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| 24 | </div> | |
| 25 | <div class="section"> | |
| 26 | <div class="titlepage"><div><div><h4 class="title"> | |
| 27 | <a name="fiber.when_any.when_any.when_any__heterogeneous_types"></a><a class="link" href="when_any__heterogeneous_types.html" title="when_any, heterogeneous types">when_any, | |
| 28 | heterogeneous types</a> | |
| 29 | </h4></div></div></div> | |
| 30 | <p> | |
| 31 | We would be remiss to ignore the case in which the various task functions | |
| 32 | have distinct return types. That means that the value returned by the first | |
| 33 | of them might have any one of those types. We can express that with <a href="http://www.boost.org/doc/libs/release/doc/html/variant.html" target="_top">Boost.Variant</a>. | |
| 34 | </p> | |
| 35 | <p> | |
| 36 | To keep the example simple, we'll revert to pretending that none of them | |
| 37 | can throw an exception. That makes <code class="computeroutput"><span class="identifier">wait_first_value_het</span><span class="special">()</span></code> strongly resemble <a class="link" href="when_any__return_value.html#wait_first_value"><code class="computeroutput"><span class="identifier">wait_first_value</span><span class="special">()</span></code></a>. | |
| 38 | We can actually reuse <a class="link" href="when_any__return_value.html#wait_first_value_impl"><code class="computeroutput"><span class="identifier">wait_first_value_impl</span><span class="special">()</span></code></a>, | |
| 39 | merely passing <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">variant</span><span class="special"><</span><span class="identifier">T0</span><span class="special">,</span> <span class="identifier">T1</span><span class="special">,</span> <span class="special">...></span></code> as the channel's value type rather | |
| 40 | than the common <code class="computeroutput"><span class="identifier">T</span></code>! | |
| 41 | </p> | |
| 42 | <p> | |
| 43 | Naturally this could be extended to use <a class="link" href="when_any__produce_first_success.html#wait_first_success"><code class="computeroutput"><span class="identifier">wait_first_success</span><span class="special">()</span></code></a> | |
| 44 | semantics instead. | |
| 45 | </p> | |
| 46 | <p> | |
| 47 | </p> | |
| 48 | <pre class="programlisting"><span class="comment">// No need to break out the first Fn for interface function: let the compiler</span> | |
| 49 | <span class="comment">// complain if empty.</span> | |
| 50 | <span class="comment">// Our functions have different return types, and we might have to return any</span> | |
| 51 | <span class="comment">// of them. Use a variant, expanding std::result_of<Fn()>::type for each Fn in</span> | |
| 52 | <span class="comment">// parameter pack.</span> | |
| 53 | <span class="keyword">template</span><span class="special"><</span> <span class="keyword">typename</span> <span class="special">...</span> <span class="identifier">Fns</span> <span class="special">></span> | |
| 54 | <span class="identifier">boost</span><span class="special">::</span><span class="identifier">variant</span><span class="special"><</span> <span class="keyword">typename</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">result_of</span><span class="special"><</span> <span class="identifier">Fns</span><span class="special">()</span> <span class="special">>::</span><span class="identifier">type</span> <span class="special">...</span> <span class="special">></span> | |
| 55 | <span class="identifier">wait_first_value_het</span><span class="special">(</span> <span class="identifier">Fns</span> <span class="special">&&</span> <span class="special">...</span> <span class="identifier">functions</span><span class="special">)</span> <span class="special">{</span> | |
| 56 | <span class="comment">// Use unbounded_channel<boost::variant<T1, T2, ...>>; see remarks above.</span> | |
| 57 | <span class="keyword">typedef</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">variant</span><span class="special"><</span> <span class="keyword">typename</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">result_of</span><span class="special"><</span> <span class="identifier">Fns</span><span class="special">()</span> <span class="special">>::</span><span class="identifier">type</span> <span class="special">...</span> <span class="special">></span> <span class="identifier">return_t</span><span class="special">;</span> | |
