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<div class="section">
<div class="titlepage"><div><div><h3 class="title">
<a name="asio.reference.Scheduler"></a><a class="link" href="Scheduler.html" title="Scheduler concept">Scheduler concept</a>
</h3></div></div></div>
<pre class="programlisting"><span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">class</span> <span class="identifier">S</span><span class="special">&gt;</span>
<span class="identifier">concept</span> <span class="identifier">scheduler</span> <span class="special">=</span>
<span class="identifier">copy_constructible</span><span class="special">&lt;</span><span class="identifier">remove_cvref_t</span><span class="special">&lt;</span><span class="identifier">S</span><span class="special">&gt;&gt;</span> <span class="special">&amp;&amp;</span>
<span class="identifier">equality_comparable</span><span class="special">&lt;</span><span class="identifier">remove_cvref_t</span><span class="special">&lt;</span><span class="identifier">S</span><span class="special">&gt;&gt;</span> <span class="special">&amp;&amp;</span>
<span class="identifier">requires</span><span class="special">(</span><span class="identifier">E</span><span class="special">&amp;&amp;</span> <span class="identifier">e</span><span class="special">)</span> <span class="special">{</span>
<span class="identifier">execution</span><span class="special">::</span><span class="identifier">schedule</span><span class="special">((</span><span class="identifier">E</span><span class="special">&amp;&amp;)</span><span class="identifier">e</span><span class="special">);</span>
<span class="special">};</span>
</pre>
<p>
None of a scheduler's copy constructor, destructor, equality comparison,
or <code class="computeroutput"><span class="identifier">swap</span></code> operation shall exit
via an exception.
</p>
<p>
None of these operations, nor a scheduler type's <code class="computeroutput"><span class="identifier">schedule</span></code>
function, or associated query functions shall introduce data races as a result
of concurrent invocations of those functions from different threads.
</p>
<p>
For any two (possibly const) values <code class="computeroutput"><span class="identifier">x1</span></code>
and <code class="computeroutput"><span class="identifier">x2</span></code> of some scheduler
type <code class="computeroutput"><span class="identifier">X</span></code>, <code class="computeroutput"><span class="identifier">x1</span>
<span class="special">==</span> <span class="identifier">x2</span></code>
shall return <code class="computeroutput"><span class="keyword">true</span></code> only if <code class="computeroutput"><span class="identifier">asio</span><span class="special">::</span><span class="identifier">query</span><span class="special">(</span><span class="identifier">x1</span><span class="special">,</span> <span class="identifier">p</span><span class="special">)</span> <span class="special">==</span> <span class="identifier">asio</span><span class="special">::</span><span class="identifier">query</span><span class="special">(</span><span class="identifier">x2</span><span class="special">,</span>
<span class="identifier">p</span><span class="special">)</span></code>
for every property <code class="computeroutput"><span class="identifier">p</span></code> where
both <code class="computeroutput"><span class="identifier">asio</span><span class="special">::</span><span class="identifier">query</span><span class="special">(</span><span class="identifier">x1</span><span class="special">,</span> <span class="identifier">p</span><span class="special">)</span></code> and <code class="computeroutput"><span class="identifier">asio</span><span class="special">::</span><span class="identifier">query</span><span class="special">(</span><span class="identifier">x2</span><span class="special">,</span>
<span class="identifier">p</span><span class="special">)</span></code>
are well-formed and result in a non-void type that is <code class="computeroutput"><span class="identifier">EqualityComparable</span></code>
(C++Std [equalitycomparable]). [<span class="emphasis"><em>Note:</em></span> The above requirements
imply that <code class="computeroutput"><span class="identifier">x1</span> <span class="special">==</span>
<span class="identifier">x2</span></code> returns <code class="computeroutput"><span class="keyword">true</span></code>
if <code class="computeroutput"><span class="identifier">x1</span></code> and <code class="computeroutput"><span class="identifier">x2</span></code>
can be interchanged with identical effects. A scheduler may conceptually
contain additional properties which are not exposed by a named property type
that can be observed via <code class="computeroutput"><span class="identifier">asio</span><span class="special">::</span><span class="identifier">query</span></code>;
in this case, it is up to the concrete scheduler implementation to decide
if these properties affect equality. Returning <code class="computeroutput"><span class="keyword">false</span></code>
does not necessarily imply that the effects are not identical. —<span class="emphasis"><em>end
note</em></span>]
</p>
<p>
A scheduler type's destructor shall not block pending completion of any receivers
submitted to the sender objects returned from <code class="computeroutput"><span class="identifier">schedule</span></code>.
[<span class="emphasis"><em>Note:</em></span> The ability to wait for completion of submitted
function objects may be provided by the execution context that produced the
scheduler. —<span class="emphasis"><em>end note</em></span>]
</p>
<p>
In addition to the above requirements, type <code class="computeroutput"><span class="identifier">S</span></code>
models <code class="computeroutput"><span class="identifier">scheduler</span></code> only if
it satisfies the requirements in the Table below.
