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<a name="proto.appendices"></a><a class="link" href="appendices.html" title="Appendices">Appendices</a>
</h2></div></div></div>
<div class="toc"><dl>
<dt><span class="section"><a href="appendices.html#boost_proto.appendices.release_notes">Appendix A: Release
Notes</a></span></dt>
<dt><span class="section"><a href="appendices.html#boost_proto.appendices.history">Appendix B: History</a></span></dt>
<dt><span class="section"><a href="appendices.html#boost_proto.appendices.rationale">Appendix C: Rationale</a></span></dt>
<dt><span class="section"><a href="appendices.html#boost_proto.appendices.implementation">Appendix D: Implementation
Notes</a></span></dt>
<dt><span class="section"><a href="appendices.html#boost_proto.appendices.acknowledgements">Appendix E:
Acknowledgements</a></span></dt>
</dl></div>
<div class="section">
<div class="titlepage"><div><div><h3 class="title">
<a name="boost_proto.appendices.release_notes"></a><a class="link" href="appendices.html#boost_proto.appendices.release_notes" title="Appendix A: Release Notes">Appendix A: Release
Notes</a>
</h3></div></div></div>
<h5>
<a name="boost_proto.appendices.release_notes.h0"></a>
<span><a name="boost_proto.appendices.release_notes.boost_1_51"></a></span><a class="link" href="appendices.html#boost_proto.appendices.release_notes.boost_1_51">Boost
1.51</a>
</h5>
<p>
<span class="bold"><strong>Unpacking Expressions</strong></span>
</p>
<p>
In Boost 1.51, Proto got simple unpacking patterns. When working with Proto
transforms, unpacking expressions are useful for unpacking the children of
an expression into a function call or an object constructor, while optionally
applying some transformations to each child in turn.
</p>
<p>
See the <a class="link" href="users_guide.html#boost_proto.users_guide.back_end.expression_transformation.unpacking_expressions" title="Unpacking Expressions">Unpacking
Expressions</a> section for more information.
</p>
<h5>
<a name="boost_proto.appendices.release_notes.h1"></a>
<span><a name="boost_proto.appendices.release_notes.boost_1_44"></a></span><a class="link" href="appendices.html#boost_proto.appendices.release_notes.boost_1_44">Boost
1.44</a>
</h5>
<p>
<span class="bold"><strong>Behavior Change: proto::and_<></strong></span>
</p>
<p>
In Boost 1.44, the behavior of <code class="computeroutput"><a class="link" href="../boost/proto/and_.html" title="Struct template and_">proto::and_<></a></code>
as a transform changed. Previously, it only applied the transform associated
with the last grammar in the set. Now, it applies all the transforms but
only returns the result of the last. That makes it behave like C++'s comma
operator. For example, a grammar such as:
</p>
<pre class="programlisting"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">and_</span><span class="special"><</span> <span class="identifier">G0</span><span class="special">,</span> <span class="identifier">G1</span><span class="special">,</span> <span class="identifier">G2</span> <span class="special">></span>
</pre>
<p>
when evaluated with an expression <code class="computeroutput"><span class="identifier">e</span></code>
now behaves like this:
</p>
<pre class="programlisting"><span class="special">((</span><span class="keyword">void</span><span class="special">)</span><span class="identifier">G0</span><span class="special">()(</span><span class="identifier">e</span><span class="special">),</span> <span class="special">(</span><span class="keyword">void</span><span class="special">)</span><span class="identifier">G1</span><span class="special">()(</span><span class="identifier">e</span><span class="special">),</span> <span class="identifier">G2</span><span class="special">()(</span><span class="identifier">e</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>
Why the void casts? It's to avoid argument-dependent lookup, which might
find an overloaded comma operator.
</p></td></tr>
</table></div>
<p>
<span class="bold"><strong>Behavior Change: proto::as_expr() and proto::as_child()</strong></span>
</p>
<p>
The functions <code class="computeroutput"><a class="link" href="../boost/proto/as_expr_idp308826416.html" title="Function as_expr">proto::as_expr()</a></code> and <code class="computeroutput"><a class="link" href="../boost/proto/as_child_idp308848384.html" title="Function as_child">proto::as_child()</a></code>
are used to guarantee that an object is a Proto expression by turning it
into one if it is not already, using an optionally specified domain. In previous
releases, when these functions were passed a Proto expression in a domain
different to the one specified, they would apply the specified domain's generator,
resulting in a twice-wrapped expression. This behavior was surprising to
some users.
</p>
<p>
The new behavior of these two functions is to always leave Proto expressions
alone, regardless of the expressions' domains.
</p>
<p>
<span class="bold"><strong>Behavior Change: proto::(pod_)generator<> and
proto::basic_expr<></strong></span>
</p>
<p>
Users familiar with Proto's extension mechanism have probably used either
<code class="computeroutput"><a class="link" href="../boost/proto/generator.html" title="Struct template generator">proto::generator<></a></code> or <code class="computeroutput"><a class="link" href="../boost/proto/pod_generator.html" title="Struct template pod_generator">proto::pod_generator<></a></code>
with a wrapper template when defining their domain. In the past, Proto would
instantiate your wrapper template with instances of <code class="computeroutput"><a class="link" href="../boost/proto/expr.html" title="Struct template expr">proto::expr<></a></code>.
