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_TRAIT_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-trait-def.html" class="navigation-link">Prev</a> <a href="./configuration.html" class="navigation-link">Next</a></span><span class="navigation-group-separator"> | </span><span class="navigation-group"><a href="./has-xxx-trait-def.html" class="navigation-link">Back</a> Along</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-trait-named-def.html" class="navigation-link">BOOST_MPL_HAS_XXX_TRAIT_NAMED_DEF</a></td> 13</tr></table><div class="header-separator"></div> 14<div class="section" id="boost-mpl-has-xxx-trait-named-def"> 15<h1><a class="toc-backref" href="./introspection.html#id1596">BOOST_MPL_HAS_XXX_TRAIT_NAMED_DEF</a></h1> 16<div class="section" id="id1336"> 17<h3><a class="subsection-title" href="#synopsis" name="synopsis">Synopsis</a></h3> 18<pre class="literal-block"> 19#define <a href="./has-xxx-trait-named-def.html" class="identifier">BOOST_MPL_HAS_XXX_TRAIT_NAMED_DEF</a>(trait, name, default_) \ 20 <em>unspecified token sequence</em> \ 21/**/ 22</pre> 23</div> 24<div class="section" id="id1337"> 25<h3><a class="subsection-title" href="#description" name="description">Description</a></h3> 26<p>Expands into a definition of a boolean unary <a class="reference internal" href="./metafunction.html">Metafunction</a> <tt class="literal"><span class="pre">trait</span></tt> 27such 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 28if <tt class="literal"><span class="pre">x</span></tt> is a class type and has a nested type memeber <tt class="literal"><span class="pre">x::name</span></tt>.</p> 29<p>On the deficient compilers not capabale of performing the detection, 30<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>. 31A boolean configuraion macro, <a class="reference internal" href="./cfg-no-has-xxx.html">BOOST_MPL_CFG_NO_HAS_XXX</a>, is provided 32to signal or override the "deficient" status of a particular compiler. 33[<em>Note:</em> The fallback value can also be provided at the point of the 34metafunction invocation; see the <cite>Expression semantics</cite> section for 35details — <em>end note</em>]</p> 36</div> 37<div class="section" id="id1338"> 38<h3><a class="subsection-title" href="#header" name="header">Header</a></h3> 39<pre class="literal-block"> 40#include <<a href="../../../../boost/mpl/has_xxx.hpp" class="header">boost/mpl/has_xxx.hpp</a>> 41</pre> 42</div> 43<div class="section" id="id1339"> 44<h3><a class="subsection-title" href="#parameters" name="parameters">Parameters</a></h3> 45<table border="1" class="docutils table"> 46<colgroup> 47<col width="15%" /> 48<col width="32%" /> 49<col width="53%" /> 50</colgroup> 51<thead valign="bottom"> 52<tr><th class="head">Parameter</th> 53<th class="head">Requirement</th> 54<th class="head">Description</th> 55</tr> 56</thead> 57<tbody valign="top"> 58<tr><td><tt class="literal"><span class="pre">trait</span></tt></td> 59<td>A legal identifier token</td> 60<td>A name of the metafunction to be generated.</td> 61</tr> 62<tr><td><tt class="literal"><span class="pre">name</span></tt></td> 63<td>A legal identifier token</td> 64<td>A name of the member being detected.</td> 65</tr> 66<tr><td><tt class="literal"><span class="pre">default_</span></tt></td> 67<td>An boolean constant</td> 68<td>A fallback value for the deficient compilers.</td> 69</tr> 70</tbody> 71</table> 72</div> 73<div class="section" id="id1340"> 74<h3><a class="subsection-title" href="#expression-semantics" name="expression-semantics">Expression semantics</a></h3> 75<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>, 76boolean <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> 77<pre class="literal-block"> 78<a href="./has-xxx-trait-named-def.html" class="identifier">BOOST_MPL_HAS_XXX_TRAIT_NAMED_DEF</a>(trait, name, c1) 79</pre> 80<table class="docutils field-list" frame="void" rules="none"> 81<col class="field-name" /> 82<col class="field-body" /> 83<tbody valign="top"> 84<tr class="field"><th class="field-name">Precondition:</th><td class="field-body"><p class="first">Appears at namespace scope.</p> 85</td> 86</tr> 87<tr class="field"><th class="field-name">Return type:</th><td class="field-body"><p class="first">None.</p> 88</td> 89</tr> 90<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> 91<pre class="literal-block"> 92template< typename X, typename fallback = boost::mpl::<a href="./bool.html" class="identifier">bool_</a><c1> > 93struct trait 94{ 95 // <em>unspecified</em> 96 // ... 