87 lines
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87 lines
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<meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
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<title>Deferred Operations</title>
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<meta name="generator" content="DocBook XSL Stylesheets V1.75.2">
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<link rel="home" href="../../../index.html" title="Asio">
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<link rel="up" href="../composition.html" title="Composition and Completion Tokens">
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<link rel="prev" href="coro.html" title="Resumable C++ 20 Coroutines">
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<body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF">
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<table cellpadding="2" width="100%"><tr><td valign="top"><img alt="asio C++ library" width="250" height="60" src="../../../asio.png"></td></tr></table>
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<div class="spirit-nav">
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<a accesskey="p" href="coro.html"><img src="../../../prev.png" alt="Prev"></a><a accesskey="u" href="../composition.html"><img src="../../../up.png" alt="Up"></a><a accesskey="h" href="../../../index.html"><img src="../../../home.png" alt="Home"></a><a accesskey="n" href="promises.html"><img src="../../../next.png" alt="Next"></a>
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</div>
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<div class="section">
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<div class="titlepage"><div><div><h4 class="title">
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<a name="asio.overview.composition.deferred"></a><a class="link" href="deferred.html" title="Deferred Operations">Deferred Operations</a>
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</h4></div></div></div>
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<p>
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The <a class="link" href="../../reference/deferred.html" title="deferred"><code class="computeroutput"><span class="identifier">deferred</span></code></a>,
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completion token takes a call to an asynchronous operation's initiating
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function and turns it into a function object that accepts a completion
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token. For example:
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</p>
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<pre class="programlisting"><span class="keyword">auto</span> <span class="identifier">deferred_op</span> <span class="special">=</span>
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<span class="identifier">timer</span><span class="special">.</span><span class="identifier">async_wait</span><span class="special">(</span>
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<span class="identifier">asio</span><span class="special">::</span><span class="identifier">deferred</span><span class="special">);</span>
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<span class="special">...</span>
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<span class="identifier">std</span><span class="special">::</span><span class="identifier">move</span><span class="special">(</span><span class="identifier">deferred_op</span><span class="special">)(</span>
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<span class="special">[](</span><span class="identifier">asio</span><span class="special">::</span><span class="identifier">error_code</span> <span class="identifier">ec</span><span class="special">){</span> <span class="special">...</span> <span class="special">});</span>
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</pre>
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<p>
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or:
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</p>
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<pre class="programlisting"><span class="keyword">auto</span> <span class="identifier">deferred_op</span> <span class="special">=</span>
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<span class="identifier">timer</span><span class="special">.</span><span class="identifier">async_wait</span><span class="special">(</span>
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<span class="identifier">asio</span><span class="special">::</span><span class="identifier">deferred</span><span class="special">);</span>
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<span class="special">...</span>
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<span class="identifier">std</span><span class="special">::</span><span class="identifier">future</span><span class="special"><</span><span class="keyword">void</span><span class="special">></span> <span class="special">=</span>
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<span class="identifier">std</span><span class="special">::</span><span class="identifier">move</span><span class="special">(</span><span class="identifier">deferred_op</span><span class="special">)(</span>
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<span class="identifier">asio</span><span class="special">::</span><span class="identifier">use_future</span><span class="special">);</span>
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</pre>
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<p>
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The deferred token also supports chaining, to create simple compositions:
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</p>
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<pre class="programlisting"><span class="keyword">auto</span> <span class="identifier">deferred_op</span> <span class="special">=</span>
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<span class="identifier">timer</span><span class="special">.</span><span class="identifier">async_wait</span><span class="special">(</span>
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<span class="identifier">asio</span><span class="special">::</span><span class="identifier">deferred</span><span class="special">(</span>
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<span class="special">[&](</span><span class="identifier">asio</span><span class="special">::</span><span class="identifier">error_code</span> <span class="identifier">ec</span><span class="special">)</span>
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<span class="special">{</span>
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<span class="identifier">timer</span><span class="special">.</span><span class="identifier">expires_after</span><span class="special">(</span>
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<span class="identifier">std</span><span class="special">::</span><span class="identifier">chrono</span><span class="special">::</span><span class="identifier">seconds</span><span class="special">(</span><span class="number">1</span><span class="special">));</span>
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<span class="keyword">return</span> <span class="identifier">timer</span><span class="special">.</span><span class="identifier">async_wait</span><span class="special">(</span>
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<span class="identifier">asio</span><span class="special">::</span><span class="identifier">deferred</span><span class="special">);</span>
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<span class="special">});</span>
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<span class="special">...</span>
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<span class="identifier">std</span><span class="special">::</span><span class="identifier">future</span><span class="special"><</span><span class="keyword">void</span><span class="special">></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">deferred_op</span><span class="special">)(</span><span class="identifier">asio</span><span class="special">::</span><span class="identifier">use_future</span><span class="special">);</span>
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</pre>
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<h6>
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<a name="asio.overview.composition.deferred.h0"></a>
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<span><a name="asio.overview.composition.deferred.see_also"></a></span><a class="link" href="deferred.html#asio.overview.composition.deferred.see_also">See
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Also</a>
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</h6>
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<p>
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<a class="link" href="../../reference/deferred.html" title="deferred">deferred</a>, <a class="link" href="../../reference/deferred_t.html" title="deferred_t">deferred_t</a>,
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<a class="link" href="../../examples/cpp14_examples.html#asio.examples.cpp14_examples.deferred">Deferred examples
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(C++11)</a>, <a class="link" href="../../examples/cpp14_examples.html#asio.examples.cpp14_examples.deferred">Deferred
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examples (C++14)</a>.
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</p>
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</div>
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<div class="copyright-footer">Copyright © 2003-2023 Christopher M. Kohlhoff<p>
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Distributed under the Boost Software License, Version 1.0. (See accompanying
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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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</p>
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</div>
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<hr>
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