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<div class="titlepage"><div><div><h3 class="title">
<a name="boost_typetraits.reference.is_assignable"></a><a class="link" href="is_assignable.html" title="is_assignable">is_assignable</a>
</h3></div></div></div>
<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>
<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>
</pre>
<p>
        <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>
        <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>
        then inherits from <a class="link" href="integral_constant.html" title="integral_constant">true_type</a>,
        otherwise from <a class="link" href="integral_constant.html" title="integral_constant">false_type</a>.
        Type <code class="computeroutput"><span class="identifier">T</span></code> must be a complete
        type.
      </p>
<p>
        Note that this trait is somewhat tricky to use correctly: for example:
      </p>
<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>
</pre>
<p>
        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>
        is an <span class="emphasis"><em>xvalue</em></span> which can not be assigned to!
      </p>
<p>
        If you're intention is to check for copy-assignment from some type U then
        use:
      </p>
<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>
</pre>
<p>
        If you're intention is to check for move-assignment then use:
      </p>
<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>
</pre>
<p>
        or simply:
      </p>
<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>
</pre>
<p>
        <span class="bold"><strong>Compiler Compatibility:</strong></span> Requires the C++11
        features <code class="computeroutput"><span class="keyword">decltype</span></code> and SFINAE-expressions
        for full support.
      </p>
<p>
        <span class="bold"><strong>Header:</strong></span> <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">is_assignable</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span></code>
        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>
      </p>
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