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  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">&lt;</span> <span class="identifier">Response</span> <span class="special">&gt;</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">&amp;</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">&amp;</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">&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span> <span class="special">&gt;</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">&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span> <span class="special">&gt;</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">&lt;&gt;</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">&amp;</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">&lt;</span> <span class="identifier">PromiseResponse</span> <span class="special">&gt;()</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">&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span> <span class="special">&gt;</span> <span class="identifier">future</span><span class="special">(</span> <span class="identifier">promisep</span><span class="special">-&gt;</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 &#169; 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>
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