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<h2><span class="refentrytitle">Macro BOOST_TTI_TRAIT_MEMBER_TYPE</span></h2>
<p>BOOST_TTI_TRAIT_MEMBER_TYPE — A macro expands to a metafunction whose typedef 'type' is either the named type or a marker type. </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/member_type_hpp.html" title="Header &lt;boost/tti/member_type.hpp&gt;">boost/tti/member_type.hpp</a>&gt;
</span>BOOST_TTI_TRAIT_MEMBER_TYPE(trait, name)</pre></div>
<div class="refsect1">
<a name="idm46191511137824"></a><h2>Description</h2>
<p>BOOST_TTI_TRAIT_MEMBER_TYPE is a macro which expands to a metafunction. The metafunction tests whether an inner type with a particular name exists by returning the inner type or a marker type. The macro takes the form of BOOST_TTI_TRAIT_MEMBER_TYPE(trait,name) where</p>
<p>trait = the name of the metafunction <br>
name = the name of the inner type.</p>
<p>BOOST_TTI_TRAIT_MEMBER_TYPE generates a metafunction called "trait" where 'trait' 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_MARKER_TYPE</span> <span class="special">=</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">tti</span><span class="special">::</span><span class="identifier">detail</span><span class="special">::</span><span class="identifier">notype</span><span class="special">&gt;</span>
<span class="keyword">struct</span> <span class="identifier">trait</span>
<span class="special">{</span>
<span class="keyword">typedef</span> <span class="identifier">unspecified</span> <span class="identifier">type</span><span class="special">;</span>
<span class="keyword">typedef</span> <span class="identifier">BOOST_TTI_TP_MARKER_TYPE</span> <span class="identifier">boost_tti_marker_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="special">.</span>
<span class="identifier">The</span> <span class="identifier">enclosing</span> <span class="identifier">type</span> <span class="identifier">can</span> <span class="identifier">be</span> <span class="identifier">a</span> <span class="keyword">class</span><span class="special">,</span> <span class="keyword">struct</span><span class="special">,</span> <span class="identifier">or</span> <span class="keyword">union</span><span class="special">.</span>
<span class="identifier">BOOST_TTI_TP_MARKER_TYPE</span> <span class="special">=</span> <span class="special">(</span><span class="identifier">optional</span><span class="special">)</span> <span class="identifier">a</span> <span class="identifier">type</span> <span class="identifier">to</span> <span class="identifier">use</span> <span class="identifier">as</span> <span class="identifier">the</span> <span class="identifier">marker</span> <span class="identifier">type</span><span class="special">.</span>
<span class="identifier">defaults</span> <span class="identifier">to</span> <span class="identifier">the</span> <span class="identifier">internal</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">tti</span><span class="special">::</span><span class="identifier">detail</span><span class="special">::</span><span class="identifier">notype</span><span class="special">.</span>
<span class="identifier">returns</span> <span class="special">=</span> <span class="char">'type'</span> <span class="identifier">is</span> <span class="identifier">the</span> <span class="identifier">inner</span> <span class="identifier">type</span> <span class="identifier">of</span> <span class="char">'name'</span> <span class="keyword">if</span> <span class="identifier">the</span> <span class="identifier">inner</span> <span class="identifier">type</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="special">,</span> <span class="keyword">else</span> <span class="char">'type'</span> <span class="identifier">is</span> <span class="identifier">a</span> <span class="identifier">marker</span> <span class="identifier">type</span><span class="special">.</span>
<span class="keyword">if</span> <span class="identifier">the</span> <span class="identifier">end</span><span class="special">-</span><span class="identifier">user</span> <span class="identifier">does</span> <span class="identifier">not</span> <span class="identifier">specify</span> <span class="identifier">a</span> <span class="identifier">marker</span> <span class="identifier">type</span> <span class="identifier">then</span>
<span class="identifier">an</span> <span class="identifier">internal</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">tti</span><span class="special">::</span><span class="identifier">detail</span><span class="special">::</span><span class="identifier">notype</span> <span class="identifier">marker</span> <span class="identifier">type</span><span class="special">,</span>
<span class="identifier">which</span> <span class="identifier">is</span> <span class="identifier">empty</span><span class="special">,</span> <span class="identifier">is</span> <span class="identifier">used</span><span class="special">.</span>
<span class="identifier">The</span> <span class="identifier">metafunction</span> <span class="identifier">also</span> <span class="identifier">encapsulates</span> <span class="identifier">the</span> <span class="identifier">type</span> <span class="identifier">of</span> <span class="identifier">the</span> <span class="identifier">marker</span> <span class="identifier">type</span> <span class="identifier">as</span>
<span class="identifier">a</span> <span class="identifier">nested</span> <span class="char">'boost_tti_marker_type'</span><span class="special">.</span>
</pre>
<p>The purpose of this macro is to encapsulate the 'name' type as the typedef 'type' of a metafunction, but only if it exists within the enclosing type. This allows for an evaluation of inner type existence, without generating a compiler error, which can be used by other metafunctions in this library. </p>
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