <p>
                    <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">plus_assign</span><span class="special">&lt;&gt;</span></code>
                  </p>
                </td>
</tr>
<tr>
<td>
                  <p>
                    binary <code class="computeroutput"><span class="special">-=</span></code>
                  </p>
                </td>
<td>
                  <p>
                    <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">minus_assign</span></code>
                  </p>
                </td>
<td>
                  <p>
                    <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">minus_assign</span><span class="special">&lt;&gt;</span></code>
                  </p>
                </td>
</tr>
<tr>
<td>
                  <p>
                    binary <code class="computeroutput"><span class="special">&amp;=</span></code>
                  </p>
                </td>
<td>
                  <p>
                    <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">bitwise_and_assign</span></code>
                  </p>
                </td>
<td>
                  <p>
                    <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">bitwise_and_assign</span><span class="special">&lt;&gt;</span></code>
                  </p>
                </td>
</tr>
<tr>
<td>
                  <p>
                    binary <code class="computeroutput"><span class="special">|=</span></code>
                  </p>
                </td>
<td>
                  <p>
                    <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">bitwise_or_assign</span></code>
                  </p>
                </td>
<td>
                  <p>
                    <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">bitwise_or_assign</span><span class="special">&lt;&gt;</span></code>
                  </p>
                </td>
</tr>
<tr>
<td>
                  <p>
                    binary <code class="computeroutput"><span class="special">^=</span></code>
                  </p>
                </td>
<td>
                  <p>
                    <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">bitwise_xor_assign</span></code>
                  </p>
                </td>
<td>
                  <p>
                    <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">bitwise_xor_assign</span><span class="special">&lt;&gt;</span></code>
                  </p>
                </td>
</tr>
<tr>
<td>
                  <p>
                    binary subscript
                  </p>
                </td>
<td>
                  <p>
                    <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">subscript</span></code>
                  </p>
                </td>
<td>
                  <p>
                    <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">subscript</span><span class="special">&lt;&gt;</span></code>
                  </p>
                </td>
</tr>
<tr>
<td>
                  <p>
                    ternary <code class="computeroutput"><span class="special">?:</span></code>
                  </p>
                </td>
<td>
                  <p>
                    <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">if_else_</span></code>
                  </p>
                </td>
<td>
                  <p>
                    <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">if_else_</span><span class="special">&lt;&gt;</span></code>
                  </p>
                </td>
</tr>
<tr>
<td>
                  <p>
                    n-ary function call
                  </p>
                </td>
<td>
                  <p>
                    <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">function</span></code>
                  </p>
                </td>
<td>
                  <p>
                    <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">function</span><span class="special">&lt;&gt;</span></code>
                  </p>
                </td>
</tr>
</tbody>
</table></div>
</div>
<br class="table-break">
</div>
<div class="section">
<div class="titlepage"><div><div><h4 class="title">
<a name="boost_proto.users_guide.intermediate_form.expressions_as_fusion_sequences"></a><a class="link" href="users_guide.html#boost_proto.users_guide.intermediate_form.expressions_as_fusion_sequences" title="Expressions as Fusion Sequences">Expressions
        as Fusion Sequences</a>
</h4></div></div></div>
<p>
          Boost.Fusion is a library of iterators, algorithms, containers and adaptors
          for manipulating heterogeneous sequences. In essence, a Proto expression
          is just a heterogeneous sequence of its child expressions, and so Proto
          expressions are valid Fusion random-access sequences. That means you can
          apply Fusion algorithms to them, transform them, apply Fusion filters and
          views to them, and access their elements using <code class="computeroutput"><span class="identifier">fusion</span><span class="special">::</span><span class="identifier">at</span><span class="special">()</span></code>. The things Fusion can do to heterogeneous
          sequences are beyond the scope of this users' guide, but below is a simple
          example. It takes a lazy function invocation like <code class="computeroutput"><span class="identifier">fun</span><span class="special">(</span><span class="number">1</span><span class="special">,</span><span class="number">2</span><span class="special">,</span><span class="number">3</span><span class="special">,</span><span class="number">4</span><span class="special">)</span></code>
          and uses Fusion to print the function arguments in order.
        </p>
<pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">display</span>
<span class="special">{</span>
    <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">T</span><span class="special">&gt;</span>
    <span class="keyword">void</span> <span class="keyword">operator</span><span class="special">()(</span><span class="identifier">T</span> <span class="keyword">const</span> <span class="special">&amp;</span><span class="identifier">t</span><span class="special">)</span> <span class="keyword">const</span>
    <span class="special">{</span>
        <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="identifier">t</span> <span class="special">&lt;&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span>
    <span class="special">}</span>
<span class="special">};</span>

<span class="keyword">struct</span> <span class="identifier">fun_t</span> <span class="special">{};</span>
<span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span><span class="identifier">fun_t</span><span class="special">&gt;::</span><span class="identifier">type</span> <span class="keyword">const</span> <span class="identifier">fun</span> <span class="special">=</span> <span class="special">{{}};</span>

<span class="comment">// ...</span>
<span class="identifier">fusion</span><span class="special">::</span><span class="identifier">for_each</span><span class="special">(</span>
    <span class="identifier">fusion</span><span class="special">::</span><span class="identifier">transform</span><span class="special">(</span>
        <span class="comment">// pop_front() removes the "fun" child</span>
        <span class="identifier">fusion</span><span class="special">::</span><span class="identifier">pop_front</span><span class="special">(</span><span class="identifier">fun</span><span class="special">(</span><span class="number">1</span><span class="special">,</span><span class="number">2</span><span class="special">,</span><span class="number">3</span><span class="special">,</span><span class="number">4</span><span class="special">))</span>
        <span class="comment">// Extract the ints from the terminal nodes</span>
      <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">functional</span><span class="special">::</span><span class="identifier">value</span><span class="special">()</span>
    <span class="special">)</span>
  <span class="special">,</span> <span class="identifier">display</span><span class="special">()</span>
<span class="special">);</span>
</pre>
<p>
          Recall from the Introduction that types in the <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">functional</span></code>
          namespace define function objects that correspond to Proto's free functions.
          So <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">functional</span><span class="special">::</span><span class="identifier">value</span><span class="special">()</span></code>
          creates a function object that is equivalent to the <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">value</span><span class="special">()</span></code> function. The above invocation of <code class="computeroutput"><span class="identifier">fusion</span><span class="special">::</span><span class="identifier">for_each</span><span class="special">()</span></code>
          displays the following:
        </p>
<pre class="programlisting">1
2
3
4
</pre>
<p>
          Terminals are also valid Fusion sequences. They contain exactly one element:
          their value.
        </p>
<h6>
<a name="boost_proto.users_guide.intermediate_form.expressions_as_fusion_sequences.h0"></a>
          <span class="phrase"><a name="boost_proto.users_guide.intermediate_form.expressions_as_fusion_sequences.flattening_proto_expression_tress"></a></span><a class="link" href="users_guide.html#boost_proto.users_guide.intermediate_form.expressions_as_fusion_sequences.flattening_proto_expression_tress">Flattening
          Proto Expression Tress</a>
        </h6>
<p>
          Imagine a slight variation of the above example where, instead of iterating
          over the arguments of a lazy function invocation, we would like to iterate
          over the terminals in an addition expression:
        </p>
<pre class="programlisting"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">&gt;::</span><span class="identifier">type</span> <span class="keyword">const</span> <span class="identifier">_1</span> <span class="special">=</span> <span class="special">{</span><span class="number">1</span><span class="special">};</span>

<span class="comment">// ERROR: this doesn't work! Why?</span>
<span class="identifier">fusion</span><span class="special">::</span><span class="identifier">for_each</span><span class="special">(</span>
    <span class="identifier">fusion</span><span class="special">::</span><span class="identifier">transform</span><span class="special">(</span>
        <span class="identifier">_1</span> <span class="special">+</span> <span class="number">2</span> <span class="special">+</span> <span class="number">3</span> <span class="special">+</span> <span class="number">4</span>
      <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">functional</span><span class="special">::</span><span class="identifier">value</span><span class="special">()</span>
    <span class="special">)</span>
  <span class="special">,</span> <span class="identifier">display</span><span class="special">()</span>
<span class="special">);</span>
</pre>
<p>
          The reason this doesn't work is because the expression <code class="computeroutput"><span class="identifier">_1</span>
          <span class="special">+</span> <span class="number">2</span> <span class="special">+</span> <span class="number">3</span> <span class="special">+</span>
          <span class="number">4</span></code> does not describe a flat sequence
          of terminals --- it describes a binary tree. We can treat it as a flat
          sequence of terminals, however, using Proto's <code class="computeroutput"><a class="link" href="../boost/proto/flatten_idp728095472.html" title="Function flatten">proto::flatten()</a></code>
          function. <code class="computeroutput"><a class="link" href="../boost/proto/flatten_idp728095472.html" title="Function flatten">proto::flatten()</a></code> returns a view which makes
          a tree appear as a flat Fusion sequence. If the top-most node has a tag
          type <code class="computeroutput"><span class="identifier">T</span></code>, then the elements
          of the flattened sequence are the child nodes that do <span class="emphasis"><em>not</em></span>
          have tag type <code class="computeroutput"><span class="identifier">T</span></code>. This process
          is evaluated recursively. So the above can correctly be written as:
        </p>
<pre class="programlisting"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">&gt;::</span><span class="identifier">type</span> <span class="keyword">const</span> <span class="identifier">_1</span> <span class="special">=</span> <span class="special">{</span><span class="number">1</span><span class="special">};</span>

<span class="comment">// OK, iterate over a flattened view</span>
<span class="identifier">fusion</span><span class="special">::</span><span class="identifier">for_each</span><span class="special">(</span>
    <span class="identifier">fusion</span><span class="special">::</span><span class="identifier">transform</span><span class="special">(</span>
        <span class="identifier">proto</span><span class="special">::</span><span class="identifier">flatten</span><span class="special">(</span><span class="identifier">_1</span> <span class="special">+</span> <span class="number">2</span> <span class="special">+</span> <span class="number">3</span> <span class="special">+</span> <span class="number">4</span><span class="special">)</span>
      <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">functional</span><span class="special">::</span><span class="identifier">value</span><span class="special">()</span>
    <span class="special">)</span>
  <span class="special">,</span> <span class="identifier">display</span><span class="special">()</span>
<span class="special">);</span>
</pre>
<p>
          The above invocation of <code class="computeroutput"><span class="identifier">fusion</span><span class="special">::</span><span class="identifier">for_each</span><span class="special">()</span></code> displays the following:
        </p>
<pre class="programlisting">1
2
3
4
</pre>
</div>
<div class="section">
<div class="titlepage"><div><div><h4 class="title">
<a name="boost_proto.users_guide.intermediate_form.expression_introspection"></a><a class="link" href="users_guide.html#boost_proto.users_guide.intermediate_form.expression_introspection" title="Expression Introspection: Defining a Grammar">Expression
        Introspection: Defining a Grammar</a>
</h4></div></div></div>
<div class="toc"><dl class="toc">
<dt><span class="section"><a href="users_guide.html#boost_proto.users_guide.intermediate_form.expression_introspection.patterns">Finding
          Patterns in Expressions</a></span></dt>
<dt><span class="section"><a href="users_guide.html#boost_proto.users_guide.intermediate_form.expression_introspection.fuzzy_and_exact_matches_of_terminals">Fuzzy
          and Exact Matches of Terminals</a></span></dt>
<dt><span class="section"><a href="users_guide.html#boost_proto.users_guide.intermediate_form.expression_introspection.if_and_not"><code class="literal">if_&lt;&gt;</code>,
          <code class="literal">and_&lt;&gt;</code>, and <code class="literal">not_&lt;&gt;</code></a></span></dt>
<dt><span class="section"><a href="users_guide.html#boost_proto.users_guide.intermediate_form.expression_introspection.switch">Improving
          Compile Times With <code class="literal">switch_&lt;&gt;</code></a></span></dt>
<dt><span class="section"><a href="users_guide.html#boost_proto.users_guide.intermediate_form.expression_introspection.matching_vararg_expressions">Matching
          Vararg Expressions</a></span></dt>
<dt><span class="section"><a href="users_guide.html#boost_proto.users_guide.intermediate_form.expression_introspection.defining_edsl_grammars">Defining
          EDSL Grammars</a></span></dt>
</dl></div>
<p>
          Expression trees can have a very rich and complicated structure. Often,
          you need to know some things about an expression's structure before you
          can process it. This section describes the tools Proto provides for peering
          inside an expression tree and discovering its structure. And as you'll
          see in later sections, all the really interesting things you can do with
          Proto begin right here.
        </p>
<div class="section">
<div class="titlepage"><div><div><h5 class="title">
<a name="boost_proto.users_guide.intermediate_form.expression_introspection.patterns"></a><a class="link" href="users_guide.html#boost_proto.users_guide.intermediate_form.expression_introspection.patterns" title="Finding Patterns in Expressions">Finding
          Patterns in Expressions</a>
</h5></div></div></div>
<p>
            Imagine your EDSL is a miniature I/O facility, with iostream operations
            that execute lazily. You might want expressions representing input operations
            to be processed by one function, and output operations to be processed
            by a different function. How would you do that?
          </p>
<p>
            The answer is to write patterns (a.k.a, <span class="emphasis"><em>grammars</em></span>)
            that match the structure of input and output expressions. Proto provides
            utilities for defining the grammars, and the <code class="computeroutput"><a class="link" href="../boost/proto/matches.html" title="Struct template matches">proto::matches&lt;&gt;</a></code>
            template for checking whether a given expression type matches the grammar.
          </p>
<p>
            First, let's define some terminals we can use in our lazy I/O expressions:
          </p>
<pre class="programlisting"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">istream</span> <span class="special">&amp;</span> <span class="special">&gt;::</span><span class="identifier">type</span> <span class="identifier">cin_</span> <span class="special">=</span> <span class="special">{</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">cin</span> <span class="special">};</span>
<span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">ostream</span> <span class="special">&amp;</span> <span class="special">&gt;::</span><span class="identifier">type</span> <span class="identifier">cout_</span> <span class="special">=</span> <span class="special">{</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">};</span>
</pre>
<p>
            Now, we can use <code class="computeroutput"><span class="identifier">cout_</span></code>
            instead of <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span></code>, and get I/O expression trees
            that we can execute later. To define grammars that match input and output
            expressions of the form <code class="computeroutput"><span class="identifier">cin_</span>
            <span class="special">&gt;&gt;</span> <span class="identifier">i</span></code>
            and <code class="computeroutput"><span class="identifier">cout_</span> <span class="special">&lt;&lt;</span>
            <span class="number">1</span></code> we do this:
          </p>
<pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">Input</span>
  <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">shift_right</span><span class="special">&lt;</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">istream</span> <span class="special">&amp;</span> <span class="special">&gt;,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_</span> <span class="special">&gt;</span>
<span class="special">{};</span>

<span class="keyword">struct</span> <span class="identifier">Output</span>
  <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">shift_left</span><span class="special">&lt;</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">ostream</span> <span class="special">&amp;</span> <span class="special">&gt;,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_</span> <span class="special">&gt;</span>
<span class="special">{};</span>
</pre>
<p>
            We've seen the template <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;&gt;</span></code> before, but here we're using
            it without accessing the nested <code class="computeroutput"><span class="special">::</span><span class="identifier">type</span></code>. When used like this, it is a
            very simple grammar, as are <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">shift_right</span><span class="special">&lt;&gt;</span></code> and <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">shift_left</span><span class="special">&lt;&gt;</span></code>. The newcomer here is <code class="computeroutput"><span class="identifier">_</span></code> in the <code class="computeroutput"><span class="identifier">proto</span></code>
            namespace. It is a wildcard that matches anything. The <code class="computeroutput"><span class="identifier">Input</span></code> struct is a grammar that matches
            any right-shift expression that has a <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">istream</span></code>
            terminal as its left operand.
          </p>
<p>
            We can use these grammars together with the <code class="computeroutput"><a class="link" href="../boost/proto/matches.html" title="Struct template matches">proto::matches&lt;&gt;</a></code>
            template to query at compile time whether a given I/O expression type
            is an input or output operation. Consider the following:
          </p>
<pre class="programlisting"><span class="keyword">template</span><span class="special">&lt;</span> <span class="keyword">typename</span> <span class="identifier">Expr</span> <span class="special">&gt;</span>
<span class="keyword">void</span> <span class="identifier">input_output</span><span class="special">(</span> <span class="identifier">Expr</span> <span class="keyword">const</span> <span class="special">&amp;</span> <span class="identifier">expr</span> <span class="special">)</span>
<span class="special">{</span>
    <span class="keyword">if</span><span class="special">(</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">matches</span><span class="special">&lt;</span> <span class="identifier">Expr</span><span class="special">,</span> <span class="identifier">Input</span> <span class="special">&gt;::</span><span class="identifier">value</span> <span class="special">)</span>
    <span class="special">{</span>
        <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="string">"Input!\n"</span><span class="special">;</span>
    <span class="special">}</span>

    <span class="keyword">if</span><span class="special">(</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">matches</span><span class="special">&lt;</span> <span class="identifier">Expr</span><span class="special">,</span> <span class="identifier">Output</span> <span class="special">&gt;::</span><span class="identifier">value</span> <span class="special">)</span>
    <span class="special">{</span>
        <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="string">"Output!\n"</span><span class="special">;</span>
    <span class="special">}</span>
<span class="special">}</span>

<span class="keyword">int</span> <span class="identifier">main</span><span class="special">()</span>
<span class="special">{</span>
    <span class="keyword">int</span> <span class="identifier">i</span> <span class="special">=</span> <span class="number">0</span><span class="special">;</span>
    <span class="identifier">input_output</span><span class="special">(</span> <span class="identifier">cout_</span> <span class="special">&lt;&lt;</span> <span class="number">1</span> <span class="special">);</span>
    <span class="identifier">input_output</span><span class="special">(</span> <span class="identifier">cin_</span> <span class="special">&gt;&gt;</span> <span class="identifier">i</span> <span class="special">);</span>

    <span class="keyword">return</span> <span class="number">0</span><span class="special">;</span>
<span class="special">}</span>
</pre>
<p>
            This program prints the following:
          </p>
<pre class="programlisting">Output!
Input!
</pre>
<p>
            If we wanted to break the <code class="computeroutput"><span class="identifier">input_output</span><span class="special">()</span></code> function into two functions, one that
            handles input expressions and one for output expressions, we can use
            <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">enable_if</span><span class="special">&lt;&gt;</span></code>,
            as follows:
          </p>
<pre class="programlisting"><span class="keyword">template</span><span class="special">&lt;</span> <span class="keyword">typename</span> <span class="identifier">Expr</span> <span class="special">&gt;</span>
<span class="keyword">typename</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">enable_if</span><span class="special">&lt;</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">matches</span><span class="special">&lt;</span> <span class="identifier">Expr</span><span class="special">,</span> <span class="identifier">Input</span> <span class="special">&gt;</span> <span class="special">&gt;::</span><span class="identifier">type</span>
<span class="identifier">input_output</span><span class="special">(</span> <span class="identifier">Expr</span> <span class="keyword">const</span> <span class="special">&amp;</span> <span class="identifier">expr</span> <span class="special">)</span>
<span class="special">{</span>
    <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="string">"Input!\n"</span><span class="special">;</span>
<span class="special">}</span>

<span class="keyword">template</span><span class="special">&lt;</span> <span class="keyword">typename</span> <span class="identifier">Expr</span> <span class="special">&gt;</span>
<span class="keyword">typename</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">enable_if</span><span class="special">&lt;</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">matches</span><span class="special">&lt;</span> <span class="identifier">Expr</span><span class="special">,</span> <span class="identifier">Output</span> <span class="special">&gt;</span> <span class="special">&gt;::</span><span class="identifier">type</span>
<span class="identifier">input_output</span><span class="special">(</span> <span class="identifier">Expr</span> <span class="keyword">const</span> <span class="special">&amp;</span> <span class="identifier">expr</span> <span class="special">)</span>
<span class="special">{</span>
    <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="string">"Output!\n"</span><span class="special">;</span>
<span class="special">}</span>
</pre>
<p>
            This works as the previous version did. However, the following does not
            compile at all:
          </p>
<pre class="programlisting"><span class="identifier">input_output</span><span class="special">(</span> <span class="identifier">cout_</span> <span class="special">&lt;&lt;</span> <span class="number">1</span> <span class="special">&lt;&lt;</span> <span class="number">2</span> <span class="special">);</span> <span class="comment">// oops!</span>
</pre>
<p>
            What's wrong? The problem is that this expression does not match our
            grammar. The expression groups as if it were written like <code class="computeroutput"><span class="special">(</span><span class="identifier">cout_</span> <span class="special">&lt;&lt;</span> <span class="number">1</span><span class="special">)</span> <span class="special">&lt;&lt;</span> <span class="number">2</span></code>. It will not match the <code class="computeroutput"><span class="identifier">Output</span></code> grammar, which expects the left
            operand to be a terminal, not another left-shift operation. We need to
            fix the grammar.
          </p>
<p>
            We notice that in order to verify an expression as input or output, we'll
            need to recurse down to the bottom-left-most leaf and check that it is
            a <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">istream</span></code> or <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">ostream</span></code>.
            When we get to the terminal, we must stop recursing. We can express this
            in our grammar using <code class="computeroutput"><a class="link" href="../boost/proto/or_.html" title="Struct template or_">proto::or_&lt;&gt;</a></code>.
            Here are the correct <code class="computeroutput"><span class="identifier">Input</span></code>
            and <code class="computeroutput"><span class="identifier">Output</span></code> grammars:
          </p>
<pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">Input</span>
  <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">or_</span><span class="special">&lt;</span>
        <span class="identifier">proto</span><span class="special">::</span><span class="identifier">shift_right</span><span class="special">&lt;</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">istream</span> <span class="special">&amp;</span> <span class="special">&gt;,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_</span> <span class="special">&gt;</span>
      <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">shift_right</span><span class="special">&lt;</span> <span class="identifier">Input</span><span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_</span> <span class="special">&gt;</span>
    <span class="special">&gt;</span>
<span class="special">{};</span>

<span class="keyword">struct</span> <span class="identifier">Output</span>
  <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">or_</span><span class="special">&lt;</span>
        <span class="identifier">proto</span><span class="special">::</span><span class="identifier">shift_left</span><span class="special">&lt;</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">ostream</span> <span class="special">&amp;</span> <span class="special">&gt;,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_</span> <span class="special">&gt;</span>
      <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">shift_left</span><span class="special">&lt;</span> <span class="identifier">Output</span><span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_</span> <span class="special">&gt;</span>
    <span class="special">&gt;</span>
<span class="special">{};</span>
</pre>
<p>
            This may look a little odd at first. We seem to be defining the <code class="computeroutput"><span class="identifier">Input</span></code> and <code class="computeroutput"><span class="identifier">Output</span></code>
            types in terms of themselves. This is perfectly OK, actually. At the
            point in the grammar that the <code class="computeroutput"><span class="identifier">Input</span></code>
            and <code class="computeroutput"><span class="identifier">Output</span></code> types are
            being used, they are <span class="emphasis"><em>incomplete</em></span>, but by the time
            we actually evaluate the grammar with <code class="computeroutput"><a class="link" href="../boost/proto/matches.html" title="Struct template matches">proto::matches&lt;&gt;</a></code>,
            the types will be complete. These are recursive grammars, and rightly
            so because they must match a recursive data structure!
          </p>
<p>
            Matching an expression such as <code class="computeroutput"><span class="identifier">cout_</span>
            <span class="special">&lt;&lt;</span> <span class="number">1</span>
            <span class="special">&lt;&lt;</span> <span class="number">2</span></code>
            against the <code class="computeroutput"><span class="identifier">Output</span></code> grammar
            procedes as follows:
          </p>
<div class="orderedlist"><ol class="orderedlist" type="1">
<li class="listitem">
                The first alternate of the <code class="computeroutput"><a class="link" href="../boost/proto/or_.html" title="Struct template or_">proto::or_&lt;&gt;</a></code>
                is tried first. It will fail, because the expression <code class="computeroutput"><span class="identifier">cout_</span> <span class="special">&lt;&lt;</span>
                <span class="number">1</span> <span class="special">&lt;&lt;</span>
                <span class="number">2</span></code> does not match the grammar
                <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">shift_left</span><span class="special">&lt;</span>
                <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span>
                <span class="identifier">std</span><span class="special">::</span><span class="identifier">ostream</span> <span class="special">&amp;</span>
                <span class="special">&gt;,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_</span> <span class="special">&gt;</span></code>.
              </li>
<li class="listitem">
                Then the second alternate is tried next. We match the expression
                against <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">shift_left</span><span class="special">&lt;</span>
                <span class="identifier">Output</span><span class="special">,</span>
                <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_</span> <span class="special">&gt;</span></code>.
                The expression is a left-shift, so we next try to match the operands.
              </li>
<li class="listitem">
                The right operand <code class="computeroutput"><span class="number">2</span></code> matches
                <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_</span></code> trivially.
              </li>
<li class="listitem">
                To see if the left operand <code class="computeroutput"><span class="identifier">cout_</span>
                <span class="special">&lt;&lt;</span> <span class="number">1</span></code>
                matches <code class="computeroutput"><span class="identifier">Output</span></code>, we
                must recursively evaluate the <code class="computeroutput"><span class="identifier">Output</span></code>
                grammar. This time we succeed, because <code class="computeroutput"><span class="identifier">cout_</span>
                <span class="special">&lt;&lt;</span> <span class="number">1</span></code>
                will match the first alternate of the <code class="computeroutput"><a class="link" href="../boost/proto/or_.html" title="Struct template or_">proto::or_&lt;&gt;</a></code>.
              </li>
</ol></div>
<p>
            We're done -- the grammar matches successfully.
          </p>
</div>
<div class="section">
<div class="titlepage"><div><div><h5 class="title">
<a name="boost_proto.users_guide.intermediate_form.expression_introspection.fuzzy_and_exact_matches_of_terminals"></a><a class="link" href="users_guide.html#boost_proto.users_guide.intermediate_form.expression_introspection.fuzzy_and_exact_matches_of_terminals" title="Fuzzy and Exact Matches of Terminals">Fuzzy
          and Exact Matches of Terminals</a>
</h5></div></div></div>
<p>
            The terminals in an expression tree could be const or non-const references,
            or they might not be references at all. When writing grammars, you usually
            don't have to worry about it because <code class="computeroutput"><a class="link" href="../boost/proto/matches.html" title="Struct template matches">proto::matches&lt;&gt;</a></code>
            gives you a little wiggle room when matching terminals. A grammar such
            as <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">&gt;</span></code>
            will match a terminal of type <code class="computeroutput"><span class="keyword">int</span></code>,
            <code class="computeroutput"><span class="keyword">int</span> <span class="special">&amp;</span></code>,
            or <code class="computeroutput"><span class="keyword">int</span> <span class="keyword">const</span>
            <span class="special">&amp;</span></code>.
          </p>
<p>
            You can explicitly specify that you want to match a reference type. If
            you do, the type must match exactly. For instance, a grammar such as
            <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span><span class="keyword">int</span> <span class="special">&amp;&gt;</span></code>
            will only match an <code class="computeroutput"><span class="keyword">int</span> <span class="special">&amp;</span></code>. It will not match an <code class="computeroutput"><span class="keyword">int</span></code> or an <code class="computeroutput"><span class="keyword">int</span>
            <span class="keyword">const</span> <span class="special">&amp;</span></code>.
          </p>
<p>
            The table below shows how Proto matches terminals. The simple rule is:
            if you want to match only reference types, you must specify the reference
            in your grammar. Otherwise, leave it off and Proto will ignore const
            and references.
          </p>
<div class="table">
<a name="boost_proto.users_guide.intermediate_form.expression_introspection.fuzzy_and_exact_matches_of_terminals.t0"></a><p class="title"><b>Table&#160;32.7.&#160;proto::matches&lt;&gt; and Reference / CV-Qualification of Terminals</b></p>
<div class="table-contents"><table class="table" summary="proto::matches&lt;&gt; and Reference / CV-Qualification of Terminals">
<colgroup>
<col>
<col>
<col>
</colgroup>
<thead><tr>
<th>
                    <p>
                      Terminal
                    </p>
                  </th>
<th>
                    <p>
                      Grammar
                    </p>
                  </th>
<th>
                    <p>
                      Matches?
                    </p>
                  </th>
</tr></thead>
<tbody>
<tr>
<td>
                    <p>
                      T
                    </p>
                  </td>
<td>
                    <p>
                      T
                    </p>
                  </td>
<td>
                    <p>
                      yes
                    </p>
                  </td>
</tr>
<tr>
<td>
                    <p>
                      T &amp;
                    </p>
                  </td>
<td>
                    <p>
                      T
                    </p>
                  </td>
<td>
                    <p>
                      yes
                    </p>
                  </td>
</tr>
<tr>
<td>
                    <p>
                      T const &amp;
                    </p>
                  </td>
<td>
                    <p>
                      T
                    </p>
                  </td>
<td>
                    <p>
                      yes
                    </p>
                  </td>
</tr>
<tr>
<td>
                    <p>
                      T
                    </p>
                  </td>
<td>
                    <p>
                      T &amp;
                    </p>
                  </td>
<td>
                    <p>
                      no
                    </p>
                  </td>
</tr>
<tr>
<td>
                    <p>
                      T &amp;
                    </p>
                  </td>
<td>
                    <p>
                      T &amp;
                    </p>
                  </td>
<td>
                    <p>
                      yes
                    </p>
                  </td>
</tr>
<tr>
<td>
                    <p>
                      T const &amp;
                    </p>
                  </td>
<td>
                    <p>
                      T &amp;
                    </p>
                  </td>
<td>
                    <p>
                      no
                    </p>
                  </td>
</tr>
<tr>
<td>
                    <p>
                      T
                    </p>
                  </td>
<td>
                    <p>
                      T const &amp;
                    </p>
                  </td>
<td>
                    <p>
                      no
                    </p>
                  </td>
</tr>
<tr>
<td>
                    <p>
                      T &amp;
                    </p>
                  </td>
<td>
                    <p>
                      T const &amp;
                    </p>
                  </td>
<td>
                    <p>
                      no
                    </p>
                  </td>
</tr>
<tr>
<td>
                    <p>
                      T const &amp;
                    </p>
                  </td>
<td>
                    <p>
                      T const &amp;
                    </p>
                  </td>
<td>
                    <p>
                      yes
                    </p>
                  </td>
</tr>
</tbody>
</table></div>
</div>
<br class="table-break"><p>
            This begs the question: What if you want to match an <code class="computeroutput"><span class="keyword">int</span></code>,
            but not an <code class="computeroutput"><span class="keyword">int</span> <span class="special">&amp;</span></code>
            or an <code class="computeroutput"><span class="keyword">int</span> <span class="keyword">const</span>
            <span class="special">&amp;</span></code>? For forcing exact matches,
            Proto provides the <code class="computeroutput"><a class="link" href="../boost/proto/exact.html" title="Struct template exact">proto::exact&lt;&gt;</a></code>
            template. For instance, <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">exact</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">&gt;</span> <span class="special">&gt;</span></code>
            would only match an <code class="computeroutput"><span class="keyword">int</span></code>
            held by value.
          </p>
<p>
            Proto gives you extra wiggle room when matching array types. Array types
            match themselves or the pointer types they decay to. This is especially
            useful with character arrays. The type returned by <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">as_expr</span><span class="special">(</span><span class="string">"hello"</span><span class="special">)</span></code> is <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span><span class="keyword">char</span> <span class="keyword">const</span><span class="special">[</span><span class="number">6</span><span class="special">]&gt;::</span><span class="identifier">type</span></code>. That's a terminal containing
            a 6-element character array. Naturally, you can match this terminal with
            the grammar <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span><span class="keyword">char</span> <span class="keyword">const</span><span class="special">[</span><span class="number">6</span><span class="special">]&gt;</span></code>,
            but the grammar <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span><span class="keyword">char</span> <span class="keyword">const</span> <span class="special">*&gt;</span></code>
            will match it as well, as the following code fragment illustrates.
          </p>
<pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">CharString</span>
  <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span> <span class="keyword">char</span> <span class="keyword">const</span> <span class="special">*</span> <span class="special">&gt;</span>
<span class="special">{};</span>

<span class="keyword">typedef</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span> <span class="keyword">char</span> <span class="keyword">const</span><span class="special">[</span><span class="number">6</span><span class="special">]</span> <span class="special">&gt;::</span><span class="identifier">type</span> <span class="identifier">char_array</span><span class="special">;</span>

<span class="identifier">BOOST_MPL_ASSERT</span><span class="special">((</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">matches</span><span class="special">&lt;</span> <span class="identifier">char_array</span><span class="special">,</span> <span class="identifier">CharString</span> <span class="special">&gt;</span> <span class="special">));</span>
</pre>
<p>
            What if we only wanted <code class="computeroutput"><span class="identifier">CharString</span></code>
            to match terminals of exactly the type <code class="computeroutput"><span class="keyword">char</span>
            <span class="keyword">const</span> <span class="special">*</span></code>?
            You can use <code class="computeroutput"><a class="link" href="../boost/proto/exact.html" title="Struct template exact">proto::exact&lt;&gt;</a></code> here to turn off
            the fuzzy matching of terminals, as follows:
          </p>
<pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">CharString</span>
  <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">exact</span><span class="special">&lt;</span> <span class="keyword">char</span> <span class="keyword">const</span> <span class="special">*</span> <span class="special">&gt;</span> <span class="special">&gt;</span>
<span class="special">{};</span>

<span class="keyword">typedef</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span><span class="keyword">char</span> <span class="keyword">const</span><span class="special">[</span><span class="number">6</span><span class="special">]&gt;::</span><span class="identifier">type</span> <span class="identifier">char_array</span><span class="special">;</span>
<span class="keyword">typedef</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span><span class="keyword">char</span> <span class="keyword">const</span> <span class="special">*&gt;::</span><span class="identifier">type</span>  <span class="identifier">char_string</span><span class="special">;</span>

<span class="identifier">BOOST_MPL_ASSERT</span><span class="special">((</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">matches</span><span class="special">&lt;</span> <span class="identifier">char_string</span><span class="special">,</span> <span class="identifier">CharString</span> <span class="special">&gt;</span> <span class="special">));</span>
<span class="identifier">BOOST_MPL_ASSERT_NOT</span><span class="special">((</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">matches</span><span class="special">&lt;</span> <span class="identifier">char_array</span><span class="special">,</span> <span class="identifier">CharString</span> <span class="special">&gt;</span> <span class="special">));</span>
</pre>
<p>
            Now, <code class="computeroutput"><span class="identifier">CharString</span></code> does
            not match array types, only character string pointers.
          </p>
<p>
            The inverse problem is a little trickier: what if you wanted to match
            all character arrays, but not character pointers? As mentioned above,
            the expression <code class="computeroutput"><span class="identifier">as_expr</span><span class="special">(</span><span class="string">"hello"</span><span class="special">)</span></code> has the type <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span> <span class="keyword">char</span> <span class="keyword">const</span><span class="special">[</span> <span class="number">6</span> <span class="special">]</span> <span class="special">&gt;::</span><span class="identifier">type</span></code>. If you wanted to match character
            arrays of arbitrary size, you could use <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">N</span></code>,
            which is an array-size wildcard. The following grammar would match any
            string literal: <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span> <span class="keyword">char</span> <span class="keyword">const</span><span class="special">[</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">N</span> <span class="special">]</span> <span class="special">&gt;</span></code>.
          </p>
<p>
            Sometimes you need even more wiggle room when matching terminals. For
            example, maybe you're building a calculator EDSL and you want to allow
            any terminals that are convertible to <code class="computeroutput"><span class="keyword">double</span></code>.
            For that, Proto provides the <code class="computeroutput"><a class="link" href="../boost/proto/convertible_to.html" title="Struct template convertible_to">proto::convertible_to&lt;&gt;</a></code>
            template. You can use it as: <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">convertible_to</span><span class="special">&lt;</span> <span class="keyword">double</span>
            <span class="special">&gt;</span> <span class="special">&gt;</span></code>.
          </p>
<p>
            There is one more way you can perform a fuzzy match on terminals. Consider
            the problem of trying to match a <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">complex</span><span class="special">&lt;&gt;</span></code> terminal. You can easily match
            a <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">complex</span><span class="special">&lt;</span><span class="keyword">float</span><span class="special">&gt;</span></code>
            or a <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">complex</span><span class="special">&lt;</span><span class="keyword">double</span><span class="special">&gt;</span></code>,
            but how would you match any instantiation of <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">complex</span><span class="special">&lt;&gt;</span></code>? You can use <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_</span></code>
            here to solve this problem. Here is the grammar to match any <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">complex</span><span class="special">&lt;&gt;</span></code>
            instantiation:
          </p>
<pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">StdComplex</span>
  <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">complex</span><span class="special">&lt;</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_</span> <span class="special">&gt;</span> <span class="special">&gt;</span>
<span class="special">{};</span>
</pre>
<p>
            When given a grammar like this, Proto will deconstruct the grammar and
            the terminal it is being matched against and see if it can match all
            the constituents.
          </p>
</div>
<div class="section">
<div class="titlepage"><div><div><h5 class="title">
<a name="boost_proto.users_guide.intermediate_form.expression_introspection.if_and_not"></a><a class="link" href="users_guide.html#boost_proto.users_guide.intermediate_form.expression_introspection.if_and_not" title="if_&lt;&gt;, and_&lt;&gt;, and not_&lt;&gt;"><code class="literal">if_&lt;&gt;</code>,
          <code class="literal">and_&lt;&gt;</code>, and <code class="literal">not_&lt;&gt;</code></a>
</h5></div></div></div>
<p>
            We've already seen how to use expression generators like <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;&gt;</span></code>
            and <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">shift_right</span><span class="special">&lt;&gt;</span></code>
            as grammars. We've also seen <code class="computeroutput"><a class="link" href="../boost/proto/or_.html" title="Struct template or_">proto::or_&lt;&gt;</a></code>,
            which we can use to express a set of alternate grammars. There are a
            few others of interest; in particular, <code class="computeroutput"><a class="link" href="../boost/proto/if_.html" title="Struct template if_">proto::if_&lt;&gt;</a></code>,
            <code class="computeroutput"><a class="link" href="../boost/proto/and_.html" title="Struct template and_">proto::and_&lt;&gt;</a></code> and <code class="computeroutput"><a class="link" href="../boost/proto/not_.html" title="Struct template not_">proto::not_&lt;&gt;</a></code>.
          </p>
<p>
            The <code class="computeroutput"><a class="link" href="../boost/proto/not_.html" title="Struct template not_">proto::not_&lt;&gt;</a></code> template is the simplest.
            It takes a grammar as a template parameter and logically negates it;
            <code class="computeroutput"><span class="identifier">not_</span><span class="special">&lt;</span><span class="identifier">Grammar</span><span class="special">&gt;</span></code>
            will match any expression that <code class="computeroutput"><span class="identifier">Grammar</span></code>
            does <span class="emphasis"><em>not</em></span> match.
          </p>
<p>
            The <code class="computeroutput"><a class="link" href="../boost/proto/if_.html" title="Struct template if_">proto::if_&lt;&gt;</a></code> template is used
            together with a Proto transform that is evaluated against expression
            types to find matches. (Proto transforms will be described later.)
          </p>
<p>
            The <code class="computeroutput"><a class="link" href="../boost/proto/and_.html" title="Struct template and_">proto::and_&lt;&gt;</a></code> template is like
            <code class="computeroutput"><a class="link" href="../boost/proto/or_.html" title="Struct template or_">proto::or_&lt;&gt;</a></code>, except that each
            argument of the <code class="computeroutput"><a class="link" href="../boost/proto/and_.html" title="Struct template and_">proto::and_&lt;&gt;</a></code> must match in order
            for the <code class="computeroutput"><a class="link" href="../boost/proto/and_.html" title="Struct template and_">proto::and_&lt;&gt;</a></code> to match. As an example,
            consider the definition of <code class="computeroutput"><span class="identifier">CharString</span></code>
            above that uses <code class="computeroutput"><a class="link" href="../boost/proto/exact.html" title="Struct template exact">proto::exact&lt;&gt;</a></code>. It could have been
            written without <code class="computeroutput"><a class="link" href="../boost/proto/exact.html" title="Struct template exact">proto::exact&lt;&gt;</a></code> as follows:
          </p>
<pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">CharString</span>
  <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">and_</span><span class="special">&lt;</span>
        <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_</span> <span class="special">&gt;</span>
      <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">if_</span><span class="special">&lt;</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">is_same</span><span class="special">&lt;</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_value</span><span class="special">,</span> <span class="keyword">char</span> <span class="keyword">const</span> <span class="special">*</span> <span class="special">&gt;()</span> <span class="special">&gt;</span>
    <span class="special">&gt;</span>
<span class="special">{};</span>
</pre>
<p>
            This says that a <code class="computeroutput"><span class="identifier">CharString</span></code>
            must be a terminal, <span class="emphasis"><em>and</em></span> its value type must be the
            same as <code class="computeroutput"><span class="keyword">char</span> <span class="keyword">const</span>
            <span class="special">*</span></code>. Notice the template argument
            of <code class="computeroutput"><a class="link" href="../boost/proto/if_.html" title="Struct template if_">proto::if_&lt;&gt;</a></code>: <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">is_same</span><span class="special">&lt;</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_value</span><span class="special">,</span> <span class="keyword">char</span> <span class="keyword">const</span> <span class="special">*</span> <span class="special">&gt;()</span></code>. This is Proto transform that compares
            the value type of a terminal to <code class="computeroutput"><span class="keyword">char</span>
            <span class="keyword">const</span> <span class="special">*</span></code>.
          </p>
<p>
            The <code class="computeroutput"><a class="link" href="../boost/proto/if_.html" title="Struct template if_">proto::if_&lt;&gt;</a></code> template has a couple
            of variants. In addition to <code class="computeroutput"><span class="identifier">if_</span><span class="special">&lt;</span><span class="identifier">Condition</span><span class="special">&gt;</span></code> you can also say <code class="computeroutput"><span class="identifier">if_</span><span class="special">&lt;</span><span class="identifier">Condition</span><span class="special">,</span> <span class="identifier">ThenGrammar</span><span class="special">&gt;</span></code> and <code class="computeroutput"><span class="identifier">if_</span><span class="special">&lt;</span><span class="identifier">Condition</span><span class="special">,</span> <span class="identifier">ThenGrammar</span><span class="special">,</span> <span class="identifier">ElseGrammar</span><span class="special">&gt;</span></code>. These let you select one sub-grammar
            or another based on the <code class="computeroutput"><span class="identifier">Condition</span></code>.
          </p>
</div>
<div class="section">
<div class="titlepage"><div><div><h5 class="title">
<a name="boost_proto.users_guide.intermediate_form.expression_introspection.switch"></a><a class="link" href="users_guide.html#boost_proto.users_guide.intermediate_form.expression_introspection.switch" title="Improving Compile Times With switch_&lt;&gt;">Improving
          Compile Times With <code class="literal">switch_&lt;&gt;</code></a>
</h5></div></div></div>
<p>
            When your Proto grammar gets large, you'll start to run into some scalability
            problems with <code class="computeroutput"><a class="link" href="../boost/proto/or_.html" title="Struct template or_">proto::or_&lt;&gt;</a></code>, the construct you
            use to specify alternate sub-grammars. First, due to limitations in C++,
            <code class="computeroutput"><a class="link" href="../boost/proto/or_.html" title="Struct template or_">proto::or_&lt;&gt;</a></code> can only accept up
            to a certain number of sub-grammars, controlled by the <code class="computeroutput"><span class="identifier">BOOST_PROTO_MAX_LOGICAL_ARITY</span></code> macro.
            This macro defaults to eight, and you can set it higher, but doing so
            will aggravate another scalability problem: long compile times. With
            <code class="computeroutput"><a class="link" href="../boost/proto/or_.html" title="Struct template or_">proto::or_&lt;&gt;</a></code>, alternate sub-grammars
            are tried in order -- like a series of cascading <code class="computeroutput"><span class="keyword">if</span></code>'s
            -- leading to lots of unnecessary template instantiations. What you would
            prefer instead is something like <code class="computeroutput"><span class="keyword">switch</span></code>
            that avoids the expense of cascading <code class="computeroutput"><span class="keyword">if</span></code>'s.
            That's the purpose of <code class="computeroutput"><a class="link" href="../boost/proto/switch_.html" title="Struct template switch_">proto::switch_&lt;&gt;</a></code>;
            although less convenient than <code class="computeroutput"><a class="link" href="../boost/proto/or_.html" title="Struct template or_">proto::or_&lt;&gt;</a></code>,
            it improves compile times for larger grammars and does not have an arbitrary
            fixed limit on the number of sub-grammars.
          </p>
<p>
            Let's illustrate how to use <code class="computeroutput"><a class="link" href="../boost/proto/switch_.html" title="Struct template switch_">proto::switch_&lt;&gt;</a></code>
            by first writing a big grammar with <code class="computeroutput"><a class="link" href="../boost/proto/or_.html" title="Struct template or_">proto::or_&lt;&gt;</a></code>
            and then translating it to an equivalent grammar using <code class="computeroutput"><a class="link" href="../boost/proto/switch_.html" title="Struct template switch_">proto::switch_&lt;&gt;</a></code>:
          </p>
<pre class="programlisting"><span class="comment">// Here is a big, inefficient grammar</span>
<span class="keyword">struct</span> <span class="identifier">ABigGrammar</span>
  <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">or_</span><span class="special">&lt;</span>
        <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">&gt;</span>
      <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span><span class="keyword">double</span><span class="special">&gt;</span>
      <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">unary_plus</span><span class="special">&lt;</span><span class="identifier">ABigGrammar</span><span class="special">&gt;</span>
      <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">negate</span><span class="special">&lt;</span><span class="identifier">ABigGrammar</span><span class="special">&gt;</span>
      <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">complement</span><span class="special">&lt;</span><span class="identifier">ABigGrammar</span><span class="special">&gt;</span>
      <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">plus</span><span class="special">&lt;</span><span class="identifier">ABigGrammar</span><span class="special">,</span> <span class="identifier">ABigGrammar</span><span class="special">&gt;</span>
      <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">minus</span><span class="special">&lt;</span><span class="identifier">ABigGrammar</span><span class="special">,</span> <span class="identifier">ABigGrammar</span><span class="special">&gt;</span>
      <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">or_</span><span class="special">&lt;</span>
            <span class="identifier">proto</span><span class="special">::</span><span class="identifier">multiplies</span><span class="special">&lt;</span><span class="identifier">ABigGrammar</span><span class="special">,</span> <span class="identifier">ABigGrammar</span><span class="special">&gt;</span>
          <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">divides</span><span class="special">&lt;</span><span class="identifier">ABigGrammar</span><span class="special">,</span> <span class="identifier">ABigGrammar</span><span class="special">&gt;</span>
          <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">modulus</span><span class="special">&lt;</span><span class="identifier">ABigGrammar</span><span class="special">,</span> <span class="identifier">ABigGrammar</span><span class="special">&gt;</span>
        <span class="special">&gt;</span>
    <span class="special">&gt;</span>
<span class="special">{};</span>
</pre>
<p>
            The above might be the grammar to a more elaborate calculator EDSL. Notice
            that since there are more than eight sub-grammars, we had to chain the
            sub-grammars with a nested <code class="computeroutput"><a class="link" href="../boost/proto/or_.html" title="Struct template or_">proto::or_&lt;&gt;</a></code>
            -- not very nice.
          </p>
<p>
            The idea behind <code class="computeroutput"><a class="link" href="../boost/proto/switch_.html" title="Struct template switch_">proto::switch_&lt;&gt;</a></code>
            is to dispatch based on an expression's tag type to a sub-grammar that
            handles expressions of that type. To use <code class="computeroutput"><a class="link" href="../boost/proto/switch_.html" title="Struct template switch_">proto::switch_&lt;&gt;</a></code>,
            you define a struct with a nested <code class="computeroutput"><span class="identifier">case_</span><span class="special">&lt;&gt;</span></code> template, specialized on tag
            types. The above grammar can be expressed using <code class="computeroutput"><a class="link" href="../boost/proto/switch_.html" title="Struct template switch_">proto::switch_&lt;&gt;</a></code>
            as follows. It is described below.
          </p>
<pre class="programlisting"><span class="comment">// Redefine ABigGrammar more efficiently using proto::switch_&lt;&gt;</span>
<span class="keyword">struct</span> <span class="identifier">ABigGrammar</span><span class="special">;</span>

<span class="keyword">struct</span> <span class="identifier">ABigGrammarCases</span>
<span class="special">{</span>
    <span class="comment">// The primary template matches nothing:</span>
    <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">Tag</span><span class="special">&gt;</span>
    <span class="keyword">struct</span> <span class="identifier">case_</span>
      <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">not_</span><span class="special">&lt;</span><span class="identifier">_</span><span class="special">&gt;</span>
    <span class="special">{};</span>
<span class="special">};</span>

<span class="comment">// Terminal expressions are handled here</span>
<span class="keyword">template</span><span class="special">&lt;&gt;</span>
<span class="keyword">struct</span> <span class="identifier">ABigGrammarCases</span><span class="special">::</span><span class="identifier">case_</span><span class="special">&lt;</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&gt;</span>
  <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">or_</span><span class="special">&lt;</span>
        <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">&gt;</span>
      <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span><span class="keyword">double</span><span class="special">&gt;</span>
    <span class="special">&gt;</span>
<span class="special">{};</span>

<span class="comment">// Non-terminals are handled similarly</span>
<span class="keyword">template</span><span class="special">&lt;&gt;</span>
<span class="keyword">struct</span> <span class="identifier">ABigGrammarCases</span><span class="special">::</span><span class="identifier">case_</span><span class="special">&lt;</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">unary_plus</span><span class="special">&gt;</span>
  <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">unary_plus</span><span class="special">&lt;</span><span class="identifier">ABigGrammar</span><span class="special">&gt;</span>
<span class="special">{};</span>

<span class="keyword">template</span><span class="special">&lt;&gt;</span>
<span class="keyword">struct</span> <span class="identifier">ABigGrammarCases</span><span class="special">::</span><span class="identifier">case_</span><span class="special">&lt;</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">negate</span><span class="special">&gt;</span>
  <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">negate</span><span class="special">&lt;</span><span class="identifier">ABigGrammar</span><span class="special">&gt;</span>
<span class="special">{};</span>

<span class="keyword">template</span><span class="special">&lt;&gt;</span>
<span class="keyword">struct</span> <span class="identifier">ABigGrammarCases</span><span class="special">::</span><span class="identifier">case_</span><span class="special">&lt;</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">complement</span><span class="special">&gt;</span>
  <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">complement</span><span class="special">&lt;</span><span class="identifier">ABigGrammar</span><span class="special">&gt;</span>
<span class="special">{};</span>

<span class="keyword">template</span><span class="special">&lt;&gt;</span>
<span class="keyword">struct</span> <span class="identifier">ABigGrammarCases</span><span class="special">::</span><span class="identifier">case_</span><span class="special">&lt;</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">plus</span><span class="special">&gt;</span>
  <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">plus</span><span class="special">&lt;</span><span class="identifier">ABigGrammar</span><span class="special">,</span> <span class="identifier">ABigGrammar</span><span class="special">&gt;</span>
<span class="special">{};</span>

<span class="keyword">template</span><span class="special">&lt;&gt;</span>
<span class="keyword">struct</span> <span class="identifier">ABigGrammarCases</span><span class="special">::</span><span class="identifier">case_</span><span class="special">&lt;</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">minus</span><span class="special">&gt;</span>
  <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">minus</span><span class="special">&lt;</span><span class="identifier">ABigGrammar</span><span class="special">,</span> <span class="identifier">ABigGrammar</span><span class="special">&gt;</span>
<span class="special">{};</span>

<span class="keyword">template</span><span class="special">&lt;&gt;</span>
<span class="keyword">struct</span> <span class="identifier">ABigGrammarCases</span><span class="special">::</span><span class="identifier">case_</span><span class="special">&lt;</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">multiplies</span><span class="special">&gt;</span>
  <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">multiplies</span><span class="special">&lt;</span><span class="identifier">ABigGrammar</span><span class="special">,</span> <span class="identifier">ABigGrammar</span><span class="special">&gt;</span>
<span class="special">{};</span>

<span class="keyword">template</span><span class="special">&lt;&gt;</span>
<span class="keyword">struct</span> <span class="identifier">ABigGrammarCases</span><span class="special">::</span><span class="identifier">case_</span><span class="special">&lt;</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">divides</span><span class="special">&gt;</span>
  <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">divides</span><span class="special">&lt;</span><span class="identifier">ABigGrammar</span><span class="special">,</span> <span class="identifier">ABigGrammar</span><span class="special">&gt;</span>
<span class="special">{};</span>

<span class="keyword">template</span><span class="special">&lt;&gt;</span>
<span class="keyword">struct</span> <span class="identifier">ABigGrammarCases</span><span class="special">::</span><span class="identifier">case_</span><span class="special">&lt;</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">modulus</span><span class="special">&gt;</span>
  <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">modulus</span><span class="special">&lt;</span><span class="identifier">ABigGrammar</span><span class="special">,</span> <span class="identifier">ABigGrammar</span><span class="special">&gt;</span>
<span class="special">{};</span>

<span class="comment">// Define ABigGrammar in terms of ABigGrammarCases</span>
<span class="comment">// using proto::switch_&lt;&gt;</span>
<span class="keyword">struct</span> <span class="identifier">ABigGrammar</span>
  <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">switch_</span><span class="special">&lt;</span><span class="identifier">ABigGrammarCases</span><span class="special">&gt;</span>
<span class="special">{};</span>
</pre>
<p>
            Matching an expression type <code class="computeroutput"><span class="identifier">E</span></code>
            against <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">switch_</span><span class="special">&lt;</span><span class="identifier">C</span><span class="special">&gt;</span></code>
            is equivalent to matching it against <code class="computeroutput"><span class="identifier">C</span><span class="special">::</span><span class="identifier">case_</span><span class="special">&lt;</span><span class="identifier">E</span><span class="special">::</span><span class="identifier">proto_tag</span><span class="special">&gt;</span></code>. By dispatching on the expression's
            tag type, we can jump to the sub-grammar that handles expressions of
            that type, skipping over all the other sub-grammars that couldn't possibly
            match. If there is no specialization of <code class="computeroutput"><span class="identifier">case_</span><span class="special">&lt;&gt;</span></code> for a particular tag type, we
            select the primary template. In this case, the primary template inherits
            from <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">not_</span><span class="special">&lt;</span><span class="identifier">_</span><span class="special">&gt;</span></code>
            which matches no expressions.
          </p>
<p>
            Notice the specialization that handles terminals:
          </p>
<pre class="programlisting"><span class="comment">// Terminal expressions are handled here</span>
<span class="keyword">template</span><span class="special">&lt;&gt;</span>
<span class="keyword">struct</span> <span class="identifier">ABigGrammarCases</span><span class="special">::</span><span class="identifier">case_</span><span class="special">&lt;</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&gt;</span>
  <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">or_</span><span class="special">&lt;</span>
        <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">&gt;</span>
      <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span><span class="keyword">double</span><span class="special">&gt;</span>
    <span class="special">&gt;</span>
<span class="special">{};</span>
</pre>
<p>
            The <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">terminal</span></code> type by itself isn't enough
            to select an appropriate sub-grammar, so we use <code class="computeroutput"><a class="link" href="../boost/proto/or_.html" title="Struct template or_">proto::or_&lt;&gt;</a></code>
            to list the alternate sub-grammars that match terminals.
          </p>
<div class="note"><table border="0" summary="Note">
<tr>
<td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../doc/src/images/note.png"></td>
<th align="left">Note</th>
</tr>
<tr><td align="left" valign="top">
<p>
              You might be tempted to define your <code class="computeroutput"><span class="identifier">case_</span><span class="special">&lt;&gt;</span></code> specializations <span class="emphasis"><em>in
              situ</em></span> as follows:
            </p>
<p>
</p>
<pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">ABigGrammarCases</span>
<span class="special">{</span>
    <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">Tag</span><span class="special">&gt;</span>
    <span class="keyword">struct</span> <span class="identifier">case_</span> <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">not_</span><span class="special">&lt;</span><span class="identifier">_</span><span class="special">&gt;</span> <span class="special">{};</span>

    <span class="comment">// ERROR: not legal C++</span>
    <span class="keyword">template</span><span class="special">&lt;&gt;</span>
    <span class="keyword">struct</span> <span class="identifier">case_</span><span class="special">&lt;</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&gt;</span>
      <span class="comment">/* ... */</span>
<span class="special">};</span>
</pre>
<p>
            </p>
<p>
              Unfortunately, for arcane reasons, it is not legal to define an explicit
              nested specialization <span class="emphasis"><em>in situ</em></span> like this. It is,
              however, perfectly legal to define <span class="emphasis"><em>partial</em></span> specializations
              <span class="emphasis"><em>in situ</em></span>, so you can add a extra dummy template
              parameter that has a default, as follows:
            </p>
<p>
</p>
<pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">ABigGrammarCases</span>
<span class="special">{</span>
    <span class="comment">// Note extra "Dummy" template parameter here:</span>
    <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">Tag</span><span class="special">,</span> <span class="keyword">int</span> <span class="identifier">Dummy</span> <span class="special">=</span> <span class="number">0</span><span class="special">&gt;</span>
    <span class="keyword">struct</span> <span class="identifier">case_</span> <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">not_</span><span class="special">&lt;</span><span class="identifier">_</span><span class="special">&gt;</span> <span class="special">{};</span>

    <span class="comment">// OK: "Dummy" makes this a partial specialization</span>
    <span class="comment">// instead of an explicit specialization.</span>
    <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">int</span> <span class="identifier">Dummy</span><span class="special">&gt;</span>
    <span class="keyword">struct</span> <span class="identifier">case_</span><span class="special">&lt;</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">,</span> <span class="identifier">Dummy</span><span class="special">&gt;</span>
      <span class="comment">/* ... */</span>
<span class="special">};</span>
</pre>
<p>
            </p>
<p>
              You might find this cleaner than defining explicit <code class="computeroutput"><span class="identifier">case_</span><span class="special">&lt;&gt;</span></code> specializations outside of
              their enclosing struct.
            </p>
</td></tr>
</table></div>
</div>
<div class="section">
<div class="titlepage"><div><div><h5 class="title">
<a name="boost_proto.users_guide.intermediate_form.expression_introspection.matching_vararg_expressions"></a><a class="link" href="users_guide.html#boost_proto.users_guide.intermediate_form.expression_introspection.matching_vararg_expressions" title="Matching Vararg Expressions">Matching
          Vararg Expressions</a>
</h5></div></div></div>
<p>
            Not all of C++'s overloadable operators are unary or binary. There is
            the oddball <code class="computeroutput"><span class="keyword">operator</span><span class="special">()</span></code>
            -- the function call operator -- which can have any number of arguments.
            Likewise, with Proto you may define your own "operators" that
            could also take more that two arguments. As a result, there may be nodes
            in your Proto expression tree that have an arbitrary number of children
            (up to <code class="literal"><code class="computeroutput"><a class="link" href="../BOOST_PROTO_MAX_ARITY.html" title="Macro BOOST_PROTO_MAX_ARITY">BOOST_PROTO_MAX_ARITY</a></code></code>,
            which is configurable). How do you write a grammar to match such a node?
          </p>
<p>
            For such cases, Proto provides the <code class="computeroutput"><a class="link" href="../boost/proto/vararg.html" title="Struct template vararg">proto::vararg&lt;&gt;</a></code>
            class template. Its template argument is a grammar, and the <code class="computeroutput"><a class="link" href="../boost/proto/vararg.html" title="Struct template vararg">proto::vararg&lt;&gt;</a></code> will match the grammar
            zero or more times. Consider a Proto lazy function called <code class="computeroutput"><span class="identifier">fun</span><span class="special">()</span></code>
            that can take zero or more characters as arguments, as follows:
          </p>
<pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">fun_tag</span> <span class="special">{};</span>
<span class="keyword">struct</span> <span class="identifier">FunTag</span> <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span> <span class="identifier">fun_tag</span> <span class="special">&gt;</span> <span class="special">{};</span>
<span class="identifier">FunTag</span><span class="special">::</span><span class="identifier">type</span> <span class="keyword">const</span> <span class="identifier">fun</span> <span class="special">=</span> <span class="special">{{}};</span>

<span class="comment">// example usage:</span>
<span class="identifier">fun</span><span class="special">();</span>
<span class="identifier">fun</span><span class="special">(</span><span class="char">'a'</span><span class="special">);</span>
<span class="identifier">fun</span><span class="special">(</span><span class="char">'a'</span><span class="special">,</span> <span class="char">'b'</span><span class="special">);</span>
<span class="special">...</span>
</pre>
<p>
            Below is the grammar that matches all the allowable invocations of <code class="computeroutput"><span class="identifier">fun</span><span class="special">()</span></code>:
          </p>
<pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">FunCall</span>
  <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">function</span><span class="special">&lt;</span> <span class="identifier">FunTag</span><span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">vararg</span><span class="special">&lt;</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span> <span class="keyword">char</span> <span class="special">&gt;</span> <span class="special">&gt;</span> <span class="special">&gt;</span>
<span class="special">{};</span>
</pre>
<p>
            The <code class="computeroutput"><span class="identifier">FunCall</span></code> grammar uses
            <code class="computeroutput"><a class="link" href="../boost/proto/vararg.html" title="Struct template vararg">proto::vararg&lt;&gt;</a></code> to match zero or
            more character literals as arguments of the <code class="computeroutput"><span class="identifier">fun</span><span class="special">()</span></code> function.
          </p>
<p>
            As another example, can you guess what the following grammar matches?
          </p>
<pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">Foo</span>
  <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">or_</span><span class="special">&lt;</span>
        <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_</span> <span class="special">&gt;</span>
      <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">nary_expr</span><span class="special">&lt;</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_</span><span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">vararg</span><span class="special">&lt;</span> <span class="identifier">Foo</span> <span class="special">&gt;</span> <span class="special">&gt;</span>
    <span class="special">&gt;</span>
<span class="special">{};</span>
</pre>
<p>
            Here's a hint: the first template parameter to <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">nary_expr</span><span class="special">&lt;&gt;</span></code> represents the node type, and
            any additional template parameters represent child nodes. The answer
            is that this is a degenerate grammar that matches every possible expression
            tree, from root to leaves.
          </p>
</div>
<div class="section">
<div class="titlepage"><div><div><h5 class="title">
<a name="boost_proto.users_guide.intermediate_form.expression_introspection.defining_edsl_grammars"></a><a class="link" href="users_guide.html#boost_proto.users_guide.intermediate_form.expression_introspection.defining_edsl_grammars" title="Defining EDSL Grammars">Defining
          EDSL Grammars</a>
</h5></div></div></div>
<p>
            In this section we'll see how to use Proto to define a grammar for your
            EDSL and use it to validate expression templates, giving short, readable
            compile-time errors for invalid expressions.
          </p>
<div class="tip"><table border="0" summary="Tip">
<tr>
<td rowspan="2" align="center" valign="top" width="25"><img alt="[Tip]" src="../../../doc/src/images/tip.png"></td>
<th align="left">Tip</th>
</tr>
<tr><td align="left" valign="top">
<p>
              You might think that this is a backwards way of doing things. <span class="quote">&#8220;<span class="quote">If
              Proto let me select which operators to overload, my users wouldn't
              be able to create invalid expressions in the first place, and I wouldn't
              need a grammar at all!</span>&#8221;</span> That may be true, but there are reasons
              for preferring to do things this way.
            </p>
<p>
              First, it lets you develop your EDSL rapidly -- all the operators are
              there for you already! -- and worry about invalid syntax later.
            </p>
<p>
              Second, it might be the case that some operators are only allowed in
              certain contexts within your EDSL. This is easy to express with a grammar,
              and hard to do with straight operator overloading.
            </p>
<p>
              Third, using an EDSL grammar to flag invalid expressions can often
              yield better errors than manually selecting the overloaded operators.
            </p>
<p>
              Fourth, the grammar can be used for more than just validation. You
              can use your grammar to define <span class="emphasis"><em>tree transformations</em></span>
              that convert expression templates into other more useful objects.
            </p>
<p>
              If none of the above convinces you, you actually <span class="emphasis"><em>can</em></span>
              use Proto to control which operators are overloaded within your domain.
              And to do it, you need to define a grammar!
            </p>
</td></tr>
</table></div>
<p>
            In a previous section, we used Proto to define an EDSL for a lazily evaluated
            calculator that allowed any combination of placeholders, floating-point
            literals, addition, subtraction, multiplication, division and grouping.
            If we were to write the grammar for this EDSL in <a href="http://en.wikipedia.org/wiki/Extended_Backus_Naur_Form" target="_top">EBNF</a>,
            it might look like this:
          </p>
<pre class="programlisting">group       ::= '(' expression ')'
factor      ::= double | '_1' | '_2' | group
term        ::= factor (('*' factor) | ('/' factor))*
expression  ::= term (('+' term) | ('-' term))*
</pre>
<p>
            This captures the syntax, associativity and precedence rules of a calculator.
            Writing the grammar for our calculator EDSL using Proto is <span class="emphasis"><em>even
            simpler</em></span>. Since we are using C++ as the host language, we are
            bound to the associativity and precedence rules for the C++ operators.
            Our grammar can assume them. Also, in C++ grouping is already handled
            for us with the use of parenthesis, so we don't have to code that into
            our grammar.
          </p>
<p>
            Let's begin our grammar for forward-declaring it:
          </p>
<pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">CalculatorGrammar</span><span class="special">;</span>
</pre>
<p>
            It's an incomplete type at this point, but we'll still be able to use
            it to define the rules of our grammar. Let's define grammar rules for
            the terminals:
          </p>
<pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">Double</span>
  <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">convertible_to</span><span class="special">&lt;</span> <span class="keyword">double</span> <span class="special">&gt;</span> <span class="special">&gt;</span>
<span class="special">{};</span>

<span class="keyword">struct</span> <span class="identifier">Placeholder1</span>
  <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span> <span class="identifier">placeholder</span><span class="special">&lt;</span><span class="number">0</span><span class="special">&gt;</span> <span class="special">&gt;</span>
<span class="special">{};</span>

<span class="keyword">struct</span> <span class="identifier">Placeholder2</span>
  <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span> <span class="identifier">placeholder</span><span class="special">&lt;</span><span class="number">1</span><span class="special">&gt;</span> <span class="special">&gt;</span>
<span class="special">{};</span>

<span class="keyword">struct</span> <span class="identifier">Terminal</span>
  <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">or_</span><span class="special">&lt;</span> <span class="identifier">Double</span><span class="special">,</span> <span class="identifier">Placeholder1</span><span class="special">,</span> <span class="identifier">Placeholder2</span> <span class="special">&gt;</span>
<span class="special">{};</span>
</pre>
<p>
            Now let's define the rules for addition, subtraction, multiplication
            and division. Here, we can ignore issues of associativity and precedence
            -- the C++ compiler will enforce that for us. We only must enforce that
            the arguments to the operators must themselves conform to the <code class="computeroutput"><span class="identifier">CalculatorGrammar</span></code> that we forward-declared
            above.
          </p>
<pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">Plus</span>
  <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">plus</span><span class="special">&lt;</span> <span class="identifier">CalculatorGrammar</span><span class="special">,</span> <span class="identifier">CalculatorGrammar</span> <span class="special">&gt;</span>
<span class="special">{};</span>

<span class="keyword">struct</span> <span class="identifier">Minus</span>
  <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">minus</span><span class="special">&lt;</span> <span class="identifier">CalculatorGrammar</span><span class="special">,</span> <span class="identifier">CalculatorGrammar</span> <span class="special">&gt;</span>
<span class="special">{};</span>

