1<?xml version="1.0" encoding="utf-8" ?> 2<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"> 3<html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en" lang="en"> 4<head> 5<meta http-equiv="Content-Type" content="text/html; charset=utf-8" /> 6<meta name="generator" content="Docutils 0.7: http://docutils.sourceforge.net/" /> 7<title>The MPL Reference Manual: BOOST_MPL_HAS_XXX_TEMPLATE_NAMED_DEF</title> 8<link rel="stylesheet" href="../style.css" type="text/css" /> 9</head> 10<body class="docframe refmanual"> 11<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> 12<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> 13</tr></table><div class="header-separator"></div> 14<div class="section" id="boost-mpl-has-xxx-template-named-def"> 15<h1><a class="toc-backref" href="./introspection.html#id1594">BOOST_MPL_HAS_XXX_TEMPLATE_NAMED_DEF</a></h1> 16<div class="section" id="id1322"> 17<h3><a class="subsection-title" href="#synopsis" name="synopsis">Synopsis</a></h3> 18<pre class="literal-block"> 19#define <a href="./has-xxx-template-named-def.html" class="identifier">BOOST_MPL_HAS_XXX_TEMPLATE_NAMED_DEF</a>(trait, name, default_) \ 20 <em>unspecified token sequence</em> \ 21/**/ 22</pre> 23</div> 24<div class="section" id="id1323"> 25<h3><a class="subsection-title" href="#description" name="description">Description</a></h3> 26<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 27that 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 28<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> 29<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> 30parameters.</p> 31<p>On deficient compilers not capable of performing the detection, 32<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 33boolean configuration macro, <a class="reference internal" href="./cfg-no-has-xxx-template.html">BOOST_MPL_CFG_NO_HAS_XXX_TEMPLATE</a>, is 34provided to signal or override the "deficient" status of a particular 35compiler. [<em>Note:</em> The fallback value can also be provided at the 36point of the metafunction invocation; see the <cite>Expression semantics</cite> 37section for details — <em>end note</em>]</p> 38</div> 39<div class="section" id="id1324"> 40<h3><a class="subsection-title" href="#header" name="header">Header</a></h3> 41<pre class="literal-block"> 42#include <<a href="../../../../boost/mpl/has_xxx.hpp" class="header">boost/mpl/has_xxx.hpp</a>> 43</pre> 44</div> 45<div class="section" id="id1325"> 46<h3><a class="subsection-title" href="#parameters" name="parameters">Parameters</a></h3> 47<table border="1" class="docutils table"> 48<colgroup> 49<col width="15%" /> 50<col width="32%" /> 51<col width="53%" /> 52</colgroup> 53<thead valign="bottom"> 54<tr><th class="head">Parameter</th> 55<th class="head">Requirement</th> 56<th class="head">Description</th> 57</tr> 58</thead> 59<tbody valign="top"> 60<tr><td><tt class="literal"><span class="pre">trait</span></tt></td> 61<td>A legal identifier token</td> 62<td>A name of the metafunction to be generated.</td> 63</tr> 64<tr><td><tt class="literal"><span class="pre">name</span></tt></td> 65<td>A legal identifier token</td> 66<td>A name of the member being detected.</td> 67</tr> 68<tr><td><tt class="literal"><span class="pre">default_</span></tt></td> 69<td>An boolean constant</td> 70<td>A fallback value for the deficient compilers.</td> 71</tr> 72</tbody> 73</table> 74</div> 75<div class="section" id="id1326"> 76<h3><a class="subsection-title" href="#expression-semantics" name="expression-semantics">Expression semantics</a></h3> 77<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 78expression <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 79type <tt class="literal"><span class="pre">x</span></tt>:</p> 80<pre class="literal-block"> 81<a href="./has-xxx-template-named-def.html" class="identifier">BOOST_MPL_HAS_XXX_TEMPLATE_NAMED_DEF</a>(trait, name, c1) 82</pre> 83<table class="docutils field-list" frame="void" rules="none"> 84<col class="field-name" /> 85<col class="field-body" /> 86<tbody valign="top"> 87<tr class="field"><th class="field-name">Precondition:</th><td class="field-body"><p class="first">Appears at namespace scope.