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  24 <a name="declval"></a>Declval</h2></div>
  25 <div><div class="authorgroup">
  26 <div class="author"><h3 class="author">
  27 <span class="firstname">Howard</span> <span class="surname">Hinnant</span>
  28 </h3></div>
  29 <div class="author"><h3 class="author">
  30 <span class="firstname">Vicente J.</span> <span class="surname">Botet Escriba</span>
  31 </h3></div>
  32 </div></div>
  33 <div><p class="copyright">Copyright &#169; 2008 Howard Hinnant</p></div>
  34 <div><p class="copyright">Copyright &#169; 2009-2012 Vicente
  35       J. Botet Escriba</p></div>
  36 <div><div class="legalnotice">
  37 <a name="declval.legal"></a><p>
  38         Distributed under the Boost Software License, Version 1.0. (See accompanying
  39         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>)
  40       </p>
  41 </div></div>
  42 </div>
  43 <hr>
  44 </div>
  45 <div class="toc">
  46 <p><b>Table of Contents</b></p>
  47 <dl class="toc">
  48 <dt><span class="section"><a href="declval.html#declval.overview">Overview</a></span></dt>
  49 <dt><span class="section"><a href="declval.html#declval.reference">Reference </a></span></dt>
  50 <dt><span class="section"><a href="declval.html#declval.history">History</a></span></dt>
  51 </dl>
  52 </div>
  53 <div class="section">
  54 <div class="titlepage"><div><div><h2 class="title" style="clear: both">
  55 <a name="declval.overview"></a><a class="link" href="declval.html#declval.overview" title="Overview">Overview</a>
  56 </h2></div></div></div>
  57 <p>
  58       The motivation for <code class="computeroutput"><span class="identifier">declval</span></code>
  59       was introduced in <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2009/n2958.html#Value" target="_top">N2958:
  60       Moving Swap Forward</a>. Here follows a rewording of this chapter.
  61     </p>
  62 <p>
  63       With the provision of decltype, late-specified return types, and default template-arguments
  64       for function templates a new generation of SFINAE patterns will emerge to at
  65       least partially compensate the lack of concepts on the C++0x timescale. Using
  66       this technique, it is sometimes necessary to obtain an object of a known type
  67       in a non-using context, e.g. given the declaration
  68     </p>
  69 <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>
  70 <span class="identifier">T</span><span class="special">&amp;&amp;</span> <span class="identifier">declval</span><span class="special">();</span> <span class="comment">// not used</span>
  71 </pre>
  72 <p>
  73       as part of the function template declaration
  74     </p>
  75 <pre class="programlisting"><span class="keyword">template</span><span class="special">&lt;</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">&gt;</span>
  76 <span class="keyword">decltype</span><span class="special">(</span><span class="keyword">static_cast</span><span class="special">&lt;</span><span class="identifier">To</span><span class="special">&gt;(</span><span class="identifier">declval</span><span class="special">&lt;</span><span class="identifier">From</span><span class="special">&gt;()))</span> <span class="identifier">convert</span><span class="special">(</span><span class="identifier">From</span><span class="special">&amp;&amp;);</span>
  77 </pre>
  78 <p>
  79       or as part of a class template definition
  80     </p>
  81 <pre class="programlisting"><span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">class</span><span class="special">&gt;</span> <span class="keyword">class</span> <span class="identifier">result_of</span><span class="special">;</span>
  82
  83 <span class="keyword">template</span><span class="special">&lt;</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">&gt;</span>
  84 <span class="keyword">struct</span> <span class="identifier">result_of</span><span class="special">&lt;</span><span class="identifier">Fn</span><span class="special">(</span><span class="identifier">ArgTypes</span><span class="special">...)&gt;</span>
  85 <span class="special">{</span>
  86   <span class="keyword">typedef</span> <span class="keyword">decltype</span><span class="special">(</span><span class="identifier">declval</span><span class="special">&lt;</span><span class="identifier">Fn</span><span class="special">&gt;()(</span><span class="identifier">declval</span><span class="special">&lt;</span><span class="identifier">ArgTypes</span><span class="special">&gt;()...))</span> <span class="identifier">type</span><span class="special">;</span>
  87 <span class="special">};</span>
  88 </pre>
  89 <p>
  90       The role of the function template declval() is a transformation of a type T
  91       into a value without using or evaluating this function. The name is supposed
  92       to direct the reader's attention to the fact that the expression <code class="computeroutput"><span class="identifier">declval</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;()</span></code> is
  93       an lvalue if and only if T is an lvalue-reference, otherwise an rvalue. To
  94       extend the domain of this function we can do a bit better by changing its declaration
  95       to
  96     </p>
  97 <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>
  98 <span class="keyword">typename</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">add_rvalue_reference</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;::</span><span class="identifier">type</span> <span class="identifier">declval</span><span class="special">();</span> <span class="comment">// not used</span>
  99 </pre>
 100 <p>
 101       which ensures that we can also use cv void as template parameter. The careful
 102       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
 103       part of the definition of the semantics of the type trait is_convertible in
 104       the C++0x standard.