In Boost 1.44, Proto now instantiates your wrapper template with instances
of a new type: <code class="computeroutput"><a class="link" href="../boost/proto/basic_expr.html" title="Struct template basic_expr">proto::basic_expr<></a></code>.
</p>
<p>
For instance:
</p>
<pre class="programlisting"><span class="comment">// An expression wrapper</span>
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">Expr</span><span class="special">></span>
<span class="keyword">struct</span> <span class="identifier">my_expr_wrapper</span><span class="special">;</span>
<span class="comment">// A domain</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"><</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">generator</span><span class="special"><</span> <span class="identifier">my_expr_wrapper</span> <span class="special">></span> <span class="special">></span>
<span class="special">{};</span>
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">Expr</span><span class="special">></span>
<span class="keyword">struct</span> <span class="identifier">my_expr_wrapper</span>
<span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">extends</span><span class="special"><</span><span class="identifier">Expr</span><span class="special">,</span> <span class="identifier">my_expr_wrapper</span><span class="special"><</span><span class="identifier">Expr</span><span class="special">>,</span> <span class="identifier">my_domain</span><span class="special">></span>
<span class="special">{</span>
<span class="comment">// Before 1.44, Expr was an instance of proto::expr<></span>
<span class="comment">// In 1.44, Expr is an instance of proto::basic_expr<></span>
<span class="special">};</span>
</pre>
<p>
The motivation for this change was to improve compile times. <code class="computeroutput"><a class="link" href="../boost/proto/expr.html" title="Struct template expr">proto::expr<></a></code>
is an expensive type to instantiate because it defines a host of member functions.
When defining your own expression wrapper, the instance of <code class="computeroutput"><a class="link" href="../boost/proto/expr.html" title="Struct template expr">proto::expr<></a></code>
sits as a hidden data member function in your wrapper and the members of
<code class="computeroutput"><a class="link" href="../boost/proto/expr.html" title="Struct template expr">proto::expr<></a></code> go unused. Therefore,
the cost of those member functions is wasted. In contrast, <code class="computeroutput"><a class="link" href="../boost/proto/basic_expr.html" title="Struct template basic_expr">proto::basic_expr<></a></code>
is a very lightweight type with no member functions at all.
</p>
<p>
The vast majority of programs should recompile without any source changes.
However, if somewhere you are assuming that you will be given instances specifically
of <code class="computeroutput"><a class="link" href="../boost/proto/expr.html" title="Struct template expr">proto::expr<></a></code>, your code will break.
</p>
<p>
<span class="bold"><strong>New Feature: Sub-domains</strong></span>
</p>
<p>
In Boost 1.44, Proto introduces an important new feature called "sub-domains".
This gives you a way to spcify that one domain is compatible with another
such that expressions in one domain can be freely mixed with expressions
in another. You can define one domain to be the sub-domain of another by
using the third template parameter of <code class="computeroutput"><a class="link" href="../boost/proto/domain.html" title="Struct template domain">proto::domain<></a></code>.
</p>
<p>
For instance:
</p>
<pre class="programlisting"><span class="comment">// Not shown: define some expression</span>
<span class="comment">// generators genA and genB</span>
<span class="keyword">struct</span> <span class="identifier">A</span>
<span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">domain</span><span class="special"><</span> <span class="identifier">genA</span><span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_</span> <span class="special">></span>
<span class="special">{};</span>
<span class="comment">// Define a domain B that is the sub-domain</span>
<span class="comment">// of domain A.</span>
<span class="keyword">struct</span> <span class="identifier">B</span>
<span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">domain</span><span class="special"><</span> <span class="identifier">genB</span><span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_</span><span class="special">,</span> <span class="identifier">A</span> <span class="special">></span>
<span class="special">{};</span>
</pre>
<p>
Expressions in domains <code class="computeroutput"><span class="identifier">A</span></code>
and <code class="computeroutput"><span class="identifier">B</span></code> can have different
wrappers (hence, different interfaces), but they can be combined into larger
expressions. Without a sub-domain relationship, this would have been an error.
The domain of the resulting expression in this case would be <code class="computeroutput"><span class="identifier">A</span></code>.
</p>
<p>
The complete description of sub-domains can be found in the reference sections
for <code class="computeroutput"><a class="link" href="../boost/proto/domain.html" title="Struct template domain">proto::domain<></a></code> and <code class="computeroutput"><a class="link" href="../boost/proto/deduce_domain.html" title="Struct deduce_domain">proto::deduce_domain</a></code>.
</p>
<p>
<span class="bold"><strong>New Feature: Domain-specific as_expr() and as_child()</strong></span>
</p>
<p>
Proto has always allowed users to customize expressions post-hoc by specifying
a Generator when defining their domain. But it has never allowed users to
control how Proto assembles sub-expressions in the first place. As of Boost
1.44, users now have this power.
</p>
<p>
Users defining their own domain can now specify how <code class="computeroutput"><a class="link" href="../boost/proto/as_expr_idp308826416.html" title="Function as_expr">proto::as_expr()</a></code>
and <code class="computeroutput"><a class="link" href="../boost/proto/as_child_idp308848384.html" title="Function as_child">proto::as_child()</a></code> work in their domain. They
can do this easily by defining nested class templates named <code class="computeroutput"><span class="identifier">as_expr</span></code> and/or <code class="computeroutput"><span class="identifier">as_child</span></code>
within their domain class.
