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<div class="section" id="boost-mpl-has-xxx-template-named-def">
<h1><a class="toc-backref" href="./introspection.html#id1594">BOOST_MPL_HAS_XXX_TEMPLATE_NAMED_DEF</a></h1>
<div class="section" id="id1322">
<h3><a class="subsection-title" href="#synopsis" name="synopsis">Synopsis</a></h3>
<pre class="literal-block">
#define <a href="./has-xxx-template-named-def.html" class="identifier">BOOST_MPL_HAS_XXX_TEMPLATE_NAMED_DEF</a>(trait, name, default_) \
    <em>unspecified token sequence</em> \
/**/
</pre>
</div>
<div class="section" id="id1323">
<h3><a class="subsection-title" href="#description" name="description">Description</a></h3>
<p>Expands into the definition of a boolean <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 template member <tt class="literal"><span class="pre">x::template</span>
<span class="pre">name</span></tt> with no more than <a class="reference internal" href="./limit-metafunction-arity.html">BOOST_MPL_LIMIT_METAFUNCTION_ARITY</a>
parameters.</p>
<p>On deficient compilers not capable 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 configuration macro, <a class="reference internal" href="./cfg-no-has-xxx-template.html">BOOST_MPL_CFG_NO_HAS_XXX_TEMPLATE</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="id1324">
<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="id1325">
<h3><a class="subsection-title" href="#parameters" name="parameters">Parameters</a></h3>
<table border="1" class="docutils table">
<colgroup>
<col width="15%" />
<col width="32%" />
<col width="53%" />
</colgroup>
<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="id1326">
<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-template-named-def.html" class="identifier">BOOST_MPL_HAS_XXX_TEMPLATE_NAMED_DEF</a>(trait, name, c1)
</pre>
<table class="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">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>
<table class="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"><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-template.html">BOOST_MPL_CFG_NO_HAS_XXX_TEMPLATE</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 template member <tt class="literal"><span class="pre">x::template</span>
<span class="pre">name</span></tt> with no more than <a class="reference internal" href="./limit-metafunction-arity.html">BOOST_MPL_LIMIT_METAFUNCTION_ARITY</a>.</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-template.html">BOOST_MPL_CFG_NO_HAS_XXX_TEMPLATE</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>
</td>
</tr>
</tbody>
</table>
</td>
</tr>
</tbody>
</table>
</div>
<div class="section" id="id1327">
<h3><a class="subsection-title" href="#example" name="example">Example</a></h3>
<pre class="literal-block">
<a href="./has-xxx-template-named-def.html" class="identifier">BOOST_MPL_HAS_XXX_TEMPLATE_NAMED_DEF</a>(
    has_xxx, xxx, false
)

struct test1  {};
struct test2  { void xxx(); };
struct test3  { int xxx; };
struct test4  { static int xxx(); };
struct test5  { typedef int xxx; };
struct test6  { struct xxx; };
struct test7  { typedef void (*xxx)(); };
struct test8  { typedef void (xxx)(); };
struct test9  { template&lt; class T &gt; struct 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; ));
<a href="./assert-not.html" class="identifier">BOOST_MPL_ASSERT_NOT</a>(( has_xxx&lt;test6&gt; ));
<a href="./assert-not.html" class="identifier">BOOST_MPL_ASSERT_NOT</a>(( has_xxx&lt;test7&gt; ));
<a href="./assert-not.html" class="identifier">BOOST_MPL_ASSERT_NOT</a>(( has_xxx&lt;test8&gt; ));

#if !defined(<a href="./cfg-no-has-xxx-template.html" class="identifier">BOOST_MPL_CFG_NO_HAS_XXX_TEMPLATE</a>)
<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;test9, <a href="./bool.html" class="identifier">true_</a>&gt; ));
</pre>
</div>
<div class="section" id="id1328">
<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-template-def.html">BOOST_MPL_HAS_XXX_TEMPLATE_DEF</a>,
<a class="reference internal" href="./cfg-no-has-xxx-template.html">BOOST_MPL_CFG_NO_HAS_XXX_TEMPLATE</a>, <a class="reference internal" href="./limit-metafunction-arity.html">BOOST_MPL_LIMIT_METAFUNCTION_ARITY</a></p>
<!-- Macros/Introspection//BOOST_MPL_HAS_XXX_TRAIT_DEF -->
</div>
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