<span class="keyword">struct</span> <span class="identifier">Multiplies</span>
  <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">multiplies</span><span class="special">&lt;</span> <span class="identifier">CalculatorGrammar</span><span class="special">,</span> <span class="identifier">CalculatorGrammar</span> <span class="special">&gt;</span>
<span class="special">{};</span>

<span class="keyword">struct</span> <span class="identifier">Divides</span>
  <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">divides</span><span class="special">&lt;</span> <span class="identifier">CalculatorGrammar</span><span class="special">,</span> <span class="identifier">CalculatorGrammar</span> <span class="special">&gt;</span>
<span class="special">{};</span>
</pre>
<p>
            Now that we've defined all the parts of the grammar, we can define <code class="computeroutput"><span class="identifier">CalculatorGrammar</span></code>:
          </p>
<pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">CalculatorGrammar</span>
  <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">or_</span><span class="special">&lt;</span>
        <span class="identifier">Terminal</span>
      <span class="special">,</span> <span class="identifier">Plus</span>
      <span class="special">,</span> <span class="identifier">Minus</span>
      <span class="special">,</span> <span class="identifier">Multiplies</span>
      <span class="special">,</span> <span class="identifier">Divides</span>
    <span class="special">&gt;</span>
<span class="special">{};</span>
</pre>
<p>
            That's it! Now we can use <code class="computeroutput"><span class="identifier">CalculatorGrammar</span></code>
            to enforce that an expression template conforms to our grammar. We can
            use <code class="computeroutput"><a class="link" href="../boost/proto/matches.html" title="Struct template matches">proto::matches&lt;&gt;</a></code> and <code class="computeroutput"><span class="identifier">BOOST_MPL_ASSERT</span><span class="special">()</span></code>
            to issue readable compile-time errors for invalid expressions, as below:
          </p>
<pre class="programlisting"><span class="keyword">template</span><span class="special">&lt;</span> <span class="keyword">typename</span> <span class="identifier">Expr</span> <span class="special">&gt;</span>
<span class="keyword">void</span> <span class="identifier">evaluate</span><span class="special">(</span> <span class="identifier">Expr</span> <span class="keyword">const</span> <span class="special">&amp;</span> <span class="identifier">expr</span> <span class="special">)</span>
<span class="special">{</span>
    <span class="identifier">BOOST_MPL_ASSERT</span><span class="special">((</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">matches</span><span class="special">&lt;</span> <span class="identifier">Expr</span><span class="special">,</span> <span class="identifier">CalculatorGrammar</span> <span class="special">&gt;</span> <span class="special">));</span>
    <span class="comment">// ...</span>
<span class="special">}</span>
</pre>
</div>
</div>
</div>
<div class="section">
<div class="titlepage"><div><div><h3 class="title">
<a name="boost_proto.users_guide.back_end"></a><a class="link" href="users_guide.html#boost_proto.users_guide.back_end" title="Back Ends: Making Expression Templates Do Useful Work">Back Ends: Making Expression
      Templates Do Useful Work</a>
</h3></div></div></div>
<div class="toc"><dl class="toc">
<dt><span class="section"><a href="users_guide.html#boost_proto.users_guide.back_end.expression_evaluation">Expression
        Evaluation: Imparting Behaviors with a Context</a></span></dt>
<dt><span class="section"><a href="users_guide.html#boost_proto.users_guide.back_end.expression_transformation">Expression
        Transformation: Semantic Actions</a></span></dt>
</dl></div>
<p>
        Now that you've written the front end for your EDSL compiler, and you've
        learned a bit about the intermediate form it produces, it's time to think
        about what to <span class="emphasis"><em>do</em></span> with the intermediate form. This is
        where you put your domain-specific algorithms and optimizations. Proto gives
        you two ways to evaluate and manipulate expression templates: contexts and
        transforms.
      </p>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
<li class="listitem">
            A <span class="emphasis"><em>context</em></span> is like a function object that you pass
            along with an expression to the <code class="computeroutput"><a class="link" href="../boost/proto/eval_idp727214672.html" title="Function eval">proto::eval()</a></code>
            function. It associates behaviors with node types. <code class="computeroutput"><a class="link" href="../boost/proto/eval_idp727214672.html" title="Function eval">proto::eval()</a></code>
            walks the expression and invokes your context at each node.
          </li>
<li class="listitem">
            A <span class="emphasis"><em>transform</em></span> is a way to associate behaviors, not
            with node types in an expression, but with rules in a Proto grammar.
            In this way, they are like semantic actions in other compiler-construction
            toolkits.
          </li>
</ul></div>
<p>
        Two ways to evaluate expressions! How to choose? Since contexts are largely
        procedural, they are a bit simpler to understand and debug so they are a
        good place to start. But although transforms are more advanced, they are
        also more powerful; since they are associated with rules in your grammar,
        you can select the proper transform based on the entire <span class="emphasis"><em>structure</em></span>
        of a sub-expression rather than simply on the type of its top-most node.
      </p>
<p>
        Also, transforms have a concise and declarative syntax that can be confusing
        at first, but highly expressive and fungible once you become accustomed to
        it. And -- this is admittedly very subjective -- the author finds programming
        with Proto transforms to be an inordinate amount of <span class="emphasis"><em>fun!</em></span>
        Your mileage may vary.
      </p>
<div class="section">
<div class="titlepage"><div><div><h4 class="title">
<a name="boost_proto.users_guide.back_end.expression_evaluation"></a><a class="link" href="users_guide.html#boost_proto.users_guide.back_end.expression_evaluation" title="Expression Evaluation: Imparting Behaviors with a Context">Expression
        Evaluation: Imparting Behaviors with a Context</a>
</h4></div></div></div>
<div class="toc"><dl class="toc">
<dt><span class="section"><a href="users_guide.html#boost_proto.users_guide.back_end.expression_evaluation.proto_eval">Evaluating
          an Expression with <code class="literal">proto::eval()</code></a></span></dt>
<dt><span class="section"><a href="users_guide.html#boost_proto.users_guide.back_end.expression_evaluation.contexts">Defining
          an Evaluation Context</a></span></dt>
<dt><span class="section"><a href="users_guide.html#boost_proto.users_guide.back_end.expression_evaluation.canned_contexts">Proto's
          Built-In Contexts</a></span></dt>
</dl></div>
<p>
          Once you have constructed a Proto expression tree, either by using Proto's
          operator overloads or with <code class="computeroutput"><a class="link" href="../boost/proto/make_expr_idp728389200.html" title="Function make_expr">proto::make_expr()</a></code>
          and friends, you probably want to actually <span class="emphasis"><em>do</em></span> something
          with it. The simplest option is to use <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">eval</span><span class="special">()</span></code>, a generic expression evaluator. To use
          <code class="computeroutput"><a class="link" href="../boost/proto/eval_idp727214672.html" title="Function eval">proto::eval()</a></code>, you'll need to define
          a <span class="emphasis"><em>context</em></span> that tells <code class="computeroutput"><a class="link" href="../boost/proto/eval_idp727214672.html" title="Function eval">proto::eval()</a></code>
          how each node should be evaluated. This section goes through the nuts and
          bolts of using <code class="computeroutput"><a class="link" href="../boost/proto/eval_idp727214672.html" title="Function eval">proto::eval()</a></code>, defining evaluation contexts,
          and using the contexts that Proto provides.
        </p>
<div class="note"><table border="0" summary="Note">
<tr>
<td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../doc/src/images/note.png"></td>
<th align="left">Note</th>
</tr>
<tr><td align="left" valign="top"><p>
            <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">eval</span><span class="special">()</span></code>
            is a less powerful but easier-to-use evaluation technique than Proto
            transforms, which are covered later. Although very powerful, transforms
            have a steep learning curve and can be more difficult to debug. <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">eval</span><span class="special">()</span></code>
            is a rather weak tree traversal algorithm. Dan Marsden has been working
            on a more general and powerful tree traversal library. When it is ready,
            I anticipate that it will eliminate the need for <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">eval</span><span class="special">()</span></code>.
          </p></td></tr>
</table></div>
<div class="section">
<div class="titlepage"><div><div><h5 class="title">
<a name="boost_proto.users_guide.back_end.expression_evaluation.proto_eval"></a><a class="link" href="users_guide.html#boost_proto.users_guide.back_end.expression_evaluation.proto_eval" title="Evaluating an Expression with proto::eval()">Evaluating
          an Expression with <code class="literal">proto::eval()</code></a>
</h5></div></div></div>
<div class="blockquote"><blockquote class="blockquote"><p>
              <span class="bold"><strong>Synopsis:</strong></span>
            </p></blockquote></div>
<pre class="programlisting"><span class="keyword">namespace</span> <span class="identifier">proto</span>
<span class="special">{</span>
    <span class="keyword">namespace</span> <span class="identifier">result_of</span>
    <span class="special">{</span>
        <span class="comment">// A metafunction for calculating the return</span>
        <span class="comment">// type of proto::eval() given certain Expr</span>
        <span class="comment">// and Context types.</span>
        <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">Expr</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">Context</span><span class="special">&gt;</span>
        <span class="keyword">struct</span> <span class="identifier">eval</span>
        <span class="special">{</span>
            <span class="keyword">typedef</span>
                <span class="keyword">typename</span> <span class="identifier">Context</span><span class="special">::</span><span class="keyword">template</span> <span class="identifier">eval</span><span class="special">&lt;</span><span class="identifier">Expr</span><span class="special">&gt;::</span><span class="identifier">result_type</span>
            <span class="identifier">type</span><span class="special">;</span>
        <span class="special">};</span>
    <span class="special">}</span>

    <span class="keyword">namespace</span> <span class="identifier">functional</span>
    <span class="special">{</span>
        <span class="comment">// A callable function object type for evaluating</span>
        <span class="comment">// a Proto expression with a certain context.</span>
        <span class="keyword">struct</span> <span class="identifier">eval</span> <span class="special">:</span> <span class="identifier">callable</span>
        <span class="special">{</span>
            <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">Sig</span><span class="special">&gt;</span>
            <span class="keyword">struct</span> <span class="identifier">result</span><span class="special">;</span>

            <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">Expr</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">Context</span><span class="special">&gt;</span>
            <span class="keyword">typename</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">result_of</span><span class="special">::</span><span class="identifier">eval</span><span class="special">&lt;</span><span class="identifier">Expr</span><span class="special">,</span> <span class="identifier">Context</span><span class="special">&gt;::</span><span class="identifier">type</span>
            <span class="keyword">operator</span> <span class="special">()(</span><span class="identifier">Expr</span> <span class="special">&amp;</span><span class="identifier">expr</span><span class="special">,</span> <span class="identifier">Context</span> <span class="special">&amp;</span><span class="identifier">context</span><span class="special">)</span> <span class="keyword">const</span><span class="special">;</span>

            <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">Expr</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">Context</span><span class="special">&gt;</span>
            <span class="keyword">typename</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">result_of</span><span class="special">::</span><span class="identifier">eval</span><span class="special">&lt;</span><span class="identifier">Expr</span><span class="special">,</span> <span class="identifier">Context</span><span class="special">&gt;::</span><span class="identifier">type</span>
            <span class="keyword">operator</span> <span class="special">()(</span><span class="identifier">Expr</span> <span class="special">&amp;</span><span class="identifier">expr</span><span class="special">,</span> <span class="identifier">Context</span> <span class="keyword">const</span> <span class="special">&amp;</span><span class="identifier">context</span><span class="special">)</span> <span class="keyword">const</span><span class="special">;</span>
        <span class="special">};</span>
    <span class="special">}</span>

    <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">Expr</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">Context</span><span class="special">&gt;</span>
    <span class="keyword">typename</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">result_of</span><span class="special">::</span><span class="identifier">eval</span><span class="special">&lt;</span><span class="identifier">Expr</span><span class="special">,</span> <span class="identifier">Context</span><span class="special">&gt;::</span><span class="identifier">type</span>
    <span class="identifier">eval</span><span class="special">(</span><span class="identifier">Expr</span> <span class="special">&amp;</span><span class="identifier">expr</span><span class="special">,</span> <span class="identifier">Context</span> <span class="special">&amp;</span><span class="identifier">context</span><span class="special">);</span>

    <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">Expr</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">Context</span><span class="special">&gt;</span>
    <span class="keyword">typename</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">result_of</span><span class="special">::</span><span class="identifier">eval</span><span class="special">&lt;</span><span class="identifier">Expr</span><span class="special">,</span> <span class="identifier">Context</span><span class="special">&gt;::</span><span class="identifier">type</span>
    <span class="identifier">eval</span><span class="special">(</span><span class="identifier">Expr</span> <span class="special">&amp;</span><span class="identifier">expr</span><span class="special">,</span> <span class="identifier">Context</span> <span class="keyword">const</span> <span class="special">&amp;</span><span class="identifier">context</span><span class="special">);</span>
<span class="special">}</span>
</pre>
<p>
            Given an expression and an evaluation context, using <code class="computeroutput"><a class="link" href="../boost/proto/eval_idp727214672.html" title="Function eval">proto::eval()</a></code>
            is quite simple. Simply pass the expression and the context to <code class="computeroutput"><a class="link" href="../boost/proto/eval_idp727214672.html" title="Function eval">proto::eval()</a></code> and it does the rest
            and returns the result. You can use the <code class="computeroutput"><span class="identifier">eval</span><span class="special">&lt;&gt;</span></code> metafunction in the <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">result_of</span></code> namespace to compute the
            return type of <code class="computeroutput"><a class="link" href="../boost/proto/eval_idp727214672.html" title="Function eval">proto::eval()</a></code>. The following demonstrates
            a use of <code class="computeroutput"><a class="link" href="../boost/proto/eval_idp727214672.html" title="Function eval">proto::eval()</a></code>:
          </p>
<pre class="programlisting"><span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">Expr</span><span class="special">&gt;</span>
<span class="keyword">typename</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">result_of</span><span class="special">::</span><span class="identifier">eval</span><span class="special">&lt;</span><span class="identifier">Expr</span> <span class="keyword">const</span><span class="special">,</span> <span class="identifier">MyContext</span><span class="special">&gt;::</span><span class="identifier">type</span>
<span class="identifier">MyEvaluate</span><span class="special">(</span><span class="identifier">Expr</span> <span class="keyword">const</span> <span class="special">&amp;</span><span class="identifier">expr</span><span class="special">)</span>
<span class="special">{</span>
    <span class="comment">// Some user-defined context type</span>
    <span class="identifier">MyContext</span> <span class="identifier">ctx</span><span class="special">;</span>

    <span class="comment">// Evaluate an expression with the context</span>
    <span class="keyword">return</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">eval</span><span class="special">(</span><span class="identifier">expr</span><span class="special">,</span> <span class="identifier">ctx</span><span class="special">);</span>
<span class="special">}</span>
</pre>
<p>
            What <code class="computeroutput"><a class="link" href="../boost/proto/eval_idp727214672.html" title="Function eval">proto::eval()</a></code> does is also very simple.
            It defers most of the work to the context itself. Here essentially is
            the implementation of <code class="computeroutput"><a class="link" href="../boost/proto/eval_idp727214672.html" title="Function eval">proto::eval()</a></code>:
          </p>
<pre class="programlisting"><span class="comment">// eval() dispatches to a nested "eval&lt;&gt;" function</span>
<span class="comment">// object within the Context:</span>
<span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">Expr</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">Context</span><span class="special">&gt;</span>
<span class="keyword">typename</span> <span class="identifier">Context</span><span class="special">::</span><span class="keyword">template</span> <span class="identifier">eval</span><span class="special">&lt;</span><span class="identifier">Expr</span><span class="special">&gt;::</span><span class="identifier">result_type</span>
<span class="identifier">eval</span><span class="special">(</span><span class="identifier">Expr</span> <span class="special">&amp;</span><span class="identifier">expr</span><span class="special">,</span> <span class="identifier">Context</span> <span class="special">&amp;</span><span class="identifier">ctx</span><span class="special">)</span>
<span class="special">{</span>
    <span class="keyword">typename</span> <span class="identifier">Context</span><span class="special">::</span><span class="keyword">template</span> <span class="identifier">eval</span><span class="special">&lt;</span><span class="identifier">Expr</span><span class="special">&gt;</span> <span class="identifier">eval_fun</span><span class="special">;</span>
    <span class="keyword">return</span> <span class="identifier">eval_fun</span><span class="special">(</span><span class="identifier">expr</span><span class="special">,</span> <span class="identifier">ctx</span><span class="special">);</span>
<span class="special">}</span>
</pre>
<p>
            Really, <code class="computeroutput"><a class="link" href="../boost/proto/eval_idp727214672.html" title="Function eval">proto::eval()</a></code> is nothing more than
            a thin wrapper that dispatches to the appropriate handler within the
            context class. In the next section, we'll see how to implement a context
            class from scratch.
          </p>
</div>
<div class="section">
<div class="titlepage"><div><div><h5 class="title">
<a name="boost_proto.users_guide.back_end.expression_evaluation.contexts"></a><a class="link" href="users_guide.html#boost_proto.users_guide.back_end.expression_evaluation.contexts" title="Defining an Evaluation Context">Defining
          an Evaluation Context</a>
</h5></div></div></div>
<p>
            As we saw in the previous section, there is really not much to the <code class="computeroutput"><a class="link" href="../boost/proto/eval_idp727214672.html" title="Function eval">proto::eval()</a></code> function. Rather, all
            the interesting expression evaluation goes on within a context class.
            This section shows how to implement one from scratch.
          </p>
<p>
            All context classes have roughly the following form:
          </p>
<pre class="programlisting"><span class="comment">// A prototypical user-defined context.</span>
<span class="keyword">struct</span> <span class="identifier">MyContext</span>
<span class="special">{</span>
    <span class="comment">// A nested eval&lt;&gt; class template</span>
    <span class="keyword">template</span><span class="special">&lt;</span>
        <span class="keyword">typename</span> <span class="identifier">Expr</span>
      <span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">Tag</span> <span class="special">=</span> <span class="keyword">typename</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag_of</span><span class="special">&lt;</span><span class="identifier">Expr</span><span class="special">&gt;::</span><span class="identifier">type</span>
    <span class="special">&gt;</span>
    <span class="keyword">struct</span> <span class="identifier">eval</span><span class="special">;</span>

    <span class="comment">// Handle terminal nodes here...</span>
    <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">Expr</span><span class="special">&gt;</span>
    <span class="keyword">struct</span> <span class="identifier">eval</span><span class="special">&lt;</span><span class="identifier">Expr</span><span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&gt;</span>
    <span class="special">{</span>
        <span class="comment">// Must have a nested result_type typedef.</span>
        <span class="keyword">typedef</span> <span class="special">...</span> <span class="identifier">result_type</span><span class="special">;</span>

        <span class="comment">// Must have a function call operator that takes</span>
        <span class="comment">// an expression and the context.</span>
        <span class="identifier">result_type</span> <span class="keyword">operator</span><span class="special">()(</span><span class="identifier">Expr</span> <span class="special">&amp;</span><span class="identifier">expr</span><span class="special">,</span> <span class="identifier">MyContext</span> <span class="special">&amp;</span><span class="identifier">ctx</span><span class="special">)</span> <span class="keyword">const</span>
        <span class="special">{</span>
            <span class="keyword">return</span> <span class="special">...;</span>
        <span class="special">}</span>
    <span class="special">};</span>

    <span class="comment">// ... other specializations of struct eval&lt;&gt; ...</span>
<span class="special">};</span>
</pre>
<p>
            Context classes are nothing more than a collection of specializations
            of a nested <code class="computeroutput"><span class="identifier">eval</span><span class="special">&lt;&gt;</span></code>
            class template. Each specialization handles a different expression type.
          </p>
<p>
            In the <a class="link" href="users_guide.html#boost_proto.users_guide.getting_started.hello_calculator" title="Hello Calculator">Hello
            Calculator</a> section, we saw an example of a user-defined context
            class for evaluating calculator expressions. That context class was implemented
            with the help of Proto's <code class="computeroutput"><a class="link" href="../boost/proto/context/callable_context.html" title="Struct template callable_context">proto::callable_context&lt;&gt;</a></code>.
            If we were to implement it from scratch, it would look something like
            this:
          </p>
<pre class="programlisting"><span class="comment">// The calculator_context from the "Hello Calculator" section,</span>
<span class="comment">// implemented from scratch.</span>
<span class="keyword">struct</span> <span class="identifier">calculator_context</span>
<span class="special">{</span>
    <span class="comment">// The values with which we'll replace the placeholders</span>
    <span class="identifier">std</span><span class="special">::</span><span class="identifier">vector</span><span class="special">&lt;</span><span class="keyword">double</span><span class="special">&gt;</span> <span class="identifier">args</span><span class="special">;</span>

    <span class="keyword">template</span><span class="special">&lt;</span>
        <span class="keyword">typename</span> <span class="identifier">Expr</span>
        <span class="comment">// defaulted template parameters, so we can</span>
        <span class="comment">// specialize on the expressions that need</span>
        <span class="comment">// special handling.</span>
      <span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">Tag</span> <span class="special">=</span> <span class="keyword">typename</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag_of</span><span class="special">&lt;</span><span class="identifier">Expr</span><span class="special">&gt;::</span><span class="identifier">type</span>
      <span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">Arg0</span> <span class="special">=</span> <span class="keyword">typename</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">child_c</span><span class="special">&lt;</span><span class="identifier">Expr</span><span class="special">,</span> <span class="number">0</span><span class="special">&gt;::</span><span class="identifier">type</span>
    <span class="special">&gt;</span>
    <span class="keyword">struct</span> <span class="identifier">eval</span><span class="special">;</span>

    <span class="comment">// Handle placeholder terminals here...</span>
    <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">Expr</span><span class="special">,</span> <span class="keyword">int</span> <span class="identifier">I</span><span class="special">&gt;</span>
    <span class="keyword">struct</span> <span class="identifier">eval</span><span class="special">&lt;</span><span class="identifier">Expr</span><span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">,</span> <span class="identifier">placeholder</span><span class="special">&lt;</span><span class="identifier">I</span><span class="special">&gt;</span> <span class="special">&gt;</span>
    <span class="special">{</span>
        <span class="keyword">typedef</span> <span class="keyword">double</span> <span class="identifier">result_type</span><span class="special">;</span>

        <span class="identifier">result_type</span> <span class="keyword">operator</span><span class="special">()(</span><span class="identifier">Expr</span> <span class="special">&amp;,</span> <span class="identifier">MyContext</span> <span class="special">&amp;</span><span class="identifier">ctx</span><span class="special">)</span> <span class="keyword">const</span>
        <span class="special">{</span>
            <span class="keyword">return</span> <span class="identifier">ctx</span><span class="special">.</span><span class="identifier">args</span><span class="special">[</span><span class="identifier">I</span><span class="special">];</span>
        <span class="special">}</span>
    <span class="special">};</span>

    <span class="comment">// Handle other terminals here...</span>
    <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">Expr</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">Arg0</span><span class="special">&gt;</span>
    <span class="keyword">struct</span> <span class="identifier">eval</span><span class="special">&lt;</span><span class="identifier">Expr</span><span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">,</span> <span class="identifier">Arg0</span><span class="special">&gt;</span>
    <span class="special">{</span>
        <span class="keyword">typedef</span> <span class="keyword">double</span> <span class="identifier">result_type</span><span class="special">;</span>

        <span class="identifier">result_type</span> <span class="keyword">operator</span><span class="special">()(</span><span class="identifier">Expr</span> <span class="special">&amp;</span><span class="identifier">expr</span><span class="special">,</span> <span class="identifier">MyContext</span> <span class="special">&amp;)</span> <span class="keyword">const</span>
        <span class="special">{</span>
            <span class="keyword">return</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">child</span><span class="special">(</span><span class="identifier">expr</span><span class="special">);</span>
        <span class="special">}</span>
    <span class="special">};</span>

    <span class="comment">// Handle addition here...</span>
    <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">Expr</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">Arg0</span><span class="special">&gt;</span>
    <span class="keyword">struct</span> <span class="identifier">eval</span><span class="special">&lt;</span><span class="identifier">Expr</span><span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">plus</span><span class="special">,</span> <span class="identifier">Arg0</span><span class="special">&gt;</span>
    <span class="special">{</span>
        <span class="keyword">typedef</span> <span class="keyword">double</span> <span class="identifier">result_type</span><span class="special">;</span>

        <span class="identifier">result_type</span> <span class="keyword">operator</span><span class="special">()(</span><span class="identifier">Expr</span> <span class="special">&amp;</span><span class="identifier">expr</span><span class="special">,</span> <span class="identifier">MyContext</span> <span class="special">&amp;</span><span class="identifier">ctx</span><span class="special">)</span> <span class="keyword">const</span>
        <span class="special">{</span>
            <span class="keyword">return</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">eval</span><span class="special">(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">left</span><span class="special">(</span><span class="identifier">expr</span><span class="special">),</span> <span class="identifier">ctx</span><span class="special">)</span>
                 <span class="special">+</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">eval</span><span class="special">(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">right</span><span class="special">(</span><span class="identifier">expr</span><span class="special">),</span> <span class="identifier">ctx</span><span class="special">);</span>
        <span class="special">}</span>
    <span class="special">};</span>

    <span class="comment">// ... other eval&lt;&gt; specializations for other node types ...</span>
<span class="special">};</span>
</pre>
<p>
            Now we can use <code class="computeroutput"><a class="link" href="../boost/proto/eval_idp727214672.html" title="Function eval">proto::eval()</a></code> with the context class
            above to evaluate calculator expressions as follows:
          </p>
<pre class="programlisting"><span class="comment">// Evaluate an expression with a calculator_context</span>
<span class="identifier">calculator_context</span> <span class="identifier">ctx</span><span class="special">;</span>
<span class="identifier">ctx</span><span class="special">.</span><span class="identifier">args</span><span class="special">.</span><span class="identifier">push_back</span><span class="special">(</span><span class="number">5</span><span class="special">);</span>
<span class="identifier">ctx</span><span class="special">.</span><span class="identifier">args</span><span class="special">.</span><span class="identifier">push_back</span><span class="special">(</span><span class="number">6</span><span class="special">);</span>
<span class="keyword">double</span> <span class="identifier">d</span> <span class="special">=</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">eval</span><span class="special">(</span><span class="identifier">_1</span> <span class="special">+</span> <span class="identifier">_2</span><span class="special">,</span> <span class="identifier">ctx</span><span class="special">);</span>
<span class="identifier">assert</span><span class="special">(</span><span class="number">11</span> <span class="special">==</span> <span class="identifier">d</span><span class="special">);</span>
</pre>
<p>
            Defining a context from scratch this way is tedious and verbose, but
            it gives you complete control over how the expression is evaluated. The
            context class in the <a class="link" href="users_guide.html#boost_proto.users_guide.getting_started.hello_calculator" title="Hello Calculator">Hello
            Calculator</a> example was much simpler. In the next section we'll
            see the helper class Proto provides to ease the job of implementing context
            classes.
          </p>
</div>
<div class="section">
<div class="titlepage"><div><div><h5 class="title">
<a name="boost_proto.users_guide.back_end.expression_evaluation.canned_contexts"></a><a class="link" href="users_guide.html#boost_proto.users_guide.back_end.expression_evaluation.canned_contexts" title="Proto's Built-In Contexts">Proto's
          Built-In Contexts</a>
</h5></div></div></div>
<p>
            Proto provides some ready-made context classes that you can use as-is,
            or that you can use to help while implementing your own contexts. They
            are:
          </p>
<div class="variablelist">
<p class="title"><b></b></p>
<dl class="variablelist">
<dt><span class="term"><a class="link" href="users_guide.html#boost_proto.users_guide.back_end.expression_evaluation.canned_contexts.default_context" title="default_context"><code class="literal">default_context</code></a></span></dt>
<dd><p>
                  An evaluation context that assigns the usual C++ meanings to all
                  the operators. For example, addition nodes are handled by evaluating
                  the left and right children and then adding the results. The <code class="computeroutput"><a class="link" href="../boost/proto/context/default_context.html" title="Struct default_context">proto::default_context</a></code>
                  uses Boost.Typeof to deduce the types of the expressions it evaluates.
                </p></dd>
<dt><span class="term"><a class="link" href="users_guide.html#boost_proto.users_guide.back_end.expression_evaluation.canned_contexts.null_context" title="null_context"><code class="literal">null_context</code></a></span></dt>
<dd><p>
                  A simple context that recursively evaluates children but does not
                  combine the results in any way and returns void.
                </p></dd>
<dt><span class="term"><a class="link" href="users_guide.html#boost_proto.users_guide.back_end.expression_evaluation.canned_contexts.callable_context" title="callable_context&lt;&gt;"><code class="literal">callable_context&lt;&gt;</code></a></span></dt>
<dd><p>
                  A helper that simplifies the job of writing context classes. Rather
                  than writing template specializations, with <code class="computeroutput"><a class="link" href="../boost/proto/context/callable_context.html" title="Struct template callable_context">proto::callable_context&lt;&gt;</a></code>
                  you write a function object with an overloaded function call operator.
                  Any expressions not handled by an overload are automatically dispatched
                  to a default evaluation context that you can specify.
                </p></dd>
</dl>
</div>
<div class="section">
<div class="titlepage"><div><div><h6 class="title">
<a name="boost_proto.users_guide.back_end.expression_evaluation.canned_contexts.default_context"></a><a class="link" href="users_guide.html#boost_proto.users_guide.back_end.expression_evaluation.canned_contexts.default_context" title="default_context"><code class="literal">default_context</code></a>
</h6></div></div></div>
<p>
              The <code class="computeroutput"><a class="link" href="../boost/proto/context/default_context.html" title="Struct default_context">proto::default_context</a></code> is an
              evaluation context that assigns the usual C++ meanings to all the operators.
              For example, addition nodes are handled by evaluating the left and
              right children and then adding the results. The <code class="computeroutput"><a class="link" href="../boost/proto/context/default_context.html" title="Struct default_context">proto::default_context</a></code> uses
              Boost.Typeof to deduce the types of the expressions it evaluates.
            </p>
<p>
              For example, consider the following "Hello World" example:
            </p>
<pre class="programlisting"><span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">iostream</span><span class="special">&gt;</span>
<span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">proto</span><span class="special">/</span><span class="identifier">proto</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
<span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">proto</span><span class="special">/</span><span class="identifier">context</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
<span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">typeof</span><span class="special">/</span><span class="identifier">std</span><span class="special">/</span><span class="identifier">ostream</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
<span class="keyword">using</span> <span class="keyword">namespace</span> <span class="identifier">boost</span><span class="special">;</span>

<span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">ostream</span> <span class="special">&amp;</span> <span class="special">&gt;::</span><span class="identifier">type</span> <span class="identifier">cout_</span> <span class="special">=</span> <span class="special">{</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">};</span>

<span class="keyword">template</span><span class="special">&lt;</span> <span class="keyword">typename</span> <span class="identifier">Expr</span> <span class="special">&gt;</span>
<span class="keyword">void</span> <span class="identifier">evaluate</span><span class="special">(</span> <span class="identifier">Expr</span> <span class="keyword">const</span> <span class="special">&amp;</span> <span class="identifier">expr</span> <span class="special">)</span>
<span class="special">{</span>
    <span class="comment">// Evaluate the expression with default_context,</span>
    <span class="comment">// to give the operators their C++ meanings:</span>
    <span class="identifier">proto</span><span class="special">::</span><span class="identifier">default_context</span> <span class="identifier">ctx</span><span class="special">;</span>
    <span class="identifier">proto</span><span class="special">::</span><span class="identifier">eval</span><span class="special">(</span><span class="identifier">expr</span><span class="special">,</span> <span class="identifier">ctx</span><span class="special">);</span>
<span class="special">}</span>

<span class="keyword">int</span> <span class="identifier">main</span><span class="special">()</span>
<span class="special">{</span>
    <span class="identifier">evaluate</span><span class="special">(</span> <span class="identifier">cout_</span> <span class="special">&lt;&lt;</span> <span class="string">"hello"</span> <span class="special">&lt;&lt;</span> <span class="char">','</span> <span class="special">&lt;&lt;</span> <span class="string">" world"</span> <span class="special">);</span>
    <span class="keyword">return</span> <span class="number">0</span><span class="special">;</span>
<span class="special">}</span>
</pre>
<p>
              This program outputs the following:
            </p>
<pre class="programlisting">hello, world
</pre>
<p>
              <code class="computeroutput"><a class="link" href="../boost/proto/context/default_context.html" title="Struct default_context">proto::default_context</a></code> is trivially
              defined in terms of a <code class="computeroutput"><span class="identifier">default_eval</span><span class="special">&lt;&gt;</span></code> template, as follows:
            </p>
<pre class="programlisting"><span class="comment">// Definition of default_context</span>
<span class="keyword">struct</span> <span class="identifier">default_context</span>
<span class="special">{</span>
    <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">Expr</span><span class="special">&gt;</span>
    <span class="keyword">struct</span> <span class="identifier">eval</span>
      <span class="special">:</span> <span class="identifier">default_eval</span><span class="special">&lt;</span>
            <span class="identifier">Expr</span>
          <span class="special">,</span> <span class="identifier">default_context</span> <span class="keyword">const</span>
          <span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">tag_of</span><span class="special">&lt;</span><span class="identifier">Expr</span><span class="special">&gt;::</span><span class="identifier">type</span>
        <span class="special">&gt;</span>
    <span class="special">{};</span>
<span class="special">};</span>
</pre>
<p>
              There are a bunch of <code class="computeroutput"><span class="identifier">default_eval</span><span class="special">&lt;&gt;</span></code> specializations, each of which
              handles a different C++ operator. Here, for instance, is the specialization
              for binary addition:
            </p>
<pre class="programlisting"><span class="comment">// A default expression evaluator for binary addition</span>
<span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">Expr</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">Context</span><span class="special">&gt;</span>
<span class="keyword">struct</span> <span class="identifier">default_eval</span><span class="special">&lt;</span><span class="identifier">Expr</span><span class="special">,</span> <span class="identifier">Context</span><span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">plus</span><span class="special">&gt;</span>
<span class="special">{</span>
<span class="keyword">private</span><span class="special">:</span>
    <span class="keyword">static</span> <span class="identifier">Expr</span>    <span class="special">&amp;</span> <span class="identifier">s_expr</span><span class="special">;</span>
    <span class="keyword">static</span> <span class="identifier">Context</span> <span class="special">&amp;</span> <span class="identifier">s_ctx</span><span class="special">;</span>

<span class="keyword">public</span><span class="special">:</span>
    <span class="keyword">typedef</span>
        <span class="keyword">decltype</span><span class="special">(</span>
            <span class="identifier">proto</span><span class="special">::</span><span class="identifier">eval</span><span class="special">(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">child_c</span><span class="special">&lt;</span><span class="number">0</span><span class="special">&gt;(</span><span class="identifier">s_expr</span><span class="special">),</span> <span class="identifier">s_ctx</span><span class="special">)</span>
          <span class="special">+</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">eval</span><span class="special">(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">child_c</span><span class="special">&lt;</span><span class="number">1</span><span class="special">&gt;(</span><span class="identifier">s_expr</span><span class="special">),</span> <span class="identifier">s_ctx</span><span class="special">)</span>
        <span class="special">)</span>
    <span class="identifier">result_type</span><span class="special">;</span>

    <span class="identifier">result_type</span> <span class="keyword">operator</span> <span class="special">()(</span><span class="identifier">Expr</span> <span class="special">&amp;</span><span class="identifier">expr</span><span class="special">,</span> <span class="identifier">Context</span> <span class="special">&amp;</span><span class="identifier">ctx</span><span class="special">)</span> <span class="keyword">const</span>
    <span class="special">{</span>
        <span class="keyword">return</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">eval</span><span class="special">(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">child_c</span><span class="special">&lt;</span><span class="number">0</span><span class="special">&gt;(</span><span class="identifier">expr</span><span class="special">),</span> <span class="identifier">ctx</span><span class="special">)</span>
             <span class="special">+</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">eval</span><span class="special">(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">child_c</span><span class="special">&lt;</span><span class="number">1</span><span class="special">&gt;(</span><span class="identifier">expr</span><span class="special">),</span> <span class="identifier">ctx</span><span class="special">);</span>
    <span class="special">}</span>
<span class="special">};</span>
</pre>
<p>
              The above code uses <code class="computeroutput"><span class="keyword">decltype</span></code>
              to calculate the return type of the function call operator. <code class="computeroutput"><span class="keyword">decltype</span></code> is a new keyword in the next
              version of C++ that gets the type of any expression. Most compilers
              do not yet support <code class="computeroutput"><span class="keyword">decltype</span></code>
              directly, so <code class="computeroutput"><span class="identifier">default_eval</span><span class="special">&lt;&gt;</span></code> uses the Boost.Typeof library
              to emulate it. On some compilers, that may mean that <code class="computeroutput"><span class="identifier">default_context</span></code> either doesn't work
              or that it requires you to register your types with the Boost.Typeof
              library. Check the documentation for Boost.Typeof to see.
            </p>
</div>
<div class="section">
<div class="titlepage"><div><div><h6 class="title">
<a name="boost_proto.users_guide.back_end.expression_evaluation.canned_contexts.null_context"></a><a class="link" href="users_guide.html#boost_proto.users_guide.back_end.expression_evaluation.canned_contexts.null_context" title="null_context"><code class="literal">null_context</code></a>
</h6></div></div></div>
<p>
              The <code class="computeroutput"><a class="link" href="../boost/proto/context/null_context.html" title="Struct null_context">proto::null_context&lt;&gt;</a></code>
              is a simple context that recursively evaluates children but does not
              combine the results in any way and returns void. It is useful in conjunction
              with <code class="computeroutput"><span class="identifier">callable_context</span><span class="special">&lt;&gt;</span></code>, or when defining your own
              contexts which mutate an expression tree in-place rather than accumulate
              a result, as we'll see below.
            </p>
<p>
              <code class="computeroutput"><a class="link" href="../boost/proto/context/null_context.html" title="Struct null_context">proto::null_context&lt;&gt;</a></code>
              is trivially implemented in terms of <code class="computeroutput"><span class="identifier">null_eval</span><span class="special">&lt;&gt;</span></code> as follows:
            </p>
<pre class="programlisting"><span class="comment">// Definition of null_context</span>
<span class="keyword">struct</span> <span class="identifier">null_context</span>
<span class="special">{</span>
    <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">Expr</span><span class="special">&gt;</span>
    <span class="keyword">struct</span> <span class="identifier">eval</span>
      <span class="special">:</span> <span class="identifier">null_eval</span><span class="special">&lt;</span><span class="identifier">Expr</span><span class="special">,</span> <span class="identifier">null_context</span> <span class="keyword">const</span><span class="special">,</span> <span class="identifier">Expr</span><span class="special">::</span><span class="identifier">proto_arity</span><span class="special">::</span><span class="identifier">value</span><span class="special">&gt;</span>
    <span class="special">{};</span>
<span class="special">};</span>
</pre>
<p>
              And <code class="computeroutput"><span class="identifier">null_eval</span><span class="special">&lt;&gt;</span></code>
              is also trivially implemented. Here, for instance is a binary <code class="computeroutput"><span class="identifier">null_eval</span><span class="special">&lt;&gt;</span></code>:
            </p>
<pre class="programlisting"><span class="comment">// Binary null_eval&lt;&gt;</span>
<span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">Expr</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">Context</span><span class="special">&gt;</span>
<span class="keyword">struct</span> <span class="identifier">null_eval</span><span class="special">&lt;</span><span class="identifier">Expr</span><span class="special">,</span> <span class="identifier">Context</span><span class="special">,</span> <span class="number">2</span><span class="special">&gt;</span>
<span class="special">{</span>
    <span class="keyword">typedef</span> <span class="keyword">void</span> <span class="identifier">result_type</span><span class="special">;</span>

    <span class="keyword">void</span> <span class="keyword">operator</span><span class="special">()(</span><span class="identifier">Expr</span> <span class="special">&amp;</span><span class="identifier">expr</span><span class="special">,</span> <span class="identifier">Context</span> <span class="special">&amp;</span><span class="identifier">ctx</span><span class="special">)</span> <span class="keyword">const</span>
    <span class="special">{</span>
        <span class="identifier">proto</span><span class="special">::</span><span class="identifier">eval</span><span class="special">(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">child_c</span><span class="special">&lt;</span><span class="number">0</span><span class="special">&gt;(</span><span class="identifier">expr</span><span class="special">),</span> <span class="identifier">ctx</span><span class="special">);</span>
        <span class="identifier">proto</span><span class="special">::</span><span class="identifier">eval</span><span class="special">(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">child_c</span><span class="special">&lt;</span><span class="number">1</span><span class="special">&gt;(</span><span class="identifier">expr</span><span class="special">),</span> <span class="identifier">ctx</span><span class="special">);</span>
    <span class="special">}</span>
<span class="special">};</span>
</pre>
<p>
              When would such classes be useful? Imagine you have an expression tree
              with integer terminals, and you would like to increment each integer
              in-place. You might define an evaluation context as follows:
            </p>
<pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">increment_ints</span>
<span class="special">{</span>
    <span class="comment">// By default, just evaluate all children by delegating</span>
    <span class="comment">// to the null_eval&lt;&gt;</span>
    <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">Expr</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">Arg</span> <span class="special">=</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">result_of</span><span class="special">::</span><span class="identifier">child</span><span class="special">&lt;</span><span class="identifier">Expr</span><span class="special">&gt;::</span><span class="identifier">type</span><span class="special">&gt;</span>
    <span class="keyword">struct</span> <span class="identifier">eval</span>
      <span class="special">:</span> <span class="identifier">null_eval</span><span class="special">&lt;</span><span class="identifier">Expr</span><span class="special">,</span> <span class="identifier">increment_ints</span> <span class="keyword">const</span><span class="special">&gt;</span>
    <span class="special">{};</span>

    <span class="comment">// Increment integer terminals</span>
    <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">Expr</span><span class="special">&gt;</span>
    <span class="keyword">struct</span> <span class="identifier">eval</span><span class="special">&lt;</span><span class="identifier">Expr</span><span class="special">,</span> <span class="keyword">int</span><span class="special">&gt;</span>
    <span class="special">{</span>
        <span class="keyword">typedef</span> <span class="keyword">void</span> <span class="identifier">result_type</span><span class="special">;</span>

        <span class="keyword">void</span> <span class="keyword">operator</span><span class="special">()(</span><span class="identifier">Expr</span> <span class="special">&amp;</span><span class="identifier">expr</span><span class="special">,</span> <span class="identifier">increment_ints</span> <span class="keyword">const</span> <span class="special">&amp;)</span> <span class="keyword">const</span>
        <span class="special">{</span>
            <span class="special">++</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">child</span><span class="special">(</span><span class="identifier">expr</span><span class="special">);</span>
        <span class="special">}</span>
    <span class="special">};</span>
<span class="special">};</span>
</pre>
<p>
              In the next section on <code class="computeroutput"><a class="link" href="../boost/proto/context/callable_context.html" title="Struct template callable_context">proto::callable_context&lt;&gt;</a></code>,
              we'll see an even simpler way to achieve the same thing.
            </p>
</div>
<div class="section">
<div class="titlepage"><div><div><h6 class="title">
<a name="boost_proto.users_guide.back_end.expression_evaluation.canned_contexts.callable_context"></a><a class="link" href="users_guide.html#boost_proto.users_guide.back_end.expression_evaluation.canned_contexts.callable_context" title="callable_context&lt;&gt;"><code class="literal">callable_context&lt;&gt;</code></a>
</h6></div></div></div>
<p>
              The <code class="computeroutput"><a class="link" href="../boost/proto/context/callable_context.html" title="Struct template callable_context">proto::callable_context&lt;&gt;</a></code>
              is a helper that simplifies the job of writing context classes. Rather
              than writing template specializations, with <code class="computeroutput"><a class="link" href="../boost/proto/context/callable_context.html" title="Struct template callable_context">proto::callable_context&lt;&gt;</a></code>
              you write a function object with an overloaded function call operator.
              Any expressions not handled by an overload are automatically dispatched
              to a default evaluation context that you can specify.
            </p>
<p>
              Rather than an evaluation context in its own right, <code class="computeroutput"><a class="link" href="../boost/proto/context/callable_context.html" title="Struct template callable_context">proto::callable_context&lt;&gt;</a></code>
              is more properly thought of as a context adaptor. To use it, you must
              define your own context that inherits from <code class="computeroutput"><a class="link" href="../boost/proto/context/callable_context.html" title="Struct template callable_context">proto::callable_context&lt;&gt;</a></code>.
            </p>
<p>
              In the <a class="link" href="users_guide.html#boost_proto.users_guide.back_end.expression_evaluation.canned_contexts.null_context" title="null_context"><code class="literal">null_context</code></a>
              section, we saw how to implement an evaluation context that increments
              all the integers within an expression tree. Here is how to do the same
              thing with the <code class="computeroutput"><a class="link" href="../boost/proto/context/callable_context.html" title="Struct template callable_context">proto::callable_context&lt;&gt;</a></code>:
            </p>
<pre class="programlisting"><span class="comment">// An evaluation context that increments all</span>
<span class="comment">// integer terminals in-place.</span>
<span class="keyword">struct</span> <span class="identifier">increment_ints</span>
  <span class="special">:</span> <span class="identifier">callable_context</span><span class="special">&lt;</span>
        <span class="identifier">increment_ints</span> <span class="keyword">const</span> <span class="comment">// derived context</span>
      <span class="special">,</span> <span class="identifier">null_context</span> <span class="keyword">const</span>  <span class="comment">// fall-back context</span>
    <span class="special">&gt;</span>
<span class="special">{</span>
    <span class="keyword">typedef</span> <span class="keyword">void</span> <span class="identifier">result_type</span><span class="special">;</span>

    <span class="comment">// Handle int terminals here:</span>
    <span class="keyword">void</span> <span class="keyword">operator</span><span class="special">()(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">,</span> <span class="keyword">int</span> <span class="special">&amp;</span><span class="identifier">i</span><span class="special">)</span> <span class="keyword">const</span>
    <span class="special">{</span>
        <span class="special">++</span><span class="identifier">i</span><span class="special">;</span>
    <span class="special">}</span>
<span class="special">};</span>
</pre>
<p>
              With such a context, we can do the following:
            </p>
<pre class="programlisting"><span class="identifier">literal</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">&gt;</span> <span class="identifier">i</span> <span class="special">=</span> <span class="number">0</span><span class="special">,</span> <span class="identifier">j</span> <span class="special">=</span> <span class="number">10</span><span class="special">;</span>
<span class="identifier">proto</span><span class="special">::</span><span class="identifier">eval</span><span class="special">(</span> <span class="identifier">i</span> <span class="special">-</span> <span class="identifier">j</span> <span class="special">*</span> <span class="number">3.14</span><span class="special">,</span> <span class="identifier">increment_ints</span><span class="special">()</span> <span class="special">);</span>

<span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="string">"i = "</span> <span class="special">&lt;&lt;</span> <span class="identifier">i</span><span class="special">.</span><span class="identifier">get</span><span class="special">()</span> <span class="special">&lt;&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="string">"j = "</span> <span class="special">&lt;&lt;</span> <span class="identifier">j</span><span class="special">.</span><span class="identifier">get</span><span class="special">()</span> <span class="special">&lt;&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span>
</pre>
<p>
              This program outputs the following, which shows that the integers
              <code class="computeroutput"><span class="identifier">i</span></code> and <code class="computeroutput"><span class="identifier">j</span></code> have been incremented by <code class="computeroutput"><span class="number">1</span></code>:
            </p>
<pre class="programlisting">i = 1
j = 11
</pre>
<p>
              In the <code class="computeroutput"><span class="identifier">increment_ints</span></code>
              context, we didn't have to define any nested <code class="computeroutput"><span class="identifier">eval</span><span class="special">&lt;&gt;</span></code> templates. That's because
              <code class="computeroutput"><a class="link" href="../boost/proto/context/callable_context.html" title="Struct template callable_context">proto::callable_context&lt;&gt;</a></code>
              implements them for us. <code class="computeroutput"><a class="link" href="../boost/proto/context/callable_context.html" title="Struct template callable_context">proto::callable_context&lt;&gt;</a></code>
              takes two template parameters: the derived context and a fall-back
              context. For each node in the expression tree being evaluated, <code class="computeroutput"><a class="link" href="../boost/proto/context/callable_context.html" title="Struct template callable_context">proto::callable_context&lt;&gt;</a></code> checks to see if
              there is an overloaded <code class="computeroutput"><span class="keyword">operator</span><span class="special">()</span></code> in the derived context that accepts
              it. Given some expression <code class="computeroutput"><span class="identifier">expr</span></code>
              of type <code class="computeroutput"><span class="identifier">Expr</span></code>, and a
              context <code class="computeroutput"><span class="identifier">ctx</span></code>, it attempts
              to call:
            </p>
<pre class="programlisting"><span class="identifier">ctx</span><span class="special">(</span>
    <span class="keyword">typename</span> <span class="identifier">Expr</span><span class="special">::</span><span class="identifier">proto_tag</span><span class="special">()</span>
  <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">child_c</span><span class="special">&lt;</span><span class="number">0</span><span class="special">&gt;(</span><span class="identifier">expr</span><span class="special">)</span>
  <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">child_c</span><span class="special">&lt;</span><span class="number">1</span><span class="special">&gt;(</span><span class="identifier">expr</span><span class="special">)</span>
    <span class="special">...</span>
<span class="special">);</span>
</pre>
<p>
              Using function overloading and metaprogramming tricks, <code class="computeroutput"><a class="link" href="../boost/proto/context/callable_context.html" title="Struct template callable_context">proto::callable_context&lt;&gt;</a></code>
              can detect at compile-time whether such a function exists or not. If
              so, that function is called. If not, the current expression is passed
              to the fall-back evaluation context to be processed.
            </p>
<p>
              We saw another example of the <code class="computeroutput"><a class="link" href="../boost/proto/context/callable_context.html" title="Struct template callable_context">proto::callable_context&lt;&gt;</a></code>
              when we looked at the simple calculator expression evaluator. There,
              we wanted to customize the evaluation of placeholder terminals, and
              delegate the handling of all other nodes to the <code class="computeroutput"><a class="link" href="../boost/proto/context/default_context.html" title="Struct default_context">proto::default_context</a></code>. We did
              that as follows:
            </p>
<pre class="programlisting"><span class="comment">// An evaluation context for calculator expressions that</span>
<span class="comment">// explicitly handles placeholder terminals, but defers the</span>
<span class="comment">// processing of all other nodes to the default_context.</span>
<span class="keyword">struct</span> <span class="identifier">calculator_context</span>
  <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">callable_context</span><span class="special">&lt;</span> <span class="identifier">calculator_context</span> <span class="keyword">const</span> <span class="special">&gt;</span>
<span class="special">{</span>
    <span class="identifier">std</span><span class="special">::</span><span class="identifier">vector</span><span class="special">&lt;</span><span class="keyword">double</span><span class="special">&gt;</span> <span class="identifier">args</span><span class="special">;</span>

    <span class="comment">// Define the result type of the calculator.</span>
    <span class="keyword">typedef</span> <span class="keyword">double</span> <span class="identifier">result_type</span><span class="special">;</span>

    <span class="comment">// Handle the placeholders:</span>
    <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">int</span> <span class="identifier">I</span><span class="special">&gt;</span>
    <span class="keyword">double</span> <span class="keyword">operator</span><span class="special">()(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">,</span> <span class="identifier">placeholder</span><span class="special">&lt;</span><span class="identifier">I</span><span class="special">&gt;)</span> <span class="keyword">const</span>
    <span class="special">{</span>
        <span class="keyword">return</span> <span class="keyword">this</span><span class="special">-&gt;</span><span class="identifier">args</span><span class="special">[</span><span class="identifier">I</span><span class="special">];</span>
    <span class="special">}</span>
<span class="special">};</span>
</pre>
<p>
              In this case, we didn't specify a fall-back context. In that case,
              <code class="computeroutput"><a class="link" href="../boost/proto/context/callable_context.html" title="Struct template callable_context">proto::callable_context&lt;&gt;</a></code>
              uses the <code class="computeroutput"><a class="link" href="../boost/proto/context/default_context.html" title="Struct default_context">proto::default_context</a></code>. With
              the above <code class="computeroutput"><span class="identifier">calculator_context</span></code>
              and a couple of appropriately defined placeholder terminals, we can
              evaluate calculator expressions, as demonstrated below:
            </p>
<pre class="programlisting"><span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">int</span> <span class="identifier">I</span><span class="special">&gt;</span>
<span class="keyword">struct</span> <span class="identifier">placeholder</span>
<span class="special">{};</span>

<span class="identifier">terminal</span><span class="special">&lt;</span><span class="identifier">placeholder</span><span class="special">&lt;</span><span class="number">0</span><span class="special">&gt;</span> <span class="special">&gt;::</span><span class="identifier">type</span> <span class="keyword">const</span> <span class="identifier">_1</span> <span class="special">=</span> <span class="special">{{}};</span>
<span class="identifier">terminal</span><span class="special">&lt;</span><span class="identifier">placeholder</span><span class="special">&lt;</span><span class="number">1</span><span class="special">&gt;</span> <span class="special">&gt;::</span><span class="identifier">type</span> <span class="keyword">const</span> <span class="identifier">_2</span> <span class="special">=</span> <span class="special">{{}};</span>
<span class="comment">// ...</span>

<span class="identifier">calculator_context</span> <span class="identifier">ctx</span><span class="special">;</span>
<span class="identifier">ctx</span><span class="special">.</span><span class="identifier">args</span><span class="special">.</span><span class="identifier">push_back</span><span class="special">(</span><span class="number">4</span><span class="special">);</span>
<span class="identifier">ctx</span><span class="special">.</span><span class="identifier">args</span><span class="special">.</span><span class="identifier">push_back</span><span class="special">(</span><span class="number">5</span><span class="special">);</span>

<span class="keyword">double</span> <span class="identifier">j</span> <span class="special">=</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">eval</span><span class="special">(</span> <span class="special">(</span><span class="identifier">_2</span> <span class="special">-</span> <span class="identifier">_1</span><span class="special">)</span> <span class="special">/</span> <span class="identifier">_2</span> <span class="special">*</span> <span class="number">100</span><span class="special">,</span> <span class="identifier">ctx</span> <span class="special">);</span>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="string">"j = "</span> <span class="special">&lt;&lt;</span> <span class="identifier">j</span> <span class="special">&lt;&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span>
</pre>
<p>
              The above code displays the following:
            </p>
<pre class="programlisting">j = 20
</pre>
</div>
</div>
</div>
<div class="section">
<div class="titlepage"><div><div><h4 class="title">
<a name="boost_proto.users_guide.back_end.expression_transformation"></a><a class="link" href="users_guide.html#boost_proto.users_guide.back_end.expression_transformation" title="Expression Transformation: Semantic Actions">Expression
        Transformation: Semantic Actions</a>
</h4></div></div></div>
<div class="toc"><dl class="toc">
<dt><span class="section"><a href="users_guide.html#boost_proto.users_guide.back_end.expression_transformation.__activating__your_grammars"><span class="quote">&#8220;<span class="quote">Activating</span>&#8221;</span>
          Your Grammars</a></span></dt>
<dt><span class="section"><a href="users_guide.html#boost_proto.users_guide.back_end.expression_transformation.handling_alternation_and_recursion">Handling
          Alternation and Recursion</a></span></dt>
<dt><span class="section"><a href="users_guide.html#boost_proto.users_guide.back_end.expression_transformation.callable_transforms">Callable
          Transforms</a></span></dt>
<dt><span class="section"><a href="users_guide.html#boost_proto.users_guide.back_end.expression_transformation.object_transforms">Object
          Transforms</a></span></dt>
<dt><span class="section"><a href="users_guide.html#boost_proto.users_guide.back_end.expression_transformation.example__calculator_arity">Example:
          Calculator Arity</a></span></dt>
<dt><span class="section"><a href="users_guide.html#boost_proto.users_guide.back_end.expression_transformation.state">Transforms
          With State Accumulation</a></span></dt>
<dt><span class="section"><a href="users_guide.html#boost_proto.users_guide.back_end.expression_transformation.data">Passing
          Auxiliary Data to Transforms</a></span></dt>
<dt><span class="section"><a href="users_guide.html#boost_proto.users_guide.back_end.expression_transformation.implicit_params">Implicit
          Parameters to Primitive Transforms</a></span></dt>
<dt><span class="section"><a href="users_guide.html#boost_proto.users_guide.back_end.expression_transformation.unpacking_expressions">Unpacking
          Expressions</a></span></dt>
<dt><span class="section"><a href="users_guide.html#boost_proto.users_guide.back_end.expression_transformation.external_transforms">Separating
          Grammars And Transforms</a></span></dt>
<dt><span class="section"><a href="users_guide.html#boost_proto.users_guide.back_end.expression_transformation.canned_transforms">Proto's
          Built-In Transforms</a></span></dt>
<dt><span class="section"><a href="users_guide.html#boost_proto.users_guide.back_end.expression_transformation.primitives">Building
          Custom Primitive Transforms</a></span></dt>
<dt><span class="section"><a href="users_guide.html#boost_proto.users_guide.back_end.expression_transformation.is_callable">Making
          Your Transform Callable</a></span></dt>
</dl></div>
<p>
          If you have ever built a parser with the help of a tool like Antlr, yacc
          or Boost.Spirit, you might be familiar with <span class="emphasis"><em>semantic actions</em></span>.
          In addition to allowing you to define the grammar of the language recognized
          by the parser, these tools let you embed code within your grammar that
          executes when parts of the grammar participate in a parse. Proto has the
          equivalent of semantic actions. They are called <span class="emphasis"><em>transforms</em></span>.
          This section describes how to embed transforms within your Proto grammars,
          turning your grammars into function objects that can manipulate or evaluate
          expressions in powerful ways.
        </p>
<p>
          Proto transforms are an advanced topic. We'll take it slow, using examples
          to illustrate the key concepts, starting simple.
        </p>
<div class="section">
<div class="titlepage"><div><div><h5 class="title">
<a name="boost_proto.users_guide.back_end.expression_transformation.__activating__your_grammars"></a><a class="link" href="users_guide.html#boost_proto.users_guide.back_end.expression_transformation.__activating__your_grammars" title="&#8220;Activating&#8221; Your Grammars"><span class="quote">&#8220;<span class="quote">Activating</span>&#8221;</span>
          Your Grammars</a>
</h5></div></div></div>
<p>
            The Proto grammars we've seen so far are static. You can check at compile-time
            to see if an expression type matches a grammar, but that's it. Things
            get more interesting when you give them runtime behaviors. A grammar
            with embedded transforms is more than just a static grammar. It is a
            function object that accepts expressions that match the grammar and does
            <span class="emphasis"><em>something</em></span> with them.
          </p>
<p>
            Below is a very simple grammar. It matches terminal expressions.
          </p>
<pre class="programlisting"><span class="comment">// A simple Proto grammar that matches all terminals</span>
<span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span> <span class="identifier">_</span> <span class="special">&gt;</span>
</pre>
<p>
            Here is the same grammar with a transform that extracts the value from
            the terminal:
          </p>
<pre class="programlisting"><span class="comment">// A simple Proto grammar that matches all terminals</span>
<span class="comment">// *and* a function object that extracts the value from</span>
<span class="comment">// the terminal</span>
<span class="identifier">proto</span><span class="special">::</span><span class="identifier">when</span><span class="special">&lt;</span>
    <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span> <span class="identifier">_</span> <span class="special">&gt;</span>
  <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_value</span>          <span class="comment">// &lt;-- Look, a transform!</span>
<span class="special">&gt;</span>
</pre>
<p>
            You can read this as follows: when you match a terminal expression, extract
            the value. The type <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_value</span></code>
            is a so-called transform. Later we'll see what makes it a transform,
            but for now just think of it as a kind of function object. Note the use
            of <code class="computeroutput"><a class="link" href="../boost/proto/when.html" title="Struct template when">proto::when&lt;&gt;</a></code>: the first template
            parameter is the grammar to match and the second is the transform to
            execute. The result is both a grammar that matches terminal expressions
            and a function object that accepts terminal expressions and extracts
            their values.
          </p>
<p>
            As with ordinary grammars, we can define an empty struct that inherits
            from a grammar+transform to give us an easy way to refer back to the
            thing we're defining, as follows:
          </p>
<pre class="programlisting"><span class="comment">// A grammar and a function object, as before</span>
<span class="keyword">struct</span> <span class="identifier">Value</span>
  <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">when</span><span class="special">&lt;</span>
        <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span> <span class="identifier">_</span> <span class="special">&gt;</span>
      <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_value</span>
    <span class="special">&gt;</span>
<span class="special">{};</span>

<span class="comment">// "Value" is a grammar that matches terminal expressions</span>
<span class="identifier">BOOST_MPL_ASSERT</span><span class="special">((</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">matches</span><span class="special">&lt;</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">&gt;::</span><span class="identifier">type</span><span class="special">,</span> <span class="identifier">Value</span> <span class="special">&gt;</span> <span class="special">));</span>

<span class="comment">// "Value" also defines a function object that accepts terminals</span>
<span class="comment">// and extracts their value.</span>
<span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">&gt;::</span><span class="identifier">type</span> <span class="identifier">answer</span> <span class="special">=</span> <span class="special">{</span><span class="number">42</span><span class="special">};</span>
<span class="identifier">Value</span> <span class="identifier">get_value</span><span class="special">;</span>
<span class="keyword">int</span> <span class="identifier">i</span> <span class="special">=</span> <span class="identifier">get_value</span><span class="special">(</span> <span class="identifier">answer</span> <span class="special">);</span>
</pre>
<p>
            As already mentioned, <code class="computeroutput"><span class="identifier">Value</span></code>
            is a grammar that matches terminal expressions and a function object
            that operates on terminal expressions. It would be an error to pass a
            non-terminal expression to the <code class="computeroutput"><span class="identifier">Value</span></code>
            function object. This is a general property of grammars with transforms;
            when using them as function objects, expressions passed to them must
            match the grammar.
          </p>
<p>
            Proto grammars are valid TR1-style function objects. That means you can
            use <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">result_of</span><span class="special">&lt;&gt;</span></code>
            to ask a grammar what its return type will be, given a particular expression
            type. For instance, we can access the <code class="computeroutput"><span class="identifier">Value</span></code>
            grammar's return type as follows:
          </p>
<pre class="programlisting"><span class="comment">// We can use boost::result_of&lt;&gt; to get the return type</span>
<span class="comment">// of a Proto grammar.</span>
<span class="keyword">typedef</span>
    <span class="keyword">typename</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">result_of</span><span class="special">&lt;</span><span class="identifier">Value</span><span class="special">(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">&gt;::</span><span class="identifier">type</span><span class="special">)&gt;::</span><span class="identifier">type</span>
<span class="identifier">result_type</span><span class="special">;</span>

<span class="comment">// Check that we got the type we expected</span>
<span class="identifier">BOOST_MPL_ASSERT</span><span class="special">((</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">is_same</span><span class="special">&lt;</span><span class="identifier">result_type</span><span class="special">,</span> <span class="keyword">int</span><span class="special">&gt;</span> <span class="special">));</span>
</pre>
<div class="note"><table border="0" summary="Note">
<tr>
<td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../doc/src/images/note.png"></td>
<th align="left">Note</th>
</tr>
<tr><td align="left" valign="top"><p>
              A grammar with embedded transforms is both a grammar and a function
              object. Calling these things "grammars with transforms" would
              get tedious. We could call them something like "active grammars",
              but as we'll see <span class="emphasis"><em>every</em></span> grammar that you can define
              with Proto is "active"; that is, every grammar has some behavior
              when used as a function object. So we'll continue calling these things
              plain "grammars". The term "transform" is reserved
              for the thing that is used as the second parameter to the <code class="computeroutput"><a class="link" href="../boost/proto/when.html" title="Struct template when">proto::when&lt;&gt;</a></code> template.
            </p></td></tr>
</table></div>
</div>
<div class="section">
<div class="titlepage"><div><div><h5 class="title">
<a name="boost_proto.users_guide.back_end.expression_transformation.handling_alternation_and_recursion"></a><a class="link" href="users_guide.html#boost_proto.users_guide.back_end.expression_transformation.handling_alternation_and_recursion" title="Handling Alternation and Recursion">Handling
          Alternation and Recursion</a>
</h5></div></div></div>
<p>
            Most grammars are a little more complicated than the one in the preceding
            section. For the sake of illustration, let's define a rather nonsensical
            grammar that matches any expression and recurses to the leftmost terminal
            and returns its value. It will demonstrate how two key concepts of Proto
            grammars -- alternation and recursion -- interact with transforms. The
            grammar is described below.
          </p>
<pre class="programlisting"><span class="comment">// A grammar that matches any expression, and a function object</span>
<span class="comment">// that returns the value of the leftmost terminal.</span>
<span class="keyword">struct</span> <span class="identifier">LeftmostLeaf</span>
  <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">or_</span><span class="special">&lt;</span>
        <span class="comment">// If the expression is a terminal, return its value</span>
        <span class="identifier">proto</span><span class="special">::</span><span class="identifier">when</span><span class="special">&lt;</span>
            <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span> <span class="identifier">_</span> <span class="special">&gt;</span>
          <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_value</span>
        <span class="special">&gt;</span>
        <span class="comment">// Otherwise, it is a non-terminal. Return the result</span>
        <span class="comment">// of invoking LeftmostLeaf on the 0th (leftmost) child.</span>
      <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">when</span><span class="special">&lt;</span>
            <span class="identifier">_</span>
          <span class="special">,</span> <span class="identifier">LeftmostLeaf</span><span class="special">(</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_child0</span> <span class="special">)</span>
        <span class="special">&gt;</span>
    <span class="special">&gt;</span>
<span class="special">{};</span>

<span class="comment">// A Proto terminal wrapping std::cout</span>
<span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">ostream</span> <span class="special">&amp;</span> <span class="special">&gt;::</span><span class="identifier">type</span> <span class="identifier">cout_</span> <span class="special">=</span> <span class="special">{</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">};</span>