</p>
<p>
For example:
</p>
<pre class="programlisting"><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"><</span> <span class="identifier">my_generator</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">domain</span><span class="special"><</span> <span class="identifier">my_generator</span> <span class="special">></span>
<span class="identifier">base_domain</span><span class="special">;</span>
<span class="comment">// For my_domain, as_child does the same as</span>
<span class="comment">// what as_expr does by default.</span>
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">></span>
<span class="keyword">struct</span> <span class="identifier">as_child</span>
<span class="special">:</span> <span class="identifier">base_domain</span><span class="special">::</span><span class="identifier">as_expr</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span>
<span class="special">{};</span>
<span class="special">};</span>
</pre>
<p>
In the above example, <code class="computeroutput"><span class="identifier">my_domain</span><span class="special">::</span><span class="identifier">as_child</span><span class="special"><></span></code> simply defers to <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">domain</span><span class="special">::</span><span class="identifier">as_expr</span><span class="special"><></span></code>. This has the nice effect of causing
all terminals to be captured by value instead of by reference, and to likewise
store child expressions by value. The result is that expressions in <code class="computeroutput"><span class="identifier">my_domain</span></code> are safe to store in <code class="computeroutput"><span class="keyword">auto</span></code> variables because they will not have
dangling references to intermediate temporary expressions. (Naturally, it
also means that expression construction has extra runtime overhead of copying
that the compiler may or may not be able to optimize away.)
</p>
<h5>
<a name="boost_proto.appendices.release_notes.h2"></a>
<span><a name="boost_proto.appendices.release_notes.boost_1_43"></a></span><a class="link" href="appendices.html#boost_proto.appendices.release_notes.boost_1_43">Boost
1.43</a>
</h5>
<p>
In Boost 1.43, the recommended usage of <code class="computeroutput"><a class="link" href="../boost/proto/extends.html" title="Struct template extends">proto::extends<></a></code>
changed slightly. The new usage looks like this:
</p>
<pre class="programlisting"><span class="comment">// my_expr is an expression extension of the Expr parameter</span>
<span class="keyword">template</span><span class="special"><</span><span class="keyword">typename</span> <span class="identifier">Expr</span><span class="special">></span>
<span class="keyword">struct</span> <span class="identifier">my_expr</span>
<span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">extends</span><span class="special"><</span><span class="identifier">Expr</span><span class="special">,</span> <span class="identifier">my_expr</span><span class="special"><</span><span class="identifier">Expr</span><span class="special">>,</span> <span class="identifier">my_domain</span><span class="special">></span>
<span class="special">{</span>
<span class="identifier">my_expr</span><span class="special">(</span><span class="identifier">Expr</span> <span class="keyword">const</span> <span class="special">&</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">proto</span><span class="special">::</span><span class="identifier">extends</span><span class="special"><</span><span class="identifier">Expr</span><span class="special">,</span> <span class="identifier">my_expr</span><span class="special">,</span> <span class="identifier">my_domain</span><span class="special">>(</span><span class="identifier">expr</span><span class="special">)</span>
<span class="special">{}</span>
<span class="comment">// NEW: use the following macro to bring</span>
<span class="comment">// proto::extends::operator= into scope.</span>
<span class="identifier">BOOST_PROTO_EXTENDS_USING_ASSIGN</span><span class="special">(</span><span class="identifier">my_expr</span><span class="special">)</span>
<span class="special">};</span>
</pre>
<p>
The new thing is the use of the <code class="literal"><code class="computeroutput"><a class="link" href="../BOOST_PROTO_EXTENDS_USING_ASSIGN.html" title="Macro BOOST_PROTO_EXTENDS_USING_ASSIGN">BOOST_PROTO_EXTENDS_USING_ASSIGN</a></code>()</code>
macro. To allow assignment operators to build expression trees, <code class="computeroutput"><a class="link" href="../boost/proto/extends.html" title="Struct template extends">proto::extends<></a></code> overloads the assignment
operator. However, for the <code class="computeroutput"><span class="identifier">my_expr</span></code>
template, the compiler generates a default copy assignment operator that
hides the ones in <code class="computeroutput"><a class="link" href="../boost/proto/extends.html" title="Struct template extends">proto::extends<></a></code>. This is often not desired
(although it depends on the syntax you want to allow).
