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<title>Chapter 42. Boost.Typeof</title>
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<div><h2 class="title">
<a name="typeof"></a>Chapter 42. Boost.Typeof</h2></div>
<div><div class="author"><h3 class="author">
<span class="firstname">Arkadiy</span> <span class="surname">Vertleyb</span>
</h3></div></div>
<div><div class="author"><h3 class="author">
<span class="firstname">Peder</span> <span class="surname">Holt</span>
</h3></div></div>
<div><p class="copyright">Copyright © 2004, 2005 Arkadiy Vertleyb, Peder Holt</p></div>
<div><div class="legalnotice">
<a name="typeof.legal"></a><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>)
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<div class="toc">
<p><b>Table of Contents</b></p>
<dl class="toc">
<dt><span class="section"><a href="typeof.html#typeof.moti">Motivation</a></span></dt>
<dt><span class="section"><a href="typeof/tuto.html">Tutorial</a></span></dt>
<dt><span class="section"><a href="typeof/refe.html">Reference</a></span></dt>
<dd><dl>
<dt><span class="section"><a href="typeof/refe.html#typeof.auto">AUTO, AUTO_TPL</a></span></dt>
<dt><span class="section"><a href="typeof/refe.html#typeof.compl">COMPLIANT</a></span></dt>
<dt><span class="section"><a href="typeof/refe.html#typeof.incr">INCREMENT_REGISTRATION_GROUP</a></span></dt>
<dt><span class="section"><a href="typeof/refe.html#typeof.inte">INTEGRAL</a></span></dt>
<dt><span class="section"><a href="typeof/refe.html#typeof.limit_func">LIMIT_FUNCTION_ARITY</a></span></dt>
<dt><span class="section"><a href="typeof/refe.html#typeof.messages">MESSAGES</a></span></dt>
<dt><span class="section"><a href="typeof/refe.html#typeof.limit_size">LIMIT_SIZE</a></span></dt>
<dt><span class="section"><a href="typeof/refe.html#typeof.regtype">REGISTER_TYPE</a></span></dt>
<dt><span class="section"><a href="typeof/refe.html#typeof.regtemp">REGISTER_TEMPLATE</a></span></dt>
<dt><span class="section"><a href="typeof/refe.html#typeof.temp">TEMPLATE</a></span></dt>
<dt><span class="section"><a href="typeof/refe.html#typeof.typo">TYPEOF, TYPEOF_TPL</a></span></dt>
<dt><span class="section"><a href="typeof/refe.html#typeof.typn">TYPEOF_NESTED_TYPEDEF, TYPEOF_NESTED_TYPEDEF_TPL</a></span></dt>
</dl></dd>
<dt><span class="section"><a href="typeof/other.html">Other considerations and tips</a></span></dt>
<dd><dl>
<dt><span class="section"><a href="typeof/other.html#typeof.natem">Native typeof support and emulation</a></span></dt>
<dt><span class="section"><a href="typeof/other.html#typeof.parties">The three participating parties</a></span></dt>
<dt><span class="section"><a href="typeof/other.html#typeof.features">Supported features</a></span></dt>
<dt><span class="section"><a href="typeof/other.html#typeof.what">What needs to be registered?</a></span></dt>
<dt><span class="section"><a href="typeof/other.html#typeof.limi">Limitations</a></span></dt>
</dl></dd>
<dt><span class="section"><a href="typeof/cont.html">Contributed By:</a></span></dt>
<dt><span class="section"><a href="typeof/ackn.html">Acknowledgements</a></span></dt>
</dl>
</div>
<div class="section">
<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="typeof.moti"></a>Motivation</h2></div></div></div>
<p>
Today many template libraries supply object generators to simplify object creation
by utilizing the C++ template argument deduction facility. Consider <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">pair</span></code>.
In order to instantiate this class template and create a temporary object of
this instantiation, one has to supply template parameters, as well as parameters
to the constructor:
</p>
<pre class="programlisting"><span class="identifier">std</span><span class="special">::</span><span class="identifier">pair</span><span class="special"><</span><span class="keyword">int</span><span class="special">,</span> <span class="keyword">double</span><span class="special">>(</span><span class="number">5</span><span class="special">,</span> <span class="number">3.14159</span><span class="special">);</span>
</pre>
<p>
To avoid this duplication, STL supplies the <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">make_pair</span></code>
object generator. When it is used, the types of template parameters are deduced
from supplied function arguments:
</p>
<pre class="programlisting"><span class="identifier">std</span><span class="special">::</span><span class="identifier">make_pair</span><span class="special">(</span><span class="number">5</span><span class="special">,</span> <span class="number">3.14159</span><span class="special">);</span>
</pre>
<p>
For the temporary objects it is enough. However, when a named object needs
to be allocated, the problem appears again:
</p>
<pre class="programlisting"><span class="identifier">std</span><span class="special">::</span><span class="identifier">pair</span><span class="special"><</span><span class="keyword">int</span><span class="special">,</span> <span class="keyword">double</span><span class="special">></span> <span class="identifier">p</span><span class="special">(</span><span class="number">5</span><span class="special">,</span> <span class="number">3.14159</span><span class="special">);</span>
</pre>
<p>
The object generator no longer helps:
</p>
<pre class="programlisting"><span class="identifier">std</span><span class="special">::</span><span class="identifier">pair</span><span class="special"><</span><span class="keyword">int</span><span class="special">,</span> <span class="keyword">double</span><span class="special">></span> <span class="identifier">p</span> <span class="special">=</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">make_pair</span><span class="special">(</span><span class="number">5</span><span class="special">,</span> <span class="number">3.14159</span><span class="special">);</span>
</pre>
<p>
It would be nice to deduce the type of the object (on the left) from the expression
it is initialized with (on the right), but the current C++ syntax does not
allow for this.
