1<html> 2<head> 3<meta http-equiv="Content-Type" content="text/html; charset=UTF-8"> 4<title>Compressed_Pair</title> 5<link rel="stylesheet" href="../../../../doc/src/boostbook.css" type="text/css"> 6<meta name="generator" content="DocBook XSL Stylesheets V1.79.1"> 7<link rel="home" href="compressed_pair.html" title="Compressed_Pair"> 8</head> 9<body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"> 10<table cellpadding="2" width="100%"><tr> 11<td valign="top"><img alt="Boost C++ Libraries" width="277" height="86" src="../../../../boost.png"></td> 12<td align="center"><a href="../../../../index.html">Home</a></td> 13<td align="center"><a href="../../../../libs/libraries.htm">Libraries</a></td> 14<td align="center"><a href="http://www.boost.org/users/people.html">People</a></td> 15<td align="center"><a href="http://www.boost.org/users/faq.html">FAQ</a></td> 16<td align="center"><a href="../../../../more/index.htm">More</a></td> 17</tr></table> 18<hr> 19<div class="spirit-nav"></div> 20<div class="article"> 21<div class="titlepage"> 22<div> 23<div><h2 class="title"> 24<a name="compressed_pair"></a>Compressed_Pair</h2></div> 25<div><div class="authorgroup"> 26<div class="author"><h3 class="author"> 27<span class="firstname">Steve</span> <span class="surname">Cleary</span> 28</h3></div> 29<div class="author"><h3 class="author"> 30<span class="firstname">Beman</span> <span class="surname">Dawes</span> 31</h3></div> 32<div class="author"><h3 class="author"> 33<span class="firstname">Howard</span> <span class="surname">Hinnant</span> 34</h3></div> 35<div class="author"><h3 class="author"> 36<span class="firstname">John</span> <span class="surname">Maddock</span> 37</h3></div> 38</div></div> 39<div><p class="copyright">Copyright © 2000 Steve Cleary, Beman Dawes, Howard Hinnant & John 40 Maddock</p></div> 41<div><div class="legalnotice"> 42<a name="compressed_pair.legal"></a><p> 43 Distributed under the Boost Software License, Version 1.0. (See accompanying 44 file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>) 45 </p> 46</div></div> 47</div> 48<hr> 49</div> 50<div class="toc"> 51<p><b>Table of Contents</b></p> 52<dl class="toc"> 53<dt><span class="section"><a href="compressed_pair.html#compressed_pair.overview">Overview</a></span></dt> 54<dt><span class="section"><a href="compressed_pair.html#compressed_pair.synopsis">Synopsis</a></span></dt> 55<dt><span class="section"><a href="compressed_pair.html#compressed_pair.acknowledgments">Acknowledgments</a></span></dt> 56</dl> 57</div> 58<div class="section"> 59<div class="titlepage"><div><div><h2 class="title" style="clear: both"> 60<a name="compressed_pair.overview"></a><a class="link" href="compressed_pair.html#compressed_pair.overview" title="Overview">Overview</a> 61</h2></div></div></div> 62<p> 63 All of the contents of <code class="computeroutput"><span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">compressed_pair</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span></code> 64 are defined inside <code class="computeroutput"><span class="keyword">namespace</span> <span class="identifier">boost</span></code>. 65 </p> 66<p> 67 The class <code class="computeroutput"><span class="identifier">compressed_pair</span></code> is 68 very similar to <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">pair</span></code>, but if either of the template arguments 69 are empty classes, then the <span class="emphasis"><em>empty base-class optimisation</em></span> 70 is applied to compress the size of the pair. 71 </p> 72</div> 73<div class="section"> 74<div class="titlepage"><div><div><h2 class="title" style="clear: both"> 75<a name="compressed_pair.synopsis"></a><a class="link" href="compressed_pair.html#compressed_pair.synopsis" title="Synopsis">Synopsis</a> 76</h2></div></div></div> 77<pre class="programlisting"><span class="keyword">template</span> <span class="special"><</span><span class="keyword">class</span> <span class="identifier">T1</span><span class="special">,</span> <span class="keyword">class</span> <span class="identifier">T2</span><span class="special">></span> 78<span class="keyword">class</span> <span class="identifier">compressed_pair</span> 79<span class="special">{</span> 80<span class="keyword">public</span><span class="special">:</span> 81 <span class="keyword">typedef</span> <span class="identifier">T1</span> <span class="identifier">first_type</span><span class="special">;</span> 82 <span class="keyword">typedef</span> <span class="identifier">T2</span> <span class="identifier">second_type</span><span class="special">;</span> 83 <span class="keyword">typedef</span> <span class="keyword">typename</span> <span class="identifier">call_traits</span><span class="special"><</span><span class="identifier">first_type</span><span class="special">>::</span><span class="identifier">param_type</span> <span class="identifier">first_param_type</span><span class="special">;</span> 84 <span class="keyword">typedef</span> <span class="keyword">typename</span> <span class="identifier">call_traits</span><span class="special"><</span><span class="identifier">second_type</span><span class="special">>::</span><span class="identifier">param_type</span> <span class="identifier">second_param_type</span><span class="special">;</span> 85 <span class="keyword">typedef</span> <span class="keyword">typename</span> <span class="identifier">call_traits</span><span class="special"><</span><span class="identifier">first_type</span><span class="special">>::</span><span class="identifier">reference</span> <span class="identifier">first_reference</span><span class="special">;</span> 86 <span class="keyword">typedef</span> <span class="keyword">typename</span> <span class="identifier">call_traits</span><span class="special"><</span><span class="identifier">second_type</span><span class="special">>::</span><span class="identifier">reference</span> <span