<span class="comment">// Create an expression and use LeftmostLeaf to extract the</span>
<span class="comment">// value of the leftmost terminal, which will be std::cout.</span>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">ostream</span> <span class="special">&amp;</span> <span class="identifier">sout</span> <span class="special">=</span> <span class="identifier">LeftmostLeaf</span><span class="special">()(</span> <span class="identifier">cout_</span> <span class="special">&lt;&lt;</span> <span class="string">"the answer: "</span> <span class="special">&lt;&lt;</span> <span class="number">42</span> <span class="special">&lt;&lt;</span> <span class="char">'\n'</span> <span class="special">);</span>
</pre>
<p>
            We've seen <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">or_</span><span class="special">&lt;&gt;</span></code>
            before. Here it is serving two roles. First, it is a grammar that matches
            any of its alternate sub-grammars; in this case, either a terminal or
            a non-terminal. Second, it is also a function object that accepts an
            expression, finds the alternate sub-grammar that matches the expression,
            and applies its transform. And since <code class="computeroutput"><span class="identifier">LeftmostLeaf</span></code>
            inherits from <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">or_</span><span class="special">&lt;&gt;</span></code>,
            <code class="computeroutput"><span class="identifier">LeftmostLeaf</span></code> is also
            both a grammar and a function object.
          </p>
<div class="note"><table border="0" summary="Note">
<tr>
<td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../doc/src/images/note.png"></td>
<th align="left">Note</th>
</tr>
<tr><td align="left" valign="top"><p>
              The second alternate uses <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_</span></code>
              as its grammar. Recall that <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_</span></code>
              is the wildcard grammar that matches any expression. Since alternates
              in <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">or_</span><span class="special">&lt;&gt;</span></code>
              are tried in order, and since the first alternate handles all terminals,
              the second alternate handles all (and only) non-terminals. Often enough,
              <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">when</span><span class="special">&lt;</span>
              <span class="identifier">_</span><span class="special">,</span>
              <em class="replaceable"><code>some-transform</code></em> <span class="special">&gt;</span></code>
              is the last alternate in a grammar, so for improved readability, you
              could use the equivalent <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">otherwise</span><span class="special">&lt;</span> <em class="replaceable"><code>some-transform</code></em>
              <span class="special">&gt;</span></code>.
            </p></td></tr>
</table></div>
<p>
            The next section describes this grammar further.
          </p>
</div>
<div class="section">
<div class="titlepage"><div><div><h5 class="title">
<a name="boost_proto.users_guide.back_end.expression_transformation.callable_transforms"></a><a class="link" href="users_guide.html#boost_proto.users_guide.back_end.expression_transformation.callable_transforms" title="Callable Transforms">Callable
          Transforms</a>
</h5></div></div></div>
<p>
            In the grammar defined in the preceding section, the transform associated
            with non-terminals is a little strange-looking:
          </p>
<pre class="programlisting"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">when</span><span class="special">&lt;</span>
    <span class="identifier">_</span>
  <span class="special">,</span> <span class="bold"><strong>LeftmostLeaf( proto::_child0 )</strong></span>   <span class="comment">// &lt;-- a "callable" transform</span>
<span class="special">&gt;</span>
</pre>
<p>
            It has the effect of accepting non-terminal expressions, taking the 0th
            (leftmost) child and recursively invoking the <code class="computeroutput"><span class="identifier">LeftmostLeaf</span></code>
            function on it. But <code class="computeroutput"><span class="identifier">LeftmostLeaf</span><span class="special">(</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_child0</span>
            <span class="special">)</span></code> is actually a <span class="emphasis"><em>function
            type</em></span>. Literally, it is the type of a function that accepts
            an object of type <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_child0</span></code>
            and returns an object of type <code class="computeroutput"><span class="identifier">LeftmostLeaf</span></code>.
            So how do we make sense of this transform? Clearly, there is no function
            that actually has this signature, nor would such a function be useful.
            The key is in understanding how <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">when</span><span class="special">&lt;&gt;</span></code> <span class="emphasis"><em>interprets</em></span>
            its second template parameter.
          </p>
<p>
            When the second template parameter to <code class="computeroutput"><a class="link" href="../boost/proto/when.html" title="Struct template when">proto::when&lt;&gt;</a></code>
            is a function type, <code class="computeroutput"><a class="link" href="../boost/proto/when.html" title="Struct template when">proto::when&lt;&gt;</a></code>
            interprets the function type as a transform. In this case, <code class="computeroutput"><span class="identifier">LeftmostLeaf</span></code> is treated as the type
            of a function object to invoke, and <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_child0</span></code>
            is treated as a transform. First, <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_child0</span></code>
            is applied to the current expression (the non-terminal that matched this
            alternate sub-grammar), and the result (the 0th child) is passed as an
            argument to <code class="computeroutput"><span class="identifier">LeftmostLeaf</span></code>.
          </p>
<div class="note"><table border="0" summary="Note">
<tr>
<td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../doc/src/images/note.png"></td>
<th align="left">Note</th>
</tr>
<tr><td align="left" valign="top">
<p>
              <span class="bold"><strong>Transforms are a Domain-Specific Language</strong></span>
            </p>
<p>
              <code class="computeroutput"><span class="identifier">LeftmostLeaf</span><span class="special">(</span>
              <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_child0</span> <span class="special">)</span></code>
              <span class="emphasis"><em>looks</em></span> like an invocation of the <code class="computeroutput"><span class="identifier">LeftmostLeaf</span></code> function object, but
              it's not, but then it actually is! Why this confusing subterfuge? Function
              types give us a natural and concise syntax for composing more complicated
              transforms from simpler ones. The fact that the syntax is suggestive
              of a function invocation is on purpose. It is an embedded domain-specific
              language for defining expression transformations. If the subterfuge
              worked, it may have fooled you into thinking the transform is doing
              exactly what it actually does! And that's the point.
            </p>
</td></tr>
</table></div>
<p>
            The type <code class="computeroutput"><span class="identifier">LeftmostLeaf</span><span class="special">(</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_child0</span>
            <span class="special">)</span></code> is an example of a <span class="emphasis"><em>callable
            transform</em></span>. It is a function type that represents a function
            object to call and its arguments. The types <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_child0</span></code>
            and <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_value</span></code> are <span class="emphasis"><em>primitive transforms</em></span>.
            They are plain structs, not unlike function objects, from which callable
            transforms can be composed. There is one other type of transform, <span class="emphasis"><em>object
            transforms</em></span>, that we'll encounter next.
          </p>
</div>
<div class="section">
<div class="titlepage"><div><div><h5 class="title">
<a name="boost_proto.users_guide.back_end.expression_transformation.object_transforms"></a><a class="link" href="users_guide.html#boost_proto.users_guide.back_end.expression_transformation.object_transforms" title="Object Transforms">Object
          Transforms</a>
</h5></div></div></div>
<p>
            The very first transform we looked at simply extracted the value of terminals.
            Let's do the same thing, but this time we'll promote all ints to longs
            first. (Please forgive the contrived-ness of the examples so far; they
            get more interesting later.) Here's the grammar:
          </p>
<pre class="programlisting"><span class="comment">// A simple Proto grammar that matches all terminals,</span>
<span class="comment">// and a function object that extracts the value from</span>
<span class="comment">// the terminal, promoting ints to longs:</span>
<span class="keyword">struct</span> <span class="identifier">ValueWithPomote</span>
  <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">or_</span><span class="special">&lt;</span>
        <span class="identifier">proto</span><span class="special">::</span><span class="identifier">when</span><span class="special">&lt;</span>
            <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span> <span class="keyword">int</span> <span class="special">&gt;</span>
          <span class="special">,</span> <span class="keyword">long</span><span class="special">(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_value</span><span class="special">)</span>     <span class="comment">// &lt;-- an "object" transform</span>
        <span class="special">&gt;</span>
      <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">when</span><span class="special">&lt;</span>
            <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span> <span class="identifier">_</span> <span class="special">&gt;</span>
          <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_value</span>
        <span class="special">&gt;</span>
    <span class="special">&gt;</span>
<span class="special">{};</span>
</pre>
<p>
            You can read the above grammar as follows: when you match an int terminal,
            extract the value from the terminal and use it to initialize a long;
            otherwise, when you match another kind of terminal, just extract the
            value. The type <code class="computeroutput"><span class="keyword">long</span><span class="special">(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_value</span><span class="special">)</span></code>
            is a so-called <span class="emphasis"><em>object</em></span> transform. It looks like the
            creation of a temporary long, but it's really a function type. Just as
            a callable transform is a function type that represents a function to
            call and its arguments, an object transforms is a function type that
            represents an object to construct and the arguments to its constructor.
          </p>
<div class="note"><table border="0" summary="Note">
<tr>
<td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../doc/src/images/note.png"></td>
<th align="left">Note</th>
</tr>
<tr><td align="left" valign="top">
<p>
              <span class="bold"><strong>Object Transforms vs. Callable Transforms</strong></span>
            </p>
<p>
              When using function types as Proto transforms, they can either represent
              an object to construct or a function to call. It is similar to "normal"
              C++ where the syntax <code class="computeroutput"><span class="identifier">foo</span><span class="special">(</span><span class="string">"arg"</span><span class="special">)</span></code> can either be interpreted as an object
              to construct or a function to call, depending on whether <code class="computeroutput"><span class="identifier">foo</span></code> is a type or a function. But
              consider two of the transforms we've seen so far:
            </p>
<p>
</p>
<pre class="programlisting"><span class="identifier">LeftmostLeaf</span><span class="special">(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_child0</span><span class="special">)</span>  <span class="comment">// &lt;-- a callable transform</span>
<span class="keyword">long</span><span class="special">(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_value</span><span class="special">)</span>           <span class="comment">// &lt;-- an object transform</span>
</pre>
<p>
            </p>
<p>
              Proto can't know in general which is which, so it uses a trait, <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">is_callable</span><span class="special">&lt;&gt;</span></code>,
              to differentiate. <code class="computeroutput"><span class="identifier">is_callable</span><span class="special">&lt;</span> <span class="keyword">long</span>
              <span class="special">&gt;::</span><span class="identifier">value</span></code>
              is false so <code class="computeroutput"><span class="keyword">long</span><span class="special">(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_value</span><span class="special">)</span></code>
              is an object to construct, but <code class="computeroutput"><span class="identifier">is_callable</span><span class="special">&lt;</span> <span class="identifier">LeftmostLeaf</span>
              <span class="special">&gt;::</span><span class="identifier">value</span></code>
              is true so <code class="computeroutput"><span class="identifier">LeftmostLeaf</span><span class="special">(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_child0</span><span class="special">)</span></code> is a function to call. Later on, we'll
              see how Proto recognizes a type as "callable".
            </p>
</td></tr>
</table></div>
</div>
<div class="section">
<div class="titlepage"><div><div><h5 class="title">
<a name="boost_proto.users_guide.back_end.expression_transformation.example__calculator_arity"></a><a class="link" href="users_guide.html#boost_proto.users_guide.back_end.expression_transformation.example__calculator_arity" title="Example: Calculator Arity">Example:
          Calculator Arity</a>
</h5></div></div></div>
<p>
            Now that we have the basics of Proto transforms down, let's consider
            a slightly more realistic example. We can use transforms to improve the
            type-safety of the <a class="link" href="users_guide.html#boost_proto.users_guide.getting_started.hello_calculator" title="Hello Calculator">calculator
            EDSL</a>. If you recall, it lets you write infix arithmetic expressions
            involving argument placeholders like <code class="computeroutput"><span class="identifier">_1</span></code>
            and <code class="computeroutput"><span class="identifier">_2</span></code> and pass them
            to STL algorithms as function objects, as follows:
          </p>
<pre class="programlisting"><span class="keyword">double</span> <span class="identifier">a1</span><span class="special">[</span><span class="number">4</span><span class="special">]</span> <span class="special">=</span> <span class="special">{</span> <span class="number">56</span><span class="special">,</span> <span class="number">84</span><span class="special">,</span> <span class="number">37</span><span class="special">,</span> <span class="number">69</span> <span class="special">};</span>
<span class="keyword">double</span> <span class="identifier">a2</span><span class="special">[</span><span class="number">4</span><span class="special">]</span> <span class="special">=</span> <span class="special">{</span> <span class="number">65</span><span class="special">,</span> <span class="number">120</span><span class="special">,</span> <span class="number">60</span><span class="special">,</span> <span class="number">70</span> <span class="special">};</span>
<span class="keyword">double</span> <span class="identifier">a3</span><span class="special">[</span><span class="number">4</span><span class="special">]</span> <span class="special">=</span> <span class="special">{</span> <span class="number">0</span> <span class="special">};</span>

<span class="comment">// Use std::transform() and a calculator expression</span>
<span class="comment">// to calculate percentages given two input sequences:</span>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">transform</span><span class="special">(</span><span class="identifier">a1</span><span class="special">,</span> <span class="identifier">a1</span><span class="special">+</span><span class="number">4</span><span class="special">,</span> <span class="identifier">a2</span><span class="special">,</span> <span class="identifier">a3</span><span class="special">,</span> <span class="special">(</span><span class="identifier">_2</span> <span class="special">-</span> <span class="identifier">_1</span><span class="special">)</span> <span class="special">/</span> <span class="identifier">_2</span> <span class="special">*</span> <span class="number">100</span><span class="special">);</span>
</pre>
<p>
            This works because we gave calculator expressions an <code class="computeroutput"><span class="keyword">operator</span><span class="special">()</span></code> that evaluates the expression, replacing
            the placeholders with the arguments to <code class="computeroutput"><span class="keyword">operator</span><span class="special">()</span></code>. The overloaded <code class="computeroutput"><span class="identifier">calculator</span><span class="special">&lt;&gt;::</span><span class="keyword">operator</span><span class="special">()</span></code> looked like this:
          </p>
<pre class="programlisting"><span class="comment">// Overload operator() to invoke proto::eval() with</span>
<span class="comment">// our calculator_context.</span>
<span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">Expr</span><span class="special">&gt;</span>
<span class="keyword">double</span>
<span class="identifier">calculator</span><span class="special">&lt;</span><span class="identifier">Expr</span><span class="special">&gt;::</span><span class="keyword">operator</span><span class="special">()(</span><span class="keyword">double</span> <span class="identifier">a1</span> <span class="special">=</span> <span class="number">0</span><span class="special">,</span> <span class="keyword">double</span> <span class="identifier">a2</span> <span class="special">=</span> <span class="number">0</span><span class="special">)</span> <span class="keyword">const</span>
<span class="special">{</span>
    <span class="identifier">calculator_context</span> <span class="identifier">ctx</span><span class="special">;</span>
    <span class="identifier">ctx</span><span class="special">.</span><span class="identifier">args</span><span class="special">.</span><span class="identifier">push_back</span><span class="special">(</span><span class="identifier">a1</span><span class="special">);</span>
    <span class="identifier">ctx</span><span class="special">.</span><span class="identifier">args</span><span class="special">.</span><span class="identifier">push_back</span><span class="special">(</span><span class="identifier">a2</span><span class="special">);</span>

    <span class="keyword">return</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">eval</span><span class="special">(*</span><span class="keyword">this</span><span class="special">,</span> <span class="identifier">ctx</span><span class="special">);</span>
<span class="special">}</span>
</pre>
<p>
            Although this works, it's not ideal because it doesn't warn users if
            they supply too many or too few arguments to a calculator expression.
            Consider the following mistakes:
          </p>
<pre class="programlisting"><span class="special">(</span><span class="identifier">_1</span> <span class="special">*</span> <span class="identifier">_1</span><span class="special">)(</span><span class="number">4</span><span class="special">,</span> <span class="number">2</span><span class="special">);</span>  <span class="comment">// Oops, too many arguments!</span>
<span class="special">(</span><span class="identifier">_2</span> <span class="special">*</span> <span class="identifier">_2</span><span class="special">)(</span><span class="number">42</span><span class="special">);</span>    <span class="comment">// Oops, too few arguments!</span>
</pre>
<p>
            The expression <code class="computeroutput"><span class="identifier">_1</span> <span class="special">*</span>
            <span class="identifier">_1</span></code> defines a unary calculator
            expression; it takes one argument and squares it. If we pass more than
            one argument, the extra arguments will be silently ignored, which might
            be surprising to users. The next expression, <code class="computeroutput"><span class="identifier">_2</span>
            <span class="special">*</span> <span class="identifier">_2</span></code>
            defines a binary calculator expression; it takes two arguments, ignores
            the first and squares the second. If we only pass one argument, the code
            silently fills in <code class="computeroutput"><span class="number">0.0</span></code> for
            the second argument, which is also probably not what users expect. What
            can be done?
          </p>
<p>
            We can say that the <span class="emphasis"><em>arity</em></span> of a calculator expression
            is the number of arguments it expects, and it is equal to the largest
            placeholder in the expression. So, the arity of <code class="computeroutput"><span class="identifier">_1</span>
            <span class="special">*</span> <span class="identifier">_1</span></code>
            is one, and the arity of <code class="computeroutput"><span class="identifier">_2</span>
            <span class="special">*</span> <span class="identifier">_2</span></code>
            is two. We can increase the type-safety of our calculator EDSL by making
            sure the arity of an expression equals the actual number of arguments
            supplied. Computing the arity of an expression is simple with the help
            of Proto transforms.
          </p>
<p>
            It's straightforward to describe in words how the arity of an expression
            should be calculated. Consider that calculator expressions can be made
            of <code class="computeroutput"><span class="identifier">_1</span></code>, <code class="computeroutput"><span class="identifier">_2</span></code>, literals, unary expressions and
            binary expressions. The following table shows the arities for each of
            these 5 constituents.
          </p>
<div class="table">
<a name="boost_proto.users_guide.back_end.expression_transformation.example__calculator_arity.t0"></a><p class="title"><b>Table&#160;32.8.&#160;Calculator Sub-Expression Arities</b></p>
<div class="table-contents"><table class="table" summary="Calculator Sub-Expression Arities">
<colgroup>
<col>
<col>
</colgroup>
<thead><tr>
<th>
                    <p>
                      Sub-Expression
                    </p>
                  </th>
<th>
                    <p>
                      Arity
                    </p>
                  </th>
</tr></thead>
<tbody>
<tr>
<td>
                    <p>
                      Placeholder 1
                    </p>
                  </td>
<td>
                    <p>
                      <code class="computeroutput"><span class="number">1</span></code>
                    </p>
                  </td>
</tr>
<tr>
<td>
                    <p>
                      Placeholder 2
                    </p>
                  </td>
<td>
                    <p>
                      <code class="computeroutput"><span class="number">2</span></code>
                    </p>
                  </td>
</tr>
<tr>
<td>
                    <p>
                      Literal
                    </p>
                  </td>
<td>
                    <p>
                      <code class="computeroutput"><span class="number">0</span></code>
                    </p>
                  </td>
</tr>
<tr>
<td>
                    <p>
                      Unary Expression
                    </p>
                  </td>
<td>
                    <p>
                      <span class="emphasis"><em>arity of the operand</em></span>
                    </p>
                  </td>
</tr>
<tr>
<td>
                    <p>
                      Binary Expression
                    </p>
                  </td>
<td>
                    <p>
                      <span class="emphasis"><em>max arity of the two operands</em></span>
                    </p>
                  </td>
</tr>
</tbody>
</table></div>
</div>
<br class="table-break"><p>
            Using this information, we can write the grammar for calculator expressions
            and attach transforms for computing the arity of each constituent. The
            code below computes the expression arity as a compile-time integer, using
            integral wrappers and metafunctions from the Boost MPL Library. The grammar
            is described below.
          </p>
<p>
</p>
<pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">CalcArity</span>
  <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">or_</span><span class="special">&lt;</span>
        <span class="identifier">proto</span><span class="special">::</span><span class="identifier">when</span><span class="special">&lt;</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span> <span class="identifier">placeholder</span><span class="special">&lt;</span><span class="number">0</span><span class="special">&gt;</span> <span class="special">&gt;,</span>
            <span class="identifier">mpl</span><span class="special">::</span><span class="identifier">int_</span><span class="special">&lt;</span><span class="number">1</span><span class="special">&gt;()</span>
        <span class="special">&gt;</span>
      <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">when</span><span class="special">&lt;</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span> <span class="identifier">placeholder</span><span class="special">&lt;</span><span class="number">1</span><span class="special">&gt;</span> <span class="special">&gt;,</span>
            <span class="identifier">mpl</span><span class="special">::</span><span class="identifier">int_</span><span class="special">&lt;</span><span class="number">2</span><span class="special">&gt;()</span>
        <span class="special">&gt;</span>
      <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">when</span><span class="special">&lt;</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span><span class="identifier">_</span><span class="special">&gt;,</span>
            <span class="identifier">mpl</span><span class="special">::</span><span class="identifier">int_</span><span class="special">&lt;</span><span class="number">0</span><span class="special">&gt;()</span>
        <span class="special">&gt;</span>
      <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">when</span><span class="special">&lt;</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">unary_expr</span><span class="special">&lt;</span><span class="identifier">_</span><span class="special">,</span> <span class="identifier">CalcArity</span><span class="special">&gt;,</span>
            <span class="identifier">CalcArity</span><span class="special">(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_child</span><span class="special">)</span>
        <span class="special">&gt;</span>
      <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">when</span><span class="special">&lt;</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">binary_expr</span><span class="special">&lt;</span><span class="identifier">_</span><span class="special">,</span> <span class="identifier">CalcArity</span><span class="special">,</span> <span class="identifier">CalcArity</span><span class="special">&gt;,</span>
            <span class="identifier">mpl</span><span class="special">::</span><span class="identifier">max</span><span class="special">&lt;</span><span class="identifier">CalcArity</span><span class="special">(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_left</span><span class="special">),</span>
                     <span class="identifier">CalcArity</span><span class="special">(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_right</span><span class="special">)&gt;()</span>
        <span class="special">&gt;</span>
    <span class="special">&gt;</span>
<span class="special">{};</span>
</pre>
<p>
          </p>
<p>
            When we find a placeholder terminal or a literal, we use an <span class="emphasis"><em>object
            transform</em></span> such as <code class="computeroutput"><span class="identifier">mpl</span><span class="special">::</span><span class="identifier">int_</span><span class="special">&lt;</span><span class="number">1</span><span class="special">&gt;()</span></code>
            to create a (default-constructed) compile-time integer representing the
            arity of that terminal.
          </p>
<p>
            For unary expressions, we use <code class="computeroutput"><span class="identifier">CalcArity</span><span class="special">(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_child</span><span class="special">)</span></code> which is a <span class="emphasis"><em>callable transform</em></span>
            that computes the arity of the expression's child.
          </p>
<p>
            The transform for binary expressions has a few new tricks. Let's look
            more closely:
          </p>
<pre class="programlisting"><span class="comment">// Compute the left and right arities and</span>
<span class="comment">// take the larger of the two.</span>
<span class="identifier">mpl</span><span class="special">::</span><span class="identifier">max</span><span class="special">&lt;</span><span class="identifier">CalcArity</span><span class="special">(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_left</span><span class="special">),</span>
         <span class="identifier">CalcArity</span><span class="special">(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_right</span><span class="special">)&gt;()</span>
</pre>
<p>
            This is an object transform; it default-constructs ... what exactly?
            The <code class="computeroutput"><span class="identifier">mpl</span><span class="special">::</span><span class="identifier">max</span><span class="special">&lt;&gt;</span></code>
            template is an MPL metafunction that accepts two compile-time integers.
            It has a nested <code class="computeroutput"><span class="special">::</span><span class="identifier">type</span></code>
            typedef (not shown) that is the maximum of the two. But here, we appear
            to be passing it two things that are <span class="emphasis"><em>not</em></span> compile-time
            integers; they're Proto callable transforms. Proto is smart enough to
            recognize that fact. It first evaluates the two nested callable transforms,
            computing the arities of the left and right child expressions. Then it
            puts the resulting integers into <code class="computeroutput"><span class="identifier">mpl</span><span class="special">::</span><span class="identifier">max</span><span class="special">&lt;&gt;</span></code> and evaluates the metafunction
            by asking for the nested <code class="computeroutput"><span class="special">::</span><span class="identifier">type</span></code>. That is the type of the object
            that gets default-constructed and returned.
          </p>
<p>
            More generally, when evaluating object transforms, Proto looks at the
            object type and checks whether it is a template specialization, like
            <code class="computeroutput"><span class="identifier">mpl</span><span class="special">::</span><span class="identifier">max</span><span class="special">&lt;&gt;</span></code>.
            If it is, Proto looks for nested transforms that it can evaluate. After
            any nested transforms have been evaluated and substituted back into the
            template, the new template specialization is the result type, unless
            that type has a nested <code class="computeroutput"><span class="special">::</span><span class="identifier">type</span></code>, in which case that becomes the
            result.
          </p>
<p>
            Now that we can calculate the arity of a calculator expression, let's
            redefine the <code class="computeroutput"><span class="identifier">calculator</span><span class="special">&lt;&gt;</span></code> expression wrapper we wrote in
            the Getting Started guide to use the <code class="computeroutput"><span class="identifier">CalcArity</span></code>
            grammar and some macros from Boost.MPL to issue compile-time errors when
            users specify too many or too few arguments.
          </p>
<pre class="programlisting"><span class="comment">// The calculator expression wrapper, as defined in the Hello</span>
<span class="comment">// Calculator example in the Getting Started guide. It behaves</span>
<span class="comment">// just like the expression it wraps, but with extra operator()</span>
<span class="comment">// member functions that evaluate the expression.</span>
<span class="comment">//   NEW: Use the CalcArity grammar to ensure that the correct</span>
<span class="comment">//   number of arguments are supplied.</span>
<span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">Expr</span><span class="special">&gt;</span>
<span class="keyword">struct</span> <span class="identifier">calculator</span>
  <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">extends</span><span class="special">&lt;</span><span class="identifier">Expr</span><span class="special">,</span> <span class="identifier">calculator</span><span class="special">&lt;</span><span class="identifier">Expr</span><span class="special">&gt;,</span> <span class="identifier">calculator_domain</span><span class="special">&gt;</span>
<span class="special">{</span>
    <span class="keyword">typedef</span>
        <span class="identifier">proto</span><span class="special">::</span><span class="identifier">extends</span><span class="special">&lt;</span><span class="identifier">Expr</span><span class="special">,</span> <span class="identifier">calculator</span><span class="special">&lt;</span><span class="identifier">Expr</span><span class="special">&gt;,</span> <span class="identifier">calculator_domain</span><span class="special">&gt;</span>
    <span class="identifier">base_type</span><span class="special">;</span>

    <span class="identifier">calculator</span><span class="special">(</span><span class="identifier">Expr</span> <span class="keyword">const</span> <span class="special">&amp;</span><span class="identifier">expr</span> <span class="special">=</span> <span class="identifier">Expr</span><span class="special">())</span>
      <span class="special">:</span> <span class="identifier">base_type</span><span class="special">(</span><span class="identifier">expr</span><span class="special">)</span>
    <span class="special">{}</span>

    <span class="keyword">typedef</span> <span class="keyword">double</span> <span class="identifier">result_type</span><span class="special">;</span>

    <span class="comment">// Use CalcArity to compute the arity of Expr: </span>
    <span class="keyword">static</span> <span class="keyword">int</span> <span class="keyword">const</span> <span class="identifier">arity</span> <span class="special">=</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">result_of</span><span class="special">&lt;</span><span class="identifier">CalcArity</span><span class="special">(</span><span class="identifier">Expr</span><span class="special">)&gt;::</span><span class="identifier">type</span><span class="special">::</span><span class="identifier">value</span><span class="special">;</span>

    <span class="keyword">double</span> <span class="keyword">operator</span><span class="special">()()</span> <span class="keyword">const</span>
    <span class="special">{</span>
        <span class="identifier">BOOST_MPL_ASSERT_RELATION</span><span class="special">(</span><span class="number">0</span><span class="special">,</span> <span class="special">==,</span> <span class="identifier">arity</span><span class="special">);</span>
        <span class="identifier">calculator_context</span> <span class="identifier">ctx</span><span class="special">;</span>
        <span class="keyword">return</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">eval</span><span class="special">(*</span><span class="keyword">this</span><span class="special">,</span> <span class="identifier">ctx</span><span class="special">);</span>
    <span class="special">}</span>

    <span class="keyword">double</span> <span class="keyword">operator</span><span class="special">()(</span><span class="keyword">double</span> <span class="identifier">a1</span><span class="special">)</span> <span class="keyword">const</span>
    <span class="special">{</span>
        <span class="identifier">BOOST_MPL_ASSERT_RELATION</span><span class="special">(</span><span class="number">1</span><span class="special">,</span> <span class="special">==,</span> <span class="identifier">arity</span><span class="special">);</span>
        <span class="identifier">calculator_context</span> <span class="identifier">ctx</span><span class="special">;</span>
        <span class="identifier">ctx</span><span class="special">.</span><span class="identifier">args</span><span class="special">.</span><span class="identifier">push_back</span><span class="special">(</span><span class="identifier">a1</span><span class="special">);</span>
        <span class="keyword">return</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">eval</span><span class="special">(*</span><span class="keyword">this</span><span class="special">,</span> <span class="identifier">ctx</span><span class="special">);</span>
    <span class="special">}</span>

    <span class="keyword">double</span> <span class="keyword">operator</span><span class="special">()(</span><span class="keyword">double</span> <span class="identifier">a1</span><span class="special">,</span> <span class="keyword">double</span> <span class="identifier">a2</span><span class="special">)</span> <span class="keyword">const</span>
    <span class="special">{</span>
        <span class="identifier">BOOST_MPL_ASSERT_RELATION</span><span class="special">(</span><span class="number">2</span><span class="special">,</span> <span class="special">==,</span> <span class="identifier">arity</span><span class="special">);</span>
        <span class="identifier">calculator_context</span> <span class="identifier">ctx</span><span class="special">;</span>
        <span class="identifier">ctx</span><span class="special">.</span><span class="identifier">args</span><span class="special">.</span><span class="identifier">push_back</span><span class="special">(</span><span class="identifier">a1</span><span class="special">);</span>
        <span class="identifier">ctx</span><span class="special">.</span><span class="identifier">args</span><span class="special">.</span><span class="identifier">push_back</span><span class="special">(</span><span class="identifier">a2</span><span class="special">);</span>
        <span class="keyword">return</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">eval</span><span class="special">(*</span><span class="keyword">this</span><span class="special">,</span> <span class="identifier">ctx</span><span class="special">);</span>
    <span class="special">}</span>
<span class="special">};</span>
</pre>
<p>
            Note the use of <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">result_of</span><span class="special">&lt;&gt;</span></code> to access the return type of
            the <code class="computeroutput"><span class="identifier">CalcArity</span></code> function
            object. Since we used compile-time integers in our transforms, the arity
            of the expression is encoded in the return type of the <code class="computeroutput"><span class="identifier">CalcArity</span></code> function object. Proto grammars
            are valid TR1-style function objects, so you can use <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">result_of</span><span class="special">&lt;&gt;</span></code> to figure out their return types.
          </p>
<p>
            With our compile-time assertions in place, when users provide too many
            or too few arguments to a calculator expression, as in:
          </p>
<pre class="programlisting"><span class="special">(</span><span class="identifier">_2</span> <span class="special">*</span> <span class="identifier">_2</span><span class="special">)(</span><span class="number">42</span><span class="special">);</span> <span class="comment">// Oops, too few arguments!</span>
</pre>
<p>
            ... they will get a compile-time error message on the line with the assertion
            that reads something like this<a href="#ftn.boost_proto.users_guide.back_end.expression_transformation.example__calculator_arity.f0" class="footnote" name="boost_proto.users_guide.back_end.expression_transformation.example__calculator_arity.f0"><sup class="footnote">[29]</sup></a>:
          </p>
<pre class="programlisting">c:\boost\org\trunk\libs\proto\scratch\main.cpp(97) : error C2664: 'boost::mpl::asse
rtion_failed' : cannot convert parameter 1 from 'boost::mpl::failed ************boo
st::mpl::assert_relation&lt;x,y,__formal&gt;::************' to 'boost::mpl::assert&lt;false&gt;
::type'
   with
   [
       x=1,
       y=2,
       __formal=bool boost::mpl::operator==(boost::mpl::failed,boost::mpl::failed)
   ]
</pre>
<p>
            The point of this exercise was to show that we can write a fairly simple
            Proto grammar with embedded transforms that is declarative and readable
            and can compute interesting properties of arbitrarily complicated expressions.
            But transforms can do more than that. Boost.Xpressive uses transforms
            to turn expressions into finite state automata for matching regular expressions,
            and Boost.Spirit uses transforms to build recursive descent parser generators.
            Proto comes with a collection of built-in transforms that you can use
            to perform very sophisticated expression manipulations like these. In
            the next few sections we'll see some of them in action.
          </p>
</div>
<div class="section">
<div class="titlepage"><div><div><h5 class="title">
<a name="boost_proto.users_guide.back_end.expression_transformation.state"></a><a class="link" href="users_guide.html#boost_proto.users_guide.back_end.expression_transformation.state" title="Transforms With State Accumulation">Transforms
          With State Accumulation</a>
</h5></div></div></div>
<p>
            So far, we've only seen examples of grammars with transforms that accept
            one argument: the expression to transform. But consider for a moment
            how, in ordinary procedural code, you would turn a binary tree into a
            linked list. You would start with an empty list. Then, you would recursively
            convert the right branch to a list, and use the result as the initial
            state while converting the left branch to a list. That is, you would
            need a function that takes two parameters: the current node and the list
            so far. These sorts of <span class="emphasis"><em>accumulation</em></span> problems are
            quite common when processing trees. The linked list is an example of
            an accumulation variable or <span class="emphasis"><em>state</em></span>. Each iteration
            of the algorithm takes the current element and state, applies some binary
            function to the two and creates a new state. In the STL, this algorithm
            is called <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">accumulate</span><span class="special">()</span></code>.
            In many other languages, it is called <span class="emphasis"><em>fold</em></span>. Let's
            see how to implement a fold algorithm with Proto transforms.
          </p>
<p>
            All Proto grammars can optionally accept a state parameter in addition
            to the expression to transform. If you want to fold a tree to a list,
            you'll need to make use of the state parameter to pass around the list
            you've built so far. As for the list, the Boost.Fusion library provides
            a <code class="computeroutput"><span class="identifier">fusion</span><span class="special">::</span><span class="identifier">cons</span><span class="special">&lt;&gt;</span></code>
            type from which you can build heterogeneous lists. The type <code class="computeroutput"><span class="identifier">fusion</span><span class="special">::</span><span class="identifier">nil</span></code> represents an empty list.
          </p>
<p>
            Below is a grammar that recognizes output expressions like <code class="computeroutput"><span class="identifier">cout_</span> <span class="special">&lt;&lt;</span>
            <span class="number">42</span> <span class="special">&lt;&lt;</span>
            <span class="char">'\n'</span></code> and puts the arguments into
            a Fusion list. It is explained below.
          </p>
<pre class="programlisting"><span class="comment">// Fold the terminals in output statements like</span>
<span class="comment">// "cout_ &lt;&lt; 42 &lt;&lt; '\n'" into a Fusion cons-list.</span>
<span class="keyword">struct</span> <span class="identifier">FoldToList</span>
  <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">or_</span><span class="special">&lt;</span>
        <span class="comment">// Don't add the ostream terminal to the list</span>
        <span class="identifier">proto</span><span class="special">::</span><span class="identifier">when</span><span class="special">&lt;</span>
            <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">ostream</span> <span class="special">&amp;</span> <span class="special">&gt;</span>
          <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_state</span>
        <span class="special">&gt;</span>
        <span class="comment">// Put all other terminals at the head of the</span>
        <span class="comment">// list that we're building in the "state" parameter</span>
      <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">when</span><span class="special">&lt;</span>
            <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span><span class="identifier">_</span><span class="special">&gt;</span>
          <span class="special">,</span> <span class="identifier">fusion</span><span class="special">::</span><span class="identifier">cons</span><span class="special">&lt;</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_value</span><span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_state</span><span class="special">&gt;(</span>
                <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_value</span><span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_state</span>
            <span class="special">)</span>
        <span class="special">&gt;</span>
        <span class="comment">// For left-shift operations, first fold the right</span>
        <span class="comment">// child to a list using the current state. Use</span>
        <span class="comment">// the result as the state parameter when folding</span>
        <span class="comment">// the left child to a list.</span>
      <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">when</span><span class="special">&lt;</span>
            <span class="identifier">proto</span><span class="special">::</span><span class="identifier">shift_left</span><span class="special">&lt;</span><span class="identifier">FoldToList</span><span class="special">,</span> <span class="identifier">FoldToList</span><span class="special">&gt;</span>
          <span class="special">,</span> <span class="identifier">FoldToList</span><span class="special">(</span>
                <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_left</span>
              <span class="special">,</span> <span class="identifier">FoldToList</span><span class="special">(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_right</span><span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_state</span><span class="special">)</span>
            <span class="special">)</span>
        <span class="special">&gt;</span>
    <span class="special">&gt;</span>
<span class="special">{};</span>
</pre>
<p>
            Before reading on, see if you can apply what you know already about object,
            callable and primitive transforms to figure out how this grammar works.
          </p>
<p>
            When you use the <code class="computeroutput"><span class="identifier">FoldToList</span></code>
            function, you'll need to pass two arguments: the expression to fold,
            and the initial state: an empty list. Those two arguments get passed
            around to each transform. We learned previously that <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_value</span></code>
            is a primitive transform that accepts a terminal expression and extracts
            its value. What we didn't know until now was that it also accepts the
            current state <span class="emphasis"><em>and ignores it</em></span>. <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_state</span></code>
            is also a primitive transform. It accepts the current expression, which
            it ignores, and the current state, which it returns.
          </p>
<p>
            When we find a terminal, we stick it at the head of the cons list, using
            the current state as the tail of the list. (The first alternate causes
            the <code class="computeroutput"><span class="identifier">ostream</span></code> to be skipped.
            We don't want <code class="computeroutput"><span class="identifier">cout</span></code> in
            the list.) When we find a shift-left node, we apply the following transform:
          </p>
<pre class="programlisting"><span class="comment">// Fold the right child and use the result as</span>
<span class="comment">// state while folding the right.</span>
<span class="identifier">FoldToList</span><span class="special">(</span>
    <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_left</span>
  <span class="special">,</span> <span class="identifier">FoldToList</span><span class="special">(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_right</span><span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_state</span><span class="special">)</span>
<span class="special">)</span>
</pre>
<p>
            You can read this transform as follows: using the current state, fold
            the right child to a list. Use the new list as the state while folding
            the left child to a list.
          </p>
<div class="tip"><table border="0" summary="Tip">
<tr>
<td rowspan="2" align="center" valign="top" width="25"><img alt="[Tip]" src="../../../doc/src/images/tip.png"></td>
<th align="left">Tip</th>
</tr>
<tr><td align="left" valign="top">
<p>
              If your compiler is Microsoft Visual C++, you'll find that the above
              transform does not compile. The compiler has bugs with its handling
              of nested function types. You can work around the bug by wrapping the
              inner transform in <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">call</span><span class="special">&lt;&gt;</span></code> as follows:
            </p>
<p>
</p>
<pre class="programlisting"><span class="identifier">FoldToList</span><span class="special">(</span>
    <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_left</span>
  <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">call</span><span class="special">&lt;</span><span class="identifier">FoldToList</span><span class="special">(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_right</span><span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_state</span><span class="special">)&gt;</span>
<span class="special">)</span>
</pre>
<p>
            </p>
<p>
              <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">call</span><span class="special">&lt;&gt;</span></code>
              turns a callable transform into a primitive transform, but more on
              that later.
            </p>
</td></tr>
</table></div>
<p>
            Now that we have defined the <code class="computeroutput"><span class="identifier">FoldToList</span></code>
            function object, we can use it to turn output expressions into lists
            as follows:
          </p>
<pre class="programlisting"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">ostream</span> <span class="special">&amp;&gt;::</span><span class="identifier">type</span> <span class="keyword">const</span> <span class="identifier">cout_</span> <span class="special">=</span> <span class="special">{</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span><span class="special">};</span>

<span class="comment">// This is the type of the list we build below</span>
<span class="keyword">typedef</span>
    <span class="identifier">fusion</span><span class="special">::</span><span class="identifier">cons</span><span class="special">&lt;</span>
        <span class="keyword">int</span>
      <span class="special">,</span> <span class="identifier">fusion</span><span class="special">::</span><span class="identifier">cons</span><span class="special">&lt;</span>
            <span class="keyword">double</span>
          <span class="special">,</span> <span class="identifier">fusion</span><span class="special">::</span><span class="identifier">cons</span><span class="special">&lt;</span>
                <span class="keyword">char</span>
              <span class="special">,</span> <span class="identifier">fusion</span><span class="special">::</span><span class="identifier">nil</span>
            <span class="special">&gt;</span>
        <span class="special">&gt;</span>
    <span class="special">&gt;</span>
<span class="identifier">result_type</span><span class="special">;</span>

<span class="comment">// Fold an output expression into a Fusion list, using</span>
<span class="comment">// fusion::nil as the initial state of the transformation.</span>
<span class="identifier">FoldToList</span> <span class="identifier">to_list</span><span class="special">;</span>
<span class="identifier">result_type</span> <span class="identifier">args</span> <span class="special">=</span> <span class="identifier">to_list</span><span class="special">(</span><span class="identifier">cout_</span> <span class="special">&lt;&lt;</span> <span class="number">1</span> <span class="special">&lt;&lt;</span> <span class="number">3.14</span> <span class="special">&lt;&lt;</span> <span class="char">'\n'</span><span class="special">,</span> <span class="identifier">fusion</span><span class="special">::</span><span class="identifier">nil</span><span class="special">());</span>

<span class="comment">// Now "args" is the list: {1, 3.14, '\n'}</span>
</pre>
<p>
            When writing transforms, "fold" is such a basic operation that
            Proto provides a number of built-in fold transforms. We'll get to them
            later. For now, rest assured that you won't always have to stretch your
            brain so far to do such basic things.
          </p>
</div>
<div class="section">
<div class="titlepage"><div><div><h5 class="title">
<a name="boost_proto.users_guide.back_end.expression_transformation.data"></a><a class="link" href="users_guide.html#boost_proto.users_guide.back_end.expression_transformation.data" title="Passing Auxiliary Data to Transforms">Passing
          Auxiliary Data to Transforms</a>
</h5></div></div></div>
<p>
            In the last section, we saw that we can pass a second parameter to grammars
            with transforms: an accumulation variable or <span class="emphasis"><em>state</em></span>
            that gets updated as your transform executes. There are times when your
            transforms will need to access auxiliary data that does <span class="emphasis"><em>not</em></span>
            accumulate, so bundling it with the state parameter is impractical. Instead,
            you can pass auxiliary data as a third parameter, known as the <span class="emphasis"><em>data</em></span>
            parameter.
          </p>
<p>
            Let's modify our previous example so that it writes each terminal to
            <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span></code> before it puts it into a list.
            This could be handy for debugging your transforms, for instance. We can
            make it general by passing a <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">ostream</span></code>
            into the transform in the data parameter. Within the transform itself,
            we can retrieve the <code class="computeroutput"><span class="identifier">ostream</span></code>
            with the <code class="computeroutput"><a class="link" href="../boost/proto/_data.html" title="Struct _data">proto::_data</a></code>
            transform. The strategy is as follows: use the <code class="computeroutput"><a class="link" href="../boost/proto/and_.html" title="Struct template and_">proto::and_&lt;&gt;</a></code>
            transform to chain two actions. The second action will create the <code class="computeroutput"><span class="identifier">fusion</span><span class="special">::</span><span class="identifier">cons</span><span class="special">&lt;&gt;</span></code>
            node as before. The first action, however, will display the current expression.
            For that, we first construct an instance of <code class="computeroutput"><a class="link" href="../boost/proto/functional/display_expr.html" title="Struct display_expr">proto::functional::display_expr</a></code> and then call
            it.
          </p>
<pre class="programlisting"><span class="comment">// Fold the terminals in output statements like</span>
<span class="comment">// "cout_ &lt;&lt; 42 &lt;&lt; '\n'" into a Fusion cons-list.</span>
<span class="keyword">struct</span> <span class="identifier">FoldToList</span>
  <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">or_</span><span class="special">&lt;</span>
        <span class="comment">// Don't add the ostream terminal to the list</span>
        <span class="identifier">proto</span><span class="special">::</span><span class="identifier">when</span><span class="special">&lt;</span>
            <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">ostream</span> <span class="special">&amp;</span> <span class="special">&gt;</span>
          <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_state</span>
        <span class="special">&gt;</span>
        <span class="comment">// Put all other terminals at the head of the</span>
        <span class="comment">// list that we're building in the "state" parameter</span>
      <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">when</span><span class="special">&lt;</span>
            <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span><span class="identifier">_</span><span class="special">&gt;</span>
          <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">and_</span><span class="special">&lt;</span>
                <span class="comment">// First, write the terminal to an ostream passed</span>
                <span class="comment">// in the data parameter</span>
                <span class="identifier">proto</span><span class="special">::</span><span class="identifier">lazy</span><span class="special">&lt;</span>
                    <span class="identifier">proto</span><span class="special">::</span><span class="identifier">make</span><span class="special">&lt;</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">functional</span><span class="special">::</span><span class="identifier">display_expr</span><span class="special">(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_data</span><span class="special">)&gt;(</span><span class="identifier">_</span><span class="special">)</span>
                <span class="special">&gt;</span>
                <span class="comment">// Then, constuct the new cons list.</span>
              <span class="special">,</span> <span class="identifier">fusion</span><span class="special">::</span><span class="identifier">cons</span><span class="special">&lt;</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_value</span><span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_state</span><span class="special">&gt;(</span>
                    <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_value</span><span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_state</span>
                <span class="special">)</span>
            <span class="special">&gt;</span>
        <span class="special">&gt;</span>
        <span class="comment">// For left-shift operations, first fold the right</span>
        <span class="comment">// child to a list using the current state. Use</span>
        <span class="comment">// the result as the state parameter when folding</span>
        <span class="comment">// the left child to a list.</span>
      <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">when</span><span class="special">&lt;</span>
            <span class="identifier">proto</span><span class="special">::</span><span class="identifier">shift_left</span><span class="special">&lt;</span><span class="identifier">FoldToList</span><span class="special">,</span> <span class="identifier">FoldToList</span><span class="special">&gt;</span>
          <span class="special">,</span> <span class="identifier">FoldToList</span><span class="special">(</span>
                <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_left</span>
              <span class="special">,</span> <span class="identifier">FoldToList</span><span class="special">(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_right</span><span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_state</span><span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_data</span><span class="special">)</span>
              <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_data</span>
            <span class="special">)</span>
        <span class="special">&gt;</span>
    <span class="special">&gt;</span>
<span class="special">{};</span>
</pre>
<p>
            This is a lot to take in, no doubt. But focus on the second <code class="computeroutput"><span class="identifier">when</span></code> clause above. It says: when you
            find a terminal, first display the terminal using the <code class="computeroutput"><span class="identifier">ostream</span></code>
            you find in the data parameter, then take the value of the terminal and
            the current state to build a new <code class="computeroutput"><span class="identifier">cons</span></code>
            list. The function object <code class="computeroutput"><span class="identifier">display_expr</span></code>
            does the job of printing the terminal, and <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">and_</span><span class="special">&lt;&gt;</span></code> chains the actions together and
            executes them in sequence, returning the result of the last one.
          </p>
<div class="note"><table border="0" summary="Note">
<tr>
<td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../doc/src/images/note.png"></td>
<th align="left">Note</th>
</tr>
<tr><td align="left" valign="top"><p>
              Also new is <code class="computeroutput"><a class="link" href="../boost/proto/lazy.html" title="Struct template lazy">proto::lazy&lt;&gt;</a></code>. Sometimes you
              don't have a ready-made callable object to execute. Instead, you want
              to first make one and <span class="emphasis"><em>then</em></span> execute it. Above,
              we need to create a <code class="computeroutput"><span class="identifier">display_expr</span></code>,
              initializing it with our <code class="computeroutput"><span class="identifier">ostream</span></code>.
              After that, we want to invoke it by passing it the current expression.
              It's as if we were doing <code class="computeroutput"><span class="identifier">display_expr</span><span class="special">(</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span><span class="special">)(</span><span class="emphasis"><em>the-expr</em></span><span class="special">)</span></code>.
              We achieve this two-phase evaluation using <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">lazy</span><span class="special">&lt;&gt;</span></code>. If this doesn't make sense
              yet, don't worry about it.
            </p></td></tr>
</table></div>
<p>
            We can use the above transform as before, but now we can pass an <code class="computeroutput"><span class="identifier">ostream</span></code> as the third parameter and
            get to watch the transform in action. Here's a sample usage:
          </p>
<pre class="programlisting"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">ostream</span> <span class="special">&amp;&gt;::</span><span class="identifier">type</span> <span class="keyword">const</span> <span class="identifier">cout_</span> <span class="special">=</span> <span class="special">{</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span><span class="special">};</span>

<span class="comment">// This is the type of the list we build below</span>
<span class="keyword">typedef</span>
    <span class="identifier">fusion</span><span class="special">::</span><span class="identifier">cons</span><span class="special">&lt;</span>
        <span class="keyword">int</span>
      <span class="special">,</span> <span class="identifier">fusion</span><span class="special">::</span><span class="identifier">cons</span><span class="special">&lt;</span>
            <span class="keyword">double</span>
          <span class="special">,</span> <span class="identifier">fusion</span><span class="special">::</span><span class="identifier">cons</span><span class="special">&lt;</span>
                <span class="keyword">char</span>
              <span class="special">,</span> <span class="identifier">fusion</span><span class="special">::</span><span class="identifier">nil</span>
            <span class="special">&gt;</span>
        <span class="special">&gt;</span>
    <span class="special">&gt;</span>
<span class="identifier">result_type</span><span class="special">;</span>

<span class="comment">// Fold an output expression into a Fusion list, using</span>
<span class="comment">// fusion::nil as the initial state of the transformation.</span>
<span class="comment">// Pass std::cout as the data parameter so that we can track</span>
<span class="comment">// the progress of the transform on the console.</span>
<span class="identifier">FoldToList</span> <span class="identifier">to_list</span><span class="special">;</span>
<span class="identifier">result_type</span> <span class="identifier">args</span> <span class="special">=</span> <span class="identifier">to_list</span><span class="special">(</span><span class="identifier">cout_</span> <span class="special">&lt;&lt;</span> <span class="number">1</span> <span class="special">&lt;&lt;</span> <span class="number">3.14</span> <span class="special">&lt;&lt;</span> <span class="char">'\n'</span><span class="special">,</span> <span class="identifier">fusion</span><span class="special">::</span><span class="identifier">nil</span><span class="special">(),</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span><span class="special">);</span>

<span class="comment">// Now "args" is the list: {1, 3.14, '\n'}</span>
</pre>
<p>
            This code displays the following:
          </p>
<pre class="programlisting">terminal(
)
terminal(3.14)
terminal(1)</pre>
<p>
            This is a rather round-about way of demonstrating that you can pass extra
            data to a transform as a third parameter. There are no restrictions on
            what this parameter can be, and, unlike the state parameter, Proto will
            never mess with it.
          </p>
<h6>
<a name="boost_proto.users_guide.back_end.expression_transformation.data.h0"></a>
            <span class="phrase"><a name="boost_proto.users_guide.back_end.expression_transformation.data.transform_environment_variables"></a></span><a class="link" href="users_guide.html#boost_proto.users_guide.back_end.expression_transformation.data.transform_environment_variables">Transform
            Environment Variables</a>
          </h6>
<div class="note"><table border="0" summary="Note">
<tr>
<td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../doc/src/images/note.png"></td>
<th align="left">Note</th>
</tr>
<tr><td align="left" valign="top"><p>
              <span class="emphasis"><em>This is an advanced topic. Feel free to skip if you are new
              to Proto.</em></span>
            </p></td></tr>
</table></div>
<p>
            The example above uses the data parameter as a transport mechanism for
            an unstructured blob of data; in this case, a reference to an <code class="computeroutput"><span class="identifier">ostream</span></code>. As your Proto algorithms become
            more sophisticated, you may find that an unstructured blob of data isn't
            terribly convenient to work with. Different parts of your algorithm may
            be interested in different bits of data. What you want, instead, is a
            way to pass in a collection of <span class="emphasis"><em>environment variables</em></span>
            to a transform, like a collection of key/value pairs. Then, you can easily
            get at the piece of data you want by asking the data parameter for the
            value associated with a particular key. Proto's <span class="emphasis"><em>transform environments</em></span>
            give you just that.
          </p>
<p>
            Let's start by defining a key.
          </p>
<pre class="programlisting"><span class="identifier">BOOST_PROTO_DEFINE_ENV_VAR</span><span class="special">(</span><span class="identifier">mykey_type</span><span class="special">,</span> <span class="identifier">mykey</span><span class="special">);</span>
</pre>
<p>
            This defines a global constant <code class="computeroutput"><span class="identifier">mykey</span></code>
            with the type <code class="computeroutput"><span class="identifier">mykey_type</span></code>.
            We can use <code class="computeroutput"><span class="identifier">mykey</span></code> to store
            a piece of assiciated data in a transform environment, as so:
          </p>
<pre class="programlisting"><span class="comment">// Call the MyEval algorithm with a transform environment containing</span>
<span class="comment">// two key/value pairs: one for proto::data and one for mykey</span>
<span class="identifier">MyEval</span><span class="special">()(</span> <span class="identifier">expr</span><span class="special">,</span> <span class="identifier">state</span><span class="special">,</span> <span class="special">(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">data</span> <span class="special">=</span> <span class="number">42</span><span class="special">,</span> <span class="identifier">mykey</span> <span class="special">=</span> <span class="string">"hello world"</span><span class="special">)</span> <span class="special">);</span>
</pre>
<p>
            The above means to invoke the <code class="computeroutput"><span class="identifier">MyEval</span></code>
            algorithm with three parameters: an expression, an initial state, and
            a transform environment containing two key/value pairs.
          </p>
<p>
            From within a Proto algorithm, you can access the values associated with
            different keys using the <code class="computeroutput"><a class="link" href="../boost/proto/_env_var.html" title="Struct template _env_var">proto::_env_var&lt;&gt;</a></code>
            transform. For instance, <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_env_var</span><span class="special">&lt;</span><span class="identifier">mykey_type</span><span class="special">&gt;</span></code> would fetch the value <code class="computeroutput"><span class="string">"hello world"</span></code> from the transform
            environment created above.
          </p>
<p>
            The <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_data</span></code> transform has some additional
            smarts. Rather than always returning the third parameter regarless of
            whether it is a blob or a transform environment, it checks first to see
            if it's a blob or not. If so, that's what gets returned. If not, it returns
            the value associated with the <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">data</span></code>
            key. In the above example, that would be the value <code class="computeroutput"><span class="number">42</span></code>.
          </p>
<p>
            There's a small host of functions, metafunction, and classes that you
            can use to create and manipulate transform environments, some for testing
            whether an object is a transform environment, some for coercing an object
            to be a transform environment, and some for querying a transform environment
            whether or not is has a value for a particular key. For an exhaustive
            treatment of the topic, check out the reference for the <code class="computeroutput"><a class="link" href="reference.html#header.boost.proto.transform.env_hpp" title="Header &lt;boost/proto/transform/env.hpp&gt;">boost/proto/transform/env.hpp</a></code>
            header.
          </p>
</div>
<div class="section">
<div class="titlepage"><div><div><h5 class="title">
<a name="boost_proto.users_guide.back_end.expression_transformation.implicit_params"></a><a class="link" href="users_guide.html#boost_proto.users_guide.back_end.expression_transformation.implicit_params" title="Implicit Parameters to Primitive Transforms">Implicit
          Parameters to Primitive Transforms</a>
</h5></div></div></div>
<p>
            Let's use <code class="computeroutput"><span class="identifier">FoldToList</span></code>
            example from the previous two sections to illustrate some other niceties
            of Proto transforms. We've seen that grammars, when used as function
            objects, can accept up to 3 parameters, and that when using these grammars
            in callable transforms, you can also specify up to 3 parameters. Let's
            take another look at the transform associated with non-terminals from
            the last section:
          </p>
<pre class="programlisting"><span class="identifier">FoldToList</span><span class="special">(</span>
    <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_left</span>
  <span class="special">,</span> <span class="identifier">FoldToList</span><span class="special">(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_right</span><span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_state</span><span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_data</span><span class="special">)</span>
  <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_data</span>
<span class="special">)</span>
</pre>
<p>
            Here we specify all three parameters to both invocations of the <code class="computeroutput"><span class="identifier">FoldToList</span></code> grammar. But we don't have
            to specify all three. If we don't specify a third parameter, <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_data</span></code> is assumed. Likewise for the
            second parameter and <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_state</span></code>.
            So the above transform could have been written more simply as:
          </p>
<pre class="programlisting"><span class="identifier">FoldToList</span><span class="special">(</span>
    <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_left</span>
  <span class="special">,</span> <span class="identifier">StringCopy</span><span class="special">(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_right</span><span class="special">)</span>
<span class="special">)</span>
</pre>
<p>
            The same is true for any primitive transform. The following are all equivalent:
          </p>
<div class="table">
<a name="boost_proto.users_guide.back_end.expression_transformation.implicit_params.t0"></a><p class="title"><b>Table&#160;32.9.&#160;Implicit Parameters to Primitive Transforms</b></p>
<div class="table-contents"><table class="table" summary="Implicit Parameters to Primitive Transforms">
<colgroup><col></colgroup>
<thead><tr><th>
                    <p>
                      Equivalent Transforms
                    </p>
                  </th></tr></thead>
<tbody>
<tr><td>
                    <p>
                      <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">when</span><span class="special">&lt;</span><span class="identifier">_</span><span class="special">,</span>
                      <span class="identifier">FoldToList</span><span class="special">&gt;</span></code>
                    </p>
                  </td></tr>
<tr><td>
                    <p>
                      <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">when</span><span class="special">&lt;</span><span class="identifier">_</span><span class="special">,</span>
                      <span class="identifier">FoldToList</span><span class="special">()&gt;</span></code>
                    </p>
                  </td></tr>
<tr><td>
                    <p>
                      <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">when</span><span class="special">&lt;</span><span class="identifier">_</span><span class="special">,</span>
                      <span class="identifier">FoldToList</span><span class="special">(</span><span class="identifier">_</span><span class="special">)&gt;</span></code>
                    </p>
                  </td></tr>
<tr><td>
                    <p>
                      <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">when</span><span class="special">&lt;</span><span class="identifier">_</span><span class="special">,</span>
                      <span class="identifier">FoldToList</span><span class="special">(</span><span class="identifier">_</span><span class="special">,</span>
                      <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_state</span><span class="special">)&gt;</span></code>
                    </p>
                  </td></tr>
<tr><td>
                    <p>
                      <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">when</span><span class="special">&lt;</span><span class="identifier">_</span><span class="special">,</span>
                      <span class="identifier">FoldToList</span><span class="special">(</span><span class="identifier">_</span><span class="special">,</span>
                      <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_state</span><span class="special">,</span>
                      <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_data</span><span class="special">)&gt;</span></code>
                    </p>
                  </td></tr>
</tbody>
</table></div>
</div>
<br class="table-break"><div class="note"><table border="0" summary="Note">
<tr>
<td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../doc/src/images/note.png"></td>
<th align="left">Note</th>
</tr>
<tr><td align="left" valign="top">
<p>
              <span class="bold"><strong>Grammars Are Primitive Transforms Are Function
              Objects</strong></span>
            </p>
<p>
              So far, we've said that all Proto grammars are function objects. But
              it's more accurate to say that Proto grammars are primitive transforms
              -- a special kind of function object that takes between 1 and 3 arguments,
              and that Proto knows to treat specially when used in a callable transform,
              as in the table above.
            </p>
</td></tr>
</table></div>
<div class="note"><table border="0" summary="Note">
<tr>
<td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../doc/src/images/note.png"></td>
<th align="left">Note</th>
</tr>
<tr><td align="left" valign="top">
<p>
              <span class="bold"><strong>Not All Function Objects Are Primitive Transforms</strong></span>
            </p>
<p>
              You might be tempted now to drop the <code class="computeroutput"><span class="identifier">_state</span></code>
              and <code class="computeroutput"><span class="identifier">_data</span></code> parameters
              for all your callable transforms. That would be an error. You can only
              do that for primitive transforms, and not all callables are primitive
              transforms. Later on, we'll see what distinguishes ordinary callables
              from their more powerful primitive transfor cousins, but the short
              version is this: primitive transforms inherit from <code class="computeroutput"><a class="link" href="../boost/proto/transform.html" title="Struct template transform">proto::transform&lt;&gt;</a></code>.
            </p>
</td></tr>
</table></div>
<p>
            Once you know that primitive transforms will always receive all three
            parameters -- expression, state, and data -- it makes things possible
            that wouldn't be otherwise. For instance, consider that for binary expressions,
            these two transforms are equivalent. Can you see why?
          </p>
<div class="table">
<a name="boost_proto.users_guide.back_end.expression_transformation.implicit_params.t1"></a><p class="title"><b>Table&#160;32.10.&#160;Two Equivalent Transforms</b></p>
<div class="table-contents"><table class="table" summary="Two Equivalent Transforms">
<colgroup>
<col>
<col>
</colgroup>
<thead><tr>
<th>
                    <p>
                      Without <code class="literal">proto::reverse_fold&lt;&gt;</code>
                    </p>
                  </th>
<th>
                    <p>
                      With <code class="literal">proto::reverse_fold&lt;&gt;</code>
                    </p>
                  </th>
</tr></thead>
<tbody><tr>
<td>
                    <p>
</p>
<pre class="programlisting"><span class="identifier">FoldToList</span><span class="special">(</span>
    <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_left</span>
  <span class="special">,</span> <span class="identifier">FoldToList</span><span class="special">(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_right</span><span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_state</span><span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_data</span><span class="special">)</span>
  <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_data</span>
<span class="special">)</span></pre>
<p>
                    </p>
                  </td>
<td>
                    <p>
</p>
<pre class="programlisting"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">reverse_fold</span><span class="special">&lt;</span><span class="identifier">_</span><span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_state</span><span class="special">,</span> <span class="identifier">FoldToList</span><span class="special">&gt;</span></pre>
<p>
                    </p>
                  </td>
</tr></tbody>
</table></div>
</div>
<br class="table-break">
</div>
<div class="section">
<div class="titlepage"><div><div><h5 class="title">
<a name="boost_proto.users_guide.back_end.expression_transformation.unpacking_expressions"></a><a class="link" href="users_guide.html#boost_proto.users_guide.back_end.expression_transformation.unpacking_expressions" title="Unpacking Expressions">Unpacking
          Expressions</a>
</h5></div></div></div>
<p>
            Processing expressions with an arbitrary number of children can be a
            pain. What if you want to do something to each child, then pass the results
            as arguments to some other function? Can you do it just once without
            worrying about how many children an expression has? Yes. This is where
            Proto's <span class="emphasis"><em>unpacking expressions</em></span> come in handy. Unpacking
            expressions give you a way to write callable and object transforms that
            handle <span class="emphasis"><em>n</em></span>-ary expressions.
          </p>
<div class="note"><table border="0" summary="Note">
<tr>
<td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../doc/src/images/note.png"></td>
<th align="left">Note</th>
</tr>
<tr><td align="left" valign="top">
<p>
              <span class="bold"><strong>Inspired by C++11 Variadic Templates</strong></span>
            </p>
<p>
              Proto's unpacking expressions take inspiration from the C++11 feature
              of the same name. If you are familiar with variadic functions, and
              in particular how to expand a function parameter pack, this discussion
              should seem very familiar. However, this feature doesn't actually use
              any C++11 features, so the code describe here will work with any compliant
              C++98 compiler.
            </p>
</td></tr>
</table></div>
<h6>
<a name="boost_proto.users_guide.back_end.expression_transformation.unpacking_expressions.h0"></a>
            <span class="phrase"><a name="boost_proto.users_guide.back_end.expression_transformation.unpacking_expressions.example__a_c___expression_evaluator"></a></span><a class="link" href="users_guide.html#boost_proto.users_guide.back_end.expression_transformation.unpacking_expressions.example__a_c___expression_evaluator">Example:
            A C++ Expression Evaluator</a>
          </h6>
<p>
            Proto has the built-in <code class="computeroutput"><a class="link" href="../boost/proto/_default.html" title="Struct template _default">proto::_default&lt;&gt;</a></code>
            transform for evaluating Proto expressions in a C++-ish way. But if it
            didn't, it wouldn't be too hard to implement one from scratch using Proto's
            unpacking patterns. The transform <code class="computeroutput"><span class="identifier">eval</span></code>
            below does just that.
          </p>
<pre class="programlisting"><span class="comment">// A callable polymorphic function object that takes an unpacked expression</span>
<span class="comment">// and a tag, and evaluates the expression. A plus tag and two operands adds</span>
<span class="comment">// them with operator +, for instance.</span>
<span class="keyword">struct</span> <span class="identifier">do_eval</span> <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">callable</span>
<span class="special">{</span>
    <span class="keyword">typedef</span> <span class="keyword">double</span> <span class="identifier">result_type</span><span class="special">;</span>

<span class="preprocessor">#define</span> <span class="identifier">UNARY_OP</span><span class="special">(</span><span class="identifier">TAG</span><span class="special">,</span> <span class="identifier">OP</span><span class="special">)</span>                                                       <span class="special">\</span>
    <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">Arg</span><span class="special">&gt;</span>                                                      <span class="special">\</span>
    <span class="keyword">double</span> <span class="keyword">operator</span><span class="special">()(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">TAG</span><span class="special">,</span> <span class="identifier">Arg</span> <span class="identifier">arg</span><span class="special">)</span> <span class="keyword">const</span>                           <span class="special">\</span>
    <span class="special">{</span>                                                                           <span class="special">\</span>
        <span class="keyword">return</span> <span class="identifier">OP</span> <span class="identifier">arg</span><span class="special">;</span>                                                          <span class="special">\</span>
    <span class="special">}</span>                                                                           <span class="special">\</span>
    <span class="comment">/**/</span>

<span class="preprocessor">#define</span> <span class="identifier">BINARY_OP</span><span class="special">(</span><span class="identifier">TAG</span><span class="special">,</span> <span class="identifier">OP</span><span class="special">)</span>                                                      <span class="special">\</span>
    <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">Left</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">Right</span><span class="special">&gt;</span>                                     <span class="special">\</span>
    <span class="keyword">double</span> <span class="keyword">operator</span><span class="special">()(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">TAG</span><span class="special">,</span> <span class="identifier">Left</span> <span class="identifier">left</span><span class="special">,</span> <span class="identifier">Right</span> <span class="identifier">right</span><span class="special">)</span> <span class="keyword">const</span>            <span class="special">\</span>
    <span class="special">{</span>                                                                           <span class="special">\</span>
        <span class="keyword">return</span> <span class="identifier">left</span> <span class="identifier">OP</span> <span class="identifier">right</span><span class="special">;</span>                                                   <span class="special">\</span>
    <span class="special">}</span>                                                                           <span class="special">\</span>
    <span class="comment">/**/</span>

    <span class="identifier">UNARY_OP</span><span class="special">(</span><span class="identifier">negate</span><span class="special">,</span> <span class="special">-)</span>
    <span class="identifier">BINARY_OP</span><span class="special">(</span><span class="identifier">plus</span><span class="special">,</span> <span class="special">+)</span>
    <span class="identifier">BINARY_OP</span><span class="special">(</span><span class="identifier">minus</span><span class="special">,</span> <span class="special">-)</span>
    <span class="identifier">BINARY_OP</span><span class="special">(</span><span class="identifier">multiplies</span><span class="special">,</span> <span class="special">*)</span>
    <span class="identifier">BINARY_OP</span><span class="special">(</span><span class="identifier">divides</span><span class="special">,</span> <span class="special">/)</span>
    <span class="comment">/*... others ...*/</span>
<span class="special">};</span>

<span class="keyword">struct</span> <span class="identifier">eval</span>
  <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">or_</span><span class="special">&lt;</span>
        <span class="comment">// Evaluate terminals by simply returning their value</span>
        <span class="identifier">proto</span><span class="special">::</span><span class="identifier">when</span><span class="special">&lt;</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span><span class="identifier">_</span><span class="special">&gt;,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_value</span><span class="special">&gt;</span>

