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| <div class="section"> |
| <div class="titlepage"><div><div><h2 class="title" style="clear: both"> |
| <a name="move.implementing_movable_classes"></a><a class="link" href="implementing_movable_classes.html" title="Implementing copyable and movable classes">Implementing copyable |
| and movable classes</a> |
| </h2></div></div></div> |
| <div class="toc"><dl class="toc"> |
| <dt><span class="section"><a href="implementing_movable_classes.html#move.implementing_movable_classes.copyable_and_movable_cpp0x">Copyable |
| and movable classes in C++0x</a></span></dt> |
| <dt><span class="section"><a href="implementing_movable_classes.html#move.implementing_movable_classes.copyable_and_movable_cpp03">Copyable |
| and movable classes in portable syntax for both C++03 and C++0x compilers</a></span></dt> |
| </dl></div> |
| <div class="section"> |
| <div class="titlepage"><div><div><h3 class="title"> |
| <a name="move.implementing_movable_classes.copyable_and_movable_cpp0x"></a><a class="link" href="implementing_movable_classes.html#move.implementing_movable_classes.copyable_and_movable_cpp0x" title="Copyable and movable classes in C++0x">Copyable |
| and movable classes in C++0x</a> |
| </h3></div></div></div> |
| <p> |
| Consider a simple handle class that owns a resource and also provides copy |
| semantics (copy constructor and assignment). For example a <code class="computeroutput"><span class="identifier">clone_ptr</span></code> might own a pointer, and call |
| <code class="computeroutput"><span class="identifier">clone</span><span class="special">()</span></code> |
| on it for copying purposes: |
| </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">clone_ptr</span> |
| <span class="special">{</span> |
| <span class="keyword">private</span><span class="special">:</span> |
| <span class="identifier">T</span><span class="special">*</span> <span class="identifier">ptr</span><span class="special">;</span> |
| |
| <span class="keyword">public</span><span class="special">:</span> |
| <span class="comment">// construction</span> |
| <span class="keyword">explicit</span> <span class="identifier">clone_ptr</span><span class="special">(</span><span class="identifier">T</span><span class="special">*</span> <span class="identifier">p</span> <span class="special">=</span> <span class="number">0</span><span class="special">)</span> <span class="special">:</span> <span class="identifier">ptr</span><span class="special">(</span><span class="identifier">p</span><span class="special">)</span> <span class="special">{}</span> |
| |
| <span class="comment">// destruction</span> |
| <span class="special">~</span><span class="identifier">clone_ptr</span><span class="special">()</span> <span class="special">{</span> <span class="keyword">delete</span> <span class="identifier">ptr</span><span class="special">;</span> <span class="special">}</span> |
| |
| <span class="comment">// copy semantics</span> |
| <span class="identifier">clone_ptr</span><span class="special">(</span><span class="keyword">const</span> <span class="identifier">clone_ptr</span><span class="special">&</span> <span class="identifier">p</span><span class="special">)</span> |
| <span class="special">:</span> <span class="identifier">ptr</span><span class="special">(</span><span class="identifier">p</span><span class="special">.</span><span class="identifier">ptr</span> <span class="special">?</span> <span class="identifier">p</span><span class="special">.</span><span class="identifier">ptr</span><span class="special">-></span><span class="identifier">clone</span><span class="special">()</span> <span class="special">:</span> <span class="number">0</span><span class="special">)</span> <span class="special">{}</span> |
| |
| <span class="identifier">clone_ptr</span><span class="special">&</span> <span class="keyword">operator</span><span class="special">=(</span><span class="keyword">const</span> <span class="identifier">clone_ptr</span><span class="special">&</span> <span class="identifier">p</span><span class="special">)</span> |
| <span class="special">{</span> |
| <span class="keyword">if</span> <span class="special">(</span><span class="keyword">this</span> <span class="special">!=</span> <span class="special">&</span><span class="identifier">p</span><span class="special">)</span> |
| <span class="special">{</span> |
| <span class="identifier">T</span> <span class="special">*</span><span class="identifier">p</span> <span class="special">=</span> <span class="identifier">p</span><span class="special">.</span><span class="identifier">ptr</span> <span class="special">?</span> <span class="identifier">p</span><span class="special">.</span><span class="identifier">ptr</span><span class="special">-></span><span class="identifier">clone</span><span class="special">()</span> <span class="special">:</span> <span class="number">0</span><span class="special">;</span> |