</p>
<p>
Previously, the recommended usage was to do this:
</p>
<pre class="programlisting"><span class="comment">// my_expr is an expression extension of the Expr parameter</span>
<span class="keyword">template</span><span class="special"><</span><span class="keyword">typename</span> <span class="identifier">Expr</span><span class="special">></span>
<span class="keyword">struct</span> <span class="identifier">my_expr</span>
<span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">extends</span><span class="special"><</span><span class="identifier">Expr</span><span class="special">,</span> <span class="identifier">my_expr</span><span class="special"><</span><span class="identifier">Expr</span><span class="special">>,</span> <span class="identifier">my_domain</span><span class="special">></span>
<span class="special">{</span>
<span class="identifier">my_expr</span><span class="special">(</span><span class="identifier">Expr</span> <span class="keyword">const</span> <span class="special">&</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">proto</span><span class="special">::</span><span class="identifier">extends</span><span class="special"><</span><span class="identifier">Expr</span><span class="special">,</span> <span class="identifier">my_expr</span><span class="special">,</span> <span class="identifier">my_domain</span><span class="special">>(</span><span class="identifier">expr</span><span class="special">)</span>
<span class="special">{}</span>
<span class="comment">// OLD: don't do it like this anymore.</span>
<span class="keyword">using</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">extends</span><span class="special"><</span><span class="identifier">Expr</span><span class="special">,</span> <span class="identifier">my_expr</span><span class="special">,</span> <span class="identifier">my_domain</span><span class="special">>::</span><span class="keyword">operator</span><span class="special">=;</span>
<span class="special">};</span>
</pre>
<p>
While this works in the majority of cases, it still doesn't suppress the
implicit generation of the default assignment operator. As a result, expressions
of the form <code class="computeroutput"><span class="identifier">a</span> <span class="special">=</span>
<span class="identifier">b</span></code> could either build an expression
template or do a copy assignment depending on whether the types of <code class="computeroutput"><span class="identifier">a</span></code> and <code class="computeroutput"><span class="identifier">b</span></code>
happen to be the same. That can lead to subtle bugs, so the behavior was
changed.
</p>
<p>
The <code class="literal"><code class="computeroutput"><a class="link" href="../BOOST_PROTO_EXTENDS_USING_ASSIGN.html" title="Macro BOOST_PROTO_EXTENDS_USING_ASSIGN">BOOST_PROTO_EXTENDS_USING_ASSIGN</a></code>()</code>
brings into scope the assignment operators defined in <code class="computeroutput"><a class="link" href="../boost/proto/extends.html" title="Struct template extends">proto::extends<></a></code>
as well as suppresses the generation of the copy assignment operator.
</p>
<p>
Also note that the <code class="computeroutput"><a class="link" href="../boost/proto/literal.html" title="Struct template literal">proto::literal<></a></code> class template, which
uses <code class="computeroutput"><a class="link" href="../boost/proto/extends.html" title="Struct template extends">proto::extends<></a></code>, has been chaged to use
<code class="literal"><code class="computeroutput"><a class="link" href="../BOOST_PROTO_EXTENDS_USING_ASSIGN.html" title="Macro BOOST_PROTO_EXTENDS_USING_ASSIGN">BOOST_PROTO_EXTENDS_USING_ASSIGN</a></code>()</code>.
The implications are highlighted in the sample code below:
</p>
<pre class="programlisting"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">literal</span><span class="special"><</span><span class="keyword">int</span><span class="special">></span> <span class="identifier">a</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">2</span><span class="special">);</span> <span class="comment">// two non-const proto literals</span>
<span class="identifier">proto</span><span class="special">::</span><span class="identifier">literal</span><span class="special"><</span><span class="keyword">int</span><span class="special">></span> <span class="keyword">const</span> <span class="identifier">c</span><span class="special">(</span><span class="number">3</span><span class="special">);</span> <span class="comment">// a const proto literal</span>
<span class="identifier">a</span> <span class="special">=</span> <span class="identifier">b</span><span class="special">;</span> <span class="comment">// No-op. Builds an expression tree and discards it.</span>
<span class="comment">// Same behavior in 1.42 and 1.43.</span>
<span class="identifier">a</span> <span class="special">=</span> <span class="identifier">c</span><span class="special">;</span> <span class="comment">// CHANGE! In 1.42, this performed copy assignment, causing</span>
<span class="comment">// a's value to change to 3. In 1.43, the behavior is now</span>
<span class="comment">// the same as above: build and discard an expression tree.</span>
</pre>
</div>
<div class="section">
<div class="titlepage"><div><div><h3 class="title">
<a name="boost_proto.appendices.history"></a><a class="link" href="appendices.html#boost_proto.appendices.history" title="Appendix B: History">Appendix B: History</a>
</h3></div></div></div>
<div class="variablelist">
<p class="title"><b></b></p>
<dl>
<dt><span class="term">August 13, 2010</span></dt>
<dd><p>
Boost 1.44: Proto gets sub-domains and per-domain control of <code class="computeroutput"><a class="link" href="../boost/proto/as_expr_idp308826416.html" title="Function as_expr">proto::as_expr()</a></code> and <code class="computeroutput"><a class="link" href="../boost/proto/as_child_idp308848384.html" title="Function as_child">proto::as_child()</a></code> to meet the needs
of Phoenix3.
</p></dd>
<dt><span class="term">August 11, 2008</span></dt>
<dd><p>
Proto v4 is merged to Boost trunk with more powerful transform protocol.
</p></dd>
<dt><span class="term">April 7, 2008</span></dt>
<dd><p>
Proto is accepted into Boost.
</p></dd>
<dt><span class="term">March 1, 2008</span></dt>
<dd><p>
Proto's Boost review begins.
</p></dd>
<dt><span class="term">January 11, 2008</span></dt>
<dd><p>
Boost.Proto v3 brings separation of grammars and transforms and a "round"
lambda syntax for defining transforms in-place.
</p></dd>
<dt><span class="term">April 15, 2007</span></dt>
<dd><p>
Boost.Xpressive is ported from Proto compilers to Proto transforms.
Support for old Proto compilers is dropped.
</p></dd>
<dt><span class="term">April 4, 2007</span></dt>
<dd><p>
Preliminary submission of Proto to Boost.