</p>
<p>
The above example demonstrates the essence of the problem but does not demonstrate
its scale. Many libraries, especially expression template libraries, create
objects of really complex types, and go a long way to hide this complexity
behind object generators. Consider a nit Boost.Lambda functor:
</p>
<pre class="programlisting"><span class="identifier">_1</span> <span class="special">></span> <span class="number">15</span> <span class="special">&&</span> <span class="identifier">_2</span> <span class="special"><</span> <span class="number">20</span>
</pre>
<p>
If one wanted to allocate a named copy of such an innocently looking functor,
she would have to specify something like this:
</p>
<pre class="programlisting"><span class="identifier">lambda_functor</span><span class="special"><</span>
<span class="identifier">lambda_functor_base</span><span class="special"><</span>
<span class="identifier">logical_action</span><span class="special"><</span><span class="identifier">and_action</span><span class="special">>,</span>
<span class="identifier">tuple</span><span class="special"><</span>
<span class="identifier">lambda_functor</span><span class="special"><</span>
<span class="identifier">lambda_functor_base</span><span class="special"><</span>
<span class="identifier">relational_action</span><span class="special"><</span><span class="identifier">greater_action</span><span class="special">>,</span>
<span class="identifier">tuple</span><span class="special"><</span>
<span class="identifier">lambda_functor</span><span class="special"><</span><span class="identifier">placeholder</span><span class="special"><</span><span class="number">1</span><span class="special">></span> <span class="special">>,</span>
<span class="keyword">int</span> <span class="keyword">const</span>
<span class="special">></span>
<span class="special">></span>
<span class="special">>,</span>
<span class="identifier">lambda_functor</span><span class="special"><</span>
<span class="identifier">lambda_functor_base</span><span class="special"><</span>
<span class="identifier">relational_action</span><span class="special"><</span><span class="identifier">less_action</span><span class="special">>,</span>
<span class="identifier">tuple</span><span class="special"><</span>
<span class="identifier">lambda_functor</span><span class="special"><</span><span class="identifier">placeholder</span><span class="special"><</span><span class="number">2</span><span class="special">></span> <span class="special">>,</span>
<span class="keyword">int</span> <span class="keyword">const</span>
<span class="special">></span>
<span class="special">></span>
<span class="special">></span>
<span class="special">></span>
<span class="special">></span>
<span class="special">></span>
<span class="identifier">f</span> <span class="special">=</span> <span class="identifier">_1</span> <span class="special">></span> <span class="number">15</span> <span class="special">&&</span> <span class="identifier">_2</span> <span class="special"><</span> <span class="number">20</span><span class="special">;</span>
</pre>
<p>
Not exactly elegant. To solve this problem (as well as some other problems),
the C++ standard committee is considering a few additions to the standard language,
such as <code class="computeroutput"><span class="identifier">typeof</span><span class="special">/</span><span class="keyword">decltype</span></code> and <code class="computeroutput"><span class="keyword">auto</span></code>
(see <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2004/n1607.pdf" target="_top">http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2004/n1607.pdf</a>).
</p>
<p>
The <code class="computeroutput"><span class="identifier">typeof</span></code> operator (or <code class="computeroutput"><span class="keyword">decltype</span></code>, which is a slightly different flavor
of <code class="computeroutput"><span class="identifier">typeof</span></code>) allows one to determine
the type of an expression at compile time. Using <code class="computeroutput"><span class="identifier">typeof</span></code>,
the above example can be simplified drastically:
</p>
<pre class="programlisting"><span class="identifier">typeof</span><span class="special">(</span><span class="identifier">_1</span> <span class="special">></span> <span class="number">15</span> <span class="special">&&</span> <span class="identifier">_2</span> <span class="special"><</span> <span class="number">20</span><span class="special">)</span> <span class="identifier">f</span> <span class="special">=</span> <span class="identifier">_1</span> <span class="special">></span> <span class="number">15</span> <span class="special">&&</span> <span class="identifier">_2</span> <span class="special"><</span> <span class="number">20</span><span class="special">;</span>
</pre>
<p>
Much better, but some duplication still exists. The <code class="computeroutput"><span class="keyword">auto</span></code>
type solves the rest of the problem:
</p>
<pre class="programlisting"><span class="keyword">auto</span> <span class="identifier">f</span> <span class="special">=</span> <span class="identifier">_1</span> <span class="special">></span> <span class="number">15</span> <span class="special">&&</span> <span class="identifier">_2</span> <span class="special"><</span> <span class="number">20</span><span class="special">;</span>
</pre>
<p>
The purpose of the Boost.Typeof library is to provide a library-based solution,
which could be used until the language-based facility is added to the Standard
and becomes widely available.
</p>
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