| <span class="keyword">delete</span> <span class="identifier">ptr</span><span class="special">;</span> |
| <span class="identifier">ptr</span> <span class="special">=</span> <span class="identifier">p</span><span class="special">;</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="comment">// move semantics</span> |
| <span class="identifier">clone_ptr</span><span class="special">(</span><span class="identifier">clone_ptr</span><span class="special">&&</span> <span class="identifier">p</span><span class="special">)</span> |
| <span class="special">:</span> <span class="identifier">ptr</span><span class="special">(</span><span class="identifier">p</span><span class="special">.</span><span class="identifier">ptr</span><span class="special">)</span> <span class="special">{</span> <span class="identifier">p</span><span class="special">.</span><span class="identifier">ptr</span> <span class="special">=</span> <span class="number">0</span><span class="special">;</span> <span class="special">}</span> |
| |
| <span class="identifier">clone_ptr</span><span class="special">&</span> <span class="keyword">operator</span><span class="special">=(</span><span class="identifier">clone_ptr</span><span class="special">&&</span> <span class="identifier">p</span><span class="special">)</span> |
| <span class="special">{</span> |
| <span class="keyword">if</span><span class="special">(</span><span class="keyword">this</span> <span class="special">!=</span> <span class="special">&</span><span class="identifier">p</span><span class="special">)</span> |
| <span class="special">{</span> |
| <span class="identifier">std</span><span class="special">::</span><span class="identifier">swap</span><span class="special">(</span><span class="identifier">ptr</span><span class="special">,</span> <span class="identifier">p</span><span class="special">.</span><span class="identifier">ptr</span><span class="special">);</span> |
| <span class="keyword">delete</span> <span class="identifier">p</span><span class="special">.</span><span class="identifier">ptr</span><span class="special">;</span> |
| <span class="identifier">p</span><span class="special">.</span><span class="identifier">ptr</span> <span class="special">=</span> <span class="number">0</span><span class="special">;</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="comment">// Other operations...</span> |
| <span class="special">};</span> |
| </pre> |
| <p> |
| <code class="computeroutput"><span class="identifier">clone_ptr</span></code> has expected copy |
| constructor and assignment semantics, duplicating resources when copying. |
| Note that copy constructing or assigning a <code class="computeroutput"><span class="identifier">clone_ptr</span></code> |
| is a relatively expensive operation: |
| </p> |
| <p> |
| </p> |
| <pre class="programlisting"><span class="identifier">clone_ptr</span><span class="special"><</span><span class="identifier">Base</span><span class="special">></span> <span class="identifier">p1</span><span class="special">(</span><span class="keyword">new</span> <span class="identifier">Derived</span><span class="special">());</span> |
| <span class="comment">// ...</span> |
| <span class="identifier">clone_ptr</span><span class="special"><</span><span class="identifier">Base</span><span class="special">></span> <span class="identifier">p2</span> <span class="special">=</span> <span class="identifier">p1</span><span class="special">;</span> <span class="comment">// p2 and p1 each own their own pointer</span> |
| </pre> |
| <p> |
| </p> |
| <p> |
| <code class="computeroutput"><span class="identifier">clone_ptr</span></code> is code that you |
| might find in today's books on C++, except for the part marked as <code class="computeroutput"><span class="identifier">move</span> <span class="identifier">semantics</span></code>. |
| That part is implemented in terms of C++0x <code class="computeroutput"><span class="identifier">rvalue</span> |
| <span class="identifier">references</span></code>. You can find some good |
| introduction and tutorials on rvalue references in these papers: |
| </p> |
| <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "> |
| <li class="listitem"> |
| <a href="http://www.artima.com/cppsource/rvalue.html" target="_top"><span class="emphasis"><em>A |
| Brief Introduction to Rvalue References</em></span></a> |
| </li> |
| <li class="listitem"> |
| <a href="http://blogs.msdn.com/vcblog/archive/2009/02/03/rvalue-references-c-0x-features-in-vc10-part-2.aspx" target="_top"><span class="emphasis"><em>Rvalue |
| References: C++0x Features in VC10, Part 2</em></span></a> |
| </li> |
| </ul></div> |
| <p> |
| When the source of the copy is known to be a <code class="computeroutput"><span class="identifier">rvalue</span></code> |
| (e.g.: a temporary object), one can avoid the potentially expensive <code class="computeroutput"><span class="identifier">clone</span><span class="special">()</span></code> |
