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<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="move.move_return"></a><a class="link" href="move_return.html" title="Implicit Move when returning a local object">Implicit Move when returning a local
object</a>
</h2></div></div></div>
<p>
The C++ standard specifies situations where an implicit move operation is safe
and the compiler can use it in cases were the (Named) Return Value Optimization)
can't be used. The typical use case is when a function returns a named (non-temporary)
object by value and the following code will perfectly compile in C++11:
</p>
<pre class="programlisting"><span class="comment">//Even if movable can't be copied</span>
<span class="comment">//the compiler will call the move-constructor</span>
<span class="comment">//to generate the return value</span>
<span class="comment">//</span>
<span class="comment">//The compiler can also optimize out the move</span>
<span class="comment">//and directly construct the object 'm'</span>
<span class="identifier">movable</span> <span class="identifier">factory</span><span class="special">()</span>
<span class="special">{</span>
<span class="identifier">movable</span> <span class="identifier">tmp</span><span class="special">;</span>
<span class="identifier">m</span> <span class="special">=</span> <span class="special">...</span>
<span class="comment">//(1) moved instead of copied</span>
<span class="keyword">return</span> <span class="identifier">tmp</span><span class="special">;</span>
<span class="special">};</span>
<span class="comment">//Initialize object</span>
<span class="identifier">movable</span> <span class="identifier">m</span><span class="special">(</span><span class="identifier">factory</span><span class="special">());</span>
</pre>
<p>
In compilers without rvalue references and some non-conforming compilers (such
as Visual C++ 2010/2012) the line marked with <code class="computeroutput"><span class="special">(</span><span class="number">1</span><span class="special">)</span></code> would trigger
a compilation error because <code class="computeroutput"><span class="identifier">movable</span></code>
can't be copied. Using a explicit <code class="computeroutput"><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">tmp</span><span class="special">)</span></code>
would workaround this limitation but it would code suboptimal in C++11 compilers
(as the compile could not use (N)RVO to optimize-away the copy/move).
</p>
<p>
<span class="bold"><strong>Boost.Move</strong></span> offers an additional macro called
<code class="computeroutput"><a class="link" href="../BOOST_MOVE_RET.html" title="Macro BOOST_MOVE_RET">BOOST_MOVE_RET</a></code> that can be used
to alleviate this problem obtaining portable move-on-return semantics. Let's
use the previously presented movable-only <code class="computeroutput"><span class="identifier">movable</span></code>
class with classes <code class="computeroutput"><span class="identifier">copyable</span></code>
(copy-only type), <code class="computeroutput"><span class="identifier">copy_movable</span></code>
(can be copied and moved) and <code class="computeroutput"><span class="identifier">non_copy_movable</span></code>
(non-copyable and non-movable):
</p>
<p>
</p>
<pre class="programlisting"><span class="comment">//header file "copymovable.hpp"</span>
<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">core</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span>
<span class="comment">//A copy_movable class</span>
<span class="keyword">class</span> <span class="identifier">copy_movable</span>
<span class="special">{</span>
<span class="identifier">BOOST_COPYABLE_AND_MOVABLE</span><span class="special">(</span><span class="identifier">copy_movable</span><span class="special">)</span>
<span class="keyword">int</span> <span class="identifier">value_</span><span class="special">;</span>
<span class="keyword">public</span><span class="special">:</span>
<span class="identifier">copy_movable</span><span class="special">()</span> <span class="special">:</span> <span class="identifier">value_</span><span class="special">(</span><span class="number">1</span><span class="special">){}</span>
<span class="comment">//Move constructor and assignment</span>
<span class="identifier">copy_movable</span><span class="special">(</span><span class="identifier">BOOST_RV_REF</span><span class="special">(</span><span class="identifier">copy_movable</span><span class="special">)</span> <span class="identifier">m</span><span class="special">)</span>
<span class="special">{</span> <span class="identifier">value_</span> <span class="special">=</span> <span class="identifier">m</span><span class="special">.</span><span class="identifier">value_</span><span class="special">;</span> <span class="identifier">m</span><span class="special">.</span><span class="identifier">value_</span> <span class="special">=</span> <span class="number">0</span><span class="special">;</span> <span class="special">}</span>
<span class="identifier">copy_movable</span><span class="special">(</span><span class="keyword">const</span> <span class="identifier">copy_movable</span> <span class="special">&</span><span class="identifier">m</span><span class="special">)</span>
<span class="special">{</span> <span class="identifier">value_</span> <span class="special">=</span> <span class="identifier">m</span><span class="special">.</span><span class="identifier">value_</span><span class="special">;</span> <span class="special">}</span>
