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>Movable but Non-Copyable Types</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="composition_inheritance.html" title="Composition or inheritance"> 11<link rel="next" href="move_and_containers.html" title="Containers and move semantics"> 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="composition_inheritance.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="move_and_containers.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.movable_only_classes"></a><a class="link" href="movable_only_classes.html" title="Movable but Non-Copyable Types">Movable but Non-Copyable Types</a> 29</h2></div></div></div> 30<p> 31 Some types are not amenable to copy semantics but can still be made movable. 32 For example: 33 </p> 34<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "> 35<li class="listitem"> 36 <code class="computeroutput"><span class="identifier">unique_ptr</span></code> (non-shared, 37 non-copyable ownership) 38 </li> 39<li class="listitem"> 40 A type representing a thread of execution 41 </li> 42<li class="listitem"> 43 A type representing a file descriptor 44 </li> 45</ul></div> 46<p> 47 By making such types movable (though still non-copyable) their utility is tremendously 48 increased. Movable but non-copyable types can be returned by value from factory 49 functions: 50 </p> 51<pre class="programlisting"><span class="identifier">file_descriptor</span> <span class="identifier">create_file</span><span class="special">(/*</span> <span class="special">...</span> <span class="special">*/);</span> 52<span class="comment">//...</span> 53<span class="identifier">file_descriptor</span> <span class="identifier">data_file</span><span class="special">;</span> 54<span class="comment">//...</span> 55<span class="identifier">data_file</span> <span class="special">=</span> <span class="identifier">create_file</span><span class="special">(/*</span> <span class="special">...</span> <span class="special">*/);</span> <span class="comment">// No copies!</span> 56</pre> 57<p> 58 In the above example, the underlying file handle is passed from object to object, 59 as long as the source <code class="computeroutput"><span class="identifier">file_descriptor</span></code> 60 is a rvalue. At all times, there is still only one underlying file handle, 61 and only one <code class="computeroutput"><span class="identifier">file_descriptor</span></code> 62 owns it at a time. 63 </p> 64<p> 65 To write a movable but not copyable type in portable syntax, you need to follow 66 these simple steps: 67 </p> 68<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "> 69<li class="listitem"> 70 Put the following macro in the <span class="bold"><strong>private</strong></span> 71 section: <code class="computeroutput"><a class="link" href="../BOOST_MOVABL_1_3_26_20_7_3.html" title="Macro BOOST_MOVABLE_BUT_NOT_COPYABLE">BOOST_MOVABLE_BUT_NOT_COPYABLE(classname)</a></code> 72 </li> 73<li class="listitem"> 74 Write a move constructor and a move assignment taking the parameter as 75 <code class="computeroutput"><a class="link" href="../BOOST_RV_REF.html" title="Macro BOOST_RV_REF">BOOST_RV_REF(classname)</a></code> 76 </li> 77</ul></div> 78<p> 79 Here's the definition of <code class="computeroutput"><span class="identifier">file</span> <span class="identifier">descriptor</span></code> using portable syntax: 80 </p> 81<p> 82</p> 83<pre class="programlisting"><span class="preprocessor">#include</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">utility_core</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span> 84<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">stdexcept</span><span class="special">></span> 85 86<span class="keyword">class</span> <span class="identifier">file_descriptor</span> 87<span class="special">{</span> 88 <span class="keyword">int</span> <span class="identifier">os_descr_</span><span class="special">;</span> 89 90 <span class="keyword">private</span><span class="special">:</span> 91 <span class="identifier">BOOST_MOVABLE_BUT_NOT_COPYABLE</span><span class="special">(</span><span class="identifier">file_descriptor</span><span class="special">)</span> 92 93 <span class="keyword">public</span><span class="special">:</span> 94 <span class="keyword">explicit</span> <span class="identifier">file_descriptor</span><span class="special">(</span><span class="keyword">const</span> <span class="keyword">char</span> <span class="special">*</span><span class="identifier">filename</span><span class="special">)</span> <span class="comment">//Constructor</span> 95 <span class="special">:</span> <span class="identifier">os_descr_</span><span class="special">(</span><span class="identifier">operating_system_open_file</span><span class="special">(</span><span class="identifier">filename</span><span class="special">))</span> 96 <span class="special">{</span> <span class="keyword">if</span><span class="special">(!