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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">Vicente J.</span> <span class="surname">Botet Escriba</span>
</h3></div>
</div></div>
<div><p class="copyright">Copyright &#169; 2008 Howard Hinnant</p></div>
<div><p class="copyright">Copyright &#169; 2009-2012 Vicente
      J. Botet Escriba</p></div>
<div><div class="legalnotice">
<a name="declval.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>)
      </p>
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<div class="toc">
<p><b>Table of Contents</b></p>
<dl class="toc">
<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>
<dt><span class="section"><a href="declval.html#declval.history">History</a></span></dt>
</dl>
</div>
<div class="section">
<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">&lt;</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">&gt;</span>
<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>
</pre>
<p>
      as part of the function template declaration
    </p>
<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>
<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>
</pre>
<p>
      or as part of a class template definition
    </p>
<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>

<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>
<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>
<span class="special">{</span>
  <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>
<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">&lt;</span><span class="identifier">T</span><span class="special">&gt;()</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">&lt;</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">&gt;</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">&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>
</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 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">
<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">&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>
    </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">&lt;</span><span class="keyword">typename</span> <span class="identifier">T</span><span class="special">&gt;</span>
    <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>

<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">&lt;</span><span class="keyword">typename</span> <span class="identifier">T</span><span class="special">&gt;</span>
<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>
</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">&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>
<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>
</pre>
<p>
      Declares a function template convert which only participates in overloading
      if the type From can be explicitly converted to type To.
    </p>
</div>
<div class="section">
<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="declval.history"></a><a class="link" href="declval.html#declval.history" title="History">History</a>
</h2></div></div></div>
<h4>
<a name="declval.history.h0"></a>
      <span class="phrase"><a name="declval.history.boost_1_50"></a></span><a class="link" href="declval.html#declval.history.boost_1_50">boost
      1.50</a>
    </h4>
<p>
      Fixes:
    </p>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem">
          <a href="http://svn.boost.org/trac/boost/ticket/6570" target="_top">#6570</a>
          Adding noexcept to boost::declval.
        </li></ul></div>
</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: December 10, 2019 at 00:21:47 GMT</small></p></td>
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