<span class="identifier">copy_movable</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">copy_movable</span><span class="special">)</span> <span class="identifier">m</span><span class="special">)</span>
<span class="special">{</span> <span class="identifier">value_</span> <span class="special">=</span> <span class="identifier">m</span><span class="special">.</span><span class="identifier">value_</span><span class="special">;</span> <span class="identifier">m</span><span class="special">.</span><span class="identifier">value_</span> <span class="special">=</span> <span class="number">0</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">copy_movable</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">copy_movable</span><span class="special">)</span> <span class="identifier">m</span><span class="special">)</span>
<span class="special">{</span> <span class="identifier">value_</span> <span class="special">=</span> <span class="identifier">m</span><span class="special">.</span><span class="identifier">value_</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="keyword">bool</span> <span class="identifier">moved</span><span class="special">()</span> <span class="keyword">const</span> <span class="comment">//Observer</span>
<span class="special">{</span> <span class="keyword">return</span> <span class="identifier">value_</span> <span class="special">==</span> <span class="number">0</span><span class="special">;</span> <span class="special">}</span>
<span class="special">};</span>
<span class="comment">//A copyable-only class</span>
<span class="keyword">class</span> <span class="identifier">copyable</span>
<span class="special">{};</span>
<span class="comment">//A copyable-only class</span>
<span class="keyword">class</span> <span class="identifier">non_copy_movable</span>
<span class="special">{</span>
<span class="keyword">public</span><span class="special">:</span>
<span class="identifier">non_copy_movable</span><span class="special">(){}</span>
<span class="keyword">private</span><span class="special">:</span>
<span class="identifier">non_copy_movable</span><span class="special">(</span><span class="keyword">const</span> <span class="identifier">non_copy_movable</span><span class="special">&);</span>
<span class="identifier">non_copy_movable</span><span class="special">&</span> <span class="keyword">operator</span><span class="special">=(</span><span class="keyword">const</span> <span class="identifier">non_copy_movable</span><span class="special">&);</span>
<span class="special">};</span>
</pre>
<p>
</p>
<p>
and build a generic factory function that returns a newly constructed value
or a reference to an already constructed object.
</p>
<p>
</p>
<pre class="programlisting"><span class="preprocessor">#include</span> <span class="string">"movable.hpp"</span>
<span class="preprocessor">#include</span> <span class="string">"copymovable.hpp"</span>
<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">core</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span>
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">Type</span><span class="special">></span>
<span class="keyword">struct</span> <span class="identifier">factory_functor</span>
<span class="special">{</span>
<span class="keyword">typedef</span> <span class="identifier">Type</span> <span class="identifier">return_type</span><span class="special">;</span>
<span class="identifier">Type</span> <span class="keyword">operator</span><span class="special">()()</span> <span class="keyword">const</span>
<span class="special">{</span> <span class="identifier">Type</span> <span class="identifier">t</span><span class="special">;</span> <span class="keyword">return</span> <span class="identifier">BOOST_MOVE_RET</span><span class="special">(</span><span class="identifier">Type</span><span class="special">,</span> <span class="identifier">t</span><span class="special">);</span> <span class="special">}</span>
<span class="special">};</span>
<span class="keyword">struct</span> <span class="identifier">return_reference</span>
<span class="special">{</span>
<span class="keyword">typedef</span> <span class="identifier">non_copy_movable</span> <span class="special">&</span><span class="identifier">return_type</span><span class="special">;</span>
<span class="identifier">non_copy_movable</span> <span class="special">&</span><span class="keyword">operator</span><span class="special">()()</span> <span class="keyword">const</span>
<span class="special">{</span> <span class="keyword">return</span> <span class="identifier">ncm</span><span class="special">;</span> <span class="special">}</span>
<span class="keyword">static</span> <span class="identifier">non_copy_movable</span> <span class="identifier">ncm</span><span class="special">;</span>
<span class="special">};</span>
<span class="identifier">non_copy_movable</span> <span class="identifier">return_reference</span><span class="special">::</span><span class="identifier">ncm</span><span class="special">;</span>
<span class="comment">//A wrapper that locks a mutex while the</span>
<span class="comment">//factory creates a new value.</span>
<span class="comment">//It must generically move the return value</span>
<span class="comment">//if possible both in C++03 and C++11</span>
<span class="keyword">template</span> <span class="special"><</span><span class="keyword">class</span> <span class="identifier">Factory</span><span class="special">></span>
<span class="keyword">typename</span> <span class="identifier">Factory</span><span class="special">::</span><span class="identifier">return_type</span> <span class="identifier">lock_wrapper</span><span class="special">(</span><span class="identifier">Factory</span> <span class="identifier">f</span><span class="special">)</span>
