ceph/src/boost/libs/fiber/doc/html/fiber/callbacks/return_errorcode.html

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  26 <div class="titlepage"><div><div><h3 class="title">
  27 <a name="fiber.callbacks.return_errorcode"></a><a class="link" href="return_errorcode.html" title="Return Errorcode">Return Errorcode</a>
  28 </h3></div></div></div>
  29 <p>
  30         The <code class="computeroutput"><span class="identifier">AsyncAPI</span><span class="special">::</span><span class="identifier">init_write</span><span class="special">()</span></code>
  31         callback passes only an <code class="computeroutput"><span class="identifier">errorcode</span></code>.
  32         If we simply want the blocking wrapper to return that <code class="computeroutput"><span class="identifier">errorcode</span></code>,
  33         this is an extremely straightforward use of <a class="link" href="../synchronization/futures/promise.html#class_promise"><code class="computeroutput">promise&lt;&gt;</code></a> and
  34         <a class="link" href="../synchronization/futures/future.html#class_future"><code class="computeroutput">future&lt;&gt;</code></a>:
  35       </p>
  36 <p>
  37 </p>
  38 <pre class="programlisting"><span class="identifier">AsyncAPI</span><span class="special">::</span><span class="identifier">errorcode</span> <span class="identifier">write_ec</span><span class="special">(</span> <span class="identifier">AsyncAPI</span> <span class="special">&amp;</span> <span class="identifier">api</span><span class="special">,</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span> <span class="keyword">const</span><span class="special">&amp;</span> <span class="identifier">data</span><span class="special">)</span> <span class="special">{</span>
  39     <span class="identifier">boost</span><span class="special">::</span><span class="identifier">fibers</span><span class="special">::</span><span class="identifier">promise</span><span class="special">&lt;</span> <span class="identifier">AsyncAPI</span><span class="special">::</span><span class="identifier">errorcode</span> <span class="special">&gt;</span> <span class="identifier">promise</span><span class="special">;</span>
  40     <span class="identifier">boost</span><span class="special">::</span><span class="identifier">fibers</span><span class="special">::</span><span class="identifier">future</span><span class="special">&lt;</span> <span class="identifier">AsyncAPI</span><span class="special">::</span><span class="identifier">errorcode</span> <span class="special">&gt;</span> <span class="identifier">future</span><span class="special">(</span> <span class="identifier">promise</span><span class="special">.</span><span class="identifier">get_future</span><span class="special">()</span> <span class="special">);</span>
  41     <span class="comment">// In general, even though we block waiting for future::get() and therefore</span>
  42     <span class="comment">// won't destroy 'promise' until promise::set_value() has been called, we</span>
  43     <span class="comment">// are advised that with threads it's possible for ~promise() to be</span>
  44     <span class="comment">// entered before promise::set_value() has returned. While that shouldn't</span>
  45     <span class="comment">// happen with fibers::promise, a robust way to deal with the lifespan</span>
  46     <span class="comment">// issue is to bind 'promise' into our lambda. Since promise is move-only,</span>
  47     <span class="comment">// use initialization capture.</span>
  48 <span class="preprocessor">#if</span> <span class="special">!</span> <span class="identifier">defined</span><span class="special">(</span><span class="identifier">BOOST_NO_CXX14_INITIALIZED_LAMBDA_CAPTURES</span><span class="special">)</span>
  49     <span class="identifier">api</span><span class="special">.</span><span class="identifier">init_write</span><span class="special">(</span>
  50         <span class="identifier">data</span><span class="special">,</span>
  51         <span class="special">[</span><span class="identifier">promise</span><span class="special">=</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">move</span><span class="special">(</span> <span class="identifier">promise</span><span class="special">)](</span> <span class="identifier">AsyncAPI</span><span class="special">::</span><span class="identifier">errorcode</span> <span class="identifier">ec</span><span class="special">)</span> <span class="keyword">mutable</span> <span class="special">{</span>
  52                             <span class="identifier">promise</span><span class="special">.</span><span class="identifier">set_value</span><span class="special">(</span> <span class="identifier">ec</span><span class="special">);</span>
