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<title>Composition or inheritance</title>
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<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="move.composition_inheritance"></a><a class="link" href="composition_inheritance.html" title="Composition or inheritance">Composition or inheritance</a>
</h2></div></div></div>
<p>
For classes made up of other classes (via either composition or inheritance),
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>
function:
</p>
<p>
</p>
<pre class="programlisting"><span class="keyword">class</span> <span class="identifier">Base</span>
<span class="special">{</span>
<span class="identifier">BOOST_COPYABLE_AND_MOVABLE</span><span class="special">(</span><span class="identifier">Base</span><span class="special">)</span>
<span class="keyword">public</span><span class="special">:</span>
<span class="identifier">Base</span><span class="special">(){}</span>
<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>
<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>
<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="identifier">x</span><span class="special">)</span>
<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>
<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="identifier">x</span><span class="special">)</span>
<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>
<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>
<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>
<span class="keyword">virtual</span> <span class="special">~</span><span class="identifier">Base</span><span class="special">(){}</span>
<span class="special">};</span>
<span class="keyword">class</span> <span class="identifier">Member</span>
<span class="special">{</span>
<span class="identifier">BOOST_COPYABLE_AND_MOVABLE</span><span class="special">(</span><span class="identifier">Member</span><span class="special">)</span>
<span class="keyword">public</span><span class="special">:</span>
<span class="identifier">Member</span><span class="special">(){}</span>
<span class="comment">// Compiler-generated copy constructor...</span>
<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>
<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>
<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>
<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>
<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>
<span class="special">};</span>
<span class="keyword">class</span> <span class="identifier">Derived</span> <span class="special">:</span> <span class="keyword">public</span> <span class="identifier">Base</span>
<span class="special">{</span>
<span class="identifier">BOOST_COPYABLE_AND_MOVABLE</span><span class="special">(</span><span class="identifier">Derived</span><span class="special">)</span>
<span class="identifier">Member</span> <span class="identifier">mem_</span><span class="special">;</span>
<span class="keyword">public</span><span class="special">:</span>
<span class="identifier">Derived</span><span class="special">(){}</span>
<span class="comment">// Compiler-generated copy constructor...</span>
<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>
<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"><</span><span class="identifier">Base</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="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>
<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>
<span class="special">{</span>
<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"><</span><span class="identifier">Base</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="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="keyword">return</span> <span class="special">*</span><span class="keyword">this</span><span class="special">;</span>
<span class="special">}</span>
<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>
<span class="special">{</span>
<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"><</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="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>
<span class="keyword">return</span> <span class="special">*</span><span class="keyword">this</span><span class="special">;</span>
<span class="special">}</span>
<span class="comment">// ...</span>
<span class="special">};</span>
</pre>
<p>
</p>
<div class="important"><table border="0" summary="Important">
<tr>
<td rowspan="2" align="center" valign="top" width="25"><img alt="[Important]" src="../../../doc/src/images/important.png"></td>
<th align="left">Important</th>
</tr>
<tr><td align="left" valign="top"><p>
Due to limitations in the emulation code, a cast to <code class="computeroutput"><span class="identifier">Base</span>
<span class="special">&</span></code> is needed before moving the
base part in the move constructor and call Base's move constructor instead
of the copy constructor.
</p></td></tr>
</table></div>
<p>
Each subobject will now be treated individually, calling move to bind to the
subobject's move constructors and move assignment operators. <code class="computeroutput"><span class="identifier">Member</span></code> has move operations coded (just like
our earlier <code class="computeroutput"><span class="identifier">clone_ptr</span></code> example)
which will completely avoid the tremendously more expensive copy operations:
</p>
<p>
</p>
<pre class="programlisting"><span class="identifier">Derived</span> <span class="identifier">d</span><span class="special">;</span>
<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>
<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>
</pre>
<p>
</p>
<p>
Note above that the argument x is treated as a lvalue reference. That's why
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>
instead of just x when passing down to the base class. This is a key safety
feature of move semantics designed to prevent accidently moving twice from
some named variable. All moves from lvalues occur explicitly.
</p>
</div>
<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
<td align="left"></td>
<td align="right"><div class="copyright-footer">Copyright © 2008-2012 Ion Gaztanaga<p>
Distributed under the Boost Software License, Version 1.0. (See accompanying
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>)
</p>
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