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<div class="titlepage"><div><div><h3 class="title">
<a name="asio.reference.async_compose"></a><a class="link" href="async_compose.html" title="async_compose">async_compose</a>
</h3></div></div></div>
<p>
<a class="indexterm" name="asio.indexterm.async_compose"></a>
</p>
<p>
Launch an asynchronous operation with a stateful implementation.
</p>
<pre class="programlisting"><span class="keyword">template</span><span class="special">&lt;</span>
<span class="keyword">typename</span> <span class="identifier">CompletionToken</span><span class="special">,</span>
<span class="keyword">typename</span> <span class="identifier">Signature</span><span class="special">,</span>
<span class="keyword">typename</span> <span class="identifier">Implementation</span><span class="special">,</span>
<span class="keyword">typename</span><span class="special">...</span> <span class="identifier">IoObjectsOrExecutors</span><span class="special">&gt;</span>
<a class="link" href="asynchronous_operations.html#asio.reference.asynchronous_operations.automatic_deduction_of_initiating_function_return_type"><span class="emphasis"><em>DEDUCED</em></span></a> <span class="identifier">async_compose</span><span class="special">(</span>
<span class="identifier">Implementation</span> <span class="special">&amp;&amp;</span> <span class="identifier">implementation</span><span class="special">,</span>
<span class="identifier">CompletionToken</span> <span class="special">&amp;</span> <span class="identifier">token</span><span class="special">,</span>
<span class="identifier">IoObjectsOrExecutors</span> <span class="special">&amp;&amp;...</span> <span class="identifier">io_objects_or_executors</span><span class="special">);</span>
</pre>
<p>
The async_compose function simplifies the implementation of composed asynchronous
operations automatically wrapping a stateful function object with a conforming
intermediate completion handler.
</p>
<h5>
<a name="asio.reference.async_compose.h0"></a>
<span><a name="asio.reference.async_compose.parameters"></a></span><a class="link" href="async_compose.html#asio.reference.async_compose.parameters">Parameters</a>
</h5>
<div class="variablelist">
<p class="title"><b></b></p>
<dl>
<dt><span class="term">implementation</span></dt>
<dd><p>
A function object that contains the implementation of the composed
asynchronous operation. The first argument to the function object is
a non-const reference to the enclosing intermediate completion handler.
The remaining arguments are any arguments that originate from the completion
handlers of any asynchronous operations performed by the implementation.
</p></dd>
<dt><span class="term">token</span></dt>
<dd><p>
The completion token.
</p></dd>
<dt><span class="term">io_objects_or_executors</span></dt>
<dd><p>
Zero or more I/O objects or I/O executors for which outstanding work
must be maintained.
</p></dd>
</dl>
</div>
<h5>
<a name="asio.reference.async_compose.h1"></a>
<span><a name="asio.reference.async_compose.example_"></a></span><a class="link" href="async_compose.html#asio.reference.async_compose.example_">Example:</a>
</h5>
<pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">async_echo_implementation</span>
<span class="special">{</span>
<span class="identifier">tcp</span><span class="special">::</span><span class="identifier">socket</span><span class="special">&amp;</span> <span class="identifier">local_socket</span><span class="special">;</span>
<span class="identifier">asio</span><span class="special">::</span><span class="identifier">mutable_buffer</span> <span class="identifier">buffer_</span><span class="special">;</span>
<span class="keyword">enum</span> <span class="special">{</span> <span class="identifier">starting</span><span class="special">,</span> <span class="identifier">reading</span><span class="special">,</span> <span class="identifier">writing</span> <span class="special">}</span> <span class="identifier">state_</span><span class="special">;</span>
<span class="keyword">template</span> <span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">Self</span><span class="special">&gt;</span>
<span class="keyword">void</span> <span class="keyword">operator</span><span class="special">()(</span><span class="identifier">Self</span><span class="special">&amp;</span> <span class="identifier">self</span><span class="special">,</span>
<span class="identifier">asio</span><span class="special">::</span><span class="identifier">error_code</span> <span class="identifier">error</span> <span class="special">=</span> <span class="special">{},</span>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">size_t</span> <span class="identifier">n</span> <span class="special">=</span> <span class="number">0</span><span class="special">)</span>
<span class="special">{</span>
<span class="keyword">switch</span> <span class="special">(</span><span class="identifier">state_</span><span class="special">)</span>
<span class="special">{</span>
<span class="keyword">case</span> <span class="identifier">starting</span><span class="special">:</span>
<span class="identifier">state_</span> <span class="special">=</span> <span class="identifier">reading</span><span class="special">;</span>
<span class="identifier">local_socket</span><span class="special">.</span><span class="identifier">async_read_some</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="identifier">self</span><span class="special">));</span>
