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79 lines
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<title>Futures</title>
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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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<a accesskey="p" href="spawn.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="cpp20_coroutines.html"><img src="../../../next.png" alt="Next"></a>
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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.futures"></a><a class="link" href="futures.html" title="Futures">Futures</a>
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</h4></div></div></div>
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<p>
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The <code class="computeroutput"><span class="identifier">asio</span><span class="special">::</span><span class="identifier">use_future</span></code> completion token provides
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first-class support for returning a <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">future</span></code>
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from an asynchronous operation's initiating function.
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</p>
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<p>
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To use <code class="computeroutput"><span class="identifier">asio</span><span class="special">::</span><span class="identifier">use_future</span></code>, pass it to an asynchronous
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operation instead of a completion handler. For example:
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</p>
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<pre class="programlisting"><span class="identifier">std</span><span class="special">::</span><span class="identifier">future</span><span class="special"><</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">size_t</span><span class="special">></span> <span class="identifier">length</span> <span class="special">=</span>
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<span class="identifier">my_socket</span><span class="special">.</span><span class="identifier">async_read_some</span><span class="special">(</span><span class="identifier">my_buffer</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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<p>
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Where a completion signature has the form:
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</p>
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<pre class="programlisting"><span class="keyword">void</span> <span class="identifier">handler</span><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="identifier">result_type</span> <span class="identifier">result</span><span class="special">);</span>
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</pre>
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<p>
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the initiating function returns a <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">future</span></code>
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templated on <code class="computeroutput"><span class="identifier">result_type</span></code>.
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In the above example, this is <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">size_t</span></code>.
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If the asynchronous operation fails, the <code class="computeroutput"><span class="identifier">error_code</span></code>
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is converted into a <code class="computeroutput"><span class="identifier">system_error</span></code>
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exception and passed back to the caller through the future.
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</p>
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<p>
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Where a completion signature has the form:
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</p>
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<pre class="programlisting"><span class="keyword">void</span> <span class="identifier">handler</span><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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</pre>
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<p>
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the initiating function returns <code class="computeroutput"><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></code>. As above, an error is passed back
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in the future as a <code class="computeroutput"><span class="identifier">system_error</span></code>
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exception.
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</p>
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<h6>
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<a name="asio.overview.composition.futures.h0"></a>
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<span><a name="asio.overview.composition.futures.see_also"></a></span><a class="link" href="futures.html#asio.overview.composition.futures.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/use_future.html" title="use_future">use_future</a>, <a class="link" href="../../reference/use_future_t.html" title="use_future_t">use_future_t</a>,
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<a class="link" href="../../examples/cpp11_examples.html#asio.examples.cpp11_examples.futures">Futures example (C++11)</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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