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<title>Receiver concepts</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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<div class="titlepage"><div><div><h3 class="title">
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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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</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"><span class="identifier">set_value</span></code>,
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<code class="computeroutput"><span class="identifier">set_error</span></code>, and <code class="computeroutput"><span class="identifier">set_done</span></code>. These are collectively known
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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"><span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">,</span> <span class="keyword">class</span> <span class="identifier">E</span> <span class="special">=</span> <span class="identifier">exception_ptr</span><span class="special">></span>
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<span class="identifier">concept</span> <span class="identifier">receiver</span> <span class="special">=</span>
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<span class="identifier">move_constructible</span><span class="special"><</span><span class="identifier">remove_cvref_t</span><span class="special"><</span><span class="identifier">T</span><span class="special">>></span> <span class="special">&&</span>
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<span class="identifier">constructible_from</span><span class="special"><</span><span class="identifier">remove_cvref_t</span><span class="special"><</span><span class="identifier">T</span><span class="special">>,</span> <span class="identifier">T</span><span class="special">></span> <span class="special">&&</span>
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<span class="identifier">requires</span><span class="special">(</span><span class="identifier">remove_cvref_t</span><span class="special"><</span><span class="identifier">T</span><span class="special">>&&</span> <span class="identifier">t</span><span class="special">,</span> <span class="identifier">E</span><span class="special">&&</span> <span class="identifier">e</span><span class="special">)</span> <span class="special">{</span>
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<span class="special">{</span> <span class="identifier">execution</span><span class="special">::</span><span class="identifier">set_done</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">t</span><span class="special">))</span> <span class="special">}</span> <span class="keyword">noexcept</span><span class="special">;</span>
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<span class="special">{</span> <span class="identifier">execution</span><span class="special">::</span><span class="identifier">set_error</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">t</span><span class="special">),</span> <span class="special">(</span><span class="identifier">E</span><span class="special">&&)</span> <span class="identifier">e</span><span class="special">)</span> <span class="special">}</span> <span class="keyword">noexcept</span><span class="special">;</span>
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<span class="special">};</span>
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<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">,</span> <span class="keyword">class</span><span class="special">...</span> <span class="identifier">An</span><span class="special">></span>
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<span class="identifier">concept</span> <span class="identifier">receiver_of</span> <span class="special">=</span>
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<span class="identifier">receiver</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="special">&&</span>
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<span class="identifier">requires</span><span class="special">(</span><span class="identifier">remove_cvref_t</span><span class="special"><</span><span class="identifier">T</span><span class="special">>&&</span> <span class="identifier">t</span><span class="special">,</span> <span class="identifier">An</span><span class="special">&&...</span> <span class="identifier">an</span><span class="special">)</span> <span class="special">{</span>
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<span class="identifier">execution</span><span class="special">::</span><span class="identifier">set_value</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">t</span><span class="special">),</span> <span class="special">(</span><span class="identifier">An</span><span class="special">&&)</span> <span class="identifier">an</span><span class="special">...);</span>
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<span class="special">};</span>
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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" 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"><span class="identifier">execution</span><span class="special">::</span><span class="identifier">start</span></code> has been called on the operation
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state object that was returned by <code class="computeroutput"><span class="identifier">execution</span><span class="special">::</span><span class="identifier">connect</span></code>
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to 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"><span class="identifier">execution</span><span class="special">::</span><span class="identifier">start</span></code> has been called on the operation
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state 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"><span class="identifier">execution</span><span class="special">::</span><span class="identifier">set_value</span></code> exits with an exception,
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it is still valid to call <code class="computeroutput"><span class="identifier">execution</span><span class="special">::</span><span class="identifier">set_error</span></code>
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or <code class="computeroutput"><span class="identifier">execution</span><span class="special">::</span><span class="identifier">set_done</span></code> 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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</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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