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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="comment">/*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="comment">/*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="comment">/*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_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> |
| <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_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> |
| <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="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 © 2008-2014 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> |
| </div></td> |
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