76 lines
4.1 KiB
HTML
76 lines
4.1 KiB
HTML
<html>
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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="chrono.html"><img src="../../../prev.png" alt="Prev"></a><a accesskey="u" href="../cpp2011.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="../implementation.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.cpp2011.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">asio::use_future</code> special value provides first-class support
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for returning a C++11 <code class="computeroutput">std::future</code> from an asynchronous operation's
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initiating function.
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</p>
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<p>
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To use <code class="computeroutput">asio::use_future</code>, pass it to an asynchronous operation
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instead of a normal completion handler. For example:
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</p>
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<pre class="programlisting">std::future<std::size_t> length =
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my_socket.async_read_some(my_buffer, asio::use_future);
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</pre>
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<p>
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Where a handler signature has the form:
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</p>
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<pre class="programlisting">void handler(asio::error_code ec, result_type result);
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</pre>
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<p>
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the initiating function returns a <code class="computeroutput">std::future</code> templated on
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<code class="computeroutput">result_type</code>. In the above example, this is <code class="computeroutput">std::size_t</code>.
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If the asynchronous operation fails, the <code class="computeroutput">error_code</code> is converted
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into a <code class="computeroutput">system_error</code> exception and passed back to the caller
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through the future.
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</p>
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<p>
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Where a handler signature has the form:
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</p>
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<pre class="programlisting">void handler(asio::error_code ec);
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</pre>
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<p>
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the initiating function returns <code class="computeroutput">std::future<void></code>. As
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above, an error is passed back in the future as a <code class="computeroutput">system_error</code>
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exception.
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</p>
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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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<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
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<td align="left"></td>
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<td align="right"><div class="copyright-footer">Copyright © 2003-2020 Christopher M.
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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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