        <span class="comment">// Non-terminals are handled by unpacking the expression,</span>
        <span class="comment">// recursively calling eval on each child, and passing</span>
        <span class="comment">// the results along with the expression's tag to do_eval</span>
        <span class="comment">// defined above.</span>
      <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">otherwise</span><span class="special">&lt;</span><span class="identifier">do_eval</span><span class="special">(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag_of</span><span class="special">&lt;</span><span class="identifier">_</span><span class="special">&gt;(),</span> <span class="identifier">eval</span><span class="special">(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">pack</span><span class="special">(</span><span class="identifier">_</span><span class="special">))...)&gt;</span>
        <span class="comment">// UNPACKING PATTERN HERE -------------------^^^^^^^^^^^^^^^^^^^^^^^^</span>
    <span class="special">&gt;</span>
<span class="special">{};</span>
</pre>
<p>
            The bulk of the above code is devoted to the <code class="computeroutput"><span class="identifier">do_eval</span></code>
            function object that maps tag types to behaviors, but the interesting
            bit is the definition of the <code class="computeroutput"><span class="identifier">eval</span></code>
            algorithm at the bottom. Terminals are handled quite simply, but non-terminals
            could be unary, binary, ternary, even <span class="emphasis"><em>n</em></span>-ary if we
            consider function call expressions. The <code class="computeroutput"><span class="identifier">eval</span></code>
            algorithm handles this uniformly with the help of an unpacking pattern.
          </p>
<p>
            Non-terminals are evaluated with this callable transform:
          </p>
<pre class="programlisting"><span class="identifier">do_eval</span><span class="special">(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag_of</span><span class="special">&lt;</span><span class="identifier">_</span><span class="special">&gt;(),</span> <span class="identifier">eval</span><span class="special">(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">pack</span><span class="special">(</span><span class="identifier">_</span><span class="special">))...)</span>
</pre>
<p>
            You can read this as: call the <code class="computeroutput"><span class="identifier">do_eval</span></code>
            function object with the tag of the current expression and all its children
            after they have each been evaluated with <code class="computeroutput"><span class="identifier">eval</span></code>.
            The unpacking pattern is the bit just before the ellipsis: <code class="computeroutput"><span class="identifier">eval</span><span class="special">(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">pack</span><span class="special">(</span><span class="identifier">_</span><span class="special">))</span></code>.
          </p>
<p>
            What's going on here is this. The unpacking expression gets repeated
            once for each child in the expression currently being evaluated. In each
            repetition, the type <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">pack</span><span class="special">(</span><span class="identifier">_</span><span class="special">)</span></code> gets replaced with <code class="literal">proto::_child_c&lt;<span class="emphasis"><em>N</em></span>&gt;</code>.
            So, if a unary expression is passed to <code class="computeroutput"><span class="identifier">eval</span></code>,
            it actually gets evaluated like this:
          </p>
<pre class="programlisting"><span class="comment">// After the unpacking pattern is expanded for a unary expression</span>
<span class="identifier">do_eval</span><span class="special">(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag_of</span><span class="special">&lt;</span><span class="identifier">_</span><span class="special">&gt;(),</span> <span class="identifier">eval</span><span class="special">(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_child_c</span><span class="special">&lt;</span><span class="number">0</span><span class="special">&gt;))</span>
</pre>
<p>
            And when passed a binary expression, the unpacking pattern expands like
            this:
          </p>
<pre class="programlisting"><span class="comment">// After the unpacking pattern is expanded for a binary expression</span>
<span class="identifier">do_eval</span><span class="special">(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag_of</span><span class="special">&lt;</span><span class="identifier">_</span><span class="special">&gt;(),</span> <span class="identifier">eval</span><span class="special">(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_child_c</span><span class="special">&lt;</span><span class="number">0</span><span class="special">&gt;),</span> <span class="identifier">eval</span><span class="special">(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_child_c</span><span class="special">&lt;</span><span class="number">1</span><span class="special">&gt;))</span>
</pre>
<p>
            Although it can't happen in our example, when passed a terminal, the
            unpacking pattern expands such that it extracts the value from the terminal
            instead of the children. So it gets handled like this:
          </p>
<pre class="programlisting"><span class="comment">// If a terminal were passed to this transform, Proto would try</span>
<span class="comment">// to evaluate it like this, which would fail:</span>
<span class="identifier">do_eval</span><span class="special">(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag_of</span><span class="special">&lt;</span><span class="identifier">_</span><span class="special">&gt;(),</span> <span class="identifier">eval</span><span class="special">(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_value</span><span class="special">))</span>
</pre>
<p>
            That doesn't make sense. <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_value</span></code>
            would return something that isn't a Proto expression, and <code class="computeroutput"><span class="identifier">eval</span></code> wouldn't be able to evaluate it.
            Proto algorithms don't work unless you pass them Proto expressions.
          </p>
<div class="note"><table border="0" summary="Note">
<tr>
<td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../doc/src/images/note.png"></td>
<th align="left">Note</th>
</tr>
<tr><td align="left" valign="top">
<p>
              <span class="bold"><strong>Kickin' It Old School</strong></span>
            </p>
<p>
              You may be thinking, my compiler doesn't support C++11 variadic templates!
              How can this possibly work? The answer is simple: The <code class="computeroutput"><span class="special">...</span></code> above isn't a C++11 pack expansion.
              It's actually an old-school C-style vararg. Remember that callable
              and object transforms are <span class="emphasis"><em>function types</em></span>. A transform
              with one of these pseudo-pack expansions is really just the type of
              a boring, old vararg function. Proto just interprets it differently.
            </p>
</td></tr>
</table></div>
<p>
            Unpacking patterns are very expressive. Any callable or object transform
            can be used as an unpacking pattern, so long as <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">pack</span><span class="special">(</span><span class="identifier">_</span><span class="special">)</span></code> appears exactly once somewhere within
            it. This gives you a lot of flexibility in how you want to process the
            children of an expression before passing them on to some function object
            or object constructor.
          </p>
</div>
<div class="section">
<div class="titlepage"><div><div><h5 class="title">
<a name="boost_proto.users_guide.back_end.expression_transformation.external_transforms"></a><a class="link" href="users_guide.html#boost_proto.users_guide.back_end.expression_transformation.external_transforms" title="Separating Grammars And Transforms">Separating
          Grammars And Transforms</a>
</h5></div></div></div>
<div class="note"><table border="0" summary="Note">
<tr>
<td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../doc/src/images/note.png"></td>
<th align="left">Note</th>
</tr>
<tr><td align="left" valign="top"><p>
              This is an advanced topic that is only necessary for people defining
              large EDSLs. Feel free to skip this if you're just getting started
              with Proto.
            </p></td></tr>
</table></div>
<p>
            So far, we've seen examples of grammars with embedded transforms. In
            practice, grammars can get pretty large, and you may want to use them
            to drive several different computations. For instance, you may have a
            grammar for a linear algebra domain, and you may want to use it to compute
            the shape of the result (vector or matrix?) and also to compute the result
            optimally. You don't want to have to copy and paste the whole shebang
            just to tweak one of the embedded transforms. What you want instead is
            to define the grammar once, and specify the transforms later when you're
            ready to evaluate an expression. For that, you use <span class="emphasis"><em>external
            transforms</em></span>. The pattern you'll use is this: replace one or
            more of the transforms in your grammar with the special placeholder
            <code class="computeroutput"><a class="link" href="../boost/proto/external_transform.html" title="Struct external_transform">proto::external_transform</a></code>.
            Then, you'll create a bundle of transforms that you will pass to the
            grammar in the data parameter (the 3rd parameter after the expression
            and state) when evaluating it.
          </p>
<p>
            To illustrate external transforms, we'll build a calculator evaluator
            that can be configured to throw an exception on division by zero. Here
            is a bare-bones front end that defines a domain, a grammar, an expression
            wrapper, and some placeholder terminals.
          </p>
<pre class="programlisting"><span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">assert</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
<span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">mpl</span><span class="special">/</span><span class="keyword">int</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
<span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">fusion</span><span class="special">/</span><span class="identifier">container</span><span class="special">/</span><span class="identifier">vector</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
<span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">fusion</span><span class="special">/</span><span class="identifier">container</span><span class="special">/</span><span class="identifier">generation</span><span class="special">/</span><span class="identifier">make_vector</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
<span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">proto</span><span class="special">/</span><span class="identifier">proto</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
<span class="keyword">namespace</span> <span class="identifier">mpl</span> <span class="special">=</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">mpl</span><span class="special">;</span>
<span class="keyword">namespace</span> <span class="identifier">proto</span> <span class="special">=</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">proto</span><span class="special">;</span>
<span class="keyword">namespace</span> <span class="identifier">fusion</span> <span class="special">=</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">fusion</span><span class="special">;</span>

<span class="comment">// The argument placeholder type</span>
<span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">I</span><span class="special">&gt;</span> <span class="keyword">struct</span> <span class="identifier">placeholder</span> <span class="special">:</span> <span class="identifier">I</span> <span class="special">{};</span>

<span class="comment">// The grammar for valid calculator expressions</span>
<span class="keyword">struct</span> <span class="identifier">calc_grammar</span>
  <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">or_</span><span class="special">&lt;</span>
        <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span><span class="identifier">placeholder</span><span class="special">&lt;</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_</span><span class="special">&gt;</span> <span class="special">&gt;</span>
      <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">&gt;</span>
      <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">plus</span><span class="special">&lt;</span><span class="identifier">calc_grammar</span><span class="special">,</span> <span class="identifier">calc_grammar</span><span class="special">&gt;</span>
      <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">minus</span><span class="special">&lt;</span><span class="identifier">calc_grammar</span><span class="special">,</span> <span class="identifier">calc_grammar</span><span class="special">&gt;</span>
      <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">multiplies</span><span class="special">&lt;</span><span class="identifier">calc_grammar</span><span class="special">,</span> <span class="identifier">calc_grammar</span><span class="special">&gt;</span>
      <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">divides</span><span class="special">&lt;</span><span class="identifier">calc_grammar</span><span class="special">,</span> <span class="identifier">calc_grammar</span><span class="special">&gt;</span>
    <span class="special">&gt;</span>
<span class="special">{};</span>

<span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">E</span><span class="special">&gt;</span> <span class="keyword">struct</span> <span class="identifier">calc_expr</span><span class="special">;</span>
<span class="keyword">struct</span> <span class="identifier">calc_domain</span> <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">domain</span><span class="special">&lt;</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">generator</span><span class="special">&lt;</span><span class="identifier">calc_expr</span><span class="special">&gt;</span> <span class="special">&gt;</span> <span class="special">{};</span>

<span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">E</span><span class="special">&gt;</span>
<span class="keyword">struct</span> <span class="identifier">calc_expr</span>
  <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">extends</span><span class="special">&lt;</span><span class="identifier">E</span><span class="special">,</span> <span class="identifier">calc_expr</span><span class="special">&lt;</span><span class="identifier">E</span><span class="special">&gt;,</span> <span class="identifier">calc_domain</span><span class="special">&gt;</span>
<span class="special">{</span>
    <span class="identifier">calc_expr</span><span class="special">(</span><span class="identifier">E</span> <span class="keyword">const</span> <span class="special">&amp;</span><span class="identifier">e</span> <span class="special">=</span> <span class="identifier">E</span><span class="special">())</span> <span class="special">:</span> <span class="identifier">calc_expr</span><span class="special">::</span><span class="identifier">proto_extends</span><span class="special">(</span><span class="identifier">e</span><span class="special">)</span> <span class="special">{}</span>
<span class="special">};</span>

<span class="identifier">calc_expr</span><span class="special">&lt;</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span><span class="identifier">placeholder</span><span class="special">&lt;</span><span class="identifier">mpl</span><span class="special">::</span><span class="identifier">int_</span><span class="special">&lt;</span><span class="number">0</span><span class="special">&gt;</span> <span class="special">&gt;</span> <span class="special">&gt;::</span><span class="identifier">type</span><span class="special">&gt;</span> <span class="identifier">_1</span><span class="special">;</span>
<span class="identifier">calc_expr</span><span class="special">&lt;</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span><span class="identifier">placeholder</span><span class="special">&lt;</span><span class="identifier">mpl</span><span class="special">::</span><span class="identifier">int_</span><span class="special">&lt;</span><span class="number">1</span><span class="special">&gt;</span> <span class="special">&gt;</span> <span class="special">&gt;::</span><span class="identifier">type</span><span class="special">&gt;</span> <span class="identifier">_2</span><span class="special">;</span>

<span class="keyword">int</span> <span class="identifier">main</span><span class="special">()</span>
<span class="special">{</span>
    <span class="comment">// Build a calculator expression, and do nothing with it.</span>
    <span class="special">(</span><span class="identifier">_1</span> <span class="special">+</span> <span class="identifier">_2</span><span class="special">);</span>
<span class="special">}</span>
</pre>
<p>
            Now, let's embed transforms into <code class="computeroutput"><span class="identifier">calc_grammar</span></code>
            so that we can use it to evaluate calculator expressions:
          </p>
<pre class="programlisting"><span class="comment">// The calculator grammar with embedded transforms for evaluating expression.</span>
<span class="keyword">struct</span> <span class="identifier">calc_grammar</span>
  <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">or_</span><span class="special">&lt;</span>
        <span class="identifier">proto</span><span class="special">::</span><span class="identifier">when</span><span class="special">&lt;</span>
            <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span><span class="identifier">placeholder</span><span class="special">&lt;</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_</span><span class="special">&gt;</span> <span class="special">&gt;</span>
          <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">functional</span><span class="special">::</span><span class="identifier">at</span><span class="special">(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_state</span><span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_value</span><span class="special">)</span>
        <span class="special">&gt;</span>
      <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">when</span><span class="special">&lt;</span>
            <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">&gt;</span>
          <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_value</span>
        <span class="special">&gt;</span>
      <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">when</span><span class="special">&lt;</span>
            <span class="identifier">proto</span><span class="special">::</span><span class="identifier">plus</span><span class="special">&lt;</span><span class="identifier">calc_grammar</span><span class="special">,</span> <span class="identifier">calc_grammar</span><span class="special">&gt;</span>
          <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_default</span><span class="special">&lt;</span><span class="identifier">calc_grammar</span><span class="special">&gt;</span>
        <span class="special">&gt;</span>
      <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">when</span><span class="special">&lt;</span>
            <span class="identifier">proto</span><span class="special">::</span><span class="identifier">minus</span><span class="special">&lt;</span><span class="identifier">calc_grammar</span><span class="special">,</span> <span class="identifier">calc_grammar</span><span class="special">&gt;</span>
          <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_default</span><span class="special">&lt;</span><span class="identifier">calc_grammar</span><span class="special">&gt;</span>
        <span class="special">&gt;</span>
      <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">when</span><span class="special">&lt;</span>
            <span class="identifier">proto</span><span class="special">::</span><span class="identifier">multiplies</span><span class="special">&lt;</span><span class="identifier">calc_grammar</span><span class="special">,</span> <span class="identifier">calc_grammar</span><span class="special">&gt;</span>
          <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_default</span><span class="special">&lt;</span><span class="identifier">calc_grammar</span><span class="special">&gt;</span>
        <span class="special">&gt;</span>
      <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">when</span><span class="special">&lt;</span>
            <span class="identifier">proto</span><span class="special">::</span><span class="identifier">divides</span><span class="special">&lt;</span><span class="identifier">calc_grammar</span><span class="special">,</span> <span class="identifier">calc_grammar</span><span class="special">&gt;</span>
          <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_default</span><span class="special">&lt;</span><span class="identifier">calc_grammar</span><span class="special">&gt;</span>
        <span class="special">&gt;</span>
    <span class="special">&gt;</span>
<span class="special">{};</span>
</pre>
<p>
            With this definition of <code class="computeroutput"><span class="identifier">calc_grammar</span></code>
            we can evaluate expressions by passing along a Fusion vector containing
            the values to use for the <code class="computeroutput"><span class="identifier">_1</span></code>
            and <code class="computeroutput"><span class="identifier">_2</span></code> placeholders:
          </p>
<pre class="programlisting"><span class="keyword">int</span> <span class="identifier">result</span> <span class="special">=</span> <span class="identifier">calc_grammar</span><span class="special">()(</span><span class="identifier">_1</span> <span class="special">+</span> <span class="identifier">_2</span><span class="special">,</span> <span class="identifier">fusion</span><span class="special">::</span><span class="identifier">make_vector</span><span class="special">(</span><span class="number">3</span><span class="special">,</span> <span class="number">4</span><span class="special">));</span>
<span class="identifier">BOOST_ASSERT</span><span class="special">(</span><span class="identifier">result</span> <span class="special">==</span> <span class="number">7</span><span class="special">);</span>
</pre>
<p>
            We also want an alternative evaluation strategy that checks for division
            by zero and throws an exception. Just how ridiculous would it be to copy
            the entire <code class="computeroutput"><span class="identifier">calc_grammar</span></code>
            just to change the one line that transforms division expressions?! External
            transforms are ideally suited to this problem.
          </p>
<p>
            First, we give the division rule in our grammar a "name"; that
            is, we make it a struct. We'll use this unique type later to dispatch
            to the right transforms.
          </p>
<pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">calc_grammar</span><span class="special">;</span>
<span class="keyword">struct</span> <span class="identifier">divides_rule</span> <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">divides</span><span class="special">&lt;</span><span class="identifier">calc_grammar</span><span class="special">,</span> <span class="identifier">calc_grammar</span><span class="special">&gt;</span> <span class="special">{};</span>
</pre>
<p>
            Next, we change <code class="computeroutput"><span class="identifier">calc_grammar</span></code>
            to make the handling of division expressions external.
          </p>
<pre class="programlisting"><span class="comment">// The calculator grammar with an external transform for evaluating</span>
<span class="comment">// division expressions.</span>
<span class="keyword">struct</span> <span class="identifier">calc_grammar</span>
  <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">or_</span><span class="special">&lt;</span>
        <span class="comment">/* ... as before ... */</span>
      <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">when</span><span class="special">&lt;</span>
            <span class="identifier">divides_rule</span>
          <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">external_transform</span>
        <span class="special">&gt;</span>
    <span class="special">&gt;</span>
<span class="special">{};</span>
</pre>
<p>
            The use of <code class="computeroutput"><a class="link" href="../boost/proto/external_transform.html" title="Struct external_transform">proto::external_transform</a></code> above
            makes the handling of division expressions externally parameterizeable.
          </p>
<p>
            Next, we use <code class="computeroutput"><a class="link" href="../boost/proto/external_transforms.html" title="Struct template external_transforms">proto::external_transforms&lt;&gt;</a></code>
            (note the trailing 's') to capture our evaluation strategy in a bundle
            that we can pass along to the transform in the data parameter. Read on
            for the explanation.
          </p>
<pre class="programlisting"><span class="comment">// Evaluate division nodes as before</span>
<span class="keyword">struct</span> <span class="identifier">non_checked_division</span>
  <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">external_transforms</span><span class="special">&lt;</span>
        <span class="identifier">proto</span><span class="special">::</span><span class="identifier">when</span><span class="special">&lt;</span> <span class="identifier">divides_rule</span><span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_default</span><span class="special">&lt;</span><span class="identifier">calc_grammar</span><span class="special">&gt;</span> <span class="special">&gt;</span>
    <span class="special">&gt;</span>
<span class="special">{};</span>

<span class="comment">/* ... */</span>

<span class="identifier">non_checked_division</span> <span class="identifier">non_checked</span><span class="special">;</span>
<span class="keyword">int</span> <span class="identifier">result2</span> <span class="special">=</span> <span class="identifier">calc_grammar</span><span class="special">()(</span><span class="identifier">_1</span> <span class="special">/</span> <span class="identifier">_2</span><span class="special">,</span> <span class="identifier">fusion</span><span class="special">::</span><span class="identifier">make_vector</span><span class="special">(</span><span class="number">6</span><span class="special">,</span> <span class="number">2</span><span class="special">),</span> <span class="identifier">non_checked</span><span class="special">);</span>
</pre>
<p>
            The struct <code class="computeroutput"><span class="identifier">non_cecked_division</span></code>
            associates the transform <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_default</span><span class="special">&lt;</span><span class="identifier">calc_grammar</span><span class="special">&gt;</span></code> with the <code class="computeroutput"><span class="identifier">divides_rule</span></code>
            grammar rule. An instance of that struct is passed along as the third
            parameter when invoking <code class="computeroutput"><span class="identifier">calc_grammar</span></code>.
          </p>
<p>
            Now, let's implement checked division. The rest should be unsurprising.
          </p>
<pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">division_by_zero</span> <span class="special">:</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">exception</span> <span class="special">{};</span>

<span class="keyword">struct</span> <span class="identifier">do_checked_divide</span> <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">callable</span>
<span class="special">{</span>
    <span class="keyword">typedef</span> <span class="keyword">int</span> <span class="identifier">result_type</span><span class="special">;</span>
    <span class="keyword">int</span> <span class="keyword">operator</span><span class="special">()(</span><span class="keyword">int</span> <span class="identifier">left</span><span class="special">,</span> <span class="keyword">int</span> <span class="identifier">right</span><span class="special">)</span> <span class="keyword">const</span>
    <span class="special">{</span>
        <span class="keyword">if</span> <span class="special">(</span><span class="identifier">right</span> <span class="special">==</span> <span class="number">0</span><span class="special">)</span> <span class="keyword">throw</span> <span class="identifier">division_by_zero</span><span class="special">();</span>
        <span class="keyword">return</span> <span class="identifier">left</span> <span class="special">/</span> <span class="identifier">right</span><span class="special">;</span>
    <span class="special">}</span>
<span class="special">};</span>

<span class="keyword">struct</span> <span class="identifier">checked_division</span>
  <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">external_transforms</span><span class="special">&lt;</span>
        <span class="identifier">proto</span><span class="special">::</span><span class="identifier">when</span><span class="special">&lt;</span>
            <span class="identifier">divides_rule</span>
          <span class="special">,</span> <span class="identifier">do_checked_divide</span><span class="special">(</span><span class="identifier">calc_grammar</span><span class="special">(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_left</span><span class="special">),</span> <span class="identifier">calc_grammar</span><span class="special">(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_right</span><span class="special">))</span>
        <span class="special">&gt;</span>
    <span class="special">&gt;</span>
<span class="special">{};</span>

<span class="comment">/* ... */</span>

<span class="keyword">try</span>
<span class="special">{</span>
    <span class="identifier">checked_division</span> <span class="identifier">checked</span><span class="special">;</span>
    <span class="keyword">int</span> <span class="identifier">result3</span> <span class="special">=</span> <span class="identifier">calc_grammar_extern</span><span class="special">()(</span><span class="identifier">_1</span> <span class="special">/</span> <span class="identifier">_2</span><span class="special">,</span> <span class="identifier">fusion</span><span class="special">::</span><span class="identifier">make_vector</span><span class="special">(</span><span class="number">6</span><span class="special">,</span> <span class="number">0</span><span class="special">),</span> <span class="identifier">checked</span><span class="special">);</span>
<span class="special">}</span>
<span class="keyword">catch</span><span class="special">(</span><span class="identifier">division_by_zero</span><span class="special">)</span>
<span class="special">{</span>
    <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="string">"caught division by zero!\n"</span><span class="special">;</span>
<span class="special">}</span>
</pre>
<p>
            The above code demonstrates how a single grammar can be used with different
            transforms specified externally. This makes it possible to reuse a grammar
            to drive several different computations.
          </p>
<h6>
<a name="boost_proto.users_guide.back_end.expression_transformation.external_transforms.h0"></a>
            <span class="phrase"><a name="boost_proto.users_guide.back_end.expression_transformation.external_transforms.separating_data_from_external_transforms"></a></span><a class="link" href="users_guide.html#boost_proto.users_guide.back_end.expression_transformation.external_transforms.separating_data_from_external_transforms">Separating
            Data From External Transforms</a>
          </h6>
<p>
            As described above, the external transforms feature usurps the data parameter,
            which is intended to be a place where you can pass arbitrary data, and
            gives it a specific meaning. But what if you are already using the data
            parameter for something else? The answer is to use a transform environment.
            By associating your external transforms with the <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">transforms</span></code>
            key, you are free to pass arbitrary data in other slots.
          </p>
<p>
            To continue the above example, what if we also needed to pass a piece
            of data into our transform along with the external transforms? It would
            look like this:
          </p>
<pre class="programlisting"><span class="keyword">int</span> <span class="identifier">result3</span> <span class="special">=</span> <span class="identifier">calc_grammar_extern</span><span class="special">()(</span>
    <span class="identifier">_1</span> <span class="special">/</span> <span class="identifier">_2</span>
  <span class="special">,</span> <span class="identifier">fusion</span><span class="special">::</span><span class="identifier">make_vector</span><span class="special">(</span><span class="number">6</span><span class="special">,</span> <span class="number">0</span><span class="special">)</span>
  <span class="special">,</span> <span class="special">(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">data</span> <span class="special">=</span> <span class="number">42</span><span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">transforms</span> <span class="special">=</span> <span class="identifier">checked</span><span class="special">)</span>
<span class="special">);</span>
</pre>
<p>
            In the above invocation of the <code class="computeroutput"><span class="identifier">calc_grammar_extern</span></code>
            algorithm, the map of external transforms is associated with the <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">transforms</span></code> key and passed to the algorithm
            in a transform environment. Also in the transform environment is a key/value
            pair that associates the value <code class="computeroutput"><span class="number">42</span></code>
            with the <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">data</span></code> key.
          </p>
</div>
<div class="section">
<div class="titlepage"><div><div><h5 class="title">
<a name="boost_proto.users_guide.back_end.expression_transformation.canned_transforms"></a><a class="link" href="users_guide.html#boost_proto.users_guide.back_end.expression_transformation.canned_transforms" title="Proto's Built-In Transforms">Proto's
          Built-In Transforms</a>
</h5></div></div></div>
<p>
            Primitive transforms are the building blocks for more interesting composite
            transforms. Proto defines a bunch of generally useful primitive transforms.
            They are summarized below.
          </p>
<div class="variablelist">
<p class="title"><b></b></p>
<dl class="variablelist">
<dt><span class="term"> <code class="computeroutput"><a class="link" href="../boost/proto/_value.html" title="Struct _value">proto::_value</a></code></span></dt>
<dd><p>
                  Given a terminal expression, return the value of the terminal.
                </p></dd>
<dt><span class="term"> <code class="computeroutput"><a class="link" href="../boost/proto/_child_c.html" title="Struct template _child_c">proto::_child_c&lt;&gt;</a></code></span></dt>
<dd><p>
                  Given a non-terminal expression, <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_child_c</span><span class="special">&lt;</span><em class="replaceable"><code>N</code></em><span class="special">&gt;</span></code> returns the <em class="replaceable"><code>N</code></em>-th
                  child.
                </p></dd>
<dt><span class="term"> <code class="computeroutput"><a class="link" href="reference.html#boost.proto._child">proto::_child</a></code></span></dt>
<dd><p>
                  A synonym for <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_child_c</span><span class="special">&lt;</span><span class="number">0</span><span class="special">&gt;</span></code>.
                </p></dd>
<dt><span class="term"> <code class="computeroutput"><a class="link" href="reference.html#boost.proto._left">proto::_left</a></code></span></dt>
<dd><p>
                  A synonym for <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_child_c</span><span class="special">&lt;</span><span class="number">0</span><span class="special">&gt;</span></code>.
                </p></dd>
<dt><span class="term"> <code class="computeroutput"><a class="link" href="reference.html#boost.proto._right">proto::_right</a></code></span></dt>
<dd><p>
                  A synonym for <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_child_c</span><span class="special">&lt;</span><span class="number">1</span><span class="special">&gt;</span></code>.
                </p></dd>
<dt><span class="term"> <code class="computeroutput"><a class="link" href="../boost/proto/_expr.html" title="Struct _expr">proto::_expr</a></code></span></dt>
<dd><p>
                  Returns the current expression unmodified.
                </p></dd>
<dt><span class="term"> <code class="computeroutput"><a class="link" href="../boost/proto/_state.html" title="Struct _state">proto::_state</a></code></span></dt>
<dd><p>
                  Returns the current state unmodified.
                </p></dd>
<dt><span class="term"> <code class="computeroutput"><a class="link" href="../boost/proto/_data.html" title="Struct _data">proto::_data</a></code></span></dt>
<dd><p>
                  Returns the current data unmodified.
                </p></dd>
<dt><span class="term"> <code class="computeroutput"><a class="link" href="../boost/proto/call.html" title="Struct template call">proto::call&lt;&gt;</a></code></span></dt>
<dd><p>
                  For a given callable transform <code class="computeroutput"><em class="replaceable"><code>CT</code></em></code>,
                  <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">call</span><span class="special">&lt;</span><em class="replaceable"><code>CT</code></em><span class="special">&gt;</span></code> turns the callable transform
                  into a primitive transform. This is useful for disambiguating callable
                  transforms from object transforms, and also for working around
                  compiler bugs with nested function types.
                </p></dd>
<dt><span class="term"> <code class="computeroutput"><a class="link" href="../boost/proto/make.html" title="Struct template make">proto::make&lt;&gt;</a></code></span></dt>
<dd><p>
                  For a given object transform <code class="computeroutput"><em class="replaceable"><code>OT</code></em></code>,
                  <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">make</span><span class="special">&lt;</span><em class="replaceable"><code>OT</code></em><span class="special">&gt;</span></code> turns the object transform
                  into a primitive transform. This is useful for disambiguating object
                  transforms from callable transforms, and also for working around
                  compiler bugs with nested function types.
                </p></dd>
<dt><span class="term"> <code class="computeroutput"><a class="link" href="../boost/proto/_default.html" title="Struct template _default">proto::_default&lt;&gt;</a></code></span></dt>
<dd><p>
                  Given a grammar <em class="replaceable"><code>G</code></em>, <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_default</span><span class="special">&lt;</span><em class="replaceable"><code>G</code></em><span class="special">&gt;</span></code> evaluates the current node
                  according to the standard C++ meaning of the operation the node
                  represents. For instance, if the current node is a binary plus
                  node, the two children will both be evaluated according to <code class="computeroutput"><em class="replaceable"><code>G</code></em></code>
                  and the results will be added and returned. The return type is
                  deduced with the help of the Boost.Typeof library.
                </p></dd>
<dt><span class="term"> <code class="computeroutput"><a class="link" href="../boost/proto/fold.html" title="Struct template fold">proto::fold&lt;&gt;</a></code></span></dt>
<dd><p>
                  Given three transforms <code class="computeroutput"><em class="replaceable"><code>ET</code></em></code>,
                  <code class="computeroutput"><em class="replaceable"><code>ST</code></em></code>, and <code class="computeroutput"><em class="replaceable"><code>FT</code></em></code>,
                  <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">fold</span><span class="special">&lt;</span><em class="replaceable"><code>ET</code></em><span class="special">,</span> <em class="replaceable"><code>ST</code></em><span class="special">,</span> <em class="replaceable"><code>FT</code></em><span class="special">&gt;</span></code> first evaluates <code class="computeroutput"><em class="replaceable"><code>ET</code></em></code>
                  to obtain a Fusion sequence and <code class="computeroutput"><em class="replaceable"><code>ST</code></em></code>
                  to obtain an initial state for the fold, and then evaluates <code class="computeroutput"><em class="replaceable"><code>FT</code></em></code>
                  for each element in the sequence to generate the next state from
                  the previous.
                </p></dd>
<dt><span class="term"> <code class="computeroutput"><a class="link" href="../boost/proto/reverse_fold.html" title="Struct template reverse_fold">proto::reverse_fold&lt;&gt;</a></code></span></dt>
<dd><p>
                  Like <code class="computeroutput"><a class="link" href="../boost/proto/fold.html" title="Struct template fold">proto::fold&lt;&gt;</a></code>, except the
                  elements in the Fusion sequence are iterated in reverse order.
                </p></dd>
<dt><span class="term"> <code class="computeroutput"><a class="link" href="../boost/proto/fold_tree.html" title="Struct template fold_tree">proto::fold_tree&lt;&gt;</a></code></span></dt>
<dd><p>
                  Like <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">fold</span><span class="special">&lt;</span><em class="replaceable"><code>ET</code></em><span class="special">,</span> <em class="replaceable"><code>ST</code></em><span class="special">,</span> <em class="replaceable"><code>FT</code></em><span class="special">&gt;</span></code>, except that the result of
                  the <code class="computeroutput"><em class="replaceable"><code>ET</code></em></code> transform is treated
                  as an expression tree that is <span class="emphasis"><em>flattened</em></span> to
                  generate the sequence to be folded. Flattening an expression tree
                  causes child nodes with the same tag type as the parent to be put
                  into sequence. For instance, <code class="computeroutput"><span class="identifier">a</span>
                  <span class="special">&gt;&gt;</span> <span class="identifier">b</span>
                  <span class="special">&gt;&gt;</span> <span class="identifier">c</span></code>
                  would be flattened to the sequence [<code class="computeroutput"><span class="identifier">a</span></code>,
                  <code class="computeroutput"><span class="identifier">b</span></code>, <code class="computeroutput"><span class="identifier">c</span></code>], and this is the sequence
                  that would be folded.
                </p></dd>
<dt><span class="term"> <code class="computeroutput"><a class="link" href="../boost/proto/reverse_fold_tree.html" title="Struct template reverse_fold_tree">proto::reverse_fold_tree&lt;&gt;</a></code></span></dt>
<dd><p>
                  Like <code class="computeroutput"><a class="link" href="../boost/proto/fold_tree.html" title="Struct template fold_tree">proto::fold_tree&lt;&gt;</a></code>, except that
                  the flattened sequence is iterated in reverse order.
                </p></dd>
<dt><span class="term"> <code class="computeroutput"><a class="link" href="../boost/proto/lazy.html" title="Struct template lazy">proto::lazy&lt;&gt;</a></code></span></dt>
<dd><p>
                  A combination of <code class="computeroutput"><a class="link" href="../boost/proto/make.html" title="Struct template make">proto::make&lt;&gt;</a></code>
                  and <code class="computeroutput"><a class="link" href="../boost/proto/call.html" title="Struct template call">proto::call&lt;&gt;</a></code> that is useful
                  when the nature of the transform depends on the expression, state
                  and/or data parameters. <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">lazy</span><span class="special">&lt;</span><span class="identifier">R</span><span class="special">(</span><span class="identifier">A0</span><span class="special">,</span><span class="identifier">A1</span><span class="special">...</span><span class="identifier">An</span><span class="special">)&gt;</span></code> first evaluates <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">make</span><span class="special">&lt;</span><span class="identifier">R</span><span class="special">()&gt;</span></code>
                  to compute a callable type <code class="computeroutput"><span class="identifier">R2</span></code>.
                  Then, it evaluates <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">call</span><span class="special">&lt;</span><span class="identifier">R2</span><span class="special">(</span><span class="identifier">A0</span><span class="special">,</span><span class="identifier">A1</span><span class="special">...</span><span class="identifier">An</span><span class="special">)&gt;</span></code>.
                </p></dd>
</dl>
</div>
<h6>
<a name="boost_proto.users_guide.back_end.expression_transformation.canned_transforms.h0"></a>
            <span class="phrase"><a name="boost_proto.users_guide.back_end.expression_transformation.canned_transforms.all_grammars_are_primitive_transforms"></a></span><a class="link" href="users_guide.html#boost_proto.users_guide.back_end.expression_transformation.canned_transforms.all_grammars_are_primitive_transforms">All
            Grammars Are Primitive Transforms</a>
          </h6>
<p>
            In addition to the above primitive transforms, all of Proto's grammar
            elements are also primitive transforms. Their behaviors are described
            below.
          </p>
<div class="variablelist">
<p class="title"><b></b></p>
<dl class="variablelist">
<dt><span class="term"> <code class="computeroutput"><a class="link" href="../boost/proto/_.html" title="Struct _">proto::_</a></code></span></dt>
<dd><p>
                  Return the current expression unmodified.
                </p></dd>
<dt><span class="term"> <code class="computeroutput"><a class="link" href="../boost/proto/or_.html" title="Struct template or_">proto::or_&lt;&gt;</a></code></span></dt>
<dd><p>
                  For the specified set of alternate sub-grammars, find the one that
                  matches the given expression and apply its associated transform.
                </p></dd>
<dt><span class="term"> <code class="computeroutput"><a class="link" href="../boost/proto/and_.html" title="Struct template and_">proto::and_&lt;&gt;</a></code></span></dt>
<dd><p>
                  For the given set of sub-grammars, apply all the associated transforms
                  and return the result of the last.
                </p></dd>
<dt><span class="term"> <code class="computeroutput"><a class="link" href="../boost/proto/not_.html" title="Struct template not_">proto::not_&lt;&gt;</a></code></span></dt>
<dd><p>
                  Return the current expression unmodified.
                </p></dd>
<dt><span class="term"> <code class="computeroutput"><a class="link" href="../boost/proto/if_.html" title="Struct template if_">proto::if_&lt;&gt;</a></code></span></dt>
<dd><p>
                  Given three transforms, evaluate the first and treat the result
                  as a compile-time Boolean value. If it is true, evaluate the second
                  transform. Otherwise, evaluate the third.
                </p></dd>
<dt><span class="term"> <code class="computeroutput"><a class="link" href="../boost/proto/switch_.html" title="Struct template switch_">proto::switch_&lt;&gt;</a></code></span></dt>
<dd><p>
                  As with <code class="computeroutput"><a class="link" href="../boost/proto/or_.html" title="Struct template or_">proto::or_&lt;&gt;</a></code>, find the sub-grammar
                  that matches the given expression and apply its associated transform.
                </p></dd>
<dt><span class="term"> <code class="computeroutput"><a class="link" href="../boost/proto/terminal.html" title="Struct template terminal">proto::terminal&lt;&gt;</a></code></span></dt>
<dd><p>
                  Return the current terminal expression unmodified.
                </p></dd>
<dt><span class="term"> <code class="computeroutput"><a class="link" href="../boost/proto/plus.html" title="Struct template plus">proto::plus&lt;&gt;</a></code>, <code class="computeroutput"><a class="link" href="../boost/proto/nary_expr.html" title="Struct template nary_expr">proto::nary_expr&lt;&gt;</a></code>,
              et. al.</span></dt>
<dd><p>
                  A Proto grammar that matches a non-terminal such as <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">plus</span><span class="special">&lt;</span><em class="replaceable"><code>G0</code></em><span class="special">,</span> <em class="replaceable"><code>G1</code></em><span class="special">&gt;</span></code>, when used as a primitive transform,
                  creates a new plus node where the left child is transformed according
                  to <code class="computeroutput"><em class="replaceable"><code>G0</code></em></code> and the right child
                  with <code class="computeroutput"><em class="replaceable"><code>G1</code></em></code>.
                </p></dd>
</dl>
</div>
<h6>
<a name="boost_proto.users_guide.back_end.expression_transformation.canned_transforms.h1"></a>
            <span class="phrase"><a name="boost_proto.users_guide.back_end.expression_transformation.canned_transforms.the_pass_through_transform"></a></span><a class="link" href="users_guide.html#boost_proto.users_guide.back_end.expression_transformation.canned_transforms.the_pass_through_transform">The
            Pass-Through Transform</a>
          </h6>
<p>
            Note the primitive transform associated with grammar elements such as
            <code class="computeroutput"><a class="link" href="../boost/proto/plus.html" title="Struct template plus">proto::plus&lt;&gt;</a></code> described above.
            They possess a so-called <span class="emphasis"><em>pass-through</em></span> transform.
            The pass-through transform accepts an expression of a certain tag type
            (say, <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">plus</span></code>) and creates a new expression
            of the same tag type, where each child expression is transformed according
            to the corresponding child grammar of the pass-through transform. So
            for instance this grammar ...
          </p>
<pre class="programlisting"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">function</span><span class="special">&lt;</span> <span class="identifier">X</span><span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">vararg</span><span class="special">&lt;</span><span class="identifier">Y</span><span class="special">&gt;</span> <span class="special">&gt;</span>
</pre>
<p>
            ... matches function expressions where the first child matches the <code class="computeroutput"><span class="identifier">X</span></code> grammar and the rest match the <code class="computeroutput"><span class="identifier">Y</span></code> grammar. When used as a transform,
            the above grammar will create a new function expression where the first
            child is transformed according to <code class="computeroutput"><span class="identifier">X</span></code>
            and the rest are transformed according to <code class="computeroutput"><span class="identifier">Y</span></code>.
          </p>
<p>
            The following class templates in Proto can be used as grammars with pass-through
            transforms:
          </p>
<div class="table">
<a name="boost_proto.users_guide.back_end.expression_transformation.canned_transforms.t0"></a><p class="title"><b>Table&#160;32.11.&#160;Class Templates With Pass-Through Transforms</b></p>
<div class="table-contents"><table class="table" summary="Class Templates With Pass-Through Transforms">
<colgroup><col></colgroup>
<thead><tr><th>
                    <p>
                      Templates with Pass-Through Transforms
                    </p>
                  </th></tr></thead>
<tbody>
<tr><td>
                    <p>
                      <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">unary_plus</span><span class="special">&lt;&gt;</span></code>
                    </p>
                  </td></tr>
<tr><td>
                    <p>
                      <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">negate</span><span class="special">&lt;&gt;</span></code>
                    </p>
                  </td></tr>
<tr><td>
                    <p>
                      <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">dereference</span><span class="special">&lt;&gt;</span></code>
                    </p>
                  </td></tr>
<tr><td>
                    <p>
                      <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">complement</span><span class="special">&lt;&gt;</span></code>
                    </p>
                  </td></tr>
<tr><td>
                    <p>
                      <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">address_of</span><span class="special">&lt;&gt;</span></code>
                    </p>
                  </td></tr>
<tr><td>
                    <p>
                      <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">logical_not</span><span class="special">&lt;&gt;</span></code>
                    </p>
                  </td></tr>
<tr><td>
                    <p>
                      <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">pre_inc</span><span class="special">&lt;&gt;</span></code>
                    </p>
                  </td></tr>
<tr><td>
                    <p>
                      <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">pre_dec</span><span class="special">&lt;&gt;</span></code>
                    </p>
                  </td></tr>
<tr><td>
                    <p>
                      <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">post_inc</span><span class="special">&lt;&gt;</span></code>
                    </p>
                  </td></tr>
<tr><td>
                    <p>
                      <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">post_dec</span><span class="special">&lt;&gt;</span></code>
                    </p>
                  </td></tr>
<tr><td>
                    <p>
                      <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">shift_left</span><span class="special">&lt;&gt;</span></code>
                    </p>
                  </td></tr>
<tr><td>
                    <p>
                      <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">shift_right</span><span class="special">&lt;&gt;</span></code>
                    </p>
                  </td></tr>
<tr><td>
                    <p>
                      <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">multiplies</span><span class="special">&lt;&gt;</span></code>
                    </p>
                  </td></tr>
<tr><td>
                    <p>
                      <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">divides</span><span class="special">&lt;&gt;</span></code>
                    </p>
                  </td></tr>
<tr><td>
                    <p>
                      <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">modulus</span><span class="special">&lt;&gt;</span></code>
                    </p>
                  </td></tr>
<tr><td>
                    <p>
                      <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">plus</span><span class="special">&lt;&gt;</span></code>
                    </p>
                  </td></tr>
<tr><td>
                    <p>
                      <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">minus</span><span class="special">&lt;&gt;</span></code>
                    </p>
                  </td></tr>
<tr><td>
                    <p>
                      <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">less</span><span class="special">&lt;&gt;</span></code>
                    </p>
                  </td></tr>
<tr><td>
                    <p>
                      <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">greater</span><span class="special">&lt;&gt;</span></code>
                    </p>
                  </td></tr>
<tr><td>
                    <p>
                      <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">less_equal</span><span class="special">&lt;&gt;</span></code>
                    </p>
                  </td></tr>
<tr><td>
                    <p>
                      <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">greater_equal</span><span class="special">&lt;&gt;</span></code>
                    </p>
                  </td></tr>
<tr><td>
                    <p>
                      <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">equal_to</span><span class="special">&lt;&gt;</span></code>
                    </p>
                  </td></tr>
<tr><td>
                    <p>
                      <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">not_equal_to</span><span class="special">&lt;&gt;</span></code>
                    </p>
                  </td></tr>
<tr><td>
                    <p>
                      <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">logical_or</span><span class="special">&lt;&gt;</span></code>
                    </p>
                  </td></tr>
<tr><td>
                    <p>
                      <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">logical_and</span><span class="special">&lt;&gt;</span></code>
                    </p>
                  </td></tr>
<tr><td>
                    <p>
                      <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">bitwise_and</span><span class="special">&lt;&gt;</span></code>
                    </p>
                  </td></tr>
<tr><td>
                    <p>
                      <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">bitwise_or</span><span class="special">&lt;&gt;</span></code>
                    </p>
                  </td></tr>
<tr><td>
                    <p>
                      <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">bitwise_xor</span><span class="special">&lt;&gt;</span></code>
                    </p>
                  </td></tr>
<tr><td>
                    <p>
                      <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">comma</span><span class="special">&lt;&gt;</span></code>
                    </p>
                  </td></tr>
<tr><td>
                    <p>
                      <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">mem_ptr</span><span class="special">&lt;&gt;</span></code>
                    </p>
                  </td></tr>
<tr><td>
                    <p>
                      <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">assign</span><span class="special">&lt;&gt;</span></code>
                    </p>
                  </td></tr>
<tr><td>
                    <p>
                      <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">shift_left_assign</span><span class="special">&lt;&gt;</span></code>
                    </p>
                  </td></tr>
<tr><td>
                    <p>
                      <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">shift_right_assign</span><span class="special">&lt;&gt;</span></code>
                    </p>
                  </td></tr>
<tr><td>
                    <p>
                      <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">multiplies_assign</span><span class="special">&lt;&gt;</span></code>
                    </p>
                  </td></tr>
<tr><td>
                    <p>
                      <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">divides_assign</span><span class="special">&lt;&gt;</span></code>
                    </p>
                  </td></tr>
<tr><td>
                    <p>
                      <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">modulus_assign</span><span class="special">&lt;&gt;</span></code>
                    </p>
                  </td></tr>
<tr><td>
                    <p>
                      <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">plus_assign</span><span class="special">&lt;&gt;</span></code>
                    </p>
                  </td></tr>
<tr><td>
                    <p>
                      <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">minus_assign</span><span class="special">&lt;&gt;</span></code>
                    </p>
                  </td></tr>
<tr><td>
                    <p>
                      <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">bitwise_and_assign</span><span class="special">&lt;&gt;</span></code>
                    </p>
                  </td></tr>
<tr><td>
                    <p>
                      <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">bitwise_or_assign</span><span class="special">&lt;&gt;</span></code>
                    </p>
                  </td></tr>
<tr><td>
                    <p>
                      <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">bitwise_xor_assign</span><span class="special">&lt;&gt;</span></code>
                    </p>
                  </td></tr>
<tr><td>
                    <p>
                      <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">subscript</span><span class="special">&lt;&gt;</span></code>
                    </p>
                  </td></tr>
<tr><td>
                    <p>
                      <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">if_else_</span><span class="special">&lt;&gt;</span></code>
                    </p>
                  </td></tr>
<tr><td>
                    <p>
                      <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">function</span><span class="special">&lt;&gt;</span></code>
                    </p>
                  </td></tr>
<tr><td>
                    <p>
                      <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">unary_expr</span><span class="special">&lt;&gt;</span></code>
                    </p>
                  </td></tr>
<tr><td>
                    <p>
                      <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">binary_expr</span><span class="special">&lt;&gt;</span></code>
                    </p>
                  </td></tr>
<tr><td>
                    <p>
                      <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">nary_expr</span><span class="special">&lt;&gt;</span></code>
                    </p>
                  </td></tr>
</tbody>
</table></div>
</div>
<br class="table-break"><h6>
<a name="boost_proto.users_guide.back_end.expression_transformation.canned_transforms.h2"></a>
            <span class="phrase"><a name="boost_proto.users_guide.back_end.expression_transformation.canned_transforms.the_many_roles_of_proto_operator_metafunctions"></a></span><a class="link" href="users_guide.html#boost_proto.users_guide.back_end.expression_transformation.canned_transforms.the_many_roles_of_proto_operator_metafunctions">The
            Many Roles of Proto Operator Metafunctions</a>
          </h6>
<p>
            We've seen templates such as <code class="computeroutput"><a class="link" href="../boost/proto/terminal.html" title="Struct template terminal">proto::terminal&lt;&gt;</a></code>,
            <code class="computeroutput"><a class="link" href="../boost/proto/plus.html" title="Struct template plus">proto::plus&lt;&gt;</a></code> and <code class="computeroutput"><a class="link" href="../boost/proto/nary_expr.html" title="Struct template nary_expr">proto::nary_expr&lt;&gt;</a></code>
            fill many roles. They are metafunction that generate expression types.
            They are grammars that match expression types. And they are primitive
            transforms. The following code samples show examples of each.
          </p>
<p>
            <span class="bold"><strong>As Metafunctions ...</strong></span>
          </p>
<pre class="programlisting"><span class="comment">// proto::terminal&lt;&gt; and proto::plus&lt;&gt; are metafunctions</span>
<span class="comment">// that generate expression types:</span>
<span class="keyword">typedef</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">&gt;::</span><span class="identifier">type</span> <span class="identifier">int_</span><span class="special">;</span>
<span class="keyword">typedef</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">plus</span><span class="special">&lt;</span><span class="identifier">int_</span><span class="special">,</span> <span class="identifier">int_</span><span class="special">&gt;::</span><span class="identifier">type</span> <span class="identifier">plus_</span><span class="special">;</span>

<span class="identifier">int_</span> <span class="identifier">i</span> <span class="special">=</span> <span class="special">{</span><span class="number">42</span><span class="special">},</span> <span class="identifier">j</span> <span class="special">=</span> <span class="special">{</span><span class="number">24</span><span class="special">};</span>
<span class="identifier">plus_</span> <span class="identifier">p</span> <span class="special">=</span> <span class="special">{</span><span class="identifier">i</span><span class="special">,</span> <span class="identifier">j</span><span class="special">};</span>
</pre>
<p>
            <span class="bold"><strong>As Grammars ...</strong></span>
          </p>
<pre class="programlisting"><span class="comment">// proto::terminal&lt;&gt; and proto::plus&lt;&gt; are grammars that</span>
<span class="comment">// match expression types</span>
<span class="keyword">struct</span> <span class="identifier">Int</span> <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">&gt;</span> <span class="special">{};</span>
<span class="keyword">struct</span> <span class="identifier">Plus</span> <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">plus</span><span class="special">&lt;</span><span class="identifier">Int</span><span class="special">,</span> <span class="identifier">Int</span><span class="special">&gt;</span> <span class="special">{};</span>

<span class="identifier">BOOST_MPL_ASSERT</span><span class="special">((</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">matches</span><span class="special">&lt;</span> <span class="identifier">int_</span><span class="special">,</span> <span class="identifier">Int</span> <span class="special">&gt;</span> <span class="special">));</span>
<span class="identifier">BOOST_MPL_ASSERT</span><span class="special">((</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">matches</span><span class="special">&lt;</span> <span class="identifier">plus_</span><span class="special">,</span> <span class="identifier">Plus</span> <span class="special">&gt;</span> <span class="special">));</span>
</pre>
<p>
            <span class="bold"><strong>As Primitive Transforms ...</strong></span>
          </p>
<pre class="programlisting"><span class="comment">// A transform that removes all unary_plus nodes in an expression</span>
<span class="keyword">struct</span> <span class="identifier">RemoveUnaryPlus</span>
  <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">or_</span><span class="special">&lt;</span>
        <span class="identifier">proto</span><span class="special">::</span><span class="identifier">when</span><span class="special">&lt;</span>
            <span class="identifier">proto</span><span class="special">::</span><span class="identifier">unary_plus</span><span class="special">&lt;</span><span class="identifier">RemoveUnaryPlus</span><span class="special">&gt;</span>
          <span class="special">,</span> <span class="identifier">RemoveUnaryPlus</span><span class="special">(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_child</span><span class="special">)</span>
        <span class="special">&gt;</span>
        <span class="comment">// Use proto::terminal&lt;&gt; and proto::nary_expr&lt;&gt;</span>
        <span class="comment">// both as grammars and as primitive transforms.</span>
      <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span><span class="identifier">_</span><span class="special">&gt;</span>
      <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">nary_expr</span><span class="special">&lt;</span><span class="identifier">_</span><span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">vararg</span><span class="special">&lt;</span><span class="identifier">RemoveUnaryPlus</span><span class="special">&gt;</span> <span class="special">&gt;</span>
    <span class="special">&gt;</span>
<span class="special">{};</span>

<span class="keyword">int</span> <span class="identifier">main</span><span class="special">()</span>
<span class="special">{</span>
    <span class="identifier">proto</span><span class="special">::</span><span class="identifier">literal</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">&gt;</span> <span class="identifier">i</span><span class="special">(</span><span class="number">0</span><span class="special">);</span>

    <span class="identifier">proto</span><span class="special">::</span><span class="identifier">display_expr</span><span class="special">(</span>
        <span class="special">+</span><span class="identifier">i</span> <span class="special">-</span> <span class="special">+(</span><span class="identifier">i</span> <span class="special">-</span> <span class="special">+</span><span class="identifier">i</span><span class="special">)</span>
    <span class="special">);</span>

    <span class="identifier">proto</span><span class="special">::</span><span class="identifier">display_expr</span><span class="special">(</span>
        <span class="identifier">RemoveUnaryPlus</span><span class="special">()(</span> <span class="special">+</span><span class="identifier">i</span> <span class="special">-</span> <span class="special">+(</span><span class="identifier">i</span> <span class="special">-</span> <span class="special">+</span><span class="identifier">i</span><span class="special">)</span> <span class="special">)</span>
    <span class="special">);</span>
<span class="special">}</span>
</pre>
<p>
            The above code displays the following, which shows that unary plus nodes
            have been stripped from the expression:
          </p>
<pre class="programlisting">minus(
    unary_plus(
        terminal(0)
    )
  , unary_plus(
        minus(
            terminal(0)
          , unary_plus(
                terminal(0)
            )
        )
    )
)
minus(
    terminal(0)
  , minus(
        terminal(0)
      , terminal(0)
    )
)
</pre>
</div>
<div class="section">
<div class="titlepage"><div><div><h5 class="title">
<a name="boost_proto.users_guide.back_end.expression_transformation.primitives"></a><a class="link" href="users_guide.html#boost_proto.users_guide.back_end.expression_transformation.primitives" title="Building Custom Primitive Transforms">Building
          Custom Primitive Transforms</a>
</h5></div></div></div>
<p>
            In previous sections, we've seen how to compose larger transforms out
            of smaller transforms using function types. The smaller transforms from
            which larger transforms are composed are <span class="emphasis"><em>primitive transforms</em></span>,
            and Proto provides a bunch of common ones such as <code class="computeroutput"><span class="identifier">_child0</span></code>
            and <code class="computeroutput"><span class="identifier">_value</span></code>. In this section
            we'll see how to author your own primitive transforms.
          </p>
<div class="note"><table border="0" summary="Note">
<tr>
<td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../doc/src/images/note.png"></td>
<th align="left">Note</th>
</tr>
<tr><td align="left" valign="top"><p>
              There are a few reasons why you might want to write your own primitive
              transforms. For instance, your transform may be complicated, and composing
              it out of primitives becomes unwieldy. You might also need to work
              around compiler bugs on legacy compilers that make composing transforms
              using function types problematic. Finally, you might also decide to
              define your own primitive transforms to improve compile times. Since
              Proto can simply invoke a primitive transform directly without having
              to process arguments or differentiate callable transforms from object
              transforms, primitive transforms are more efficient.
            </p></td></tr>
</table></div>
<p>
            Primitive transforms inherit from <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">transform</span><span class="special">&lt;&gt;</span></code> and have a nested <code class="computeroutput"><span class="identifier">impl</span><span class="special">&lt;&gt;</span></code>
            template that inherits from <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">transform_impl</span><span class="special">&lt;&gt;</span></code>. For example, this is how Proto
            defines the <code class="computeroutput"><span class="identifier">_child_c</span><span class="special">&lt;</span><em class="replaceable"><code>N</code></em><span class="special">&gt;</span></code>
            transform, which returns the <em class="replaceable"><code>N</code></em>-th child of
            the current expression:
          </p>
<pre class="programlisting"><span class="keyword">namespace</span> <span class="identifier">boost</span> <span class="special">{</span> <span class="keyword">namespace</span> <span class="identifier">proto</span>
<span class="special">{</span>
    <span class="comment">// A primitive transform that returns N-th child</span>
    <span class="comment">// of the current expression.</span>
    <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">int</span> <span class="identifier">N</span><span class="special">&gt;</span>
    <span class="keyword">struct</span> <span class="identifier">_child_c</span> <span class="special">:</span> <span class="identifier">transform</span><span class="special">&lt;</span><span class="identifier">_child_c</span><span class="special">&lt;</span><span class="identifier">N</span><span class="special">&gt;</span> <span class="special">&gt;</span>
    <span class="special">{</span>
        <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">Expr</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">State</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">Data</span><span class="special">&gt;</span>
        <span class="keyword">struct</span> <span class="identifier">impl</span> <span class="special">:</span> <span class="identifier">transform_impl</span><span class="special">&lt;</span><span class="identifier">Expr</span><span class="special">,</span> <span class="identifier">State</span><span class="special">,</span> <span class="identifier">Data</span><span class="special">&gt;</span>
        <span class="special">{</span>
            <span class="keyword">typedef</span>
                <span class="keyword">typename</span> <span class="identifier">result_of</span><span class="special">::</span><span class="identifier">child_c</span><span class="special">&lt;</span><span class="identifier">Expr</span><span class="special">,</span> <span class="identifier">N</span><span class="special">&gt;::</span><span class="identifier">type</span>
            <span class="identifier">result_type</span><span class="special">;</span>

            <span class="identifier">result_type</span> <span class="keyword">operator</span> <span class="special">()(</span>
                <span class="keyword">typename</span> <span class="identifier">impl</span><span class="special">::</span><span class="identifier">expr_param</span> <span class="identifier">expr</span>
              <span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">impl</span><span class="special">::</span><span class="identifier">state_param</span> <span class="identifier">state</span>
              <span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">impl</span><span class="special">::</span><span class="identifier">data_param</span> <span class="identifier">data</span>
            <span class="special">)</span> <span class="keyword">const</span>
            <span class="special">{</span>
                <span class="keyword">return</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">child_c</span><span class="special">&lt;</span><span class="identifier">N</span><span class="special">&gt;(</span><span class="identifier">expr</span><span class="special">);</span>
            <span class="special">}</span>
        <span class="special">};</span>
    <span class="special">};</span>

    <span class="comment">// Note that _child_c&lt;N&gt; is callable, so that</span>
    <span class="comment">// it can be used in callable transforms, as:</span>
    <span class="comment">//   _child_c&lt;0&gt;(_child_c&lt;1&gt;)</span>
    <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">int</span> <span class="identifier">N</span><span class="special">&gt;</span>
    <span class="keyword">struct</span> <span class="identifier">is_callable</span><span class="special">&lt;</span><span class="identifier">_child_c</span><span class="special">&lt;</span><span class="identifier">N</span><span class="special">&gt;</span> <span class="special">&gt;</span>
      <span class="special">:</span> <span class="identifier">mpl</span><span class="special">::</span><span class="identifier">true_</span>
    <span class="special">{};</span>
<span class="special">}}</span>
</pre>
<p>
            The <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">transform</span><span class="special">&lt;&gt;</span></code>
            base class provides the <code class="computeroutput"><span class="keyword">operator</span><span class="special">()</span></code> overloads and the nested <code class="computeroutput"><span class="identifier">result</span><span class="special">&lt;&gt;</span></code>
            template that make your transform a valid function object. These are
            implemented in terms of the nested <code class="computeroutput"><span class="identifier">impl</span><span class="special">&lt;&gt;</span></code> template you define.
          </p>
<p>
            The <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">transform_impl</span><span class="special">&lt;&gt;</span></code>
            base class is a convenience. It provides some nested typedefs that are
            generally useful. They are specified in the table below:
          </p>
<div class="table">
<a name="boost_proto.users_guide.back_end.expression_transformation.primitives.t0"></a><p class="title"><b>Table&#160;32.12.&#160;proto::transform_impl&lt;Expr, State, Data&gt; typedefs</b></p>
<div class="table-contents"><table class="table" summary="proto::transform_impl&lt;Expr, State, Data&gt; typedefs">
<colgroup>
<col>
<col>
</colgroup>
<thead><tr>
<th>
                    <p>
                      typedef
                    </p>
                  </th>
<th>
                    <p>
                      Equivalent To
                    </p>
                  </th>
</tr></thead>
<tbody>
<tr>
<td>
                    <p>
                      <code class="computeroutput"><span class="identifier">expr</span></code>
                    </p>
                  </td>
<td>
                    <p>
                      <code class="computeroutput"><span class="keyword">typename</span> <span class="identifier">remove_reference</span><span class="special">&lt;</span><span class="identifier">Expr</span><span class="special">&gt;::</span><span class="identifier">type</span></code>
                    </p>
                  </td>
</tr>
<tr>
<td>
                    <p>
                      <code class="computeroutput"><span class="identifier">state</span></code>
                    </p>
                  </td>
<td>
                    <p>
                      <code class="computeroutput"><span class="keyword">typename</span> <span class="identifier">remove_reference</span><span class="special">&lt;</span><span class="identifier">State</span><span class="special">&gt;::</span><span class="identifier">type</span></code>
                    </p>
                  </td>
</tr>
<tr>
<td>
                    <p>
                      <code class="computeroutput"><span class="identifier">data</span></code>
                    </p>
                  </td>
<td>
                    <p>
                      <code class="computeroutput"><span class="keyword">typename</span> <span class="identifier">remove_reference</span><span class="special">&lt;</span><span class="identifier">Data</span><span class="special">&gt;::</span><span class="identifier">type</span></code>
                    </p>
                  </td>
</tr>
<tr>
<td>
                    <p>
                      <code class="computeroutput"><span class="identifier">expr_param</span></code>
                    </p>
                  </td>
<td>
                    <p>
                      <code class="computeroutput"><span class="keyword">typename</span> <span class="identifier">add_reference</span><span class="special">&lt;</span><span class="keyword">typename</span>
                      <span class="identifier">add_const</span><span class="special">&lt;</span><span class="identifier">Expr</span><span class="special">&gt;::</span><span class="identifier">type</span><span class="special">&gt;::</span><span class="identifier">type</span></code>
                    </p>
                  </td>
</tr>
<tr>
<td>
                    <p>
                      <code class="computeroutput"><span class="identifier">state_param</span></code>
                    </p>
                  </td>
<td>
                    <p>
                      <code class="computeroutput"><span class="keyword">typename</span> <span class="identifier">add_reference</span><span class="special">&lt;</span><span class="keyword">typename</span>
                      <span class="identifier">add_const</span><span class="special">&lt;</span><span class="identifier">State</span><span class="special">&gt;::</span><span class="identifier">type</span><span class="special">&gt;::</span><span class="identifier">type</span></code>
                    </p>
                  </td>
</tr>
<tr>
<td>
                    <p>
                      <code class="computeroutput"><span class="identifier">data_param</span></code>
                    </p>
                  </td>
<td>
                    <p>
                      <code class="computeroutput"><span class="keyword">typename</span> <span class="identifier">add_reference</span><span class="special">&lt;</span><span class="keyword">typename</span>
                      <span class="identifier">add_const</span><span class="special">&lt;</span><span class="identifier">Data</span><span class="special">&gt;::</span><span class="identifier">type</span><span class="special">&gt;::</span><span class="identifier">type</span></code>
                    </p>
                  </td>
</tr>
</tbody>
</table></div>
</div>
<br class="table-break"><p>
            You'll notice that <code class="computeroutput"><span class="identifier">_child_c</span><span class="special">::</span><span class="identifier">impl</span><span class="special">::</span><span class="keyword">operator</span><span class="special">()</span></code> takes arguments of types <code class="computeroutput"><span class="identifier">expr_param</span></code>, <code class="computeroutput"><span class="identifier">state_param</span></code>,
            and <code class="computeroutput"><span class="identifier">data_param</span></code>. The typedefs
            make it easy to accept arguments by reference or const reference accordingly.
          </p>
<p>
            The only other interesting bit is the <code class="computeroutput"><span class="identifier">is_callable</span><span class="special">&lt;&gt;</span></code> specialization, which will be
            described in the <a class="link" href="users_guide.html#boost_proto.users_guide.back_end.expression_transformation.is_callable" title="Making Your Transform Callable">next
            section</a>.
          </p>
</div>
<div class="section">
<div class="titlepage"><div><div><h5 class="title">
<a name="boost_proto.users_guide.back_end.expression_transformation.is_callable"></a><a class="link" href="users_guide.html#boost_proto.users_guide.back_end.expression_transformation.is_callable" title="Making Your Transform Callable">Making
          Your Transform Callable</a>
</h5></div></div></div>
<p>
            Transforms are typically of the form <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">when</span><span class="special">&lt;</span> <span class="identifier">Something</span><span class="special">,</span> <span class="identifier">R</span><span class="special">(</span><span class="identifier">A0</span><span class="special">,</span><span class="identifier">A1</span><span class="special">,...)</span> <span class="special">&gt;</span></code>.
            The question is whether <code class="computeroutput"><span class="identifier">R</span></code>
            represents a function to call or an object to construct, and the answer
            determines how <code class="computeroutput"><a class="link" href="../boost/proto/when.html" title="Struct template when">proto::when&lt;&gt;</a></code> evaluates the transform.
            <code class="computeroutput"><a class="link" href="../boost/proto/when.html" title="Struct template when">proto::when&lt;&gt;</a></code> uses the <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">is_callable</span><span class="special">&lt;&gt;</span></code>
            trait to disambiguate between the two. Proto does its best to guess whether
            a type is callable or not, but it doesn't always get it right. It's best
            to know the rules Proto uses, so that you know when you need to be more
            explicit.
          </p>
<p>
            For most types <code class="computeroutput"><span class="identifier">R</span></code>, <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">is_callable</span><span class="special">&lt;</span><span class="identifier">R</span><span class="special">&gt;</span></code>
            checks for inheritance from <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">callable</span></code>.
            However, if the type <code class="computeroutput"><span class="identifier">R</span></code>
            is a template specialization, Proto assumes that it is <span class="emphasis"><em>not</em></span>
            callable <span class="emphasis"><em>even if the template inherits from <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">callable</span></code></em></span>.
            We'll see why in a minute. Consider the following erroneous callable
            object:
          </p>
<pre class="programlisting"><span class="comment">// Proto can't tell this defines something callable!</span>
<span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">T</span><span class="special">&gt;</span>
<span class="keyword">struct</span> <span class="identifier">times2</span> <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">callable</span>
<span class="special">{</span>
    <span class="keyword">typedef</span> <span class="identifier">T</span> <span class="identifier">result_type</span><span class="special">;</span>

    <span class="identifier">T</span> <span class="keyword">operator</span><span class="special">()(</span><span class="identifier">T</span> <span class="identifier">i</span><span class="special">)</span> <span class="keyword">const</span>
    <span class="special">{</span>
        <span class="keyword">return</span> <span class="identifier">i</span> <span class="special">*</span> <span class="number">2</span><span class="special">;</span>
    <span class="special">}</span>
<span class="special">};</span>

