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193 lines
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<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
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<meta name="generator" content="Docutils 0.7: http://docutils.sourceforge.net/" />
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<title>The MPL Reference Manual: BOOST_MPL_HAS_XXX_TEMPLATE_NAMED_DEF</title>
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<body class="docframe refmanual">
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<table class="header"><tr class="header"><td class="header-group navigation-bar"><span class="navigation-group"><a href="./has-xxx-template-def.html" class="navigation-link">Prev</a> <a href="./has-xxx-trait-def.html" class="navigation-link">Next</a></span><span class="navigation-group-separator"> | </span><span class="navigation-group"><a href="./has-xxx-template-def.html" class="navigation-link">Back</a> <a href="./has-xxx-trait-def.html" class="navigation-link">Along</a></span><span class="navigation-group-separator"> | </span><span class="navigation-group"><a href="./introspection.html" class="navigation-link">Up</a> <a href="../refmanual.html" class="navigation-link">Home</a></span><span class="navigation-group-separator"> | </span><span class="navigation-group"><a href="./refmanual_toc.html" class="navigation-link">Full TOC</a></span></td>
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<td class="header-group page-location"><a href="../refmanual.html" class="navigation-link">Front Page</a> / <a href="./macros.html" class="navigation-link">Macros</a> / <a href="./introspection.html" class="navigation-link">Introspection</a> / <a href="./has-xxx-template-named-def.html" class="navigation-link">BOOST_MPL_HAS_XXX_TEMPLATE_NAMED_DEF</a></td>
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</tr></table><div class="header-separator"></div>
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<div class="section" id="boost-mpl-has-xxx-template-named-def">
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<h1><a class="toc-backref" href="./introspection.html#id1594">BOOST_MPL_HAS_XXX_TEMPLATE_NAMED_DEF</a></h1>
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<div class="section" id="id1322">
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<h3><a class="subsection-title" href="#synopsis" name="synopsis">Synopsis</a></h3>
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<pre class="literal-block">
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#define <a href="./has-xxx-template-named-def.html" class="identifier">BOOST_MPL_HAS_XXX_TEMPLATE_NAMED_DEF</a>(trait, name, default_) \
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<em>unspecified token sequence</em> \
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/**/
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</pre>
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</div>
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<div class="section" id="id1323">
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<h3><a class="subsection-title" href="#description" name="description">Description</a></h3>
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<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
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that for any type <tt class="literal"><span class="pre">x</span></tt> <tt class="literal"><span class="pre">trait<x>::value</span> <span class="pre">==</span> <span class="pre">true</span></tt> if and only if
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<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>
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<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>
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parameters.</p>
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<p>On deficient compilers not capable of performing the detection,
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<tt class="literal"><span class="pre">trait<x>::value</span></tt> always returns a fallback value <tt class="literal"><span class="pre">default_</span></tt>. A
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boolean configuration macro, <a class="reference internal" href="./cfg-no-has-xxx-template.html">BOOST_MPL_CFG_NO_HAS_XXX_TEMPLATE</a>, is
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provided to signal or override the "deficient" status of a particular
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compiler. [<em>Note:</em> The fallback value can also be provided at the
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point of the metafunction invocation; see the <cite>Expression semantics</cite>
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section for details — <em>end note</em>]</p>
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</div>
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<div class="section" id="id1324">
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<h3><a class="subsection-title" href="#header" name="header">Header</a></h3>
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<pre class="literal-block">
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#include <<a href="../../../../boost/mpl/has_xxx.hpp" class="header">boost/mpl/has_xxx.hpp</a>>
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</pre>
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</div>
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<div class="section" id="id1325">
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<h3><a class="subsection-title" href="#parameters" name="parameters">Parameters</a></h3>
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<table border="1" class="docutils table">
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<colgroup>
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<col width="15%" />
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<col width="32%" />
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<col width="53%" />
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</colgroup>
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<thead valign="bottom">
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<tr><th class="head">Parameter</th>
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<th class="head">Requirement</th>
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<th class="head">Description</th>
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</tr>
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</thead>
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<tbody valign="top">
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<tr><td><tt class="literal"><span class="pre">trait</span></tt></td>
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<td>A legal identifier token</td>
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<td>A name of the metafunction to be generated.</td>
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</tr>
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<tr><td><tt class="literal"><span class="pre">name</span></tt></td>
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<td>A legal identifier token</td>
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<td>A name of the member being detected.</td>
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</tr>
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<tr><td><tt class="literal"><span class="pre">default_</span></tt></td>
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<td>An boolean constant</td>
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<td>A fallback value for the deficient compilers.</td>
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</tr>
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</tbody>
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</table>
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</div>
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<div class="section" id="id1326">
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<h3><a class="subsection-title" href="#expression-semantics" name="expression-semantics">Expression semantics</a></h3>
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<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
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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
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type <tt class="literal"><span class="pre">x</span></tt>:</p>
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<pre class="literal-block">
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<a href="./has-xxx-template-named-def.html" class="identifier">BOOST_MPL_HAS_XXX_TEMPLATE_NAMED_DEF</a>(trait, name, c1)
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</pre>
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<table class="docutils field-list" frame="void" rules="none">
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<col class="field-name" />
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<col class="field-body" />
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<tbody valign="top">
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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>
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</td>
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</tr>
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<tr class="field"><th class="field-name">Return type:</th><td class="field-body"><p class="first">None.</p>
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</td>
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</tr>
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<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
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definition</p>
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<pre class="literal-block">
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template<
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typename X
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, typename fallback = boost::mpl::<a href="./bool.html" class="identifier">bool_</a><c1>
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>
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struct trait
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{
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// <em>unspecified</em>
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// ...