</p></dd>
<dt><span class="term">December 11, 2006</span></dt>
<dd><p>
The idea for transforms that decorate grammar rules is born in a private
email discussion with Joel de Guzman and Hartmut Kaiser. The first
transforms are committed to CVS 5 days later on December 16.
</p></dd>
<dt><span class="term">November 1, 2006</span></dt>
<dd><p>
The idea for <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">matches</span><span class="special"><></span></code> and the whole grammar facility
is hatched during a discussion with Hartmut Kaiser on the spirit-devel
list. The first version of <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">matches</span><span class="special"><></span></code> is checked into CVS 3 days
later. Message is <a href="http://osdir.com/ml/parsers.spirit.devel/2006-11/msg00003.html" target="_top">here</a>.
</p></dd>
<dt><span class="term">October 28, 2006</span></dt>
<dd><p>
Proto is reborn, this time with a uniform expression types that are
POD. Announcement is <a href="http://lists.boost.org/Archives/boost/2006/10/112453.php" target="_top">here</a>.
</p></dd>
<dt><span class="term">April 20, 2005</span></dt>
<dd><p>
Proto is born as a major refactorization of Boost.Xpressive's meta-programming.
Proto offers expression types, operator overloads and "compilers",
an early formulation of what later became transforms. Announcement
is <a href="http://lists.boost.org/Archives/boost/2005/04/85256.php" target="_top">here</a>.
</p></dd>
</dl>
</div>
</div>
<div class="section">
<div class="titlepage"><div><div><h3 class="title">
<a name="boost_proto.appendices.rationale"></a><a class="link" href="appendices.html#boost_proto.appendices.rationale" title="Appendix C: Rationale">Appendix C: Rationale</a>
</h3></div></div></div>
<div class="toc"><dl>
<dt><span class="section"><a href="appendices.html#boost_proto.appendices.rationale.static_initialization">Static
Initialization</a></span></dt>
<dt><span class="section"><a href="appendices.html#boost_proto.appendices.rationale.preprocessor">Why
Not Reuse MPL, Fusion, et cetera?</a></span></dt>
</dl></div>
<div class="section">
<div class="titlepage"><div><div><h4 class="title">
<a name="boost_proto.appendices.rationale.static_initialization"></a><a class="link" href="appendices.html#boost_proto.appendices.rationale.static_initialization" title="Static Initialization">Static
Initialization</a>
</h4></div></div></div>
<p>
Proto expression types are PODs (Plain Old Data), and do not have constructors.
They are brace-initialized, as follows:
</p>
<pre class="programlisting"><span class="identifier">terminal</span><span class="special"><</span><span class="keyword">int</span><span class="special">>::</span><span class="identifier">type</span> <span class="keyword">const</span> <span class="identifier">_i</span> <span class="special">=</span> <span class="special">{</span><span class="number">1</span><span class="special">};</span>
</pre>
<p>
The reason is so that expression objects like <code class="computeroutput"><span class="identifier">_i</span></code>
above can be <span class="emphasis"><em>statically initialized</em></span>. Why is static
initialization important? The terminals of many embedded domain-specific
languages are likely to be global const objects, like <code class="computeroutput"><span class="identifier">_1</span></code>
and <code class="computeroutput"><span class="identifier">_2</span></code> from the Boost Lambda
Library. Were these object to require run-time initialization, it might
be possible to use these objects before they are initialized. That would
be bad. Statically initialized objects cannot be misused that way.
</p>
</div>
<div class="section">
<div class="titlepage"><div><div><h4 class="title">
<a name="boost_proto.appendices.rationale.preprocessor"></a><a class="link" href="appendices.html#boost_proto.appendices.rationale.preprocessor" title="Why Not Reuse MPL, Fusion, et cetera?">Why
Not Reuse MPL, Fusion, et cetera?</a>
</h4></div></div></div>
<p>
Anyone who has peeked at Proto's source code has probably wondered, "Why
all the dirty preprocessor gunk? Couldn't this have been all implemented
cleanly on top of libraries like MPL and Fusion?" The answer is that
Proto could have been implemented this way, and in fact was at one point.
The problem is that template metaprogramming (TMP) makes for longer compile
times. As a foundation upon which other TMP-heavy libraries will be built,
Proto itself should be as lightweight as possible. That is achieved by
prefering preprocessor metaprogramming to template metaprogramming. Expanding
a macro is far more efficient than instantiating a template. In some cases,
the "clean" version takes 10x longer to compile than the "dirty"
version.
</p>
<p>
The "clean and slow" version of Proto can still be found at http://svn.boost.org/svn/boost/branches/proto/v3.
Anyone who is interested can download it and verify that it is, in fact,
unusably slow to compile. Note that this branch's development was abandoned,
and it does not conform exactly with Proto's current interface.
