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<div class="titlepage"><div><div><h3 class="title">
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<a name="boost_asio.reference.Receiver"></a><a class="link" href="Receiver.html" title="Receiver concepts">Receiver concepts</a>
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</h3></div></div></div>
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<p>
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A receiver represents the continuation of an asynchronous operation. An asynchronous
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operation may complete with a (possibly empty) set of values, an error, or
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it may be cancelled. A receiver has three principal operations corresponding
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to the three ways an asynchronous operation may complete: <code class="computeroutput">set_value</code>,
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<code class="computeroutput">set_error</code>, and <code class="computeroutput">set_done</code>. These are collectively
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known as a receiver’s <span class="emphasis"><em>completion-signal operations</em></span>.
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</p>
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<pre class="programlisting">template<class T, class E = exception_ptr>
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concept receiver =
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move_constructible<remove_cvref_t<T>> &&
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constructible_from<remove_cvref_t<T>, T> &&
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requires(remove_cvref_t<T>&& t, E&& e) {
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{ execution::set_done(std::move(t)) } noexcept;
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{ execution::set_error(std::move(t), (E&&) e) } noexcept;
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};
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template<class T, class... An>
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concept receiver_of =
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receiver<T> &&
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requires(remove_cvref_t<T>&& t, An&&... an) {
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execution::set_value(std::move(t), (An&&) an...);
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};
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</pre>
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<p>
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The receiver’s completion-signal operations have semantic requirements
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that are collectively known as the <span class="emphasis"><em>receiver contract</em></span>,
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described below:
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</p>
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<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
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<li class="listitem">
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None of a receiver’s completion-signal operations shall be invoked
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before <code class="computeroutput">execution::start</code> has been called on the operation
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state object that was returned by <code class="computeroutput">execution::connect</code> to
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connect that receiver to a sender.
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</li>
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<li class="listitem">
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Once <code class="computeroutput">execution::start</code> has been called on the operation state
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object, exactly one of the receiver’s completion-signal operations
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shall complete non-exceptionally before the receiver is destroyed.
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</li>
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<li class="listitem">
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If <code class="computeroutput">execution::set_value</code> exits with an exception, it is still
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valid to call <code class="computeroutput">execution::set_error</code> or <code class="computeroutput">execution::set_done</code>
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on the receiver.
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</li>
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</ul></div>
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<p>
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Once one of a receiver’s completion-signal operations has completed non-exceptionally,
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the receiver contract has been satisfied.
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</p>
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<td align="left"></td>
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<td align="right"><div class="copyright-footer">Copyright © 2003-2021 Christopher
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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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