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| <div><h2 class="title"> |
| <a name="declval"></a>Declval</h2></div> |
| <div><div class="authorgroup"> |
| <div class="author"><h3 class="author"> |
| <span class="firstname">Howard</span> <span class="surname">Hinnant</span> |
| </h3></div> |
| <div class="author"><h3 class="author"> |
| <span class="firstname">Beman</span> <span class="surname">Dawes</span> |
| </h3></div> |
| <div class="author"><h3 class="author"> |
| <span class="firstname">Vicente J.</span> <span class="surname">Botet Escriba</span> |
| </h3></div> |
| </div></div> |
| <div><p class="copyright">Copyright © 2008 Howard Hinnant</p></div> |
| <div><p class="copyright">Copyright © 2008 Beman Dawes</p></div> |
| <div><p class="copyright">Copyright © 2009 -2010 Vicente J. Botet Escriba</p></div> |
| <div><div class="legalnotice"> |
| <a name="id879409"></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> |
| <dt><span class="section"><a href="declval.html#declval.overview">Overview</a></span></dt> |
| <dt><span class="section"><a href="declval.html#declval.reference"> Reference </a></span></dt> |
| </dl> |
| </div> |
| <div class="section" lang="en"> |
| <div class="titlepage"><div><div><h2 class="title" style="clear: both"> |
| <a name="declval.overview"></a><a class="link" href="declval.html#declval.overview" title="Overview">Overview</a> |
| </h2></div></div></div> |
| <p> |
| The motivation for <code class="computeroutput"><span class="identifier">declval</span></code> |
| was introduced in <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2009/n2958.html#Value" target="_top">N2958: |
| Moving Swap Forward</a>. Here follows a rewording of this chapter. |
| </p> |
| <p> |
| With the provision of decltype, late-specified return types, and default template-arguments |
| for function templates a new generation of SFINAE patterns will emerge to at |
| least partially compensate the lack of concepts on the C++0x timescale. Using |
| this technique, it is sometimes necessary to obtain an object of a known type |
| in a non-using context, e.g. given the declaration |
| </p> |
| <pre class="programlisting"><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="identifier">T</span><span class="special">&&</span> <span class="identifier">declval</span><span class="special">();</span> <span class="comment">// not used |
| </span></pre> |
| <p> |
| as part of the function template declaration |
| </p> |
| <pre class="programlisting"><span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">To</span><span class="special">,</span> <span class="keyword">class</span> <span class="identifier">From</span><span class="special">></span> |
| <span class="identifier">decltype</span><span class="special">(</span><span class="keyword">static_cast</span><span class="special"><</span><span class="identifier">To</span><span class="special">>(</span><span class="identifier">declval</span><span class="special"><</span><span class="identifier">From</span><span class="special">>()))</span> <span class="identifier">convert</span><span class="special">(</span><span class="identifier">From</span><span class="special">&&);</span> |
| </pre> |
| <p> |
| or as part of a class template definition |
| </p> |
| <pre class="programlisting"><span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span><span class="special">></span> <span class="keyword">class</span> <span class="identifier">result_of</span><span class="special">;</span> |
| |
| <span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">Fn</span><span class="special">,</span> <span class="keyword">class</span><span class="special">...</span> <span class="identifier">ArgTypes</span><span class="special">></span> |
| <span class="keyword">struct</span> <span class="identifier">result_of</span><span class="special"><</span><span class="identifier">Fn</span><span class="special">(</span><span class="identifier">ArgTypes</span><span class="special">...)></span> |
| <span class="special">{</span> |
| <span class="keyword">typedef</span> <span class="identifier">decltype</span><span class="special">(</span><span class="identifier">declval</span><span class="special"><</span><span class="identifier">Fn</span><span class="special">>()(</span><span class="identifier">declval</span><span class="special"><</span><span class="identifier">ArgTypes</span><span class="special">>()...))</span> <span class="identifier">type</span><span class="special">;</span> |
| <span class="special">};</span> |
| </pre> |
| <p> |
| The role of the function template declval() is a transformation of a type T |
| into a value without using or evaluating this function. The name is supposed |