</p>
</div>
</div>
<div class="section">
<div class="titlepage"><div><div><h3 class="title">
<a name="boost_proto.appendices.implementation"></a><a class="link" href="appendices.html#boost_proto.appendices.implementation" title="Appendix D: Implementation Notes">Appendix D: Implementation
Notes</a>
</h3></div></div></div>
<div class="toc"><dl>
<dt><span class="section"><a href="appendices.html#boost_proto.appendices.implementation.sfinae">Quick-n-Dirty
Type Categorization</a></span></dt>
<dt><span class="section"><a href="appendices.html#boost_proto.appendices.implementation.function_arity">Detecting
the Arity of Function Objects</a></span></dt>
</dl></div>
<div class="section">
<div class="titlepage"><div><div><h4 class="title">
<a name="boost_proto.appendices.implementation.sfinae"></a><a class="link" href="appendices.html#boost_proto.appendices.implementation.sfinae" title="Quick-n-Dirty Type Categorization">Quick-n-Dirty
Type Categorization</a>
</h4></div></div></div>
<p>
Much has already been written about dispatching on type traits using SFINAE
(Substitution Failure Is Not An Error) techniques in C++. There is a Boost
library, Boost.Enable_if, to make the technique idiomatic. Proto dispatches
on type traits extensively, but it doesn't use <code class="computeroutput"><span class="identifier">enable_if</span><span class="special"><></span></code> very often. Rather, it dispatches
based on the presence or absence of nested types, often typedefs for void.
</p>
<p>
Consider the implementation of <code class="computeroutput"><span class="identifier">is_expr</span><span class="special"><></span></code>. It could have been written as
something like this:
</p>
<pre class="programlisting"><span class="keyword">template</span><span class="special"><</span><span class="keyword">typename</span> <span class="identifier">T</span><span class="special">></span>
<span class="keyword">struct</span> <span class="identifier">is_expr</span>
<span class="special">:</span> <span class="identifier">is_base_and_derived</span><span class="special"><</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">some_expr_base</span><span class="special">,</span> <span class="identifier">T</span><span class="special">></span>
<span class="special">{};</span>
</pre>
<p>
Rather, it is implemented as this:
</p>
<pre class="programlisting"><span class="keyword">template</span><span class="special"><</span><span class="keyword">typename</span> <span class="identifier">T</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">Void</span> <span class="special">=</span> <span class="keyword">void</span><span class="special">></span>
<span class="keyword">struct</span> <span class="identifier">is_expr</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"><</span><span class="keyword">typename</span> <span class="identifier">T</span><span class="special">></span>
<span class="keyword">struct</span> <span class="identifier">is_expr</span><span class="special"><</span><span class="identifier">T</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">T</span><span class="special">::</span><span class="identifier">proto_is_expr_</span><span class="special">></span>
<span class="special">:</span> <span class="identifier">mpl</span><span class="special">::</span><span class="identifier">true_</span>
<span class="special">{};</span>
</pre>
<p>
This relies on the fact that the specialization will be preferred if <code class="computeroutput"><span class="identifier">T</span></code> has a nested <code class="computeroutput"><span class="identifier">proto_is_expr_</span></code>
that is a typedef for <code class="computeroutput"><span class="keyword">void</span></code>.
All Proto expression types have such a nested typedef.
</p>
<p>
Why does Proto do it this way? The reason is because, after running extensive
benchmarks while trying to improve compile times, I have found that this
approach compiles faster. It requires exactly one template instantiation.
The other approach requires at least 2: <code class="computeroutput"><span class="identifier">is_expr</span><span class="special"><></span></code> and <code class="computeroutput"><span class="identifier">is_base_and_derived</span><span class="special"><></span></code>, plus whatever templates <code class="computeroutput"><span class="identifier">is_base_and_derived</span><span class="special"><></span></code>
may instantiate.
</p>
</div>
<div class="section">
<div class="titlepage"><div><div><h4 class="title">
<a name="boost_proto.appendices.implementation.function_arity"></a><a class="link" href="appendices.html#boost_proto.appendices.implementation.function_arity" title="Detecting the Arity of Function Objects">Detecting
the Arity of Function Objects</a>
</h4></div></div></div>
<p>
In several places, Proto needs to know whether or not a function object
<code class="computeroutput"><span class="identifier">Fun</span></code> can be called with
certain parameters and take a fallback action if not. This happens in
<code class="computeroutput"><a class="link" href="../boost/proto/context/callable_context.html" title="Struct template callable_context">proto::callable_context<></a></code>
and in the <code class="computeroutput"><a class="link" href="../boost/proto/call.html" title="Struct template call">proto::call<></a></code> transform. How does
Proto know? It involves some tricky metaprogramming. Here's how.