  53                   <span class="special">});</span>
  54
  55 <span class="preprocessor">#else</span> <span class="comment">// defined(BOOST_NO_CXX14_INITIALIZED_LAMBDA_CAPTURES)</span>
  56     <span class="identifier">api</span><span class="special">.</span><span class="identifier">init_write</span><span class="special">(</span>
  57         <span class="identifier">data</span><span class="special">,</span>
  58         <span class="identifier">std</span><span class="special">::</span><span class="identifier">bind</span><span class="special">([](</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">fibers</span><span class="special">::</span><span class="identifier">promise</span><span class="special">&lt;</span> <span class="identifier">AsyncAPI</span><span class="special">::</span><span class="identifier">errorcode</span> <span class="special">&gt;</span> <span class="special">&amp;</span> <span class="identifier">promise</span><span class="special">,</span>
  59                      <span class="identifier">AsyncAPI</span><span class="special">::</span><span class="identifier">errorcode</span> <span class="identifier">ec</span><span class="special">)</span> <span class="special">{</span>
  60             <span class="identifier">promise</span><span class="special">.</span><span class="identifier">set_value</span><span class="special">(</span> <span class="identifier">ec</span><span class="special">);</span>
  61         <span class="special">},</span>
  62         <span class="identifier">std</span><span class="special">::</span><span class="identifier">move</span><span class="special">(</span> <span class="identifier">promise</span><span class="special">),</span>
  63         <span class="identifier">std</span><span class="special">::</span><span class="identifier">placeholders</span><span class="special">::</span><span class="identifier">_1</span><span class="special">)</span> <span class="special">);</span>
  64 <span class="preprocessor">#endif</span> <span class="comment">// BOOST_NO_CXX14_INITIALIZED_LAMBDA_CAPTURES</span>
  65
  66     <span class="keyword">return</span> <span class="identifier">future</span><span class="special">.</span><span class="identifier">get</span><span class="special">();</span>
  67 <span class="special">}</span>
  68 </pre>
  69 <p>
  70       </p>
  71 <p>
  72         All we have to do is:
  73       </p>
  74 <div class="orderedlist"><ol class="orderedlist" type="1">
  75 <li class="listitem">
  76             Instantiate a <code class="computeroutput"><span class="identifier">promise</span><span class="special">&lt;&gt;</span></code> of correct type.
  77           </li>
  78 <li class="listitem">
  79             Obtain its <code class="computeroutput"><span class="identifier">future</span><span class="special">&lt;&gt;</span></code>.
  80           </li>
  81 <li class="listitem">
  82             Arrange for the callback to call <a class="link" href="../synchronization/futures/promise.html#promise_set_value"><code class="computeroutput">promise::set_value()</code></a>.
  83           </li>
  84 <li class="listitem">
  85             Block on <a class="link" href="../synchronization/futures/future.html#future_get"><code class="computeroutput">future::get()</code></a>.
  86           </li>
  87 </ol></div>
  88 <div class="note"><table border="0" summary="Note">
  89 <tr>
  90 <td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../../../../doc/src/images/note.png"></td>
  91 <th align="left">Note</th>
  92 </tr>
  93 <tr><td align="left" valign="top"><p>
  94           This tactic for resuming a pending fiber works even if the callback is
  95           called on a different thread than the one on which the initiating fiber
  96           is running. In fact, <a href="../../../../examples/adapt_callbacks.cpp" target="_top">the
  97           example program&#8217;s</a> dummy <code class="computeroutput"><span class="identifier">AsyncAPI</span></code>
  98           implementation illustrates that: it simulates async I/O by launching a
  99           new thread that sleeps briefly and then calls the relevant callback.
 100         </p></td></tr>
 101 </table></div>
 102 </div>
 103 <table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
 104 <td align="left"></td>
 105 <td align="right"><div class="copyright-footer">Copyright &#169; 2013 Oliver Kowalke<p>
 106         Distributed under the Boost Software License, Version 1.0. (See accompanying
 107         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>)
 108       </p>
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