| operation by pilfering source's pointer (no one will notice!). The move constructor |
| above does exactly that, leaving the rvalue in a default constructed state. |
| The move assignment operator simply does the same freeing old resources. |
| </p> |
| <p> |
| Now when code tries to copy a rvalue <code class="computeroutput"><span class="identifier">clone_ptr</span></code>, |
| or if that code explicitly gives permission to consider the source of the |
| copy a rvalue (using <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">move</span></code>), |
| the operation will execute much faster. |
| </p> |
| <p> |
| </p> |
| <pre class="programlisting"><span class="identifier">clone_ptr</span><span class="special"><</span><span class="identifier">Base</span><span class="special">></span> <span class="identifier">p1</span><span class="special">(</span><span class="keyword">new</span> <span class="identifier">Derived</span><span class="special">());</span> |
| <span class="comment">// ...</span> |
| <span class="identifier">clone_ptr</span><span class="special"><</span><span class="identifier">Base</span><span class="special">></span> <span class="identifier">p2</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">p1</span><span class="special">);</span> <span class="comment">// p2 now owns the pointer instead of p1</span> |
| <span class="identifier">p2</span> <span class="special">=</span> <span class="identifier">clone_ptr</span><span class="special"><</span><span class="identifier">Base</span><span class="special">>(</span><span class="keyword">new</span> <span class="identifier">Derived</span><span class="special">());</span> <span class="comment">// temporary is moved to p2</span> |
| <span class="special">}</span> |
| </pre> |
| <p> |
| </p> |
| </div> |
| <div class="section"> |
| <div class="titlepage"><div><div><h3 class="title"> |
| <a name="move.implementing_movable_classes.copyable_and_movable_cpp03"></a><a class="link" href="implementing_movable_classes.html#move.implementing_movable_classes.copyable_and_movable_cpp03" title="Copyable and movable classes in portable syntax for both C++03 and C++0x compilers">Copyable |
| and movable classes in portable syntax for both C++03 and C++0x compilers</a> |
| </h3></div></div></div> |
| <p> |
| Many aspects of move semantics can be emulated for compilers not supporting |
| <code class="computeroutput"><span class="identifier">rvalue</span> <span class="identifier">references</span></code> |
| and <span class="bold"><strong>Boost.Move</strong></span> offers tools for that purpose. |
| With <span class="bold"><strong>Boost.Move</strong></span> we can write <code class="computeroutput"><span class="identifier">clone_ptr</span></code> so that it will work both in |
| compilers with rvalue references and those who conform to C++03. You just |
| 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_COPYABLE_AND_MOVABLE.html" title="Macro BOOST_COPYABLE_AND_MOVABLE">BOOST_COPYABLE_AND_MOVABLE(classname)</a></code> |
| </li> |
| <li class="listitem"> |
| Leave copy constructor as is. |
| </li> |
| <li class="listitem"> |
| Write a copy assignment taking the parameter as <code class="computeroutput"><a class="link" href="../BOOST_COPY_ASSIGN_REF.html" title="Macro BOOST_COPY_ASSIGN_REF">BOOST_COPY_ASSIGN_REF(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> |
| Let's see how are applied to <code class="computeroutput"><span class="identifier">clone_ptr</span></code>: |
| </p> |
| <p> |
| </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">clone_ptr</span> |
| <span class="special">{</span> |
| <span class="keyword">private</span><span class="special">:</span> |
| <span class="comment">// Mark this class copyable and movable</span> |
| <span class="identifier">BOOST_COPYABLE_AND_MOVABLE</span><span class="special">(</span><span class="identifier">clone_ptr</span><span class="special">)</span> |
| <span class="identifier">T</span><span class="special">*</span> <span class="identifier">ptr</span><span class="special">;</span> |
| |
| <span class="keyword">public</span><span class="special">:</span> |
| <span class="comment">// Construction</span> |
| <span class="keyword">explicit</span> <span class="identifier">clone_ptr</span><span class="special">(</span><span class="identifier">T</span><span class="special">*</span> <span class="identifier">p</span> <span class="special">=</span> <span class="number">0</span><span class="special">)</span> <span class="special">:</span> <span class="identifier">ptr</span><span class="special">(</span><span class="identifier">p</span><span class="special">)</span> <span class="special">{}</span> |
| |
| <span class="comment">// Destruction</span> |
