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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>&nbsp;<a href="./has-xxx-trait-def.html" class="navigation-link">Next</a></span><span class="navigation-group-separator">&nbsp;|&nbsp;</span><span class="navigation-group"><a href="./has-xxx-template-def.html" class="navigation-link">Back</a>&nbsp;<a href="./has-xxx-trait-def.html" class="navigation-link">Along</a></span><span class="navigation-group-separator">&nbsp;|&nbsp;</span><span class="navigation-group"><a href="./introspection.html" class="navigation-link">Up</a>&nbsp;<a href="../refmanual.html" class="navigation-link">Home</a></span><span class="navigation-group-separator">&nbsp;|&nbsp;</span><span class="navigation-group"><a href="./refmanual_toc.html" class="navigation-link">Full TOC</a></span></td>
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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&lt;x&gt;::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&lt;x&gt;::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 &quot;deficient&quot; 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 &lt;<a href="../../../../boost/mpl/has_xxx.hpp" class="header">boost/mpl/has_xxx.hpp</a>&gt;
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&lt;
97 typename X
98 , typename fallback = boost::mpl::<a href="./bool.html" class="identifier">bool_</a>&lt;c1&gt;
99&gt;
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&lt;x&gt;::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&lt; x, c2 &gt;::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&lt;x&gt;::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&lt; class T &gt; struct xxx {}; };
164
165<a href="./assert-not.html" class="identifier">BOOST_MPL_ASSERT_NOT</a>(( has_xxx&lt;test1&gt; ));
166<a href="./assert-not.html" class="identifier">BOOST_MPL_ASSERT_NOT</a>(( has_xxx&lt;test2&gt; ));
167<a href="./assert-not.html" class="identifier">BOOST_MPL_ASSERT_NOT</a>(( has_xxx&lt;test3&gt; ));
168<a href="./assert-not.html" class="identifier">BOOST_MPL_ASSERT_NOT</a>(( has_xxx&lt;test4&gt; ));
169<a href="./assert-not.html" class="identifier">BOOST_MPL_ASSERT_NOT</a>(( has_xxx&lt;test5&gt; ));
170<a href="./assert-not.html" class="identifier">BOOST_MPL_ASSERT_NOT</a>(( has_xxx&lt;test6&gt; ));
171<a href="./assert-not.html" class="identifier">BOOST_MPL_ASSERT_NOT</a>(( has_xxx&lt;test7&gt; ));
172<a href="./assert-not.html" class="identifier">BOOST_MPL_ASSERT_NOT</a>(( has_xxx&lt;test8&gt; ));
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&lt;test9&gt; ));
176#endif
177
178<a href="./assert.html" class="identifier">BOOST_MPL_ASSERT</a>(( has_xxx&lt;test9, <a href="./bool.html" class="identifier">true_</a>&gt; ));
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
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