Planet
navi homePPSaboutscreenshotsdownloaddevelopmentforum

source: downloads/boost_1_34_1/libs/mpl/doc/refmanual/eval-if.html @ 29

Last change on this file since 29 was 29, checked in by landauf, 17 years ago

updated boost from 1_33_1 to 1_34_1
File size: 7.6 KB
Line 
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<!-- Copyright Aleksey Gurtovoy 2006. Distributed under the Boost -->
5<!-- Software License, Version 1.0. (See accompanying -->
6<!-- file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt) -->
7<head>
8<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
9<meta name="generator" content="Docutils 0.3.6: http://docutils.sourceforge.net/" />
10<title>The MPL Reference Manual: eval_if</title>
11<link rel="stylesheet" href="../style.css" type="text/css" />
12</head>
13<body class="docframe refmanual">
14<table class="header"><tr class="header"><td class="header-group navigation-bar"><span class="navigation-group"><a href="./if-c.html" class="navigation-link">Prev</a>&nbsp;<a href="./eval-if-c.html" class="navigation-link">Next</a></span><span class="navigation-group-separator">&nbsp;|&nbsp;</span><span class="navigation-group"><a href="./if-c.html" class="navigation-link">Back</a>&nbsp;<a href="./eval-if-c.html" class="navigation-link">Along</a></span><span class="navigation-group-separator">&nbsp;|&nbsp;</span><span class="navigation-group"><a href="./type-selection.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>
15<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>
16</tr></table><div class="header-separator"></div>
17<div class="section" id="eval-if">
18<h1><a class="toc-backref" href="./type-selection.html#id448" name="eval-if">eval_if</a></h1>
19<div class="section" id="eval-synopsis">
20<h3><a class="subsection-title" href="#synopsis" name="synopsis">Synopsis</a></h3>
21<pre class="literal-block">
22template&lt; 
23      typename C
24    , typename F1
25    , typename F2
26    &gt;
27struct <a href="./eval-if.html" class="identifier">eval_if</a>
28{
29    typedef <em>unspecified</em> type;
30};
31</pre>
32</div>
33<div class="section" id="eval-description">
34<h3><a class="subsection-title" href="#description" name="description">Description</a></h3>
35<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
36on the value <tt class="literal"><span class="pre">C</span></tt>.</p>
37</div>
38<div class="section" id="eval-header">
39<h3><a class="subsection-title" href="#header" name="header">Header</a></h3>
40<pre class="literal-block">
41#include &lt;<a href="../../../../boost/mpl/eval_if.hpp" class="header">boost/mpl/eval_if.hpp</a>&gt;
42</pre>
43</div>
44<div class="section" id="eval-parameters">
45<h3><a class="subsection-title" href="#parameters" name="parameters">Parameters</a></h3>
46<table border="1" class="table">
47<colgroup>
48<col width="15%" />
49<col width="36%" />
50<col width="48%" />
51</colgroup>
52<thead valign="bottom">
53<tr><th>Parameter</th>
54<th>Requirement</th>
55<th>Description</th>
56</tr>
57</thead>
58<tbody valign="top">
59<tr><td><tt class="literal"><span class="pre">C</span></tt></td>
60<td><a class="reference" href="./integral-constant.html">Integral Constant</a></td>
61<td>An evaluation condition.</td>
62</tr>
63<tr><td><tt class="literal"><span class="pre">F1</span></tt>, <tt class="literal"><span class="pre">F2</span></tt></td>
64<td>Nullary <a class="reference" href="./metafunction.html">Metafunction</a></td>
65<td>Metafunctions to select for evaluation from.</td>
66</tr>
67</tbody>
68</table>
69</div>
70<div class="section" id="eval-expression-semantics">
71<h3><a class="subsection-title" href="#expression-semantics" name="expression-semantics">Expression semantics</a></h3>
72<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>
73<pre class="literal-block">
74typedef <a href="./eval-if.html" class="identifier">eval_if</a>&lt;c,f1,f2&gt;::type t;
75</pre>
76<table class="field-list" frame="void" rules="none">
77<col class="field-name" />
78<col class="field-body" />
79<tbody valign="top">
80<tr class="field"><th class="field-name">Return type:</th><td class="field-body">Any type.</td>
81</tr>
82<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
83identical to <tt class="literal"><span class="pre">f2::type</span></tt>.</td>
84</tr>
85</tbody>
86</table>
87</div>
88<div class="section" id="eval-example">
89<h3><a class="subsection-title" href="#example" name="example">Example</a></h3>
90<pre class="literal-block">
91typedef <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;
92typedef <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;
93
94<a href="./assert.html" class="identifier">BOOST_MPL_ASSERT</a>(( is_same&lt;t1,char&gt; ));
95<a href="./assert.html" class="identifier">BOOST_MPL_ASSERT</a>(( is_same&lt;t2,long&gt; ));
96</pre>
97</div>
98<div class="section" id="eval-see-also">
99<h3><a class="subsection-title" href="#see-also" name="see-also">See also</a></h3>
100<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>
101<!-- modtime: November 10, 2004 04:42:29 +0000 -->
102<!-- Metafunctions/Type Selection//eval_if_c |40 -->
103</div>
104</div>
105
106<div class="footer-separator"></div>
107<table class="footer"><tr class="footer"><td class="header-group navigation-bar"><span class="navigation-group"><a href="./if-c.html" class="navigation-link">Prev</a>&nbsp;<a href="./eval-if-c.html" class="navigation-link">Next</a></span><span class="navigation-group-separator">&nbsp;|&nbsp;</span><span class="navigation-group"><a href="./if-c.html" class="navigation-link">Back</a>&nbsp;<a href="./eval-if-c.html" class="navigation-link">Along</a></span><span class="navigation-group-separator">&nbsp;|&nbsp;</span><span class="navigation-group"><a href="./type-selection.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>
108</tr></table></body>
109</html>
Note: See TracBrowser for help on using the repository browser.