</p>
<p>
Another way of framing the question is by trying to implement the following
<code class="computeroutput"><span class="identifier">can_be_called</span><span class="special"><></span></code>
Boolean metafunction, which checks to see if a function object <code class="computeroutput"><span class="identifier">Fun</span></code> can be called with parameters of
type <code class="computeroutput"><span class="identifier">A</span></code> and <code class="computeroutput"><span class="identifier">B</span></code>:
</p>
<pre class="programlisting"><span class="keyword">template</span><span class="special"><</span><span class="keyword">typename</span> <span class="identifier">Fun</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">B</span><span class="special">></span>
<span class="keyword">struct</span> <span class="identifier">can_be_called</span><span class="special">;</span>
</pre>
<p>
First, we define the following <code class="computeroutput"><span class="identifier">dont_care</span></code>
struct, which has an implicit conversion from anything. And not just any
implicit conversion; it has a ellipsis conversion, which is the worst possible
conversion for the purposes of overload resolution:
</p>
<pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">dont_care</span>
<span class="special">{</span>
<span class="identifier">dont_care</span><span class="special">(...);</span>
<span class="special">};</span>
</pre>
<p>
We also need some private type known only to us with an overloaded comma
operator (!), and some functions that detect the presence of this type
and return types with different sizes, as follows:
</p>
<pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">private_type</span>
<span class="special">{</span>
<span class="identifier">private_type</span> <span class="keyword">const</span> <span class="special">&</span><span class="keyword">operator</span><span class="special">,(</span><span class="keyword">int</span><span class="special">)</span> <span class="keyword">const</span><span class="special">;</span>
<span class="special">};</span>
<span class="keyword">typedef</span> <span class="keyword">char</span> <span class="identifier">yes_type</span><span class="special">;</span> <span class="comment">// sizeof(yes_type) == 1</span>
<span class="keyword">typedef</span> <span class="keyword">char</span> <span class="special">(&</span><span class="identifier">no_type</span><span class="special">)[</span><span class="number">2</span><span class="special">];</span> <span class="comment">// sizeof(no_type) == 2</span>
<span class="keyword">template</span><span class="special"><</span><span class="keyword">typename</span> <span class="identifier">T</span><span class="special">></span>
<span class="identifier">no_type</span> <span class="identifier">is_private_type</span><span class="special">(</span><span class="identifier">T</span> <span class="keyword">const</span> <span class="special">&);</span>
<span class="identifier">yes_type</span> <span class="identifier">is_private_type</span><span class="special">(</span><span class="identifier">private_type</span> <span class="keyword">const</span> <span class="special">&);</span>
</pre>
<p>
Next, we implement a binary function object wrapper with a very strange
conversion operator, whose meaning will become clear later.
</p>
<pre class="programlisting"><span class="keyword">template</span><span class="special"><</span><span class="keyword">typename</span> <span class="identifier">Fun</span><span class="special">></span>
<span class="keyword">struct</span> <span class="identifier">funwrap2</span> <span class="special">:</span> <span class="identifier">Fun</span>
<span class="special">{</span>
<span class="identifier">funwrap2</span><span class="special">();</span>
<span class="keyword">typedef</span> <span class="identifier">private_type</span> <span class="keyword">const</span> <span class="special">&(*</span><span class="identifier">pointer_to_function</span><span class="special">)(</span><span class="identifier">dont_care</span><span class="special">,</span> <span class="identifier">dont_care</span><span class="special">);</span>
<span class="keyword">operator</span> <span class="identifier">pointer_to_function</span><span class="special">()</span> <span class="keyword">const</span><span class="special">;</span>
<span class="special">};</span>
</pre>
<p>
With all of these bits and pieces, we can implement <code class="computeroutput"><span class="identifier">can_be_called</span><span class="special"><></span></code> as follows:
</p>
<pre class="programlisting"><span class="keyword">template</span><span class="special"><</span><span class="keyword">typename</span> <span class="identifier">Fun</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">B</span><span class="special">></span>
<span class="keyword">struct</span> <span class="identifier">can_be_called</span>
<span class="special">{</span>
<span class="keyword">static</span> <span class="identifier">funwrap2</span><span class="special"><</span><span class="identifier">Fun</span><span class="special">></span> <span class="special">&</span><span class="identifier">fun</span><span class="special">;</span>
<span class="keyword">static</span> <span class="identifier">A</span> <span class="special">&</span><span class="identifier">a</span><span class="special">;</span>
<span class="keyword">static</span> <span class="identifier">B</span> <span class="special">&</span><span class="identifier">b</span><span class="special">;</span>
<span class="keyword">static</span> <span class="keyword">bool</span> <span class="keyword">const</span> <span class="identifier">value</span> <span class="special">=</span> <span class="special">(</span>
<span class="keyword">sizeof</span><span class="special">(</span><span class="identifier">no_type</span><span class="special">)</span> <span class="special">==</span> <span class="keyword">sizeof</span><span class="special">(</span><span class="identifier">is_private_type</span><span class="special">(</span> <span class="special">(</span><span class="identifier">fun</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="number">0</span><span class="special">)</span> <span class="special">))</span>
<span class="special">);</span>
<span class="keyword">typedef</span> <span class="identifier">mpl</span><span class="special">::</span><span class="identifier">bool_</span><span class="special"><</span><span class="identifier">value</span><span class="special">></span> <span class="identifier">type</span><span class="special">;</span>
<span class="special">};</span>
</pre>
<p>
The idea is to make it so that <code class="computeroutput"><span class="identifier">fun</span><span class="special">(</span><span class="identifier">a</span><span class="special">,</span><span class="identifier">b</span><span class="special">)</span></code> will
always compile by adding our own binary function overload, but doing it
in such a way that we can detect whether our overload was selected or not.
And we rig it so that our overload is selected if there is really no better
option. What follows is a description of how <code class="computeroutput"><span class="identifier">can_be_called</span><span class="special"><></span></code> works.
</p>
<p>
We wrap <code class="computeroutput"><span class="identifier">Fun</span></code> in a type that
has an implicit conversion to a pointer to a binary function. An object
<code class="computeroutput"><span class="identifier">fun</span></code> of class type can be
invoked as <code class="computeroutput"><span class="identifier">fun</span><span class="special">(</span><span class="identifier">a</span><span class="special">,</span> <span class="identifier">b</span><span class="special">)</span></code> if it has such a conversion operator,
but since it involves a user-defined conversion operator, it is less preferred
than an overloaded <code class="computeroutput"><span class="keyword">operator</span><span class="special">()</span></code>, which requires no such conversion.
