Receiver.html
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<a name="asio.reference.Receiver"></a><a class="link" href="Receiver.html" title="Receiver concepts">Receiver concepts</a>
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<p>
A receiver represents the continuation of an asynchronous operation. An asynchronous
operation may complete with a (possibly empty) set of values, an error, or
it may be cancelled. A receiver has three principal operations corresponding
to the three ways an asynchronous operation may complete: <code class="computeroutput">set_value</code>,
<code class="computeroutput">set_error</code>, and <code class="computeroutput">set_done</code>. These are collectively
known as a receiver’s <span class="emphasis"><em>completion-signal operations</em></span>.
</p>
<pre class="programlisting">template<class T, class E = exception_ptr>
concept receiver =
move_constructible<remove_cvref_t<T>> &&
constructible_from<remove_cvref_t<T>, T> &&
requires(remove_cvref_t<T>&& t, E&& e) {
{ execution::set_done(std::move(t)) } noexcept;
{ execution::set_error(std::move(t), (E&&) e) } noexcept;
};
template<class T, class... An>
concept receiver_of =
receiver<T> &&
requires(remove_cvref_t<T>&& t, An&&... an) {
execution::set_value(std::move(t), (An&&) an...);
};
</pre>
<p>
The receiver’s completion-signal operations have semantic requirements
that are collectively known as the <span class="emphasis"><em>receiver contract</em></span>,
described below:
</p>
<div class="itemizedlist"><ul class="itemizedlist" type="disc">
<li class="listitem">
None of a receiver’s completion-signal operations shall be invoked
before <code class="computeroutput">execution::start</code> has been called on the operation
state object that was returned by <code class="computeroutput">execution::connect</code> to
connect that receiver to a sender.
</li>
<li class="listitem">
Once <code class="computeroutput">execution::start</code> has been called on the operation state
object, exactly one of the receiver’s completion-signal operations
shall complete non-exceptionally before the receiver is destroyed.
</li>
<li class="listitem">
If <code class="computeroutput">execution::set_value</code> exits with an exception, it is still
valid to call <code class="computeroutput">execution::set_error</code> or <code class="computeroutput">execution::set_done</code>
on the receiver.
</li>
</ul></div>
<p>
Once one of a receiver’s completion-signal operations has completed non-exceptionally,
the receiver contract has been satisfied.
</p>
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<td align="right"><div class="copyright-footer">Copyright © 2003-2020 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>)
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