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<div class="section">
<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="move.how_the_library_works"></a><a class="link" href="how_the_library_works.html" title="How the library works">How the library works</a>
</h2></div></div></div>
<p>
<span class="bold"><strong>Boost.Move</strong></span> is based on macros that are expanded
to true rvalue references in C++0x compilers and emulated rvalue reference
classes and conversion operators in C++03 compilers.
</p>
<p>
In C++03 compilers <span class="bold"><strong>Boost.Move</strong></span> defines a class
named <code class="computeroutput"><span class="special">::</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">rv</span></code>:
</p>
<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">class</span> <span class="identifier">rv</span> <span class="special">:</span> <span class="keyword">public</span> <span class="identifier">T</span>
<span class="special">{</span>
<span class="identifier">rv</span><span class="special">();</span>
<span class="special">~</span><span class="identifier">rv</span><span class="special">();</span>
<span class="identifier">rv</span><span class="special">(</span><span class="identifier">rv</span> <span class="keyword">const</span><span class="special">&);</span>
<span class="keyword">void</span> <span class="keyword">operator</span><span class="special">=(</span><span class="identifier">rv</span> <span class="keyword">const</span><span class="special">&);</span>
<span class="special">};</span>
</pre>
<p>
which is convertible to the movable base class (usual C++ derived to base conversion).
When users mark their classes as <code class="computeroutput"><a class="link" href="../BOOST_MOVABLE_BUT_NOT_COPYABLE.html" title="Macro BOOST_MOVABLE_BUT_NOT_COPYABLE">BOOST_MOVABLE_BUT_NOT_COPYABLE</a></code>
or <code class="computeroutput"><a class="link" href="../BOOST_COPYABLE_AND_MOVABLE.html" title="Macro BOOST_COPYABLE_AND_MOVABLE">BOOST_COPYABLE_AND_MOVABLE</a></code>,
these macros define conversion operators to references to <code class="computeroutput"><span class="special">::</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">rv</span></code>:
</p>
<pre class="programlisting"><span class="preprocessor">#define</span> <span class="identifier">BOOST_MOVABLE_BUT_NOT_COPYABLE</span><span class="special">(</span><span class="identifier">TYPE</span><span class="special">)\</span>
<span class="keyword">public</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">rv</span><span class="special"><</span><span class="identifier">TYPE</span><span class="special">>&()</span> <span class="special">\</span>
<span class="special">{</span> <span class="keyword">return</span> <span class="special">*</span><span class="keyword">static_cast</span><span class="special"><</span> <span class="special">::</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">rv</span><span class="special"><</span><span class="identifier">TYPE</span><span class="special">>*</span> <span class="special">>(</span><span class="keyword">this</span><span class="special">);</span> <span class="special">}\</span>
<span class="keyword">operator</span> <span class="keyword">const</span> <span class="special">::</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">rv</span><span class="special"><</span><span class="identifier">TYPE</span><span class="special">>&()</span> <span class="keyword">const</span> <span class="special">\</span>
<span class="special">{</span> <span class="keyword">return</span> <span class="keyword">static_cast</span><span class="special"><</span><span class="keyword">const</span> <span class="special">::</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">rv</span><span class="special"><</span><span class="identifier">TYPE</span><span class="special">>*</span> <span class="special">>(</span><span class="keyword">this</span><span class="special">);</span> <span class="special">}\</span>
<span class="keyword">private</span><span class="special">:\</span>
<span class="comment">//More stuff...</span>
</pre>
<p>
<code class="computeroutput"><a class="link" href="../BOOST_MOVABLE_BUT_NOT_COPYABLE.html" title="Macro BOOST_MOVABLE_BUT_NOT_COPYABLE">BOOST_MOVABLE_BUT_NOT_COPYABLE</a></code>
also declares a private copy constructor and assignment. <code class="computeroutput"><a class="link" href="../BOOST_COPYABLE_AND_MOVABLE.html" title="Macro BOOST_COPYABLE_AND_MOVABLE">BOOST_COPYABLE_AND_MOVABLE</a></code>
defines a non-const copy constructor <code class="computeroutput"><span class="identifier">TYPE</span>
<span class="special">&</span><span class="keyword">operator</span><span class="special">=(</span><span class="identifier">TYPE</span><span class="special">&)</span></code>
that forwards to a const version:
</p>
<pre class="programlisting"><span class="preprocessor">#define</span> <span class="identifier">BOOST_COPYABLE_AND_MOVABLE</span><span class="special">(</span><span class="identifier">TYPE</span><span class="special">)\</span>
<span class="keyword">public</span><span class="special">:\</span>
<span class="identifier">TYPE</span><span class="special">&</span> <span class="keyword">operator</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="keyword">this</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="special">::</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">rv</span><span class="special"><</span><span class="identifier">TYPE</span><span class="special">></span> <span class="special">&>(</span><span class="keyword">const_cast</span><span class="special"><</span><span class="keyword">const</span> <span class="identifier">TYPE</span> <span class="special">&>(</span><span class="identifier">t</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">//More stuff...</span>
</pre>
<p>
In C++0x compilers <code class="computeroutput"><span class="identifier">BOOST_COPYABLE_AND_MOVABLE</span></code>
expands to nothing and <code class="computeroutput"><span class="identifier">BOOST_MOVABLE_BUT_NOT_COPYABLE</span></code>
declares copy constructor and assigment operator private.