<span class="special">{</span>
<span class="keyword">typedef</span> <span class="keyword">typename</span> <span class="identifier">Factory</span><span class="special">::</span><span class="identifier">return_type</span> <span class="identifier">return_type</span><span class="special">;</span>
<span class="comment">//LOCK();</span>
<span class="identifier">return_type</span> <span class="identifier">r</span> <span class="special">=</span> <span class="identifier">f</span><span class="special">();</span>
<span class="comment">//UNLOCK();</span>
<span class="comment">//In C++03: boost::move() if R is not a reference and</span>
<span class="comment">//has move emulation enabled. In C++11: just return r.</span>
<span class="keyword">return</span> <span class="identifier">BOOST_MOVE_RET</span><span class="special">(</span><span class="identifier">return_type</span><span class="special">,</span> <span class="identifier">r</span><span class="special">);</span>
<span class="special">}</span>
<span class="keyword">int</span> <span class="identifier">main</span><span class="special">()</span>
<span class="special">{</span>
<span class="identifier">movable</span> <span class="identifier">m</span> <span class="special">=</span> <span class="identifier">lock_wrapper</span><span class="special">(</span><span class="identifier">factory_functor</span><span class="special"><</span><span class="identifier">movable</span><span class="special">></span> <span class="special">());</span>
<span class="identifier">copy_movable</span> <span class="identifier">cm</span> <span class="special">=</span> <span class="identifier">lock_wrapper</span><span class="special">(</span><span class="identifier">factory_functor</span><span class="special"><</span><span class="identifier">copy_movable</span><span class="special">>());</span>
<span class="identifier">copyable</span> <span class="identifier">c</span> <span class="special">=</span> <span class="identifier">lock_wrapper</span><span class="special">(</span><span class="identifier">factory_functor</span><span class="special"><</span><span class="identifier">copyable</span><span class="special">></span> <span class="special">());</span>
<span class="identifier">non_copy_movable</span> <span class="special">&</span><span class="identifier">mr</span> <span class="special">=</span> <span class="identifier">lock_wrapper</span><span class="special">(</span><span class="identifier">return_reference</span> <span class="special">());</span>
<span class="keyword">return</span> <span class="number">0</span><span class="special">;</span>
<span class="special">}</span>
</pre>
<p>
</p>
<p>
<span class="bold"><strong>Caution</strong></span>: When using this macro in a non-conforming
or C++03 compilers, a move will be performed even if the C++11 standard does
not allow it (e.g. returning a static variable). The user is responsible for
using this macro only used to return local objects that met C++11 criteria.
E.g.:
</p>
<pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">foo</span>
<span class="special">{</span>
<span class="identifier">copy_movable</span> <span class="keyword">operator</span><span class="special">()()</span> <span class="keyword">const</span>
<span class="special">{</span>
<span class="comment">//ERROR! The Standard does not allow implicit move returns when the object to be returned </span>
<span class="comment">//does not met the criteria for elision of a copy operation (such as returning a static member data)</span>
<span class="comment">//In C++03 compilers this will MOVE resources from cm</span>
<span class="comment">//In C++11 compilers this will COPY resources from cm</span>
<span class="comment">//so DON'T use use BOOST_MOVE_RET without care.</span>
<span class="keyword">return</span> <span class="identifier">BOOST_MOVE_RET</span><span class="special">(</span><span class="identifier">copy_movable</span><span class="special">,</span> <span class="identifier">cm</span><span class="special">);</span>
<span class="special">}</span>
<span class="keyword">static</span> <span class="identifier">copy_movable</span> <span class="identifier">cm</span><span class="special">;</span>
<span class="special">};</span>
</pre>
<p>
<span class="bold"><strong>Note</strong></span>: When returning a temporary object <code class="computeroutput"><span class="identifier">BOOST_MOVE_REF</span></code> is not needed as copy ellision
rules will work on both C++03 and C++11 compilers.
</p>
<pre class="programlisting"><span class="comment">//Note: no BOOST_MOVE_RET</span>
<span class="identifier">movable</span> <span class="identifier">get_movable</span><span class="special">()</span>
<span class="special">{</span> <span class="keyword">return</span> <span class="identifier">movable</span><span class="special">();</span> <span class="special">}</span>
<span class="identifier">copy_movable</span> <span class="identifier">get_copy_movable</span><span class="special">()</span>
<span class="special">{</span> <span class="keyword">return</span> <span class="identifier">copy_movable</span><span class="special">();</span> <span class="special">}</span>
<span class="identifier">copyable</span> <span class="identifier">get_copyable</span><span class="special">()</span>
<span class="special">{</span> <span class="keyword">return</span> <span class="identifier">copyable</span><span class="special">();</span> <span class="special">}</span>
</pre>
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<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>
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