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| <div class="section"> |
| <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_MOVABLE_BUT_NOT_COPYABLE.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"><</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">></span> |
| <span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">stdexcept</span><span class="special">></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">&</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> |
| <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>) |
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