| to direct the reader's attention to the fact that the expression <code class="computeroutput"><span class="identifier">declval</span><span class="special"><</span><span class="identifier">T</span><span class="special">>()</span></code> is |
| an lvalue if and only if T is an lvalue-reference, otherwise an rvalue. To |
| extend the domain of this function we can do a bit better by changing its declaration |
| to |
| </p> |
| <pre class="programlisting"><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">typename</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">add_rvalue_reference</span><span class="special"><</span><span class="identifier">T</span><span class="special">>::</span><span class="identifier">type</span> <span class="identifier">declval</span><span class="special">();</span> <span class="comment">// not used |
| </span></pre> |
| <p> |
| which ensures that we can also use cv void as template parameter. The careful |
| reader might have noticed that <code class="computeroutput"><span class="identifier">declval</span><span class="special">()</span></code> already exists under the name create() as |
| part of the definition of the semantics of the type trait is_convertible in |
| the C==0x standard. |
| </p> |
| <p> |
| The provision of a new library component that allows the production of values |
| in unevaluated expressions is considered as important to realize constrained |
| templates in C++0x where concepts are not available. This extremely light-weight |
| function is expected to be part of the daily tool-box of the C++0x programmer. |
| </p> |
| </div> |
| <div class="section" lang="en"> |
| <div class="titlepage"><div><div><h2 class="title" style="clear: both"> |
| <a name="declval.reference"></a><a class="link" href="declval.html#declval.reference" title="Reference"> Reference </a> |
| </h2></div></div></div> |
| <p> |
| <code class="computeroutput"><span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">utility</span><span class="special">/</span><span class="identifier">declval</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span></code> |
| </p> |
| <pre class="programlisting"><span class="keyword">namespace</span> <span class="identifier">boost</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">typename</span> <span class="identifier">add_rvalue_reference</span><span class="special"><</span><span class="identifier">T</span><span class="special">>::</span><span class="identifier">type</span> <span class="identifier">declval</span><span class="special">();</span> <span class="comment">//noexcept; // as unevaluated operand |
| </span> |
| <span class="special">}</span> <span class="comment">// namespace boost |
| </span></pre> |
| <p> |
| The library provides the function template declval to simplify the definition |
| of expressions which occur as unevaluated operands. |
| </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">add_rvalue_reference</span><span class="special"><</span><span class="identifier">T</span><span class="special">>::</span><span class="identifier">type</span> <span class="identifier">declval</span><span class="special">();</span> |
| </pre> |
| <p> |
| <span class="bold"><strong>Remarks:</strong></span> If this function is used, the program |
| is ill-formed. |
| </p> |
| <p> |
| <span class="bold"><strong>Remarks:</strong></span> The template parameter T of declval |
| may be an incomplete type. |
| </p> |
| <p> |
| <span class="bold"><strong>Example:</strong></span> |
| </p> |
| <pre class="programlisting"><span class="keyword">template</span> <span class="special"><</span><span class="keyword">class</span> <span class="identifier">To</span><span class="special">,</span> <span class="keyword">class</span> <span class="identifier">From</span><span class="special">></span> |
| <span class="identifier">decltype</span><span class="special">(</span><span class="keyword">static_cast</span><span class="special"><</span><span class="identifier">To</span><span class="special">>(</span><span class="identifier">declval</span><span class="special"><</span><span class="identifier">From</span><span class="special">>()))</span> <span class="identifier">convert</span><span class="special">(</span><span class="identifier">From</span><span class="special">&&);</span> |
| </pre> |
| <p> |
| Declares a function template convert which only participats in overloading |
| if the type From can be explicitly converted to type To. |
| </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: September 16, 2010 at 16:19:10 GMT</small></p></td> |
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