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<div class="titlepage"><div><div><h3 class="title">
<a name="boost_typetraits.reference.is_union"></a><a class="link" href="is_union.html" title="is_union">is_union</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">&gt;</span>
<span class="keyword">struct</span> <span class="identifier">is_union</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 T is a (possibly cv-qualified)
        union type then inherits from <a class="link" href="integral_constant.html" title="integral_constant">true_type</a>,
        otherwise inherits from <a class="link" href="integral_constant.html" title="integral_constant">false_type</a>.
        Currently requires some kind of compiler support, otherwise unions are identified
        as classes.
      </p>
<p>
        <span class="bold"><strong>C++ Standard Reference:</strong></span> 3.9.2 and 9.5.
      </p>
<p>
        <span class="bold"><strong>Compiler Compatibility:</strong></span> Without (some as
        yet unspecified) help from the compiler, we cannot distinguish between union
        and class types using only standard C++, as a result this type will never
        inherit from <a class="link" href="integral_constant.html" title="integral_constant">true_type</a>,
        unless the user explicitly specializes the template for their user-defined
        union types, or unless the compiler supplies some unspecified intrinsic that
        implements this functionality. Currently (June 2015) compilers more recent
        than Visual C++ 8, clang, GCC-4.3, Greenhills 6.0, Intel-11.0, and Codegear
        have the necessary compiler <a class="link" href="../intrinsics.html" title="Support for Compiler Intrinsics">intrinsics</a>
        to ensure that this trait "just works". You may also test to see
        if the necessary <a class="link" href="../intrinsics.html" title="Support for Compiler Intrinsics">intrinsics</a>
        are available by checking to see if the macro <code class="computeroutput"><span class="identifier">BOOST_IS_UNION</span></code>
        is defined.
      </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_union</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>
<p>
        <span class="bold"><strong>Examples:</strong></span>
      </p>
<p>
        Given <code class="computeroutput"><span class="keyword">union</span> <span class="identifier">my_union</span>
        <span class="special">{};</span></code> then:
      </p>
<div class="blockquote"><blockquote class="blockquote"><p>
          <code class="computeroutput"><span class="identifier">is_union</span><span class="special">&lt;</span><span class="identifier">my_union</span><span class="special">&gt;</span></code>
          inherits from <code class="computeroutput"><a class="link" href="integral_constant.html" title="integral_constant">true_type</a></code>.
        </p></blockquote></div>
<div class="blockquote"><blockquote class="blockquote"><p>
          <code class="computeroutput"><span class="identifier">is_union</span><span class="special">&lt;</span><span class="keyword">const</span> <span class="identifier">my_union</span><span class="special">&gt;::</span><span class="identifier">type</span></code>
          is the type <code class="computeroutput"><a class="link" href="integral_constant.html" title="integral_constant">true_type</a></code>.
        </p></blockquote></div>
<div class="blockquote"><blockquote class="blockquote"><p>
          <code class="computeroutput"><span class="identifier">is_union</span><span class="special">&lt;</span><span class="identifier">my_union</span><span class="special">&gt;::</span><span class="identifier">value</span></code> is an integral constant expression
          that evaluates to <span class="emphasis"><em>true</em></span>.
        </p></blockquote></div>
<div class="blockquote"><blockquote class="blockquote"><p>
          <code class="computeroutput"><span class="identifier">is_union</span><span class="special">&lt;</span><span class="identifier">my_union</span><span class="special">*&gt;::</span><span class="identifier">value</span></code> is an integral constant expression
          that evaluates to <span class="emphasis"><em>false</em></span>.
        </p></blockquote></div>
<div class="blockquote"><blockquote class="blockquote"><p>
          <code class="computeroutput"><span class="identifier">is_union</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;::</span><span class="identifier">value_type</span></code> is the type <code class="computeroutput"><span class="keyword">bool</span></code>.
        </p></blockquote></div>
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