1<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd"> 2<html> 3<head> 4<meta http-equiv="Content-Type" content="text/html; charset=UTF-8"> 5<title>Introduction</title> 6<link rel="stylesheet" href="../../../doc/src/boostbook.css" type="text/css"> 7<meta name="generator" content="DocBook XSL Stylesheets V1.79.1"> 8<link rel="home" href="../index.html" title="The Boost C++ Libraries BoostBook Documentation Subset"> 9<link rel="up" href="../move.html" title="Chapter 25. Boost.Move"> 10<link rel="prev" href="what_is_boost_move.html" title="What is Boost.Move?"> 11<link rel="next" href="implementing_movable_classes.html" title="Implementing copyable and movable classes"> 12</head> 13<body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"> 14<table cellpadding="2" width="100%"><tr> 15<td valign="top"><img alt="Boost C++ Libraries" width="277" height="86" src="../../../boost.png"></td> 16<td align="center"><a href="../../../index.html">Home</a></td> 17<td align="center"><a href="../../../libs/libraries.htm">Libraries</a></td> 18<td align="center"><a href="http://www.boost.org/users/people.html">People</a></td> 19<td align="center"><a href="http://www.boost.org/users/faq.html">FAQ</a></td> 20<td align="center"><a href="../../../more/index.htm">More</a></td> 21</tr></table> 22<hr> 23<div class="spirit-nav"> 24<a accesskey="p" href="what_is_boost_move.html"><img src="../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../move.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="implementing_movable_classes.html"><img src="../../../doc/src/images/next.png" alt="Next"></a> 25</div> 26<div class="section"> 27<div class="titlepage"><div><div><h2 class="title" style="clear: both"> 28<a name="move.introduction"></a><a class="link" href="introduction.html" title="Introduction">Introduction</a> 29</h2></div></div></div> 30<div class="note"><table border="0" summary="Note"> 31<tr> 32<td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../doc/src/images/note.png"></td> 33<th align="left">Note</th> 34</tr> 35<tr><td align="left" valign="top"><p> 36 The first 3 chapters are the adapted from the article <a href="http://www.artima.com/cppsource/rvalue.html" target="_top"><span class="emphasis"><em>A 37 Brief Introduction to Rvalue References</em></span></a> by Howard E. Hinnant, 38 Bjarne Stroustrup, and Bronek Kozicki 39 </p></td></tr> 40</table></div> 41<p> 42 Copying can be expensive. For example, for vectors <code class="computeroutput"><span class="identifier">v2</span><span class="special">=</span><span class="identifier">v1</span></code> typically 43 involves a function call, a memory allocation, and a loop. This is of course 44 acceptable where we actually need two copies of a vector, but in many cases, 45 we don't: We often copy a <code class="computeroutput"><span class="identifier">vector</span></code> 46 from one place to another, just to proceed to overwrite the old copy. Consider: 47 </p> 48<pre class="programlisting"><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">a</span><span class="special">,</span> <span class="identifier">T</span><span class="special">&</span> <span class="identifier">b</span><span class="special">)</span> 49<span class="special">{</span> 50 <span class="identifier">T</span> <span class="identifier">tmp</span><span class="special">(</span><span class="identifier">a</span><span class="special">);</span> <span class="comment">// now we have two copies of a</span> 51 <span class="identifier">a</span> <span class="special">=</span> <span class="identifier">b</span><span class="special">;</span> <span class="comment">// now we have two copies of b</span> 52 <span class="identifier">b</span> <span class="special">=</span> <span class="identifier">tmp</span><span class="special">;</span> <span class="comment">// now we have two copies of tmp (aka a)</span> 53<span class="special">}</span> 54</pre> 55<p> 56 But, we didn't want to have any copies of a or b, we just wanted to swap them. 57 Let's try again: 58 </p> 59<pre class="programlisting"><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">a</span><span class="special">,</span> <span class="identifier">T</span><span class="special">&</span> <span class="identifier">b</span><span class="special">)</span> 60<span class="special">{</span> 61 <span class="identifier">T</span> <span class="identifier">tmp</span><span class="special">(::</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">move</span><span class="special">(</span><span class="identifier">a</span><span class="special">));</span> 62 <span class="identifier">a</span> <span class="special">=</span> <span class="special">::</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">move</span><span class="special">(</span><span class="identifier">b</span><span class="special">);</span> 63 <span class="identifier">b</span> <span class="special">=</span> <span class="special">::</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">move</span><span class="special">(</span><span class="identifier">tmp</span><span class="special">);</span> 64<span class="special">}</span> 65</pre> 66<p> 67 This <code class="computeroutput"><span class="identifier">move</span><span class="special">()</span></code> 68 gives its target the value of its argument, but is not obliged to preserve 69 the value of its source. So, for a <code class="computeroutput"><span class="identifier">vector</span></code>, 70 <code class="computeroutput"><span class="identifier">move</span><span class="special">()</span></code> 71 could reasonably be expected to leave its argument as a zero-capacity vector 72 to avoid having to copy all the elements. In other words, <span class="bold"><strong>move 73 is a potentially destructive copy</strong></span>. 74 </p> 75<p> 76 In this particular case, we could have optimized swap by a specialization. 77 However, we can't specialize every function that copies a large object just 78 before it deletes or overwrites it. That would be unmanageable. 79 </p> 80<p> 81 In C++0x, move semantics are implemented with the introduction of rvalue references. 82 They allow us to implement <code class="computeroutput"><span class="identifier">move</span><span class="special">()</span></code> without verbosity or runtime overhead. <span class="bold"><strong>Boost.Move</strong></span> is a library that offers tools to implement 83 those move semantics not only in compilers with <code class="computeroutput"><span class="identifier">rvalue</span> 84 <span class="identifier">references</span></code> but also in compilers 85 conforming to C++03. 86 </p> 87</div> 88<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr> 89<td align="left"></td> 90<td align="right"><div class="copyright-footer">Copyright © 2008-2014 Ion Gaztanaga<p> 91 Distributed under the Boost Software License, Version 1.0. (See accompanying 92 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>) 93 </p> 94</div></td> 95</tr></table> 96<hr> 97<div class="spirit-nav"> 98<a accesskey="p" href="what_is_boost_move.html"><img src="../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../move.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="implementing_movable_classes.html"><img src="../../../doc/src/images/next.png" alt="Next"></a> 99</div> 100</body> 101</html> 102