| <span class="special">~</span><span class="identifier">clone_ptr</span><span class="special">()</span> <span class="special">{</span> <span class="keyword">delete</span> <span class="identifier">ptr</span><span class="special">;</span> <span class="special">}</span> |
| |
| <span class="identifier">clone_ptr</span><span class="special">(</span><span class="keyword">const</span> <span class="identifier">clone_ptr</span><span class="special">&</span> <span class="identifier">p</span><span class="special">)</span> <span class="comment">// Copy constructor (as usual)</span> |
| <span class="special">:</span> <span class="identifier">ptr</span><span class="special">(</span><span class="identifier">p</span><span class="special">.</span><span class="identifier">ptr</span> <span class="special">?</span> <span class="identifier">p</span><span class="special">.</span><span class="identifier">ptr</span><span class="special">-></span><span class="identifier">clone</span><span class="special">()</span> <span class="special">:</span> <span class="number">0</span><span class="special">)</span> <span class="special">{}</span> |
| |
| <span class="identifier">clone_ptr</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">clone_ptr</span><span class="special">)</span> <span class="identifier">p</span><span class="special">)</span> <span class="comment">// Copy assignment</span> |
| <span class="special">{</span> |
| <span class="keyword">if</span> <span class="special">(</span><span class="keyword">this</span> <span class="special">!=</span> <span class="special">&</span><span class="identifier">p</span><span class="special">){</span> |
| <span class="identifier">T</span> <span class="special">*</span><span class="identifier">tmp_p</span> <span class="special">=</span> <span class="identifier">p</span><span class="special">.</span><span class="identifier">ptr</span> <span class="special">?</span> <span class="identifier">p</span><span class="special">.</span><span class="identifier">ptr</span><span class="special">-></span><span class="identifier">clone</span><span class="special">()</span> <span class="special">:</span> <span class="number">0</span><span class="special">;</span> |
| <span class="keyword">delete</span> <span class="identifier">ptr</span><span class="special">;</span> |
| <span class="identifier">ptr</span> <span class="special">=</span> <span class="identifier">tmp_p</span><span class="special">;</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="comment">//Move semantics...</span> |
| <span class="identifier">clone_ptr</span><span class="special">(</span><span class="identifier">BOOST_RV_REF</span><span class="special">(</span><span class="identifier">clone_ptr</span><span class="special">)</span> <span class="identifier">p</span><span class="special">)</span> <span class="comment">//Move constructor</span> |
| <span class="special">:</span> <span class="identifier">ptr</span><span class="special">(</span><span class="identifier">p</span><span class="special">.</span><span class="identifier">ptr</span><span class="special">)</span> <span class="special">{</span> <span class="identifier">p</span><span class="special">.</span><span class="identifier">ptr</span> <span class="special">=</span> <span class="number">0</span><span class="special">;</span> <span class="special">}</span> |
| |
| <span class="identifier">clone_ptr</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">clone_ptr</span><span class="special">)</span> <span class="identifier">p</span><span class="special">)</span> <span class="comment">//Move assignment</span> |
| <span class="special">{</span> |
| <span class="keyword">if</span> <span class="special">(</span><span class="keyword">this</span> <span class="special">!=</span> <span class="special">&</span><span class="identifier">p</span><span class="special">){</span> |
| <span class="keyword">delete</span> <span class="identifier">ptr</span><span class="special">;</span> |
| <span class="identifier">ptr</span> <span class="special">=</span> <span class="identifier">p</span><span class="special">.</span><span class="identifier">ptr</span><span class="special">;</span> |
| <span class="identifier">p</span><span class="special">.</span><span class="identifier">ptr</span> <span class="special">=</span> <span class="number">0</span><span class="special">;</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="special">};</span> |
| </pre> |
| <p> |
| </p> |
| </div> |
| <p> |
| <span class="bold"><strong>Question</strong></span>: What about types that don't own |
| resources? (E.g. <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">complex</span></code>?) |
| </p> |
| <p> |
| No work needs to be done in that case. The copy constructor is already optimal. |
| </p> |
| </div> |
| <table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr> |
| <td align="left"></td> |
| <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> |
| </div></td> |
| </tr></table> |
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