 105     </p>
 106 <p>
 107       The provision of a new library component that allows the production of values
 108       in unevaluated expressions is considered important to realize constrained templates
 109       in C++0x where concepts are not available. This extremely light-weight function
 110       is expected to be part of the daily tool-box of the C++0x programmer.
 111     </p>
 112 </div>
 113 <div class="section">
 114 <div class="titlepage"><div><div><h2 class="title" style="clear: both">
 115 <a name="declval.reference"></a><a class="link" href="declval.html#declval.reference" title="Reference">Reference </a>
 116 </h2></div></div></div>
 117 <p>
 118       <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">utility</span><span class="special">/</span><span class="identifier">declval</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span></code>
 119     </p>
 120 <pre class="programlisting"><span class="keyword">namespace</span> <span class="identifier">boost</span> <span class="special">{</span>
 121
 122     <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>
 123     <span class="keyword">typename</span> <span class="identifier">add_rvalue_reference</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;::</span><span class="identifier">type</span> <span class="identifier">declval</span><span class="special">()</span> <span class="keyword">noexcept</span><span class="special">;</span> <span class="comment">// as unevaluated operand</span>
 124
 125 <span class="special">}</span>  <span class="comment">// namespace boost</span>
 126 </pre>
 127 <p>
 128       The library provides the function template declval to simplify the definition
 129       of expressions which occur as unevaluated operands.
 130     </p>
 131 <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">&gt;</span>
 132 <span class="keyword">typename</span> <span class="identifier">add_rvalue_reference</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;::</span><span class="identifier">type</span> <span class="identifier">declval</span><span class="special">();</span>
 133 </pre>
 134 <p>
 135       <span class="bold"><strong>Remarks:</strong></span> If this function is used, the program
 136       is ill-formed.
 137     </p>
 138 <p>
 139       <span class="bold"><strong>Remarks:</strong></span> The template parameter T of declval
 140       may be an incomplete type.
 141     </p>
 142 <p>
 143       <span class="bold"><strong>Example:</strong></span>
 144     </p>
 145 <pre class="programlisting"><span class="keyword">template</span> <span class="special">&lt;</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">&gt;</span>
 146 <span class="keyword">decltype</span><span class="special">(</span><span class="keyword">static_cast</span><span class="special">&lt;</span><span class="identifier">To</span><span class="special">&gt;(</span><span class="identifier">declval</span><span class="special">&lt;</span><span class="identifier">From</span><span class="special">&gt;()))</span> <span class="identifier">convert</span><span class="special">(</span><span class="identifier">From</span><span class="special">&amp;&amp;);</span>
 147 </pre>
 148 <p>
 149       Declares a function template convert which only participates in overloading
 150       if the type From can be explicitly converted to type To.
 151     </p>
 152 </div>
 153 <div class="section">
 154 <div class="titlepage"><div><div><h2 class="title" style="clear: both">
 155 <a name="declval.history"></a><a class="link" href="declval.html#declval.history" title="History">History</a>
 156 </h2></div></div></div>
 157 <h4>
 158 <a name="declval.history.h0"></a>
 159       <span class="phrase"><a name="declval.history.boost_1_50"></a></span><a class="link" href="declval.html#declval.history.boost_1_50">boost
 160       1.50</a>
 161     </h4>
 162 <p>
 163       Fixes:
 164     </p>
 165 <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem">
 166           <a href="http://svn.boost.org/trac/boost/ticket/6570" target="_top">#6570</a>
 167           Adding noexcept to boost::declval.
 168         </li></ul></div>
 169 </div>
 170 </div>
 171 <table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
 172 <td align="left"><p><small>Last revised: October 30, 2014 at 10:17:15 GMT</small></p></td>
 173 <td align="right"><div class="copyright-footer"></div></td>
 174 </tr></table>
 175 <hr>
 176 <div class="spirit-nav"></div>
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