1<html> 2<head> 3<meta http-equiv="Content-Type" content="text/html; charset=UTF-8"> 4<title>Return Errorcode</title> 5<link rel="stylesheet" href="../../../../../../doc/src/boostbook.css" type="text/css"> 6<meta name="generator" content="DocBook XSL Stylesheets V1.79.1"> 7<link rel="home" href="../../index.html" title="Chapter 1. Fiber"> 8<link rel="up" href="../callbacks.html" title="Integrating Fibers with Asynchronous Callbacks"> 9<link rel="prev" href="overview.html" title="Overview"> 10<link rel="next" href="success_or_exception.html" title="Success or Exception"> 11</head> 12<body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"> 13<table cellpadding="2" width="100%"><tr> 14<td valign="top"><img alt="Boost C++ Libraries" width="277" height="86" src="../../../../../../boost.png"></td> 15<td align="center"><a href="../../../../../../index.html">Home</a></td> 16<td align="center"><a href="../../../../../../libs/libraries.htm">Libraries</a></td> 17<td align="center"><a href="http://www.boost.org/users/people.html">People</a></td> 18<td align="center"><a href="http://www.boost.org/users/faq.html">FAQ</a></td> 19<td align="center"><a href="../../../../../../more/index.htm">More</a></td> 20</tr></table> 21<hr> 22<div class="spirit-nav"> 23<a accesskey="p" href="overview.html"><img src="../../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../callbacks.html"><img src="../../../../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../../index.html"><img src="../../../../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="success_or_exception.html"><img src="../../../../../../doc/src/images/next.png" alt="Next"></a> 24</div> 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> 110</tr></table> 111<hr> 112<div class="spirit-nav"> 113<a accesskey="p" href="overview.html"><img src="../../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../callbacks.html"><img src="../../../../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../../index.html"><img src="../../../../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="success_or_exception.html"><img src="../../../../../../doc/src/images/next.png" alt="Next"></a> 114</div> 115</body> 116</html> 117
