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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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| <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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