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<h2><span class="refentrytitle">Macro BOOST_TTI_HAS_UNION</span></h2>
<p>BOOST_TTI_HAS_UNION — A macro which expands to a metafunction which tests whether an inner union with a particular name exists. </p>
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<h2 xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" class="refsynopsisdiv-title">Synopsis</h2>
<div xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" class="refsynopsisdiv"><pre class="synopsis"><span class="comment">// In header: &lt;<a class="link" href="header/boost/tti/has_union_hpp.html" title="Header &lt;boost/tti/has_union.hpp&gt;">boost/tti/has_union.hpp</a>&gt;
</span>BOOST_TTI_HAS_UNION(name)</pre></div>
<div class="refsect1">
<a name="idm46191511554336"></a><h2>Description</h2>
<p>BOOST_TTI_HAS_UNION is a macro which expands to a metafunction. The metafunction tests whether an inner union with a particular name exists and, optionally, whether an MPL lambda expression invoked with the inner union is true or not. The macro takes the form of BOOST_TTI_HAS_UNION(name) where</p>
<p>name = the name of the inner union.</p>
<p>BOOST_TTI_HAS_UNION generates a metafunction called "has_union_'name'" where 'name' is the macro parameter.</p>
<pre class="programlisting"><span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">class</span> <span class="identifier">BOOST_TTI_TP_T</span><span class="special">,</span><span class="keyword">class</span> <span class="identifier">BOOST_TTI_TP_U</span><span class="special">&gt;</span>
<span class="keyword">struct</span> <span class="identifier">has_union_</span><span class="char">'name'</span>
<span class="special">{</span>
<span class="keyword">static</span> <span class="keyword">const</span> <span class="identifier">value</span> <span class="special">=</span> <span class="identifier">unspecified</span><span class="special">;</span>
<span class="keyword">typedef</span> <span class="identifier">mpl</span><span class="special">::</span><span class="identifier">bool_</span><span class="special">&lt;</span><span class="keyword">true</span><span class="special">-</span><span class="identifier">or</span><span class="special">-</span><span class="keyword">false</span><span class="special">&gt;</span> <span class="identifier">type</span><span class="special">;</span>
<span class="special">}</span><span class="special">;</span>
<span class="identifier">The</span> <span class="identifier">metafunction</span> <span class="identifier">types</span> <span class="identifier">and</span> <span class="keyword">return</span><span class="special">:</span>
<span class="identifier">BOOST_TTI_TP_T</span> <span class="special">=</span> <span class="identifier">the</span> <span class="identifier">enclosing</span> <span class="identifier">type</span> <span class="identifier">in</span> <span class="identifier">which</span> <span class="identifier">to</span> <span class="identifier">look</span> <span class="keyword">for</span> <span class="identifier">our</span> <span class="char">'name'</span><span class="special">.</span>
<span class="identifier">BOOST_TTI_TP_U</span> <span class="special">=</span> <span class="special">(</span><span class="identifier">optional</span><span class="special">)</span> <span class="identifier">An</span> <span class="identifier">optional</span> <span class="keyword">template</span> <span class="identifier">parameter</span><span class="special">,</span> <span class="identifier">defaulting</span> <span class="identifier">to</span> <span class="identifier">a</span> <span class="identifier">marker</span> <span class="identifier">type</span><span class="special">.</span>
<span class="identifier">If</span> <span class="identifier">specified</span> <span class="identifier">it</span> <span class="identifier">is</span> <span class="identifier">an</span> <span class="identifier">MPL</span> <span class="identifier">lambda</span> <span class="identifier">expression</span> <span class="identifier">which</span> <span class="identifier">is</span> <span class="identifier">invoked</span>
<span class="identifier">with</span> <span class="identifier">the</span> <span class="identifier">inner</span> <span class="keyword">union</span> <span class="identifier">found</span> <span class="identifier">and</span> <span class="identifier">must</span> <span class="keyword">return</span> <span class="identifier">a</span> <span class="identifier">constant</span> <span class="identifier">boolean</span>
<span class="identifier">value</span><span class="special">.</span>