<span class="comment">// ERROR! This is not going to multiply the int by 2:</span>
<span class="keyword">struct</span> <span class="identifier">IntTimes2</span>
  <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">when</span><span class="special">&lt;</span>
        <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">&gt;</span>
      <span class="special">,</span> <span class="identifier">times2</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">&gt;(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_value</span><span class="special">)</span>
    <span class="special">&gt;</span>
<span class="special">{};</span>
</pre>
<p>
            The problem is that Proto doesn't know that <code class="computeroutput"><span class="identifier">times2</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">&gt;</span></code> is callable, so rather that invoking
            the <code class="computeroutput"><span class="identifier">times2</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">&gt;</span></code>
            function object, Proto will try to construct a <code class="computeroutput"><span class="identifier">times2</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">&gt;</span></code> object and initialize it will an
            <code class="computeroutput"><span class="keyword">int</span></code>. That will not compile.
          </p>
<div class="note"><table border="0" summary="Note">
<tr>
<td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../doc/src/images/note.png"></td>
<th align="left">Note</th>
</tr>
<tr><td align="left" valign="top"><p>
              Why can't Proto tell that <code class="computeroutput"><span class="identifier">times2</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">&gt;</span></code> is callable? After all, it inherits
              from <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">callable</span></code>, and that is detectable,
              right? The problem is that merely asking whether some type <code class="computeroutput"><span class="identifier">X</span><span class="special">&lt;</span><span class="identifier">Y</span><span class="special">&gt;</span></code>
              inherits from <code class="computeroutput"><span class="identifier">callable</span></code>
              will cause the template <code class="computeroutput"><span class="identifier">X</span><span class="special">&lt;</span><span class="identifier">Y</span><span class="special">&gt;</span></code> to be instantiated. That's a problem
              for a type like <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">vector</span><span class="special">&lt;</span><span class="identifier">_value</span><span class="special">(</span><span class="identifier">_child1</span><span class="special">)&gt;</span></code>. <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">vector</span><span class="special">&lt;&gt;</span></code> will not suffer to be instantiated
              with <code class="computeroutput"><span class="identifier">_value</span><span class="special">(</span><span class="identifier">_child1</span><span class="special">)</span></code>
              as a template parameter. Since merely asking the question will sometimes
              result in a hard error, Proto can't ask; it has to assume that <code class="computeroutput"><span class="identifier">X</span><span class="special">&lt;</span><span class="identifier">Y</span><span class="special">&gt;</span></code>
              represents an object to construct and not a function to call.
            </p></td></tr>
</table></div>
<p>
            There are a couple of solutions to the <code class="computeroutput"><span class="identifier">times2</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">&gt;</span></code> problem. One solution is to wrap
            the transform in <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">call</span><span class="special">&lt;&gt;</span></code>. This forces Proto to treat
            <code class="computeroutput"><span class="identifier">times2</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">&gt;</span></code>
            as callable:
          </p>
<pre class="programlisting"><span class="comment">// OK, calls times2&lt;int&gt;</span>
<span class="keyword">struct</span> <span class="identifier">IntTimes2</span>
  <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">when</span><span class="special">&lt;</span>
        <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">&gt;</span>
      <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">call</span><span class="special">&lt;</span><span class="identifier">times2</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">&gt;(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_value</span><span class="special">)&gt;</span>
    <span class="special">&gt;</span>
<span class="special">{};</span>
</pre>
<p>
            This can be a bit of a pain, because we need to wrap every use of <code class="computeroutput"><span class="identifier">times2</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">&gt;</span></code>,
            which can be tedious and error prone, and makes our grammar cluttered
            and harder to read.
          </p>
<p>
            Another solution is to specialize <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">is_callable</span><span class="special">&lt;&gt;</span></code> on our <code class="computeroutput"><span class="identifier">times2</span><span class="special">&lt;&gt;</span></code> template:
          </p>
<pre class="programlisting"><span class="keyword">namespace</span> <span class="identifier">boost</span> <span class="special">{</span> <span class="keyword">namespace</span> <span class="identifier">proto</span>
<span class="special">{</span>
    <span class="comment">// Tell Proto that times2&lt;&gt; is callable</span>
    <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">T</span><span class="special">&gt;</span>
    <span class="keyword">struct</span> <span class="identifier">is_callable</span><span class="special">&lt;</span><span class="identifier">times2</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;</span> <span class="special">&gt;</span>
      <span class="special">:</span> <span class="identifier">mpl</span><span class="special">::</span><span class="identifier">true_</span>
    <span class="special">{};</span>
<span class="special">}}</span>

<span class="comment">// OK, times2&lt;&gt; is callable</span>
<span class="keyword">struct</span> <span class="identifier">IntTimes2</span>
  <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">when</span><span class="special">&lt;</span>
        <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">&gt;</span>
      <span class="special">,</span> <span class="identifier">times2</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">&gt;(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_value</span><span class="special">)</span>
    <span class="special">&gt;</span>
<span class="special">{};</span>
</pre>
<p>
            This is better, but still a pain because of the need to open Proto's
            namespace.
          </p>
<p>
            You could simply make sure that the callable type is not a template specialization.
            Consider the following:
          </p>
<pre class="programlisting"><span class="comment">// No longer a template specialization!</span>
<span class="keyword">struct</span> <span class="identifier">times2int</span> <span class="special">:</span> <span class="identifier">times2</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">&gt;</span> <span class="special">{};</span>

<span class="comment">// OK, times2int is callable</span>
<span class="keyword">struct</span> <span class="identifier">IntTimes2</span>
  <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">when</span><span class="special">&lt;</span>
        <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">&gt;</span>
      <span class="special">,</span> <span class="identifier">times2int</span><span class="special">(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_value</span><span class="special">)</span>
    <span class="special">&gt;</span>
<span class="special">{};</span>
</pre>
<p>
            This works because now Proto can tell that <code class="computeroutput"><span class="identifier">times2int</span></code>
            inherits (indirectly) from <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">callable</span></code>.
            Any non-template types can be safely checked for inheritance because,
            as they are not templates, there is no worry about instantiation errors.
          </p>
<p>
            There is one last way to tell Proto that <code class="computeroutput"><span class="identifier">times2</span><span class="special">&lt;&gt;</span></code> is callable. You could add an
            extra dummy template parameter that defaults to <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">callable</span></code>:
          </p>
<pre class="programlisting"><span class="comment">// Proto will recognize this as callable</span>
<span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">T</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">Callable</span> <span class="special">=</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">callable</span><span class="special">&gt;</span>
<span class="keyword">struct</span> <span class="identifier">times2</span> <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">callable</span>
<span class="special">{</span>
    <span class="keyword">typedef</span> <span class="identifier">T</span> <span class="identifier">result_type</span><span class="special">;</span>

    <span class="identifier">T</span> <span class="keyword">operator</span><span class="special">()(</span><span class="identifier">T</span> <span class="identifier">i</span><span class="special">)</span> <span class="keyword">const</span>
    <span class="special">{</span>
        <span class="keyword">return</span> <span class="identifier">i</span> <span class="special">*</span> <span class="number">2</span><span class="special">;</span>
    <span class="special">}</span>
<span class="special">};</span>

<span class="comment">// OK, this works!</span>
<span class="keyword">struct</span> <span class="identifier">IntTimes2</span>
  <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">when</span><span class="special">&lt;</span>
        <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">&gt;</span>
      <span class="special">,</span> <span class="identifier">times2</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">&gt;(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_value</span><span class="special">)</span>
    <span class="special">&gt;</span>
<span class="special">{};</span>
</pre>
<p>
            Note that in addition to the extra template parameter, <code class="computeroutput"><span class="identifier">times2</span><span class="special">&lt;&gt;</span></code>
            still inherits from <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">callable</span></code>.
            That's not necessary in this example but it is good style because any
            types derived from <code class="computeroutput"><span class="identifier">times2</span><span class="special">&lt;&gt;</span></code> (as <code class="computeroutput"><span class="identifier">times2int</span></code>
            defined above) will still be considered callable.
          </p>
</div>
</div>
</div>
<div class="section">
<div class="titlepage"><div><div><h3 class="title">
<a name="boost_proto.users_guide.examples"></a><a class="link" href="users_guide.html#boost_proto.users_guide.examples" title="Examples">Examples</a>
</h3></div></div></div>
<div class="toc"><dl class="toc">
<dt><span class="section"><a href="users_guide.html#boost_proto.users_guide.examples.hello_world">Hello
        World: Building an Expression Template and Evaluating It</a></span></dt>
<dt><span class="section"><a href="users_guide.html#boost_proto.users_guide.examples.calc1">Calc1: Defining
        an Evaluation Context</a></span></dt>
<dt><span class="section"><a href="users_guide.html#boost_proto.users_guide.examples.calc2">Calc2: Adding
        Members Using <code class="literal">proto::extends&lt;&gt;</code></a></span></dt>
<dt><span class="section"><a href="users_guide.html#boost_proto.users_guide.examples.calc3">Calc3: Defining
        a Simple Transform</a></span></dt>
<dt><span class="section"><a href="users_guide.html#boost_proto.users_guide.examples.lazy_vector">Lazy
        Vector: Controlling Operator Overloads</a></span></dt>
<dt><span class="section"><a href="users_guide.html#boost_proto.users_guide.examples.rgb">RGB: Type Manipulations
        with Proto Transforms</a></span></dt>
<dt><span class="section"><a href="users_guide.html#boost_proto.users_guide.examples.tarray">TArray: A
        Simple Linear Algebra Library</a></span></dt>
<dt><span class="section"><a href="users_guide.html#boost_proto.users_guide.examples.vec3">Vec3: Computing
        With Transforms and Contexts</a></span></dt>
<dt><span class="section"><a href="users_guide.html#boost_proto.users_guide.examples.vector">Vector: Adapting
        a Non-Proto Terminal Type</a></span></dt>
<dt><span class="section"><a href="users_guide.html#boost_proto.users_guide.examples.mixed">Mixed: Adapting
        Several Non-Proto Terminal Types</a></span></dt>
<dt><span class="section"><a href="users_guide.html#boost_proto.users_guide.examples.map_assign">Map Assign:
        An Intermediate Transform</a></span></dt>
<dt><span class="section"><a href="users_guide.html#boost_proto.users_guide.examples.future_group">Future
        Group: A More Advanced Transform</a></span></dt>
<dt><span class="section"><a href="users_guide.html#boost_proto.users_guide.examples.lambda">Lambda: A
        Simple Lambda Library with Proto</a></span></dt>
<dt><span class="section"><a href="users_guide.html#boost_proto.users_guide.examples.checked_calc">Checked
        Calculator: A Simple Example of External Transforms</a></span></dt>
</dl></div>
<p>
        A code example is worth a thousand words ...
      </p>
<div class="section">
<div class="titlepage"><div><div><h4 class="title">
<a name="boost_proto.users_guide.examples.hello_world"></a><a class="link" href="users_guide.html#boost_proto.users_guide.examples.hello_world" title="Hello World: Building an Expression Template and Evaluating It">Hello
        World: Building an Expression Template and Evaluating It</a>
</h4></div></div></div>
<p>
          A trivial example which builds and expression template and evaluates it.
        </p>
<p>
</p>
<pre class="programlisting"><span class="comment">////////////////////////////////////////////////////////////////////</span>
<span class="comment">//  Copyright 2008 Eric Niebler. Distributed under the Boost</span>
<span class="comment">//  Software License, Version 1.0. (See accompanying file</span>
<span class="comment">//  LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)</span>

<span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">iostream</span><span class="special">&gt;</span>
<span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">proto</span><span class="special">/</span><span class="identifier">core</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
<span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">proto</span><span class="special">/</span><span class="identifier">context</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
<span class="comment">// This #include is only needed for compilers that use typeof emulation:</span>
<span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">typeof</span><span class="special">/</span><span class="identifier">std</span><span class="special">/</span><span class="identifier">ostream</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
<span class="keyword">namespace</span> <span class="identifier">proto</span> <span class="special">=</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">proto</span><span class="special">;</span>

<span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">ostream</span> <span class="special">&amp;</span> <span class="special">&gt;::</span><span class="identifier">type</span> <span class="identifier">cout_</span> <span class="special">=</span> <span class="special">{</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span><span class="special">};</span>

<span class="keyword">template</span><span class="special">&lt;</span> <span class="keyword">typename</span> <span class="identifier">Expr</span> <span class="special">&gt;</span>
<span class="keyword">void</span> <span class="identifier">evaluate</span><span class="special">(</span> <span class="identifier">Expr</span> <span class="keyword">const</span> <span class="special">&amp;</span> <span class="identifier">expr</span> <span class="special">)</span>
<span class="special">{</span>
    <span class="identifier">proto</span><span class="special">::</span><span class="identifier">default_context</span> <span class="identifier">ctx</span><span class="special">;</span>
    <span class="identifier">proto</span><span class="special">::</span><span class="identifier">eval</span><span class="special">(</span><span class="identifier">expr</span><span class="special">,</span> <span class="identifier">ctx</span><span class="special">);</span>
<span class="special">}</span>

<span class="keyword">int</span> <span class="identifier">main</span><span class="special">()</span>
<span class="special">{</span>
    <span class="identifier">evaluate</span><span class="special">(</span> <span class="identifier">cout_</span> <span class="special">&lt;&lt;</span> <span class="string">"hello"</span> <span class="special">&lt;&lt;</span> <span class="char">','</span> <span class="special">&lt;&lt;</span> <span class="string">" world"</span> <span class="special">);</span>
    <span class="keyword">return</span> <span class="number">0</span><span class="special">;</span>
<span class="special">}</span>
</pre>
<p>
        </p>
</div>
<div class="section">
<div class="titlepage"><div><div><h4 class="title">
<a name="boost_proto.users_guide.examples.calc1"></a><a class="link" href="users_guide.html#boost_proto.users_guide.examples.calc1" title="Calc1: Defining an Evaluation Context">Calc1: Defining
        an Evaluation Context</a>
</h4></div></div></div>
<p>
          A simple example that builds a miniature embedded domain-specific language
          for lazy arithmetic expressions, with TR1 bind-style argument placeholders.
        </p>
<p>
</p>
<pre class="programlisting"><span class="comment">//  Copyright 2008 Eric Niebler. Distributed under the Boost</span>
<span class="comment">//  Software License, Version 1.0. (See accompanying file</span>
<span class="comment">//  LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)</span>
<span class="comment">//</span>
<span class="comment">// This is a simple example of how to build an arithmetic expression</span>
<span class="comment">// evaluator with placeholders.</span>

<span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">iostream</span><span class="special">&gt;</span>
<span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">proto</span><span class="special">/</span><span class="identifier">core</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
<span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">proto</span><span class="special">/</span><span class="identifier">context</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
<span class="keyword">namespace</span> <span class="identifier">proto</span> <span class="special">=</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">proto</span><span class="special">;</span>
<span class="keyword">using</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_</span><span class="special">;</span>

<span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">int</span> <span class="identifier">I</span><span class="special">&gt;</span> <span class="keyword">struct</span> <span class="identifier">placeholder</span> <span class="special">{};</span>

<span class="comment">// Define some placeholders</span>
<span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span> <span class="identifier">placeholder</span><span class="special">&lt;</span> <span class="number">1</span> <span class="special">&gt;</span> <span class="special">&gt;::</span><span class="identifier">type</span> <span class="keyword">const</span> <span class="identifier">_1</span> <span class="special">=</span> <span class="special">{{}};</span>
<span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span> <span class="identifier">placeholder</span><span class="special">&lt;</span> <span class="number">2</span> <span class="special">&gt;</span> <span class="special">&gt;::</span><span class="identifier">type</span> <span class="keyword">const</span> <span class="identifier">_2</span> <span class="special">=</span> <span class="special">{{}};</span>

<span class="comment">// Define a calculator context, for evaluating arithmetic expressions</span>
<span class="keyword">struct</span> <span class="identifier">calculator_context</span>
  <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">callable_context</span><span class="special">&lt;</span> <span class="identifier">calculator_context</span> <span class="keyword">const</span> <span class="special">&gt;</span>
<span class="special">{</span>
    <span class="comment">// The values bound to the placeholders</span>
    <span class="keyword">double</span> <span class="identifier">d</span><span class="special">[</span><span class="number">2</span><span class="special">];</span>

    <span class="comment">// The result of evaluating arithmetic expressions</span>
    <span class="keyword">typedef</span> <span class="keyword">double</span> <span class="identifier">result_type</span><span class="special">;</span>

    <span class="keyword">explicit</span> <span class="identifier">calculator_context</span><span class="special">(</span><span class="keyword">double</span> <span class="identifier">d1</span> <span class="special">=</span> <span class="number">0.</span><span class="special">,</span> <span class="keyword">double</span> <span class="identifier">d2</span> <span class="special">=</span> <span class="number">0.</span><span class="special">)</span>
    <span class="special">{</span>
        <span class="identifier">d</span><span class="special">[</span><span class="number">0</span><span class="special">]</span> <span class="special">=</span> <span class="identifier">d1</span><span class="special">;</span>
        <span class="identifier">d</span><span class="special">[</span><span class="number">1</span><span class="special">]</span> <span class="special">=</span> <span class="identifier">d2</span><span class="special">;</span>
    <span class="special">}</span>

    <span class="comment">// Handle the evaluation of the placeholder terminals</span>
    <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">int</span> <span class="identifier">I</span><span class="special">&gt;</span>
    <span class="keyword">double</span> <span class="keyword">operator</span> <span class="special">()(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">,</span> <span class="identifier">placeholder</span><span class="special">&lt;</span><span class="identifier">I</span><span class="special">&gt;)</span> <span class="keyword">const</span>
    <span class="special">{</span>
        <span class="keyword">return</span> <span class="identifier">d</span><span class="special">[</span> <span class="identifier">I</span> <span class="special">-</span> <span class="number">1</span> <span class="special">];</span>
    <span class="special">}</span>
<span class="special">};</span>

<span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">Expr</span><span class="special">&gt;</span>
<span class="keyword">double</span> <span class="identifier">evaluate</span><span class="special">(</span> <span class="identifier">Expr</span> <span class="keyword">const</span> <span class="special">&amp;</span><span class="identifier">expr</span><span class="special">,</span> <span class="keyword">double</span> <span class="identifier">d1</span> <span class="special">=</span> <span class="number">0.</span><span class="special">,</span> <span class="keyword">double</span> <span class="identifier">d2</span> <span class="special">=</span> <span class="number">0.</span> <span class="special">)</span>
<span class="special">{</span>
    <span class="comment">// Create a calculator context with d1 and d2 substituted for _1 and _2</span>
    <span class="identifier">calculator_context</span> <span class="keyword">const</span> <span class="identifier">ctx</span><span class="special">(</span><span class="identifier">d1</span><span class="special">,</span> <span class="identifier">d2</span><span class="special">);</span>

    <span class="comment">// Evaluate the calculator expression with the calculator_context</span>
    <span class="keyword">return</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">eval</span><span class="special">(</span><span class="identifier">expr</span><span class="special">,</span> <span class="identifier">ctx</span><span class="special">);</span>
<span class="special">}</span>

<span class="keyword">int</span> <span class="identifier">main</span><span class="special">()</span>
<span class="special">{</span>
    <span class="comment">// Displays "5"</span>
    <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="identifier">evaluate</span><span class="special">(</span> <span class="identifier">_1</span> <span class="special">+</span> <span class="number">2.0</span><span class="special">,</span> <span class="number">3.0</span> <span class="special">)</span> <span class="special">&lt;&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span>

    <span class="comment">// Displays "6"</span>
    <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="identifier">evaluate</span><span class="special">(</span> <span class="identifier">_1</span> <span class="special">*</span> <span class="identifier">_2</span><span class="special">,</span> <span class="number">3.0</span><span class="special">,</span> <span class="number">2.0</span> <span class="special">)</span> <span class="special">&lt;&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span>

    <span class="comment">// Displays "0.5"</span>
    <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="identifier">evaluate</span><span class="special">(</span> <span class="special">(</span><span class="identifier">_1</span> <span class="special">-</span> <span class="identifier">_2</span><span class="special">)</span> <span class="special">/</span> <span class="identifier">_2</span><span class="special">,</span> <span class="number">3.0</span><span class="special">,</span> <span class="number">2.0</span> <span class="special">)</span> <span class="special">&lt;&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span>

    <span class="keyword">return</span> <span class="number">0</span><span class="special">;</span>
<span class="special">}</span>
</pre>
<p>
        </p>
</div>
<div class="section">
<div class="titlepage"><div><div><h4 class="title">
<a name="boost_proto.users_guide.examples.calc2"></a><a class="link" href="users_guide.html#boost_proto.users_guide.examples.calc2" title="Calc2: Adding Members Using proto::extends&lt;&gt;">Calc2: Adding
        Members Using <code class="literal">proto::extends&lt;&gt;</code></a>
</h4></div></div></div>
<p>
          An extension of the Calc1 example that uses <code class="computeroutput"><a class="link" href="../boost/proto/extends.html" title="Struct template extends">proto::extends&lt;&gt;</a></code>
          to make calculator expressions valid function objects that can be used
          with STL algorithms.
        </p>
<p>
</p>
<pre class="programlisting"><span class="comment">//  Copyright 2008 Eric Niebler. Distributed under the Boost</span>
<span class="comment">//  Software License, Version 1.0. (See accompanying file</span>
<span class="comment">//  LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)</span>
<span class="comment">//</span>
<span class="comment">// This example enhances the simple arithmetic expression evaluator</span>
<span class="comment">// in calc1.cpp by using proto::extends to make arithmetic</span>
<span class="comment">// expressions immediately evaluable with operator (), a-la a</span>
<span class="comment">// function object</span>

<span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">iostream</span><span class="special">&gt;</span>
<span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">proto</span><span class="special">/</span><span class="identifier">core</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
<span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">proto</span><span class="special">/</span><span class="identifier">context</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
<span class="keyword">namespace</span> <span class="identifier">proto</span> <span class="special">=</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">proto</span><span class="special">;</span>
<span class="keyword">using</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_</span><span class="special">;</span>

<span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">Expr</span><span class="special">&gt;</span>
<span class="keyword">struct</span> <span class="identifier">calculator_expression</span><span class="special">;</span>

<span class="comment">// Tell proto how to generate expressions in the calculator_domain</span>
<span class="keyword">struct</span> <span class="identifier">calculator_domain</span>
  <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">domain</span><span class="special">&lt;</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">generator</span><span class="special">&lt;</span><span class="identifier">calculator_expression</span><span class="special">&gt;</span> <span class="special">&gt;</span>
<span class="special">{};</span>

<span class="comment">// Will be used to define the placeholders _1 and _2</span>
<span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">int</span> <span class="identifier">I</span><span class="special">&gt;</span> <span class="keyword">struct</span> <span class="identifier">placeholder</span> <span class="special">{};</span>

<span class="comment">// Define a calculator context, for evaluating arithmetic expressions</span>
<span class="comment">// (This is as before, in calc1.cpp)</span>
<span class="keyword">struct</span> <span class="identifier">calculator_context</span>
  <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">callable_context</span><span class="special">&lt;</span> <span class="identifier">calculator_context</span> <span class="keyword">const</span> <span class="special">&gt;</span>
<span class="special">{</span>
    <span class="comment">// The values bound to the placeholders</span>
    <span class="keyword">double</span> <span class="identifier">d</span><span class="special">[</span><span class="number">2</span><span class="special">];</span>

    <span class="comment">// The result of evaluating arithmetic expressions</span>
    <span class="keyword">typedef</span> <span class="keyword">double</span> <span class="identifier">result_type</span><span class="special">;</span>

    <span class="keyword">explicit</span> <span class="identifier">calculator_context</span><span class="special">(</span><span class="keyword">double</span> <span class="identifier">d1</span> <span class="special">=</span> <span class="number">0.</span><span class="special">,</span> <span class="keyword">double</span> <span class="identifier">d2</span> <span class="special">=</span> <span class="number">0.</span><span class="special">)</span>
    <span class="special">{</span>
        <span class="identifier">d</span><span class="special">[</span><span class="number">0</span><span class="special">]</span> <span class="special">=</span> <span class="identifier">d1</span><span class="special">;</span>
        <span class="identifier">d</span><span class="special">[</span><span class="number">1</span><span class="special">]</span> <span class="special">=</span> <span class="identifier">d2</span><span class="special">;</span>
    <span class="special">}</span>

    <span class="comment">// Handle the evaluation of the placeholder terminals</span>
    <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">int</span> <span class="identifier">I</span><span class="special">&gt;</span>
    <span class="keyword">double</span> <span class="keyword">operator</span> <span class="special">()(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">,</span> <span class="identifier">placeholder</span><span class="special">&lt;</span><span class="identifier">I</span><span class="special">&gt;)</span> <span class="keyword">const</span>
    <span class="special">{</span>
        <span class="keyword">return</span> <span class="identifier">d</span><span class="special">[</span> <span class="identifier">I</span> <span class="special">-</span> <span class="number">1</span> <span class="special">];</span>
    <span class="special">}</span>
<span class="special">};</span>

<span class="comment">// Wrap all calculator expressions in this type, which defines</span>
<span class="comment">// operator () to evaluate the expression.</span>
<span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">Expr</span><span class="special">&gt;</span>
<span class="keyword">struct</span> <span class="identifier">calculator_expression</span>
  <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">extends</span><span class="special">&lt;</span><span class="identifier">Expr</span><span class="special">,</span> <span class="identifier">calculator_expression</span><span class="special">&lt;</span><span class="identifier">Expr</span><span class="special">&gt;,</span> <span class="identifier">calculator_domain</span><span class="special">&gt;</span>
<span class="special">{</span>
    <span class="keyword">explicit</span> <span class="identifier">calculator_expression</span><span class="special">(</span><span class="identifier">Expr</span> <span class="keyword">const</span> <span class="special">&amp;</span><span class="identifier">expr</span> <span class="special">=</span> <span class="identifier">Expr</span><span class="special">())</span>
      <span class="special">:</span> <span class="identifier">calculator_expression</span><span class="special">::</span><span class="identifier">proto_extends</span><span class="special">(</span><span class="identifier">expr</span><span class="special">)</span>
    <span class="special">{}</span>

    <span class="identifier">BOOST_PROTO_EXTENDS_USING_ASSIGN</span><span class="special">(</span><span class="identifier">calculator_expression</span><span class="special">&lt;</span><span class="identifier">Expr</span><span class="special">&gt;)</span>

    <span class="comment">// Override operator () to evaluate the expression</span>
    <span class="keyword">double</span> <span class="keyword">operator</span> <span class="special">()()</span> <span class="keyword">const</span>
    <span class="special">{</span>
        <span class="identifier">calculator_context</span> <span class="keyword">const</span> <span class="identifier">ctx</span><span class="special">;</span>
        <span class="keyword">return</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">eval</span><span class="special">(*</span><span class="keyword">this</span><span class="special">,</span> <span class="identifier">ctx</span><span class="special">);</span>
    <span class="special">}</span>

    <span class="keyword">double</span> <span class="keyword">operator</span> <span class="special">()(</span><span class="keyword">double</span> <span class="identifier">d1</span><span class="special">)</span> <span class="keyword">const</span>
    <span class="special">{</span>
        <span class="identifier">calculator_context</span> <span class="keyword">const</span> <span class="identifier">ctx</span><span class="special">(</span><span class="identifier">d1</span><span class="special">);</span>
        <span class="keyword">return</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">eval</span><span class="special">(*</span><span class="keyword">this</span><span class="special">,</span> <span class="identifier">ctx</span><span class="special">);</span>
    <span class="special">}</span>

    <span class="keyword">double</span> <span class="keyword">operator</span> <span class="special">()(</span><span class="keyword">double</span> <span class="identifier">d1</span><span class="special">,</span> <span class="keyword">double</span> <span class="identifier">d2</span><span class="special">)</span> <span class="keyword">const</span>
    <span class="special">{</span>
        <span class="identifier">calculator_context</span> <span class="keyword">const</span> <span class="identifier">ctx</span><span class="special">(</span><span class="identifier">d1</span><span class="special">,</span> <span class="identifier">d2</span><span class="special">);</span>
        <span class="keyword">return</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">eval</span><span class="special">(*</span><span class="keyword">this</span><span class="special">,</span> <span class="identifier">ctx</span><span class="special">);</span>
    <span class="special">}</span>
<span class="special">};</span>

<span class="comment">// Define some placeholders (notice they're wrapped in calculator_expression&lt;&gt;)</span>
<span class="identifier">calculator_expression</span><span class="special">&lt;</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span> <span class="identifier">placeholder</span><span class="special">&lt;</span> <span class="number">1</span> <span class="special">&gt;</span> <span class="special">&gt;::</span><span class="identifier">type</span><span class="special">&gt;</span> <span class="keyword">const</span> <span class="identifier">_1</span><span class="special">;</span>
<span class="identifier">calculator_expression</span><span class="special">&lt;</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span> <span class="identifier">placeholder</span><span class="special">&lt;</span> <span class="number">2</span> <span class="special">&gt;</span> <span class="special">&gt;::</span><span class="identifier">type</span><span class="special">&gt;</span> <span class="keyword">const</span> <span class="identifier">_2</span><span class="special">;</span>

<span class="comment">// Now, our arithmetic expressions are immediately executable function objects:</span>
<span class="keyword">int</span> <span class="identifier">main</span><span class="special">()</span>
<span class="special">{</span>
    <span class="comment">// Displays "5"</span>
    <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="special">(</span><span class="identifier">_1</span> <span class="special">+</span> <span class="number">2.0</span><span class="special">)(</span> <span class="number">3.0</span> <span class="special">)</span> <span class="special">&lt;&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span>

    <span class="comment">// Displays "6"</span>
    <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="special">(</span> <span class="identifier">_1</span> <span class="special">*</span> <span class="identifier">_2</span> <span class="special">)(</span> <span class="number">3.0</span><span class="special">,</span> <span class="number">2.0</span> <span class="special">)</span> <span class="special">&lt;&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span>

    <span class="comment">// Displays "0.5"</span>
    <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="special">(</span> <span class="special">(</span><span class="identifier">_1</span> <span class="special">-</span> <span class="identifier">_2</span><span class="special">)</span> <span class="special">/</span> <span class="identifier">_2</span> <span class="special">)(</span> <span class="number">3.0</span><span class="special">,</span> <span class="number">2.0</span> <span class="special">)</span> <span class="special">&lt;&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span>

    <span class="keyword">return</span> <span class="number">0</span><span class="special">;</span>
<span class="special">}</span>
</pre>
<p>
        </p>
</div>
<div class="section">
<div class="titlepage"><div><div><h4 class="title">
<a name="boost_proto.users_guide.examples.calc3"></a><a class="link" href="users_guide.html#boost_proto.users_guide.examples.calc3" title="Calc3: Defining a Simple Transform">Calc3: Defining
        a Simple Transform</a>
</h4></div></div></div>
<p>
          An extension of the Calc2 example that uses a Proto transform to calculate
          the arity of a calculator expression and statically assert that the correct
          number of arguments are passed.
        </p>
<p>
</p>
<pre class="programlisting"><span class="comment">//  Copyright 2008 Eric Niebler. Distributed under the Boost</span>
<span class="comment">//  Software License, Version 1.0. (See accompanying file</span>
<span class="comment">//  LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)</span>
<span class="comment">//</span>
<span class="comment">// This example enhances the arithmetic expression evaluator</span>
<span class="comment">// in calc2.cpp by using a proto transform to calculate the</span>
<span class="comment">// number of arguments an expression requires and using a</span>
<span class="comment">// compile-time assert to guarantee that the right number of</span>
<span class="comment">// arguments are actually specified.</span>

<span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">iostream</span><span class="special">&gt;</span>
<span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">mpl</span><span class="special">/</span><span class="keyword">int</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
<span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">mpl</span><span class="special">/</span><span class="identifier">assert</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
<span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">mpl</span><span class="special">/</span><span class="identifier">min_max</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
<span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">proto</span><span class="special">/</span><span class="identifier">core</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
<span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">proto</span><span class="special">/</span><span class="identifier">context</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
<span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">proto</span><span class="special">/</span><span class="identifier">transform</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
<span class="keyword">namespace</span> <span class="identifier">mpl</span> <span class="special">=</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">mpl</span><span class="special">;</span>
<span class="keyword">namespace</span> <span class="identifier">proto</span> <span class="special">=</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">proto</span><span class="special">;</span>
<span class="keyword">using</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_</span><span class="special">;</span>

<span class="comment">// Will be used to define the placeholders _1 and _2</span>
<span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">I</span><span class="special">&gt;</span> <span class="keyword">struct</span> <span class="identifier">placeholder</span> <span class="special">:</span> <span class="identifier">I</span> <span class="special">{};</span>

<span class="comment">// This grammar basically says that a calculator expression is one of:</span>
<span class="comment">//   - A placeholder terminal</span>
<span class="comment">//   - Some other terminal</span>
<span class="comment">//   - Some non-terminal whose children are calculator expressions</span>
<span class="comment">// In addition, it has transforms that say how to calculate the</span>
<span class="comment">// expression arity for each of the three cases.</span>
<span class="keyword">struct</span> <span class="identifier">CalculatorGrammar</span>
  <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">or_</span><span class="special">&lt;</span>

        <span class="comment">// placeholders have a non-zero arity ...</span>
        <span class="identifier">proto</span><span class="special">::</span><span class="identifier">when</span><span class="special">&lt;</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span> <span class="identifier">placeholder</span><span class="special">&lt;</span><span class="identifier">_</span><span class="special">&gt;</span> <span class="special">&gt;,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_value</span> <span class="special">&gt;</span>

        <span class="comment">// Any other terminals have arity 0 ...</span>
      <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">when</span><span class="special">&lt;</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span><span class="identifier">_</span><span class="special">&gt;,</span> <span class="identifier">mpl</span><span class="special">::</span><span class="identifier">int_</span><span class="special">&lt;</span><span class="number">0</span><span class="special">&gt;()</span> <span class="special">&gt;</span>

        <span class="comment">// For any non-terminals, find the arity of the children and</span>
        <span class="comment">// take the maximum. This is recursive.</span>
      <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">when</span><span class="special">&lt;</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">nary_expr</span><span class="special">&lt;</span><span class="identifier">_</span><span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">vararg</span><span class="special">&lt;</span><span class="identifier">_</span><span class="special">&gt;</span> <span class="special">&gt;</span>
             <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">fold</span><span class="special">&lt;</span><span class="identifier">_</span><span class="special">,</span> <span class="identifier">mpl</span><span class="special">::</span><span class="identifier">int_</span><span class="special">&lt;</span><span class="number">0</span><span class="special">&gt;(),</span> <span class="identifier">mpl</span><span class="special">::</span><span class="identifier">max</span><span class="special">&lt;</span><span class="identifier">CalculatorGrammar</span><span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_state</span><span class="special">&gt;()</span> <span class="special">&gt;</span> <span class="special">&gt;</span>

    <span class="special">&gt;</span>
<span class="special">{};</span>

<span class="comment">// Simple wrapper for calculating a calculator expression's arity.</span>
<span class="comment">// It specifies mpl::int_&lt;0&gt; as the initial state. The data, which</span>
<span class="comment">// is not used, is mpl::void_.</span>
<span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">Expr</span><span class="special">&gt;</span>
<span class="keyword">struct</span> <span class="identifier">calculator_arity</span>
  <span class="special">:</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">result_of</span><span class="special">&lt;</span><span class="identifier">CalculatorGrammar</span><span class="special">(</span><span class="identifier">Expr</span><span class="special">)&gt;</span>
<span class="special">{};</span>

<span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">Expr</span><span class="special">&gt;</span>
<span class="keyword">struct</span> <span class="identifier">calculator_expression</span><span class="special">;</span>

<span class="comment">// Tell proto how to generate expressions in the calculator_domain</span>
<span class="keyword">struct</span> <span class="identifier">calculator_domain</span>
  <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">domain</span><span class="special">&lt;</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">generator</span><span class="special">&lt;</span><span class="identifier">calculator_expression</span><span class="special">&gt;</span> <span class="special">&gt;</span>
<span class="special">{};</span>

<span class="comment">// Define a calculator context, for evaluating arithmetic expressions</span>
<span class="comment">// (This is as before, in calc1.cpp and calc2.cpp)</span>
<span class="keyword">struct</span> <span class="identifier">calculator_context</span>
  <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">callable_context</span><span class="special">&lt;</span> <span class="identifier">calculator_context</span> <span class="keyword">const</span> <span class="special">&gt;</span>
<span class="special">{</span>
    <span class="comment">// The values bound to the placeholders</span>
    <span class="keyword">double</span> <span class="identifier">d</span><span class="special">[</span><span class="number">2</span><span class="special">];</span>

    <span class="comment">// The result of evaluating arithmetic expressions</span>
    <span class="keyword">typedef</span> <span class="keyword">double</span> <span class="identifier">result_type</span><span class="special">;</span>

    <span class="keyword">explicit</span> <span class="identifier">calculator_context</span><span class="special">(</span><span class="keyword">double</span> <span class="identifier">d1</span> <span class="special">=</span> <span class="number">0.</span><span class="special">,</span> <span class="keyword">double</span> <span class="identifier">d2</span> <span class="special">=</span> <span class="number">0.</span><span class="special">)</span>
    <span class="special">{</span>
        <span class="identifier">d</span><span class="special">[</span><span class="number">0</span><span class="special">]</span> <span class="special">=</span> <span class="identifier">d1</span><span class="special">;</span>
        <span class="identifier">d</span><span class="special">[</span><span class="number">1</span><span class="special">]</span> <span class="special">=</span> <span class="identifier">d2</span><span class="special">;</span>
    <span class="special">}</span>

    <span class="comment">// Handle the evaluation of the placeholder terminals</span>
    <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">I</span><span class="special">&gt;</span>
    <span class="keyword">double</span> <span class="keyword">operator</span> <span class="special">()(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">,</span> <span class="identifier">placeholder</span><span class="special">&lt;</span><span class="identifier">I</span><span class="special">&gt;)</span> <span class="keyword">const</span>
    <span class="special">{</span>
        <span class="keyword">return</span> <span class="identifier">d</span><span class="special">[</span> <span class="identifier">I</span><span class="special">()</span> <span class="special">-</span> <span class="number">1</span> <span class="special">];</span>
    <span class="special">}</span>
<span class="special">};</span>

<span class="comment">// Wrap all calculator expressions in this type, which defines</span>
<span class="comment">// operator () to evaluate the expression.</span>
<span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">Expr</span><span class="special">&gt;</span>
<span class="keyword">struct</span> <span class="identifier">calculator_expression</span>
  <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">extends</span><span class="special">&lt;</span><span class="identifier">Expr</span><span class="special">,</span> <span class="identifier">calculator_expression</span><span class="special">&lt;</span><span class="identifier">Expr</span><span class="special">&gt;,</span> <span class="identifier">calculator_domain</span><span class="special">&gt;</span>
<span class="special">{</span>
    <span class="keyword">typedef</span>
        <span class="identifier">proto</span><span class="special">::</span><span class="identifier">extends</span><span class="special">&lt;</span><span class="identifier">Expr</span><span class="special">,</span> <span class="identifier">calculator_expression</span><span class="special">&lt;</span><span class="identifier">Expr</span><span class="special">&gt;,</span> <span class="identifier">calculator_domain</span><span class="special">&gt;</span>
    <span class="identifier">base_type</span><span class="special">;</span>

    <span class="keyword">explicit</span> <span class="identifier">calculator_expression</span><span class="special">(</span><span class="identifier">Expr</span> <span class="keyword">const</span> <span class="special">&amp;</span><span class="identifier">expr</span> <span class="special">=</span> <span class="identifier">Expr</span><span class="special">())</span>
      <span class="special">:</span> <span class="identifier">base_type</span><span class="special">(</span><span class="identifier">expr</span><span class="special">)</span>
    <span class="special">{}</span>

    <span class="identifier">BOOST_PROTO_EXTENDS_USING_ASSIGN</span><span class="special">(</span><span class="identifier">calculator_expression</span><span class="special">&lt;</span><span class="identifier">Expr</span><span class="special">&gt;)</span>

    <span class="comment">// Override operator () to evaluate the expression</span>
    <span class="keyword">double</span> <span class="keyword">operator</span> <span class="special">()()</span> <span class="keyword">const</span>
    <span class="special">{</span>
        <span class="comment">// Assert that the expression has arity 0</span>
        <span class="identifier">BOOST_MPL_ASSERT_RELATION</span><span class="special">(</span><span class="number">0</span><span class="special">,</span> <span class="special">==,</span> <span class="identifier">calculator_arity</span><span class="special">&lt;</span><span class="identifier">Expr</span><span class="special">&gt;::</span><span class="identifier">type</span><span class="special">::</span><span class="identifier">value</span><span class="special">);</span>
        <span class="identifier">calculator_context</span> <span class="keyword">const</span> <span class="identifier">ctx</span><span class="special">;</span>
        <span class="keyword">return</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">eval</span><span class="special">(*</span><span class="keyword">this</span><span class="special">,</span> <span class="identifier">ctx</span><span class="special">);</span>
    <span class="special">}</span>

    <span class="keyword">double</span> <span class="keyword">operator</span> <span class="special">()(</span><span class="keyword">double</span> <span class="identifier">d1</span><span class="special">)</span> <span class="keyword">const</span>
    <span class="special">{</span>
        <span class="comment">// Assert that the expression has arity 1</span>
        <span class="identifier">BOOST_MPL_ASSERT_RELATION</span><span class="special">(</span><span class="number">1</span><span class="special">,</span> <span class="special">==,</span> <span class="identifier">calculator_arity</span><span class="special">&lt;</span><span class="identifier">Expr</span><span class="special">&gt;::</span><span class="identifier">type</span><span class="special">::</span><span class="identifier">value</span><span class="special">);</span>
        <span class="identifier">calculator_context</span> <span class="keyword">const</span> <span class="identifier">ctx</span><span class="special">(</span><span class="identifier">d1</span><span class="special">);</span>
        <span class="keyword">return</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">eval</span><span class="special">(*</span><span class="keyword">this</span><span class="special">,</span> <span class="identifier">ctx</span><span class="special">);</span>
    <span class="special">}</span>

    <span class="keyword">double</span> <span class="keyword">operator</span> <span class="special">()(</span><span class="keyword">double</span> <span class="identifier">d1</span><span class="special">,</span> <span class="keyword">double</span> <span class="identifier">d2</span><span class="special">)</span> <span class="keyword">const</span>
    <span class="special">{</span>
        <span class="comment">// Assert that the expression has arity 2</span>
        <span class="identifier">BOOST_MPL_ASSERT_RELATION</span><span class="special">(</span><span class="number">2</span><span class="special">,</span> <span class="special">==,</span> <span class="identifier">calculator_arity</span><span class="special">&lt;</span><span class="identifier">Expr</span><span class="special">&gt;::</span><span class="identifier">type</span><span class="special">::</span><span class="identifier">value</span><span class="special">);</span>
        <span class="identifier">calculator_context</span> <span class="keyword">const</span> <span class="identifier">ctx</span><span class="special">(</span><span class="identifier">d1</span><span class="special">,</span> <span class="identifier">d2</span><span class="special">);</span>
        <span class="keyword">return</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">eval</span><span class="special">(*</span><span class="keyword">this</span><span class="special">,</span> <span class="identifier">ctx</span><span class="special">);</span>
    <span class="special">}</span>
<span class="special">};</span>

<span class="comment">// Define some placeholders (notice they're wrapped in calculator_expression&lt;&gt;)</span>
<span class="identifier">calculator_expression</span><span class="special">&lt;</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span> <span class="identifier">placeholder</span><span class="special">&lt;</span> <span class="identifier">mpl</span><span class="special">::</span><span class="identifier">int_</span><span class="special">&lt;</span><span class="number">1</span><span class="special">&gt;</span> <span class="special">&gt;</span> <span class="special">&gt;::</span><span class="identifier">type</span><span class="special">&gt;</span> <span class="keyword">const</span> <span class="identifier">_1</span><span class="special">;</span>
<span class="identifier">calculator_expression</span><span class="special">&lt;</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span> <span class="identifier">placeholder</span><span class="special">&lt;</span> <span class="identifier">mpl</span><span class="special">::</span><span class="identifier">int_</span><span class="special">&lt;</span><span class="number">2</span><span class="special">&gt;</span> <span class="special">&gt;</span> <span class="special">&gt;::</span><span class="identifier">type</span><span class="special">&gt;</span> <span class="keyword">const</span> <span class="identifier">_2</span><span class="special">;</span>

<span class="comment">// Now, our arithmetic expressions are immediately executable function objects:</span>
<span class="keyword">int</span> <span class="identifier">main</span><span class="special">()</span>
<span class="special">{</span>
    <span class="comment">// Displays "5"</span>
    <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="special">(</span><span class="identifier">_1</span> <span class="special">+</span> <span class="number">2.0</span><span class="special">)(</span> <span class="number">3.0</span> <span class="special">)</span> <span class="special">&lt;&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span>

    <span class="comment">// Displays "6"</span>
    <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="special">(</span> <span class="identifier">_1</span> <span class="special">*</span> <span class="identifier">_2</span> <span class="special">)(</span> <span class="number">3.0</span><span class="special">,</span> <span class="number">2.0</span> <span class="special">)</span> <span class="special">&lt;&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span>

    <span class="comment">// Displays "0.5"</span>
    <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="special">(</span> <span class="special">(</span><span class="identifier">_1</span> <span class="special">-</span> <span class="identifier">_2</span><span class="special">)</span> <span class="special">/</span> <span class="identifier">_2</span> <span class="special">)(</span> <span class="number">3.0</span><span class="special">,</span> <span class="number">2.0</span> <span class="special">)</span> <span class="special">&lt;&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span>

    <span class="comment">// This won't compile because the arity of the</span>
    <span class="comment">// expression doesn't match the number of arguments</span>
    <span class="comment">// ( (_1 - _2) / _2 )( 3.0 );</span>

    <span class="keyword">return</span> <span class="number">0</span><span class="special">;</span>
<span class="special">}</span>
</pre>
<p>
        </p>
</div>
<div class="section">
<div class="titlepage"><div><div><h4 class="title">
<a name="boost_proto.users_guide.examples.lazy_vector"></a><a class="link" href="users_guide.html#boost_proto.users_guide.examples.lazy_vector" title="Lazy Vector: Controlling Operator Overloads">Lazy
        Vector: Controlling Operator Overloads</a>
</h4></div></div></div>
<p>
          This example constructs a mini-library for linear algebra, using expression
          templates to eliminate the need for temporaries when adding vectors of
          numbers.
        </p>
<p>
          This example uses a domain with a grammar to prune the set of overloaded
          operators. Only those operators that produce valid lazy vector expressions
          are allowed.
        </p>
<p>
</p>
<pre class="programlisting"><span class="comment">///////////////////////////////////////////////////////////////////////////////</span>
<span class="comment">//  Copyright 2008 Eric Niebler. Distributed under the Boost</span>
<span class="comment">//  Software License, Version 1.0. (See accompanying file</span>
<span class="comment">//  LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)</span>
<span class="comment">//</span>
<span class="comment">// This example constructs a mini-library for linear algebra, using</span>
<span class="comment">// expression templates to eliminate the need for temporaries when</span>
<span class="comment">// adding vectors of numbers.</span>
<span class="comment">//</span>
<span class="comment">// This example uses a domain with a grammar to prune the set</span>
<span class="comment">// of overloaded operators. Only those operators that produce</span>
<span class="comment">// valid lazy vector expressions are allowed.</span>

<span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">vector</span><span class="special">&gt;</span>
<span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">iostream</span><span class="special">&gt;</span>
<span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">mpl</span><span class="special">/</span><span class="keyword">int</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
<span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">proto</span><span class="special">/</span><span class="identifier">core</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
<span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">proto</span><span class="special">/</span><span class="identifier">context</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
<span class="keyword">namespace</span> <span class="identifier">mpl</span> <span class="special">=</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">mpl</span><span class="special">;</span>
<span class="keyword">namespace</span> <span class="identifier">proto</span> <span class="special">=</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">proto</span><span class="special">;</span>
<span class="keyword">using</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_</span><span class="special">;</span>

<span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">Expr</span><span class="special">&gt;</span>
<span class="keyword">struct</span> <span class="identifier">lazy_vector_expr</span><span class="special">;</span>

<span class="comment">// This grammar describes which lazy vector expressions</span>
<span class="comment">// are allowed; namely, vector terminals and addition</span>
<span class="comment">// and subtraction of lazy vector expressions.</span>
<span class="keyword">struct</span> <span class="identifier">LazyVectorGrammar</span>
  <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">or_</span><span class="special">&lt;</span>
        <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">vector</span><span class="special">&lt;</span><span class="identifier">_</span><span class="special">&gt;</span> <span class="special">&gt;</span>
      <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">plus</span><span class="special">&lt;</span> <span class="identifier">LazyVectorGrammar</span><span class="special">,</span> <span class="identifier">LazyVectorGrammar</span> <span class="special">&gt;</span>
      <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">minus</span><span class="special">&lt;</span> <span class="identifier">LazyVectorGrammar</span><span class="special">,</span> <span class="identifier">LazyVectorGrammar</span> <span class="special">&gt;</span>
    <span class="special">&gt;</span>
<span class="special">{};</span>

<span class="comment">// Tell proto that in the lazy_vector_domain, all</span>
<span class="comment">// expressions should be wrapped in laxy_vector_expr&lt;&gt;</span>
<span class="comment">// and must conform to the lazy vector grammar.</span>
<span class="keyword">struct</span> <span class="identifier">lazy_vector_domain</span>
  <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">domain</span><span class="special">&lt;</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">generator</span><span class="special">&lt;</span><span class="identifier">lazy_vector_expr</span><span class="special">&gt;,</span> <span class="identifier">LazyVectorGrammar</span><span class="special">&gt;</span>
<span class="special">{};</span>

<span class="comment">// Here is an evaluation context that indexes into a lazy vector</span>
<span class="comment">// expression, and combines the result.</span>
<span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">Size</span> <span class="special">=</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">size_t</span><span class="special">&gt;</span>
<span class="keyword">struct</span> <span class="identifier">lazy_subscript_context</span>
<span class="special">{</span>
    <span class="identifier">lazy_subscript_context</span><span class="special">(</span><span class="identifier">Size</span> <span class="identifier">subscript</span><span class="special">)</span>
      <span class="special">:</span> <span class="identifier">subscript_</span><span class="special">(</span><span class="identifier">subscript</span><span class="special">)</span>
    <span class="special">{}</span>

    <span class="comment">// Use default_eval for all the operations ...</span>
    <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">Expr</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">Tag</span> <span class="special">=</span> <span class="keyword">typename</span> <span class="identifier">Expr</span><span class="special">::</span><span class="identifier">proto_tag</span><span class="special">&gt;</span>
    <span class="keyword">struct</span> <span class="identifier">eval</span>
      <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">default_eval</span><span class="special">&lt;</span><span class="identifier">Expr</span><span class="special">,</span> <span class="identifier">lazy_subscript_context</span><span class="special">&gt;</span>
    <span class="special">{};</span>

    <span class="comment">// ... except for terminals, which we index with our subscript</span>
    <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">Expr</span><span class="special">&gt;</span>
    <span class="keyword">struct</span> <span class="identifier">eval</span><span class="special">&lt;</span><span class="identifier">Expr</span><span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&gt;</span>
    <span class="special">{</span>
        <span class="keyword">typedef</span> <span class="keyword">typename</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">result_of</span><span class="special">::</span><span class="identifier">value</span><span class="special">&lt;</span><span class="identifier">Expr</span><span class="special">&gt;::</span><span class="identifier">type</span><span class="special">::</span><span class="identifier">value_type</span> <span class="identifier">result_type</span><span class="special">;</span>

        <span class="identifier">result_type</span> <span class="keyword">operator</span> <span class="special">()(</span> <span class="identifier">Expr</span> <span class="keyword">const</span> <span class="special">&amp;</span> <span class="identifier">expr</span><span class="special">,</span> <span class="identifier">lazy_subscript_context</span> <span class="special">&amp;</span> <span class="identifier">ctx</span> <span class="special">)</span> <span class="keyword">const</span>
        <span class="special">{</span>
            <span class="keyword">return</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">value</span><span class="special">(</span> <span class="identifier">expr</span> <span class="special">)[</span> <span class="identifier">ctx</span><span class="special">.</span><span class="identifier">subscript_</span> <span class="special">];</span>
        <span class="special">}</span>
    <span class="special">};</span>

    <span class="identifier">Size</span> <span class="identifier">subscript_</span><span class="special">;</span>
<span class="special">};</span>

<span class="comment">// Here is the domain-specific expression wrapper, which overrides</span>
<span class="comment">// operator [] to evaluate the expression using the lazy_subscript_context.</span>
<span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">Expr</span><span class="special">&gt;</span>
<span class="keyword">struct</span> <span class="identifier">lazy_vector_expr</span>
  <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">extends</span><span class="special">&lt;</span><span class="identifier">Expr</span><span class="special">,</span> <span class="identifier">lazy_vector_expr</span><span class="special">&lt;</span><span class="identifier">Expr</span><span class="special">&gt;,</span> <span class="identifier">lazy_vector_domain</span><span class="special">&gt;</span>
<span class="special">{</span>
    <span class="identifier">lazy_vector_expr</span><span class="special">(</span> <span class="identifier">Expr</span> <span class="keyword">const</span> <span class="special">&amp;</span> <span class="identifier">expr</span> <span class="special">=</span> <span class="identifier">Expr</span><span class="special">()</span> <span class="special">)</span>
      <span class="special">:</span> <span class="identifier">lazy_vector_expr</span><span class="special">::</span><span class="identifier">proto_extends</span><span class="special">(</span> <span class="identifier">expr</span> <span class="special">)</span>
    <span class="special">{}</span>

    <span class="comment">// Use the lazy_subscript_context&lt;&gt; to implement subscripting</span>
    <span class="comment">// of a lazy vector expression tree.</span>
    <span class="keyword">template</span><span class="special">&lt;</span> <span class="keyword">typename</span> <span class="identifier">Size</span> <span class="special">&gt;</span>
    <span class="keyword">typename</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">result_of</span><span class="special">::</span><span class="identifier">eval</span><span class="special">&lt;</span> <span class="identifier">Expr</span><span class="special">,</span> <span class="identifier">lazy_subscript_context</span><span class="special">&lt;</span><span class="identifier">Size</span><span class="special">&gt;</span> <span class="special">&gt;::</span><span class="identifier">type</span>
    <span class="keyword">operator</span> <span class="special">[](</span> <span class="identifier">Size</span> <span class="identifier">subscript</span> <span class="special">)</span> <span class="keyword">const</span>
    <span class="special">{</span>
        <span class="identifier">lazy_subscript_context</span><span class="special">&lt;</span><span class="identifier">Size</span><span class="special">&gt;</span> <span class="identifier">ctx</span><span class="special">(</span><span class="identifier">subscript</span><span class="special">);</span>
        <span class="keyword">return</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">eval</span><span class="special">(*</span><span class="keyword">this</span><span class="special">,</span> <span class="identifier">ctx</span><span class="special">);</span>
    <span class="special">}</span>
<span class="special">};</span>

<span class="comment">// Here is our lazy_vector terminal, implemented in terms of lazy_vector_expr</span>
<span class="keyword">template</span><span class="special">&lt;</span> <span class="keyword">typename</span> <span class="identifier">T</span> <span class="special">&gt;</span>
<span class="keyword">struct</span> <span class="identifier">lazy_vector</span>
  <span class="special">:</span> <span class="identifier">lazy_vector_expr</span><span class="special">&lt;</span> <span class="keyword">typename</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">vector</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;</span> <span class="special">&gt;::</span><span class="identifier">type</span> <span class="special">&gt;</span>
<span class="special">{</span>
    <span class="keyword">typedef</span> <span class="keyword">typename</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">vector</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;</span> <span class="special">&gt;::</span><span class="identifier">type</span> <span class="identifier">expr_type</span><span class="special">;</span>

    <span class="identifier">lazy_vector</span><span class="special">(</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">size_t</span> <span class="identifier">size</span> <span class="special">=</span> <span class="number">0</span><span class="special">,</span> <span class="identifier">T</span> <span class="keyword">const</span> <span class="special">&amp;</span> <span class="identifier">value</span> <span class="special">=</span> <span class="identifier">T</span><span class="special">()</span> <span class="special">)</span>
      <span class="special">:</span> <span class="identifier">lazy_vector_expr</span><span class="special">&lt;</span><span class="identifier">expr_type</span><span class="special">&gt;(</span> <span class="identifier">expr_type</span><span class="special">::</span><span class="identifier">make</span><span class="special">(</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">vector</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;(</span> <span class="identifier">size</span><span class="special">,</span> <span class="identifier">value</span> <span class="special">)</span> <span class="special">)</span> <span class="special">)</span>
    <span class="special">{}</span>

    <span class="comment">// Here we define a += operator for lazy vector terminals that</span>
    <span class="comment">// takes a lazy vector expression and indexes it. expr[i] here</span>
    <span class="comment">// uses lazy_subscript_context&lt;&gt; under the covers.</span>
    <span class="keyword">template</span><span class="special">&lt;</span> <span class="keyword">typename</span> <span class="identifier">Expr</span> <span class="special">&gt;</span>
    <span class="identifier">lazy_vector</span> <span class="special">&amp;</span><span class="keyword">operator</span> <span class="special">+=</span> <span class="special">(</span><span class="identifier">Expr</span> <span class="keyword">const</span> <span class="special">&amp;</span> <span class="identifier">expr</span><span class="special">)</span>
    <span class="special">{</span>
        <span class="identifier">std</span><span class="special">::</span><span class="identifier">size_t</span> <span class="identifier">size</span> <span class="special">=</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">value</span><span class="special">(*</span><span class="keyword">this</span><span class="special">).</span><span class="identifier">size</span><span class="special">();</span>
        <span class="keyword">for</span><span class="special">(</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">size_t</span> <span class="identifier">i</span> <span class="special">=</span> <span class="number">0</span><span class="special">;</span> <span class="identifier">i</span> <span class="special">&lt;</span> <span class="identifier">size</span><span class="special">;</span> <span class="special">++</span><span class="identifier">i</span><span class="special">)</span>
        <span class="special">{</span>
            <span class="identifier">proto</span><span class="special">::</span><span class="identifier">value</span><span class="special">(*</span><span class="keyword">this</span><span class="special">)[</span><span class="identifier">i</span><span class="special">]</span> <span class="special">+=</span> <span class="identifier">expr</span><span class="special">[</span><span class="identifier">i</span><span class="special">];</span>
        <span class="special">}</span>
        <span class="keyword">return</span> <span class="special">*</span><span class="keyword">this</span><span class="special">;</span>
    <span class="special">}</span>
<span class="special">};</span>

<span class="keyword">int</span> <span class="identifier">main</span><span class="special">()</span>
<span class="special">{</span>
    <span class="comment">// lazy_vectors with 4 elements each.</span>
    <span class="identifier">lazy_vector</span><span class="special">&lt;</span> <span class="keyword">double</span> <span class="special">&gt;</span> <span class="identifier">v1</span><span class="special">(</span> <span class="number">4</span><span class="special">,</span> <span class="number">1.0</span> <span class="special">),</span> <span class="identifier">v2</span><span class="special">(</span> <span class="number">4</span><span class="special">,</span> <span class="number">2.0</span> <span class="special">),</span> <span class="identifier">v3</span><span class="special">(</span> <span class="number">4</span><span class="special">,</span> <span class="number">3.0</span> <span class="special">);</span>

    <span class="comment">// Add two vectors lazily and get the 2nd element.</span>
    <span class="keyword">double</span> <span class="identifier">d1</span> <span class="special">=</span> <span class="special">(</span> <span class="identifier">v2</span> <span class="special">+</span> <span class="identifier">v3</span> <span class="special">)[</span> <span class="number">2</span> <span class="special">];</span>   <span class="comment">// Look ma, no temporaries!</span>
    <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="identifier">d1</span> <span class="special">&lt;&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span>

    <span class="comment">// Subtract two vectors and add the result to a third vector.</span>
    <span class="identifier">v1</span> <span class="special">+=</span> <span class="identifier">v2</span> <span class="special">-</span> <span class="identifier">v3</span><span class="special">;</span>                  <span class="comment">// Still no temporaries!</span>
    <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="char">'{'</span> <span class="special">&lt;&lt;</span> <span class="identifier">v1</span><span class="special">[</span><span class="number">0</span><span class="special">]</span> <span class="special">&lt;&lt;</span> <span class="char">','</span> <span class="special">&lt;&lt;</span> <span class="identifier">v1</span><span class="special">[</span><span class="number">1</span><span class="special">]</span>
              <span class="special">&lt;&lt;</span> <span class="char">','</span> <span class="special">&lt;&lt;</span> <span class="identifier">v1</span><span class="special">[</span><span class="number">2</span><span class="special">]</span> <span class="special">&lt;&lt;</span> <span class="char">','</span> <span class="special">&lt;&lt;</span> <span class="identifier">v1</span><span class="special">[</span><span class="number">3</span><span class="special">]</span> <span class="special">&lt;&lt;</span> <span class="char">'}'</span> <span class="special">&lt;&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span>

    <span class="comment">// This expression is disallowed because it does not conform</span>
    <span class="comment">// to the LazyVectorGrammar</span>
    <span class="comment">//(v2 + v3) += v1;</span>

    <span class="keyword">return</span> <span class="number">0</span><span class="special">;</span>
<span class="special">}</span>
</pre>
<p>
        </p>
</div>
<div class="section">
<div class="titlepage"><div><div><h4 class="title">
<a name="boost_proto.users_guide.examples.rgb"></a><a class="link" href="users_guide.html#boost_proto.users_guide.examples.rgb" title="RGB: Type Manipulations with Proto Transforms">RGB: Type Manipulations
        with Proto Transforms</a>
</h4></div></div></div>
<p>
          This is a simple example of doing arbitrary type manipulations with Proto
          transforms. It takes some expression involving primary colors and combines
          the colors according to arbitrary rules. It is a port of the RGB example
          from <a href="http://acts.nersc.gov/formertools/pete/index.html" target="_top">PETE</a>.
        </p>
<p>
</p>
<pre class="programlisting"><span class="comment">///////////////////////////////////////////////////////////////////////////////</span>
<span class="comment">//  Copyright 2008 Eric Niebler. Distributed under the Boost</span>
<span class="comment">//  Software License, Version 1.0. (See accompanying file</span>
<span class="comment">//  LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)</span>
<span class="comment">//</span>
<span class="comment">// This is a simple example of doing arbitrary type manipulations with proto</span>
<span class="comment">// transforms. It takes some expression involving primary colors and combines</span>
<span class="comment">// the colors according to arbitrary rules. It is a port of the RGB example</span>
<span class="comment">// from PETE (http://www.codesourcery.com/pooma/download.html).</span>

<span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">iostream</span><span class="special">&gt;</span>
<span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">proto</span><span class="special">/</span><span class="identifier">core</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
<span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">proto</span><span class="special">/</span><span class="identifier">transform</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
<span class="keyword">namespace</span> <span class="identifier">proto</span> <span class="special">=</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">proto</span><span class="special">;</span>

<span class="keyword">struct</span> <span class="identifier">RedTag</span>
<span class="special">{</span>
    <span class="keyword">friend</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">ostream</span> <span class="special">&amp;</span><span class="keyword">operator</span> <span class="special">&lt;&lt;(</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">ostream</span> <span class="special">&amp;</span><span class="identifier">sout</span><span class="special">,</span> <span class="identifier">RedTag</span><span class="special">)</span>
    <span class="special">{</span>
        <span class="keyword">return</span> <span class="identifier">sout</span> <span class="special">&lt;&lt;</span> <span class="string">"This expression is red."</span><span class="special">;</span>
    <span class="special">}</span>
<span class="special">};</span>

<span class="keyword">struct</span> <span class="identifier">BlueTag</span>
<span class="special">{</span>
    <span class="keyword">friend</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">ostream</span> <span class="special">&amp;</span><span class="keyword">operator</span> <span class="special">&lt;&lt;(</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">ostream</span> <span class="special">&amp;</span><span class="identifier">sout</span><span class="special">,</span> <span class="identifier">BlueTag</span><span class="special">)</span>
    <span class="special">{</span>
        <span class="keyword">return</span> <span class="identifier">sout</span> <span class="special">&lt;&lt;</span> <span class="string">"This expression is blue."</span><span class="special">;</span>
    <span class="special">}</span>
<span class="special">};</span>

<span class="keyword">struct</span> <span class="identifier">GreenTag</span>
<span class="special">{</span>
    <span class="keyword">friend</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">ostream</span> <span class="special">&amp;</span><span class="keyword">operator</span> <span class="special">&lt;&lt;(</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">ostream</span> <span class="special">&amp;</span><span class="identifier">sout</span><span class="special">,</span> <span class="identifier">GreenTag</span><span class="special">)</span>
    <span class="special">{</span>
        <span class="keyword">return</span> <span class="identifier">sout</span> <span class="special">&lt;&lt;</span> <span class="string">"This expression is green."</span><span class="special">;</span>
    <span class="special">}</span>
<span class="special">};</span>

<span class="keyword">typedef</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span><span class="identifier">RedTag</span><span class="special">&gt;::</span><span class="identifier">type</span> <span class="identifier">RedT</span><span class="special">;</span>
<span class="keyword">typedef</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span><span class="identifier">BlueTag</span><span class="special">&gt;::</span><span class="identifier">type</span> <span class="identifier">BlueT</span><span class="special">;</span>
<span class="keyword">typedef</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span><span class="identifier">GreenTag</span><span class="special">&gt;::</span><span class="identifier">type</span> <span class="identifier">GreenT</span><span class="special">;</span>

<span class="keyword">struct</span> <span class="identifier">Red</span><span class="special">;</span>
<span class="keyword">struct</span> <span class="identifier">Blue</span><span class="special">;</span>
<span class="keyword">struct</span> <span class="identifier">Green</span><span class="special">;</span>

<span class="comment">///////////////////////////////////////////////////////////////////////////////</span>
<span class="comment">// A transform that produces new colors according to some arbitrary rules:</span>
<span class="comment">// red &amp; green give blue, red &amp; blue give green, blue and green give red.</span>
<span class="keyword">struct</span> <span class="identifier">Red</span>
  <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">or_</span><span class="special">&lt;</span>
        <span class="identifier">proto</span><span class="special">::</span><span class="identifier">plus</span><span class="special">&lt;</span><span class="identifier">Green</span><span class="special">,</span> <span class="identifier">Blue</span><span class="special">&gt;</span>
      <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">plus</span><span class="special">&lt;</span><span class="identifier">Blue</span><span class="special">,</span> <span class="identifier">Green</span><span class="special">&gt;</span>
      <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">plus</span><span class="special">&lt;</span><span class="identifier">Red</span><span class="special">,</span> <span class="identifier">Red</span><span class="special">&gt;</span>
      <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span><span class="identifier">RedTag</span><span class="special">&gt;</span>
    <span class="special">&gt;</span>
<span class="special">{};</span>

<span class="keyword">struct</span> <span class="identifier">Green</span>
  <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">or_</span><span class="special">&lt;</span>
        <span class="identifier">proto</span><span class="special">::</span><span class="identifier">plus</span><span class="special">&lt;</span><span class="identifier">Red</span><span class="special">,</span> <span class="identifier">Blue</span><span class="special">&gt;</span>
      <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">plus</span><span class="special">&lt;</span><span class="identifier">Blue</span><span class="special">,</span> <span class="identifier">Red</span><span class="special">&gt;</span>
      <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">plus</span><span class="special">&lt;</span><span class="identifier">Green</span><span class="special">,</span> <span class="identifier">Green</span><span class="special">&gt;</span>
      <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span><span class="identifier">GreenTag</span><span class="special">&gt;</span>
    <span class="special">&gt;</span>
<span class="special">{};</span>

<span class="keyword">struct</span> <span class="identifier">Blue</span>
  <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">or_</span><span class="special">&lt;</span>
        <span class="identifier">proto</span><span class="special">::</span><span class="identifier">plus</span><span class="special">&lt;</span><span class="identifier">Red</span><span class="special">,</span> <span class="identifier">Green</span><span class="special">&gt;</span>
      <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">plus</span><span class="special">&lt;</span><span class="identifier">Green</span><span class="special">,</span> <span class="identifier">Red</span><span class="special">&gt;</span>
      <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">plus</span><span class="special">&lt;</span><span class="identifier">Blue</span><span class="special">,</span> <span class="identifier">Blue</span><span class="special">&gt;</span>
      <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span><span class="identifier">BlueTag</span><span class="special">&gt;</span>
    <span class="special">&gt;</span>
<span class="special">{};</span>

<span class="keyword">struct</span> <span class="identifier">RGB</span>
  <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">or_</span><span class="special">&lt;</span>
        <span class="identifier">proto</span><span class="special">::</span><span class="identifier">when</span><span class="special">&lt;</span> <span class="identifier">Red</span><span class="special">,</span> <span class="identifier">RedTag</span><span class="special">()</span> <span class="special">&gt;</span>
      <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">when</span><span class="special">&lt;</span> <span class="identifier">Blue</span><span class="special">,</span> <span class="identifier">BlueTag</span><span class="special">()</span> <span class="special">&gt;</span>
      <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">when</span><span class="special">&lt;</span> <span class="identifier">Green</span><span class="special">,</span> <span class="identifier">GreenTag</span><span class="special">()</span> <span class="special">&gt;</span>
    <span class="special">&gt;</span>
<span class="special">{};</span>