</p>
<p>
When users define the <code class="computeroutput"><a class="link" href="../BOOST_RV_REF.html" title="Macro BOOST_RV_REF">BOOST_RV_REF</a></code>
overload of a copy constructor/assignment, in C++0x compilers it is expanded
to a rvalue reference (<code class="computeroutput"><span class="identifier">T</span><span class="special">&&</span></code>)
overload and in C++03 compilers it is expanded to a <code class="computeroutput"><span class="special">::</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">rv</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span>
<span class="special">&</span></code> overload:
</p>
<pre class="programlisting"><span class="preprocessor">#define</span> <span class="identifier">BOOST_RV_REF</span><span class="special">(</span><span class="identifier">TYPE</span><span class="special">)</span> <span class="special">::</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">rv</span><span class="special"><</span> <span class="identifier">TYPE</span> <span class="special">>&</span> <span class="special">\</span>
</pre>
<p>
When users define the <code class="computeroutput"><a class="link" href="../BOOST_COPY_ASSIGN_REF.html" title="Macro BOOST_COPY_ASSIGN_REF">BOOST_COPY_ASSIGN_REF</a></code>
overload, it is expanded to a usual copy assignment (<code class="computeroutput"><span class="keyword">const</span>
<span class="identifier">T</span> <span class="special">&</span></code>)
overload in C++0x compilers and to a <code class="computeroutput"><span class="keyword">const</span>
<span class="special">::</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">rv</span> <span class="special">&</span></code>
overload in C++03 compilers:
</p>
<pre class="programlisting"><span class="preprocessor">#define</span> <span class="identifier">BOOST_COPY_ASSIGN_REF</span><span class="special">(</span><span class="identifier">TYPE</span><span class="special">)</span> <span class="keyword">const</span> <span class="special">::</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">rv</span><span class="special"><</span> <span class="identifier">TYPE</span> <span class="special">>&</span>
</pre>
<p>
As seen, in <span class="bold"><strong>Boost.Move</strong></span> generates efficient
and clean code for C++0x move semantics, without modifying any resolution overload.
For C++03 compilers when overload resolution is performed these are the bindings:
</p>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
<li class="listitem">
a) non-const rvalues (e.g.: temporaries), bind to <code class="computeroutput"><span class="special">::</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">rv</span><span class="special"><</span> <span class="identifier">TYPE</span> <span class="special">>&</span></code>
</li>
<li class="listitem">
b) const rvalue and lvalues, bind to <code class="computeroutput"><span class="keyword">const</span>
<span class="special">::</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">rv</span><span class="special"><</span> <span class="identifier">TYPE</span> <span class="special">>&</span></code>
</li>
<li class="listitem">
c) non-const lvalues (e.g. non-const references) bind to <code class="computeroutput"><span class="identifier">TYPE</span><span class="special">&</span></code>
</li>
</ul></div>
<p>
The library does not define the equivalent of <code class="computeroutput"><a class="link" href="../BOOST_COPY_ASSIGN_REF.html" title="Macro BOOST_COPY_ASSIGN_REF">BOOST_COPY_ASSIGN_REF</a></code>
for copy construction (say, <code class="computeroutput"><span class="identifier">BOOST_COPY_CTOR_REF</span></code>)
because nearly all modern compilers implement RVO and this is much more efficient
than any move emulation. <code class="computeroutput"><a class="link" href="../boost/move_idp683900176.html" title="Function template move">move</a></code>
just casts <code class="computeroutput"><span class="identifier">TYPE</span> <span class="special">&</span></code>
into <code class="computeroutput"><span class="special">::</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">rv</span><span class="special"><</span><span class="identifier">TYPE</span><span class="special">></span> <span class="special">&</span></code>.
