1<html> 2<head> 3<meta http-equiv="Content-Type" content="text/html; charset=UTF-8"> 4<title>Data or Exception</title> 5<link rel="stylesheet" href="../../../../../../doc/src/boostbook.css" type="text/css"> 6<meta name="generator" content="DocBook XSL Stylesheets V1.79.1"> 7<link rel="home" href="../../index.html" title="Chapter 1. Fiber"> 8<link rel="up" href="../callbacks.html" title="Integrating Fibers with Asynchronous Callbacks"> 9<link rel="prev" href="return_errorcode_or_data.html" title="Return Errorcode or Data"> 10<link rel="next" href="success_error_virtual_methods.html" title="Success/Error Virtual Methods"> 11</head> 12<body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"> 13<table cellpadding="2" width="100%"><tr> 14<td valign="top"><img alt="Boost C++ Libraries" width="277" height="86" src="../../../../../../boost.png"></td> 15<td align="center"><a href="../../../../../../index.html">Home</a></td> 16<td align="center"><a href="../../../../../../libs/libraries.htm">Libraries</a></td> 17<td align="center"><a href="http://www.boost.org/users/people.html">People</a></td> 18<td align="center"><a href="http://www.boost.org/users/faq.html">FAQ</a></td> 19<td align="center"><a href="../../../../../../more/index.htm">More</a></td> 20</tr></table> 21<hr> 22<div class="spirit-nav"> 23<a accesskey="p" href="return_errorcode_or_data.html"><img src="../../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../callbacks.html"><img src="../../../../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../../index.html"><img src="../../../../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="success_error_virtual_methods.html"><img src="../../../../../../doc/src/images/next.png" alt="Next"></a> 24</div> 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> 89</tr></table> 90<hr> 91<div class="spirit-nav"> 92<a accesskey="p" href="return_errorcode_or_data.html"><img src="../../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../callbacks.html"><img src="../../../../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../../index.html"><img src="../../../../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="success_error_virtual_methods.html"><img src="../../../../../../doc/src/images/next.png" alt="Next"></a> 93</div> 94</body> 95</html> 96