<span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">Expr</span><span class="special">&gt;</span>
<span class="keyword">void</span> <span class="identifier">printColor</span><span class="special">(</span><span class="identifier">Expr</span> <span class="keyword">const</span> <span class="special">&amp;</span> <span class="identifier">expr</span><span class="special">)</span>
<span class="special">{</span>
    <span class="keyword">int</span> <span class="identifier">i</span> <span class="special">=</span> <span class="number">0</span><span class="special">;</span> <span class="comment">// dummy state and data parameter, not used</span>
    <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="identifier">RGB</span><span class="special">()(</span><span class="identifier">expr</span><span class="special">,</span> <span class="identifier">i</span><span class="special">,</span> <span class="identifier">i</span><span class="special">)</span> <span class="special">&lt;&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span>
<span class="special">}</span>

<span class="keyword">int</span> <span class="identifier">main</span><span class="special">()</span>
<span class="special">{</span>
    <span class="identifier">printColor</span><span class="special">(</span><span class="identifier">RedT</span><span class="special">()</span> <span class="special">+</span> <span class="identifier">GreenT</span><span class="special">());</span>
    <span class="identifier">printColor</span><span class="special">(</span><span class="identifier">RedT</span><span class="special">()</span> <span class="special">+</span> <span class="identifier">GreenT</span><span class="special">()</span> <span class="special">+</span> <span class="identifier">BlueT</span><span class="special">());</span>
    <span class="identifier">printColor</span><span class="special">(</span><span class="identifier">RedT</span><span class="special">()</span> <span class="special">+</span> <span class="special">(</span><span class="identifier">GreenT</span><span class="special">()</span> <span class="special">+</span> <span class="identifier">BlueT</span><span class="special">()));</span>

    <span class="keyword">return</span> <span class="number">0</span><span class="special">;</span>
<span class="special">}</span>
</pre>
<p>
        </p>
</div>
<div class="section">
<div class="titlepage"><div><div><h4 class="title">
<a name="boost_proto.users_guide.examples.tarray"></a><a class="link" href="users_guide.html#boost_proto.users_guide.examples.tarray" title="TArray: A Simple Linear Algebra Library">TArray: A
        Simple Linear Algebra Library</a>
</h4></div></div></div>
<p>
          This example constructs a mini-library for linear algebra, using expression
          templates to eliminate the need for temporaries when adding arrays of numbers.
          It duplicates the TArray example from <a href="http://acts.nersc.gov/formertools/pete/index.html" target="_top">PETE</a>.
        </p>
<p>
</p>
<pre class="programlisting"><span class="comment">///////////////////////////////////////////////////////////////////////////////</span>
<span class="comment">//  Copyright 2008 Eric Niebler. Distributed under the Boost</span>
<span class="comment">//  Software License, Version 1.0. (See accompanying file</span>
<span class="comment">//  LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)</span>
<span class="comment">//</span>
<span class="comment">// This example constructs a mini-library for linear algebra, using</span>
<span class="comment">// expression templates to eliminate the need for temporaries when</span>
<span class="comment">// adding arrays of numbers. It duplicates the TArray example from</span>
<span class="comment">// PETE (http://www.codesourcery.com/pooma/download.html)</span>

<span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">iostream</span><span class="special">&gt;</span>
<span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">mpl</span><span class="special">/</span><span class="keyword">int</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
<span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">proto</span><span class="special">/</span><span class="identifier">core</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
<span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">proto</span><span class="special">/</span><span class="identifier">context</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
<span class="keyword">namespace</span> <span class="identifier">mpl</span> <span class="special">=</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">mpl</span><span class="special">;</span>
<span class="keyword">namespace</span> <span class="identifier">proto</span> <span class="special">=</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">proto</span><span class="special">;</span>
<span class="keyword">using</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_</span><span class="special">;</span>

<span class="comment">// This grammar describes which TArray expressions</span>
<span class="comment">// are allowed; namely, int and array terminals</span>
<span class="comment">// plus, minus, multiplies and divides of TArray expressions.</span>
<span class="keyword">struct</span> <span class="identifier">TArrayGrammar</span>
  <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">or_</span><span class="special">&lt;</span>
        <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span> <span class="keyword">int</span> <span class="special">&gt;</span>
      <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span> <span class="keyword">int</span><span class="special">[</span><span class="number">3</span><span class="special">]</span> <span class="special">&gt;</span>
      <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">plus</span><span class="special">&lt;</span> <span class="identifier">TArrayGrammar</span><span class="special">,</span> <span class="identifier">TArrayGrammar</span> <span class="special">&gt;</span>
      <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">minus</span><span class="special">&lt;</span> <span class="identifier">TArrayGrammar</span><span class="special">,</span> <span class="identifier">TArrayGrammar</span> <span class="special">&gt;</span>
      <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">multiplies</span><span class="special">&lt;</span> <span class="identifier">TArrayGrammar</span><span class="special">,</span> <span class="identifier">TArrayGrammar</span> <span class="special">&gt;</span>
      <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">divides</span><span class="special">&lt;</span> <span class="identifier">TArrayGrammar</span><span class="special">,</span> <span class="identifier">TArrayGrammar</span> <span class="special">&gt;</span>
    <span class="special">&gt;</span>
<span class="special">{};</span>

<span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">Expr</span><span class="special">&gt;</span>
<span class="keyword">struct</span> <span class="identifier">TArrayExpr</span><span class="special">;</span>

<span class="comment">// Tell proto that in the TArrayDomain, all</span>
<span class="comment">// expressions should be wrapped in TArrayExpr&lt;&gt; and</span>
<span class="comment">// must conform to the TArrayGrammar</span>
<span class="keyword">struct</span> <span class="identifier">TArrayDomain</span>
  <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">domain</span><span class="special">&lt;</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">generator</span><span class="special">&lt;</span><span class="identifier">TArrayExpr</span><span class="special">&gt;,</span> <span class="identifier">TArrayGrammar</span><span class="special">&gt;</span>
<span class="special">{};</span>

<span class="comment">// Here is an evaluation context that indexes into a TArray</span>
<span class="comment">// expression, and combines the result.</span>
<span class="keyword">struct</span> <span class="identifier">TArraySubscriptCtx</span>
  <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">callable_context</span><span class="special">&lt;</span> <span class="identifier">TArraySubscriptCtx</span> <span class="keyword">const</span> <span class="special">&gt;</span>
<span class="special">{</span>
    <span class="keyword">typedef</span> <span class="keyword">int</span> <span class="identifier">result_type</span><span class="special">;</span>

    <span class="identifier">TArraySubscriptCtx</span><span class="special">(</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">ptrdiff_t</span> <span class="identifier">i</span><span class="special">)</span>
      <span class="special">:</span> <span class="identifier">i_</span><span class="special">(</span><span class="identifier">i</span><span class="special">)</span>
    <span class="special">{}</span>

    <span class="comment">// Index array terminals with our subscript. Everything</span>
    <span class="comment">// else will be handled by the default evaluation context.</span>
    <span class="keyword">int</span> <span class="keyword">operator</span> <span class="special">()(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">,</span> <span class="keyword">int</span> <span class="keyword">const</span> <span class="special">(&amp;</span><span class="identifier">data</span><span class="special">)[</span><span class="number">3</span><span class="special">])</span> <span class="keyword">const</span>
    <span class="special">{</span>
        <span class="keyword">return</span> <span class="identifier">data</span><span class="special">[</span><span class="keyword">this</span><span class="special">-&gt;</span><span class="identifier">i_</span><span class="special">];</span>
    <span class="special">}</span>

    <span class="identifier">std</span><span class="special">::</span><span class="identifier">ptrdiff_t</span> <span class="identifier">i_</span><span class="special">;</span>
<span class="special">};</span>

<span class="comment">// Here is an evaluation context that prints a TArray expression.</span>
<span class="keyword">struct</span> <span class="identifier">TArrayPrintCtx</span>
  <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">callable_context</span><span class="special">&lt;</span> <span class="identifier">TArrayPrintCtx</span> <span class="keyword">const</span> <span class="special">&gt;</span>
<span class="special">{</span>
    <span class="keyword">typedef</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">ostream</span> <span class="special">&amp;</span><span class="identifier">result_type</span><span class="special">;</span>

    <span class="identifier">TArrayPrintCtx</span><span class="special">()</span> <span class="special">{}</span>

    <span class="identifier">std</span><span class="special">::</span><span class="identifier">ostream</span> <span class="special">&amp;</span><span class="keyword">operator</span> <span class="special">()(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">,</span> <span class="keyword">int</span> <span class="identifier">i</span><span class="special">)</span> <span class="keyword">const</span>
    <span class="special">{</span>
        <span class="keyword">return</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="identifier">i</span><span class="special">;</span>
    <span class="special">}</span>

    <span class="identifier">std</span><span class="special">::</span><span class="identifier">ostream</span> <span class="special">&amp;</span><span class="keyword">operator</span> <span class="special">()(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">,</span> <span class="keyword">int</span> <span class="keyword">const</span> <span class="special">(&amp;</span><span class="identifier">arr</span><span class="special">)[</span><span class="number">3</span><span class="special">])</span> <span class="keyword">const</span>
    <span class="special">{</span>
        <span class="keyword">return</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="char">'{'</span> <span class="special">&lt;&lt;</span> <span class="identifier">arr</span><span class="special">[</span><span class="number">0</span><span class="special">]</span> <span class="special">&lt;&lt;</span> <span class="string">", "</span> <span class="special">&lt;&lt;</span> <span class="identifier">arr</span><span class="special">[</span><span class="number">1</span><span class="special">]</span> <span class="special">&lt;&lt;</span> <span class="string">", "</span> <span class="special">&lt;&lt;</span> <span class="identifier">arr</span><span class="special">[</span><span class="number">2</span><span class="special">]</span> <span class="special">&lt;&lt;</span> <span class="char">'}'</span><span class="special">;</span>
    <span class="special">}</span>

    <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">L</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">R</span><span class="special">&gt;</span>
    <span class="identifier">std</span><span class="special">::</span><span class="identifier">ostream</span> <span class="special">&amp;</span><span class="keyword">operator</span> <span class="special">()(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">plus</span><span class="special">,</span> <span class="identifier">L</span> <span class="keyword">const</span> <span class="special">&amp;</span><span class="identifier">l</span><span class="special">,</span> <span class="identifier">R</span> <span class="keyword">const</span> <span class="special">&amp;</span><span class="identifier">r</span><span class="special">)</span> <span class="keyword">const</span>
    <span class="special">{</span>
        <span class="keyword">return</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="char">'('</span> <span class="special">&lt;&lt;</span> <span class="identifier">l</span> <span class="special">&lt;&lt;</span> <span class="string">" + "</span> <span class="special">&lt;&lt;</span> <span class="identifier">r</span> <span class="special">&lt;&lt;</span> <span class="char">')'</span><span class="special">;</span>
    <span class="special">}</span>

    <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">L</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">R</span><span class="special">&gt;</span>
    <span class="identifier">std</span><span class="special">::</span><span class="identifier">ostream</span> <span class="special">&amp;</span><span class="keyword">operator</span> <span class="special">()(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">minus</span><span class="special">,</span> <span class="identifier">L</span> <span class="keyword">const</span> <span class="special">&amp;</span><span class="identifier">l</span><span class="special">,</span> <span class="identifier">R</span> <span class="keyword">const</span> <span class="special">&amp;</span><span class="identifier">r</span><span class="special">)</span> <span class="keyword">const</span>
    <span class="special">{</span>
        <span class="keyword">return</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="char">'('</span> <span class="special">&lt;&lt;</span> <span class="identifier">l</span> <span class="special">&lt;&lt;</span> <span class="string">" - "</span> <span class="special">&lt;&lt;</span> <span class="identifier">r</span> <span class="special">&lt;&lt;</span> <span class="char">')'</span><span class="special">;</span>
    <span class="special">}</span>

    <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">L</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">R</span><span class="special">&gt;</span>
    <span class="identifier">std</span><span class="special">::</span><span class="identifier">ostream</span> <span class="special">&amp;</span><span class="keyword">operator</span> <span class="special">()(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">multiplies</span><span class="special">,</span> <span class="identifier">L</span> <span class="keyword">const</span> <span class="special">&amp;</span><span class="identifier">l</span><span class="special">,</span> <span class="identifier">R</span> <span class="keyword">const</span> <span class="special">&amp;</span><span class="identifier">r</span><span class="special">)</span> <span class="keyword">const</span>
    <span class="special">{</span>
        <span class="keyword">return</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="identifier">l</span> <span class="special">&lt;&lt;</span> <span class="string">" * "</span> <span class="special">&lt;&lt;</span> <span class="identifier">r</span><span class="special">;</span>
    <span class="special">}</span>

    <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">L</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">R</span><span class="special">&gt;</span>
    <span class="identifier">std</span><span class="special">::</span><span class="identifier">ostream</span> <span class="special">&amp;</span><span class="keyword">operator</span> <span class="special">()(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">divides</span><span class="special">,</span> <span class="identifier">L</span> <span class="keyword">const</span> <span class="special">&amp;</span><span class="identifier">l</span><span class="special">,</span> <span class="identifier">R</span> <span class="keyword">const</span> <span class="special">&amp;</span><span class="identifier">r</span><span class="special">)</span> <span class="keyword">const</span>
    <span class="special">{</span>
        <span class="keyword">return</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="identifier">l</span> <span class="special">&lt;&lt;</span> <span class="string">" / "</span> <span class="special">&lt;&lt;</span> <span class="identifier">r</span><span class="special">;</span>
    <span class="special">}</span>
<span class="special">};</span>

<span class="comment">// Here is the domain-specific expression wrapper, which overrides</span>
<span class="comment">// operator [] to evaluate the expression using the TArraySubscriptCtx.</span>
<span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">Expr</span><span class="special">&gt;</span>
<span class="keyword">struct</span> <span class="identifier">TArrayExpr</span>
  <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">extends</span><span class="special">&lt;</span><span class="identifier">Expr</span><span class="special">,</span> <span class="identifier">TArrayExpr</span><span class="special">&lt;</span><span class="identifier">Expr</span><span class="special">&gt;,</span> <span class="identifier">TArrayDomain</span><span class="special">&gt;</span>
<span class="special">{</span>
    <span class="keyword">typedef</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">extends</span><span class="special">&lt;</span><span class="identifier">Expr</span><span class="special">,</span> <span class="identifier">TArrayExpr</span><span class="special">&lt;</span><span class="identifier">Expr</span><span class="special">&gt;,</span> <span class="identifier">TArrayDomain</span><span class="special">&gt;</span> <span class="identifier">base_type</span><span class="special">;</span>

    <span class="identifier">TArrayExpr</span><span class="special">(</span> <span class="identifier">Expr</span> <span class="keyword">const</span> <span class="special">&amp;</span> <span class="identifier">expr</span> <span class="special">=</span> <span class="identifier">Expr</span><span class="special">()</span> <span class="special">)</span>
      <span class="special">:</span> <span class="identifier">base_type</span><span class="special">(</span> <span class="identifier">expr</span> <span class="special">)</span>
    <span class="special">{}</span>

    <span class="comment">// Use the TArraySubscriptCtx to implement subscripting</span>
    <span class="comment">// of a TArray expression tree.</span>
    <span class="keyword">int</span> <span class="keyword">operator</span> <span class="special">[](</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">ptrdiff_t</span> <span class="identifier">i</span> <span class="special">)</span> <span class="keyword">const</span>
    <span class="special">{</span>
        <span class="identifier">TArraySubscriptCtx</span> <span class="keyword">const</span> <span class="identifier">ctx</span><span class="special">(</span><span class="identifier">i</span><span class="special">);</span>
        <span class="keyword">return</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">eval</span><span class="special">(*</span><span class="keyword">this</span><span class="special">,</span> <span class="identifier">ctx</span><span class="special">);</span>
    <span class="special">}</span>

    <span class="comment">// Use the TArrayPrintCtx to display a TArray expression tree.</span>
    <span class="keyword">friend</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">ostream</span> <span class="special">&amp;</span><span class="keyword">operator</span> <span class="special">&lt;&lt;(</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">ostream</span> <span class="special">&amp;</span><span class="identifier">sout</span><span class="special">,</span> <span class="identifier">TArrayExpr</span><span class="special">&lt;</span><span class="identifier">Expr</span><span class="special">&gt;</span> <span class="keyword">const</span> <span class="special">&amp;</span><span class="identifier">expr</span><span class="special">)</span>
    <span class="special">{</span>
        <span class="identifier">TArrayPrintCtx</span> <span class="keyword">const</span> <span class="identifier">ctx</span><span class="special">;</span>
        <span class="keyword">return</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">eval</span><span class="special">(</span><span class="identifier">expr</span><span class="special">,</span> <span class="identifier">ctx</span><span class="special">);</span>
    <span class="special">}</span>
<span class="special">};</span>

<span class="comment">// Here is our TArray terminal, implemented in terms of TArrayExpr</span>
<span class="comment">// It is basically just an array of 3 integers.</span>
<span class="keyword">struct</span> <span class="identifier">TArray</span>
  <span class="special">:</span> <span class="identifier">TArrayExpr</span><span class="special">&lt;</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span> <span class="keyword">int</span><span class="special">[</span><span class="number">3</span><span class="special">]</span> <span class="special">&gt;::</span><span class="identifier">type</span> <span class="special">&gt;</span>
<span class="special">{</span>
    <span class="keyword">explicit</span> <span class="identifier">TArray</span><span class="special">(</span> <span class="keyword">int</span> <span class="identifier">i</span> <span class="special">=</span> <span class="number">0</span><span class="special">,</span> <span class="keyword">int</span> <span class="identifier">j</span> <span class="special">=</span> <span class="number">0</span><span class="special">,</span> <span class="keyword">int</span> <span class="identifier">k</span> <span class="special">=</span> <span class="number">0</span> <span class="special">)</span>
    <span class="special">{</span>
        <span class="special">(*</span><span class="keyword">this</span><span class="special">)[</span><span class="number">0</span><span class="special">]</span> <span class="special">=</span> <span class="identifier">i</span><span class="special">;</span>
        <span class="special">(*</span><span class="keyword">this</span><span class="special">)[</span><span class="number">1</span><span class="special">]</span> <span class="special">=</span> <span class="identifier">j</span><span class="special">;</span>
        <span class="special">(*</span><span class="keyword">this</span><span class="special">)[</span><span class="number">2</span><span class="special">]</span> <span class="special">=</span> <span class="identifier">k</span><span class="special">;</span>
    <span class="special">}</span>

    <span class="comment">// Here we override operator [] to give read/write access to</span>
    <span class="comment">// the elements of the array. (We could use the TArrayExpr</span>
    <span class="comment">// operator [] if we made the subscript context smarter about</span>
    <span class="comment">// returning non-const reference when appropriate.)</span>
    <span class="keyword">int</span> <span class="special">&amp;</span><span class="keyword">operator</span> <span class="special">[](</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">ptrdiff_t</span> <span class="identifier">i</span><span class="special">)</span>
    <span class="special">{</span>
        <span class="keyword">return</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">value</span><span class="special">(*</span><span class="keyword">this</span><span class="special">)[</span><span class="identifier">i</span><span class="special">];</span>
    <span class="special">}</span>

    <span class="keyword">int</span> <span class="keyword">const</span> <span class="special">&amp;</span><span class="keyword">operator</span> <span class="special">[](</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">ptrdiff_t</span> <span class="identifier">i</span><span class="special">)</span> <span class="keyword">const</span>
    <span class="special">{</span>
        <span class="keyword">return</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">value</span><span class="special">(*</span><span class="keyword">this</span><span class="special">)[</span><span class="identifier">i</span><span class="special">];</span>
    <span class="special">}</span>

    <span class="comment">// Here we define a operator = for TArray terminals that</span>
    <span class="comment">// takes a TArray expression.</span>
    <span class="keyword">template</span><span class="special">&lt;</span> <span class="keyword">typename</span> <span class="identifier">Expr</span> <span class="special">&gt;</span>
    <span class="identifier">TArray</span> <span class="special">&amp;</span><span class="keyword">operator</span> <span class="special">=(</span><span class="identifier">Expr</span> <span class="keyword">const</span> <span class="special">&amp;</span> <span class="identifier">expr</span><span class="special">)</span>
    <span class="special">{</span>
        <span class="comment">// proto::as_expr&lt;TArrayDomain&gt;(expr) is the same as</span>
        <span class="comment">// expr unless expr is an integer, in which case it</span>
        <span class="comment">// is made into a TArrayExpr terminal first.</span>
        <span class="keyword">return</span> <span class="keyword">this</span><span class="special">-&gt;</span><span class="identifier">assign</span><span class="special">(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">as_expr</span><span class="special">&lt;</span><span class="identifier">TArrayDomain</span><span class="special">&gt;(</span><span class="identifier">expr</span><span class="special">));</span>
    <span class="special">}</span>

    <span class="keyword">template</span><span class="special">&lt;</span> <span class="keyword">typename</span> <span class="identifier">Expr</span> <span class="special">&gt;</span>
    <span class="identifier">TArray</span> <span class="special">&amp;</span><span class="identifier">printAssign</span><span class="special">(</span><span class="identifier">Expr</span> <span class="keyword">const</span> <span class="special">&amp;</span> <span class="identifier">expr</span><span class="special">)</span>
    <span class="special">{</span>
        <span class="special">*</span><span class="keyword">this</span> <span class="special">=</span> <span class="identifier">expr</span><span class="special">;</span>
        <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="special">*</span><span class="keyword">this</span> <span class="special">&lt;&lt;</span> <span class="string">" = "</span> <span class="special">&lt;&lt;</span> <span class="identifier">expr</span> <span class="special">&lt;&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span>
        <span class="keyword">return</span> <span class="special">*</span><span class="keyword">this</span><span class="special">;</span>
    <span class="special">}</span>

<span class="keyword">private</span><span class="special">:</span>
    <span class="keyword">template</span><span class="special">&lt;</span> <span class="keyword">typename</span> <span class="identifier">Expr</span> <span class="special">&gt;</span>
    <span class="identifier">TArray</span> <span class="special">&amp;</span><span class="identifier">assign</span><span class="special">(</span><span class="identifier">Expr</span> <span class="keyword">const</span> <span class="special">&amp;</span> <span class="identifier">expr</span><span class="special">)</span>
    <span class="special">{</span>
        <span class="comment">// expr[i] here uses TArraySubscriptCtx under the covers.</span>
        <span class="special">(*</span><span class="keyword">this</span><span class="special">)[</span><span class="number">0</span><span class="special">]</span> <span class="special">=</span> <span class="identifier">expr</span><span class="special">[</span><span class="number">0</span><span class="special">];</span>
        <span class="special">(*</span><span class="keyword">this</span><span class="special">)[</span><span class="number">1</span><span class="special">]</span> <span class="special">=</span> <span class="identifier">expr</span><span class="special">[</span><span class="number">1</span><span class="special">];</span>
        <span class="special">(*</span><span class="keyword">this</span><span class="special">)[</span><span class="number">2</span><span class="special">]</span> <span class="special">=</span> <span class="identifier">expr</span><span class="special">[</span><span class="number">2</span><span class="special">];</span>
        <span class="keyword">return</span> <span class="special">*</span><span class="keyword">this</span><span class="special">;</span>
    <span class="special">}</span>
<span class="special">};</span>

<span class="keyword">int</span> <span class="identifier">main</span><span class="special">()</span>
<span class="special">{</span>
    <span class="identifier">TArray</span> <span class="identifier">a</span><span class="special">(</span><span class="number">3</span><span class="special">,</span><span class="number">1</span><span class="special">,</span><span class="number">2</span><span class="special">);</span>

    <span class="identifier">TArray</span> <span class="identifier">b</span><span class="special">;</span>

    <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="identifier">a</span> <span class="special">&lt;&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span>
    <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="identifier">b</span> <span class="special">&lt;&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span>

    <span class="identifier">b</span><span class="special">[</span><span class="number">0</span><span class="special">]</span> <span class="special">=</span> <span class="number">7</span><span class="special">;</span> <span class="identifier">b</span><span class="special">[</span><span class="number">1</span><span class="special">]</span> <span class="special">=</span> <span class="number">33</span><span class="special">;</span> <span class="identifier">b</span><span class="special">[</span><span class="number">2</span><span class="special">]</span> <span class="special">=</span> <span class="special">-</span><span class="number">99</span><span class="special">;</span>

    <span class="identifier">TArray</span> <span class="identifier">c</span><span class="special">(</span><span class="identifier">a</span><span class="special">);</span>

    <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="identifier">c</span> <span class="special">&lt;&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span>

    <span class="identifier">a</span> <span class="special">=</span> <span class="number">0</span><span class="special">;</span>

    <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="identifier">a</span> <span class="special">&lt;&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span>
    <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="identifier">b</span> <span class="special">&lt;&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span>
    <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="identifier">c</span> <span class="special">&lt;&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span>

    <span class="identifier">a</span> <span class="special">=</span> <span class="identifier">b</span> <span class="special">+</span> <span class="identifier">c</span><span class="special">;</span>

    <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="identifier">a</span> <span class="special">&lt;&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span>

    <span class="identifier">a</span><span class="special">.</span><span class="identifier">printAssign</span><span class="special">(</span><span class="identifier">b</span><span class="special">+</span><span class="identifier">c</span><span class="special">*(</span><span class="identifier">b</span> <span class="special">+</span> <span class="number">3</span><span class="special">*</span><span class="identifier">c</span><span class="special">));</span>

    <span class="keyword">return</span> <span class="number">0</span><span class="special">;</span>
<span class="special">}</span>
</pre>
<p>
        </p>
</div>
<div class="section">
<div class="titlepage"><div><div><h4 class="title">
<a name="boost_proto.users_guide.examples.vec3"></a><a class="link" href="users_guide.html#boost_proto.users_guide.examples.vec3" title="Vec3: Computing With Transforms and Contexts">Vec3: Computing
        With Transforms and Contexts</a>
</h4></div></div></div>
<p>
          This is a simple example using <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">extends</span><span class="special">&lt;&gt;</span></code> to extend a terminal type with
          additional behaviors, and using custom contexts and <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">eval</span><span class="special">()</span></code> for evaluating expressions. It is a port
          of the Vec3 example from <a href="http://acts.nersc.gov/formertools/pete/index.html" target="_top">PETE</a>.
        </p>
<p>
</p>
<pre class="programlisting"><span class="comment">///////////////////////////////////////////////////////////////////////////////</span>
<span class="comment">//  Copyright 2008 Eric Niebler. Distributed under the Boost</span>
<span class="comment">//  Software License, Version 1.0. (See accompanying file</span>
<span class="comment">//  LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)</span>
<span class="comment">//</span>
<span class="comment">// This is a simple example using proto::extends to extend a terminal type with</span>
<span class="comment">// additional behaviors, and using custom contexts and proto::eval for</span>
<span class="comment">// evaluating expressions. It is a port of the Vec3 example</span>
<span class="comment">// from PETE (http://www.codesourcery.com/pooma/download.html).</span>

<span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">iostream</span><span class="special">&gt;</span>
<span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">functional</span><span class="special">&gt;</span>
<span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">assert</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
<span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">mpl</span><span class="special">/</span><span class="keyword">int</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
<span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">proto</span><span class="special">/</span><span class="identifier">core</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
<span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">proto</span><span class="special">/</span><span class="identifier">context</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
<span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">proto</span><span class="special">/</span><span class="identifier">proto_typeof</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
<span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">proto</span><span class="special">/</span><span class="identifier">transform</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
<span class="keyword">namespace</span> <span class="identifier">mpl</span> <span class="special">=</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">mpl</span><span class="special">;</span>
<span class="keyword">namespace</span> <span class="identifier">proto</span> <span class="special">=</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">proto</span><span class="special">;</span>
<span class="keyword">using</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_</span><span class="special">;</span>

<span class="comment">// Here is an evaluation context that indexes into a Vec3</span>
<span class="comment">// expression, and combines the result.</span>
<span class="keyword">struct</span> <span class="identifier">Vec3SubscriptCtx</span>
  <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">callable_context</span><span class="special">&lt;</span> <span class="identifier">Vec3SubscriptCtx</span> <span class="keyword">const</span> <span class="special">&gt;</span>
<span class="special">{</span>
    <span class="keyword">typedef</span> <span class="keyword">int</span> <span class="identifier">result_type</span><span class="special">;</span>

    <span class="identifier">Vec3SubscriptCtx</span><span class="special">(</span><span class="keyword">int</span> <span class="identifier">i</span><span class="special">)</span>
      <span class="special">:</span> <span class="identifier">i_</span><span class="special">(</span><span class="identifier">i</span><span class="special">)</span>
    <span class="special">{}</span>

    <span class="comment">// Index array terminals with our subscript. Everything</span>
    <span class="comment">// else will be handled by the default evaluation context.</span>
    <span class="keyword">int</span> <span class="keyword">operator</span> <span class="special">()(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">,</span> <span class="keyword">int</span> <span class="keyword">const</span> <span class="special">(&amp;</span><span class="identifier">arr</span><span class="special">)[</span><span class="number">3</span><span class="special">])</span> <span class="keyword">const</span>
    <span class="special">{</span>
        <span class="keyword">return</span> <span class="identifier">arr</span><span class="special">[</span><span class="keyword">this</span><span class="special">-&gt;</span><span class="identifier">i_</span><span class="special">];</span>
    <span class="special">}</span>

    <span class="keyword">int</span> <span class="identifier">i_</span><span class="special">;</span>
<span class="special">};</span>

<span class="comment">// Here is an evaluation context that counts the number</span>
<span class="comment">// of Vec3 terminals in an expression.</span>
<span class="keyword">struct</span> <span class="identifier">CountLeavesCtx</span>
  <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">callable_context</span><span class="special">&lt;</span> <span class="identifier">CountLeavesCtx</span><span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">null_context</span> <span class="special">&gt;</span>
<span class="special">{</span>
    <span class="identifier">CountLeavesCtx</span><span class="special">()</span>
      <span class="special">:</span> <span class="identifier">count</span><span class="special">(</span><span class="number">0</span><span class="special">)</span>
      <span class="special">{}</span>

      <span class="keyword">typedef</span> <span class="keyword">void</span> <span class="identifier">result_type</span><span class="special">;</span>

      <span class="keyword">void</span> <span class="keyword">operator</span> <span class="special">()(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">,</span> <span class="keyword">int</span> <span class="keyword">const</span><span class="special">(&amp;)[</span><span class="number">3</span><span class="special">])</span>
      <span class="special">{</span>
          <span class="special">++</span><span class="keyword">this</span><span class="special">-&gt;</span><span class="identifier">count</span><span class="special">;</span>
      <span class="special">}</span>

      <span class="keyword">int</span> <span class="identifier">count</span><span class="special">;</span>
<span class="special">};</span>

<span class="keyword">struct</span> <span class="identifier">iplus</span> <span class="special">:</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">plus</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">&gt;,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">callable</span> <span class="special">{};</span>

<span class="comment">// Here is a transform that does the same thing as the above context.</span>
<span class="comment">// It demonstrates the use of the std::plus&lt;&gt; function object</span>
<span class="comment">// with the fold transform. With minor modifications, this</span>
<span class="comment">// transform could be used to calculate the leaf count at compile</span>
<span class="comment">// time, rather than at runtime.</span>
<span class="keyword">struct</span> <span class="identifier">CountLeaves</span>
  <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">or_</span><span class="special">&lt;</span>
        <span class="comment">// match a Vec3 terminal, return 1</span>
        <span class="identifier">proto</span><span class="special">::</span><span class="identifier">when</span><span class="special">&lt;</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">[</span><span class="number">3</span><span class="special">]&gt;,</span> <span class="identifier">mpl</span><span class="special">::</span><span class="identifier">int_</span><span class="special">&lt;</span><span class="number">1</span><span class="special">&gt;()</span> <span class="special">&gt;</span>
        <span class="comment">// match a terminal, return int() (which is 0)</span>
      <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">when</span><span class="special">&lt;</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span><span class="identifier">_</span><span class="special">&gt;,</span> <span class="keyword">int</span><span class="special">()</span> <span class="special">&gt;</span>
        <span class="comment">// fold everything else, using std::plus&lt;&gt; to add</span>
        <span class="comment">// the leaf count of each child to the accumulated state.</span>
      <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">otherwise</span><span class="special">&lt;</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">fold</span><span class="special">&lt;</span><span class="identifier">_</span><span class="special">,</span> <span class="keyword">int</span><span class="special">(),</span> <span class="identifier">iplus</span><span class="special">(</span><span class="identifier">CountLeaves</span><span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_state</span><span class="special">)</span> <span class="special">&gt;</span> <span class="special">&gt;</span>
    <span class="special">&gt;</span>
<span class="special">{};</span>

<span class="comment">// Here is the Vec3 struct, which is a vector of 3 integers.</span>
<span class="keyword">struct</span> <span class="identifier">Vec3</span>
  <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">extends</span><span class="special">&lt;</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">[</span><span class="number">3</span><span class="special">]&gt;::</span><span class="identifier">type</span><span class="special">,</span> <span class="identifier">Vec3</span><span class="special">&gt;</span>
<span class="special">{</span>
    <span class="keyword">explicit</span> <span class="identifier">Vec3</span><span class="special">(</span><span class="keyword">int</span> <span class="identifier">i</span><span class="special">=</span><span class="number">0</span><span class="special">,</span> <span class="keyword">int</span> <span class="identifier">j</span><span class="special">=</span><span class="number">0</span><span class="special">,</span> <span class="keyword">int</span> <span class="identifier">k</span><span class="special">=</span><span class="number">0</span><span class="special">)</span>
    <span class="special">{</span>
        <span class="special">(*</span><span class="keyword">this</span><span class="special">)[</span><span class="number">0</span><span class="special">]</span> <span class="special">=</span> <span class="identifier">i</span><span class="special">;</span>
        <span class="special">(*</span><span class="keyword">this</span><span class="special">)[</span><span class="number">1</span><span class="special">]</span> <span class="special">=</span> <span class="identifier">j</span><span class="special">;</span>
        <span class="special">(*</span><span class="keyword">this</span><span class="special">)[</span><span class="number">2</span><span class="special">]</span> <span class="special">=</span> <span class="identifier">k</span><span class="special">;</span>
    <span class="special">}</span>

    <span class="keyword">int</span> <span class="special">&amp;</span><span class="keyword">operator</span> <span class="special">[](</span><span class="keyword">int</span> <span class="identifier">i</span><span class="special">)</span>
    <span class="special">{</span>
        <span class="keyword">return</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">value</span><span class="special">(*</span><span class="keyword">this</span><span class="special">)[</span><span class="identifier">i</span><span class="special">];</span>
    <span class="special">}</span>

    <span class="keyword">int</span> <span class="keyword">const</span> <span class="special">&amp;</span><span class="keyword">operator</span> <span class="special">[](</span><span class="keyword">int</span> <span class="identifier">i</span><span class="special">)</span> <span class="keyword">const</span>
    <span class="special">{</span>
        <span class="keyword">return</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">value</span><span class="special">(*</span><span class="keyword">this</span><span class="special">)[</span><span class="identifier">i</span><span class="special">];</span>
    <span class="special">}</span>

    <span class="comment">// Here we define a operator = for Vec3 terminals that</span>
    <span class="comment">// takes a Vec3 expression.</span>
    <span class="keyword">template</span><span class="special">&lt;</span> <span class="keyword">typename</span> <span class="identifier">Expr</span> <span class="special">&gt;</span>
    <span class="identifier">Vec3</span> <span class="special">&amp;</span><span class="keyword">operator</span> <span class="special">=(</span><span class="identifier">Expr</span> <span class="keyword">const</span> <span class="special">&amp;</span> <span class="identifier">expr</span><span class="special">)</span>
    <span class="special">{</span>
        <span class="keyword">typedef</span> <span class="identifier">Vec3SubscriptCtx</span> <span class="keyword">const</span> <span class="identifier">CVec3SubscriptCtx</span><span class="special">;</span>
        <span class="special">(*</span><span class="keyword">this</span><span class="special">)[</span><span class="number">0</span><span class="special">]</span> <span class="special">=</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">eval</span><span class="special">(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">as_expr</span><span class="special">(</span><span class="identifier">expr</span><span class="special">),</span> <span class="identifier">CVec3SubscriptCtx</span><span class="special">(</span><span class="number">0</span><span class="special">));</span>
        <span class="special">(*</span><span class="keyword">this</span><span class="special">)[</span><span class="number">1</span><span class="special">]</span> <span class="special">=</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">eval</span><span class="special">(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">as_expr</span><span class="special">(</span><span class="identifier">expr</span><span class="special">),</span> <span class="identifier">CVec3SubscriptCtx</span><span class="special">(</span><span class="number">1</span><span class="special">));</span>
        <span class="special">(*</span><span class="keyword">this</span><span class="special">)[</span><span class="number">2</span><span class="special">]</span> <span class="special">=</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">eval</span><span class="special">(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">as_expr</span><span class="special">(</span><span class="identifier">expr</span><span class="special">),</span> <span class="identifier">CVec3SubscriptCtx</span><span class="special">(</span><span class="number">2</span><span class="special">));</span>
        <span class="keyword">return</span> <span class="special">*</span><span class="keyword">this</span><span class="special">;</span>
    <span class="special">}</span>

    <span class="comment">// This copy-assign is needed because a template is never</span>
    <span class="comment">// considered for copy assignment.</span>
    <span class="identifier">Vec3</span> <span class="special">&amp;</span><span class="keyword">operator</span><span class="special">=(</span><span class="identifier">Vec3</span> <span class="keyword">const</span> <span class="special">&amp;</span><span class="identifier">that</span><span class="special">)</span>
    <span class="special">{</span>
        <span class="special">(*</span><span class="keyword">this</span><span class="special">)[</span><span class="number">0</span><span class="special">]</span> <span class="special">=</span> <span class="identifier">that</span><span class="special">[</span><span class="number">0</span><span class="special">];</span>
        <span class="special">(*</span><span class="keyword">this</span><span class="special">)[</span><span class="number">1</span><span class="special">]</span> <span class="special">=</span> <span class="identifier">that</span><span class="special">[</span><span class="number">1</span><span class="special">];</span>
        <span class="special">(*</span><span class="keyword">this</span><span class="special">)[</span><span class="number">2</span><span class="special">]</span> <span class="special">=</span> <span class="identifier">that</span><span class="special">[</span><span class="number">2</span><span class="special">];</span>
        <span class="keyword">return</span> <span class="special">*</span><span class="keyword">this</span><span class="special">;</span>
    <span class="special">}</span>

    <span class="keyword">void</span> <span class="identifier">print</span><span class="special">()</span> <span class="keyword">const</span>
    <span class="special">{</span>
        <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="char">'{'</span> <span class="special">&lt;&lt;</span> <span class="special">(*</span><span class="keyword">this</span><span class="special">)[</span><span class="number">0</span><span class="special">]</span>
                  <span class="special">&lt;&lt;</span> <span class="string">", "</span> <span class="special">&lt;&lt;</span> <span class="special">(*</span><span class="keyword">this</span><span class="special">)[</span><span class="number">1</span><span class="special">]</span>
                  <span class="special">&lt;&lt;</span> <span class="string">", "</span> <span class="special">&lt;&lt;</span> <span class="special">(*</span><span class="keyword">this</span><span class="special">)[</span><span class="number">2</span><span class="special">]</span>
                  <span class="special">&lt;&lt;</span> <span class="char">'}'</span> <span class="special">&lt;&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span>
    <span class="special">}</span>
<span class="special">};</span>

<span class="comment">// The count_leaves() function uses the CountLeaves transform and</span>
<span class="comment">// to count the number of leaves in an expression.</span>
<span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">Expr</span><span class="special">&gt;</span>
<span class="keyword">int</span> <span class="identifier">count_leaves</span><span class="special">(</span><span class="identifier">Expr</span> <span class="keyword">const</span> <span class="special">&amp;</span><span class="identifier">expr</span><span class="special">)</span>
<span class="special">{</span>
    <span class="comment">// Count the number of Vec3 terminals using the</span>
    <span class="comment">// CountLeavesCtx evaluation context.</span>
    <span class="identifier">CountLeavesCtx</span> <span class="identifier">ctx</span><span class="special">;</span>
    <span class="identifier">proto</span><span class="special">::</span><span class="identifier">eval</span><span class="special">(</span><span class="identifier">expr</span><span class="special">,</span> <span class="identifier">ctx</span><span class="special">);</span>

    <span class="comment">// This is another way to count the leaves using a transform.</span>
    <span class="keyword">int</span> <span class="identifier">i</span> <span class="special">=</span> <span class="number">0</span><span class="special">;</span>
    <span class="identifier">BOOST_ASSERT</span><span class="special">(</span> <span class="identifier">CountLeaves</span><span class="special">()(</span><span class="identifier">expr</span><span class="special">,</span> <span class="identifier">i</span><span class="special">,</span> <span class="identifier">i</span><span class="special">)</span> <span class="special">==</span> <span class="identifier">ctx</span><span class="special">.</span><span class="identifier">count</span> <span class="special">);</span>

    <span class="keyword">return</span> <span class="identifier">ctx</span><span class="special">.</span><span class="identifier">count</span><span class="special">;</span>
<span class="special">}</span>

<span class="keyword">int</span> <span class="identifier">main</span><span class="special">()</span>
<span class="special">{</span>
    <span class="identifier">Vec3</span> <span class="identifier">a</span><span class="special">,</span> <span class="identifier">b</span><span class="special">,</span> <span class="identifier">c</span><span class="special">;</span>

    <span class="identifier">c</span> <span class="special">=</span> <span class="number">4</span><span class="special">;</span>

    <span class="identifier">b</span><span class="special">[</span><span class="number">0</span><span class="special">]</span> <span class="special">=</span> <span class="special">-</span><span class="number">1</span><span class="special">;</span>
    <span class="identifier">b</span><span class="special">[</span><span class="number">1</span><span class="special">]</span> <span class="special">=</span> <span class="special">-</span><span class="number">2</span><span class="special">;</span>
    <span class="identifier">b</span><span class="special">[</span><span class="number">2</span><span class="special">]</span> <span class="special">=</span> <span class="special">-</span><span class="number">3</span><span class="special">;</span>

    <span class="identifier">a</span> <span class="special">=</span> <span class="identifier">b</span> <span class="special">+</span> <span class="identifier">c</span><span class="special">;</span>

    <span class="identifier">a</span><span class="special">.</span><span class="identifier">print</span><span class="special">();</span>

    <span class="identifier">Vec3</span> <span class="identifier">d</span><span class="special">;</span>
    <span class="identifier">BOOST_PROTO_AUTO</span><span class="special">(</span><span class="identifier">expr1</span><span class="special">,</span> <span class="identifier">b</span> <span class="special">+</span> <span class="identifier">c</span><span class="special">);</span>
    <span class="identifier">d</span> <span class="special">=</span> <span class="identifier">expr1</span><span class="special">;</span>
    <span class="identifier">d</span><span class="special">.</span><span class="identifier">print</span><span class="special">();</span>

    <span class="keyword">int</span> <span class="identifier">num</span> <span class="special">=</span> <span class="identifier">count_leaves</span><span class="special">(</span><span class="identifier">expr1</span><span class="special">);</span>
    <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="identifier">num</span> <span class="special">&lt;&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span>

    <span class="identifier">BOOST_PROTO_AUTO</span><span class="special">(</span><span class="identifier">expr2</span><span class="special">,</span> <span class="identifier">b</span> <span class="special">+</span> <span class="number">3</span> <span class="special">*</span> <span class="identifier">c</span><span class="special">);</span>
    <span class="identifier">num</span> <span class="special">=</span> <span class="identifier">count_leaves</span><span class="special">(</span><span class="identifier">expr2</span><span class="special">);</span>
    <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="identifier">num</span> <span class="special">&lt;&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span>

    <span class="identifier">BOOST_PROTO_AUTO</span><span class="special">(</span><span class="identifier">expr3</span><span class="special">,</span> <span class="identifier">b</span> <span class="special">+</span> <span class="identifier">c</span> <span class="special">*</span> <span class="identifier">d</span><span class="special">);</span>
    <span class="identifier">num</span> <span class="special">=</span> <span class="identifier">count_leaves</span><span class="special">(</span><span class="identifier">expr3</span><span class="special">);</span>
    <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="identifier">num</span> <span class="special">&lt;&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span>

    <span class="keyword">return</span> <span class="number">0</span><span class="special">;</span>
<span class="special">}</span>
</pre>
<p>
        </p>
</div>
<div class="section">
<div class="titlepage"><div><div><h4 class="title">
<a name="boost_proto.users_guide.examples.vector"></a><a class="link" href="users_guide.html#boost_proto.users_guide.examples.vector" title="Vector: Adapting a Non-Proto Terminal Type">Vector: Adapting
        a Non-Proto Terminal Type</a>
</h4></div></div></div>
<p>
          This is an example of using <code class="computeroutput"><span class="identifier">BOOST_PROTO_DEFINE_OPERATORS</span><span class="special">()</span></code> to Protofy expressions using <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">vector</span><span class="special">&lt;&gt;</span></code>,
          a non-Proto type. It is a port of the Vector example from <a href="http://acts.nersc.gov/formertools/pete/index.html" target="_top">PETE</a>.
        </p>
<p>
</p>
<pre class="programlisting"><span class="comment">///////////////////////////////////////////////////////////////////////////////</span>
<span class="comment">//  Copyright 2008 Eric Niebler. Distributed under the Boost</span>
<span class="comment">//  Software License, Version 1.0. (See accompanying file</span>
<span class="comment">//  LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)</span>
<span class="comment">//</span>
<span class="comment">// This is an example of using BOOST_PROTO_DEFINE_OPERATORS to Protofy</span>
<span class="comment">// expressions using std::vector&lt;&gt;, a non-proto type. It is a port of the</span>
<span class="comment">// Vector example from PETE (http://www.codesourcery.com/pooma/download.html).</span>

<span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">vector</span><span class="special">&gt;</span>
<span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">iostream</span><span class="special">&gt;</span>
<span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">stdexcept</span><span class="special">&gt;</span>
<span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">mpl</span><span class="special">/</span><span class="keyword">bool</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
<span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">proto</span><span class="special">/</span><span class="identifier">core</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
<span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">proto</span><span class="special">/</span><span class="identifier">debug</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
<span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">proto</span><span class="special">/</span><span class="identifier">context</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
<span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">utility</span><span class="special">/</span><span class="identifier">enable_if</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
<span class="keyword">namespace</span> <span class="identifier">mpl</span> <span class="special">=</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">mpl</span><span class="special">;</span>
<span class="keyword">namespace</span> <span class="identifier">proto</span> <span class="special">=</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">proto</span><span class="special">;</span>
<span class="keyword">using</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_</span><span class="special">;</span>

<span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">Expr</span><span class="special">&gt;</span>
<span class="keyword">struct</span> <span class="identifier">VectorExpr</span><span class="special">;</span>

<span class="comment">// Here is an evaluation context that indexes into a std::vector</span>
<span class="comment">// expression and combines the result.</span>
<span class="keyword">struct</span> <span class="identifier">VectorSubscriptCtx</span>
<span class="special">{</span>
    <span class="identifier">VectorSubscriptCtx</span><span class="special">(</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">size_t</span> <span class="identifier">i</span><span class="special">)</span>
      <span class="special">:</span> <span class="identifier">i_</span><span class="special">(</span><span class="identifier">i</span><span class="special">)</span>
    <span class="special">{}</span>

    <span class="comment">// Unless this is a vector terminal, use the</span>
    <span class="comment">// default evaluation context</span>
    <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">Expr</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">EnableIf</span> <span class="special">=</span> <span class="keyword">void</span><span class="special">&gt;</span>
    <span class="keyword">struct</span> <span class="identifier">eval</span>
      <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">default_eval</span><span class="special">&lt;</span><span class="identifier">Expr</span><span class="special">,</span> <span class="identifier">VectorSubscriptCtx</span> <span class="keyword">const</span><span class="special">&gt;</span>
    <span class="special">{};</span>

    <span class="comment">// Index vector terminals with our subscript.</span>
    <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">Expr</span><span class="special">&gt;</span>
    <span class="keyword">struct</span> <span class="identifier">eval</span><span class="special">&lt;</span>
        <span class="identifier">Expr</span>
      <span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">enable_if</span><span class="special">&lt;</span>
            <span class="identifier">proto</span><span class="special">::</span><span class="identifier">matches</span><span class="special">&lt;</span><span class="identifier">Expr</span><span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">vector</span><span class="special">&lt;</span><span class="identifier">_</span><span class="special">,</span> <span class="identifier">_</span><span class="special">&gt;</span> <span class="special">&gt;</span> <span class="special">&gt;</span>
        <span class="special">&gt;::</span><span class="identifier">type</span>
    <span class="special">&gt;</span>
    <span class="special">{</span>
        <span class="keyword">typedef</span> <span class="keyword">typename</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">result_of</span><span class="special">::</span><span class="identifier">value</span><span class="special">&lt;</span><span class="identifier">Expr</span><span class="special">&gt;::</span><span class="identifier">type</span><span class="special">::</span><span class="identifier">value_type</span> <span class="identifier">result_type</span><span class="special">;</span>

        <span class="identifier">result_type</span> <span class="keyword">operator</span> <span class="special">()(</span><span class="identifier">Expr</span> <span class="special">&amp;</span><span class="identifier">expr</span><span class="special">,</span> <span class="identifier">VectorSubscriptCtx</span> <span class="keyword">const</span> <span class="special">&amp;</span><span class="identifier">ctx</span><span class="special">)</span> <span class="keyword">const</span>
        <span class="special">{</span>
            <span class="keyword">return</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">value</span><span class="special">(</span><span class="identifier">expr</span><span class="special">)[</span><span class="identifier">ctx</span><span class="special">.</span><span class="identifier">i_</span><span class="special">];</span>
        <span class="special">}</span>
    <span class="special">};</span>

    <span class="identifier">std</span><span class="special">::</span><span class="identifier">size_t</span> <span class="identifier">i_</span><span class="special">;</span>
<span class="special">};</span>

<span class="comment">// Here is an evaluation context that verifies that all the</span>
<span class="comment">// vectors in an expression have the same size.</span>
<span class="keyword">struct</span> <span class="identifier">VectorSizeCtx</span>
<span class="special">{</span>
    <span class="identifier">VectorSizeCtx</span><span class="special">(</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">size_t</span> <span class="identifier">size</span><span class="special">)</span>
      <span class="special">:</span> <span class="identifier">size_</span><span class="special">(</span><span class="identifier">size</span><span class="special">)</span>
    <span class="special">{}</span>

    <span class="comment">// Unless this is a vector terminal, use the</span>
    <span class="comment">// null evaluation context</span>
    <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">Expr</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">EnableIf</span> <span class="special">=</span> <span class="keyword">void</span><span class="special">&gt;</span>
    <span class="keyword">struct</span> <span class="identifier">eval</span>
      <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">null_eval</span><span class="special">&lt;</span><span class="identifier">Expr</span><span class="special">,</span> <span class="identifier">VectorSizeCtx</span> <span class="keyword">const</span><span class="special">&gt;</span>
    <span class="special">{};</span>

    <span class="comment">// Index array terminals with our subscript. Everything</span>
    <span class="comment">// else will be handled by the default evaluation context.</span>
    <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">Expr</span><span class="special">&gt;</span>
    <span class="keyword">struct</span> <span class="identifier">eval</span><span class="special">&lt;</span>
        <span class="identifier">Expr</span>
      <span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">enable_if</span><span class="special">&lt;</span>
            <span class="identifier">proto</span><span class="special">::</span><span class="identifier">matches</span><span class="special">&lt;</span><span class="identifier">Expr</span><span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">vector</span><span class="special">&lt;</span><span class="identifier">_</span><span class="special">,</span> <span class="identifier">_</span><span class="special">&gt;</span> <span class="special">&gt;</span> <span class="special">&gt;</span>
        <span class="special">&gt;::</span><span class="identifier">type</span>
    <span class="special">&gt;</span>
    <span class="special">{</span>
        <span class="keyword">typedef</span> <span class="keyword">void</span> <span class="identifier">result_type</span><span class="special">;</span>

        <span class="identifier">result_type</span> <span class="keyword">operator</span> <span class="special">()(</span><span class="identifier">Expr</span> <span class="special">&amp;</span><span class="identifier">expr</span><span class="special">,</span> <span class="identifier">VectorSizeCtx</span> <span class="keyword">const</span> <span class="special">&amp;</span><span class="identifier">ctx</span><span class="special">)</span> <span class="keyword">const</span>
        <span class="special">{</span>
            <span class="keyword">if</span><span class="special">(</span><span class="identifier">ctx</span><span class="special">.</span><span class="identifier">size_</span> <span class="special">!=</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">value</span><span class="special">(</span><span class="identifier">expr</span><span class="special">).</span><span class="identifier">size</span><span class="special">())</span>
            <span class="special">{</span>
                <span class="keyword">throw</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">runtime_error</span><span class="special">(</span><span class="string">"LHS and RHS are not compatible"</span><span class="special">);</span>
            <span class="special">}</span>
        <span class="special">}</span>
    <span class="special">};</span>

    <span class="identifier">std</span><span class="special">::</span><span class="identifier">size_t</span> <span class="identifier">size_</span><span class="special">;</span>
<span class="special">};</span>

<span class="comment">// A grammar which matches all the assignment operators,</span>
<span class="comment">// so we can easily disable them.</span>
<span class="keyword">struct</span> <span class="identifier">AssignOps</span>
  <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">switch_</span><span class="special">&lt;</span><span class="keyword">struct</span> <span class="identifier">AssignOpsCases</span><span class="special">&gt;</span>
<span class="special">{};</span>

<span class="comment">// Here are the cases used by the switch_ above.</span>
<span class="keyword">struct</span> <span class="identifier">AssignOpsCases</span>
<span class="special">{</span>
    <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">Tag</span><span class="special">,</span> <span class="keyword">int</span> <span class="identifier">D</span> <span class="special">=</span> <span class="number">0</span><span class="special">&gt;</span> <span class="keyword">struct</span> <span class="identifier">case_</span>  <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">not_</span><span class="special">&lt;</span><span class="identifier">_</span><span class="special">&gt;</span> <span class="special">{};</span>

    <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">int</span> <span class="identifier">D</span><span class="special">&gt;</span> <span class="keyword">struct</span> <span class="identifier">case_</span><span class="special">&lt;</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">plus_assign</span><span class="special">,</span> <span class="identifier">D</span> <span class="special">&gt;</span>         <span class="special">:</span> <span class="identifier">_</span> <span class="special">{};</span>
    <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">int</span> <span class="identifier">D</span><span class="special">&gt;</span> <span class="keyword">struct</span> <span class="identifier">case_</span><span class="special">&lt;</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">minus_assign</span><span class="special">,</span> <span class="identifier">D</span> <span class="special">&gt;</span>        <span class="special">:</span> <span class="identifier">_</span> <span class="special">{};</span>
    <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">int</span> <span class="identifier">D</span><span class="special">&gt;</span> <span class="keyword">struct</span> <span class="identifier">case_</span><span class="special">&lt;</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">multiplies_assign</span><span class="special">,</span> <span class="identifier">D</span> <span class="special">&gt;</span>   <span class="special">:</span> <span class="identifier">_</span> <span class="special">{};</span>
    <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">int</span> <span class="identifier">D</span><span class="special">&gt;</span> <span class="keyword">struct</span> <span class="identifier">case_</span><span class="special">&lt;</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">divides_assign</span><span class="special">,</span> <span class="identifier">D</span> <span class="special">&gt;</span>      <span class="special">:</span> <span class="identifier">_</span> <span class="special">{};</span>
    <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">int</span> <span class="identifier">D</span><span class="special">&gt;</span> <span class="keyword">struct</span> <span class="identifier">case_</span><span class="special">&lt;</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">modulus_assign</span><span class="special">,</span> <span class="identifier">D</span> <span class="special">&gt;</span>      <span class="special">:</span> <span class="identifier">_</span> <span class="special">{};</span>
    <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">int</span> <span class="identifier">D</span><span class="special">&gt;</span> <span class="keyword">struct</span> <span class="identifier">case_</span><span class="special">&lt;</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">shift_left_assign</span><span class="special">,</span> <span class="identifier">D</span> <span class="special">&gt;</span>   <span class="special">:</span> <span class="identifier">_</span> <span class="special">{};</span>
    <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">int</span> <span class="identifier">D</span><span class="special">&gt;</span> <span class="keyword">struct</span> <span class="identifier">case_</span><span class="special">&lt;</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">shift_right_assign</span><span class="special">,</span> <span class="identifier">D</span> <span class="special">&gt;</span>  <span class="special">:</span> <span class="identifier">_</span> <span class="special">{};</span>
    <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">int</span> <span class="identifier">D</span><span class="special">&gt;</span> <span class="keyword">struct</span> <span class="identifier">case_</span><span class="special">&lt;</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">bitwise_and_assign</span><span class="special">,</span> <span class="identifier">D</span> <span class="special">&gt;</span>  <span class="special">:</span> <span class="identifier">_</span> <span class="special">{};</span>
    <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">int</span> <span class="identifier">D</span><span class="special">&gt;</span> <span class="keyword">struct</span> <span class="identifier">case_</span><span class="special">&lt;</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">bitwise_or_assign</span><span class="special">,</span> <span class="identifier">D</span> <span class="special">&gt;</span>   <span class="special">:</span> <span class="identifier">_</span> <span class="special">{};</span>
    <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">int</span> <span class="identifier">D</span><span class="special">&gt;</span> <span class="keyword">struct</span> <span class="identifier">case_</span><span class="special">&lt;</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">bitwise_xor_assign</span><span class="special">,</span> <span class="identifier">D</span> <span class="special">&gt;</span>  <span class="special">:</span> <span class="identifier">_</span> <span class="special">{};</span>
<span class="special">};</span>

<span class="comment">// A vector grammar is a terminal or some op that is not an</span>
<span class="comment">// assignment op. (Assignment will be handled specially.)</span>
<span class="keyword">struct</span> <span class="identifier">VectorGrammar</span>
  <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">or_</span><span class="special">&lt;</span>
        <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span><span class="identifier">_</span><span class="special">&gt;</span>
      <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">and_</span><span class="special">&lt;</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">nary_expr</span><span class="special">&lt;</span><span class="identifier">_</span><span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">vararg</span><span class="special">&lt;</span><span class="identifier">VectorGrammar</span><span class="special">&gt;</span> <span class="special">&gt;,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">not_</span><span class="special">&lt;</span><span class="identifier">AssignOps</span><span class="special">&gt;</span> <span class="special">&gt;</span>
    <span class="special">&gt;</span>
<span class="special">{};</span>

<span class="comment">// Expressions in the vector domain will be wrapped in VectorExpr&lt;&gt;</span>
<span class="comment">// and must conform to the VectorGrammar</span>
<span class="keyword">struct</span> <span class="identifier">VectorDomain</span>
  <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">domain</span><span class="special">&lt;</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">generator</span><span class="special">&lt;</span><span class="identifier">VectorExpr</span><span class="special">&gt;,</span> <span class="identifier">VectorGrammar</span><span class="special">&gt;</span>
<span class="special">{};</span>

<span class="comment">// Here is VectorExpr, which extends a proto expr type by</span>
<span class="comment">// giving it an operator [] which uses the VectorSubscriptCtx</span>
<span class="comment">// to evaluate an expression with a given index.</span>
<span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">Expr</span><span class="special">&gt;</span>
<span class="keyword">struct</span> <span class="identifier">VectorExpr</span>
  <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">extends</span><span class="special">&lt;</span><span class="identifier">Expr</span><span class="special">,</span> <span class="identifier">VectorExpr</span><span class="special">&lt;</span><span class="identifier">Expr</span><span class="special">&gt;,</span> <span class="identifier">VectorDomain</span><span class="special">&gt;</span>
<span class="special">{</span>
    <span class="keyword">explicit</span> <span class="identifier">VectorExpr</span><span class="special">(</span><span class="identifier">Expr</span> <span class="keyword">const</span> <span class="special">&amp;</span><span class="identifier">expr</span><span class="special">)</span>
      <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">extends</span><span class="special">&lt;</span><span class="identifier">Expr</span><span class="special">,</span> <span class="identifier">VectorExpr</span><span class="special">&lt;</span><span class="identifier">Expr</span><span class="special">&gt;,</span> <span class="identifier">VectorDomain</span><span class="special">&gt;(</span><span class="identifier">expr</span><span class="special">)</span>
    <span class="special">{}</span>

    <span class="comment">// Use the VectorSubscriptCtx to implement subscripting</span>
    <span class="comment">// of a Vector expression tree.</span>
    <span class="keyword">typename</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">result_of</span><span class="special">::</span><span class="identifier">eval</span><span class="special">&lt;</span><span class="identifier">Expr</span> <span class="keyword">const</span><span class="special">,</span> <span class="identifier">VectorSubscriptCtx</span> <span class="keyword">const</span><span class="special">&gt;::</span><span class="identifier">type</span>
    <span class="keyword">operator</span> <span class="special">[](</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">size_t</span> <span class="identifier">i</span> <span class="special">)</span> <span class="keyword">const</span>
    <span class="special">{</span>
        <span class="identifier">VectorSubscriptCtx</span> <span class="keyword">const</span> <span class="identifier">ctx</span><span class="special">(</span><span class="identifier">i</span><span class="special">);</span>
        <span class="keyword">return</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">eval</span><span class="special">(*</span><span class="keyword">this</span><span class="special">,</span> <span class="identifier">ctx</span><span class="special">);</span>
    <span class="special">}</span>
<span class="special">};</span>

<span class="comment">// Define a trait type for detecting vector terminals, to</span>
<span class="comment">// be used by the BOOST_PROTO_DEFINE_OPERATORS macro below.</span>
<span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">T</span><span class="special">&gt;</span>
<span class="keyword">struct</span> <span class="identifier">IsVector</span>
  <span class="special">:</span> <span class="identifier">mpl</span><span class="special">::</span><span class="identifier">false_</span>
<span class="special">{};</span>

<span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">T</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">A</span><span class="special">&gt;</span>
<span class="keyword">struct</span> <span class="identifier">IsVector</span><span class="special">&lt;</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">vector</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">,</span> <span class="identifier">A</span><span class="special">&gt;</span> <span class="special">&gt;</span>
  <span class="special">:</span> <span class="identifier">mpl</span><span class="special">::</span><span class="identifier">true_</span>
<span class="special">{};</span>

<span class="keyword">namespace</span> <span class="identifier">VectorOps</span>
<span class="special">{</span>
    <span class="comment">// This defines all the overloads to make expressions involving</span>
    <span class="comment">// std::vector to build expression templates.</span>
    <span class="identifier">BOOST_PROTO_DEFINE_OPERATORS</span><span class="special">(</span><span class="identifier">IsVector</span><span class="special">,</span> <span class="identifier">VectorDomain</span><span class="special">)</span>

    <span class="keyword">typedef</span> <span class="identifier">VectorSubscriptCtx</span> <span class="keyword">const</span> <span class="identifier">CVectorSubscriptCtx</span><span class="special">;</span>

    <span class="comment">// Assign to a vector from some expression.</span>
    <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">T</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">A</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">Expr</span><span class="special">&gt;</span>
    <span class="identifier">std</span><span class="special">::</span><span class="identifier">vector</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">,</span> <span class="identifier">A</span><span class="special">&gt;</span> <span class="special">&amp;</span><span class="identifier">assign</span><span class="special">(</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">vector</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">,</span> <span class="identifier">A</span><span class="special">&gt;</span> <span class="special">&amp;</span><span class="identifier">arr</span><span class="special">,</span> <span class="identifier">Expr</span> <span class="keyword">const</span> <span class="special">&amp;</span><span class="identifier">expr</span><span class="special">)</span>
    <span class="special">{</span>
        <span class="identifier">VectorSizeCtx</span> <span class="keyword">const</span> <span class="identifier">size</span><span class="special">(</span><span class="identifier">arr</span><span class="special">.</span><span class="identifier">size</span><span class="special">());</span>
        <span class="identifier">proto</span><span class="special">::</span><span class="identifier">eval</span><span class="special">(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">as_expr</span><span class="special">&lt;</span><span class="identifier">VectorDomain</span><span class="special">&gt;(</span><span class="identifier">expr</span><span class="special">),</span> <span class="identifier">size</span><span class="special">);</span> <span class="comment">// will throw if the sizes don't match</span>
        <span class="keyword">for</span><span class="special">(</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">size_t</span> <span class="identifier">i</span> <span class="special">=</span> <span class="number">0</span><span class="special">;</span> <span class="identifier">i</span> <span class="special">&lt;</span> <span class="identifier">arr</span><span class="special">.</span><span class="identifier">size</span><span class="special">();</span> <span class="special">++</span><span class="identifier">i</span><span class="special">)</span>
        <span class="special">{</span>
            <span class="identifier">arr</span><span class="special">[</span><span class="identifier">i</span><span class="special">]</span> <span class="special">=</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">as_expr</span><span class="special">&lt;</span><span class="identifier">VectorDomain</span><span class="special">&gt;(</span><span class="identifier">expr</span><span class="special">)[</span><span class="identifier">i</span><span class="special">];</span>
        <span class="special">}</span>
        <span class="keyword">return</span> <span class="identifier">arr</span><span class="special">;</span>
    <span class="special">}</span>

    <span class="comment">// Add-assign to a vector from some expression.</span>
    <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">T</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">A</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">Expr</span><span class="special">&gt;</span>
    <span class="identifier">std</span><span class="special">::</span><span class="identifier">vector</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">,</span> <span class="identifier">A</span><span class="special">&gt;</span> <span class="special">&amp;</span><span class="keyword">operator</span> <span class="special">+=(</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">vector</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">,</span> <span class="identifier">A</span><span class="special">&gt;</span> <span class="special">&amp;</span><span class="identifier">arr</span><span class="special">,</span> <span class="identifier">Expr</span> <span class="keyword">const</span> <span class="special">&amp;</span><span class="identifier">expr</span><span class="special">)</span>
    <span class="special">{</span>
        <span class="identifier">VectorSizeCtx</span> <span class="keyword">const</span> <span class="identifier">size</span><span class="special">(</span><span class="identifier">arr</span><span class="special">.</span><span class="identifier">size</span><span class="special">());</span>
        <span class="identifier">proto</span><span class="special">::</span><span class="identifier">eval</span><span class="special">(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">as_expr</span><span class="special">&lt;</span><span class="identifier">VectorDomain</span><span class="special">&gt;(</span><span class="identifier">expr</span><span class="special">),</span> <span class="identifier">size</span><span class="special">);</span> <span class="comment">// will throw if the sizes don't match</span>
        <span class="keyword">for</span><span class="special">(</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">size_t</span> <span class="identifier">i</span> <span class="special">=</span> <span class="number">0</span><span class="special">;</span> <span class="identifier">i</span> <span class="special">&lt;</span> <span class="identifier">arr</span><span class="special">.</span><span class="identifier">size</span><span class="special">();</span> <span class="special">++</span><span class="identifier">i</span><span class="special">)</span>
        <span class="special">{</span>
            <span class="identifier">arr</span><span class="special">[</span><span class="identifier">i</span><span class="special">]</span> <span class="special">+=</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">as_expr</span><span class="special">&lt;</span><span class="identifier">VectorDomain</span><span class="special">&gt;(</span><span class="identifier">expr</span><span class="special">)[</span><span class="identifier">i</span><span class="special">];</span>
        <span class="special">}</span>
        <span class="keyword">return</span> <span class="identifier">arr</span><span class="special">;</span>
    <span class="special">}</span>
<span class="special">}</span>

<span class="keyword">int</span> <span class="identifier">main</span><span class="special">()</span>
<span class="special">{</span>
    <span class="keyword">using</span> <span class="keyword">namespace</span> <span class="identifier">VectorOps</span><span class="special">;</span>