</p>
<p>
Here's an example that demostrates how different rlvalue objects bind to <code class="computeroutput"><span class="special">::</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">rv</span></code> references in the presence of three overloads
and the conversion operators in C++03 compilers:
</p>
<p>
</p>
<pre class="programlisting"><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="preprocessor">#include</span> <span class="special"><</span><span class="identifier">iostream</span><span class="special">></span>
<span class="keyword">class</span> <span class="identifier">sink_tester</span>
<span class="special">{</span>
<span class="keyword">public</span><span class="special">:</span> <span class="comment">//conversions provided by BOOST_COPYABLE_AND_MOVABLE</span>
<span class="keyword">operator</span> <span class="special">::</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">rv</span><span class="special"><</span><span class="identifier">sink_tester</span><span class="special">>&()</span>
<span class="special">{</span> <span class="keyword">return</span> <span class="special">*</span><span class="keyword">static_cast</span><span class="special"><</span> <span class="special">::</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">rv</span><span class="special"><</span><span class="identifier">sink_tester</span><span class="special">>*</span> <span class="special">>(</span><span class="keyword">this</span><span class="special">);</span> <span class="special">}</span>
<span class="keyword">operator</span> <span class="keyword">const</span> <span class="special">::</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">rv</span><span class="special"><</span><span class="identifier">sink_tester</span><span class="special">>&()</span> <span class="keyword">const</span>
<span class="special">{</span> <span class="keyword">return</span> <span class="special">*</span><span class="keyword">static_cast</span><span class="special"><</span><span class="keyword">const</span> <span class="special">::</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">rv</span><span class="special"><</span><span class="identifier">sink_tester</span><span class="special">>*</span> <span class="special">>(</span><span class="keyword">this</span><span class="special">);</span> <span class="special">}</span>
<span class="special">};</span>
<span class="comment">//Functions returning different r/lvalue types</span>
<span class="identifier">sink_tester</span> <span class="identifier">rvalue</span><span class="special">()</span> <span class="special">{</span> <span class="keyword">return</span> <span class="identifier">sink_tester</span><span class="special">();</span> <span class="special">}</span>
<span class="keyword">const</span> <span class="identifier">sink_tester</span> <span class="identifier">const_rvalue</span><span class="special">()</span> <span class="special">{</span> <span class="keyword">return</span> <span class="identifier">sink_tester</span><span class="special">();</span> <span class="special">}</span>
<span class="identifier">sink_tester</span> <span class="special">&</span> <span class="identifier">lvalue</span><span class="special">()</span> <span class="special">{</span> <span class="keyword">static</span> <span class="identifier">sink_tester</span> <span class="identifier">lv</span><span class="special">;</span> <span class="keyword">return</span> <span class="identifier">lv</span><span class="special">;</span> <span class="special">}</span>
<span class="keyword">const</span> <span class="identifier">sink_tester</span> <span class="special">&</span> <span class="identifier">const_lvalue</span><span class="special">()</span> <span class="special">{</span> <span class="keyword">static</span> <span class="keyword">const</span> <span class="identifier">sink_tester</span> <span class="identifier">clv</span> <span class="special">=</span> <span class="identifier">sink_tester</span><span class="special">();</span> <span class="keyword">return</span> <span class="identifier">clv</span><span class="special">;</span> <span class="special">}</span>
<span class="comment">//BOOST_RV_REF overload</span>
<span class="keyword">void</span> <span class="identifier">sink</span><span class="special">(::</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">rv</span><span class="special"><</span><span class="identifier">sink_tester</span><span class="special">></span> <span class="special">&)</span> <span class="special">{</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special"><<</span> <span class="string">"non-const rvalue catched"</span> <span class="special"><<</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span> <span class="special">}</span>
<span class="comment">//BOOST_COPY_ASSIGN_REF overload</span>
<span class="keyword">void</span> <span class="identifier">sink</span><span class="special">(</span><span class="keyword">const</span> <span class="special">::</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">rv</span><span class="special"><</span><span class="identifier">sink_tester</span><span class="special">></span> <span class="special">&){</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special"><<</span> <span class="string">"const (r-l)value catched"</span> <span class="special"><<</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span> <span class="special">}</span>
<span class="comment">//Overload provided by BOOST_COPYABLE_AND_MOVABLE</span>
<span class="keyword">void</span> <span class="identifier">sink</span><span class="special">(</span><span class="identifier">sink_tester</span> <span class="special">&)</span> <span class="special">{</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special"><<</span> <span class="string">"non-const lvalue catched"</span> <span class="special"><<</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</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">sink</span><span class="special">(</span><span class="identifier">const_rvalue</span><span class="special">());</span> <span class="comment">//"const (r-l)value catched"</span>
<span class="identifier">sink</span><span class="special">(</span><span class="identifier">const_lvalue</span><span class="special">());</span> <span class="comment">//"const (r-l)value catched"</span>
<span class="identifier">sink</span><span class="special">(</span><span class="identifier">lvalue</span><span class="special">());</span> <span class="comment">//"non-const lvalue catched"</span>
<span class="identifier">sink</span><span class="special">(</span><span class="identifier">rvalue</span><span class="special">());</span> <span class="comment">//"non-const rvalue catched"</span>
<span class="keyword">return</span> <span class="number">0</span><span class="special">;</span>
<span class="special">}</span>
</pre>
<p>
</p>
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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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