<span class="identifier">returns</span> <span class="special">=</span> <span class="char">'value'</span> <span class="identifier">depends</span> <span class="identifier">on</span> <span class="identifier">whether</span> <span class="identifier">or</span> <span class="identifier">not</span> <span class="identifier">the</span> <span class="identifier">optional</span> <span class="identifier">BOOST_TTI_TP_U</span> <span class="identifier">is</span> <span class="identifier">specified</span><span class="special">.</span>
<span class="identifier">If</span> <span class="identifier">BOOST_TTI_TP_U</span> <span class="identifier">is</span> <span class="identifier">not</span> <span class="identifier">specified</span><span class="special">,</span> <span class="identifier">then</span> <span class="char">'value'</span> <span class="identifier">is</span> <span class="keyword">true</span> <span class="keyword">if</span> <span class="identifier">the</span> <span class="char">'name'</span> <span class="keyword">union</span>
<span class="identifier">exists</span> <span class="identifier">within</span> <span class="identifier">the</span> <span class="identifier">enclosing</span> <span class="identifier">type</span> <span class="identifier">BOOST_TTI_TP_T</span><span class="special">;</span> <span class="identifier">otherwise</span> <span class="char">'value'</span> <span class="identifier">is</span> <span class="keyword">false</span><span class="special">.</span>
<span class="identifier">If</span> <span class="identifier">BOOST_TTI_TP_U</span> <span class="identifier">is</span> <span class="identifier">specified</span><span class="special">,</span> <span class="identifier">then</span> <span class="char">'value'</span> <span class="identifier">is</span> <span class="keyword">true</span> <span class="keyword">if</span> <span class="identifier">the</span> <span class="char">'name'</span> <span class="keyword">union</span> <span class="identifier">exists</span>
<span class="identifier">within</span> <span class="identifier">the</span> <span class="identifier">enclosing</span> <span class="identifier">type</span> <span class="identifier">BOOST_TTI_TP_T</span> <span class="identifier">and</span> <span class="identifier">the</span> <span class="identifier">MPL</span> <span class="identifier">lambda</span> <span class="identifier">expression</span> <span class="identifier">as</span> <span class="identifier">specified</span>
<span class="identifier">by</span> <span class="identifier">BOOST_TTI_TP_U</span><span class="special">,</span> <span class="identifier">invoked</span> <span class="identifier">by</span> <span class="identifier">passing</span> <span class="identifier">the</span> <span class="identifier">actual</span> <span class="identifier">inner</span> <span class="keyword">union</span> <span class="identifier">of</span> <span class="char">'name'</span><span class="special">,</span> <span class="identifier">returns</span>
<span class="identifier">a</span> <span class="char">'value'</span> <span class="identifier">of</span> <span class="keyword">true</span><span class="special">;</span> <span class="identifier">otherwise</span> <span class="char">'value'</span> <span class="identifier">is</span> <span class="keyword">false</span><span class="special">.</span>
<span class="identifier">The</span> <span class="identifier">action</span> <span class="identifier">taken</span> <span class="identifier">with</span> <span class="identifier">BOOST_TTI_TP_U</span> <span class="identifier">occurs</span> <span class="identifier">only</span> <span class="identifier">when</span> <span class="identifier">the</span> <span class="char">'name'</span> <span class="keyword">union</span> <span class="identifier">exists</span>
<span class="identifier">within</span> <span class="identifier">the</span> <span class="identifier">enclosing</span> <span class="identifier">type</span> <span class="identifier">BOOST_TTI_TP_T</span><span class="special">.</span>
</pre>
<p>Example usage:</p>
<pre class="programlisting"><span class="identifier">BOOST_TTI_HAS_UNION</span><span class="special">(</span><span class="identifier">MyType</span><span class="special">)</span> <span class="identifier">generates</span> <span class="identifier">the</span> <span class="identifier">metafunction</span> <span class="identifier">has_union_MyType</span> <span class="identifier">in</span> <span class="identifier">the</span> <span class="identifier">current</span> <span class="identifier">scope</span>
<span class="identifier">to</span> <span class="identifier">look</span> <span class="keyword">for</span> <span class="identifier">an</span> <span class="identifier">inner</span> <span class="keyword">union</span> <span class="identifier">called</span> <span class="identifier">MyType</span><span class="special">.</span>
