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<div class="titlepage"><div><div><h3 class="title">
<a name="asio.reference.io_context__strand"></a><a class="link" href="io_context__strand.html" title="io_context::strand">io_context::strand</a>
</h3></div></div></div>
<p>
<a class="indexterm" name="asio.indexterm.io_context__strand"></a>
</p>
<p>
Provides serialised handler execution.
</p>
<pre class="programlisting"><span class="keyword">class</span> <span class="identifier">strand</span>
</pre>
<h5>
<a name="asio.reference.io_context__strand.h0"></a>
<span><a name="asio.reference.io_context__strand.member_functions"></a></span><a class="link" href="io_context__strand.html#asio.reference.io_context__strand.member_functions">Member
Functions</a>
</h5>
<div class="informaltable"><table class="table">
<colgroup>
<col>
<col>
</colgroup>
<thead><tr>
<th>
<p>
Name
</p>
</th>
<th>
<p>
Description
</p>
</th>
</tr></thead>
<tbody>
<tr>
<td>
<p>
<a class="link" href="io_context__strand/context.html" title="io_context::strand::context"><span class="bold"><strong>context</strong></span></a>
</p>
</td>
<td>
<p>
Obtain the underlying execution context.
</p>
</td>
</tr>
<tr>
<td>
<p>
<a class="link" href="io_context__strand/defer.html" title="io_context::strand::defer"><span class="bold"><strong>defer</strong></span></a>
</p>
</td>
<td>
<p>
Request the strand to invoke the given function object.
</p>
</td>
</tr>
<tr>
<td>
<p>
<a class="link" href="io_context__strand/dispatch.html" title="io_context::strand::dispatch"><span class="bold"><strong>dispatch</strong></span></a>
</p>
</td>
<td>
<p>
Request the strand to invoke the given function object. <br>
<span class="silver"> —</span><br> (Deprecated: Use asio::dispatch().) Request the strand to
invoke the given handler.
</p>
</td>
</tr>
<tr>
<td>
<p>
<a class="link" href="io_context__strand/on_work_finished.html" title="io_context::strand::on_work_finished"><span class="bold"><strong>on_work_finished</strong></span></a>
</p>
</td>
<td>
<p>
Inform the strand that some work is no longer outstanding.
</p>
</td>
</tr>
<tr>
<td>
<p>
<a class="link" href="io_context__strand/on_work_started.html" title="io_context::strand::on_work_started"><span class="bold"><strong>on_work_started</strong></span></a>
</p>
</td>
<td>
<p>
Inform the strand that it has some outstanding work to do.
</p>
</td>
</tr>
<tr>
<td>
<p>
<a class="link" href="io_context__strand/post.html" title="io_context::strand::post"><span class="bold"><strong>post</strong></span></a>
</p>
</td>
<td>
<p>
Request the strand to invoke the given function object. <br>
<span class="silver"> —</span><br> (Deprecated: Use asio::post().) Request the strand to invoke
the given handler and return immediately.
</p>
</td>
</tr>
<tr>
<td>
<p>
<a class="link" href="io_context__strand/running_in_this_thread.html" title="io_context::strand::running_in_this_thread"><span class="bold"><strong>running_in_this_thread</strong></span></a>
</p>
</td>
<td>
<p>
Determine whether the strand is running in the current thread.
</p>
</td>
</tr>
<tr>
<td>
<p>
<a class="link" href="io_context__strand/strand.html" title="io_context::strand::strand"><span class="bold"><strong>strand</strong></span></a> <span class="silver">[constructor]</span>
</p>
</td>
<td>
<p>
Constructor.
</p>
</td>
</tr>
<tr>
<td>
<p>
<a class="link" href="io_context__strand/wrap.html" title="io_context::strand::wrap"><span class="bold"><strong>wrap</strong></span></a>
</p>
</td>
<td>
<p>
(Deprecated: Use asio::bind_executor().) Create a new handler that
automatically dispatches the wrapped handler on the strand.
