1<html> 2<head> 3<meta http-equiv="Content-Type" content="text/html; charset=UTF-8"> 4<title>Success/Error Virtual Methods</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="data_or_exception.html" title="Data or Exception"> 10<link rel="next" href="then_there_s____boost_asio__.html" title="Then There’s Boost.Asio"> 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="data_or_exception.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="then_there_s____boost_asio__.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.success_error_virtual_methods"></a><a class="link" href="success_error_virtual_methods.html" title="Success/Error Virtual Methods">Success/Error 28 Virtual Methods</a> 29</h3></div></div></div> 30<p> 31 One classic approach to completion notification is to define an abstract 32 base class with <code class="computeroutput"><span class="identifier">success</span><span class="special">()</span></code> 33 and <code class="computeroutput"><span class="identifier">error</span><span class="special">()</span></code> 34 methods. Code wishing to perform async I/O must derive a subclass, override 35 each of these methods and pass the async operation a pointer to a subclass 36 instance. The abstract base class might look like this: 37 </p> 38<p> 39</p> 40<pre class="programlisting"><span class="comment">// every async operation receives a subclass instance of this abstract base</span> 41<span class="comment">// class through which to communicate its result</span> 42<span class="keyword">struct</span> <span class="identifier">Response</span> <span class="special">{</span> 43 <span class="keyword">typedef</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">shared_ptr</span><span class="special"><</span> <span class="identifier">Response</span> <span class="special">></span> <span class="identifier">ptr</span><span class="special">;</span> 44 45 <span class="comment">// called if the operation succeeds</span> 46 <span class="keyword">virtual</span> <span class="keyword">void</span> <span class="identifier">success</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> <span class="number">0</span><span class="special">;</span> 47 48 <span class="comment">// called if the operation fails</span> 49 <span class="keyword">virtual</span> <span class="keyword">void</span> <span class="identifier">error</span><span class="special">(</span> <span class="identifier">AsyncAPIBase</span><span class="special">::</span><span class="identifier">errorcode</span> <span class="identifier">ec</span><span class="special">)</span> <span class="special">=</span> <span class="number">0</span><span class="special">;</span> 50<span class="special">};</span> 51</pre> 52<p> 53 </p> 54<p> 55 Now the <code class="computeroutput"><span class="identifier">AsyncAPI</span></code> operation 56 might look more like this: 57 </p> 58<p> 59</p> 60<pre class="programlisting"><span class="comment">// derive Response subclass, instantiate, pass Response::ptr</span> 61<span class="keyword">void</span> <span class="identifier">init_read</span><span class="special">(</span> <span class="identifier">Response</span><span class="special">::</span><span class="identifier">ptr</span><span class="special">);</span> 62</pre> 63<p> 64 </p> 65<p> 66 We can address this by writing a one-size-fits-all <code class="computeroutput"><span class="identifier">PromiseResponse</span></code>: 67 </p> 68<p> 69</p> 70<pre class="programlisting"><span class="keyword">class</span> <span class="identifier">PromiseResponse</span><span class="special">:</span> <span class="keyword">public</span> <span class="identifier">Response</span> <span class="special">{</span> 71<span class="keyword">public</span><span class="special">:</span> 72 <span class="comment">// called if the operation succeeds</span> 73 <span class="keyword">virtual</span> <span class="keyword">void</span> <span class="identifier">success</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> 74 <span class="identifier">promise_</span><span class="special">.</span><span class="identifier">set_value</span><span class="special">(</span> <span class="identifier">data</span><span class="special">);</span> 75 <span class="special">}</span> 76 77 <span class="comment">// called if the operation fails</span> 78 <span class="keyword">virtual</span> <span class="keyword">void</span> <span class="identifier">error</span><span class="special">(</span> <span class="identifier">AsyncAPIBase</span><span class="special">::</span><span class="identifier">errorcode</span> <span class="identifier">ec</span><span class="special">)</span> <span class="special">{</span> 79 <span class="identifier">promise_</span><span class="special">.</span><span class="identifier">set_exception</span><span class="special">(</span> 80 <span class="identifier">std</span><span class="special">::</span><span class="identifier">make_exception_ptr</span><span class="special">(</span> 81 <span class="identifier">make_exception</span><span class="special">(</span><span class="string">"read"</span><span class="special">,</span> <span class="identifier">ec</span><span class="special">)</span> <span class="special">)</span> <span class="special">);</span> 82 <span class="special">}</span> 83 84 <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">std</span><span class="special">::</span><span class="identifier">string</span> <span class="special">></span> <span class="identifier">get_future</span><span class="special">()</span> <span class="special">{</span> 85 <span class="keyword">return</span> <span class="identifier">promise_</span><span class="special">.</span><span class="identifier">get_future</span><span class="special">();</span> 86 <span class="special">}</span> 87 88<span class="keyword">private</span><span class="special">:</span> 89 <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">std</span><span class="special">::</span><span class="identifier">string</span> <span class="special">></span> <span class="identifier">promise_</span><span class="special">;</span> 90<span class="special">};</span> 91</pre> 92<p> 93 </p> 94<p> 95 Now we can simply obtain the <code class="computeroutput"><span class="identifier">future</span><span class="special"><></span></code> from that <code class="computeroutput"><span class="identifier">PromiseResponse</span></code> 96 and wait on its <code class="computeroutput"><span class="identifier">get</span><span class="special">()</span></code>: 97 </p> 98<p> 99</p> 100<pre class="programlisting"><span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span> <span class="identifier">read</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="special">{</span> 101 <span class="comment">// Because init_read() requires a shared_ptr, we must allocate our</span> 102 <span class="comment">// ResponsePromise on the heap, even though we know its lifespan.</span> 103 <span class="keyword">auto</span> <span class="identifier">promisep</span><span class="special">(</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">make_shared</span><span class="special"><</span> <span class="identifier">PromiseResponse</span> <span class="special">>()</span> <span class="special">);</span> 104 <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">std</span><span class="special">::</span><span class="identifier">string</span> <span class="special">></span> <span class="identifier">future</span><span class="special">(</span> <span class="identifier">promisep</span><span class="special">-></span><span class="identifier">get_future</span><span class="special">()</span> <span class="special">);</span> 105 <span class="comment">// Both 'promisep' and 'future' will survive until our lambda has been</span> 106 <span class="comment">// called.</span> 107 <span class="identifier">api</span><span class="special">.</span><span class="identifier">init_read</span><span class="special">(</span> <span class="identifier">promisep</span><span class="special">);</span> 108 <span class="keyword">return</span> <span class="identifier">future</span><span class="special">.</span><span class="identifier">get</span><span class="special">();</span> 109<span class="special">}</span> 110</pre> 111<p> 112 </p> 113<p> 114 The source code above is found in <a href="../../../../examples/adapt_callbacks.cpp" target="_top">adapt_callbacks.cpp</a> 115 and <a href="../../../../examples/adapt_method_calls.cpp" target="_top">adapt_method_calls.cpp</a>. 116 </p> 117</div> 118<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr> 119<td align="left"></td> 120<td align="right"><div class="copyright-footer">Copyright © 2013 Oliver Kowalke<p> 121 Distributed under the Boost Software License, Version 1.0. (See accompanying 122 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>) 123 </p> 124</div></td> 125</tr></table> 126<hr> 127<div class="spirit-nav"> 128<a accesskey="p" href="data_or_exception.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="then_there_s____boost_asio__.html"><img src="../../../../../../doc/src/images/next.png" alt="Next"></a> 129</div> 130</body> 131</html> 132
