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  26 <div class="titlepage"><div><div><h2 class="title" style="clear: both">
  27 <a name="move.composition_inheritance"></a><a class="link" href="composition_inheritance.html" title="Composition or inheritance">Composition or inheritance</a>
  28 </h2></div></div></div>
  29 <p>
  30       For classes made up of other classes (via either composition or inheritance),
  31       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>
  32       function:
  33     </p>
  34 <p>
  35 </p>
  36 <pre class="programlisting"><span class="keyword">class</span> <span class="identifier">Base</span>
  37 <span class="special">{</span>
  38    <span class="identifier">BOOST_COPYABLE_AND_MOVABLE</span><span class="special">(</span><span class="identifier">Base</span><span class="special">)</span>
  39
  40    <span class="keyword">public</span><span class="special">:</span>
  41    <span class="identifier">Base</span><span class="special">(){}</span>
  42
  43    <span class="identifier">Base</span><span class="special">(</span><span class="keyword">const</span> <span class="identifier">Base</span> <span class="special">&amp;</span><span class="identifier">x</span><span class="special">)</span> <span class="special">{/**/}</span>             <span class="comment">// Copy ctor</span>
  44
  45    <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="identifier">x</span><span class="special">)</span> <span class="special">{/**/}</span>      <span class="comment">// Move ctor</span>
  46
  47    <span class="identifier">Base</span><span class="special">&amp;</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="identifier">x</span><span class="special">)</span>
  48    <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>
  49
  50    <span class="identifier">Base</span><span class="special">&amp;</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="identifier">x</span><span class="special">)</span>
  51    <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>
  52
  53    <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>
  54    <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>
  55
  56    <span class="keyword">virtual</span> <span class="special">~</span><span class="identifier">Base</span><span class="special">(){}</span>
  57 <span class="special">};</span>
  58
  59 <span class="keyword">class</span> <span class="identifier">Member</span>
  60 <span class="special">{</span>
  61    <span class="identifier">BOOST_COPYABLE_AND_MOVABLE</span><span class="special">(</span><span class="identifier">Member</span><span class="special">)</span>
  62
  63    <span class="keyword">public</span><span class="special">:</span>
  64    <span class="identifier">Member</span><span class="special">(){}</span>
  65
  66    <span class="comment">// Compiler-generated copy constructor...</span>
  67
  68    <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>
  69
  70    <span class="identifier">Member</span> <span class="special">&amp;</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>
  71    <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>
  72
  73    <span class="identifier">Member</span> <span class="special">&amp;</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>
  74    <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>
  75 <span class="special">};</span>
  76
  77 <span class="keyword">class</span> <span class="identifier">Derived</span> <span class="special">:</span> <span class="keyword">public</span> <span class="identifier">Base</span>
  78 <span class="special">{</span>
  79    <span class="identifier">BOOST_COPYABLE_AND_MOVABLE</span><span class="special">(</span><span class="identifier">Derived</span><span class="special">)</span>
  80    <span class="identifier">Member</span> <span class="identifier">mem_</span><span class="special">;</span>
  81
  82    <span class="keyword">public</span><span class="special">:</span>
  83    <span class="identifier">Derived</span><span class="special">(){}</span>
  84
  85    <span class="comment">// Compiler-generated copy constructor...</span>
  86
  87    <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>
  88       <span class="special">:</span> <span class="identifier">Base</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="keyword">static_cast</span><span class="special">&lt;</span><span class="identifier">Base</span><span class="special">&amp;&gt;(</span><span class="identifier">x</span><span class="special">))),</span>
  89         <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>
  90
  91    <span class="identifier">Derived</span><span class="special">&amp;</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>
  92    <span class="special">{</span>
  93       <span class="identifier">Base</span><span class="special">::</span><span class="keyword">operator</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="keyword">static_cast</span><span class="special">&lt;</span><span class="identifier">Base</span><span class="special">&amp;&gt;(</span><span class="identifier">x</span><span class="special">)));</span>
  94       <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>
  95       <span class="keyword">return</span> <span class="special">*</span><span class="keyword">this</span><span class="special">;</span>
  96    <span class="special">}</span>
  97
  98    <span class="identifier">Derived</span><span class="special">&amp;</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>
  99    <span class="special">{</span>
 100       <span class="identifier">Base</span><span class="special">::</span><span class="keyword">operator</span><span class="special">=(</span><span class="keyword">static_cast</span><span class="special">&lt;</span><span class="keyword">const</span> <span class="identifier">Base</span><span class="special">&amp;&gt;(</span><span class="identifier">x</span><span class="special">));</span>
 101       <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>
 102       <span class="keyword">return</span> <span class="special">*</span><span class="keyword">this</span><span class="special">;</span>
 103    <span class="special">}</span>
 104    <span class="comment">// ...</span>
 105 <span class="special">};</span>
 106 </pre>
 107 <p>
 108     </p>
 109 <div class="important"><table border="0" summary="Important">
 110 <tr>
 111 <td rowspan="2" align="center" valign="top" width="25"><img alt="[Important]" src="../../../doc/src/images/important.png"></td>
 112 <th align="left">Important</th>
 113 </tr>
 114 <tr><td align="left" valign="top"><p>
 115         Due to limitations in the emulation code, a cast to <code class="computeroutput"><span class="identifier">Base</span>
 116         <span class="special">&amp;</span></code> is needed before moving the
 117         base part in the move constructor and call Base's move constructor instead
 118         of the copy constructor.
 119       </p></td></tr>
 120 </table></div>
 121 <p>
 122       Each subobject will now be treated individually, calling move to bind to the
 123       subobject's move constructors and move assignment operators. <code class="computeroutput"><span class="identifier">Member</span></code> has move operations coded (just like
 124       our earlier <code class="computeroutput"><span class="identifier">clone_ptr</span></code> example)
 125       which will completely avoid the tremendously more expensive copy operations:
 126     </p>
 127 <p>
 128 </p>
 129 <pre class="programlisting"><span class="identifier">Derived</span> <span class="identifier">d</span><span class="special">;</span>
 130 <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>
 131 <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 </pre>
 133 <p>
 134     </p>
 135 <p>
 136       Note above that the argument x is treated as a lvalue reference. That's why
 137       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>
 138       instead of just x when passing down to the base class. This is a key safety
 139       feature of move semantics designed to prevent accidently moving twice from
 140       some named variable. All moves from lvalues occur explicitly.
 141     </p>
 142 </div>
 143 <table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
 144 <td align="left"></td>
 145 <td align="right"><div class="copyright-footer">Copyright &#169; 2008-2010 Ion Gaztanaga<p>
 146         Distributed under the Boost Software License, Version 1.0. (See accompanying
 147         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>)
 148       </p>
 149 </div></td>
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