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>Composition or inheritance</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="implementing_movable_classes.html" title="Implementing copyable and movable classes"> 11<link rel="next" href="movable_only_classes.html" title="Movable but Non-Copyable Types"> 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="implementing_movable_classes.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="movable_only_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.composition_inheritance"></a><a class="link" href="composition_inheritance.html" title="Composition or inheritance">Composition or inheritance</a> 29</h2></div></div></div> 30<p> 31 For classes made up of other classes (via either composition or inheritance), 32 the move constructor and move assignment can be easily coded using the <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">move</span></code> 33 function: 34 </p> 35<p> 36</p> 37<pre class="programlisting"><span class="keyword">class</span> <span class="identifier">Base</span> 38<span class="special">{</span> 39 <span class="identifier">BOOST_COPYABLE_AND_MOVABLE</span><span class="special">(</span><span class="identifier">Base</span><span class="special">)</span> 40 41 <span class="keyword">public</span><span class="special">:</span> 42 <span class="identifier">Base</span><span class="special">(){}</span> 43 44 <span class="identifier">Base</span><span class="special">(</span><span class="keyword">const</span> <span class="identifier">Base</span> <span class="special">&/*</span><span class="identifier">x</span><span class="special">*/)</span> <span class="special">{/**/}</span> <span class="comment">// Copy ctor</span> 45 46 <span class="identifier">Base</span><span class="special">(</span><span class="identifier">BOOST_RV_REF</span><span class="special">(</span><span class="identifier">Base</span><span class="special">)</span> <span class="comment">/*x*/</span><span class="special">)</span> <span class="special">{/**/}</span> <span class="comment">// Move ctor</span> 47 48 <span class="identifier">Base</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">Base</span><span class="special">)</span> <span class="comment">/*x*/</span><span class="special">)</span> 49 <span class="special">{/**/</span> <span class="keyword">return</span> <span class="special">*</span><span class="keyword">this</span><span class="special">;}</span> <span class="comment">// Move assign</span> 50 51 <span class="identifier">Base</span><span class="special">&</span> <span class="keyword">operator</span><span class="special">=(</span><span class="identifier">BOOST_COPY_ASSIGN_REF</span><span class="special">(</span><span class="identifier">Base</span><span class="special">)</span> <span class="comment">/*x*/</span><span class="special">)</span> 52 <span class="special">{/**/</span> <span class="keyword">return</span> <span class="special">*</span><span class="keyword">this</span><span class="special">;}</span> <span class="comment">// Copy assign</span> 53 54 <span class="keyword">virtual</span> <span class="identifier">Base</span> <span class="special">*</span><span class="identifier">clone</span><span class="special">()</span> <span class="keyword">const</span> 55 <span class="special">{</span> <span class="keyword">return</span> <span class="keyword">new</span> <span class="identifier">Base</span><span class="special">(*</span><span class="keyword">this</span><span class="special">);</span> <span class="special">}</span> 56 57 <span class="keyword">virtual</span> <span class="special">~</span><span class="identifier">Base</span><span class="special">(){}</span> 58<span class="special">};</span> 59 60<span class="keyword">class</span> <span class="identifier">Member</span> 61<span class="special">{</span> 62 <span class="identifier">BOOST_COPYABLE_AND_MOVABLE</span><span class="special">(</span><span class="identifier">Member</span><span class="special">)</span> 63 64 <span class="keyword">public</span><span class="special">:</span> 65 <span class="identifier">Member</span><span class="special">(){}</span> 66 67 <span class="comment">// Compiler-generated copy constructor...</span> 68 69 <span class="identifier">Member</span><span class="special">(</span><span class="identifier">BOOST_RV_REF</span><span class="special">(</span><span class="identifier">Member</span><span class="special">))</span> <span class="special">{/**/}</span> <span class="comment">// Move ctor</span> 70 71 <span class="identifier">Member</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">Member</span><span class="special">))</span> <span class="comment">// Move assign</span> 72 <span class="special">{/**/</span> <span class="keyword">return</span> <span class="special">*</span><span class="keyword">this</span><span class="special">;</span> <span class="special">}</span> 73 74 <span class="identifier">Member</span> <span class="special">&</span><span class="keyword">operator</span><span class="special">=(</span><span class="identifier">BOOST_COPY_ASSIGN_REF</span><span class="special">(</span><span class="identifier">Member</span><span class="special">))</span> <span class="comment">// Copy assign</span> 75 <span class="special">{/**/</span> <span class="keyword">return</span> <span class="special">*</span><span class="keyword">this</span><span class="special">;</span> <span class="special">}</span> 76<span class="special">};</span> 77 78<span class="keyword">class</span> <span class="identifier">Derived</span> <span class="special">:</span> <span class="keyword">public</span> <span class="identifier">Base</span> 79<span class="special">{</span> 80 <span class="identifier">BOOST_COPYABLE_AND_MOVABLE</span><span class="special">(</span><span class="identifier">Derived</span><span class="special">)</span> 81 <span class="identifier">Member</span> <span class="identifier">mem_</span><span class="special">;</span> 82 83 <span class="keyword">public</span><span class="special">:</span> 84 <span class="identifier">Derived</span><span class="special">(){}</span> 85 86 <span class="comment">// Compiler-generated copy constructor...