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<div class="section" id="boost-mpl-has-xxx-trait-named-def">
<h1><a class="toc-backref" href="./introspection.html#id1596">BOOST_MPL_HAS_XXX_TRAIT_NAMED_DEF</a></h1>
<div class="section" id="id1336">
<h3><a class="subsection-title" href="#synopsis" name="synopsis">Synopsis</a></h3>
<pre class="literal-block">
#define <a href="./has-xxx-trait-named-def.html" class="identifier">BOOST_MPL_HAS_XXX_TRAIT_NAMED_DEF</a>(trait, name, default_) \
<em>unspecified token sequence</em> \
/**/
</pre>
</div>
<div class="section" id="id1337">
<h3><a class="subsection-title" href="#description" name="description">Description</a></h3>
<p>Expands into a definition of a boolean unary <a class="reference internal" href="./metafunction.html">Metafunction</a> <tt class="literal"><span class="pre">trait</span></tt>
such that for any type <tt class="literal"><span class="pre">x</span></tt> <tt class="literal"><span class="pre">trait&lt;x&gt;::value</span> <span class="pre">==</span> <span class="pre">true</span></tt> if and only
if <tt class="literal"><span class="pre">x</span></tt> is a class type and has a nested type memeber <tt class="literal"><span class="pre">x::name</span></tt>.</p>
<p>On the deficient compilers not capabale of performing the detection,
<tt class="literal"><span class="pre">trait&lt;x&gt;::value</span></tt> always returns a fallback value <tt class="literal"><span class="pre">default_</span></tt>.
A boolean configuraion macro, <a class="reference internal" href="./cfg-no-has-xxx.html">BOOST_MPL_CFG_NO_HAS_XXX</a>, is provided
to signal or override the &quot;deficient&quot; status of a particular compiler.
[<em>Note:</em> The fallback value can also be provided at the point of the
metafunction invocation; see the <cite>Expression semantics</cite> section for
details — <em>end note</em>]</p>
</div>
<div class="section" id="id1338">
<h3><a class="subsection-title" href="#header" name="header">Header</a></h3>
<pre class="literal-block">
#include &lt;<a href="../../../../boost/mpl/has_xxx.hpp" class="header">boost/mpl/has_xxx.hpp</a>&gt;
</pre>
</div>
<div class="section" id="id1339">
<h3><a class="subsection-title" href="#parameters" name="parameters">Parameters</a></h3>
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<col width="32%" />
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<thead valign="bottom">
<tr><th class="head">Parameter</th>
<th class="head">Requirement</th>
<th class="head">Description</th>
</tr>
</thead>
<tbody valign="top">
<tr><td><tt class="literal"><span class="pre">trait</span></tt></td>
<td>A legal identifier token</td>
<td>A name of the metafunction to be generated.</td>
</tr>
<tr><td><tt class="literal"><span class="pre">name</span></tt></td>
<td>A legal identifier token</td>
<td>A name of the member being detected.</td>
</tr>
<tr><td><tt class="literal"><span class="pre">default_</span></tt></td>
<td>An boolean constant</td>
<td>A fallback value for the deficient compilers.</td>
</tr>
</tbody>
</table>
</div>
<div class="section" id="id1340">
<h3><a class="subsection-title" href="#expression-semantics" name="expression-semantics">Expression semantics</a></h3>
<p>For any legal C++ identifiers <tt class="literal"><span class="pre">trait</span></tt> and <tt class="literal"><span class="pre">name</span></tt>, boolean constant expression <tt class="literal"><span class="pre">c1</span></tt>,
boolean <a class="reference internal" href="./integral-constant.html">Integral Constant</a> <tt class="literal"><span class="pre">c2</span></tt>, and arbitrary type <tt class="literal"><span class="pre">x</span></tt>:</p>
<pre class="literal-block">
<a href="./has-xxx-trait-named-def.html" class="identifier">BOOST_MPL_HAS_XXX_TRAIT_NAMED_DEF</a>(trait, name, c1)
</pre>
<table class="docutils field-list" frame="void" rules="none">
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<tr class="field"><th class="field-name">Precondition:</th><td class="field-body"><p class="first">Appears at namespace scope.</p>
</td>
</tr>
<tr class="field"><th class="field-name">Return type:</th><td class="field-body"><p class="first">None.</p>
</td>
</tr>
<tr class="field"><th class="field-name">Semantics:</th><td class="field-body"><p class="first">Expands into an equivalent of the following class template definition</p>
<pre class="literal-block">
template&lt; typename X, typename fallback = boost::mpl::<a href="./bool.html" class="identifier">bool_</a>&lt;c1&gt; &gt;
struct trait
{
// <em>unspecified</em>
// ...
