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25 | <div class="section"> | |
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<></code></a> and | |
34 | <a class="link" href="../synchronization/futures/future.html#class_future"><code class="computeroutput">future<></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">&</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">&</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"><</span> <span class="identifier">AsyncAPI</span><span class="special">::</span><span class="identifier">errorcode</span> <span class="special">></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"><</span> <span class="identifier">AsyncAPI</span><span class="special">::</span><span class="identifier">errorcode</span> <span class="special">></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"><</span> <span class="identifier">AsyncAPI</span><span class="special">::</span><span class="identifier">errorcode</span> <span class="special">></span> <span class="special">&</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"><></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"><></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’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 © 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> | |
109 | </div></td> | |
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