1<html> 2<head> 3<meta http-equiv="Content-Type" content="text/html; charset=UTF-8"> 4<title>swap</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="../index.html" title="Chapter 1. Boost.Core"> 8<link rel="up" href="../index.html" title="Chapter 1. Boost.Core"> 9<link rel="prev" href="scoped_enum.html" title="scoped_enum"> 10<link rel="next" href="typeinfo.html" title="typeinfo"> 11</head> 12<body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"> 13<table cellpadding="2" width="100%"><tr> 14<td valign="top"><img alt="Boost C++ Libraries" width="277" height="86" src="../../../../../boost.png"></td> 15<td align="center"><a href="../../../../../index.html">Home</a></td> 16<td align="center"><a href="../../../../../libs/libraries.htm">Libraries</a></td> 17<td align="center"><a href="http://www.boost.org/users/people.html">People</a></td> 18<td align="center"><a href="http://www.boost.org/users/faq.html">FAQ</a></td> 19<td align="center"><a href="../../../../../more/index.htm">More</a></td> 20</tr></table> 21<hr> 22<div class="spirit-nav"> 23<a accesskey="p" href="scoped_enum.html"><img src="../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../index.html"><img src="../../../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../index.html"><img src="../../../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="typeinfo.html"><img src="../../../../../doc/src/images/next.png" alt="Next"></a> 24</div> 25<div class="section"> 26<div class="titlepage"><div><div><h2 class="title" style="clear: both"> 27<a name="core.swap"></a><a class="link" href="swap.html" title="swap">swap</a> 28</h2></div></div></div> 29<div class="toc"><dl class="toc"> 30<dt><span class="section"><a href="swap.html#core.swap.header_boost_core_swap_hpp">Header <boost/core/swap.hpp></a></span></dt> 31<dt><span class="section"><a href="swap.html#core.swap.introduction">Introduction</a></span></dt> 32<dt><span class="section"><a href="swap.html#core.swap.rationale">Rationale</a></span></dt> 33<dt><span class="section"><a href="swap.html#core.swap.exception_safety">Exception Safety</a></span></dt> 34<dt><span class="section"><a href="swap.html#core.swap.requirements">Requirements</a></span></dt> 35<dt><span class="section"><a href="swap.html#core.swap.portability">Portability</a></span></dt> 36<dt><span class="section"><a href="swap.html#core.swap.credits">Credits</a></span></dt> 37</dl></div> 38<div class="simplesect"> 39<div class="titlepage"><div><div><h3 class="title"> 40<a name="idm45312825994384"></a>Authors</h3></div></div></div> 41<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "> 42<li class="listitem"> 43 Niels Dekker 44 </li> 45<li class="listitem"> 46 Joseph Gauterin 47 </li> 48<li class="listitem"> 49 Steven Watanabe 50 </li> 51<li class="listitem"> 52 Eric Niebler 53 </li> 54</ul></div> 55</div> 56<div class="section"> 57<div class="titlepage"><div><div><h3 class="title"> 58<a name="core.swap.header_boost_core_swap_hpp"></a><a class="link" href="swap.html#core.swap.header_boost_core_swap_hpp" title="Header <boost/core/swap.hpp>">Header <boost/core/swap.hpp></a> 59</h3></div></div></div> 60<p> 61 <code class="computeroutput"><span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">></span> <span class="keyword">void</span> <span class="identifier">swap</span><span class="special">(</span><span class="identifier">T</span><span class="special">&</span> <span class="identifier">left</span><span class="special">,</span> <span class="identifier">T</span><span class="special">&</span> 62 <span class="identifier">right</span><span class="special">);</span></code> 63 </p> 64</div> 65<div class="section"> 66<div class="titlepage"><div><div><h3 class="title"> 67<a name="core.swap.introduction"></a><a class="link" href="swap.html#core.swap.introduction" title="Introduction">Introduction</a> 68</h3></div></div></div> 69<p> 70 The template function <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">swap</span></code> 71 allows the values of two variables to be swapped, using argument dependent 72 lookup to select a specialized swap function if available. If no specialized 73 swap function is available, <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">swap</span></code> 74 is used. 75 </p> 76</div> 77<div class="section"> 78<div class="titlepage"><div><div><h3 class="title"> 79<a name="core.swap.rationale"></a><a class="link" href="swap.html#core.swap.rationale" title="Rationale">Rationale</a> 80</h3></div></div></div> 81<p> 82 The generic <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">swap</span></code> function requires that the elements 83 to be swapped are assignable and copy constructible. It is usually implemented 84 using one copy construction and two assignments - this is often both unnecessarily 85 restrictive and unnecessarily slow. In addition, where the generic swap implementation 86 provides only the basic guarantee, specialized swap functions are often able 87 to provide the no-throw exception guarantee (and it is considered best practice 88 to do so where possible <a href="#ftn.core.swap.rationale.f0" class="footnote" name="core.swap.rationale.f0"><sup class="footnote">[1]</sup></a>. 89 </p> 90<p> 91 The alternative to using argument dependent lookup in this situation is to 92 provide a template specialization of <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">swap</span></code> 93 for every type that requires a specialized swap. Although this is legal C++, 94 no Boost libraries use this method, whereas many Boost libraries provide 95 specialized swap functions in their own namespaces. 96 </p> 97<p> 98 <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">swap</span></code> also supports swapping built-in arrays. 