    <span class="keyword">int</span> <span class="identifier">i</span><span class="special">;</span>
    <span class="keyword">const</span> <span class="keyword">int</span> <span class="identifier">n</span> <span class="special">=</span> <span class="number">10</span><span class="special">;</span>
    <span class="identifier">std</span><span class="special">::</span><span class="identifier">vector</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">&gt;</span> <span class="identifier">a</span><span class="special">,</span><span class="identifier">b</span><span class="special">,</span><span class="identifier">c</span><span class="special">,</span><span class="identifier">d</span><span class="special">;</span>
    <span class="identifier">std</span><span class="special">::</span><span class="identifier">vector</span><span class="special">&lt;</span><span class="keyword">double</span><span class="special">&gt;</span> <span class="identifier">e</span><span class="special">(</span><span class="identifier">n</span><span class="special">);</span>

    <span class="keyword">for</span> <span class="special">(</span><span class="identifier">i</span> <span class="special">=</span> <span class="number">0</span><span class="special">;</span> <span class="identifier">i</span> <span class="special">&lt;</span> <span class="identifier">n</span><span class="special">;</span> <span class="special">++</span><span class="identifier">i</span><span class="special">)</span>
    <span class="special">{</span>
        <span class="identifier">a</span><span class="special">.</span><span class="identifier">push_back</span><span class="special">(</span><span class="identifier">i</span><span class="special">);</span>
        <span class="identifier">b</span><span class="special">.</span><span class="identifier">push_back</span><span class="special">(</span><span class="number">2</span><span class="special">*</span><span class="identifier">i</span><span class="special">);</span>
        <span class="identifier">c</span><span class="special">.</span><span class="identifier">push_back</span><span class="special">(</span><span class="number">3</span><span class="special">*</span><span class="identifier">i</span><span class="special">);</span>
        <span class="identifier">d</span><span class="special">.</span><span class="identifier">push_back</span><span class="special">(</span><span class="identifier">i</span><span class="special">);</span>
    <span class="special">}</span>

    <span class="identifier">VectorOps</span><span class="special">::</span><span class="identifier">assign</span><span class="special">(</span><span class="identifier">b</span><span class="special">,</span> <span class="number">2</span><span class="special">);</span>
    <span class="identifier">VectorOps</span><span class="special">::</span><span class="identifier">assign</span><span class="special">(</span><span class="identifier">d</span><span class="special">,</span> <span class="identifier">a</span> <span class="special">+</span> <span class="identifier">b</span> <span class="special">*</span> <span class="identifier">c</span><span class="special">);</span>
    <span class="identifier">a</span> <span class="special">+=</span> <span class="identifier">if_else</span><span class="special">(</span><span class="identifier">d</span> <span class="special">&lt;</span> <span class="number">30</span><span class="special">,</span> <span class="identifier">b</span><span class="special">,</span> <span class="identifier">c</span><span class="special">);</span>

    <span class="identifier">VectorOps</span><span class="special">::</span><span class="identifier">assign</span><span class="special">(</span><span class="identifier">e</span><span class="special">,</span> <span class="identifier">c</span><span class="special">);</span>
    <span class="identifier">e</span> <span class="special">+=</span> <span class="identifier">e</span> <span class="special">-</span> <span class="number">4</span> <span class="special">/</span> <span class="special">(</span><span class="identifier">c</span> <span class="special">+</span> <span class="number">1</span><span class="special">);</span>

    <span class="keyword">for</span> <span class="special">(</span><span class="identifier">i</span> <span class="special">=</span> <span class="number">0</span><span class="special">;</span> <span class="identifier">i</span> <span class="special">&lt;</span> <span class="identifier">n</span><span class="special">;</span> <span class="special">++</span><span class="identifier">i</span><span class="special">)</span>
    <span class="special">{</span>
        <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span>
            <span class="special">&lt;&lt;</span> <span class="string">" a("</span> <span class="special">&lt;&lt;</span> <span class="identifier">i</span> <span class="special">&lt;&lt;</span> <span class="string">") = "</span> <span class="special">&lt;&lt;</span> <span class="identifier">a</span><span class="special">[</span><span class="identifier">i</span><span class="special">]</span>
            <span class="special">&lt;&lt;</span> <span class="string">" b("</span> <span class="special">&lt;&lt;</span> <span class="identifier">i</span> <span class="special">&lt;&lt;</span> <span class="string">") = "</span> <span class="special">&lt;&lt;</span> <span class="identifier">b</span><span class="special">[</span><span class="identifier">i</span><span class="special">]</span>
            <span class="special">&lt;&lt;</span> <span class="string">" c("</span> <span class="special">&lt;&lt;</span> <span class="identifier">i</span> <span class="special">&lt;&lt;</span> <span class="string">") = "</span> <span class="special">&lt;&lt;</span> <span class="identifier">c</span><span class="special">[</span><span class="identifier">i</span><span class="special">]</span>
            <span class="special">&lt;&lt;</span> <span class="string">" d("</span> <span class="special">&lt;&lt;</span> <span class="identifier">i</span> <span class="special">&lt;&lt;</span> <span class="string">") = "</span> <span class="special">&lt;&lt;</span> <span class="identifier">d</span><span class="special">[</span><span class="identifier">i</span><span class="special">]</span>
            <span class="special">&lt;&lt;</span> <span class="string">" e("</span> <span class="special">&lt;&lt;</span> <span class="identifier">i</span> <span class="special">&lt;&lt;</span> <span class="string">") = "</span> <span class="special">&lt;&lt;</span> <span class="identifier">e</span><span class="special">[</span><span class="identifier">i</span><span class="special">]</span>
            <span class="special">&lt;&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span>
    <span class="special">}</span>
<span class="special">}</span>
</pre>
<p>
        </p>
</div>
<div class="section">
<div class="titlepage"><div><div><h4 class="title">
<a name="boost_proto.users_guide.examples.mixed"></a><a class="link" href="users_guide.html#boost_proto.users_guide.examples.mixed" title="Mixed: Adapting Several Non-Proto Terminal Types">Mixed: Adapting
        Several Non-Proto Terminal Types</a>
</h4></div></div></div>
<p>
          This is an example of using <code class="computeroutput"><span class="identifier">BOOST_PROTO_DEFINE_OPERATORS</span><span class="special">()</span></code> to Protofy expressions using <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">vector</span><span class="special">&lt;&gt;</span></code>
          and <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">list</span><span class="special">&lt;&gt;</span></code>,
          non-Proto types. It is a port of the Mixed example from <a href="http://acts.nersc.gov/formertools/pete/index.html" target="_top">PETE</a>.
        </p>
<p>
</p>
<pre class="programlisting"><span class="comment">///////////////////////////////////////////////////////////////////////////////</span>
<span class="comment">//  Copyright 2008 Eric Niebler. Distributed under the Boost</span>
<span class="comment">//  Software License, Version 1.0. (See accompanying file</span>
<span class="comment">//  LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)</span>
<span class="comment">//</span>
<span class="comment">// This is an example of using BOOST_PROTO_DEFINE_OPERATORS to Protofy</span>
<span class="comment">// expressions using std::vector&lt;&gt; and std::list, non-proto types. It is a port</span>
<span class="comment">// of the Mixed example from PETE.</span>
<span class="comment">// (http://www.codesourcery.com/pooma/download.html).</span>

<span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">list</span><span class="special">&gt;</span>
<span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">cmath</span><span class="special">&gt;</span>
<span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">vector</span><span class="special">&gt;</span>
<span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">complex</span><span class="special">&gt;</span>
<span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">iostream</span><span class="special">&gt;</span>
<span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">stdexcept</span><span class="special">&gt;</span>
<span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">proto</span><span class="special">/</span><span class="identifier">core</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
<span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">proto</span><span class="special">/</span><span class="identifier">debug</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
<span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">proto</span><span class="special">/</span><span class="identifier">context</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
<span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">proto</span><span class="special">/</span><span class="identifier">transform</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
<span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">utility</span><span class="special">/</span><span class="identifier">enable_if</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
<span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">typeof</span><span class="special">/</span><span class="identifier">std</span><span class="special">/</span><span class="identifier">list</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
<span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">typeof</span><span class="special">/</span><span class="identifier">std</span><span class="special">/</span><span class="identifier">vector</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
<span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">typeof</span><span class="special">/</span><span class="identifier">std</span><span class="special">/</span><span class="identifier">complex</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
<span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">type_traits</span><span class="special">/</span><span class="identifier">remove_reference</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
<span class="keyword">namespace</span> <span class="identifier">proto</span> <span class="special">=</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">proto</span><span class="special">;</span>
<span class="keyword">namespace</span> <span class="identifier">mpl</span> <span class="special">=</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">mpl</span><span class="special">;</span>
<span class="keyword">using</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_</span><span class="special">;</span>

<span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">Expr</span><span class="special">&gt;</span>
<span class="keyword">struct</span> <span class="identifier">MixedExpr</span><span class="special">;</span>

<span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">Iter</span><span class="special">&gt;</span>
<span class="keyword">struct</span> <span class="identifier">iterator_wrapper</span>
<span class="special">{</span>
    <span class="keyword">typedef</span> <span class="identifier">Iter</span> <span class="identifier">iterator</span><span class="special">;</span>

    <span class="keyword">explicit</span> <span class="identifier">iterator_wrapper</span><span class="special">(</span><span class="identifier">Iter</span> <span class="identifier">iter</span><span class="special">)</span>
      <span class="special">:</span> <span class="identifier">it</span><span class="special">(</span><span class="identifier">iter</span><span class="special">)</span>
    <span class="special">{}</span>

    <span class="keyword">mutable</span> <span class="identifier">Iter</span> <span class="identifier">it</span><span class="special">;</span>
<span class="special">};</span>

<span class="keyword">struct</span> <span class="identifier">begin</span> <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">callable</span>
<span class="special">{</span>
    <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">class</span> <span class="identifier">Sig</span><span class="special">&gt;</span>
    <span class="keyword">struct</span> <span class="identifier">result</span><span class="special">;</span>

    <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">class</span> <span class="identifier">This</span><span class="special">,</span> <span class="keyword">class</span> <span class="identifier">Cont</span><span class="special">&gt;</span>
    <span class="keyword">struct</span> <span class="identifier">result</span><span class="special">&lt;</span><span class="identifier">This</span><span class="special">(</span><span class="identifier">Cont</span><span class="special">)&gt;</span>
      <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">result_of</span><span class="special">::</span><span class="identifier">as_expr</span><span class="special">&lt;</span>
            <span class="identifier">iterator_wrapper</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">remove_reference</span><span class="special">&lt;</span><span class="identifier">Cont</span><span class="special">&gt;::</span><span class="identifier">type</span><span class="special">::</span><span class="identifier">const_iterator</span><span class="special">&gt;</span>
        <span class="special">&gt;</span>
    <span class="special">{};</span>

    <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">Cont</span><span class="special">&gt;</span>
    <span class="keyword">typename</span> <span class="identifier">result</span><span class="special">&lt;</span><span class="identifier">begin</span><span class="special">(</span><span class="identifier">Cont</span> <span class="keyword">const</span> <span class="special">&amp;)&gt;::</span><span class="identifier">type</span>
    <span class="keyword">operator</span> <span class="special">()(</span><span class="identifier">Cont</span> <span class="keyword">const</span> <span class="special">&amp;</span><span class="identifier">cont</span><span class="special">)</span> <span class="keyword">const</span>
    <span class="special">{</span>
        <span class="identifier">iterator_wrapper</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">Cont</span><span class="special">::</span><span class="identifier">const_iterator</span><span class="special">&gt;</span> <span class="identifier">it</span><span class="special">(</span><span class="identifier">cont</span><span class="special">.</span><span class="identifier">begin</span><span class="special">());</span>
        <span class="keyword">return</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">as_expr</span><span class="special">(</span><span class="identifier">it</span><span class="special">);</span>
    <span class="special">}</span>
<span class="special">};</span>

<span class="comment">// Here is a grammar that replaces vector and list terminals with their</span>
<span class="comment">// begin iterators</span>
<span class="keyword">struct</span> <span class="identifier">Begin</span>
  <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">or_</span><span class="special">&lt;</span>
        <span class="identifier">proto</span><span class="special">::</span><span class="identifier">when</span><span class="special">&lt;</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">vector</span><span class="special">&lt;</span><span class="identifier">_</span><span class="special">,</span> <span class="identifier">_</span><span class="special">&gt;</span> <span class="special">&gt;,</span> <span class="identifier">begin</span><span class="special">(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_value</span><span class="special">)</span> <span class="special">&gt;</span>
      <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">when</span><span class="special">&lt;</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">list</span><span class="special">&lt;</span><span class="identifier">_</span><span class="special">,</span> <span class="identifier">_</span><span class="special">&gt;</span> <span class="special">&gt;,</span> <span class="identifier">begin</span><span class="special">(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_value</span><span class="special">)</span> <span class="special">&gt;</span>
      <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">when</span><span class="special">&lt;</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span><span class="identifier">_</span><span class="special">&gt;</span> <span class="special">&gt;</span>
      <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">when</span><span class="special">&lt;</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">nary_expr</span><span class="special">&lt;</span><span class="identifier">_</span><span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">vararg</span><span class="special">&lt;</span><span class="identifier">Begin</span><span class="special">&gt;</span> <span class="special">&gt;</span> <span class="special">&gt;</span>
    <span class="special">&gt;</span>
<span class="special">{};</span>

<span class="comment">// Here is an evaluation context that dereferences iterator</span>
<span class="comment">// terminals.</span>
<span class="keyword">struct</span> <span class="identifier">DereferenceCtx</span>
<span class="special">{</span>
    <span class="comment">// Unless this is an iterator terminal, use the</span>
    <span class="comment">// default evaluation context</span>
    <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">Expr</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">EnableIf</span> <span class="special">=</span> <span class="keyword">void</span><span class="special">&gt;</span>
    <span class="keyword">struct</span> <span class="identifier">eval</span>
      <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">default_eval</span><span class="special">&lt;</span><span class="identifier">Expr</span><span class="special">,</span> <span class="identifier">DereferenceCtx</span> <span class="keyword">const</span><span class="special">&gt;</span>
    <span class="special">{};</span>

    <span class="comment">// Dereference iterator terminals.</span>
    <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">Expr</span><span class="special">&gt;</span>
    <span class="keyword">struct</span> <span class="identifier">eval</span><span class="special">&lt;</span>
        <span class="identifier">Expr</span>
      <span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">enable_if</span><span class="special">&lt;</span>
            <span class="identifier">proto</span><span class="special">::</span><span class="identifier">matches</span><span class="special">&lt;</span><span class="identifier">Expr</span><span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span><span class="identifier">iterator_wrapper</span><span class="special">&lt;</span><span class="identifier">_</span><span class="special">&gt;</span> <span class="special">&gt;</span> <span class="special">&gt;</span>
        <span class="special">&gt;::</span><span class="identifier">type</span>
    <span class="special">&gt;</span>
    <span class="special">{</span>
        <span class="keyword">typedef</span> <span class="keyword">typename</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">result_of</span><span class="special">::</span><span class="identifier">value</span><span class="special">&lt;</span><span class="identifier">Expr</span><span class="special">&gt;::</span><span class="identifier">type</span> <span class="identifier">IteratorWrapper</span><span class="special">;</span>
        <span class="keyword">typedef</span> <span class="keyword">typename</span> <span class="identifier">IteratorWrapper</span><span class="special">::</span><span class="identifier">iterator</span> <span class="identifier">iterator</span><span class="special">;</span>
        <span class="keyword">typedef</span> <span class="keyword">typename</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">iterator_traits</span><span class="special">&lt;</span><span class="identifier">iterator</span><span class="special">&gt;::</span><span class="identifier">reference</span> <span class="identifier">result_type</span><span class="special">;</span>

        <span class="identifier">result_type</span> <span class="keyword">operator</span> <span class="special">()(</span><span class="identifier">Expr</span> <span class="special">&amp;</span><span class="identifier">expr</span><span class="special">,</span> <span class="identifier">DereferenceCtx</span> <span class="keyword">const</span> <span class="special">&amp;)</span> <span class="keyword">const</span>
        <span class="special">{</span>
            <span class="keyword">return</span> <span class="special">*</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">value</span><span class="special">(</span><span class="identifier">expr</span><span class="special">).</span><span class="identifier">it</span><span class="special">;</span>
        <span class="special">}</span>
    <span class="special">};</span>
<span class="special">};</span>

<span class="comment">// Here is an evaluation context that increments iterator</span>
<span class="comment">// terminals.</span>
<span class="keyword">struct</span> <span class="identifier">IncrementCtx</span>
<span class="special">{</span>
    <span class="comment">// Unless this is an iterator terminal, use the</span>
    <span class="comment">// default evaluation context</span>
    <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">Expr</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">EnableIf</span> <span class="special">=</span> <span class="keyword">void</span><span class="special">&gt;</span>
    <span class="keyword">struct</span> <span class="identifier">eval</span>
      <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">null_eval</span><span class="special">&lt;</span><span class="identifier">Expr</span><span class="special">,</span> <span class="identifier">IncrementCtx</span> <span class="keyword">const</span><span class="special">&gt;</span>
    <span class="special">{};</span>

    <span class="comment">// advance iterator terminals.</span>
    <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">Expr</span><span class="special">&gt;</span>
    <span class="keyword">struct</span> <span class="identifier">eval</span><span class="special">&lt;</span>
        <span class="identifier">Expr</span>
      <span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">enable_if</span><span class="special">&lt;</span>
            <span class="identifier">proto</span><span class="special">::</span><span class="identifier">matches</span><span class="special">&lt;</span><span class="identifier">Expr</span><span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span><span class="identifier">iterator_wrapper</span><span class="special">&lt;</span><span class="identifier">_</span><span class="special">&gt;</span> <span class="special">&gt;</span> <span class="special">&gt;</span>
        <span class="special">&gt;::</span><span class="identifier">type</span>
    <span class="special">&gt;</span>
    <span class="special">{</span>
        <span class="keyword">typedef</span> <span class="keyword">void</span> <span class="identifier">result_type</span><span class="special">;</span>

        <span class="identifier">result_type</span> <span class="keyword">operator</span> <span class="special">()(</span><span class="identifier">Expr</span> <span class="special">&amp;</span><span class="identifier">expr</span><span class="special">,</span> <span class="identifier">IncrementCtx</span> <span class="keyword">const</span> <span class="special">&amp;)</span> <span class="keyword">const</span>
        <span class="special">{</span>
            <span class="special">++</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">value</span><span class="special">(</span><span class="identifier">expr</span><span class="special">).</span><span class="identifier">it</span><span class="special">;</span>
        <span class="special">}</span>
    <span class="special">};</span>
<span class="special">};</span>

<span class="comment">// A grammar which matches all the assignment operators,</span>
<span class="comment">// so we can easily disable them.</span>
<span class="keyword">struct</span> <span class="identifier">AssignOps</span>
  <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">switch_</span><span class="special">&lt;</span><span class="keyword">struct</span> <span class="identifier">AssignOpsCases</span><span class="special">&gt;</span>
<span class="special">{};</span>

<span class="comment">// Here are the cases used by the switch_ above.</span>
<span class="keyword">struct</span> <span class="identifier">AssignOpsCases</span>
<span class="special">{</span>
    <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">Tag</span><span class="special">,</span> <span class="keyword">int</span> <span class="identifier">D</span> <span class="special">=</span> <span class="number">0</span><span class="special">&gt;</span> <span class="keyword">struct</span> <span class="identifier">case_</span>  <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">not_</span><span class="special">&lt;</span><span class="identifier">_</span><span class="special">&gt;</span> <span class="special">{};</span>

    <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">int</span> <span class="identifier">D</span><span class="special">&gt;</span> <span class="keyword">struct</span> <span class="identifier">case_</span><span class="special">&lt;</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">plus_assign</span><span class="special">,</span> <span class="identifier">D</span> <span class="special">&gt;</span>         <span class="special">:</span> <span class="identifier">_</span> <span class="special">{};</span>
    <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">int</span> <span class="identifier">D</span><span class="special">&gt;</span> <span class="keyword">struct</span> <span class="identifier">case_</span><span class="special">&lt;</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">minus_assign</span><span class="special">,</span> <span class="identifier">D</span> <span class="special">&gt;</span>        <span class="special">:</span> <span class="identifier">_</span> <span class="special">{};</span>
    <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">int</span> <span class="identifier">D</span><span class="special">&gt;</span> <span class="keyword">struct</span> <span class="identifier">case_</span><span class="special">&lt;</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">multiplies_assign</span><span class="special">,</span> <span class="identifier">D</span> <span class="special">&gt;</span>   <span class="special">:</span> <span class="identifier">_</span> <span class="special">{};</span>
    <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">int</span> <span class="identifier">D</span><span class="special">&gt;</span> <span class="keyword">struct</span> <span class="identifier">case_</span><span class="special">&lt;</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">divides_assign</span><span class="special">,</span> <span class="identifier">D</span> <span class="special">&gt;</span>      <span class="special">:</span> <span class="identifier">_</span> <span class="special">{};</span>
    <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">int</span> <span class="identifier">D</span><span class="special">&gt;</span> <span class="keyword">struct</span> <span class="identifier">case_</span><span class="special">&lt;</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">modulus_assign</span><span class="special">,</span> <span class="identifier">D</span> <span class="special">&gt;</span>      <span class="special">:</span> <span class="identifier">_</span> <span class="special">{};</span>
    <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">int</span> <span class="identifier">D</span><span class="special">&gt;</span> <span class="keyword">struct</span> <span class="identifier">case_</span><span class="special">&lt;</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">shift_left_assign</span><span class="special">,</span> <span class="identifier">D</span> <span class="special">&gt;</span>   <span class="special">:</span> <span class="identifier">_</span> <span class="special">{};</span>
    <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">int</span> <span class="identifier">D</span><span class="special">&gt;</span> <span class="keyword">struct</span> <span class="identifier">case_</span><span class="special">&lt;</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">shift_right_assign</span><span class="special">,</span> <span class="identifier">D</span> <span class="special">&gt;</span>  <span class="special">:</span> <span class="identifier">_</span> <span class="special">{};</span>
    <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">int</span> <span class="identifier">D</span><span class="special">&gt;</span> <span class="keyword">struct</span> <span class="identifier">case_</span><span class="special">&lt;</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">bitwise_and_assign</span><span class="special">,</span> <span class="identifier">D</span> <span class="special">&gt;</span>  <span class="special">:</span> <span class="identifier">_</span> <span class="special">{};</span>
    <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">int</span> <span class="identifier">D</span><span class="special">&gt;</span> <span class="keyword">struct</span> <span class="identifier">case_</span><span class="special">&lt;</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">bitwise_or_assign</span><span class="special">,</span> <span class="identifier">D</span> <span class="special">&gt;</span>   <span class="special">:</span> <span class="identifier">_</span> <span class="special">{};</span>
    <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">int</span> <span class="identifier">D</span><span class="special">&gt;</span> <span class="keyword">struct</span> <span class="identifier">case_</span><span class="special">&lt;</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">bitwise_xor_assign</span><span class="special">,</span> <span class="identifier">D</span> <span class="special">&gt;</span>  <span class="special">:</span> <span class="identifier">_</span> <span class="special">{};</span>
<span class="special">};</span>

<span class="comment">// An expression conforms to the MixedGrammar if it is a terminal or some</span>
<span class="comment">// op that is not an assignment op. (Assignment will be handled specially.)</span>
<span class="keyword">struct</span> <span class="identifier">MixedGrammar</span>
  <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">or_</span><span class="special">&lt;</span>
        <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span><span class="identifier">_</span><span class="special">&gt;</span>
      <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">and_</span><span class="special">&lt;</span>
            <span class="identifier">proto</span><span class="special">::</span><span class="identifier">nary_expr</span><span class="special">&lt;</span><span class="identifier">_</span><span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">vararg</span><span class="special">&lt;</span><span class="identifier">MixedGrammar</span><span class="special">&gt;</span> <span class="special">&gt;</span>
          <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">not_</span><span class="special">&lt;</span><span class="identifier">AssignOps</span><span class="special">&gt;</span>
        <span class="special">&gt;</span>
    <span class="special">&gt;</span>
<span class="special">{};</span>

<span class="comment">// Expressions in the MixedDomain will be wrapped in MixedExpr&lt;&gt;</span>
<span class="comment">// and must conform to the MixedGrammar</span>
<span class="keyword">struct</span> <span class="identifier">MixedDomain</span>
  <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">domain</span><span class="special">&lt;</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">generator</span><span class="special">&lt;</span><span class="identifier">MixedExpr</span><span class="special">&gt;,</span> <span class="identifier">MixedGrammar</span><span class="special">&gt;</span>
<span class="special">{};</span>

<span class="comment">// Here is MixedExpr, a wrapper for expression types in the MixedDomain.</span>
<span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">Expr</span><span class="special">&gt;</span>
<span class="keyword">struct</span> <span class="identifier">MixedExpr</span>
  <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">extends</span><span class="special">&lt;</span><span class="identifier">Expr</span><span class="special">,</span> <span class="identifier">MixedExpr</span><span class="special">&lt;</span><span class="identifier">Expr</span><span class="special">&gt;,</span> <span class="identifier">MixedDomain</span><span class="special">&gt;</span>
<span class="special">{</span>
    <span class="keyword">explicit</span> <span class="identifier">MixedExpr</span><span class="special">(</span><span class="identifier">Expr</span> <span class="keyword">const</span> <span class="special">&amp;</span><span class="identifier">expr</span><span class="special">)</span>
      <span class="special">:</span> <span class="identifier">MixedExpr</span><span class="special">::</span><span class="identifier">proto_extends</span><span class="special">(</span><span class="identifier">expr</span><span class="special">)</span>
    <span class="special">{}</span>
<span class="keyword">private</span><span class="special">:</span>
    <span class="comment">// hide this:</span>
    <span class="keyword">using</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">extends</span><span class="special">&lt;</span><span class="identifier">Expr</span><span class="special">,</span> <span class="identifier">MixedExpr</span><span class="special">&lt;</span><span class="identifier">Expr</span><span class="special">&gt;,</span> <span class="identifier">MixedDomain</span><span class="special">&gt;::</span><span class="keyword">operator</span> <span class="special">[];</span>
<span class="special">};</span>

<span class="comment">// Define a trait type for detecting vector and list terminals, to</span>
<span class="comment">// be used by the BOOST_PROTO_DEFINE_OPERATORS macro below.</span>
<span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">T</span><span class="special">&gt;</span>
<span class="keyword">struct</span> <span class="identifier">IsMixed</span>
  <span class="special">:</span> <span class="identifier">mpl</span><span class="special">::</span><span class="identifier">false_</span>
<span class="special">{};</span>

<span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">T</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">A</span><span class="special">&gt;</span>
<span class="keyword">struct</span> <span class="identifier">IsMixed</span><span class="special">&lt;</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">list</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">,</span> <span class="identifier">A</span><span class="special">&gt;</span> <span class="special">&gt;</span>
  <span class="special">:</span> <span class="identifier">mpl</span><span class="special">::</span><span class="identifier">true_</span>
<span class="special">{};</span>

<span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">T</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">A</span><span class="special">&gt;</span>
<span class="keyword">struct</span> <span class="identifier">IsMixed</span><span class="special">&lt;</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">vector</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">,</span> <span class="identifier">A</span><span class="special">&gt;</span> <span class="special">&gt;</span>
  <span class="special">:</span> <span class="identifier">mpl</span><span class="special">::</span><span class="identifier">true_</span>
<span class="special">{};</span>

<span class="keyword">namespace</span> <span class="identifier">MixedOps</span>
<span class="special">{</span>
    <span class="comment">// This defines all the overloads to make expressions involving</span>
    <span class="comment">// std::vector to build expression templates.</span>
    <span class="identifier">BOOST_PROTO_DEFINE_OPERATORS</span><span class="special">(</span><span class="identifier">IsMixed</span><span class="special">,</span> <span class="identifier">MixedDomain</span><span class="special">)</span>

    <span class="keyword">struct</span> <span class="identifier">assign_op</span>
    <span class="special">{</span>
        <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">T</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">U</span><span class="special">&gt;</span>
        <span class="keyword">void</span> <span class="keyword">operator</span> <span class="special">()(</span><span class="identifier">T</span> <span class="special">&amp;</span><span class="identifier">t</span><span class="special">,</span> <span class="identifier">U</span> <span class="keyword">const</span> <span class="special">&amp;</span><span class="identifier">u</span><span class="special">)</span> <span class="keyword">const</span>
        <span class="special">{</span>
            <span class="identifier">t</span> <span class="special">=</span> <span class="identifier">u</span><span class="special">;</span>
        <span class="special">}</span>
    <span class="special">};</span>

    <span class="keyword">struct</span> <span class="identifier">plus_assign_op</span>
    <span class="special">{</span>
        <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">T</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">U</span><span class="special">&gt;</span>
        <span class="keyword">void</span> <span class="keyword">operator</span> <span class="special">()(</span><span class="identifier">T</span> <span class="special">&amp;</span><span class="identifier">t</span><span class="special">,</span> <span class="identifier">U</span> <span class="keyword">const</span> <span class="special">&amp;</span><span class="identifier">u</span><span class="special">)</span> <span class="keyword">const</span>
        <span class="special">{</span>
            <span class="identifier">t</span> <span class="special">+=</span> <span class="identifier">u</span><span class="special">;</span>
        <span class="special">}</span>
    <span class="special">};</span>

    <span class="keyword">struct</span> <span class="identifier">minus_assign_op</span>
    <span class="special">{</span>
        <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">T</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">U</span><span class="special">&gt;</span>
        <span class="keyword">void</span> <span class="keyword">operator</span> <span class="special">()(</span><span class="identifier">T</span> <span class="special">&amp;</span><span class="identifier">t</span><span class="special">,</span> <span class="identifier">U</span> <span class="keyword">const</span> <span class="special">&amp;</span><span class="identifier">u</span><span class="special">)</span> <span class="keyword">const</span>
        <span class="special">{</span>
            <span class="identifier">t</span> <span class="special">-=</span> <span class="identifier">u</span><span class="special">;</span>
        <span class="special">}</span>
    <span class="special">};</span>

    <span class="keyword">struct</span> <span class="identifier">sin_</span>
    <span class="special">{</span>
        <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">Sig</span><span class="special">&gt;</span>
        <span class="keyword">struct</span> <span class="identifier">result</span><span class="special">;</span>

        <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">This</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">Arg</span><span class="special">&gt;</span>
        <span class="keyword">struct</span> <span class="identifier">result</span><span class="special">&lt;</span><span class="identifier">This</span><span class="special">(</span><span class="identifier">Arg</span><span class="special">)&gt;</span>
          <span class="special">:</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">remove_const</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">remove_reference</span><span class="special">&lt;</span><span class="identifier">Arg</span><span class="special">&gt;::</span><span class="identifier">type</span><span class="special">&gt;</span>
        <span class="special">{};</span>

        <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">Arg</span><span class="special">&gt;</span>
        <span class="identifier">Arg</span> <span class="keyword">operator</span> <span class="special">()(</span><span class="identifier">Arg</span> <span class="keyword">const</span> <span class="special">&amp;</span><span class="identifier">a</span><span class="special">)</span> <span class="keyword">const</span>
        <span class="special">{</span>
            <span class="keyword">return</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">sin</span><span class="special">(</span><span class="identifier">a</span><span class="special">);</span>
        <span class="special">}</span>
    <span class="special">};</span>

    <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">A</span><span class="special">&gt;</span>
    <span class="keyword">typename</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">result_of</span><span class="special">::</span><span class="identifier">make_expr</span><span class="special">&lt;</span>
        <span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">function</span>
      <span class="special">,</span> <span class="identifier">MixedDomain</span>
      <span class="special">,</span> <span class="identifier">sin_</span> <span class="keyword">const</span>
      <span class="special">,</span> <span class="identifier">A</span> <span class="keyword">const</span> <span class="special">&amp;</span>
    <span class="special">&gt;::</span><span class="identifier">type</span> <span class="identifier">sin</span><span class="special">(</span><span class="identifier">A</span> <span class="keyword">const</span> <span class="special">&amp;</span><span class="identifier">a</span><span class="special">)</span>
    <span class="special">{</span>
        <span class="keyword">return</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">make_expr</span><span class="special">&lt;</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">function</span><span class="special">,</span> <span class="identifier">MixedDomain</span><span class="special">&gt;(</span><span class="identifier">sin_</span><span class="special">(),</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">ref</span><span class="special">(</span><span class="identifier">a</span><span class="special">));</span>
    <span class="special">}</span>

    <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">FwdIter</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">Expr</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">Op</span><span class="special">&gt;</span>
    <span class="keyword">void</span> <span class="identifier">evaluate</span><span class="special">(</span><span class="identifier">FwdIter</span> <span class="identifier">begin</span><span class="special">,</span> <span class="identifier">FwdIter</span> <span class="identifier">end</span><span class="special">,</span> <span class="identifier">Expr</span> <span class="keyword">const</span> <span class="special">&amp;</span><span class="identifier">expr</span><span class="special">,</span> <span class="identifier">Op</span> <span class="identifier">op</span><span class="special">)</span>
    <span class="special">{</span>
        <span class="identifier">IncrementCtx</span> <span class="keyword">const</span> <span class="identifier">inc</span> <span class="special">=</span> <span class="special">{};</span>
        <span class="identifier">DereferenceCtx</span> <span class="keyword">const</span> <span class="identifier">deref</span> <span class="special">=</span> <span class="special">{};</span>
        <span class="keyword">typename</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">result_of</span><span class="special">&lt;</span><span class="identifier">Begin</span><span class="special">(</span><span class="identifier">Expr</span> <span class="keyword">const</span> <span class="special">&amp;)&gt;::</span><span class="identifier">type</span> <span class="identifier">expr2</span> <span class="special">=</span> <span class="identifier">Begin</span><span class="special">()(</span><span class="identifier">expr</span><span class="special">);</span>
        <span class="keyword">for</span><span class="special">(;</span> <span class="identifier">begin</span> <span class="special">!=</span> <span class="identifier">end</span><span class="special">;</span> <span class="special">++</span><span class="identifier">begin</span><span class="special">)</span>
        <span class="special">{</span>
            <span class="identifier">op</span><span class="special">(*</span><span class="identifier">begin</span><span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">eval</span><span class="special">(</span><span class="identifier">expr2</span><span class="special">,</span> <span class="identifier">deref</span><span class="special">));</span>
            <span class="identifier">proto</span><span class="special">::</span><span class="identifier">eval</span><span class="special">(</span><span class="identifier">expr2</span><span class="special">,</span> <span class="identifier">inc</span><span class="special">);</span>
        <span class="special">}</span>
    <span class="special">}</span>

    <span class="comment">// Add-assign to a vector from some expression.</span>
    <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">T</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">A</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">Expr</span><span class="special">&gt;</span>
    <span class="identifier">std</span><span class="special">::</span><span class="identifier">vector</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">,</span> <span class="identifier">A</span><span class="special">&gt;</span> <span class="special">&amp;</span><span class="identifier">assign</span><span class="special">(</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">vector</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">,</span> <span class="identifier">A</span><span class="special">&gt;</span> <span class="special">&amp;</span><span class="identifier">arr</span><span class="special">,</span> <span class="identifier">Expr</span> <span class="keyword">const</span> <span class="special">&amp;</span><span class="identifier">expr</span><span class="special">)</span>
    <span class="special">{</span>
        <span class="identifier">evaluate</span><span class="special">(</span><span class="identifier">arr</span><span class="special">.</span><span class="identifier">begin</span><span class="special">(),</span> <span class="identifier">arr</span><span class="special">.</span><span class="identifier">end</span><span class="special">(),</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">as_expr</span><span class="special">&lt;</span><span class="identifier">MixedDomain</span><span class="special">&gt;(</span><span class="identifier">expr</span><span class="special">),</span> <span class="identifier">assign_op</span><span class="special">());</span>
        <span class="keyword">return</span> <span class="identifier">arr</span><span class="special">;</span>
    <span class="special">}</span>

    <span class="comment">// Add-assign to a list from some expression.</span>
    <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">T</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">A</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">Expr</span><span class="special">&gt;</span>
    <span class="identifier">std</span><span class="special">::</span><span class="identifier">list</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">,</span> <span class="identifier">A</span><span class="special">&gt;</span> <span class="special">&amp;</span><span class="identifier">assign</span><span class="special">(</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">list</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">,</span> <span class="identifier">A</span><span class="special">&gt;</span> <span class="special">&amp;</span><span class="identifier">arr</span><span class="special">,</span> <span class="identifier">Expr</span> <span class="keyword">const</span> <span class="special">&amp;</span><span class="identifier">expr</span><span class="special">)</span>
    <span class="special">{</span>
        <span class="identifier">evaluate</span><span class="special">(</span><span class="identifier">arr</span><span class="special">.</span><span class="identifier">begin</span><span class="special">(),</span> <span class="identifier">arr</span><span class="special">.</span><span class="identifier">end</span><span class="special">(),</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">as_expr</span><span class="special">&lt;</span><span class="identifier">MixedDomain</span><span class="special">&gt;(</span><span class="identifier">expr</span><span class="special">),</span> <span class="identifier">assign_op</span><span class="special">());</span>
        <span class="keyword">return</span> <span class="identifier">arr</span><span class="special">;</span>
    <span class="special">}</span>

    <span class="comment">// Add-assign to a vector from some expression.</span>
    <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">T</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">A</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">Expr</span><span class="special">&gt;</span>
    <span class="identifier">std</span><span class="special">::</span><span class="identifier">vector</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">,</span> <span class="identifier">A</span><span class="special">&gt;</span> <span class="special">&amp;</span><span class="keyword">operator</span> <span class="special">+=(</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">vector</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">,</span> <span class="identifier">A</span><span class="special">&gt;</span> <span class="special">&amp;</span><span class="identifier">arr</span><span class="special">,</span> <span class="identifier">Expr</span> <span class="keyword">const</span> <span class="special">&amp;</span><span class="identifier">expr</span><span class="special">)</span>
    <span class="special">{</span>
        <span class="identifier">evaluate</span><span class="special">(</span><span class="identifier">arr</span><span class="special">.</span><span class="identifier">begin</span><span class="special">(),</span> <span class="identifier">arr</span><span class="special">.</span><span class="identifier">end</span><span class="special">(),</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">as_expr</span><span class="special">&lt;</span><span class="identifier">MixedDomain</span><span class="special">&gt;(</span><span class="identifier">expr</span><span class="special">),</span> <span class="identifier">plus_assign_op</span><span class="special">());</span>
        <span class="keyword">return</span> <span class="identifier">arr</span><span class="special">;</span>
    <span class="special">}</span>

    <span class="comment">// Add-assign to a list from some expression.</span>
    <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">T</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">A</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">Expr</span><span class="special">&gt;</span>
    <span class="identifier">std</span><span class="special">::</span><span class="identifier">list</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">,</span> <span class="identifier">A</span><span class="special">&gt;</span> <span class="special">&amp;</span><span class="keyword">operator</span> <span class="special">+=(</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">list</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">,</span> <span class="identifier">A</span><span class="special">&gt;</span> <span class="special">&amp;</span><span class="identifier">arr</span><span class="special">,</span> <span class="identifier">Expr</span> <span class="keyword">const</span> <span class="special">&amp;</span><span class="identifier">expr</span><span class="special">)</span>
    <span class="special">{</span>
        <span class="identifier">evaluate</span><span class="special">(</span><span class="identifier">arr</span><span class="special">.</span><span class="identifier">begin</span><span class="special">(),</span> <span class="identifier">arr</span><span class="special">.</span><span class="identifier">end</span><span class="special">(),</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">as_expr</span><span class="special">&lt;</span><span class="identifier">MixedDomain</span><span class="special">&gt;(</span><span class="identifier">expr</span><span class="special">),</span> <span class="identifier">plus_assign_op</span><span class="special">());</span>
        <span class="keyword">return</span> <span class="identifier">arr</span><span class="special">;</span>
    <span class="special">}</span>

    <span class="comment">// Minus-assign to a vector from some expression.</span>
    <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">T</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">A</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">Expr</span><span class="special">&gt;</span>
    <span class="identifier">std</span><span class="special">::</span><span class="identifier">vector</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">,</span> <span class="identifier">A</span><span class="special">&gt;</span> <span class="special">&amp;</span><span class="keyword">operator</span> <span class="special">-=(</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">vector</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">,</span> <span class="identifier">A</span><span class="special">&gt;</span> <span class="special">&amp;</span><span class="identifier">arr</span><span class="special">,</span> <span class="identifier">Expr</span> <span class="keyword">const</span> <span class="special">&amp;</span><span class="identifier">expr</span><span class="special">)</span>
    <span class="special">{</span>
        <span class="identifier">evaluate</span><span class="special">(</span><span class="identifier">arr</span><span class="special">.</span><span class="identifier">begin</span><span class="special">(),</span> <span class="identifier">arr</span><span class="special">.</span><span class="identifier">end</span><span class="special">(),</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">as_expr</span><span class="special">&lt;</span><span class="identifier">MixedDomain</span><span class="special">&gt;(</span><span class="identifier">expr</span><span class="special">),</span> <span class="identifier">minus_assign_op</span><span class="special">());</span>
        <span class="keyword">return</span> <span class="identifier">arr</span><span class="special">;</span>
    <span class="special">}</span>

    <span class="comment">// Minus-assign to a list from some expression.</span>
    <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">T</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">A</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">Expr</span><span class="special">&gt;</span>
    <span class="identifier">std</span><span class="special">::</span><span class="identifier">list</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">,</span> <span class="identifier">A</span><span class="special">&gt;</span> <span class="special">&amp;</span><span class="keyword">operator</span> <span class="special">-=(</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">list</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">,</span> <span class="identifier">A</span><span class="special">&gt;</span> <span class="special">&amp;</span><span class="identifier">arr</span><span class="special">,</span> <span class="identifier">Expr</span> <span class="keyword">const</span> <span class="special">&amp;</span><span class="identifier">expr</span><span class="special">)</span>
    <span class="special">{</span>
        <span class="identifier">evaluate</span><span class="special">(</span><span class="identifier">arr</span><span class="special">.</span><span class="identifier">begin</span><span class="special">(),</span> <span class="identifier">arr</span><span class="special">.</span><span class="identifier">end</span><span class="special">(),</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">as_expr</span><span class="special">&lt;</span><span class="identifier">MixedDomain</span><span class="special">&gt;(</span><span class="identifier">expr</span><span class="special">),</span> <span class="identifier">minus_assign_op</span><span class="special">());</span>
        <span class="keyword">return</span> <span class="identifier">arr</span><span class="special">;</span>
    <span class="special">}</span>
<span class="special">}</span>

<span class="keyword">int</span> <span class="identifier">main</span><span class="special">()</span>
<span class="special">{</span>
    <span class="keyword">using</span> <span class="keyword">namespace</span> <span class="identifier">MixedOps</span><span class="special">;</span>

    <span class="keyword">int</span> <span class="identifier">n</span> <span class="special">=</span> <span class="number">10</span><span class="special">;</span>
    <span class="identifier">std</span><span class="special">::</span><span class="identifier">vector</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">&gt;</span> <span class="identifier">a</span><span class="special">,</span><span class="identifier">b</span><span class="special">,</span><span class="identifier">c</span><span class="special">,</span><span class="identifier">d</span><span class="special">;</span>
    <span class="identifier">std</span><span class="special">::</span><span class="identifier">list</span><span class="special">&lt;</span><span class="keyword">double</span><span class="special">&gt;</span> <span class="identifier">e</span><span class="special">;</span>
    <span class="identifier">std</span><span class="special">::</span><span class="identifier">list</span><span class="special">&lt;</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">complex</span><span class="special">&lt;</span><span class="keyword">double</span><span class="special">&gt;</span> <span class="special">&gt;</span> <span class="identifier">f</span><span class="special">;</span>

    <span class="keyword">int</span> <span class="identifier">i</span><span class="special">;</span>
    <span class="keyword">for</span><span class="special">(</span><span class="identifier">i</span> <span class="special">=</span> <span class="number">0</span><span class="special">;</span><span class="identifier">i</span> <span class="special">&lt;</span> <span class="identifier">n</span><span class="special">;</span> <span class="special">++</span><span class="identifier">i</span><span class="special">)</span>
    <span class="special">{</span>
        <span class="identifier">a</span><span class="special">.</span><span class="identifier">push_back</span><span class="special">(</span><span class="identifier">i</span><span class="special">);</span>
        <span class="identifier">b</span><span class="special">.</span><span class="identifier">push_back</span><span class="special">(</span><span class="number">2</span><span class="special">*</span><span class="identifier">i</span><span class="special">);</span>
        <span class="identifier">c</span><span class="special">.</span><span class="identifier">push_back</span><span class="special">(</span><span class="number">3</span><span class="special">*</span><span class="identifier">i</span><span class="special">);</span>
        <span class="identifier">d</span><span class="special">.</span><span class="identifier">push_back</span><span class="special">(</span><span class="identifier">i</span><span class="special">);</span>
        <span class="identifier">e</span><span class="special">.</span><span class="identifier">push_back</span><span class="special">(</span><span class="number">0.0</span><span class="special">);</span>
        <span class="identifier">f</span><span class="special">.</span><span class="identifier">push_back</span><span class="special">(</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">complex</span><span class="special">&lt;</span><span class="keyword">double</span><span class="special">&gt;(</span><span class="number">1.0</span><span class="special">,</span> <span class="number">1.0</span><span class="special">));</span>
    <span class="special">}</span>

    <span class="identifier">MixedOps</span><span class="special">::</span><span class="identifier">assign</span><span class="special">(</span><span class="identifier">b</span><span class="special">,</span> <span class="number">2</span><span class="special">);</span>
    <span class="identifier">MixedOps</span><span class="special">::</span><span class="identifier">assign</span><span class="special">(</span><span class="identifier">d</span><span class="special">,</span> <span class="identifier">a</span> <span class="special">+</span> <span class="identifier">b</span> <span class="special">*</span> <span class="identifier">c</span><span class="special">);</span>
    <span class="identifier">a</span> <span class="special">+=</span> <span class="identifier">if_else</span><span class="special">(</span><span class="identifier">d</span> <span class="special">&lt;</span> <span class="number">30</span><span class="special">,</span> <span class="identifier">b</span><span class="special">,</span> <span class="identifier">c</span><span class="special">);</span>

    <span class="identifier">MixedOps</span><span class="special">::</span><span class="identifier">assign</span><span class="special">(</span><span class="identifier">e</span><span class="special">,</span> <span class="identifier">c</span><span class="special">);</span>
    <span class="identifier">e</span> <span class="special">+=</span> <span class="identifier">e</span> <span class="special">-</span> <span class="number">4</span> <span class="special">/</span> <span class="special">(</span><span class="identifier">c</span> <span class="special">+</span> <span class="number">1</span><span class="special">);</span>

    <span class="identifier">f</span> <span class="special">-=</span> <span class="identifier">sin</span><span class="special">(</span><span class="number">0.1</span> <span class="special">*</span> <span class="identifier">e</span> <span class="special">*</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">complex</span><span class="special">&lt;</span><span class="keyword">double</span><span class="special">&gt;(</span><span class="number">0.2</span><span class="special">,</span> <span class="number">1.2</span><span class="special">));</span>

    <span class="identifier">std</span><span class="special">::</span><span class="identifier">list</span><span class="special">&lt;</span><span class="keyword">double</span><span class="special">&gt;::</span><span class="identifier">const_iterator</span> <span class="identifier">ei</span> <span class="special">=</span> <span class="identifier">e</span><span class="special">.</span><span class="identifier">begin</span><span class="special">();</span>
    <span class="identifier">std</span><span class="special">::</span><span class="identifier">list</span><span class="special">&lt;</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">complex</span><span class="special">&lt;</span><span class="keyword">double</span><span class="special">&gt;</span> <span class="special">&gt;::</span><span class="identifier">const_iterator</span> <span class="identifier">fi</span> <span class="special">=</span> <span class="identifier">f</span><span class="special">.</span><span class="identifier">begin</span><span class="special">();</span>
    <span class="keyword">for</span> <span class="special">(</span><span class="identifier">i</span> <span class="special">=</span> <span class="number">0</span><span class="special">;</span> <span class="identifier">i</span> <span class="special">&lt;</span> <span class="identifier">n</span><span class="special">;</span> <span class="special">++</span><span class="identifier">i</span><span class="special">)</span>
    <span class="special">{</span>
        <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span>
            <span class="special">&lt;&lt;</span> <span class="string">"a("</span> <span class="special">&lt;&lt;</span> <span class="identifier">i</span> <span class="special">&lt;&lt;</span> <span class="string">") = "</span> <span class="special">&lt;&lt;</span> <span class="identifier">a</span><span class="special">[</span><span class="identifier">i</span><span class="special">]</span>
            <span class="special">&lt;&lt;</span> <span class="string">" b("</span> <span class="special">&lt;&lt;</span> <span class="identifier">i</span> <span class="special">&lt;&lt;</span> <span class="string">") = "</span> <span class="special">&lt;&lt;</span> <span class="identifier">b</span><span class="special">[</span><span class="identifier">i</span><span class="special">]</span>
            <span class="special">&lt;&lt;</span> <span class="string">" c("</span> <span class="special">&lt;&lt;</span> <span class="identifier">i</span> <span class="special">&lt;&lt;</span> <span class="string">") = "</span> <span class="special">&lt;&lt;</span> <span class="identifier">c</span><span class="special">[</span><span class="identifier">i</span><span class="special">]</span>
            <span class="special">&lt;&lt;</span> <span class="string">" d("</span> <span class="special">&lt;&lt;</span> <span class="identifier">i</span> <span class="special">&lt;&lt;</span> <span class="string">") = "</span> <span class="special">&lt;&lt;</span> <span class="identifier">d</span><span class="special">[</span><span class="identifier">i</span><span class="special">]</span>
            <span class="special">&lt;&lt;</span> <span class="string">" e("</span> <span class="special">&lt;&lt;</span> <span class="identifier">i</span> <span class="special">&lt;&lt;</span> <span class="string">") = "</span> <span class="special">&lt;&lt;</span> <span class="special">*</span><span class="identifier">ei</span><span class="special">++</span>
            <span class="special">&lt;&lt;</span> <span class="string">" f("</span> <span class="special">&lt;&lt;</span> <span class="identifier">i</span> <span class="special">&lt;&lt;</span> <span class="string">") = "</span> <span class="special">&lt;&lt;</span> <span class="special">*</span><span class="identifier">fi</span><span class="special">++</span>
            <span class="special">&lt;&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span>
    <span class="special">}</span>
<span class="special">}</span>
</pre>
<p>
        </p>
</div>
<div class="section">
<div class="titlepage"><div><div><h4 class="title">
<a name="boost_proto.users_guide.examples.map_assign"></a><a class="link" href="users_guide.html#boost_proto.users_guide.examples.map_assign" title="Map Assign: An Intermediate Transform">Map Assign:
        An Intermediate Transform</a>
</h4></div></div></div>
<p>
          A demonstration of how to implement <code class="computeroutput"><span class="identifier">map_list_of</span><span class="special">()</span></code> from the Boost.Assign library using Proto.
          <code class="computeroutput"><span class="identifier">map_list_assign</span><span class="special">()</span></code>
          is used to conveniently initialize a <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">map</span><span class="special">&lt;&gt;</span></code>. By using Proto, we can avoid any
          dynamic allocation while building the intermediate representation.
        </p>
<p>
</p>
<pre class="programlisting"><span class="comment">//  Copyright 2008 Eric Niebler. Distributed under the Boost</span>
<span class="comment">//  Software License, Version 1.0. (See accompanying file</span>
<span class="comment">//  LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)</span>
<span class="comment">//</span>
<span class="comment">// This is a port of map_list_of() from the Boost.Assign library.</span>
<span class="comment">// It has the advantage of being more efficient at runtime by not</span>
<span class="comment">// building any temporary container that requires dynamic allocation.</span>

<span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">map</span><span class="special">&gt;</span>
<span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">string</span><span class="special">&gt;</span>
<span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">iostream</span><span class="special">&gt;</span>
<span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">proto</span><span class="special">/</span><span class="identifier">core</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
<span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">proto</span><span class="special">/</span><span class="identifier">transform</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
<span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">type_traits</span><span class="special">/</span><span class="identifier">add_reference</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
<span class="keyword">namespace</span> <span class="identifier">proto</span> <span class="special">=</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">proto</span><span class="special">;</span>
<span class="keyword">using</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_</span><span class="special">;</span>

<span class="keyword">struct</span> <span class="identifier">map_list_of_tag</span>
<span class="special">{};</span>

<span class="comment">// A simple callable function object that inserts a</span>
<span class="comment">// (key,value) pair into a map.</span>
<span class="keyword">struct</span> <span class="identifier">insert</span>
  <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">callable</span>
<span class="special">{</span>
    <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">Sig</span><span class="special">&gt;</span>
    <span class="keyword">struct</span> <span class="identifier">result</span><span class="special">;</span>

    <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">This</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">Map</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">Key</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">Value</span><span class="special">&gt;</span>
    <span class="keyword">struct</span> <span class="identifier">result</span><span class="special">&lt;</span><span class="identifier">This</span><span class="special">(</span><span class="identifier">Map</span><span class="special">,</span> <span class="identifier">Key</span><span class="special">,</span> <span class="identifier">Value</span><span class="special">)&gt;</span>
      <span class="special">:</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">add_reference</span><span class="special">&lt;</span><span class="identifier">Map</span><span class="special">&gt;</span>
    <span class="special">{};</span>

    <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">Map</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">Key</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">Value</span><span class="special">&gt;</span>
    <span class="identifier">Map</span> <span class="special">&amp;</span><span class="keyword">operator</span><span class="special">()(</span><span class="identifier">Map</span> <span class="special">&amp;</span><span class="identifier">map</span><span class="special">,</span> <span class="identifier">Key</span> <span class="keyword">const</span> <span class="special">&amp;</span><span class="identifier">key</span><span class="special">,</span> <span class="identifier">Value</span> <span class="keyword">const</span> <span class="special">&amp;</span><span class="identifier">value</span><span class="special">)</span> <span class="keyword">const</span>
    <span class="special">{</span>
        <span class="identifier">map</span><span class="special">.</span><span class="identifier">insert</span><span class="special">(</span><span class="keyword">typename</span> <span class="identifier">Map</span><span class="special">::</span><span class="identifier">value_type</span><span class="special">(</span><span class="identifier">key</span><span class="special">,</span> <span class="identifier">value</span><span class="special">));</span>
        <span class="keyword">return</span> <span class="identifier">map</span><span class="special">;</span>
    <span class="special">}</span>
<span class="special">};</span>

<span class="comment">// Work-arounds for Microsoft Visual C++ 7.1</span>
<span class="preprocessor">#if</span> <span class="identifier">BOOST_WORKAROUND</span><span class="special">(</span><span class="identifier">BOOST_MSVC</span><span class="special">,</span> <span class="special">==</span> <span class="number">1310</span><span class="special">)</span>
<span class="preprocessor">#define</span> <span class="identifier">MapListOf</span><span class="special">(</span><span class="identifier">x</span><span class="special">)</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">call</span><span class="special">&lt;</span><span class="identifier">MapListOf</span><span class="special">(</span><span class="identifier">x</span><span class="special">)&gt;</span>
<span class="preprocessor">#define</span> <span class="identifier">_value</span><span class="special">(</span><span class="identifier">x</span><span class="special">)</span> <span class="identifier">call</span><span class="special">&lt;</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_value</span><span class="special">(</span><span class="identifier">x</span><span class="special">)&gt;</span>
<span class="preprocessor">#endif</span>

<span class="comment">// The grammar for valid map-list expressions, and a</span>
<span class="comment">// transform that populates the map.</span>
<span class="keyword">struct</span> <span class="identifier">MapListOf</span>
  <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">or_</span><span class="special">&lt;</span>
        <span class="identifier">proto</span><span class="special">::</span><span class="identifier">when</span><span class="special">&lt;</span>
            <span class="comment">// map_list_of(a,b)</span>
            <span class="identifier">proto</span><span class="special">::</span><span class="identifier">function</span><span class="special">&lt;</span>
                <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span><span class="identifier">map_list_of_tag</span><span class="special">&gt;</span>
              <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span><span class="identifier">_</span><span class="special">&gt;</span>
              <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span><span class="identifier">_</span><span class="special">&gt;</span>
            <span class="special">&gt;</span>
          <span class="special">,</span> <span class="identifier">insert</span><span class="special">(</span>
                <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_data</span>
              <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_value</span><span class="special">(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_child1</span><span class="special">)</span>
              <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_value</span><span class="special">(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_child2</span><span class="special">)</span>
            <span class="special">)</span>
        <span class="special">&gt;</span>
      <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">when</span><span class="special">&lt;</span>
            <span class="comment">// map_list_of(a,b)(c,d)...</span>
            <span class="identifier">proto</span><span class="special">::</span><span class="identifier">function</span><span class="special">&lt;</span>
                <span class="identifier">MapListOf</span>
              <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span><span class="identifier">_</span><span class="special">&gt;</span>
              <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span><span class="identifier">_</span><span class="special">&gt;</span>
            <span class="special">&gt;</span>
          <span class="special">,</span> <span class="identifier">insert</span><span class="special">(</span>
                <span class="identifier">MapListOf</span><span class="special">(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_child0</span><span class="special">)</span>
              <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_value</span><span class="special">(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_child1</span><span class="special">)</span>
              <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_value</span><span class="special">(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_child2</span><span class="special">)</span>
            <span class="special">)</span>
        <span class="special">&gt;</span>
    <span class="special">&gt;</span>
<span class="special">{};</span>

<span class="preprocessor">#if</span> <span class="identifier">BOOST_WORKAROUND</span><span class="special">(</span><span class="identifier">BOOST_MSVC</span><span class="special">,</span> <span class="special">==</span> <span class="number">1310</span><span class="special">)</span>
<span class="preprocessor">#undef</span> <span class="identifier">MapListOf</span>
<span class="preprocessor">#undef</span> <span class="identifier">_value</span>
<span class="preprocessor">#endif</span>

<span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">Expr</span><span class="special">&gt;</span>
<span class="keyword">struct</span> <span class="identifier">map_list_of_expr</span><span class="special">;</span>

<span class="keyword">struct</span> <span class="identifier">map_list_of_dom</span>
  <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">domain</span><span class="special">&lt;</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">pod_generator</span><span class="special">&lt;</span><span class="identifier">map_list_of_expr</span><span class="special">&gt;,</span> <span class="identifier">MapListOf</span><span class="special">&gt;</span>
<span class="special">{};</span>

<span class="comment">// An expression wrapper that provides a conversion to a</span>
<span class="comment">// map that uses the MapListOf</span>
<span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">Expr</span><span class="special">&gt;</span>
<span class="keyword">struct</span> <span class="identifier">map_list_of_expr</span>
<span class="special">{</span>
    <span class="identifier">BOOST_PROTO_BASIC_EXTENDS</span><span class="special">(</span><span class="identifier">Expr</span><span class="special">,</span> <span class="identifier">map_list_of_expr</span><span class="special">,</span> <span class="identifier">map_list_of_dom</span><span class="special">)</span>
    <span class="identifier">BOOST_PROTO_EXTENDS_FUNCTION</span><span class="special">()</span>

    <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">Key</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">Value</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">Cmp</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">Al</span><span class="special">&gt;</span>
    <span class="keyword">operator</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">map</span><span class="special">&lt;</span><span class="identifier">Key</span><span class="special">,</span> <span class="identifier">Value</span><span class="special">,</span> <span class="identifier">Cmp</span><span class="special">,</span> <span class="identifier">Al</span><span class="special">&gt;</span> <span class="special">()</span> <span class="keyword">const</span>
    <span class="special">{</span>
        <span class="identifier">BOOST_MPL_ASSERT</span><span class="special">((</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">matches</span><span class="special">&lt;</span><span class="identifier">Expr</span><span class="special">,</span> <span class="identifier">MapListOf</span><span class="special">&gt;));</span>
        <span class="identifier">std</span><span class="special">::</span><span class="identifier">map</span><span class="special">&lt;</span><span class="identifier">Key</span><span class="special">,</span> <span class="identifier">Value</span><span class="special">,</span> <span class="identifier">Cmp</span><span class="special">,</span> <span class="identifier">Al</span><span class="special">&gt;</span> <span class="identifier">map</span><span class="special">;</span>
        <span class="keyword">return</span> <span class="identifier">MapListOf</span><span class="special">()(*</span><span class="keyword">this</span><span class="special">,</span> <span class="number">0</span><span class="special">,</span> <span class="identifier">map</span><span class="special">);</span>
    <span class="special">}</span>
<span class="special">};</span>

<span class="identifier">map_list_of_expr</span><span class="special">&lt;</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span><span class="identifier">map_list_of_tag</span><span class="special">&gt;::</span><span class="identifier">type</span><span class="special">&gt;</span> <span class="keyword">const</span> <span class="identifier">map_list_of</span> <span class="special">=</span> <span class="special">{{{}}};</span>

<span class="keyword">int</span> <span class="identifier">main</span><span class="special">()</span>
<span class="special">{</span>
    <span class="comment">// Initialize a map:</span>
    <span class="identifier">std</span><span class="special">::</span><span class="identifier">map</span><span class="special">&lt;</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span><span class="special">,</span> <span class="keyword">int</span><span class="special">&gt;</span> <span class="identifier">op</span> <span class="special">=</span>
        <span class="identifier">map_list_of</span>
            <span class="special">(</span><span class="string">"&lt;"</span><span class="special">,</span> <span class="number">1</span><span class="special">)</span>
            <span class="special">(</span><span class="string">"&lt;="</span><span class="special">,</span><span class="number">2</span><span class="special">)</span>
            <span class="special">(</span><span class="string">"&gt;"</span><span class="special">,</span> <span class="number">3</span><span class="special">)</span>
            <span class="special">(</span><span class="string">"&gt;="</span><span class="special">,</span><span class="number">4</span><span class="special">)</span>
            <span class="special">(</span><span class="string">"="</span><span class="special">,</span> <span class="number">5</span><span class="special">)</span>
            <span class="special">(</span><span class="string">"&lt;&gt;"</span><span class="special">,</span><span class="number">6</span><span class="special">)</span>
        <span class="special">;</span>

    <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="string">"\"&lt;\"  --&gt; "</span> <span class="special">&lt;&lt;</span> <span class="identifier">op</span><span class="special">[</span><span class="string">"&lt;"</span><span class="special">]</span> <span class="special">&lt;&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span>
    <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="string">"\"&lt;=\" --&gt; "</span> <span class="special">&lt;&lt;</span> <span class="identifier">op</span><span class="special">[</span><span class="string">"&lt;="</span><span class="special">]</span> <span class="special">&lt;&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span>
    <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="string">"\"&gt;\"  --&gt; "</span> <span class="special">&lt;&lt;</span> <span class="identifier">op</span><span class="special">[</span><span class="string">"&gt;"</span><span class="special">]</span> <span class="special">&lt;&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span>
    <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="string">"\"&gt;=\" --&gt; "</span> <span class="special">&lt;&lt;</span> <span class="identifier">op</span><span class="special">[</span><span class="string">"&gt;="</span><span class="special">]</span> <span class="special">&lt;&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span>
    <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="string">"\"=\"  --&gt; "</span> <span class="special">&lt;&lt;</span> <span class="identifier">op</span><span class="special">[</span><span class="string">"="</span><span class="special">]</span> <span class="special">&lt;&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span>
    <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="string">"\"&lt;&gt;\" --&gt; "</span> <span class="special">&lt;&lt;</span> <span class="identifier">op</span><span class="special">[</span><span class="string">"&lt;&gt;"</span><span class="special">]</span> <span class="special">&lt;&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span>

    <span class="keyword">return</span> <span class="number">0</span><span class="special">;</span>
<span class="special">}</span>
</pre>
<p>
        </p>
</div>
<div class="section">
<div class="titlepage"><div><div><h4 class="title">
<a name="boost_proto.users_guide.examples.future_group"></a><a class="link" href="users_guide.html#boost_proto.users_guide.examples.future_group" title="Future Group: A More Advanced Transform">Future
        Group: A More Advanced Transform</a>
</h4></div></div></div>
<p>
          An advanced example of a Proto transform that implements Howard Hinnant's
          design for <span class="emphasis"><em>future groups</em></span> that block for all or some
          asynchronous operations to complete and returns their results in a tuple
          of the appropriate type.
        </p>
<p>
</p>
<pre class="programlisting"><span class="comment">//  Copyright 2008 Eric Niebler. Distributed under the Boost</span>
<span class="comment">//  Software License, Version 1.0. (See accompanying file</span>
<span class="comment">//  LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)</span>
<span class="comment">//</span>
<span class="comment">// This is an example of using Proto transforms to implement</span>
<span class="comment">// Howard Hinnant's future group proposal.</span>

<span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">fusion</span><span class="special">/</span><span class="identifier">include</span><span class="special">/</span><span class="identifier">vector</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
<span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">fusion</span><span class="special">/</span><span class="identifier">include</span><span class="special">/</span><span class="identifier">as_vector</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
<span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">fusion</span><span class="special">/</span><span class="identifier">include</span><span class="special">/</span><span class="identifier">joint_view</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
<span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">fusion</span><span class="special">/</span><span class="identifier">include</span><span class="special">/</span><span class="identifier">single_view</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
<span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">proto</span><span class="special">/</span><span class="identifier">core</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
<span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">proto</span><span class="special">/</span><span class="identifier">transform</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
<span class="keyword">namespace</span> <span class="identifier">mpl</span> <span class="special">=</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">mpl</span><span class="special">;</span>
<span class="keyword">namespace</span> <span class="identifier">proto</span> <span class="special">=</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">proto</span><span class="special">;</span>
<span class="keyword">namespace</span> <span class="identifier">fusion</span> <span class="special">=</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">fusion</span><span class="special">;</span>
<span class="keyword">using</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_</span><span class="special">;</span>

<span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">class</span> <span class="identifier">L</span><span class="special">,</span><span class="keyword">class</span> <span class="identifier">R</span><span class="special">&gt;</span>
<span class="keyword">struct</span> <span class="identifier">pick_left</span>
<span class="special">{</span>
    <span class="identifier">BOOST_MPL_ASSERT</span><span class="special">((</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">is_same</span><span class="special">&lt;</span><span class="identifier">L</span><span class="special">,</span> <span class="identifier">R</span><span class="special">&gt;));</span>
    <span class="keyword">typedef</span> <span class="identifier">L</span> <span class="identifier">type</span><span class="special">;</span>
<span class="special">};</span>

<span class="comment">// Work-arounds for Microsoft Visual C++ 7.1</span>
<span class="preprocessor">#if</span> <span class="identifier">BOOST_WORKAROUND</span><span class="special">(</span><span class="identifier">BOOST_MSVC</span><span class="special">,</span> <span class="special">==</span> <span class="number">1310</span><span class="special">)</span>
<span class="preprocessor">#define</span> <span class="identifier">FutureGroup</span><span class="special">(</span><span class="identifier">x</span><span class="special">)</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">call</span><span class="special">&lt;</span><span class="identifier">FutureGroup</span><span class="special">(</span><span class="identifier">x</span><span class="special">)&gt;</span>
<span class="preprocessor">#endif</span>

