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| 4 | <title>Success/Error Virtual Methods</title> | |
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| 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> | |
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