</p>
<p>
The function pointer can accept any two arguments by virtue of the <code class="computeroutput"><span class="identifier">dont_care</span></code> type. The conversion sequence
for each argument is guaranteed to be the worst possible conversion sequence:
an implicit conversion through an ellipsis, and a user-defined conversion
to <code class="computeroutput"><span class="identifier">dont_care</span></code>. In total,
it means that <code class="computeroutput"><span class="identifier">funwrap2</span><span class="special"><</span><span class="identifier">Fun</span><span class="special">>()(</span><span class="identifier">a</span><span class="special">,</span> <span class="identifier">b</span><span class="special">)</span></code>
will always compile, but it will select our overload only if there really
is no better option.
</p>
<p>
If there is a better option --- for example if <code class="computeroutput"><span class="identifier">Fun</span></code>
has an overloaded function call operator such as <code class="computeroutput"><span class="keyword">void</span>
<span class="keyword">operator</span><span class="special">()(</span><span class="identifier">A</span> <span class="identifier">a</span><span class="special">,</span> <span class="identifier">B</span> <span class="identifier">b</span><span class="special">)</span></code> ---
then <code class="computeroutput"><span class="identifier">fun</span><span class="special">(</span><span class="identifier">a</span><span class="special">,</span> <span class="identifier">b</span><span class="special">)</span></code> will resolve to that one instead. The
question now is how to detect which function got picked by overload resolution.
</p>
<p>
Notice how <code class="computeroutput"><span class="identifier">fun</span><span class="special">(</span><span class="identifier">a</span><span class="special">,</span> <span class="identifier">b</span><span class="special">)</span></code> appears in <code class="computeroutput"><span class="identifier">can_be_called</span><span class="special"><></span></code>: <code class="computeroutput"><span class="special">(</span><span class="identifier">fun</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="number">0</span><span class="special">)</span></code>.
Why do we use the comma operator there? The reason is because we are using
this expression as the argument to a function. If the return type of <code class="computeroutput"><span class="identifier">fun</span><span class="special">(</span><span class="identifier">a</span><span class="special">,</span> <span class="identifier">b</span><span class="special">)</span></code> is <code class="computeroutput"><span class="keyword">void</span></code>,
it cannot legally be used as an argument to a function. The comma operator
sidesteps the issue.
</p>
<p>
This should also make plain the purpose of the overloaded comma operator
in <code class="computeroutput"><span class="identifier">private_type</span></code>. The return
type of the pointer to function is <code class="computeroutput"><span class="identifier">private_type</span></code>.
If overload resolution selects our overload, then the type of <code class="computeroutput"><span class="special">(</span><span class="identifier">fun</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="number">0</span><span class="special">)</span></code>
is <code class="computeroutput"><span class="identifier">private_type</span></code>. Otherwise,
it is <code class="computeroutput"><span class="keyword">int</span></code>. That fact is used
to dispatch to either overload of <code class="computeroutput"><span class="identifier">is_private_type</span><span class="special">()</span></code>, which encodes its answer in the size
of its return type.
</p>
<p>
That's how it works with binary functions. Now repeat the above process
for functions up to some predefined function arity, and you're done.
</p>
</div>
</div>
<div class="section">
<div class="titlepage"><div><div><h3 class="title">
<a name="boost_proto.appendices.acknowledgements"></a><a class="link" href="appendices.html#boost_proto.appendices.acknowledgements" title="Appendix E: Acknowledgements">Appendix E:
Acknowledgements</a>
</h3></div></div></div>
<p>
I'd like to thank Joel de Guzman and Hartmut Kaiser for being willing to
take a chance on using Proto for their work on Spirit-2 and Karma when Proto
was little more than a vision. Their requirements and feedback have been
indespensable.
</p>
<p>
Thanks also to Thomas Heller and again to Hartmut for their feedback and
suggestions during the redesign of Phoenix. That effort yielded several valuable
advanced features such as sub-domains, external transforms, and per-domain
<code class="computeroutput"><span class="identifier">as_child</span></code> customization.
</p>
<p>
Thanks to Daniel James for providing a patch to remove the dependence on
deprecated configuration macros for C++0x features.
</p>
<p>
Thanks to Joel Falcou and Christophe Henry for their enthusiasm, support,
feedback, and humor; and for volunteering to be Proto's co-maintainers.
</p>
<p>
Thanks to Dave Abrahams for an especially detailed review, and for making
a VM with msvc-7.1 available so I could track down portability issues on
that compiler.
</p>
<p>
Many thanks to Daniel Wallin who first implemented the code used to find
the common domain among a set, accounting for super- and sub-domains. Thanks
also to Jeremiah Willcock, John Bytheway and Krishna Achuthan who offered
alternate solutions to this tricky programming problem.
</p>
<p>
Thanks also to the developers of <a href="http://acts.nersc.gov/formertools/pete/index.html" target="_top">PETE</a>.
I found many good ideas there.
</p>
</div>
</div>
<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
<td align="left"></td>
<td align="right"><div class="copyright-footer">Copyright © 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>
</tr></table>
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