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};
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</pre>
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<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
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semantics:</p>
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<pre class="literal-block">
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typedef trait<x>::type r;
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</pre>
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<table class="docutils field-list" frame="void" rules="none">
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<col class="field-name" />
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<col class="field-body" />
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<tbody valign="top">
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<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>
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</tr>
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<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>
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<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>
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is a class type that has a nested template member <tt class="literal"><span class="pre">x::template</span>
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<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>
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</tr>
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</tbody>
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</table>
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<pre class="literal-block">
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typedef trait< x, c2 >::type r;
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</pre>
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<table class="last docutils field-list" frame="void" rules="none">
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<col class="field-name" />
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<col class="field-body" />
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<tbody valign="top">
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<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>
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</td>
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</tr>
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<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>
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<span class="pre">==</span> <span class="pre">c2::value</span></tt>; otherwise, equivalent to</p>
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<pre class="last literal-block">
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typedef trait<x>::type r;
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</pre>
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</td>
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</tr>
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</tbody>
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</table>
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</td>
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</tr>
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</tbody>
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</table>
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</div>
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<div class="section" id="id1327">
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<h3><a class="subsection-title" href="#example" name="example">Example</a></h3>
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<pre class="literal-block">
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<a href="./has-xxx-template-named-def.html" class="identifier">BOOST_MPL_HAS_XXX_TEMPLATE_NAMED_DEF</a>(
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has_xxx, xxx, false
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)
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struct test1 {};
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struct test2 { void xxx(); };
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struct test3 { int xxx; };
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struct test4 { static int xxx(); };
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struct test5 { typedef int xxx; };
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struct test6 { struct xxx; };
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struct test7 { typedef void (*xxx)(); };
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struct test8 { typedef void (xxx)(); };
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struct test9 { template< class T > struct xxx {}; };
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<a href="./assert-not.html" class="identifier">BOOST_MPL_ASSERT_NOT</a>(( has_xxx<test1> ));
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<a href="./assert-not.html" class="identifier">BOOST_MPL_ASSERT_NOT</a>(( has_xxx<test2> ));
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<a href="./assert-not.html" class="identifier">BOOST_MPL_ASSERT_NOT</a>(( has_xxx<test3> ));
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<a href="./assert-not.html" class="identifier">BOOST_MPL_ASSERT_NOT</a>(( has_xxx<test4> ));
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<a href="./assert-not.html" class="identifier">BOOST_MPL_ASSERT_NOT</a>(( has_xxx<test5> ));
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<a href="./assert-not.html" class="identifier">BOOST_MPL_ASSERT_NOT</a>(( has_xxx<test6> ));
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<a href="./assert-not.html" class="identifier">BOOST_MPL_ASSERT_NOT</a>(( has_xxx<test7> ));
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<a href="./assert-not.html" class="identifier">BOOST_MPL_ASSERT_NOT</a>(( has_xxx<test8> ));
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#if !defined(<a href="./cfg-no-has-xxx-template.html" class="identifier">BOOST_MPL_CFG_NO_HAS_XXX_TEMPLATE</a>)
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<a href="./assert.html" class="identifier">BOOST_MPL_ASSERT</a>(( has_xxx<test9> ));
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#endif
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<a href="./assert.html" class="identifier">BOOST_MPL_ASSERT</a>(( has_xxx<test9, <a href="./bool.html" class="identifier">true_</a>> ));
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</pre>
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</div>
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<div class="section" id="id1328">
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<h3><a class="subsection-title" href="#see-also" name="see-also">See also</a></h3>
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<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>,
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<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>
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<!-- Macros/Introspection//BOOST_MPL_HAS_XXX_TRAIT_DEF -->
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