ceph/src/boost/libs/type_traits/doc/html/boost_typetraits/reference/is_assignable.html

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  26 <div class="titlepage"><div><div><h3 class="title">
  27 <a name="boost_typetraits.reference.is_assignable"></a><a class="link" href="is_assignable.html" title="is_assignable">is_assignable</a>
  28 </h3></div></div></div>
  29 <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">,</span> <span class="keyword">class</span> <span class="identifier">U</span><span class="special">&gt;</span>
  30 <span class="keyword">struct</span> <span class="identifier">is_assignable</span> <span class="special">:</span> <span class="keyword">public</span> <em class="replaceable"><code><a class="link" href="integral_constant.html" title="integral_constant">true_type</a>-or-<a class="link" href="integral_constant.html" title="integral_constant">false_type</a></code></em> <span class="special">{};</span>
  31 </pre>
  32 <p>
  33         <span class="bold"><strong>Inherits:</strong></span> If <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">declval</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;()</span>
  34         <span class="special">=</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">declval</span><span class="special">&lt;</span><span class="identifier">U</span><span class="special">&gt;()</span></code>
  35         then inherits from <a class="link" href="integral_constant.html" title="integral_constant">true_type</a>,
  36         otherwise from __flase_type. Type <code class="computeroutput"><span class="identifier">T</span></code>
  37         must be a complete type.
  38       </p>
  39 <p>
  40         Note that this trait is somewhat tricky to use correctly: for example:
  41       </p>
  42 <pre class="programlisting"><span class="identifier">is_assignable</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">,</span> <span class="keyword">int</span><span class="special">&gt;::</span><span class="identifier">value</span>
  43 </pre>
  44 <p>
  45         is <code class="computeroutput"><span class="keyword">false</span></code> since <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">declval</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">&gt;()</span></code>
  46         is an <span class="emphasis"><em>xvalue</em></span> which can not be assigned to!
  47       </p>
  48 <p>
  49         If you're intention is to check for copy-assignment from some type U then
  50         use:
  51       </p>
  52 <pre class="programlisting"><span class="identifier">is_assignable</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&amp;,</span> <span class="keyword">const</span> <span class="identifier">U</span><span class="special">&amp;&gt;::</span><span class="identifier">value</span>
  53 </pre>
  54 <p>
  55         If you're intention is to check for move-assignment then use:
  56       </p>
  57 <pre class="programlisting"><span class="identifier">is_assignable</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&amp;,</span> <span class="identifier">U</span><span class="special">&amp;&amp;&gt;::</span><span class="identifier">value</span>
  58 </pre>
  59 <p>
  60         or simply:
  61       </p>
  62 <pre class="programlisting"><span class="identifier">is_assignable</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&amp;,</span> <span class="identifier">U</span><span class="special">&gt;::</span><span class="identifier">value</span>
  63 </pre>
  64 <p>
  65         <span class="bold"><strong>Compiler Compatibility:</strong></span> Requires the C++11
  66         features <code class="computeroutput"><span class="keyword">decltype</span></code> and SFINAE-expressions
  67         for full support.
  68       </p>
  69 <p>
  70         <span class="bold"><strong>Header:</strong></span> <code class="computeroutput"> <span class="preprocessor">#include</span>
  71         <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">type_traits</span><span class="special">/</span><span class="identifier">is_assignable</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span></code>
  72         or <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">type_traits</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span></code>
  73       </p>
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