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25 | <div class="section"> | |
26 | <div class="titlepage"><div><div><h3 class="title"> | |
27 | <a name="fiber.callbacks.data_or_exception"></a><a name="Data_or_Exception"></a><a class="link" href="data_or_exception.html" title="Data or Exception">Data | |
28 | or Exception</a> | |
29 | </h3></div></div></div> | |
30 | <p> | |
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. | |
33 | </p> | |
34 | <p> | |
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>: | |
38 | </p> | |
39 | <p> | |
40 | </p> | |
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> | |
74 | </pre> | |
75 | <p> | |
76 | </p> | |
77 | <p> | |
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. | |
80 | </p> | |
81 | </div> | |
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>) | |
87 | </p> | |
88 | </div></td> | |
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