class="identifier">second_reference</span><span class="special">;</span> 87 <span class="keyword">typedef</span> <span class="keyword">typename</span> <span class="identifier">call_traits</span><span class="special"><</span><span class="identifier">first_type</span><span class="special">>::</span><span class="identifier">const_reference</span> <span class="identifier">first_const_reference</span><span class="special">;</span> 88 <span class="keyword">typedef</span> <span class="keyword">typename</span> <span class="identifier">call_traits</span><span class="special"><</span><span class="identifier">second_type</span><span class="special">>::</span><span class="identifier">const_reference</span> <span class="identifier">second_const_reference</span><span class="special">;</span> 89 90 <span class="identifier">compressed_pair</span><span class="special">()</span> <span class="special">:</span> <span class="identifier">base</span><span class="special">()</span> <span class="special">{}</span> 91 <span class="identifier">compressed_pair</span><span class="special">(</span><span class="identifier">first_param_type</span> <span class="identifier">x</span><span class="special">,</span> <span class="identifier">second_param_type</span> <span class="identifier">y</span><span class="special">);</span> 92 <span class="keyword">explicit</span> <span class="identifier">compressed_pair</span><span class="special">(</span><span class="identifier">first_param_type</span> <span class="identifier">x</span><span class="special">);</span> 93 <span class="keyword">explicit</span> <span class="identifier">compressed_pair</span><span class="special">(</span><span class="identifier">second_param_type</span> <span class="identifier">y</span><span class="special">);</span> 94 95 <span class="identifier">compressed_pair</span><span class="special">&</span> <span class="keyword">operator</span><span class="special">=(</span><span class="keyword">const</span> <span class="identifier">compressed_pair</span><span class="special">&);</span> 96 97 <span class="identifier">first_reference</span> <span class="identifier">first</span><span class="special">();</span> 98 <span class="identifier">first_const_reference</span> <span class="identifier">first</span><span class="special">()</span> <span class="keyword">const</span><span class="special">;</span> 99 100 <span class="identifier">second_reference</span> <span class="identifier">second</span><span class="special">();</span> 101 <span class="identifier">second_const_reference</span> <span class="identifier">second</span><span class="special">()</span> <span class="keyword">const</span><span class="special">;</span> 102 103 <span class="keyword">void</span> <span class="identifier">swap</span><span class="special">(</span><span class="identifier">compressed_pair</span><span class="special">&</span> <span class="identifier">y</span><span class="special">);</span> 104<span class="special">};</span> 105</pre> 106<p> 107 The two members of the pair can be accessed using the member functions <code class="computeroutput"><span class="identifier">first</span><span class="special">()</span></code> and 108 <code class="computeroutput"><span class="identifier">second</span><span class="special">()</span></code>. 109 Note that not all member functions can be instantiated for all template parameter 110 types. In particular <code class="computeroutput"><span class="identifier">compressed_pair</span></code> 111 can be instantiated for reference and array types, however in these cases the 112 range of constructors that can be used are limited. If types <code class="computeroutput"><span class="identifier">T1</span></code> and <code class="computeroutput"><span class="identifier">T2</span></code> 113 are the same type, then there is only one version of the single-argument constructor, 114 and this constructor initialises both values in the pair to the passed value. 115 </p> 116<p> 117 Note that if either member is a POD type, then that member is not zero-initialized 118 by the <code class="computeroutput"><span class="identifier">compressed_pair</span></code> default 119 constructor: it's up to you to supply an initial value for these types if you 120 want them to have a default value. 121 </p> 122<p> 123 Note that <code class="computeroutput"><span class="identifier">compressed_pair</span></code> can 124 not be instantiated if either of the template arguments is a union type, unless 125 there is compiler support for <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">is_union</span></code>, 126 or if <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">is_union</span></code> is specialised for the union type. 127 </p> 128<p> 129 Finally, a word of caution for Visual C++ 6 users: if either argument is an 130 empty type, then assigning to that member will produce memory corruption, unless 131 the empty type has a "do nothing" assignment operator defined. This 132 is due to a bug in the way VC6 generates implicit assignment operators. 133 </p> 134</div> 135<div class="section"> 136<div class="titlepage"><div><div><h2 class="title" style="clear: both"> 137<a name="compressed_pair.acknowledgments"></a><a class="link" href="compressed_pair.html#compressed_pair.acknowledgments" title="Acknowledgments">Acknowledgments</a> 138</h2></div></div></div> 139<p> 140 Based on contributions by Steve Cleary, Beman Dawes, Howard Hinnant and John 141 Maddock. 142 </p> 143<p> 144 Maintained by <a href="mailto:john%40johnmaddock.co.uk" target="_top">John Maddock</a>. 145 </p> 146</div> 147</div> 148<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr> 149<td align="left"><p><small>Last revised: August 11, 2020 at 14:59:07 GMT</small></p></td> 150<td align="right"><div class="copyright-footer"></div></td> 151</tr></table> 152<hr> 153<div class="spirit-nav"></div> 154</body> 155</html> 156
