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<link rel="up" href="../move.html" title="Chapter 25. Boost.Move">
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<a accesskey="p" href="what_is_boost_move.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="implementing_movable_classes.html"><img src="../../../doc/src/images/next.png" alt="Next"></a>
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<div class="titlepage"><div><div><h2 class="title" style="clear: both">
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<a name="move.introduction"></a><a class="link" href="introduction.html" title="Introduction">Introduction</a>
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</h2></div></div></div>
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<div class="note"><table border="0" summary="Note">
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<tr>
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<td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../doc/src/images/note.png"></td>
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<th align="left">Note</th>
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</tr>
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<tr><td align="left" valign="top"><p>
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The first 3 chapters are the adapted from the article <a href="http://www.artima.com/cppsource/rvalue.html" target="_top"><span class="emphasis"><em>A
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Brief Introduction to Rvalue References</em></span></a> by Howard E. Hinnant,
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Bjarne Stroustrup, and Bronek Kozicki
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</p></td></tr>
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</table></div>
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<p>
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Copying can be expensive. For example, for vectors <code class="computeroutput"><span class="identifier">v2</span><span class="special">=</span><span class="identifier">v1</span></code> typically
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involves a function call, a memory allocation, and a loop. This is of course
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acceptable where we actually need two copies of a vector, but in many cases,
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we don't: We often copy a <code class="computeroutput"><span class="identifier">vector</span></code>
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from one place to another, just to proceed to overwrite the old copy. Consider:
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</p>
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<pre class="programlisting"><span class="keyword">template</span> <span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">></span> <span class="keyword">void</span> <span class="identifier">swap</span><span class="special">(</span><span class="identifier">T</span><span class="special">&</span> <span class="identifier">a</span><span class="special">,</span> <span class="identifier">T</span><span class="special">&</span> <span class="identifier">b</span><span class="special">)</span>
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<span class="special">{</span>
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<span class="identifier">T</span> <span class="identifier">tmp</span><span class="special">(</span><span class="identifier">a</span><span class="special">);</span> <span class="comment">// now we have two copies of a</span>
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<span class="identifier">a</span> <span class="special">=</span> <span class="identifier">b</span><span class="special">;</span> <span class="comment">// now we have two copies of b</span>
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<span class="identifier">b</span> <span class="special">=</span> <span class="identifier">tmp</span><span class="special">;</span> <span class="comment">// now we have two copies of tmp (aka a)</span>
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<span class="special">}</span>
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</pre>
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<p>
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But, we didn't want to have any copies of a or b, we just wanted to swap them.
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Let's try again:
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</p>
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<pre class="programlisting"><span class="keyword">template</span> <span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">></span> <span class="keyword">void</span> <span class="identifier">swap</span><span class="special">(</span><span class="identifier">T</span><span class="special">&</span> <span class="identifier">a</span><span class="special">,</span> <span class="identifier">T</span><span class="special">&</span> <span class="identifier">b</span><span class="special">)</span>
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<span class="special">{</span>
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<span class="identifier">T</span> <span class="identifier">tmp</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">a</span><span class="special">));</span>
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<span class="identifier">a</span> <span class="special">=</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">b</span><span class="special">);</span>
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<span class="identifier">b</span> <span class="special">=</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">tmp</span><span class="special">);</span>
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<span class="special">}</span>
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</pre>
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<p>
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This <code class="computeroutput"><span class="identifier">move</span><span class="special">()</span></code>
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gives its target the value of its argument, but is not obliged to preserve
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the value of its source. So, for a <code class="computeroutput"><span class="identifier">vector</span></code>,
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<code class="computeroutput"><span class="identifier">move</span><span class="special">()</span></code>
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could reasonably be expected to leave its argument as a zero-capacity vector
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to avoid having to copy all the elements. In other words, <span class="bold"><strong>move
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is a potentially destructive copy</strong></span>.
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</p>
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<p>
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In this particular case, we could have optimized swap by a specialization.
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However, we can't specialize every function that copies a large object just
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before it deletes or overwrites it. That would be unmanageable.
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</p>
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<p>
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In C++0x, move semantics are implemented with the introduction of rvalue references.
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They allow us to implement <code class="computeroutput"><span class="identifier">move</span><span class="special">()</span></code> without verbosity or runtime overhead. <span class="bold"><strong>Boost.Move</strong></span> is a library that offers tools to implement
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those move semantics not only in compilers with <code class="computeroutput"><span class="identifier">rvalue</span>
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<span class="identifier">references</span></code> but also in compilers
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conforming to C++03.
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</p>
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</div>
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<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
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<td align="left"></td>
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<td align="right"><div class="copyright-footer">Copyright © 2008-2014 Ion Gaztanaga<p>
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Distributed under the Boost Software License, Version 1.0. (See accompanying
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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>)
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</p>
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