</p> 88</td> 89</tr> 90<tr class="field"><th class="field-name">Return type:</th><td class="field-body"><p class="first">None.</p> 91</td> 92</tr> 93<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 94definition</p> 95<pre class="literal-block"> 96template< 97 typename X 98 , typename fallback = boost::mpl::<a href="./bool.html" class="identifier">bool_</a><c1> 99> 100struct trait 101{ 102 // <em>unspecified</em> 103 // ... 104}; 105</pre> 106<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 107semantics:</p> 108<pre class="literal-block"> 109typedef trait<x>::type r; 110</pre> 111<table class="docutils field-list" frame="void" rules="none"> 112<col class="field-name" /> 113<col class="field-body" /> 114<tbody valign="top"> 115<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> 116</tr> 117<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> 118<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> 119is a class type that has a nested template member <tt class="literal"><span class="pre">x::template</span> 120<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> 121</tr> 122</tbody> 123</table> 124<pre class="literal-block"> 125typedef trait< x, c2 >::type r; 126</pre> 127<table class="last docutils field-list" frame="void" rules="none"> 128<col class="field-name" /> 129<col class="field-body" /> 130<tbody valign="top"> 131<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> 132</td> 133</tr> 134<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> 135<span class="pre">==</span> <span class="pre">c2::value</span></tt>; otherwise, equivalent to</p> 136<pre class="last literal-block"> 137typedef trait<x>::type r; 138</pre> 139</td> 140</tr> 141</tbody> 142</table> 143</td> 144</tr> 145</tbody> 146</table> 147</div> 148<div class="section" id="id1327"> 149<h3><a class="subsection-title" href="#example" name="example">Example</a></h3> 150<pre class="literal-block"> 151<a href="./has-xxx-template-named-def.html" class="identifier">BOOST_MPL_HAS_XXX_TEMPLATE_NAMED_DEF</a>( 152 has_xxx, xxx, false 153) 154 155struct test1 {}; 156struct test2 { void xxx(); }; 157struct test3 { int xxx; }; 158struct test4 { static int xxx(); }; 159struct test5 { typedef int xxx; }; 160struct test6 { struct xxx; }; 161struct test7 { typedef void (*xxx)(); }; 162struct test8 { typedef void (xxx)(); }; 163struct test9 { template< class T > struct xxx {}; }; 164 165<a href="./assert-not.html" class="identifier">BOOST_MPL_ASSERT_NOT</a>(( has_xxx<test1> )); 166<a href="./assert-not.html" class="identifier">BOOST_MPL_ASSERT_NOT</a>(( has_xxx<test2> )); 167<a href="./assert-not.html" class="identifier">BOOST_MPL_ASSERT_NOT</a>(( has_xxx<test3> )); 168<a href="./assert-not.html" class="identifier">BOOST_MPL_ASSERT_NOT</a>(( has_xxx<test4> )); 169<a href="./assert-not.html" class="identifier">BOOST_MPL_ASSERT_NOT</a>(( has_xxx<test5> )); 170<a href="./assert-not.html" class="identifier">BOOST_MPL_ASSERT_NOT</a>(( has_xxx<test6> )); 171<a href="./assert-not.html" class="identifier">BOOST_MPL_ASSERT_NOT</a>(( has_xxx<test7> )); 172<a href="./assert-not.html" class="identifier">BOOST_MPL_ASSERT_NOT</a>(( has_xxx<test8> )); 173 174#if !defined(<a href="./cfg-no-has-xxx-template.html" class="identifier">BOOST_MPL_CFG_NO_HAS_XXX_TEMPLATE</a>) 175<a href="./assert.html" class="identifier">BOOST_MPL_ASSERT</a>(( has_xxx<test9> )); 176#endif 177 178<a href="./assert.html" class="identifier">BOOST_MPL_ASSERT</a>(( has_xxx<test9, <a href="./bool.html" class="identifier">true_</a>> )); 179</pre> 180</div> 181<div class="section" id="id1328"> 182<h3><a class="subsection-title" href="#see-also" name="see-also">See also</a></h3> 183<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>, 184<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> 185<!-- Macros/Introspection//BOOST_MPL_HAS_XXX_TRAIT_DEF --> 186</div> 187</div> 188 189<div class="footer-separator"></div> 190<table class="footer"><tr class="footer"><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> 191<td><div class="copyright-footer"></div></td></tr></table></body> 192</html> 193