</span> 87 88 <span class="identifier">Derived</span><span class="special">(</span><span class="identifier">BOOST_RV_REF</span><span class="special">(</span><span class="identifier">Derived</span><span class="special">)</span> <span class="identifier">x</span><span class="special">)</span> <span class="comment">// Move ctor</span> 89 <span class="special">:</span> <span class="identifier">Base</span><span class="special">(</span><span class="identifier">BOOST_MOVE_BASE</span><span class="special">(</span><span class="identifier">Base</span><span class="special">,</span> <span class="identifier">x</span><span class="special">)),</span> 90 <span class="identifier">mem_</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">x</span><span class="special">.</span><span class="identifier">mem_</span><span class="special">))</span> <span class="special">{</span> <span class="special">}</span> 91 92 <span class="identifier">Derived</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">Derived</span><span class="special">)</span> <span class="identifier">x</span><span class="special">)</span> <span class="comment">// Move assign</span> 93 <span class="special">{</span> 94 <span class="identifier">Base</span><span class="special">::</span><span class="keyword">operator</span><span class="special">=(</span><span class="identifier">BOOST_MOVE_BASE</span><span class="special">(</span><span class="identifier">Base</span><span class="special">,</span> <span class="identifier">x</span><span class="special">));</span> 95 <span class="identifier">mem_</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">x</span><span class="special">.</span><span class="identifier">mem_</span><span class="special">);</span> 96 <span class="keyword">return</span> <span class="special">*</span><span class="keyword">this</span><span class="special">;</span> 97 <span class="special">}</span> 98 99 <span class="identifier">Derived</span><span class="special">&</span> <span class="keyword">operator</span><span class="special">=(</span><span class="identifier">BOOST_COPY_ASSIGN_REF</span><span class="special">(</span><span class="identifier">Derived</span><span class="special">)</span> <span class="identifier">x</span><span class="special">)</span> <span class="comment">// Copy assign</span> 100 <span class="special">{</span> 101 <span class="identifier">Base</span><span class="special">::</span><span class="keyword">operator</span><span class="special">=(</span><span class="identifier">x</span><span class="special">);</span> 102 <span class="identifier">mem_</span> <span class="special">=</span> <span class="identifier">x</span><span class="special">.</span><span class="identifier">mem_</span><span class="special">;</span> 103 <span class="keyword">return</span> <span class="special">*</span><span class="keyword">this</span><span class="special">;</span> 104 <span class="special">}</span> 105 <span class="comment">// ...</span> 106<span class="special">};</span> 107</pre> 108<p> 109 </p> 110<div class="important"><table border="0" summary="Important"> 111<tr> 112<td rowspan="2" align="center" valign="top" width="25"><img alt="[Important]" src="../../../doc/src/images/important.png"></td> 113<th align="left">Important</th> 114</tr> 115<tr><td align="left" valign="top"><p> 116 Due to limitations in the emulation code, a cast to <code class="computeroutput"><span class="identifier">Base</span> 117 <span class="special">&</span></code> is needed before moving the 118 base part in the move constructor and call Base's move constructor instead 119 of the copy constructor. 120 </p></td></tr> 121</table></div> 122<p> 123 Each subobject will now be treated individually, calling move to bind to the 124 subobject's move constructors and move assignment operators. <code class="computeroutput"><span class="identifier">Member</span></code> has move operations coded (just like 125 our earlier <code class="computeroutput"><span class="identifier">clone_ptr</span></code> example) 126 which will completely avoid the tremendously more expensive copy operations: 127 </p> 128<p> 129</p> 130<pre class="programlisting"><span class="identifier">Derived</span> <span class="identifier">d</span><span class="special">;</span> 131<span class="identifier">Derived</span> <span class="identifier">d2</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">d</span><span class="special">));</span> 132<span class="identifier">d2</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">d</span><span class="special">);</span> 133</pre> 134<p> 135 </p> 136<p> 137 Note above that the argument x is treated as a lvalue reference. That's why 138 it is necessary to say <code class="computeroutput"><span class="identifier">move</span><span class="special">(</span><span class="identifier">x</span><span class="special">)</span></code> 139 instead of just x when passing down to the base class. This is a key safety 140 feature of move semantics designed to prevent accidently moving twice from 141 some named variable. All moves from lvalues occur explicitly. 142 </p> 143</div> 144<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr> 145<td align="left"></td> 146<td align="right"><div class="copyright-footer">Copyright © 2008-2014 Ion Gaztanaga<p> 147 Distributed under the Boost Software License, Version 1.0. (See accompanying 148 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>) 149 </p> 150</div></td> 151</tr></table> 152<hr> 153<div class="spirit-nav"> 154<a accesskey="p" href="implementing_movable_classes.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="movable_only_classes.html"><img src="../../../doc/src/images/next.png" alt="Next"></a> 155</div> 156</body> 157</html> 158
