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<div class="section">
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<div class="titlepage"><div><div><h3 class="title">
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<a name="boost_asio.reference.Scheduler"></a><a class="link" href="Scheduler.html" title="Scheduler concept">Scheduler concept</a>
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</h3></div></div></div>
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<pre class="programlisting">template<class S>
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concept scheduler =
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copy_constructible<remove_cvref_t<S>> &&
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equality_comparable<remove_cvref_t<S>> &&
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requires(E&& e) {
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execution::schedule((E&&)e);
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};
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</pre>
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<p>
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None of a scheduler's copy constructor, destructor, equality comparison,
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or <code class="computeroutput">swap</code> operation shall exit via an exception.
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</p>
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<p>
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None of these operations, nor a scheduler type's <code class="computeroutput">schedule</code> function,
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or associated query functions shall introduce data races as a result of concurrent
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invocations of those functions from different threads.
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</p>
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<p>
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For any two (possibly const) values <code class="computeroutput">x1</code> and <code class="computeroutput">x2</code> of
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some scheduler type <code class="computeroutput">X</code>, <code class="computeroutput">x1 == x2</code> shall return <code class="computeroutput">true</code>
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only if <code class="computeroutput">boost::asio::query(x1, p) == boost::asio::query(x2, p)</code>
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for every property <code class="computeroutput">p</code> where both <code class="computeroutput">boost::asio::query(x1,
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p)</code> and <code class="computeroutput">boost::asio::query(x2, p)</code> are well-formed and
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result in a non-void type that is <code class="computeroutput">EqualityComparable</code> (C++Std
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[equalitycomparable]). [<span class="emphasis"><em>Note:</em></span> The above requirements
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imply that <code class="computeroutput">x1 == x2</code> returns <code class="computeroutput">true</code> if <code class="computeroutput">x1</code>
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and <code class="computeroutput">x2</code> can be interchanged with identical effects. A scheduler
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may conceptually contain additional properties which are not exposed by a
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named property type that can be observed via <code class="computeroutput">boost::asio::query</code>;
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in this case, it is up to the concrete scheduler implementation to decide
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if these properties affect equality. Returning <code class="computeroutput">false</code> does not
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necessarily imply that the effects are not identical. —<span class="emphasis"><em>end note</em></span>]
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</p>
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<p>
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A scheduler type's destructor shall not block pending completion of any receivers
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submitted to the sender objects returned from <code class="computeroutput">schedule</code>. [<span class="emphasis"><em>Note:</em></span>
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The ability to wait for completion of submitted function objects may be provided
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by the execution context that produced the scheduler. —<span class="emphasis"><em>end note</em></span>]
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</p>
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<p>
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In addition to the above requirements, type <code class="computeroutput">S</code> models <code class="computeroutput">scheduler</code>
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only if it satisfies the requirements in the Table below.
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</p>
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<p>
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In the Table 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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<code class="computeroutput">s</code> denotes a (possibly const) scheduler object of type <code class="computeroutput">S</code>,
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</li>
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<li class="listitem">
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<code class="computeroutput">N</code> denotes a type that models <code class="computeroutput">sender</code>, and
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</li>
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<li class="listitem">
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<code class="computeroutput">n</code> denotes a sender object of type <code class="computeroutput">N</code>
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</li>
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</ul></div>
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<div class="table">
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<a name="boost_asio.reference.Scheduler.t0"></a><p class="title"><b>Table 25. Scheduler requirements</b></p>
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<div class="table-contents"><table class="table" summary="Scheduler requirements">
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<colgroup>
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<col>
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<col>
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<col>
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</colgroup>
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<thead><tr>
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<th>
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<p>
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expression
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</p>
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</th>
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<th>
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<p>
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return type
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</p>
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</th>
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<th>
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<p>
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operation semantics
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</p>
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</th>
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</tr></thead>
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<tbody><tr>
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<td>
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<p>
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<code class="computeroutput">execution::schedule(s)</code>
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</p>
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</td>
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<td>
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<p>
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<code class="computeroutput">N</code>
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</p>
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</td>
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<td>
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<p>
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Evaluates <code class="computeroutput">execution::schedule(s)</code> on the calling thread
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to create <code class="computeroutput">N</code>.
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</p>
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</td>
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</tr></tbody>
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</table></div>
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</div>
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<br class="table-break"><p>
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<code class="computeroutput">execution::start(o)</code>, where <code class="computeroutput">o</code> is the result of a
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call to <code class="computeroutput">execution::connect(N, r)</code> for some receiver object <code class="computeroutput">r</code>,
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is required to eagerly submit <code class="computeroutput">r</code> for execution on an execution
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agent that <code class="computeroutput">s</code> creates for it. Let <code class="computeroutput">rc</code> be <code class="computeroutput">r</code>
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or an object created by copy or move construction from <code class="computeroutput">r</code>. The
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semantic constraints on the <code class="computeroutput">sender</code> <code class="computeroutput">N</code> returned from
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a scheduler <code class="computeroutput">s</code>'s <code class="computeroutput">schedule</code> function are as follows:
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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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If <code class="computeroutput">rc</code>'s <code class="computeroutput">set_error</code> function is called in response
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to a submission error, scheduling error, or other internal error, let
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<code class="computeroutput">E</code> be an expression that refers to that error if <code class="computeroutput">set_error(rc,
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E)</code> is well-formed; otherwise, let <code class="computeroutput">E</code> be an <code class="computeroutput">exception_ptr</code>
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that refers to that error. [Note: <code class="computeroutput">E</code> could be the result
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of calling <code class="computeroutput">current_exception</code> or <code class="computeroutput">make_exception_ptr</code>.]
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The scheduler calls <code class="computeroutput">set_error(rc, E)</code> on an unspecified weakly-parallel
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execution agent ([Note: An invocation of <code class="computeroutput">set_error</code> on a
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receiver is required to be <code class="computeroutput">noexcept</code>]), and
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</li>
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<li class="listitem">
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If <code class="computeroutput">rc</code>'s <code class="computeroutput">set_error</code> function is called in response
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to an exception that propagates out of the invocation of <code class="computeroutput">set_value</code>
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on <code class="computeroutput">rc</code>, let <code class="computeroutput">E</code> be <code class="computeroutput">make_exception_ptr(receiver_invocation_error{})</code>
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invoked from within a catch clause that has caught the exception. The
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executor calls <code class="computeroutput">set_error(rc, E)</code> on an unspecified weakly-parallel
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execution agent, and
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</li>
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<li class="listitem">
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A call to <code class="computeroutput">set_done(rc)</code> is made on an unspecified weakly-parallel
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execution agent ([Note: An invocation of a receiver's <code class="computeroutput">set_done</code>
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function is required to be <code class="computeroutput">noexcept</code>]).
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</li>
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</ul></div>
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<p>
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[<span class="emphasis"><em>Note:</em></span> The senders returned from a scheduler's <code class="computeroutput">schedule</code>
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function have wide discretion when deciding which of the three receiver functions
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to call upon submission. —<span class="emphasis"><em>end note</em></span>]
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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 © 2003-2021 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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</p>
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</div></td>
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</tr></table>
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<hr>
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