<span class="comment">// Define the grammar of future group expression, as well as a</span>
<span class="comment">// transform to turn them into a Fusion sequence of the correct</span>
<span class="comment">// type.</span>
<span class="keyword">struct</span> <span class="identifier">FutureGroup</span>
  <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">or_</span><span class="special">&lt;</span>
        <span class="comment">// terminals become a single-element Fusion sequence</span>
        <span class="identifier">proto</span><span class="special">::</span><span class="identifier">when</span><span class="special">&lt;</span>
            <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span><span class="identifier">_</span><span class="special">&gt;</span>
          <span class="special">,</span> <span class="identifier">fusion</span><span class="special">::</span><span class="identifier">single_view</span><span class="special">&lt;</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_value</span><span class="special">&gt;(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_value</span><span class="special">)</span>
        <span class="special">&gt;</span>
        <span class="comment">// (a &amp;&amp; b) becomes a concatenation of the sequence</span>
        <span class="comment">// from 'a' and the one from 'b':</span>
      <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">when</span><span class="special">&lt;</span>
            <span class="identifier">proto</span><span class="special">::</span><span class="identifier">logical_and</span><span class="special">&lt;</span><span class="identifier">FutureGroup</span><span class="special">,</span> <span class="identifier">FutureGroup</span><span class="special">&gt;</span>
          <span class="special">,</span> <span class="identifier">fusion</span><span class="special">::</span><span class="identifier">joint_view</span><span class="special">&lt;</span>
                <span class="identifier">boost</span><span class="special">::</span><span class="identifier">add_const</span><span class="special">&lt;</span><span class="identifier">FutureGroup</span><span class="special">(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_left</span><span class="special">)</span> <span class="special">&gt;</span>
              <span class="special">,</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">add_const</span><span class="special">&lt;</span><span class="identifier">FutureGroup</span><span class="special">(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_right</span><span class="special">)</span> <span class="special">&gt;</span>
            <span class="special">&gt;(</span><span class="identifier">FutureGroup</span><span class="special">(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_left</span><span class="special">),</span> <span class="identifier">FutureGroup</span><span class="special">(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_right</span><span class="special">))</span>
        <span class="special">&gt;</span>
        <span class="comment">// (a || b) becomes the sequence for 'a', so long</span>
        <span class="comment">// as it is the same as the sequence for 'b'.</span>
      <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">when</span><span class="special">&lt;</span>
            <span class="identifier">proto</span><span class="special">::</span><span class="identifier">logical_or</span><span class="special">&lt;</span><span class="identifier">FutureGroup</span><span class="special">,</span> <span class="identifier">FutureGroup</span><span class="special">&gt;</span>
          <span class="special">,</span> <span class="identifier">pick_left</span><span class="special">&lt;</span>
                <span class="identifier">FutureGroup</span><span class="special">(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_left</span><span class="special">)</span>
              <span class="special">,</span> <span class="identifier">FutureGroup</span><span class="special">(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_right</span><span class="special">)</span>
            <span class="special">&gt;(</span><span class="identifier">FutureGroup</span><span class="special">(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_left</span><span class="special">))</span>
        <span class="special">&gt;</span>
    <span class="special">&gt;</span>
<span class="special">{};</span>

<span class="preprocessor">#if</span> <span class="identifier">BOOST_WORKAROUND</span><span class="special">(</span><span class="identifier">BOOST_MSVC</span><span class="special">,</span> <span class="special">==</span> <span class="number">1310</span><span class="special">)</span>
<span class="preprocessor">#undef</span> <span class="identifier">FutureGroup</span>
<span class="preprocessor">#endif</span>

<span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">class</span> <span class="identifier">E</span><span class="special">&gt;</span>
<span class="keyword">struct</span> <span class="identifier">future_expr</span><span class="special">;</span>

<span class="keyword">struct</span> <span class="identifier">future_dom</span>
  <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">domain</span><span class="special">&lt;</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">generator</span><span class="special">&lt;</span><span class="identifier">future_expr</span><span class="special">&gt;,</span> <span class="identifier">FutureGroup</span><span class="special">&gt;</span>
<span class="special">{};</span>

<span class="comment">// Expressions in the future group domain have a .get()</span>
<span class="comment">// member function that (ostensibly) blocks for the futures</span>
<span class="comment">// to complete and returns the results in an appropriate</span>
<span class="comment">// tuple.</span>
<span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">class</span> <span class="identifier">E</span><span class="special">&gt;</span>
<span class="keyword">struct</span> <span class="identifier">future_expr</span>
  <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">extends</span><span class="special">&lt;</span><span class="identifier">E</span><span class="special">,</span> <span class="identifier">future_expr</span><span class="special">&lt;</span><span class="identifier">E</span><span class="special">&gt;,</span> <span class="identifier">future_dom</span><span class="special">&gt;</span>
<span class="special">{</span>
    <span class="keyword">explicit</span> <span class="identifier">future_expr</span><span class="special">(</span><span class="identifier">E</span> <span class="keyword">const</span> <span class="special">&amp;</span><span class="identifier">e</span><span class="special">)</span>
      <span class="special">:</span> <span class="identifier">future_expr</span><span class="special">::</span><span class="identifier">proto_extends</span><span class="special">(</span><span class="identifier">e</span><span class="special">)</span>
    <span class="special">{}</span>

    <span class="keyword">typename</span> <span class="identifier">fusion</span><span class="special">::</span><span class="identifier">result_of</span><span class="special">::</span><span class="identifier">as_vector</span><span class="special">&lt;</span>
        <span class="keyword">typename</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">result_of</span><span class="special">&lt;</span><span class="identifier">FutureGroup</span><span class="special">(</span><span class="identifier">E</span><span class="special">)&gt;::</span><span class="identifier">type</span>
    <span class="special">&gt;::</span><span class="identifier">type</span>
    <span class="identifier">get</span><span class="special">()</span> <span class="keyword">const</span>
    <span class="special">{</span>
        <span class="keyword">return</span> <span class="identifier">fusion</span><span class="special">::</span><span class="identifier">as_vector</span><span class="special">(</span><span class="identifier">FutureGroup</span><span class="special">()(*</span><span class="keyword">this</span><span class="special">));</span>
    <span class="special">}</span>
<span class="special">};</span>

<span class="comment">// The future&lt;&gt; type has an even simpler .get()</span>
<span class="comment">// member function.</span>
<span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">&gt;</span>
<span class="keyword">struct</span> <span class="identifier">future</span>
  <span class="special">:</span> <span class="identifier">future_expr</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;::</span><span class="identifier">type</span><span class="special">&gt;</span>
<span class="special">{</span>
    <span class="identifier">future</span><span class="special">(</span><span class="identifier">T</span> <span class="keyword">const</span> <span class="special">&amp;</span><span class="identifier">t</span> <span class="special">=</span> <span class="identifier">T</span><span class="special">())</span>
      <span class="special">:</span> <span class="identifier">future</span><span class="special">::</span><span class="identifier">proto_derived_expr</span><span class="special">(</span><span class="identifier">future</span><span class="special">::</span><span class="identifier">proto_base_expr</span><span class="special">::</span><span class="identifier">make</span><span class="special">(</span><span class="identifier">t</span><span class="special">))</span>
    <span class="special">{}</span>

    <span class="identifier">T</span> <span class="identifier">get</span><span class="special">()</span> <span class="keyword">const</span>
    <span class="special">{</span>
        <span class="keyword">return</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">value</span><span class="special">(*</span><span class="keyword">this</span><span class="special">);</span>
    <span class="special">}</span>
<span class="special">};</span>

<span class="comment">// TEST CASES</span>
<span class="keyword">struct</span> <span class="identifier">A</span> <span class="special">{};</span>
<span class="keyword">struct</span> <span class="identifier">B</span> <span class="special">{};</span>
<span class="keyword">struct</span> <span class="identifier">C</span> <span class="special">{};</span>

<span class="keyword">int</span> <span class="identifier">main</span><span class="special">()</span>
<span class="special">{</span>
    <span class="keyword">using</span> <span class="identifier">fusion</span><span class="special">::</span><span class="identifier">vector</span><span class="special">;</span>
    <span class="identifier">future</span><span class="special">&lt;</span><span class="identifier">A</span><span class="special">&gt;</span> <span class="identifier">a</span><span class="special">;</span>
    <span class="identifier">future</span><span class="special">&lt;</span><span class="identifier">B</span><span class="special">&gt;</span> <span class="identifier">b</span><span class="special">;</span>
    <span class="identifier">future</span><span class="special">&lt;</span><span class="identifier">C</span><span class="special">&gt;</span> <span class="identifier">c</span><span class="special">;</span>
    <span class="identifier">future</span><span class="special">&lt;</span><span class="identifier">vector</span><span class="special">&lt;</span><span class="identifier">A</span><span class="special">,</span><span class="identifier">B</span><span class="special">&gt;</span> <span class="special">&gt;</span> <span class="identifier">ab</span><span class="special">;</span>

    <span class="comment">// Verify that various future groups have the</span>
    <span class="comment">// correct return types.</span>
    <span class="identifier">A</span>                       <span class="identifier">t0</span> <span class="special">=</span> <span class="identifier">a</span><span class="special">.</span><span class="identifier">get</span><span class="special">();</span>
    <span class="identifier">vector</span><span class="special">&lt;</span><span class="identifier">A</span><span class="special">,</span> <span class="identifier">B</span><span class="special">,</span> <span class="identifier">C</span><span class="special">&gt;</span>         <span class="identifier">t1</span> <span class="special">=</span> <span class="special">(</span><span class="identifier">a</span> <span class="special">&amp;&amp;</span> <span class="identifier">b</span> <span class="special">&amp;&amp;</span> <span class="identifier">c</span><span class="special">).</span><span class="identifier">get</span><span class="special">();</span>
    <span class="identifier">vector</span><span class="special">&lt;</span><span class="identifier">A</span><span class="special">,</span> <span class="identifier">C</span><span class="special">&gt;</span>            <span class="identifier">t2</span> <span class="special">=</span> <span class="special">((</span><span class="identifier">a</span> <span class="special">||</span> <span class="identifier">a</span><span class="special">)</span> <span class="special">&amp;&amp;</span> <span class="identifier">c</span><span class="special">).</span><span class="identifier">get</span><span class="special">();</span>
    <span class="identifier">vector</span><span class="special">&lt;</span><span class="identifier">A</span><span class="special">,</span> <span class="identifier">B</span><span class="special">,</span> <span class="identifier">C</span><span class="special">&gt;</span>         <span class="identifier">t3</span> <span class="special">=</span> <span class="special">((</span><span class="identifier">a</span> <span class="special">&amp;&amp;</span> <span class="identifier">b</span> <span class="special">||</span> <span class="identifier">a</span> <span class="special">&amp;&amp;</span> <span class="identifier">b</span><span class="special">)</span> <span class="special">&amp;&amp;</span> <span class="identifier">c</span><span class="special">).</span><span class="identifier">get</span><span class="special">();</span>
    <span class="identifier">vector</span><span class="special">&lt;</span><span class="identifier">vector</span><span class="special">&lt;</span><span class="identifier">A</span><span class="special">,</span> <span class="identifier">B</span><span class="special">&gt;,</span> <span class="identifier">C</span><span class="special">&gt;</span> <span class="identifier">t4</span> <span class="special">=</span> <span class="special">((</span><span class="identifier">ab</span> <span class="special">||</span> <span class="identifier">ab</span><span class="special">)</span> <span class="special">&amp;&amp;</span> <span class="identifier">c</span><span class="special">).</span><span class="identifier">get</span><span class="special">();</span>

    <span class="keyword">return</span> <span class="number">0</span><span class="special">;</span>
<span class="special">}</span>
</pre>
<p>
        </p>
</div>
<div class="section">
<div class="titlepage"><div><div><h4 class="title">
<a name="boost_proto.users_guide.examples.lambda"></a><a class="link" href="users_guide.html#boost_proto.users_guide.examples.lambda" title="Lambda: A Simple Lambda Library with Proto">Lambda: A
        Simple Lambda Library with Proto</a>
</h4></div></div></div>
<p>
          This is an advanced example that shows how to implement a simple lambda
          EDSL with Proto, like the Boost.Lambda_library. It uses contexts, transforms
          and expression extension.
        </p>
<p>
</p>
<pre class="programlisting"><span class="comment">///////////////////////////////////////////////////////////////////////////////</span>
<span class="comment">// Copyright 2008 Eric Niebler. Distributed under the Boost</span>
<span class="comment">// Software License, Version 1.0. (See accompanying file</span>
<span class="comment">// LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)</span>
<span class="comment">//</span>
<span class="comment">// This example builds a simple but functional lambda library using Proto.</span>

<span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">iostream</span><span class="special">&gt;</span>
<span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">algorithm</span><span class="special">&gt;</span>
<span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">mpl</span><span class="special">/</span><span class="keyword">int</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
<span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">mpl</span><span class="special">/</span><span class="identifier">min_max</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
<span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">mpl</span><span class="special">/</span><span class="identifier">eval_if</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
<span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">mpl</span><span class="special">/</span><span class="identifier">identity</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
<span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">mpl</span><span class="special">/</span><span class="identifier">next_prior</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
<span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">fusion</span><span class="special">/</span><span class="identifier">tuple</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
<span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">typeof</span><span class="special">/</span><span class="identifier">typeof</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
<span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">typeof</span><span class="special">/</span><span class="identifier">std</span><span class="special">/</span><span class="identifier">ostream</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
<span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">typeof</span><span class="special">/</span><span class="identifier">std</span><span class="special">/</span><span class="identifier">iostream</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
<span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">proto</span><span class="special">/</span><span class="identifier">core</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
<span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">proto</span><span class="special">/</span><span class="identifier">context</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
<span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">proto</span><span class="special">/</span><span class="identifier">transform</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
<span class="keyword">namespace</span> <span class="identifier">mpl</span> <span class="special">=</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">mpl</span><span class="special">;</span>
<span class="keyword">namespace</span> <span class="identifier">proto</span> <span class="special">=</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">proto</span><span class="special">;</span>
<span class="keyword">namespace</span> <span class="identifier">fusion</span> <span class="special">=</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">fusion</span><span class="special">;</span>
<span class="keyword">using</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_</span><span class="special">;</span>

<span class="comment">// Forward declaration of the lambda expression wrapper</span>
<span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">T</span><span class="special">&gt;</span>
<span class="keyword">struct</span> <span class="identifier">lambda</span><span class="special">;</span>

<span class="keyword">struct</span> <span class="identifier">lambda_domain</span>
  <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">domain</span><span class="special">&lt;</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">pod_generator</span><span class="special">&lt;</span><span class="identifier">lambda</span><span class="special">&gt;</span> <span class="special">&gt;</span>
<span class="special">{};</span>

<span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">I</span><span class="special">&gt;</span>
<span class="keyword">struct</span> <span class="identifier">placeholder</span>
<span class="special">{</span>
    <span class="keyword">typedef</span> <span class="identifier">I</span> <span class="identifier">arity</span><span class="special">;</span>
<span class="special">};</span>

<span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">T</span><span class="special">&gt;</span>
<span class="keyword">struct</span> <span class="identifier">placeholder_arity</span>
<span class="special">{</span>
    <span class="keyword">typedef</span> <span class="keyword">typename</span> <span class="identifier">T</span><span class="special">::</span><span class="identifier">arity</span> <span class="identifier">type</span><span class="special">;</span>
<span class="special">};</span>

<span class="comment">// The lambda grammar, with the transforms for calculating the max arity</span>
<span class="keyword">struct</span> <span class="identifier">lambda_arity</span>
  <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">or_</span><span class="special">&lt;</span>
        <span class="identifier">proto</span><span class="special">::</span><span class="identifier">when</span><span class="special">&lt;</span>
            <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span> <span class="identifier">placeholder</span><span class="special">&lt;</span><span class="identifier">_</span><span class="special">&gt;</span> <span class="special">&gt;</span>
          <span class="special">,</span> <span class="identifier">mpl</span><span class="special">::</span><span class="identifier">next</span><span class="special">&lt;</span><span class="identifier">placeholder_arity</span><span class="special">&lt;</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_value</span><span class="special">&gt;</span> <span class="special">&gt;()</span>
        <span class="special">&gt;</span>
      <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">when</span><span class="special">&lt;</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span><span class="identifier">_</span><span class="special">&gt;</span>
          <span class="special">,</span> <span class="identifier">mpl</span><span class="special">::</span><span class="identifier">int_</span><span class="special">&lt;</span><span class="number">0</span><span class="special">&gt;()</span>
        <span class="special">&gt;</span>
      <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">when</span><span class="special">&lt;</span>
            <span class="identifier">proto</span><span class="special">::</span><span class="identifier">nary_expr</span><span class="special">&lt;</span><span class="identifier">_</span><span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">vararg</span><span class="special">&lt;</span><span class="identifier">_</span><span class="special">&gt;</span> <span class="special">&gt;</span>
          <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">fold</span><span class="special">&lt;</span><span class="identifier">_</span><span class="special">,</span> <span class="identifier">mpl</span><span class="special">::</span><span class="identifier">int_</span><span class="special">&lt;</span><span class="number">0</span><span class="special">&gt;(),</span> <span class="identifier">mpl</span><span class="special">::</span><span class="identifier">max</span><span class="special">&lt;</span><span class="identifier">lambda_arity</span><span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_state</span><span class="special">&gt;()&gt;</span>
        <span class="special">&gt;</span>
    <span class="special">&gt;</span>
<span class="special">{};</span>

<span class="comment">// The lambda context is the same as the default context</span>
<span class="comment">// with the addition of special handling for lambda placeholders</span>
<span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">Tuple</span><span class="special">&gt;</span>
<span class="keyword">struct</span> <span class="identifier">lambda_context</span>
  <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">callable_context</span><span class="special">&lt;</span><span class="identifier">lambda_context</span><span class="special">&lt;</span><span class="identifier">Tuple</span><span class="special">&gt;</span> <span class="keyword">const</span><span class="special">&gt;</span>
<span class="special">{</span>
    <span class="identifier">lambda_context</span><span class="special">(</span><span class="identifier">Tuple</span> <span class="keyword">const</span> <span class="special">&amp;</span><span class="identifier">args</span><span class="special">)</span>
      <span class="special">:</span> <span class="identifier">args_</span><span class="special">(</span><span class="identifier">args</span><span class="special">)</span>
    <span class="special">{}</span>

    <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">Sig</span><span class="special">&gt;</span>
    <span class="keyword">struct</span> <span class="identifier">result</span><span class="special">;</span>

    <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">This</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">I</span><span class="special">&gt;</span>
    <span class="keyword">struct</span> <span class="identifier">result</span><span class="special">&lt;</span><span class="identifier">This</span><span class="special">(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">,</span> <span class="identifier">placeholder</span><span class="special">&lt;</span><span class="identifier">I</span><span class="special">&gt;</span> <span class="keyword">const</span> <span class="special">&amp;)&gt;</span>
      <span class="special">:</span> <span class="identifier">fusion</span><span class="special">::</span><span class="identifier">result_of</span><span class="special">::</span><span class="identifier">at</span><span class="special">&lt;</span><span class="identifier">Tuple</span><span class="special">,</span> <span class="identifier">I</span><span class="special">&gt;</span>
    <span class="special">{};</span>

    <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">I</span><span class="special">&gt;</span>
    <span class="keyword">typename</span> <span class="identifier">fusion</span><span class="special">::</span><span class="identifier">result_of</span><span class="special">::</span><span class="identifier">at</span><span class="special">&lt;</span><span class="identifier">Tuple</span><span class="special">,</span> <span class="identifier">I</span><span class="special">&gt;::</span><span class="identifier">type</span>
    <span class="keyword">operator</span> <span class="special">()(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">,</span> <span class="identifier">placeholder</span><span class="special">&lt;</span><span class="identifier">I</span><span class="special">&gt;</span> <span class="keyword">const</span> <span class="special">&amp;)</span> <span class="keyword">const</span>
    <span class="special">{</span>
        <span class="keyword">return</span> <span class="identifier">fusion</span><span class="special">::</span><span class="identifier">at</span><span class="special">&lt;</span><span class="identifier">I</span><span class="special">&gt;(</span><span class="keyword">this</span><span class="special">-&gt;</span><span class="identifier">args_</span><span class="special">);</span>
    <span class="special">}</span>

    <span class="identifier">Tuple</span> <span class="identifier">args_</span><span class="special">;</span>
<span class="special">};</span>

<span class="comment">// The lambda&lt;&gt; expression wrapper makes expressions polymorphic</span>
<span class="comment">// function objects</span>
<span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">T</span><span class="special">&gt;</span>
<span class="keyword">struct</span> <span class="identifier">lambda</span>
<span class="special">{</span>
    <span class="identifier">BOOST_PROTO_BASIC_EXTENDS</span><span class="special">(</span><span class="identifier">T</span><span class="special">,</span> <span class="identifier">lambda</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;,</span> <span class="identifier">lambda_domain</span><span class="special">)</span>
    <span class="identifier">BOOST_PROTO_EXTENDS_ASSIGN</span><span class="special">()</span>
    <span class="identifier">BOOST_PROTO_EXTENDS_SUBSCRIPT</span><span class="special">()</span>

    <span class="comment">// Calculate the arity of this lambda expression</span>
    <span class="keyword">static</span> <span class="keyword">int</span> <span class="keyword">const</span> <span class="identifier">arity</span> <span class="special">=</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">result_of</span><span class="special">&lt;</span><span class="identifier">lambda_arity</span><span class="special">(</span><span class="identifier">T</span><span class="special">)&gt;::</span><span class="identifier">type</span><span class="special">::</span><span class="identifier">value</span><span class="special">;</span>

    <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">Sig</span><span class="special">&gt;</span>
    <span class="keyword">struct</span> <span class="identifier">result</span><span class="special">;</span>

    <span class="comment">// Define nested result&lt;&gt; specializations to calculate the return</span>
    <span class="comment">// type of this lambda expression. But be careful not to evaluate</span>
    <span class="comment">// the return type of the nullary function unless we have a nullary</span>
    <span class="comment">// lambda!</span>
    <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">This</span><span class="special">&gt;</span>
    <span class="keyword">struct</span> <span class="identifier">result</span><span class="special">&lt;</span><span class="identifier">This</span><span class="special">()&gt;</span>
      <span class="special">:</span> <span class="identifier">mpl</span><span class="special">::</span><span class="identifier">eval_if_c</span><span class="special">&lt;</span>
            <span class="number">0</span> <span class="special">==</span> <span class="identifier">arity</span>
          <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">result_of</span><span class="special">::</span><span class="identifier">eval</span><span class="special">&lt;</span><span class="identifier">T</span> <span class="keyword">const</span><span class="special">,</span> <span class="identifier">lambda_context</span><span class="special">&lt;</span><span class="identifier">fusion</span><span class="special">::</span><span class="identifier">tuple</span><span class="special">&lt;&gt;</span> <span class="special">&gt;</span> <span class="special">&gt;</span>
          <span class="special">,</span> <span class="identifier">mpl</span><span class="special">::</span><span class="identifier">identity</span><span class="special">&lt;</span><span class="keyword">void</span><span class="special">&gt;</span>
        <span class="special">&gt;</span>
    <span class="special">{};</span>

    <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">This</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">A0</span><span class="special">&gt;</span>
    <span class="keyword">struct</span> <span class="identifier">result</span><span class="special">&lt;</span><span class="identifier">This</span><span class="special">(</span><span class="identifier">A0</span><span class="special">)&gt;</span>
      <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">result_of</span><span class="special">::</span><span class="identifier">eval</span><span class="special">&lt;</span><span class="identifier">T</span> <span class="keyword">const</span><span class="special">,</span> <span class="identifier">lambda_context</span><span class="special">&lt;</span><span class="identifier">fusion</span><span class="special">::</span><span class="identifier">tuple</span><span class="special">&lt;</span><span class="identifier">A0</span><span class="special">&gt;</span> <span class="special">&gt;</span> <span class="special">&gt;</span>
    <span class="special">{};</span>

    <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">This</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">A0</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">A1</span><span class="special">&gt;</span>
    <span class="keyword">struct</span> <span class="identifier">result</span><span class="special">&lt;</span><span class="identifier">This</span><span class="special">(</span><span class="identifier">A0</span><span class="special">,</span> <span class="identifier">A1</span><span class="special">)&gt;</span>
      <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">result_of</span><span class="special">::</span><span class="identifier">eval</span><span class="special">&lt;</span><span class="identifier">T</span> <span class="keyword">const</span><span class="special">,</span> <span class="identifier">lambda_context</span><span class="special">&lt;</span><span class="identifier">fusion</span><span class="special">::</span><span class="identifier">tuple</span><span class="special">&lt;</span><span class="identifier">A0</span><span class="special">,</span> <span class="identifier">A1</span><span class="special">&gt;</span> <span class="special">&gt;</span> <span class="special">&gt;</span>
    <span class="special">{};</span>

    <span class="comment">// Define our operator () that evaluates the lambda expression.</span>
    <span class="keyword">typename</span> <span class="identifier">result</span><span class="special">&lt;</span><span class="identifier">lambda</span><span class="special">()&gt;::</span><span class="identifier">type</span>
    <span class="keyword">operator</span> <span class="special">()()</span> <span class="keyword">const</span>
    <span class="special">{</span>
        <span class="identifier">fusion</span><span class="special">::</span><span class="identifier">tuple</span><span class="special">&lt;&gt;</span> <span class="identifier">args</span><span class="special">;</span>
        <span class="identifier">lambda_context</span><span class="special">&lt;</span><span class="identifier">fusion</span><span class="special">::</span><span class="identifier">tuple</span><span class="special">&lt;&gt;</span> <span class="special">&gt;</span> <span class="identifier">ctx</span><span class="special">(</span><span class="identifier">args</span><span class="special">);</span>
        <span class="keyword">return</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">eval</span><span class="special">(*</span><span class="keyword">this</span><span class="special">,</span> <span class="identifier">ctx</span><span class="special">);</span>
    <span class="special">}</span>

    <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">A0</span><span class="special">&gt;</span>
    <span class="keyword">typename</span> <span class="identifier">result</span><span class="special">&lt;</span><span class="identifier">lambda</span><span class="special">(</span><span class="identifier">A0</span> <span class="keyword">const</span> <span class="special">&amp;)&gt;::</span><span class="identifier">type</span>
    <span class="keyword">operator</span> <span class="special">()(</span><span class="identifier">A0</span> <span class="keyword">const</span> <span class="special">&amp;</span><span class="identifier">a0</span><span class="special">)</span> <span class="keyword">const</span>
    <span class="special">{</span>
        <span class="identifier">fusion</span><span class="special">::</span><span class="identifier">tuple</span><span class="special">&lt;</span><span class="identifier">A0</span> <span class="keyword">const</span> <span class="special">&amp;&gt;</span> <span class="identifier">args</span><span class="special">(</span><span class="identifier">a0</span><span class="special">);</span>
        <span class="identifier">lambda_context</span><span class="special">&lt;</span><span class="identifier">fusion</span><span class="special">::</span><span class="identifier">tuple</span><span class="special">&lt;</span><span class="identifier">A0</span> <span class="keyword">const</span> <span class="special">&amp;&gt;</span> <span class="special">&gt;</span> <span class="identifier">ctx</span><span class="special">(</span><span class="identifier">args</span><span class="special">);</span>
        <span class="keyword">return</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">eval</span><span class="special">(*</span><span class="keyword">this</span><span class="special">,</span> <span class="identifier">ctx</span><span class="special">);</span>
    <span class="special">}</span>

    <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">A0</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">A1</span><span class="special">&gt;</span>
    <span class="keyword">typename</span> <span class="identifier">result</span><span class="special">&lt;</span><span class="identifier">lambda</span><span class="special">(</span><span class="identifier">A0</span> <span class="keyword">const</span> <span class="special">&amp;,</span> <span class="identifier">A1</span> <span class="keyword">const</span> <span class="special">&amp;)&gt;::</span><span class="identifier">type</span>
    <span class="keyword">operator</span> <span class="special">()(</span><span class="identifier">A0</span> <span class="keyword">const</span> <span class="special">&amp;</span><span class="identifier">a0</span><span class="special">,</span> <span class="identifier">A1</span> <span class="keyword">const</span> <span class="special">&amp;</span><span class="identifier">a1</span><span class="special">)</span> <span class="keyword">const</span>
    <span class="special">{</span>
        <span class="identifier">fusion</span><span class="special">::</span><span class="identifier">tuple</span><span class="special">&lt;</span><span class="identifier">A0</span> <span class="keyword">const</span> <span class="special">&amp;,</span> <span class="identifier">A1</span> <span class="keyword">const</span> <span class="special">&amp;&gt;</span> <span class="identifier">args</span><span class="special">(</span><span class="identifier">a0</span><span class="special">,</span> <span class="identifier">a1</span><span class="special">);</span>
        <span class="identifier">lambda_context</span><span class="special">&lt;</span><span class="identifier">fusion</span><span class="special">::</span><span class="identifier">tuple</span><span class="special">&lt;</span><span class="identifier">A0</span> <span class="keyword">const</span> <span class="special">&amp;,</span> <span class="identifier">A1</span> <span class="keyword">const</span> <span class="special">&amp;&gt;</span> <span class="special">&gt;</span> <span class="identifier">ctx</span><span class="special">(</span><span class="identifier">args</span><span class="special">);</span>
        <span class="keyword">return</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">eval</span><span class="special">(*</span><span class="keyword">this</span><span class="special">,</span> <span class="identifier">ctx</span><span class="special">);</span>
    <span class="special">}</span>
<span class="special">};</span>

<span class="comment">// Define some lambda placeholders</span>
<span class="identifier">lambda</span><span class="special">&lt;</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span><span class="identifier">placeholder</span><span class="special">&lt;</span><span class="identifier">mpl</span><span class="special">::</span><span class="identifier">int_</span><span class="special">&lt;</span><span class="number">0</span><span class="special">&gt;</span> <span class="special">&gt;</span> <span class="special">&gt;::</span><span class="identifier">type</span><span class="special">&gt;</span> <span class="keyword">const</span> <span class="identifier">_1</span> <span class="special">=</span> <span class="special">{{}};</span>
<span class="identifier">lambda</span><span class="special">&lt;</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span><span class="identifier">placeholder</span><span class="special">&lt;</span><span class="identifier">mpl</span><span class="special">::</span><span class="identifier">int_</span><span class="special">&lt;</span><span class="number">1</span><span class="special">&gt;</span> <span class="special">&gt;</span> <span class="special">&gt;::</span><span class="identifier">type</span><span class="special">&gt;</span> <span class="keyword">const</span> <span class="identifier">_2</span> <span class="special">=</span> <span class="special">{{}};</span>

<span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">T</span><span class="special">&gt;</span>
<span class="identifier">lambda</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;::</span><span class="identifier">type</span><span class="special">&gt;</span> <span class="keyword">const</span> <span class="identifier">val</span><span class="special">(</span><span class="identifier">T</span> <span class="keyword">const</span> <span class="special">&amp;</span><span class="identifier">t</span><span class="special">)</span>
<span class="special">{</span>
    <span class="identifier">lambda</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;::</span><span class="identifier">type</span><span class="special">&gt;</span> <span class="identifier">that</span> <span class="special">=</span> <span class="special">{{</span><span class="identifier">t</span><span class="special">}};</span>
    <span class="keyword">return</span> <span class="identifier">that</span><span class="special">;</span>
<span class="special">}</span>

<span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">T</span><span class="special">&gt;</span>
<span class="identifier">lambda</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span><span class="identifier">T</span> <span class="special">&amp;&gt;::</span><span class="identifier">type</span><span class="special">&gt;</span> <span class="keyword">const</span> <span class="identifier">var</span><span class="special">(</span><span class="identifier">T</span> <span class="special">&amp;</span><span class="identifier">t</span><span class="special">)</span>
<span class="special">{</span>
    <span class="identifier">lambda</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span><span class="identifier">T</span> <span class="special">&amp;&gt;::</span><span class="identifier">type</span><span class="special">&gt;</span> <span class="identifier">that</span> <span class="special">=</span> <span class="special">{{</span><span class="identifier">t</span><span class="special">}};</span>
    <span class="keyword">return</span> <span class="identifier">that</span><span class="special">;</span>
<span class="special">}</span>

<span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">T</span><span class="special">&gt;</span>
<span class="keyword">struct</span> <span class="identifier">construct_helper</span>
<span class="special">{</span>
    <span class="keyword">typedef</span> <span class="identifier">T</span> <span class="identifier">result_type</span><span class="special">;</span> <span class="comment">// for TR1 result_of</span>

    <span class="identifier">T</span> <span class="keyword">operator</span><span class="special">()()</span> <span class="keyword">const</span>
    <span class="special">{</span> <span class="keyword">return</span> <span class="identifier">T</span><span class="special">();</span> <span class="special">}</span>

    <span class="comment">// Generate BOOST_PROTO_MAX_ARITY overloads of the</span>
    <span class="comment">// following function call operator.</span>
<span class="preprocessor">#define</span> <span class="identifier">BOOST_PROTO_LOCAL_MACRO</span><span class="special">(</span><span class="identifier">N</span><span class="special">,</span> <span class="identifier">typename_A</span><span class="special">,</span> <span class="identifier">A_const_ref</span><span class="special">,</span> <span class="identifier">A_const_ref_a</span><span class="special">,</span> <span class="identifier">a</span><span class="special">)\</span>
    <span class="keyword">template</span><span class="special">&lt;</span><span class="identifier">typename_A</span><span class="special">(</span><span class="identifier">N</span><span class="special">)&gt;</span>                                       <span class="special">\</span>
    <span class="identifier">T</span> <span class="keyword">operator</span><span class="special">()(</span><span class="identifier">A_const_ref_a</span><span class="special">(</span><span class="identifier">N</span><span class="special">))</span> <span class="keyword">const</span>                          <span class="special">\</span>
    <span class="special">{</span> <span class="keyword">return</span> <span class="identifier">T</span><span class="special">(</span><span class="identifier">a</span><span class="special">(</span><span class="identifier">N</span><span class="special">));</span> <span class="special">}</span>
<span class="preprocessor">#define</span> <span class="identifier">BOOST_PROTO_LOCAL_a</span> <span class="identifier">BOOST_PROTO_a</span>
<span class="preprocessor">#include</span> <span class="identifier">BOOST_PROTO_LOCAL_ITERATE</span><span class="special">()</span>
<span class="special">};</span>

<span class="comment">// Generate BOOST_PROTO_MAX_ARITY-1 overloads of the</span>
<span class="comment">// following construct() function template.</span>
<span class="preprocessor">#define</span> <span class="identifier">M0</span><span class="special">(</span><span class="identifier">N</span><span class="special">,</span> <span class="identifier">typename_A</span><span class="special">,</span> <span class="identifier">A_const_ref</span><span class="special">,</span> <span class="identifier">A_const_ref_a</span><span class="special">,</span> <span class="identifier">ref_a</span><span class="special">)</span>      <span class="special">\</span>
<span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">T</span><span class="special">,</span> <span class="identifier">typename_A</span><span class="special">(</span><span class="identifier">N</span><span class="special">)&gt;</span>                               <span class="special">\</span>
<span class="keyword">typename</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">result_of</span><span class="special">::</span><span class="identifier">make_expr</span><span class="special">&lt;</span>                             <span class="special">\</span>
    <span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">function</span>                                          <span class="special">\</span>
  <span class="special">,</span> <span class="identifier">lambda_domain</span>                                                 <span class="special">\</span>
  <span class="special">,</span> <span class="identifier">construct_helper</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;</span>                                           <span class="special">\</span>
  <span class="special">,</span> <span class="identifier">A_const_ref</span><span class="special">(</span><span class="identifier">N</span><span class="special">)</span>                                                <span class="special">\</span>
<span class="special">&gt;::</span><span class="identifier">type</span> <span class="keyword">const</span>                                                     <span class="special">\</span>
<span class="identifier">construct</span><span class="special">(</span><span class="identifier">A_const_ref_a</span><span class="special">(</span><span class="identifier">N</span><span class="special">))</span>                                       <span class="special">\</span>
<span class="special">{</span>                                                                 <span class="special">\</span>
    <span class="keyword">return</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">make_expr</span><span class="special">&lt;</span>                                      <span class="special">\</span>
        <span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">function</span>                                      <span class="special">\</span>
      <span class="special">,</span> <span class="identifier">lambda_domain</span>                                             <span class="special">\</span>
    <span class="special">&gt;(</span>                                                            <span class="special">\</span>
        <span class="identifier">construct_helper</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;()</span>                                     <span class="special">\</span>
      <span class="special">,</span> <span class="identifier">ref_a</span><span class="special">(</span><span class="identifier">N</span><span class="special">)</span>                                                  <span class="special">\</span>
    <span class="special">);</span>                                                            <span class="special">\</span>
<span class="special">}</span>
<span class="identifier">BOOST_PROTO_REPEAT_FROM_TO</span><span class="special">(</span><span class="number">1</span><span class="special">,</span> <span class="identifier">BOOST_PROTO_MAX_ARITY</span><span class="special">,</span> <span class="identifier">M0</span><span class="special">)</span>
<span class="preprocessor">#undef</span> <span class="identifier">M0</span>

<span class="keyword">struct</span> <span class="identifier">S</span>
<span class="special">{</span>
    <span class="identifier">S</span><span class="special">()</span> <span class="special">{}</span>
    <span class="identifier">S</span><span class="special">(</span><span class="keyword">int</span> <span class="identifier">i</span><span class="special">,</span> <span class="keyword">char</span> <span class="identifier">c</span><span class="special">)</span>
    <span class="special">{</span>
        <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="string">"S("</span> <span class="special">&lt;&lt;</span> <span class="identifier">i</span> <span class="special">&lt;&lt;</span> <span class="string">","</span> <span class="special">&lt;&lt;</span> <span class="identifier">c</span> <span class="special">&lt;&lt;</span> <span class="string">")\n"</span><span class="special">;</span>
    <span class="special">}</span>
<span class="special">};</span>

<span class="keyword">int</span> <span class="identifier">main</span><span class="special">()</span>
<span class="special">{</span>
    <span class="comment">// Create some lambda objects and immediately</span>
    <span class="comment">// invoke them by applying their operator():</span>
    <span class="keyword">int</span> <span class="identifier">i</span> <span class="special">=</span> <span class="special">(</span> <span class="special">(</span><span class="identifier">_1</span> <span class="special">+</span> <span class="number">2</span><span class="special">)</span> <span class="special">/</span> <span class="number">4</span> <span class="special">)(</span><span class="number">42</span><span class="special">);</span>
    <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="identifier">i</span> <span class="special">&lt;&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span> <span class="comment">// prints 11</span>

    <span class="keyword">int</span> <span class="identifier">j</span> <span class="special">=</span> <span class="special">(</span> <span class="special">(-(</span><span class="identifier">_1</span> <span class="special">+</span> <span class="number">2</span><span class="special">))</span> <span class="special">/</span> <span class="number">4</span> <span class="special">)(</span><span class="number">42</span><span class="special">);</span>
    <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="identifier">j</span> <span class="special">&lt;&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span> <span class="comment">// prints -11</span>

    <span class="keyword">double</span> <span class="identifier">d</span> <span class="special">=</span> <span class="special">(</span> <span class="special">(</span><span class="number">4</span> <span class="special">-</span> <span class="identifier">_2</span><span class="special">)</span> <span class="special">*</span> <span class="number">3</span> <span class="special">)(</span><span class="number">42</span><span class="special">,</span> <span class="number">3.14</span><span class="special">);</span>
    <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="identifier">d</span> <span class="special">&lt;&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span> <span class="comment">// prints 2.58</span>

    <span class="comment">// check non-const ref terminals</span>
    <span class="special">(</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="identifier">_1</span> <span class="special">&lt;&lt;</span> <span class="string">" -- "</span> <span class="special">&lt;&lt;</span> <span class="identifier">_2</span> <span class="special">&lt;&lt;</span> <span class="char">'\n'</span><span class="special">)(</span><span class="number">42</span><span class="special">,</span> <span class="string">"Life, the Universe and Everything!"</span><span class="special">);</span>
    <span class="comment">// prints "42 -- Life, the Universe and Everything!"</span>

    <span class="comment">// "Nullary" lambdas work too</span>
    <span class="keyword">int</span> <span class="identifier">k</span> <span class="special">=</span> <span class="special">(</span><span class="identifier">val</span><span class="special">(</span><span class="number">1</span><span class="special">)</span> <span class="special">+</span> <span class="identifier">val</span><span class="special">(</span><span class="number">2</span><span class="special">))();</span>
    <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="identifier">k</span> <span class="special">&lt;&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span> <span class="comment">// prints 3</span>

    <span class="comment">// check array indexing for kicks</span>
    <span class="keyword">int</span> <span class="identifier">integers</span><span class="special">[</span><span class="number">5</span><span class="special">]</span> <span class="special">=</span> <span class="special">{</span><span class="number">0</span><span class="special">};</span>
    <span class="special">(</span><span class="identifier">var</span><span class="special">(</span><span class="identifier">integers</span><span class="special">)[</span><span class="number">2</span><span class="special">]</span> <span class="special">=</span> <span class="number">2</span><span class="special">)();</span>
    <span class="special">(</span><span class="identifier">var</span><span class="special">(</span><span class="identifier">integers</span><span class="special">)[</span><span class="identifier">_1</span><span class="special">]</span> <span class="special">=</span> <span class="identifier">_1</span><span class="special">)(</span><span class="number">3</span><span class="special">);</span>
    <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="identifier">integers</span><span class="special">[</span><span class="number">2</span><span class="special">]</span> <span class="special">&lt;&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span> <span class="comment">// prints 2</span>
    <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="identifier">integers</span><span class="special">[</span><span class="number">3</span><span class="special">]</span> <span class="special">&lt;&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span> <span class="comment">// prints 3</span>

    <span class="comment">// Now use a lambda with an STL algorithm!</span>
    <span class="keyword">int</span> <span class="identifier">rgi</span><span class="special">[</span><span class="number">4</span><span class="special">]</span> <span class="special">=</span> <span class="special">{</span><span class="number">1</span><span class="special">,</span><span class="number">2</span><span class="special">,</span><span class="number">3</span><span class="special">,</span><span class="number">4</span><span class="special">};</span>
    <span class="keyword">char</span> <span class="identifier">rgc</span><span class="special">[</span><span class="number">4</span><span class="special">]</span> <span class="special">=</span> <span class="special">{</span><span class="char">'a'</span><span class="special">,</span><span class="char">'b'</span><span class="special">,</span><span class="char">'c'</span><span class="special">,</span><span class="char">'d'</span><span class="special">};</span>
    <span class="identifier">S</span> <span class="identifier">rgs</span><span class="special">[</span><span class="number">4</span><span class="special">];</span>

    <span class="identifier">std</span><span class="special">::</span><span class="identifier">transform</span><span class="special">(</span><span class="identifier">rgi</span><span class="special">,</span> <span class="identifier">rgi</span><span class="special">+</span><span class="number">4</span><span class="special">,</span> <span class="identifier">rgc</span><span class="special">,</span> <span class="identifier">rgs</span><span class="special">,</span> <span class="identifier">construct</span><span class="special">&lt;</span><span class="identifier">S</span><span class="special">&gt;(</span><span class="identifier">_1</span><span class="special">,</span> <span class="identifier">_2</span><span class="special">));</span>
    <span class="keyword">return</span> <span class="number">0</span><span class="special">;</span>
<span class="special">}</span>
</pre>
<p>
        </p>
</div>
<div class="section">
<div class="titlepage"><div><div><h4 class="title">
<a name="boost_proto.users_guide.examples.checked_calc"></a><a class="link" href="users_guide.html#boost_proto.users_guide.examples.checked_calc" title="Checked Calculator: A Simple Example of External Transforms">Checked
        Calculator: A Simple Example of External Transforms</a>
</h4></div></div></div>
<p>
          This is an advanced example that shows how to externally parameterize a
          grammar's transforms. It defines a calculator EDSL with a grammar that
          can perform either checked or unchecked arithmetic.
        </p>
<p>
</p>
<pre class="programlisting"><span class="comment">//  Copyright 2011 Eric Niebler. Distributed under the Boost</span>
<span class="comment">//  Software License, Version 1.0. (See accompanying file</span>
<span class="comment">//  LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)</span>
<span class="comment">//</span>
<span class="comment">// This is an example of how to specify a transform externally so</span>
<span class="comment">// that a single grammar can be used to drive multiple differnt</span>
<span class="comment">// calculations. In particular, it defines a calculator grammar</span>
<span class="comment">// that computes the result of an expression with either checked</span>
<span class="comment">// or non-checked division.</span>

<span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">iostream</span><span class="special">&gt;</span>
<span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">assert</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
<span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">mpl</span><span class="special">/</span><span class="keyword">int</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
<span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">mpl</span><span class="special">/</span><span class="identifier">next</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
<span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">mpl</span><span class="special">/</span><span class="identifier">min_max</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
<span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">fusion</span><span class="special">/</span><span class="identifier">container</span><span class="special">/</span><span class="identifier">vector</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
<span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">fusion</span><span class="special">/</span><span class="identifier">container</span><span class="special">/</span><span class="identifier">generation</span><span class="special">/</span><span class="identifier">make_vector</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
<span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">proto</span><span class="special">/</span><span class="identifier">proto</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
<span class="keyword">namespace</span> <span class="identifier">mpl</span> <span class="special">=</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">mpl</span><span class="special">;</span>
<span class="keyword">namespace</span> <span class="identifier">proto</span> <span class="special">=</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">proto</span><span class="special">;</span>
<span class="keyword">namespace</span> <span class="identifier">fusion</span> <span class="special">=</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">fusion</span><span class="special">;</span>

<span class="comment">// The argument placeholder type</span>
<span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">I</span><span class="special">&gt;</span> <span class="keyword">struct</span> <span class="identifier">placeholder</span> <span class="special">:</span> <span class="identifier">I</span> <span class="special">{};</span>

<span class="comment">// Give each rule in the grammar a "name". This is so that we</span>
<span class="comment">// can easily dispatch on it later.</span>
<span class="keyword">struct</span> <span class="identifier">calc_grammar</span><span class="special">;</span>
<span class="keyword">struct</span> <span class="identifier">divides_rule</span> <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">divides</span><span class="special">&lt;</span><span class="identifier">calc_grammar</span><span class="special">,</span> <span class="identifier">calc_grammar</span><span class="special">&gt;</span> <span class="special">{};</span>

<span class="comment">// Use external transforms in calc_gramar</span>
<span class="keyword">struct</span> <span class="identifier">calc_grammar</span>
  <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">or_</span><span class="special">&lt;</span>
        <span class="identifier">proto</span><span class="special">::</span><span class="identifier">when</span><span class="special">&lt;</span>
            <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span><span class="identifier">placeholder</span><span class="special">&lt;</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_</span><span class="special">&gt;</span> <span class="special">&gt;</span>
            <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">functional</span><span class="special">::</span><span class="identifier">at</span><span class="special">(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_state</span><span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_value</span><span class="special">)</span>
        <span class="special">&gt;</span>
      <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">when</span><span class="special">&lt;</span>
            <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">convertible_to</span><span class="special">&lt;</span><span class="keyword">double</span><span class="special">&gt;</span> <span class="special">&gt;</span>
          <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_value</span>
        <span class="special">&gt;</span>
      <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">when</span><span class="special">&lt;</span>
            <span class="identifier">proto</span><span class="special">::</span><span class="identifier">plus</span><span class="special">&lt;</span><span class="identifier">calc_grammar</span><span class="special">,</span> <span class="identifier">calc_grammar</span><span class="special">&gt;</span>
          <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_default</span><span class="special">&lt;</span><span class="identifier">calc_grammar</span><span class="special">&gt;</span>
        <span class="special">&gt;</span>
      <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">when</span><span class="special">&lt;</span>
            <span class="identifier">proto</span><span class="special">::</span><span class="identifier">minus</span><span class="special">&lt;</span><span class="identifier">calc_grammar</span><span class="special">,</span> <span class="identifier">calc_grammar</span><span class="special">&gt;</span>
          <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_default</span><span class="special">&lt;</span><span class="identifier">calc_grammar</span><span class="special">&gt;</span>
        <span class="special">&gt;</span>
      <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">when</span><span class="special">&lt;</span>
            <span class="identifier">proto</span><span class="special">::</span><span class="identifier">multiplies</span><span class="special">&lt;</span><span class="identifier">calc_grammar</span><span class="special">,</span> <span class="identifier">calc_grammar</span><span class="special">&gt;</span>
          <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_default</span><span class="special">&lt;</span><span class="identifier">calc_grammar</span><span class="special">&gt;</span>
        <span class="special">&gt;</span>
        <span class="comment">// Note that we don't specify how division nodes are</span>
        <span class="comment">// handled here. Proto::external_transform is a placeholder</span>
        <span class="comment">// for an actual transform.</span>
      <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">when</span><span class="special">&lt;</span>
            <span class="identifier">divides_rule</span>
          <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">external_transform</span>
        <span class="special">&gt;</span>
    <span class="special">&gt;</span>
<span class="special">{};</span>

<span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">E</span><span class="special">&gt;</span> <span class="keyword">struct</span> <span class="identifier">calc_expr</span><span class="special">;</span>
<span class="keyword">struct</span> <span class="identifier">calc_domain</span> <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">domain</span><span class="special">&lt;</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">generator</span><span class="special">&lt;</span><span class="identifier">calc_expr</span><span class="special">&gt;</span> <span class="special">&gt;</span> <span class="special">{};</span>

<span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">E</span><span class="special">&gt;</span>
<span class="keyword">struct</span> <span class="identifier">calc_expr</span>
  <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">extends</span><span class="special">&lt;</span><span class="identifier">E</span><span class="special">,</span> <span class="identifier">calc_expr</span><span class="special">&lt;</span><span class="identifier">E</span><span class="special">&gt;,</span> <span class="identifier">calc_domain</span><span class="special">&gt;</span>
<span class="special">{</span>
    <span class="identifier">calc_expr</span><span class="special">(</span><span class="identifier">E</span> <span class="keyword">const</span> <span class="special">&amp;</span><span class="identifier">e</span> <span class="special">=</span> <span class="identifier">E</span><span class="special">())</span> <span class="special">:</span> <span class="identifier">calc_expr</span><span class="special">::</span><span class="identifier">proto_extends</span><span class="special">(</span><span class="identifier">e</span><span class="special">)</span> <span class="special">{}</span>
<span class="special">};</span>

<span class="identifier">calc_expr</span><span class="special">&lt;</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span><span class="identifier">placeholder</span><span class="special">&lt;</span><span class="identifier">mpl</span><span class="special">::</span><span class="identifier">int_</span><span class="special">&lt;</span><span class="number">0</span><span class="special">&gt;</span> <span class="special">&gt;</span> <span class="special">&gt;::</span><span class="identifier">type</span><span class="special">&gt;</span> <span class="identifier">_1</span><span class="special">;</span>
<span class="identifier">calc_expr</span><span class="special">&lt;</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span><span class="identifier">placeholder</span><span class="special">&lt;</span><span class="identifier">mpl</span><span class="special">::</span><span class="identifier">int_</span><span class="special">&lt;</span><span class="number">1</span><span class="special">&gt;</span> <span class="special">&gt;</span> <span class="special">&gt;::</span><span class="identifier">type</span><span class="special">&gt;</span> <span class="identifier">_2</span><span class="special">;</span>

<span class="comment">// Use proto::external_transforms to map from named grammar rules to</span>
<span class="comment">// transforms.</span>
<span class="keyword">struct</span> <span class="identifier">non_checked_division</span>
  <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">external_transforms</span><span class="special">&lt;</span>
        <span class="identifier">proto</span><span class="special">::</span><span class="identifier">when</span><span class="special">&lt;</span> <span class="identifier">divides_rule</span><span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_default</span><span class="special">&lt;</span><span class="identifier">calc_grammar</span><span class="special">&gt;</span> <span class="special">&gt;</span>
    <span class="special">&gt;</span>
<span class="special">{};</span>

<span class="keyword">struct</span> <span class="identifier">division_by_zero</span> <span class="special">:</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">exception</span> <span class="special">{};</span>

<span class="keyword">struct</span> <span class="identifier">do_checked_divide</span>
  <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">callable</span>
<span class="special">{</span>
    <span class="keyword">typedef</span> <span class="keyword">int</span> <span class="identifier">result_type</span><span class="special">;</span>
    <span class="keyword">int</span> <span class="keyword">operator</span><span class="special">()(</span><span class="keyword">int</span> <span class="identifier">left</span><span class="special">,</span> <span class="keyword">int</span> <span class="identifier">right</span><span class="special">)</span> <span class="keyword">const</span>
    <span class="special">{</span>
        <span class="keyword">if</span> <span class="special">(</span><span class="identifier">right</span> <span class="special">==</span> <span class="number">0</span><span class="special">)</span> <span class="keyword">throw</span> <span class="identifier">division_by_zero</span><span class="special">();</span>
        <span class="keyword">return</span> <span class="identifier">left</span> <span class="special">/</span> <span class="identifier">right</span><span class="special">;</span>
    <span class="special">}</span>
<span class="special">};</span>

<span class="comment">// Use proto::external_transforms again, this time to map the divides_rule</span>
<span class="comment">// to a transforms that performs checked division.</span>
<span class="keyword">struct</span> <span class="identifier">checked_division</span>
  <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">external_transforms</span><span class="special">&lt;</span>
        <span class="identifier">proto</span><span class="special">::</span><span class="identifier">when</span><span class="special">&lt;</span>
            <span class="identifier">divides_rule</span>
          <span class="special">,</span> <span class="identifier">do_checked_divide</span><span class="special">(</span><span class="identifier">calc_grammar</span><span class="special">(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_left</span><span class="special">),</span> <span class="identifier">calc_grammar</span><span class="special">(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_right</span><span class="special">))</span>
        <span class="special">&gt;</span>
    <span class="special">&gt;</span>
<span class="special">{};</span>

<span class="keyword">int</span> <span class="identifier">main</span><span class="special">()</span>
<span class="special">{</span>
    <span class="identifier">non_checked_division</span> <span class="identifier">non_checked</span><span class="special">;</span>
    <span class="keyword">int</span> <span class="identifier">result2</span> <span class="special">=</span> <span class="identifier">calc_grammar</span><span class="special">()(</span><span class="identifier">_1</span> <span class="special">/</span> <span class="identifier">_2</span><span class="special">,</span> <span class="identifier">fusion</span><span class="special">::</span><span class="identifier">make_vector</span><span class="special">(</span><span class="number">6</span><span class="special">,</span> <span class="number">2</span><span class="special">),</span> <span class="identifier">non_checked</span><span class="special">);</span>
    <span class="identifier">BOOST_ASSERT</span><span class="special">(</span><span class="identifier">result2</span> <span class="special">==</span> <span class="number">3</span><span class="special">);</span>

    <span class="keyword">try</span>
    <span class="special">{</span>
        <span class="identifier">checked_division</span> <span class="identifier">checked</span><span class="special">;</span>
        <span class="comment">// This should throw</span>
        <span class="keyword">int</span> <span class="identifier">result3</span> <span class="special">=</span> <span class="identifier">calc_grammar</span><span class="special">()(</span><span class="identifier">_1</span> <span class="special">/</span> <span class="identifier">_2</span><span class="special">,</span> <span class="identifier">fusion</span><span class="special">::</span><span class="identifier">make_vector</span><span class="special">(</span><span class="number">6</span><span class="special">,</span> <span class="number">0</span><span class="special">),</span> <span class="identifier">checked</span><span class="special">);</span>
        <span class="identifier">BOOST_ASSERT</span><span class="special">(</span><span class="keyword">false</span><span class="special">);</span> <span class="comment">// shouldn't get here!</span>
    <span class="special">}</span>
    <span class="keyword">catch</span><span class="special">(</span><span class="identifier">division_by_zero</span><span class="special">)</span>
    <span class="special">{</span>
        <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="string">"caught division by zero!\n"</span><span class="special">;</span>
    <span class="special">}</span>
<span class="special">}</span>
</pre>
<p>
        </p>
</div>
</div>
<div class="section">
<div class="titlepage"><div><div><h3 class="title">
<a name="boost_proto.users_guide.resources"></a><a class="link" href="users_guide.html#boost_proto.users_guide.resources" title="Background and Resources">Background and Resources</a>
</h3></div></div></div>
<p>
        Proto was initially developed as part of <a href="../../../libs/xpressive/index.html" target="_top">Boost.Xpressive</a>
        to simplify the job of transforming an expression template into an executable
        finite state machine capable of matching a regular expression. Since then,
        Proto has found application in the redesigned and improved Spirit-2 and the
        related Karma library. As a result of these efforts, Proto evolved into a
        generic and abstract grammar and tree transformation framework applicable
        in a wide variety of EDSL scenarios.
      </p>
<p>
        The grammar and tree transformation framework is modeled on Spirit's grammar
        and semantic action framework. The expression tree data structure is similar
        to Fusion data structures in many respects, and is interoperable with Fusion's
        iterators and algorithms.
      </p>
<p>
        The syntax for the grammar-matching features of <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">matches</span><span class="special">&lt;&gt;</span></code> is inspired by MPL's lambda expressions.
      </p>
<p>
        The idea for using function types for Proto's composite transforms is inspired
        by Aleksey Gurtovoy's <a href="http://lists.boost.org/Archives/boost/2002/11/39718.php" target="_top">"round"
        lambda</a> notation.
      </p>
<h5>
<a name="boost_proto.users_guide.resources.h0"></a>
        <span class="phrase"><a name="boost_proto.users_guide.resources.references"></a></span><a class="link" href="users_guide.html#boost_proto.users_guide.resources.references">References</a>
      </h5>
<div class="blockquote"><blockquote class="blockquote"><p>
          <a name="boost_proto.users_guide.resources.SYB"></a>Ren, D. and Erwig,
          M. 2006. A generic recursion toolbox for Haskell or: scrap your boilerplate
          systematically. In <span class="emphasis"><em>Proceedings of the 2006 ACM SIGPLAN Workshop
          on Haskell</em></span> (Portland, Oregon, USA, September 17 - 17, 2006).
          Haskell '06. ACM, New York, NY, 13-24. DOI=<a href="http://doi.acm.org/10.1145/1159842.1159845" target="_top">http://doi.acm.org/10.1145/1159842.1159845</a>
        </p></blockquote></div>
<h5>
<a name="boost_proto.users_guide.resources.h1"></a>
        <span class="phrase"><a name="boost_proto.users_guide.resources.further_reading"></a></span><a class="link" href="users_guide.html#boost_proto.users_guide.resources.further_reading">Further
        Reading</a>
      </h5>
<p>
        A technical paper about an earlier version of Proto was accepted into the
        <a href="http://lcsd.cs.tamu.edu/2007/" target="_top">ACM SIGPLAN Symposium on Library-Centric
        Software Design LCSD'07</a>, and can be found at <a href="http://lcsd.cs.tamu.edu/2007/final/1/1_Paper.pdf" target="_top">http://lcsd.cs.tamu.edu/2007/final/1/1_Paper.pdf</a>.
        The tree transforms described in that paper differ from what exists today.
      </p>
</div>
<div class="section">
<div class="titlepage"><div><div><h3 class="title">
<a name="boost_proto.users_guide.glossary"></a><a class="link" href="users_guide.html#boost_proto.users_guide.glossary" title="Glossary">Glossary</a>
</h3></div></div></div>
<div class="variablelist">
<p class="title"><b></b></p>
<dl class="variablelist">
<dt><span class="term"> <a name="boost_proto.users_guide.glossary.callable_transform"></a> callable transform</span></dt>
<dd><p>
              A transform of the form <code class="computeroutput"><span class="identifier">R</span><span class="special">(</span><span class="identifier">A0</span><span class="special">,</span><span class="identifier">A1</span><span class="special">,...)</span></code> (i.e., a function type) where
              <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">is_callable</span><span class="special">&lt;</span><span class="identifier">R</span><span class="special">&gt;::</span><span class="identifier">value</span></code> is <code class="computeroutput"><span class="keyword">true</span></code>.
              <code class="computeroutput"><span class="identifier">R</span></code> is treated as a polymorphic
              function object and the arguments are treated as transforms that yield
              the arguments to the function object.
            </p></dd>
<dt><span class="term"> <a name="boost_proto.users_guide.glossary.context"></a> context</span></dt>
<dd><p>
              In Proto, the term <span class="emphasis"><em>context</em></span> refers to an object
              that can be passed, along with an expression to evaluate, to the <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">eval</span><span class="special">()</span></code>
              function. The context determines how the expression is evaluated. All
              context structs define a nested <code class="computeroutput"><span class="identifier">eval</span><span class="special">&lt;&gt;</span></code> template that, when instantiated
              with a node tag type (e.g., <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">plus</span></code>),
              is a binary polymorphic function object that accepts an expression
              of that type and the context object. In this way, contexts associate
              behaviors with expression nodes.
            </p></dd>
<dt><span class="term"> <a name="boost_proto.users_guide.glossary.domain"></a> domain</span></dt>
<dd><p>
              In Proto, the term <span class="emphasis"><em>domain</em></span> refers to a type that
              associates expressions within that domain with a <span class="emphasis"><em>generator</em></span>
              for that domain and optionally a <span class="emphasis"><em>grammar</em></span> for the
              domain. Domains are used primarily to imbue expressions within that
              domain with additional members and to restrict Proto's operator overloads
              such that expressions not conforming to the domain's grammar are never
              created. Domains are empty structs that inherit from <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">domain</span><span class="special">&lt;&gt;</span></code>.
            </p></dd>
<dt><span class="term"> <a name="boost_proto.users_guide.glossary.dsl"></a> domain-specific language</span></dt>
<dd><p>
              A programming language that targets a particular problem space by providing
              programming idioms, abstractions and constructs that match the constructs
              within that problem space.
            </p></dd>
<dt><span class="term"> <a name="boost_proto.users_guide.glossary.edsl"></a> embedded domain-specific language</span></dt>
<dd><p>
              A domain-specific language implemented as a library. The language in
              which the library is written is called the "host" language,
              and the language implemented by the library is called the "embedded"
              language.
            </p></dd>
<dt><span class="term"> <a name="boost_proto.users_guide.glossary.expression"></a> expression</span></dt>
<dd><p>
              In Proto, an <span class="emphasis"><em>expression</em></span> is a heterogeneous tree
              where each node is either an instantiation of <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">expr</span><span class="special">&lt;&gt;</span></code>, <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">basic_expr</span><span class="special">&lt;&gt;</span></code> or some type that is an extension
              (via <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">extends</span><span class="special">&lt;&gt;</span></code>
              or <code class="computeroutput"><span class="identifier">BOOST_PROTO_EXTENDS</span><span class="special">()</span></code>) of such an instantiation.
            </p></dd>
<dt><span class="term"> <a name="boost_proto.users_guide.glossary.expression_template"></a> expression template</span></dt>
<dd><p>
              A C++ technique using templates and operator overloading to cause expressions
              to build trees that represent the expression for lazy evaluation later,
              rather than evaluating the expression eagerly. Some C++ libraries use
              expression templates to build embedded domain-specific languages.
            </p></dd>
<dt><span class="term"> <a name="boost_proto.users_guide.glossary.generator"></a> generator</span></dt>
<dd><p>
              In Proto, a <span class="emphasis"><em>generator</em></span> is a unary polymorphic function
              object that you specify when defining a <span class="emphasis"><em>domain</em></span>.
              After constructing a new expression, Proto passes the expression to
              your domain's generator for further processing. Often, the generator
              wraps the expression in an extension wrapper that adds additional members
              to it.
            </p></dd>
<dt><span class="term"> <a name="boost_proto.users_guide.glossary.grammar"></a> grammar</span></dt>
<dd><p>
              In Proto, a <span class="emphasis"><em>grammar</em></span> is a type that describes a
              subset of Proto expression types. Expressions in a domain must conform
              to that domain's grammar. The <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">matches</span><span class="special">&lt;&gt;</span></code> metafunction evaluates whether
              an expression type matches a grammar. Grammars are either primitives
              such as <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_</span></code>, composites such as <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">plus</span><span class="special">&lt;&gt;</span></code>,
              control structures such as <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">or_</span><span class="special">&lt;&gt;</span></code>, or some type derived from
              a grammar.
            </p></dd>
<dt><span class="term"> <a name="boost_proto.users_guide.glossary.object_transform"></a> object transform</span></dt>
<dd><p>
              A transform of the form <code class="computeroutput"><span class="identifier">R</span><span class="special">(</span><span class="identifier">A0</span><span class="special">,</span><span class="identifier">A1</span><span class="special">,...)</span></code> (i.e., a function type) where
              <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">is_callable</span><span class="special">&lt;</span><span class="identifier">R</span><span class="special">&gt;::</span><span class="identifier">value</span></code> is <code class="computeroutput"><span class="keyword">false</span></code>.
              <code class="computeroutput"><span class="identifier">R</span></code> is treated as the
              type of an object to construct and the arguments are treated as transforms
              that yield the parameters to the constructor.
            </p></dd>
<dt><span class="term"> <a name="boost_proto.users_guide.glossary.polymorphic_function_object"></a> polymorphic function object</span></dt>
<dd><p>
              An instance of a class type with an overloaded function call operator
              and a nested <code class="computeroutput"><span class="identifier">result_type</span></code>
              typedef or <code class="computeroutput"><span class="identifier">result</span><span class="special">&lt;&gt;</span></code>
              template for calculating the return type of the function call operator.
            </p></dd>
<dt><span class="term"> <a name="boost_proto.users_guide.glossary.primitive_transform"></a> primitive transform</span></dt>
<dd><p>
              A type that defines a kind of polymorphic function object that takes
              three arguments: expression, state, and data. Primitive transforms
              can be used to compose callable transforms and object transforms.
            </p></dd>
<dt><span class="term"> <a name="boost_proto.users_guide.glossary.subdomain"></a> sub-domain</span></dt>
<dd><p>
              A sub-domain is a domain that declares another domain as its super-domain.
              Expressions in sub-domains can be combined with expressions in the
              super-domain, and the resulting expression is in the super-domain.
            </p></dd>
<dt><span class="term"> <a name="boost_proto.users_guide.glossary.transform"></a> transform</span></dt>
<dd><p>
              Transforms are used to manipulate expression trees. They come in three
              flavors: primitive transforms, callable transforms, or object transforms.
              A transform <code class="computeroutput"><em class="replaceable"><code>T</code></em></code> can be made into
              a ternary polymorphic function object with <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">when</span><span class="special">&lt;&gt;</span></code>, as in <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">when</span><span class="special">&lt;</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_</span><span class="special">,</span> <em class="replaceable"><code>T</code></em><span class="special">&gt;</span></code>.
              Such a function object accepts <span class="emphasis"><em>expression</em></span>, <span class="emphasis"><em>state</em></span>,
              and <span class="emphasis"><em>data</em></span> parameters, and computes a result from
              them.
            </p></dd>
</dl>
</div>
</div>
<div class="footnotes">
<br><hr style="width:100; text-align:left;margin-left: 0">
<div id="ftn.boost_proto.users_guide.front_end.customizing_expressions_in_your_domain.per_domain_as_child.f0" class="footnote"><p><a href="#boost_proto.users_guide.front_end.customizing_expressions_in_your_domain.per_domain_as_child.f0" class="para"><sup class="para">[28] </sup></a>
              It's not always possible to hold something by value. By default, <code class="computeroutput"><a class="link" href="../boost/proto/as_expr_idp731118320.html" title="Function as_expr">proto::as_expr()</a></code> makes an exception
              for functions, abstract types, and iostreams (types derived from <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">ios_base</span></code>). These objects are held
              by reference. All others are held by value, even arrays.
            </p></div>
<div id="ftn.boost_proto.users_guide.back_end.expression_transformation.example__calculator_arity.f0" class="footnote"><p><a href="#boost_proto.users_guide.back_end.expression_transformation.example__calculator_arity.f0" class="para"><sup class="para">[29] </sup></a>
              This error message was generated with Microsoft Visual C++ 9.0. Different
              compilers will emit different messages with varying degrees of readability.
            </p></div>
</div>
</div>
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<td align="right"><div class="copyright-footer">Copyright &#169; 2008 Eric Niebler<p>
        Distributed under the Boost Software License, Version 1.0. (See accompanying
        file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
      </p>
</div></td>
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