</p>
</td>
</tr>
<tr>
<td>
<p>
<a class="link" href="io_context__strand/_strand.html" title="io_context::strand::~strand"><span class="bold"><strong>~strand</strong></span></a> <span class="silver">[destructor]</span>
</p>
</td>
<td>
<p>
Destructor.
</p>
</td>
</tr>
</tbody>
</table></div>
<h5>
<a name="asio.reference.io_context__strand.h1"></a>
<span><a name="asio.reference.io_context__strand.friends"></a></span><a class="link" href="io_context__strand.html#asio.reference.io_context__strand.friends">Friends</a>
</h5>
<div class="informaltable"><table class="table">
<colgroup>
<col>
<col>
</colgroup>
<thead><tr>
<th>
<p>
Name
</p>
</th>
<th>
<p>
Description
</p>
</th>
</tr></thead>
<tbody>
<tr>
<td>
<p>
<a class="link" href="io_context__strand/operator_not__eq_.html" title="io_context::strand::operator!="><span class="bold"><strong>operator!=</strong></span></a>
</p>
</td>
<td>
<p>
Compare two strands for inequality.
</p>
</td>
</tr>
<tr>
<td>
<p>
<a class="link" href="io_context__strand/operator_eq__eq_.html" title="io_context::strand::operator=="><span class="bold"><strong>operator==</strong></span></a>
</p>
</td>
<td>
<p>
Compare two strands for equality.
</p>
</td>
</tr>
</tbody>
</table></div>
<p>
The <a class="link" href="io_context__strand.html" title="io_context::strand"><code class="computeroutput"><span class="identifier">io_context</span><span class="special">::</span><span class="identifier">strand</span></code></a>
class provides the ability to post and dispatch handlers with the guarantee
that none of those handlers will execute concurrently.
</p>
<h5>
<a name="asio.reference.io_context__strand.h2"></a>
<span><a name="asio.reference.io_context__strand.order_of_handler_invocation"></a></span><a class="link" href="io_context__strand.html#asio.reference.io_context__strand.order_of_handler_invocation">Order
of handler invocation</a>
</h5>
<p>
Given:
</p>
<div class="itemizedlist"><ul class="itemizedlist" type="disc">
<li class="listitem">
a strand object <code class="computeroutput"><span class="identifier">s</span></code>
</li>
<li class="listitem">
an object <code class="computeroutput"><span class="identifier">a</span></code> meeting completion
handler requirements
</li>
<li class="listitem">
an object <code class="computeroutput"><span class="identifier">a1</span></code> which is
an arbitrary copy of <code class="computeroutput"><span class="identifier">a</span></code>
made by the implementation
</li>
<li class="listitem">
an object <code class="computeroutput"><span class="identifier">b</span></code> meeting completion
handler requirements
</li>
<li class="listitem">
an object <code class="computeroutput"><span class="identifier">b1</span></code> which is
an arbitrary copy of <code class="computeroutput"><span class="identifier">b</span></code>
made by the implementation
</li>
</ul></div>
<p>
if any of the following conditions are true:
</p>
<div class="itemizedlist"><ul class="itemizedlist" type="disc">
<li class="listitem">
<code class="computeroutput"><span class="identifier">s</span><span class="special">.</span><span class="identifier">post</span><span class="special">(</span><span class="identifier">a</span><span class="special">)</span></code>
happens-before <code class="computeroutput"><span class="identifier">s</span><span class="special">.</span><span class="identifier">post</span><span class="special">(</span><span class="identifier">b</span><span class="special">)</span></code>
</li>
<li class="listitem">
<code class="computeroutput"><span class="identifier">s</span><span class="special">.</span><span class="identifier">post</span><span class="special">(</span><span class="identifier">a</span><span class="special">)</span></code>
happens-before <code class="computeroutput"><span class="identifier">s</span><span class="special">.</span><span class="identifier">dispatch</span><span class="special">(</span><span class="identifier">b</span><span class="special">)</span></code>,
where the latter is performed outside the strand
</li>
<li class="listitem">
<code class="computeroutput"><span class="identifier">s</span><span class="special">.</span><span class="identifier">dispatch</span><span class="special">(</span><span class="identifier">a</span><span class="special">)</span></code>