| 58 | <span class="keyword">typedef</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">fibers</span><span class="special">::</span><span class="identifier">unbounded_channel</span><span class="special"><</span> <span class="identifier">return_t</span> <span class="special">></span> <span class="identifier">channel_t</span><span class="special">;</span> | |
| 59 | <span class="keyword">auto</span> <span class="identifier">channelp</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">channel_t</span> <span class="special">>()</span> <span class="special">);</span> | |
| 60 | <span class="comment">// launch all the relevant fibers</span> | |
| 61 | <span class="identifier">wait_first_value_impl</span><span class="special"><</span> <span class="identifier">return_t</span> <span class="special">>(</span> <span class="identifier">channelp</span><span class="special">,</span> | |
| 62 | <span class="identifier">std</span><span class="special">::</span><span class="identifier">forward</span><span class="special"><</span> <span class="identifier">Fns</span> <span class="special">>(</span> <span class="identifier">functions</span><span class="special">)</span> <span class="special">...</span> <span class="special">);</span> | |
| 63 | <span class="comment">// retrieve the first value</span> | |
| 64 | <span class="identifier">return_t</span> <span class="identifier">value</span><span class="special">(</span> <span class="identifier">channelp</span><span class="special">-></span><span class="identifier">value_pop</span><span class="special">()</span> <span class="special">);</span> | |
| 65 | <span class="comment">// close the channel: no subsequent push() has to succeed</span> | |
| 66 | <span class="identifier">channelp</span><span class="special">-></span><span class="identifier">close</span><span class="special">();</span> | |
| 67 | <span class="keyword">return</span> <span class="identifier">value</span><span class="special">;</span> | |
| 68 | <span class="special">}</span> | |
| 69 | </pre> | |
| 70 | <p> | |
| 71 | </p> | |
| 72 | <p> | |
| 73 | It might be called like this: | |
| 74 | </p> | |
| 75 | <p> | |
| 76 | </p> | |
| 77 | <pre class="programlisting"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">variant</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="keyword">double</span><span class="special">,</span> <span class="keyword">int</span> <span class="special">></span> <span class="identifier">result</span> <span class="special">=</span> | |
| 78 | <span class="identifier">wait_first_value_het</span><span class="special">(</span> | |
| 79 | <span class="special">[](){</span> <span class="keyword">return</span> <span class="identifier">sleeper</span><span class="special">(</span><span class="string">"wfvh_third"</span><span class="special">,</span> <span class="number">150</span><span class="special">);</span> <span class="special">},</span> | |
| 80 | <span class="special">[](){</span> <span class="keyword">return</span> <span class="identifier">sleeper</span><span class="special">(</span><span class="number">3.14</span><span class="special">,</span> <span class="number">100</span><span class="special">);</span> <span class="special">},</span> | |
| 81 | <span class="special">[](){</span> <span class="keyword">return</span> <span class="identifier">sleeper</span><span class="special">(</span><span class="number">17</span><span class="special">,</span> <span class="number">50</span><span class="special">);</span> <span class="special">});</span> | |
| 82 | <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special"><<</span> <span class="string">"wait_first_value_het() => "</span> <span class="special"><<</span> <span class="identifier">result</span> <span class="special"><<</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span> | |
| 83 | <span class="identifier">assert</span><span class="special">(</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">get</span><span class="special"><</span> <span class="keyword">int</span> <span class="special">>(</span> <span class="identifier">result</span><span class="special">)</span> <span class="special">==</span> <span class="number">17</span><span class="special">);</span> | |
| 84 | </pre> | |
| 85 | <p> | |
| 86 | </p> | |
| 87 | </div> | |
| 88 | <table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr> | |
| 89 | <td align="left"></td> | |
| 90 | <td align="right"><div class="copyright-footer">Copyright © 2013 Oliver Kowalke<p> | |
| 91 | Distributed under the Boost Software License, Version 1.0. (See accompanying | |
| 92 | 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>) | |
| 93 | </p> | |
| 94 | </div></td> | |
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