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| <div class="titlepage"><div><div><h4 class="title"> |
| <a name="boost_asio.overview.cpp2011.move_handlers"></a><a class="link" href="move_handlers.html" title="Movable Handlers">Movable |
| Handlers</a> |
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| <p> |
| As an optimisation, user-defined completion handlers may provide move constructors, |
| and Boost.Asio's implementation will use a handler's move constructor in |
| preference to its copy constructor. In certain circumstances, Boost.Asio |
| may be able to eliminate all calls to a handler's copy constructor. However, |
| handler types are still required to be copy constructible. |
| </p> |
| <p> |
| When move support is enabled, asynchronous that are documented as follows: |
| </p> |
| <pre class="programlisting"><span class="keyword">template</span> <span class="special"><</span><span class="keyword">typename</span> <span class="identifier">Handler</span><span class="special">></span> |
| <span class="keyword">void</span> <span class="identifier">async_XYZ</span><span class="special">(...,</span> <span class="identifier">Handler</span> <span class="identifier">handler</span><span class="special">);</span> |
| </pre> |
| <p> |
| are actually declared as: |
| </p> |
| <pre class="programlisting"><span class="keyword">template</span> <span class="special"><</span><span class="keyword">typename</span> <span class="identifier">Handler</span><span class="special">></span> |
| <span class="keyword">void</span> <span class="identifier">async_XYZ</span><span class="special">(...,</span> <span class="identifier">Handler</span><span class="special">&&</span> <span class="identifier">handler</span><span class="special">);</span> |
| </pre> |
| <p> |
| The handler argument is perfectly forwarded and the move construction occurs |
| within the body of <code class="computeroutput"><span class="identifier">async_XYZ</span><span class="special">()</span></code>. This ensures that all other function |
| arguments are evaluated prior to the move. This is critical when the other |
| arguments to <code class="computeroutput"><span class="identifier">async_XYZ</span><span class="special">()</span></code> are members of the handler. For example: |
| </p> |
| <pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">my_operation</span> |
| <span class="special">{</span> |
| <span class="identifier">shared_ptr</span><span class="special"><</span><span class="identifier">tcp</span><span class="special">::</span><span class="identifier">socket</span><span class="special">></span> <span class="identifier">socket</span><span class="special">;</span> |
| <span class="identifier">shared_ptr</span><span class="special"><</span><span class="identifier">vector</span><span class="special"><</span><span class="keyword">char</span><span class="special">>></span> <span class="identifier">buffer</span><span class="special">;</span> |
| <span class="special">...</span> |
| <span class="keyword">void</span> <span class="keyword">operator</span><span class="special">(</span><span class="identifier">error_code</span> <span class="identifier">ec</span><span class="special">,</span> <span class="identifier">size_t</span> <span class="identifier">length</span><span class="special">)</span> |
| <span class="special">{</span> |
| <span class="special">...</span> |
| <span class="identifier">socket</span><span class="special">-></span><span class="identifier">async_read_some</span><span class="special">(</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">asio</span><span class="special">::</span><span class="identifier">buffer</span><span class="special">(*</span><span class="identifier">buffer</span><span class="special">),</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">move</span><span class="special">(*</span><span class="keyword">this</span><span class="special">));</span> |
| <span class="special">...</span> |
| <span class="special">}</span> |
| <span class="special">};</span> |
| </pre> |
| <p> |
| Move support is automatically enabled for <code class="literal">g++</code> 4.5 and |
| later, when the <code class="literal">-std=c++0x</code> or <code class="literal">-std=gnu++0x</code> |
| compiler options are used. It may be disabled by defining <code class="computeroutput"><span class="identifier">BOOST_ASIO_DISABLE_MOVE</span></code>, or explicitly |
| enabled for other compilers by defining <code class="computeroutput"><span class="identifier">BOOST_ASIO_HAS_MOVE</span></code>. |
| Note that these macros also affect the availability of <a class="link" href="move_objects.html" title="Movable I/O Objects">movable |
| I/O objects</a>. |
| </p> |
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| <td align="right"><div class="copyright-footer">Copyright © 2003-2011 Christopher |
| M. Kohlhoff<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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