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12<td class="header-group page-location"><a href="../refmanual.html" class="navigation-link">Front Page</a> / <a href="./metafunctions.html" class="navigation-link">Metafunctions</a> / <a href="./type-selection.html" class="navigation-link">Type Selection</a> / <a href="./eval-if.html" class="navigation-link">eval_if</a></td>
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14<div class="section" id="eval-if">
15<h1><a class="toc-backref" href="./type-selection.html#id448" name="eval-if">eval_if</a></h1>
16<div class="section" id="eval-synopsis">
17<h3><a class="subsection-title" href="#synopsis" name="synopsis">Synopsis</a></h3>
18<pre class="literal-block">
19template&lt; 
20      typename C
21    , typename F1
22    , typename F2
23    &gt;
24struct <a href="./eval-if.html" class="identifier">eval_if</a>
25{
26    typedef <em>unspecified</em> type;
27};
28</pre>
29</div>
30<div class="section" id="eval-description">
31<h3><a class="subsection-title" href="#description" name="description">Description</a></h3>
32<p>Evaluates one of its two <a class="reference" href="./metafunction.html#nullary-metafunction">nullary-metafunction</a> arguments, <tt class="literal"><span class="pre">F1</span></tt> or <tt class="literal"><span class="pre">F2</span></tt>, depending
33on the value <tt class="literal"><span class="pre">C</span></tt>.</p>
34</div>
35<div class="section" id="eval-header">
36<h3><a class="subsection-title" href="#header" name="header">Header</a></h3>
37<pre class="literal-block">
38#include &lt;<a href="../../../../boost/mpl/eval_if.hpp" class="header">boost/mpl/eval_if.hpp</a>&gt;
39</pre>
40</div>
41<div class="section" id="eval-parameters">
42<h3><a class="subsection-title" href="#parameters" name="parameters">Parameters</a></h3>
43<table border="1" class="table">
44<colgroup>
45<col width="15%" />
46<col width="36%" />
47<col width="48%" />
48</colgroup>
49<thead valign="bottom">
50<tr><th>Parameter</th>
51<th>Requirement</th>
52<th>Description</th>
53</tr>
54</thead>
55<tbody valign="top">
56<tr><td><tt class="literal"><span class="pre">C</span></tt></td>
57<td><a class="reference" href="./integral-constant.html">Integral Constant</a></td>
58<td>An evaluation condition.</td>
59</tr>
60<tr><td><tt class="literal"><span class="pre">F1</span></tt>, <tt class="literal"><span class="pre">F2</span></tt></td>
61<td>Nullary <a class="reference" href="./metafunction.html">Metafunction</a></td>
62<td>Metafunctions to select for evaluation from.</td>
63</tr>
64</tbody>
65</table>
66</div>
67<div class="section" id="eval-expression-semantics">
68<h3><a class="subsection-title" href="#expression-semantics" name="expression-semantics">Expression semantics</a></h3>
69<p>For any <a class="reference" href="./integral-constant.html">Integral Constant</a> <tt class="literal"><span class="pre">c</span></tt> and nullary <a class="reference" href="./metafunction.html">Metafunction</a>s <tt class="literal"><span class="pre">f1</span></tt>, <tt class="literal"><span class="pre">f2</span></tt>:</p>
70<pre class="literal-block">
71typedef <a href="./eval-if.html" class="identifier">eval_if</a>&lt;c,f1,f2&gt;::type t;
72</pre>
73<table class="field-list" frame="void" rules="none">
74<col class="field-name" />
75<col class="field-body" />
76<tbody valign="top">
77<tr class="field"><th class="field-name">Return type:</th><td class="field-body">Any type.</td>
78</tr>
79<tr class="field"><th class="field-name">Semantics:</th><td class="field-body">If <tt class="literal"><span class="pre">c::value</span> <span class="pre">==</span> <span class="pre">true</span></tt>, <tt class="literal"><span class="pre">t</span></tt> is identical to <tt class="literal"><span class="pre">f1::type</span></tt>; otherwise <tt class="literal"><span class="pre">t</span></tt> is
80identical to <tt class="literal"><span class="pre">f2::type</span></tt>.</td>
81</tr>
82</tbody>
83</table>
84</div>
85<div class="section" id="eval-example">
86<h3><a class="subsection-title" href="#example" name="example">Example</a></h3>
87<pre class="literal-block">
88typedef <a href="./eval-if.html" class="identifier">eval_if</a>&lt; true_, <a href="./identity.html" class="identifier">identity</a>&lt;char&gt;, <a href="./identity.html" class="identifier">identity</a>&lt;long&gt; &gt;::type t1;
89typedef <a href="./eval-if.html" class="identifier">eval_if</a>&lt; false_, <a href="./identity.html" class="identifier">identity</a>&lt;char&gt;, <a href="./identity.html" class="identifier">identity</a>&lt;long&gt; &gt;::type t2;
90
91<a href="./assert.html" class="identifier">BOOST_MPL_ASSERT</a>(( is_same&lt;t1,char&gt; ));
92<a href="./assert.html" class="identifier">BOOST_MPL_ASSERT</a>(( is_same&lt;t2,long&gt; ));
93</pre>
94</div>
95<div class="section" id="eval-see-also">
96<h3><a class="subsection-title" href="#see-also" name="see-also">See also</a></h3>
97<p><a class="reference" href="./metafunctions.html">Metafunctions</a>, <a class="reference" href="./integral-constant.html">Integral Constant</a>, <a class="refentry reference" href="./eval-if-c.html"><tt class="refentry literal"><span class="pre">eval_if_c</span></tt></a>, <a class="refentry reference" href="./if.html"><tt class="refentry literal"><span class="pre">if_</span></tt></a></p>
98<!-- modtime: November 10, 2004 04:42:29 +0000 -->
99<!-- Metafunctions/Type Selection//eval_if_c |40 -->
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