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<title>Movable but Non-Copyable Types</title>
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<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="move.movable_only_classes"></a><a class="link" href="movable_only_classes.html" title="Movable but Non-Copyable Types">Movable but Non-Copyable Types</a>
</h2></div></div></div>
<p>
Some types are not amenable to copy semantics but can still be made movable.
For example:
</p>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
<li class="listitem">
<code class="computeroutput"><span class="identifier">unique_ptr</span></code> (non-shared,
non-copyable ownership)
</li>
<li class="listitem">
A type representing a thread of execution
</li>
<li class="listitem">
A type representing a file descriptor
</li>
</ul></div>
<p>
By making such types movable (though still non-copyable) their utility is tremendously
increased. Movable but non-copyable types can be returned by value from factory
functions:
</p>
<pre class="programlisting"><span class="identifier">file_descriptor</span> <span class="identifier">create_file</span><span class="special">(/*</span> <span class="special">...</span> <span class="special">*/);</span>
<span class="comment">//...</span>
<span class="identifier">file_descriptor</span> <span class="identifier">data_file</span><span class="special">;</span>
<span class="comment">//...</span>
<span class="identifier">data_file</span> <span class="special">=</span> <span class="identifier">create_file</span><span class="special">(/*</span> <span class="special">...</span> <span class="special">*/);</span> <span class="comment">// No copies!</span>
</pre>
<p>
In the above example, the underlying file handle is passed from object to object,
as long as the source <code class="computeroutput"><span class="identifier">file_descriptor</span></code>
is a rvalue. At all times, there is still only one underlying file handle,
and only one <code class="computeroutput"><span class="identifier">file_descriptor</span></code>
owns it at a time.
</p>
<p>
To write a movable but not copyable type in portable syntax, you need to follow
these simple steps:
</p>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
<li class="listitem">
Put the following macro in the <span class="bold"><strong>private</strong></span>
section: <code class="computeroutput"><a class="link" href="../BOOST_MOVABL_1_3_26_20_7_3.html" title="Macro BOOST_MOVABLE_BUT_NOT_COPYABLE">BOOST_MOVABLE_BUT_NOT_COPYABLE(classname)</a></code>
</li>
<li class="listitem">
Write a move constructor and a move assignment taking the parameter as
<code class="computeroutput"><a class="link" href="../BOOST_RV_REF.html" title="Macro BOOST_RV_REF">BOOST_RV_REF(classname)</a></code>
</li>
</ul></div>
<p>
Here's the definition of <code class="computeroutput"><span class="identifier">file</span> <span class="identifier">descriptor</span></code> using portable syntax:
</p>
<p>
</p>
<pre class="programlisting"><span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">move</span><span class="special">/</span><span class="identifier">utility_core</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
<span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">stdexcept</span><span class="special">&gt;</span>
<span class="keyword">class</span> <span class="identifier">file_descriptor</span>
<span class="special">{</span>
<span class="keyword">int</span> <span class="identifier">os_descr_</span><span class="special">;</span>
<span class="keyword">private</span><span class="special">:</span>
<span class="identifier">BOOST_MOVABLE_BUT_NOT_COPYABLE</span><span class="special">(</span><span class="identifier">file_descriptor</span><span class="special">)</span>
<span class="keyword">public</span><span class="special">:</span>
<span class="keyword">explicit</span> <span class="identifier">file_descriptor</span><span class="special">(</span><span class="keyword">const</span> <span class="keyword">char</span> <span class="special">*</span><span class="identifier">filename</span><span class="special">)</span> <span class="comment">//Constructor</span>
<span class="special">:</span> <span class="identifier">os_descr_</span><span class="special">(</span><span class="identifier">operating_system_open_file</span><span class="special">(</span><span class="identifier">filename</span><span class="special">))</span>
<span class="special">{</span>
<span class="keyword">if</span><span class="special">(!</span><span class="identifier">os_descr_</span><span class="special">)</span>
<span class="keyword">throw</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">runtime_error</span><span class="special">(</span><span class="string">"file not found"</span><span class="special">);</span>
<span class="special">}</span>
<span class="special">~</span><span class="identifier">file_descriptor</span><span class="special">()</span> <span class="comment">//Destructor</span>
<span class="special">{</span> <span class="keyword">if</span><span class="special">(</span><span class="identifier">os_descr_</span><span class="special">)</span> <span class="identifier">operating_system_close_file</span><span class="special">(</span><span class="identifier">os_descr_</span><span class="special">);</span> <span class="special">}</span>
<span class="identifier">file_descriptor</span><span class="special">(</span><span class="identifier">BOOST_RV_REF</span><span class="special">(</span><span class="identifier">file_descriptor</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">os_descr_</span><span class="special">(</span><span class="identifier">x</span><span class="special">.</span><span class="identifier">os_descr_</span><span class="special">)</span>
<span class="special">{</span> <span class="identifier">x</span><span class="special">.</span><span class="identifier">os_descr_</span> <span class="special">=</span> <span class="number">0</span><span class="special">;</span> <span class="special">}</span>
<span class="identifier">file_descriptor</span><span class="special">&amp;</span> <span class="keyword">operator</span><span class="special">=(</span><span class="identifier">BOOST_RV_REF</span><span class="special">(</span><span class="identifier">file_descriptor</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="keyword">if</span><span class="special">(</span><span class="identifier">os_descr_</span><span class="special">)</span> <span class="identifier">operating_system_close_file</span><span class="special">(</span><span class="identifier">os_descr_</span><span class="special">);</span>
<span class="identifier">os_descr_</span> <span class="special">=</span> <span class="identifier">x</span><span class="special">.</span><span class="identifier">os_descr_</span><span class="special">;</span>
<span class="identifier">x</span><span class="special">.</span><span class="identifier">os_descr_</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="keyword">bool</span> <span class="identifier">empty</span><span class="special">()</span> <span class="keyword">const</span> <span class="special">{</span> <span class="keyword">return</span> <span class="identifier">os_descr_</span> <span class="special">==</span> <span class="number">0</span><span class="special">;</span> <span class="special">}</span>
<span class="special">};</span>
</pre>
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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>)
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