</span><span class="identifier">os_descr_</span><span class="special">)</span> <span class="keyword">throw</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">runtime_error</span><span class="special">(</span><span class="string">"file not found"</span><span class="special">);</span> <span class="special">}</span> 97 98 <span class="special">~</span><span class="identifier">file_descriptor</span><span class="special">()</span> <span class="comment">//Destructor</span> 99 <span class="special">{</span> <span class="keyword">if</span><span class="special">(</span><span class="identifier">os_descr_</span><span class="special">)</span> <span class="identifier">operating_system_close_file</span><span class="special">(</span><span class="identifier">os_descr_</span><span class="special">);</span> <span class="special">}</span> 100 101 <span class="identifier">file_descriptor</span><span class="special">(</span><span class="identifier">BOOST_RV_REF</span><span class="special">(</span><span class="identifier">file_descriptor</span><span class="special">)</span> <span class="identifier">x</span><span class="special">)</span> <span class="comment">// Move ctor</span> 102 <span class="special">:</span> <span class="identifier">os_descr_</span><span class="special">(</span><span class="identifier">x</span><span class="special">.</span><span class="identifier">os_descr_</span><span class="special">)</span> 103 <span class="special">{</span> <span class="identifier">x</span><span class="special">.</span><span class="identifier">os_descr_</span> <span class="special">=</span> <span class="number">0</span><span class="special">;</span> <span class="special">}</span> 104 105 <span class="identifier">file_descriptor</span><span class="special">&</span> <span class="keyword">operator</span><span class="special">=(</span><span class="identifier">BOOST_RV_REF</span><span class="special">(</span><span class="identifier">file_descriptor</span><span class="special">)</span> <span class="identifier">x</span><span class="special">)</span> <span class="comment">// Move assign</span> 106 <span class="special">{</span> 107 <span class="keyword">if</span><span class="special">(</span><span class="identifier">os_descr_</span><span class="special">)</span> <span class="identifier">operating_system_close_file</span><span class="special">(</span><span class="identifier">os_descr_</span><span class="special">);</span> 108 <span class="identifier">os_descr_</span> <span class="special">=</span> <span class="identifier">x</span><span class="special">.</span><span class="identifier">os_descr_</span><span class="special">;</span> 109 <span class="identifier">x</span><span class="special">.</span><span class="identifier">os_descr_</span> <span class="special">=</span> <span class="number">0</span><span class="special">;</span> 110 <span class="keyword">return</span> <span class="special">*</span><span class="keyword">this</span><span class="special">;</span> 111 <span class="special">}</span> 112 113 <span class="keyword">bool</span> <span class="identifier">empty</span><span class="special">()</span> <span class="keyword">const</span> <span class="special">{</span> <span class="keyword">return</span> <span class="identifier">os_descr_</span> <span class="special">==</span> <span class="number">0</span><span class="special">;</span> <span class="special">}</span> 114<span class="special">};</span> 115</pre> 116<p> 117 </p> 118</div> 119<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr> 120<td align="left"></td> 121<td align="right"><div class="copyright-footer">Copyright © 2008-2014 Ion Gaztanaga<p> 122 Distributed under the Boost Software License, Version 1.0. (See accompanying 123 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>) 124 </p> 125</div></td> 126</tr></table> 127<hr> 128<div class="spirit-nav"> 129<a accesskey="p" href="composition_inheritance.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="move_and_containers.html"><img src="../../../doc/src/images/next.png" alt="Next"></a> 130</div> 131</body> 132</html> 133