};
</pre>
<p>where <tt class="literal"><span class="pre">trait</span></tt> is a boolean <a class="reference internal" href="./metafunction.html">Metafunction</a> with the following semantics:</p>
<pre class="literal-block">
typedef trait&lt;x&gt;::type r;
</pre>
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<tbody valign="top">
<tr class="field"><th class="field-name">Return type:</th><td class="field-body"><a class="reference internal" href="./integral-constant.html">Integral Constant</a>.</td>
</tr>
<tr class="field"><th class="field-name">Semantics:</th><td class="field-body">If <a class="reference internal" href="./cfg-no-has-xxx.html">BOOST_MPL_CFG_NO_HAS_XXX</a> is defined, <tt class="literal"><span class="pre">r::value</span> <span class="pre">==</span> <span class="pre">c1</span></tt>;
otherwise, <tt class="literal"><span class="pre">r::value</span> <span class="pre">==</span> <span class="pre">true</span></tt> if and only if <tt class="literal"><span class="pre">x</span></tt> is a class type
that has a nested type memeber <tt class="literal"><span class="pre">x::name</span></tt>.</td>
</tr>
</tbody>
</table>
<pre class="literal-block">
typedef trait&lt; x,c2 &gt;::type r;
</pre>
<table class="last docutils field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name">Return type:</th><td class="field-body"><p class="first"><a class="reference internal" href="./integral-constant.html">Integral Constant</a>.</p>
</td>
</tr>
<tr class="field"><th class="field-name">Semantics:</th><td class="field-body"><p class="first">If <a class="reference internal" href="./cfg-no-has-xxx.html">BOOST_MPL_CFG_NO_HAS_XXX</a> is defined, <tt class="literal"><span class="pre">r::value</span> <span class="pre">==</span> <span class="pre">c2::value</span></tt>;
otherwise, equivalent to</p>
<pre class="last literal-block">
typedef trait&lt;x&gt;::type r;
</pre>
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</div>
<div class="section" id="id1341">
<h3><a class="subsection-title" href="#example" name="example">Example</a></h3>
<pre class="literal-block">
<a href="./has-xxx-trait-named-def.html" class="identifier">BOOST_MPL_HAS_XXX_TRAIT_NAMED_DEF</a>(has_xxx, xxx, false)
struct test1 {};
struct test2 { void xxx(); };
struct test3 { int xxx; };
struct test4 { static int xxx(); };
struct test5 { template&lt; typename T &gt; struct xxx {}; };
struct test6 { typedef int xxx; };
struct test7 { struct xxx; };
struct test8 { typedef void (*xxx)(); };
struct test9 { typedef void (xxx)(); };
<a href="./assert-not.html" class="identifier">BOOST_MPL_ASSERT_NOT</a>(( has_xxx&lt;test1&gt; ));
<a href="./assert-not.html" class="identifier">BOOST_MPL_ASSERT_NOT</a>(( has_xxx&lt;test2&gt; ));
<a href="./assert-not.html" class="identifier">BOOST_MPL_ASSERT_NOT</a>(( has_xxx&lt;test3&gt; ));
<a href="./assert-not.html" class="identifier">BOOST_MPL_ASSERT_NOT</a>(( has_xxx&lt;test4&gt; ));
<a href="./assert-not.html" class="identifier">BOOST_MPL_ASSERT_NOT</a>(( has_xxx&lt;test5&gt; ));
#if !defined(<a href="./cfg-no-has-xxx.html" class="identifier">BOOST_MPL_CFG_NO_HAS_XXX</a>)
<a href="./assert.html" class="identifier">BOOST_MPL_ASSERT</a>(( has_xxx&lt;test6&gt; ));
<a href="./assert.html" class="identifier">BOOST_MPL_ASSERT</a>(( has_xxx&lt;test7&gt; ));
<a href="./assert.html" class="identifier">BOOST_MPL_ASSERT</a>(( has_xxx&lt;test8&gt; ));
<a href="./assert.html" class="identifier">BOOST_MPL_ASSERT</a>(( has_xxx&lt;test9&gt; ));
#endif
<a href="./assert.html" class="identifier">BOOST_MPL_ASSERT</a>(( has_xxx&lt;test6,<a href="./bool.html" class="identifier">true_</a>&gt; ));
<a href="./assert.html" class="identifier">BOOST_MPL_ASSERT</a>(( has_xxx&lt;test7,<a href="./bool.html" class="identifier">true_</a>&gt; ));
<a href="./assert.html" class="identifier">BOOST_MPL_ASSERT</a>(( has_xxx&lt;test8,<a href="./bool.html" class="identifier">true_</a>&gt; ));
<a href="./assert.html" class="identifier">BOOST_MPL_ASSERT</a>(( has_xxx&lt;test9,<a href="./bool.html" class="identifier">true_</a>&gt; ));
</pre>
</div>
<div class="section" id="id1342">
<h3><a class="subsection-title" href="#see-also" name="see-also">See also</a></h3>
<p><a class="reference internal" href="./macros.html">Macros</a>, <a class="reference internal" href="./has-xxx-trait-def.html">BOOST_MPL_HAS_XXX_TRAIT_DEF</a>, <a class="reference internal" href="./cfg-no-has-xxx.html">BOOST_MPL_CFG_NO_HAS_XXX</a></p>
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