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26 <div class=
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27 <a name=
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"Success/Error Virtual Methods">Success/Error
29 </h3></div></div></div>
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:
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>
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>
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>
55 Now the
<code class=
"computeroutput"><span class=
"identifier">AsyncAPI
</span></code> operation
56 might look more like this:
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>
66 We can address this by writing a one-size-fits-all
<code class=
"computeroutput"><span class=
"identifier">PromiseResponse
</span></code>:
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>
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>
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>
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>
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>:
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>
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>.
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>)
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