<html> <head> <meta http-equiv="Content-Type" content="text/html; charset=US-ASCII"> <title>Movable Handlers</title> <link rel="stylesheet" href="../../../../../doc/src/boostbook.css" type="text/css"> <meta name="generator" content="DocBook XSL Stylesheets V1.78.1"> <link rel="home" href="../../../boost_asio.html" title="Boost.Asio"> <link rel="up" href="../cpp2011.html" title="C++ 2011 Support"> <link rel="prev" href="move_objects.html" title="Movable I/O Objects"> <link rel="next" href="variadic.html" title="Variadic Templates"> </head> <body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"> <table cellpadding="2" width="100%"><tr> <td valign="top"><img alt="Boost C++ Libraries" width="277" height="86" src="../../../../../boost.png"></td> <td align="center"><a href="../../../../../index.html">Home</a></td> <td align="center"><a href="../../../../../libs/libraries.htm">Libraries</a></td> <td align="center"><a href="http://www.boost.org/users/people.html">People</a></td> <td align="center"><a href="http://www.boost.org/users/faq.html">FAQ</a></td> <td align="center"><a href="../../../../../more/index.htm">More</a></td> </tr></table> <hr> <div class="spirit-nav"> <a accesskey="p" href="move_objects.html"><img src="../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../cpp2011.html"><img src="../../../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../../../boost_asio.html"><img src="../../../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="variadic.html"><img src="../../../../../doc/src/images/next.png" alt="Next"></a> </div> <div class="section"> <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> </h4></div></div></div> <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> </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 © 2003-2015 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>) </p> </div></td> </tr></table> <hr> <div class="spirit-nav"> <a accesskey="p" href="move_objects.html"><img src="../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../cpp2011.html"><img src="../../../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../../../boost_asio.html"><img src="../../../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="variadic.html"><img src="../../../../../doc/src/images/next.png" alt="Next"></a> </div> </body> </html>