doc/html/boost_asio/reference/strand.html


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<title>strand</title>
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<div class="section">
<div class="titlepage"><div><div><h3 class="title">
<a name="boost_asio.reference.strand"></a><a class="link" href="strand.html" title="strand">strand</a>
</h3></div></div></div>
<p>
        <a class="indexterm" name="idp201141120"></a> 
(Deprecated: Use <a class="link" href="io_service__strand.html" title="io_service::strand"><code class="computeroutput"><span class="identifier">io_service</span><span class="special">::</span><span class="identifier">strand</span></code></a>.) Typedef for backwards compatibility.
      </p>
<pre class="programlisting"><span class="keyword">typedef</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">asio</span><span class="special">::</span><span class="identifier">io_service</span><span class="special">::</span><span class="identifier">strand</span> <span class="identifier">strand</span><span class="special">;</span>
</pre>
<h5>
<a name="boost_asio.reference.strand.h0"></a>
        <span class="phrase"><a name="boost_asio.reference.strand.member_functions"></a></span><a class="link" href="strand.html#boost_asio.reference.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_service__strand/dispatch.html" title="io_service::strand::dispatch"><span class="bold"><strong>dispatch</strong></span></a>
                </p>
              </td>
<td>
                <p>
                  Request the strand to invoke the given handler.
                </p>
              </td>
</tr>
<tr>
<td>
                <p>
                  <a class="link" href="io_service__strand/get_io_service.html" title="io_service::strand::get_io_service"><span class="bold"><strong>get_io_service</strong></span></a>
                </p>
              </td>
<td>
                <p>
                  Get the io_service associated with the strand.
                </p>
              </td>
</tr>
<tr>
<td>
                <p>
                  <a class="link" href="io_service__strand/post.html" title="io_service::strand::post"><span class="bold"><strong>post</strong></span></a>
                </p>
              </td>
<td>
                <p>
                  Request the strand to invoke the given handler and return immediately.
                </p>
              </td>
</tr>
<tr>
<td>
                <p>
                  <a class="link" href="io_service__strand/running_in_this_thread.html" title="io_service::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_service__strand/strand.html" title="io_service::strand::strand"><span class="bold"><strong>strand</strong></span></a>
                </p>
              </td>
<td>
                <p>
                  Constructor.
                </p>
              </td>
</tr>
<tr>
<td>
                <p>
                  <a class="link" href="io_service__strand/wrap.html" title="io_service::strand::wrap"><span class="bold"><strong>wrap</strong></span></a>
                </p>
              </td>
<td>
                <p>
                  Create a new handler that automatically dispatches the wrapped
                  handler on the strand.
                </p>
              </td>
</tr>
<tr>
<td>
                <p>
                  <a class="link" href="io_service__strand/_strand.html" title="io_service::strand::~strand"><span class="bold"><strong>~strand</strong></span></a>
                </p>
              </td>
<td>
                <p>
                  Destructor.
                </p>
              </td>
</tr>
</tbody>
</table></div>
<p>
        The <a class="link" href="io_service__strand.html" title="io_service::strand"><code class="computeroutput"><span class="identifier">io_service</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="boost_asio.reference.strand.h1"></a>
        <span class="phrase"><a name="boost_asio.reference.strand.order_of_handler_invocation"></a></span><a class="link" href="strand.html#boost_asio.reference.strand.order_of_handler_invocation">Order of
        handler invocation</a>
      </h5>
<p>
        Given:
      </p>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-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" style="list-style-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="boost_asio.reference.strand.h2"></a>
        <span class="phrase"><a name="boost_asio.reference.strand.remarks"></a></span><a class="link" href="strand.html#boost_asio.reference.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="boost_asio.reference.strand.h3"></a>
        <span class="phrase"><a name="boost_asio.reference.strand.thread_safety"></a></span><a class="link" href="strand.html#boost_asio.reference.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="boost_asio.reference.strand.h4"></a>
        <span class="phrase"><a name="boost_asio.reference.strand.requirements"></a></span><a class="link" href="strand.html#boost_asio.reference.strand.requirements">Requirements</a>
      </h5>
<p>
        <span class="emphasis"><em>Header: </em></span><code class="literal">boost/asio/strand.hpp</code>
      </p>
<p>
        <span class="emphasis"><em>Convenience header: </em></span><code class="literal">boost/asio.hpp</code>
      </p>
</div>
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<td align="right"><div class="copyright-footer">Copyright &#169; 2003-2017 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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