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- <title>The MPL Reference Manual: BOOST_MPL_HAS_XXX_TEMPLATE_NAMED_DEF</title>
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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<x>::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<x>::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 "deficient" 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 <<a href="../../../../boost/mpl/has_xxx.hpp" class="header">boost/mpl/has_xxx.hpp</a>>
- </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<
- typename X
- , typename fallback = boost::mpl::<a href="./bool.html" class="identifier">bool_</a><c1>
- >
- 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<x>::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< x, c2 >::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<x>::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< class T > struct xxx {}; };
- <a href="./assert-not.html" class="identifier">BOOST_MPL_ASSERT_NOT</a>(( has_xxx<test1> ));
- <a href="./assert-not.html" class="identifier">BOOST_MPL_ASSERT_NOT</a>(( has_xxx<test2> ));
- <a href="./assert-not.html" class="identifier">BOOST_MPL_ASSERT_NOT</a>(( has_xxx<test3> ));
- <a href="./assert-not.html" class="identifier">BOOST_MPL_ASSERT_NOT</a>(( has_xxx<test4> ));
- <a href="./assert-not.html" class="identifier">BOOST_MPL_ASSERT_NOT</a>(( has_xxx<test5> ));
- <a href="./assert-not.html" class="identifier">BOOST_MPL_ASSERT_NOT</a>(( has_xxx<test6> ));
- <a href="./assert-not.html" class="identifier">BOOST_MPL_ASSERT_NOT</a>(( has_xxx<test7> ));
- <a href="./assert-not.html" class="identifier">BOOST_MPL_ASSERT_NOT</a>(( has_xxx<test8> ));
- #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<test9> ));
- #endif
- <a href="./assert.html" class="identifier">BOOST_MPL_ASSERT</a>(( has_xxx<test9, <a href="./bool.html" class="identifier">true_</a>> ));
- </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>
- </div>
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