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| <div><h2 class="title"> |
| <a name="typeof"></a>Chapter 25. 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="id2759176"></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>) |
| </p> |
| </div></div> |
| </div></div> |
| <div class="toc"> |
| <p><b>Table of Contents</b></p> |
| <dl> |
| <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="identifier">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="identifier">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> |
| </div> |
| </div> |
| <table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr> |
| <td align="left"><p><small>Last revised: November 01, 2010 at 13:12:40 +0000</small></p></td> |
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