<span class="identifier">has_union_MyType</span><span class="special">&lt;</span><span class="identifier">EnclosingType</span><span class="special">&gt;</span><span class="special">::</span><span class="identifier">value</span> <span class="identifier">is</span> <span class="keyword">true</span> <span class="keyword">if</span> <span class="identifier">MyType</span> <span class="identifier">is</span> <span class="identifier">an</span> <span class="identifier">inner</span> <span class="keyword">union</span> <span class="identifier">of</span> <span class="identifier">EnclosingType</span><span class="special">,</span> <span class="identifier">otherwise</span> <span class="keyword">false</span><span class="special">.</span>
<span class="identifier">has_class_MyType</span><span class="special">&lt;</span><span class="identifier">EnclosingType</span><span class="special">,</span><span class="identifier">ALambdaExpression</span><span class="special">&gt;</span><span class="special">::</span><span class="identifier">value</span> <span class="identifier">is</span> <span class="keyword">true</span> <span class="keyword">if</span> <span class="identifier">MyType</span> <span class="identifier">is</span> <span class="identifier">an</span> <span class="identifier">inner</span> <span class="keyword">union</span> <span class="identifier">of</span> <span class="identifier">EnclosingType</span>
<span class="identifier">and</span> <span class="identifier">invoking</span> <span class="identifier">ALambdaExpression</span> <span class="identifier">with</span> <span class="identifier">the</span> <span class="identifier">inner</span> <span class="keyword">union</span> <span class="identifier">returns</span> <span class="identifier">a</span> <span class="identifier">value</span> <span class="identifier">of</span> <span class="keyword">true</span><span class="special">,</span> <span class="identifier">otherwise</span> <span class="keyword">false</span><span class="special">.</span>
<span class="identifier">A</span> <span class="identifier">popular</span> <span class="identifier">use</span> <span class="identifier">of</span> <span class="identifier">the</span> <span class="identifier">optional</span> <span class="identifier">MPL</span> <span class="identifier">lambda</span> <span class="identifier">expression</span> <span class="identifier">is</span> <span class="identifier">to</span> <span class="identifier">check</span> <span class="identifier">whether</span> <span class="identifier">the</span> <span class="keyword">union</span> <span class="identifier">found</span> <span class="identifier">is</span> <span class="identifier">the</span> <span class="identifier">same</span>
<span class="identifier">as</span> <span class="identifier">another</span> <span class="identifier">type</span><span class="special">,</span> <span class="identifier">when</span> <span class="identifier">the</span> <span class="keyword">union</span> <span class="identifier">found</span> <span class="identifier">is</span> <span class="identifier">a</span> <span class="keyword">typedef</span><span class="special">.</span> <span class="identifier">In</span> <span class="identifier">that</span> <span class="keyword">case</span> <span class="identifier">our</span> <span class="identifier">example</span> <span class="identifier">would</span> <span class="identifier">be</span><span class="special">:</span>
<span class="identifier">has_union_MyType</span><span class="special">&lt;</span><span class="identifier">EnclosingType</span><span class="special">,</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">is_same</span><span class="special">&lt;</span><span class="identifier">_</span><span class="special">,</span><span class="identifier">SomeOtherType</span><span class="special">&gt;</span> <span class="special">&gt;</span><span class="special">::</span><span class="identifier">value</span> <span class="identifier">is</span> <span class="keyword">true</span> <span class="keyword">if</span> <span class="identifier">MyType</span> <span class="identifier">is</span> <span class="identifier">an</span> <span class="identifier">inner</span> <span class="keyword">union</span>
<span class="identifier">of</span> <span class="identifier">EnclosingType</span> <span class="identifier">and</span> <span class="identifier">is</span> <span class="identifier">the</span> <span class="identifier">same</span> <span class="identifier">type</span> <span class="identifier">as</span> <span class="identifier">SomeOtherType</span><span class="special">.</span>
</pre>
<p> </p>
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