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: identity</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="./miscellaneous.html" class="navigation-link">Prev</a> <a href="./always.html" class="navigation-link">Next</a></span><span class="navigation-group-separator"> | </span><span class="navigation-group">Back <a href="./always.html" class="navigation-link">Along</a></span><span class="navigation-group-separator"> | </span><span class="navigation-group"><a href="./miscellaneous.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="./metafunctions.html" class="navigation-link">Metafunctions</a> / <a href="./miscellaneous.html" class="navigation-link">Miscellaneous</a> / <a href="./identity.html" class="navigation-link">identity</a></td> 13</tr></table><div class="header-separator"></div> 14<div class="section" id="identity"> 15<h1><a class="toc-backref" href="./miscellaneous.html#id1564">identity</a></h1> 16<div class="section" id="id1146"> 17<h3><a class="subsection-title" href="#synopsis" name="synopsis">Synopsis</a></h3> 18<pre class="literal-block"> 19template< 20 typename X 21 > 22struct <a href="./identity.html" class="identifier">identity</a> 23{ 24 typedef X type; 25}; 26</pre> 27</div> 28<div class="section" id="id1147"> 29<h3><a class="subsection-title" href="#description" name="description">Description</a></h3> 30<p>The <a class="reference external" href="http://mathworld.wolfram.com/IdentityFunction.html" target="_top">identity</a> metafunction. Returns <tt class="literal"><span class="pre">X</span></tt> unchanged.</p> 31</div> 32<div class="section" id="id1149"> 33<h3><a class="subsection-title" href="#header" name="header">Header</a></h3> 34<pre class="literal-block"> 35#include <<a href="../../../../boost/mpl/identity.hpp" class="header">boost/mpl/identity.hpp</a>> 36</pre> 37</div> 38<div class="section" id="id1150"> 39<h3><a class="subsection-title" href="#model-of" name="model-of">Model of</a></h3> 40<p><a class="reference internal" href="./metafunction.html">Metafunction</a></p> 41</div> 42<div class="section" id="id1151"> 43<h3><a class="subsection-title" href="#parameters" name="parameters">Parameters</a></h3> 44<table border="1" class="docutils table"> 45<colgroup> 46<col width="22%" /> 47<col width="28%" /> 48<col width="51%" /> 49</colgroup> 50<thead valign="bottom"> 51<tr><th class="head">Parameter</th> 52<th class="head">Requirement</th> 53<th class="head">Description</th> 54</tr> 55</thead> 56<tbody valign="top"> 57<tr><td><tt class="literal"><span class="pre">X</span></tt></td> 58<td>Any type</td> 59<td>An argument to be returned.</td> 60</tr> 61</tbody> 62</table> 63</div> 64<div class="section" id="id1152"> 65<h3><a class="subsection-title" href="#expression-semantics" name="expression-semantics">Expression semantics</a></h3> 66<p>For an arbitrary type <tt class="literal"><span class="pre">x</span></tt>:</p> 67<pre class="literal-block"> 68typedef <a href="./identity.html" class="identifier">identity</a><x>::type r; 69</pre> 70<table class="docutils field-list" frame="void" rules="none"> 71<col class="field-name" /> 72<col class="field-body" /> 73<tbody valign="top"> 74<tr class="field"><th class="field-name">Return type:</th><td class="field-body"><p class="first">A type.</p> 75</td> 76</tr> 77<tr class="field"><th class="field-name">Semantics:</th><td class="field-body"><p class="first">Equivalent to</p> 78<pre class="literal-block"> 79typedef x r; 80</pre> 81</td> 82</tr> 83<tr class="field"><th class="field-name">Postcondition:</th><td class="field-body"><p class="first last"><tt class="literal"><span class="pre">is_same<r,x>::value</span> <span class="pre">==</span> <span class="pre">true</span></tt>.</p> 84</td> 85</tr> 86</tbody> 87</table> 88</div> 89<div class="section" id="id1153"> 90<h3><a class="subsection-title" href="#example" name="example">Example</a></h3> 91<pre class="literal-block"> 92typedef <a href="./apply.html" class="identifier">apply</a>< <a href="./identity.html" class="identifier">identity</a><<a href="./placeholders.html" class="identifier">_1</a>>, char >::type t1; 93typedef <a href="./apply.html" class="identifier">apply</a>< <a href="./identity.html" class="identifier">identity</a><<a href="./placeholders.html" class="identifier">_2</a>>, char,int >::type t2; 94 95<a href="./assert.html" class="identifier">BOOST_MPL_ASSERT</a>(( is_same< t1, char > )); 96<a href="./assert.html" class="identifier">BOOST_MPL_ASSERT</a>(( is_same< t2, int > )); 97</pre> 98</div> 99<div class="section" id="id1154"> 100<h3><a class="subsection-title" href="#see-also" name="see-also">See also</a></h3> 101<p><a class="reference internal" href="./metafunctions.html">Metafunctions</a>, <a class="reference internal" href="./placeholders.html">Placeholders</a>, <a class="reference internal" href="./trivial.html#label-metafunctions-trivial">Trivial Metafunctions</a>, <a class="reference internal" href="./always.html">always</a>, <a class="reference internal" href="./apply.html">apply</a></p> 102<!-- Metafunctions/Miscellaneous//always |20 --> 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="./miscellaneous.html" class="navigation-link">Prev</a> <a href="./always.html" class="navigation-link">Next</a></span><span class="navigation-group-separator"> | </span><span class="navigation-group">Back <a href="./always.html" class="navigation-link">Along</a></span><span class="navigation-group-separator"> | </span><span class="navigation-group"><a href="./miscellaneous.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> 108<td><div class="copyright-footer"><div class="copyright">Copyright © 2001-2009 Aleksey Gurtovoy and David Abrahams</div> 109Distributed under the Boost Software License, Version 1.0. (See accompanying 110file 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> 111</html> 112