99 Note that <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">swap</span></code> originally did not do so, but a request 100 to add an overload of <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">swap</span></code> 101 for built-in arrays has been accepted by the C++ Standards Committee<a href="#ftn.core.swap.rationale.f1" class="footnote" name="core.swap.rationale.f1"><sup class="footnote">[2]</sup></a>. 102 </p> 103</div> 104<div class="section"> 105<div class="titlepage"><div><div><h3 class="title"> 106<a name="core.swap.exception_safety"></a><a class="link" href="swap.html#core.swap.exception_safety" title="Exception Safety">Exception Safety</a> 107</h3></div></div></div> 108<p> 109 <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">swap</span></code> provides the same exception guarantee 110 as the underlying swap function used, with one exception; for an array of 111 type <code class="computeroutput"><span class="identifier">T</span><span class="special">[</span><span class="identifier">n</span><span class="special">]</span></code>, where 112 <code class="computeroutput"><span class="identifier">n</span> <span class="special">></span> 113 <span class="number">1</span></code> and the underlying swap function 114 for <code class="computeroutput"><span class="identifier">T</span></code> provides the strong 115 exception guarantee, <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">swap</span></code> 116 provides only the basic exception guarantee. 117 </p> 118</div> 119<div class="section"> 120<div class="titlepage"><div><div><h3 class="title"> 121<a name="core.swap.requirements"></a><a class="link" href="swap.html#core.swap.requirements" title="Requirements">Requirements</a> 122</h3></div></div></div> 123<p> 124 Either: 125 </p> 126<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "> 127<li class="listitem"> 128 T must be assignable 129 </li> 130<li class="listitem"> 131 T must be copy constructible 132 </li> 133</ul></div> 134<p> 135 Or: 136 </p> 137<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem"> 138 A function with the signature <code class="computeroutput"><span class="identifier">swap</span><span class="special">(</span><span class="identifier">T</span><span class="special">&,</span><span class="identifier">T</span><span class="special">&)</span></code> is available via argument dependent 139 lookup 140 </li></ul></div> 141<p> 142 Or: 143 </p> 144<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem"> 145 A template specialization of <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">swap</span></code> 146 exists for T 147 </li></ul></div> 148<p> 149 Or: 150 </p> 151<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem"> 152 T is a built-in array of swappable elements 153 </li></ul></div> 154</div> 155<div class="section"> 156<div class="titlepage"><div><div><h3 class="title"> 157<a name="core.swap.portability"></a><a class="link" href="swap.html#core.swap.portability" title="Portability">Portability</a> 158</h3></div></div></div> 159<p> 160 Several older compilers do not support argument dependent lookup. On these 161 compilers <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">swap</span></code> will call <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">swap</span></code>, 162 ignoring any specialized swap functions that could be found as a result of 163 argument dependent lookup. 164 </p> 165</div> 166<div class="section"> 167<div class="titlepage"><div><div><h3 class="title"> 168<a name="core.swap.credits"></a><a class="link" href="swap.html#core.swap.credits" title="Credits">Credits</a> 169</h3></div></div></div> 170<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "> 171<li class="listitem"> 172 <span class="bold"><strong>Niels Dekker</strong></span> - for implementing and 173 documenting support for built-in arrays 174 </li> 175<li class="listitem"> 176 <span class="bold"><strong>Joseph Gauterin</strong></span> - for the initial idea, 177 implementation, tests, and documentation 178 </li> 179<li class="listitem"> 180 <span class="bold"><strong>Steven Watanabe</strong></span> - for the idea to make 181 <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">swap</span></code> less specialized than <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">swap</span></code>, thereby allowing the function 182 to have the name 'swap' without introducing ambiguity 183 </li> 184</ul></div> 185</div> 186<div class="footnotes"> 187<br><hr style="width:100; text-align:left;margin-left: 0"> 188<div id="ftn.core.swap.rationale.f0" class="footnote"><p><a href="#core.swap.rationale.f0" class="para"><sup class="para">[1] </sup></a> 189 Scott Meyers, Effective C++ Third Edition, Item 25: "Consider support 190 for a non-throwing swap" 191 </p></div> 192<div id="ftn.core.swap.rationale.f1" class="footnote"><p><a href="#core.swap.rationale.f1" class="para"><sup class="para">[2] </sup></a> 193 <a href="http://open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html#809" target="_top">LWG 194 Defect Report 809: std::swap should be overloaded for array types</a> 195 </p></div> 196</div> 197</div> 198<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr> 199<td align="left"></td> 200<td align="right"><div class="copyright-footer">Copyright © 2014 Peter Dimov<br>Copyright © 2014 Glen Fernandes<br>Copyright © 2014 Andrey Semashev<p> 201 Distributed under the <a href="http://boost.org/LICENSE_1_0.txt" target="_top">Boost 202 Software License, Version 1.0</a>. 203 </p> 204</div></td> 205</tr></table> 206<hr> 207<div class="spirit-nav"> 208<a accesskey="p" href="scoped_enum.html"><img src="../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../index.html"><img src="../../../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../index.html"><img src="../../../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="typeinfo.html"><img src="../../../../../doc/src/images/next.png" alt="Next"></a> 209</div> 210</body> 211</html> 212
