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