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| 4 | <title>Return Errorcode or Data</title> | |
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| 25 | <div class="section"> | |
| 26 | <div class="titlepage"><div><div><h3 class="title"> | |
| 27 | <a name="fiber.callbacks.return_errorcode_or_data"></a><a class="link" href="return_errorcode_or_data.html" title="Return Errorcode or Data">Return Errorcode | |
| 28 | or Data</a> | |
| 29 | </h3></div></div></div> | |
| 30 | <p> | |
| 31 | Things get a bit more interesting when the async operation’s callback passes | |
| 32 | multiple data items of interest. One approach would be to use <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">pair</span><span class="special"><></span></code> to capture both: | |
| 33 | </p> | |
| 34 | <p> | |
| 35 | </p> | |
| 36 | <pre class="programlisting"><span class="identifier">std</span><span class="special">::</span><span class="identifier">pair</span><span class="special"><</span> <span class="identifier">AsyncAPI</span><span class="special">::</span><span class="identifier">errorcode</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">read_ec</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> | |
| 37 | <span class="keyword">typedef</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">pair</span><span class="special"><</span> <span class="identifier">AsyncAPI</span><span class="special">::</span><span class="identifier">errorcode</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">result_pair</span><span class="special">;</span> | |
| 38 | <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">result_pair</span> <span class="special">></span> <span class="identifier">promise</span><span class="special">;</span> | |
| 39 | <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">result_pair</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> | |
| 40 | <span class="comment">// We promise that both 'promise' and 'future' will survive until our</span> | |
| 41 | <span class="comment">// lambda has been called.</span> | |
| 42 | <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> | |
| 43 | <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">std</span><span class="special">::</span><span class="identifier">move</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> | |
| 44 | <span class="identifier">promise</span><span class="special">.</span><span class="identifier">set_value</span><span class="special">(</span> <span class="identifier">result_pair</span><span class="special">(</span> <span class="identifier">ec</span><span class="special">,</span> <span class="identifier">data</span><span class="special">)</span> <span class="special">);</span> | |
| 45 | <span class="special">});</span> | |
| 46 | <span class="preprocessor">#else</span> <span class="comment">// defined(BOOST_NO_CXX14_INITIALIZED_LAMBDA_CAPTURES)</span> | |
| 47 | <span class="identifier">api</span><span class="special">.</span><span class="identifier">init_read</span><span class="special">(</span> | |
| 48 | <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">result_pair</span> <span class="special">></span> <span class="special">&</span> <span class="identifier">promise</span><span class="special">,</span> | |
| 49 | <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> | |
| 50 | <span class="identifier">promise</span><span class="special">.</span><span class="identifier">set_value</span><span class="special">(</span> <span class="identifier">result_pair</span><span class="special">(</span> <span class="identifier">ec</span><span class="special">,</span> <span class="identifier">data</span><span class="special">)</span> <span class="special">);</span> | |
| 51 | <span class="special">},</span> | |
| 52 | <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> | |
| 53 | <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> | |
| 54 | <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> | |
| 55 | <span class="preprocessor">#endif</span> <span class="comment">// BOOST_NO_CXX14_INITIALIZED_LAMBDA_CAPTURES</span> | |
| 56 | <span class="keyword">return</span> <span class="identifier">future</span><span class="special">.</span><span class="identifier">get</span><span class="special">();</span> | |
| 57 | <span class="special">}</span> | |
| 58 | </pre> | |
| 59 | <p> | |
| 60 | </p> | |
| 61 | <p> | |
| 62 | Once you bundle the interesting data in <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">pair</span><span class="special"><></span></code>, the code is effectively identical | |
| 63 | to <code class="computeroutput"><span class="identifier">write_ec</span><span class="special">()</span></code>. | |
| 64 | You can call it like this: | |
| 65 | </p> | |
| 66 | <p> | |
| 67 | </p> | |
| 68 | <pre class="programlisting"><span class="identifier">std</span><span class="special">::</span><span class="identifier">tie</span><span class="special">(</span> <span class="identifier">ec</span><span class="special">,</span> <span class="identifier">data</span><span class="special">)</span> <span class="special">=</span> <span class="identifier">read_ec</span><span class="special">(</span> <span class="identifier">api</span><span class="special">);</span> | |
| 69 | </pre> | |
| 70 | <p> | |
| 71 | </p> | |
| 72 | </div> | |
| 73 | <table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr> | |
| 74 | <td align="left"></td> | |
| 75 | <td align="right"><div class="copyright-footer">Copyright © 2013 Oliver Kowalke<p> | |
| 76 | Distributed under the Boost Software License, Version 1.0. (See accompanying | |
| 77 | 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>) | |
| 78 | </p> | |
| 79 | </div></td> | |
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