happens-before <code class="computeroutput"><span class="identifier">s</span><span class="special">.</span><span class="identifier">post</span><span class="special">(</span><span class="identifier">b</span><span class="special">)</span></code>,
where the former is performed outside the strand
</li>
<li class="listitem">
<code class="computeroutput"><span class="identifier">s</span><span class="special">.</span><span class="identifier">dispatch</span><span class="special">(</span><span class="identifier">a</span><span class="special">)</span></code>
happens-before <code class="computeroutput"><span class="identifier">s</span><span class="special">.</span><span class="identifier">dispatch</span><span class="special">(</span><span class="identifier">b</span><span class="special">)</span></code>,
where both are performed outside the strand
</li>
</ul></div>
<p>
then <code class="computeroutput"><span class="identifier">asio_handler_invoke</span><span class="special">(</span><span class="identifier">a1</span><span class="special">,</span>
<span class="special">&amp;</span><span class="identifier">a1</span><span class="special">)</span></code> happens-before <code class="computeroutput"><span class="identifier">asio_handler_invoke</span><span class="special">(</span><span class="identifier">b1</span><span class="special">,</span>
<span class="special">&amp;</span><span class="identifier">b1</span><span class="special">)</span></code>.
</p>
<p>
Note that in the following case:
</p>
<pre class="programlisting"><span class="identifier">async_op_1</span><span class="special">(...,</span> <span class="identifier">s</span><span class="special">.</span><span class="identifier">wrap</span><span class="special">(</span><span class="identifier">a</span><span class="special">));</span>
<span class="identifier">async_op_2</span><span class="special">(...,</span> <span class="identifier">s</span><span class="special">.</span><span class="identifier">wrap</span><span class="special">(</span><span class="identifier">b</span><span class="special">));</span>
</pre>
<p>
the completion of the first async operation will perform <code class="computeroutput"><span class="identifier">s</span><span class="special">.</span><span class="identifier">dispatch</span><span class="special">(</span><span class="identifier">a</span><span class="special">)</span></code>,
and the second will perform <code class="computeroutput"><span class="identifier">s</span><span class="special">.</span><span class="identifier">dispatch</span><span class="special">(</span><span class="identifier">b</span><span class="special">)</span></code>,
but the order in which those are performed is unspecified. That is, you cannot
state whether one happens-before the other. Therefore none of the above conditions
are met and no ordering guarantee is made.
</p>
<h5>
<a name="asio.reference.io_context__strand.h3"></a>
<span><a name="asio.reference.io_context__strand.remarks"></a></span><a class="link" href="io_context__strand.html#asio.reference.io_context__strand.remarks">Remarks</a>
</h5>
<p>
The implementation makes no guarantee that handlers posted or dispatched
through different <code class="computeroutput"><span class="identifier">strand</span></code>
objects will be invoked concurrently.
</p>
<h5>
<a name="asio.reference.io_context__strand.h4"></a>
<span><a name="asio.reference.io_context__strand.thread_safety"></a></span><a class="link" href="io_context__strand.html#asio.reference.io_context__strand.thread_safety">Thread
Safety</a>
</h5>
<p>
<span class="emphasis"><em>Distinct</em></span> <span class="emphasis"><em>objects:</em></span> Safe.
</p>
<p>
<span class="emphasis"><em>Shared</em></span> <span class="emphasis"><em>objects:</em></span> Safe.
</p>
<h5>
<a name="asio.reference.io_context__strand.h5"></a>
<span><a name="asio.reference.io_context__strand.requirements"></a></span><a class="link" href="io_context__strand.html#asio.reference.io_context__strand.requirements">Requirements</a>
</h5>
<p>
<span class="emphasis"><em>Header: </em></span><code class="literal">asio/io_context_strand.hpp</code>
</p>
<p>
<span class="emphasis"><em>Convenience header: </em></span><code class="literal">asio.hpp</code>
</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>)
</p>
</div>
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