97}; 98</pre> 99<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> 100<pre class="literal-block"> 101typedef trait<x>::type r; 102</pre> 103<table class="docutils field-list" frame="void" rules="none"> 104<col class="field-name" /> 105<col class="field-body" /> 106<tbody valign="top"> 107<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> 108</tr> 109<tr class="field"><th class="field-name">Semantics:</th><td class="field-body">If <a class="reference internal" href="./cfg-no-has-xxx.html">BOOST_MPL_CFG_NO_HAS_XXX</a> is defined, <tt class="literal"><span class="pre">r::value</span> <span class="pre">==</span> <span class="pre">c1</span></tt>; 110otherwise, <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 111that has a nested type memeber <tt class="literal"><span class="pre">x::name</span></tt>.</td> 112</tr> 113</tbody> 114</table> 115<pre class="literal-block"> 116typedef trait< x,c2 >::type r; 117</pre> 118<table class="last docutils field-list" frame="void" rules="none"> 119<col class="field-name" /> 120<col class="field-body" /> 121<tbody valign="top"> 122<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> 123</td> 124</tr> 125<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.html">BOOST_MPL_CFG_NO_HAS_XXX</a> is defined, <tt class="literal"><span class="pre">r::value</span> <span class="pre">==</span> <span class="pre">c2::value</span></tt>; 126otherwise, equivalent to</p> 127<pre class="last literal-block"> 128typedef trait<x>::type r; 129</pre> 130</td> 131</tr> 132</tbody> 133</table> 134</td> 135</tr> 136</tbody> 137</table> 138</div> 139<div class="section" id="id1341"> 140<h3><a class="subsection-title" href="#example" name="example">Example</a></h3> 141<pre class="literal-block"> 142<a href="./has-xxx-trait-named-def.html" class="identifier">BOOST_MPL_HAS_XXX_TRAIT_NAMED_DEF</a>(has_xxx, xxx, false) 143 144struct test1 {}; 145struct test2 { void xxx(); }; 146struct test3 { int xxx; }; 147struct test4 { static int xxx(); }; 148struct test5 { template< typename T > struct xxx {}; }; 149struct test6 { typedef int xxx; }; 150struct test7 { struct xxx; }; 151struct test8 { typedef void (*xxx)(); }; 152struct test9 { typedef void (xxx)(); }; 153 154<a href="./assert-not.html" class="identifier">BOOST_MPL_ASSERT_NOT</a>(( has_xxx<test1> )); 155<a href="./assert-not.html" class="identifier">BOOST_MPL_ASSERT_NOT</a>(( has_xxx<test2> )); 156<a href="./assert-not.html" class="identifier">BOOST_MPL_ASSERT_NOT</a>(( has_xxx<test3> )); 157<a href="./assert-not.html" class="identifier">BOOST_MPL_ASSERT_NOT</a>(( has_xxx<test4> )); 158<a href="./assert-not.html" class="identifier">BOOST_MPL_ASSERT_NOT</a>(( has_xxx<test5> )); 159 160#if !defined(<a href="./cfg-no-has-xxx.html" class="identifier">BOOST_MPL_CFG_NO_HAS_XXX</a>) 161<a href="./assert.html" class="identifier">BOOST_MPL_ASSERT</a>(( has_xxx<test6> )); 162<a href="./assert.html" class="identifier">BOOST_MPL_ASSERT</a>(( has_xxx<test7> )); 163<a href="./assert.html" class="identifier">BOOST_MPL_ASSERT</a>(( has_xxx<test8> )); 164<a href="./assert.html" class="identifier">BOOST_MPL_ASSERT</a>(( has_xxx<test9> )); 165#endif 166 167<a href="./assert.html" class="identifier">BOOST_MPL_ASSERT</a>(( has_xxx<test6,<a href="./bool.html" class="identifier">true_</a>> )); 168<a href="./assert.html" class="identifier">BOOST_MPL_ASSERT</a>(( has_xxx<test7,<a href="./bool.html" class="identifier">true_</a>> )); 169<a href="./assert.html" class="identifier">BOOST_MPL_ASSERT</a>(( has_xxx<test8,<a href="./bool.html" class="identifier">true_</a>> )); 170<a href="./assert.html" class="identifier">BOOST_MPL_ASSERT</a>(( has_xxx<test9,<a href="./bool.html" class="identifier">true_</a>> )); 171</pre> 172</div> 173<div class="section" id="id1342"> 174<h3><a class="subsection-title" href="#see-also" name="see-also">See also</a></h3> 175<p><a class="reference internal" href="./macros.html">Macros</a>, <a class="reference internal" href="./has-xxx-trait-def.html">BOOST_MPL_HAS_XXX_TRAIT_DEF</a>, <a class="reference internal" href="./cfg-no-has-xxx.html">BOOST_MPL_CFG_NO_HAS_XXX</a></p> 176</div> 177</div> 178 179<div class="footer-separator"></div> 180<table class="footer"><tr class="footer"><td class="header-group navigation-bar"><span class="navigation-group"><a href="./has-xxx-trait-def.html" class="navigation-link">Prev</a> <a href="./configuration.html" class="navigation-link">Next</a></span><span class="navigation-group-separator"> | </span><span class="navigation-group"><a href="./has-xxx-trait-def.html" class="navigation-link">Back</a> Along</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> 181<td><div class="copyright-footer"><div class="copyright">Copyright © 2001-2009 Aleksey Gurtovoy and David Abrahams</div> 182Distributed under the Boost Software License, Version 1.0. (See accompanying 183file LICENSE_1_0.txt or copy at <a class="reference external" href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)</div></td></tr></table></body> 184</html> 185