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26 <div class=
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27 <a name=
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29 </h3></div></div></div>
31 But a more natural API for a function that obtains data is to return only
32 the data on success, throwing an exception on error.
35 As with
<code class=
"computeroutput"><span class=
"identifier">write
</span><span class=
"special">()
</span></code>
36 above, it
’s certainly possible to code a
<code class=
"computeroutput"><span class=
"identifier">read
</span><span class=
"special">()
</span></code> wrapper in terms of
<code class=
"computeroutput"><span class=
"identifier">read_ec
</span><span class=
"special">()
</span></code>. But since a given application is unlikely
37 to need both, let
’s code
<code class=
"computeroutput"><span class=
"identifier">read
</span><span class=
"special">()
</span></code> from scratch, leveraging
<a class=
"link" href=
"../synchronization/futures/promise.html#promise_set_exception"><code class=
"computeroutput">promise::set_exception()
</code></a>:
41 <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>
42 <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>
43 <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">promise
</span><span class=
"special">.
</span><span class=
"identifier">get_future
</span><span class=
"special">()
</span> <span class=
"special">);
</span>
44 <span class=
"comment">// Both 'promise' and 'future' will survive until our lambda has been
</span>
45 <span class=
"comment">// called.
</span>
46 <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>
47 <span class=
"identifier">api
</span><span class=
"special">.
</span><span class=
"identifier">init_read
</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=
"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=
"keyword">mutable
</span> <span class=
"special">{
</span>
48 <span class=
"keyword">if
</span> <span class=
"special">(
</span> <span class=
"special">!
</span> <span class=
"identifier">ec
</span><span class=
"special">)
</span> <span class=
"special">{
</span>
49 <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>
50 <span class=
"special">}
</span> <span class=
"keyword">else
</span> <span class=
"special">{
</span>
51 <span class=
"identifier">promise
</span><span class=
"special">.
</span><span class=
"identifier">set_exception
</span><span class=
"special">(
</span>
52 <span class=
"identifier">std
</span><span class=
"special">::
</span><span class=
"identifier">make_exception_ptr
</span><span class=
"special">(
</span>
53 <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>
54 <span class=
"special">}
</span>
55 <span class=
"special">});
</span>
56 <span class=
"preprocessor">#else
</span> <span class=
"comment">// defined(BOOST_NO_CXX14_INITIALIZED_LAMBDA_CAPTURES)
</span>
57 <span class=
"identifier">api
</span><span class=
"special">.
</span><span class=
"identifier">init_read
</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">std
</span><span class=
"special">::
</span><span class=
"identifier">string
</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=
"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=
"keyword">mutable
</span> <span class=
"special">{
</span>
60 <span class=
"keyword">if
</span> <span class=
"special">(
</span> <span class=
"special">!
</span> <span class=
"identifier">ec
</span><span class=
"special">)
</span> <span class=
"special">{
</span>
61 <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>
62 <span class=
"special">}
</span> <span class=
"keyword">else
</span> <span class=
"special">{
</span>
63 <span class=
"identifier">promise
</span><span class=
"special">.
</span><span class=
"identifier">set_exception
</span><span class=
"special">(
</span>
64 <span class=
"identifier">std
</span><span class=
"special">::
</span><span class=
"identifier">make_exception_ptr
</span><span class=
"special">(
</span>
65 <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>
66 <span class=
"special">}
</span>
67 <span class=
"special">},
</span>
68 <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>
69 <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>
70 <span class=
"identifier">std
</span><span class=
"special">::
</span><span class=
"identifier">placeholders
</span><span class=
"special">::
</span><span class=
"identifier">_2
</span><span class=
"special">)
</span> <span class=
"special">);
</span>
71 <span class=
"preprocessor">#endif
</span> <span class=
"comment">// BOOST_NO_CXX14_INITIALIZED_LAMBDA_CAPTURES
</span>
72 <span class=
"keyword">return
</span> <span class=
"identifier">future
</span><span class=
"special">.
</span><span class=
"identifier">get
</span><span class=
"special">();
</span>
73 <span class=
"special">}
</span>
78 <a class=
"link" href=
"../synchronization/futures/future.html#future_get"><code class=
"computeroutput">future::get()
</code></a> will do the right thing, either returning
<code class=
"computeroutput"><span class=
"identifier">std
</span><span class=
"special">::
</span><span class=
"identifier">string
</span></code>
79 or throwing an exception.
82 <table xmlns:
rev=
"http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width=
"100%"><tr>
83 <td align=
"left"></td>
84 <td align=
"right"><div class=
"copyright-footer">Copyright
© 2013 Oliver Kowalke
<p>
85 Distributed under the Boost Software License, Version
1.0. (See accompanying
86 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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"spirit-nav">
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