</p>
<p>
In the Table below,
</p>
<div class="itemizedlist"><ul class="itemizedlist" type="disc">
<li class="listitem">
<code class="computeroutput"><span class="identifier">s</span></code> denotes a (possibly
const) scheduler object of type <code class="computeroutput"><span class="identifier">S</span></code>,
</li>
<li class="listitem">
<code class="computeroutput"><span class="identifier">N</span></code> denotes a type that
models <code class="computeroutput"><span class="identifier">sender</span></code>, and
</li>
<li class="listitem">
<code class="computeroutput"><span class="identifier">n</span></code> denotes a sender object
of type <code class="computeroutput"><span class="identifier">N</span></code>
</li>
</ul></div>
<div class="table">
<a name="asio.reference.Scheduler.t0"></a><p class="title"><b>Table 27. Scheduler requirements</b></p>
<div class="table-contents"><table class="table" summary="Scheduler requirements">
<colgroup>
<col>
<col>
<col>
</colgroup>
<thead><tr>
<th>
<p>
expression
</p>
</th>
<th>
<p>
return type
</p>
</th>
<th>
<p>
operation semantics
</p>
</th>
</tr></thead>
<tbody><tr>
<td>
<p>
<code class="computeroutput"><span class="identifier">execution</span><span class="special">::</span><span class="identifier">schedule</span><span class="special">(</span><span class="identifier">s</span><span class="special">)</span></code>
</p>
</td>
<td>
<p>
<code class="computeroutput"><span class="identifier">N</span></code>
</p>
</td>
<td>
<p>
Evaluates <code class="computeroutput"><span class="identifier">execution</span><span class="special">::</span><span class="identifier">schedule</span><span class="special">(</span><span class="identifier">s</span><span class="special">)</span></code> on the calling thread to create
<code class="computeroutput"><span class="identifier">N</span></code>.
</p>
</td>
</tr></tbody>
</table></div>
</div>
<br class="table-break"><p>
<code class="computeroutput"><span class="identifier">execution</span><span class="special">::</span><span class="identifier">start</span><span class="special">(</span><span class="identifier">o</span><span class="special">)</span></code>, where
<code class="computeroutput"><span class="identifier">o</span></code> is the result of a call
to <code class="computeroutput"><span class="identifier">execution</span><span class="special">::</span><span class="identifier">connect</span><span class="special">(</span><span class="identifier">N</span><span class="special">,</span> <span class="identifier">r</span><span class="special">)</span></code> for some receiver object <code class="computeroutput"><span class="identifier">r</span></code>,
is required to eagerly submit <code class="computeroutput"><span class="identifier">r</span></code>
for execution on an execution agent that <code class="computeroutput"><span class="identifier">s</span></code>
creates for it. Let <code class="computeroutput"><span class="identifier">rc</span></code> be
<code class="computeroutput"><span class="identifier">r</span></code> or an object created by
copy or move construction from <code class="computeroutput"><span class="identifier">r</span></code>.
The semantic constraints on the <code class="computeroutput"><span class="identifier">sender</span></code>
<code class="computeroutput"><span class="identifier">N</span></code> returned from a scheduler
<code class="computeroutput"><span class="identifier">s</span></code>'s <code class="computeroutput"><span class="identifier">schedule</span></code>
function are as follows:
</p>
<div class="itemizedlist"><ul class="itemizedlist" type="disc">
<li class="listitem">
If <code class="computeroutput"><span class="identifier">rc</span></code>'s <code class="computeroutput"><span class="identifier">set_error</span></code> function is called in response
to a submission error, scheduling error, or other internal error, let
<code class="computeroutput"><span class="identifier">E</span></code> be an expression that
refers to that error if <code class="computeroutput"><span class="identifier">set_error</span><span class="special">(</span><span class="identifier">rc</span><span class="special">,</span> <span class="identifier">E</span><span class="special">)</span></code> is well-formed; otherwise, let <code class="computeroutput"><span class="identifier">E</span></code> be an <code class="computeroutput"><span class="identifier">exception_ptr</span></code>
that refers to that error. [Note: <code class="computeroutput"><span class="identifier">E</span></code>
could be the result of calling <code class="computeroutput"><span class="identifier">current_exception</span></code>
or <code class="computeroutput"><span class="identifier">make_exception_ptr</span></code>.]
The scheduler calls <code class="computeroutput"><span class="identifier">set_error</span><span class="special">(</span><span class="identifier">rc</span><span class="special">,</span> <span class="identifier">E</span><span class="special">)</span></code> on an unspecified weakly-parallel execution
agent ([Note: An invocation of <code class="computeroutput"><span class="identifier">set_error</span></code>
on a receiver is required to be <code class="computeroutput"><span class="keyword">noexcept</span></code>]),
and
</li>
<li class="listitem">
If <code class="computeroutput"><span class="identifier">rc</span></code>'s <code class="computeroutput"><span class="identifier">set_error</span></code> function is called in response
to an exception that propagates out of the invocation of <code class="computeroutput"><span class="identifier">set_value</span></code> on <code class="computeroutput"><span class="identifier">rc</span></code>,
let <code class="computeroutput"><span class="identifier">E</span></code> be <code class="computeroutput"><span class="identifier">make_exception_ptr</span><span class="special">(</span><span class="identifier">receiver_invocation_error</span><span class="special">{})</span></code>
invoked from within a catch clause that has caught the exception. The
executor calls <code class="computeroutput"><span class="identifier">set_error</span><span class="special">(</span><span class="identifier">rc</span><span class="special">,</span> <span class="identifier">E</span><span class="special">)</span></code> on an unspecified weakly-parallel execution
agent, and
</li>
<li class="listitem">
A call to <code class="computeroutput"><span class="identifier">set_done</span><span class="special">(</span><span class="identifier">rc</span><span class="special">)</span></code>
is made on an unspecified weakly-parallel execution agent ([Note: An
invocation of a receiver's <code class="computeroutput"><span class="identifier">set_done</span></code>
function is required to be <code class="computeroutput"><span class="keyword">noexcept</span></code>]).
</li>
</ul></div>
<p>
[<span class="emphasis"><em>Note:</em></span> The senders returned from a scheduler's <code class="computeroutput"><span class="identifier">schedule</span></code> function have wide discretion
when deciding which of the three receiver functions to call upon submission.
—<span class="emphasis"><em>end note</em></span>]
</p>
</div>
<div class="copyright-footer">Copyright © 2003-2023 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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