<span class="keyword">break</span><span class="special">;</span>
<span class="keyword">case</span> <span class="identifier">reading</span><span class="special">:</span>
<span class="keyword">if</span> <span class="special">(</span><span class="identifier">error</span><span class="special">)</span>
<span class="special">{</span>
<span class="identifier">self</span><span class="special">.</span><span class="identifier">complete</span><span class="special">(</span><span class="identifier">error</span><span class="special">,</span> <span class="number">0</span><span class="special">);</span>
<span class="special">}</span>
<span class="keyword">else</span>
<span class="special">{</span>
<span class="identifier">state_</span> <span class="special">=</span> <span class="identifier">writing</span><span class="special">;</span>
<span class="identifier">asio</span><span class="special">::</span><span class="identifier">async_write</span><span class="special">(</span><span class="identifier">local_socket</span><span class="special">,</span> <span class="identifier">buffer_</span><span class="special">,</span>
<span class="identifier">asio</span><span class="special">::</span><span class="identifier">transfer_exactly</span><span class="special">(</span><span class="identifier">n</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="identifier">self</span><span class="special">));</span>
<span class="special">}</span>
<span class="keyword">break</span><span class="special">;</span>
<span class="keyword">case</span> <span class="identifier">writing</span><span class="special">:</span>
<span class="identifier">self</span><span class="special">.</span><span class="identifier">complete</span><span class="special">(</span><span class="identifier">error</span><span class="special">,</span> <span class="identifier">n</span><span class="special">);</span>
<span class="keyword">break</span><span class="special">;</span>
<span class="special">}</span>
<span class="special">}</span>
<span class="special">};</span>
<span class="keyword">template</span> <span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">CompletionToken</span><span class="special">&gt;</span>
<span class="keyword">auto</span> <span class="identifier">async_echo</span><span class="special">(</span><span class="identifier">tcp</span><span class="special">::</span><span class="identifier">socket</span><span class="special">&amp;</span> <span class="identifier">socket</span><span class="special">,</span>
<span class="identifier">asio</span><span class="special">::</span><span class="identifier">mutable_buffer</span> <span class="identifier">buffer</span><span class="special">,</span>
<span class="identifier">CompletionToken</span><span class="special">&amp;&amp;</span> <span class="identifier">token</span><span class="special">)</span> <span class="special">-&gt;</span>
<span class="keyword">decltype</span><span class="special">(</span>
<span class="identifier">asio</span><span class="special">::</span><span class="identifier">async_compose</span><span class="special">&lt;</span><span class="identifier">CompletionToken</span><span class="special">,</span>
<span class="keyword">void</span><span class="special">(</span><span class="identifier">asio</span><span class="special">::</span><span class="identifier">error_code</span><span class="special">,</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">size_t</span><span class="special">)&gt;(</span>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">declval</span><span class="special">&lt;</span><span class="identifier">async_echo_implementation</span><span class="special">&gt;(),</span>
<span class="identifier">token</span><span class="special">,</span> <span class="identifier">socket</span><span class="special">))</span>
<span class="special">{</span>
<span class="keyword">return</span> <span class="identifier">asio</span><span class="special">::</span><span class="identifier">async_compose</span><span class="special">&lt;</span><span class="identifier">CompletionToken</span><span class="special">,</span>
<span class="keyword">void</span><span class="special">(</span><span class="identifier">asio</span><span class="special">::</span><span class="identifier">error_code</span><span class="special">,</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">size_t</span><span class="special">)&gt;(</span>
<span class="identifier">async_echo_implementation</span><span class="special">{</span><span class="identifier">socket</span><span class="special">,</span> <span class="identifier">buffer</span><span class="special">,</span>
<span class="identifier">async_echo_implementation</span><span class="special">::</span><span class="identifier">starting</span><span class="special">},</span>
<span class="identifier">token</span><span class="special">,</span> <span class="identifier">socket</span><span class="special">);</span>
<span class="special">}</span>
</pre>
<h5>
<a name="asio.reference.async_compose.h2"></a>
<span><a name="asio.reference.async_compose.requirements"></a></span><a class="link" href="async_compose.html#asio.reference.async_compose.requirements">Requirements</a>
</h5>
<p>
<span class="emphasis"><em>Header: </em></span><code class="literal">asio/compose.hpp</code>
</p>
<p>
<span class="emphasis"><em>Convenience header: </em></span><code class="literal">asio.hpp</code>
</p>
</div>
<div class="copyright-footer">Copyright © 2003-2023 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>
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