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| 4 | <title>when_any, produce first outcome, whether result or exception</title> | |
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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__produce_first_outcome__whether_result_or_exception"></a><a class="link" href="when_any__produce_first_outcome__whether_result_or_exception.html" title="when_any, produce first outcome, whether result or exception">when_any, | |
| 28 | produce first outcome, whether result or exception</a> | |
| 29 | </h4></div></div></div> | |
| 30 | <p> | |
| 31 | We may not be running in an environment in which we can guarantee no exception | |
| 32 | will be thrown by any of our task functions. In that case, the above implementations | |
| 33 | of <code class="computeroutput"><span class="identifier">wait_first_something</span><span class="special">()</span></code> would be naïve: as mentioned in <a class="link" href="../../fiber_mgmt.html#exceptions">the section on Fiber Management</a>, an uncaught | |
| 34 | exception in one of our task fibers would cause <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">terminate</span><span class="special">()</span></code> to be called. | |
| 35 | </p> | |
| 36 | <p> | |
| 37 | Let's at least ensure that such an exception would propagate to the fiber | |
| 38 | awaiting the first result. We can use <a class="link" href="../../synchronization/futures/future.html#class_future"><code class="computeroutput">future<></code></a> to transport | |
| 39 | either a return value or an exception. Therefore, we will change <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>'s <a class="link" href="../../synchronization/channels.html#class_unbounded_channel"><code class="computeroutput">unbounded_channel<></code></a> to | |
| 40 | hold <code class="computeroutput"><span class="identifier">future</span><span class="special"><</span> | |
| 41 | <span class="identifier">T</span> <span class="special">></span></code> | |
| 42 | items instead of simply <code class="computeroutput"><span class="identifier">T</span></code>. | |
| 43 | </p> | |
| 44 | <p> | |
| 45 | Once we have a <code class="computeroutput"><span class="identifier">future</span><span class="special"><></span></code> | |
| 46 | in hand, all we need do is call <a class="link" href="../../synchronization/futures/future.html#future_get"><code class="computeroutput">future::get()</code></a>, which will either | |
| 47 | return the value or rethrow the exception. | |
| 48 | </p> | |
| 49 | <p> | |
| 50 | <a name="wait_first_outcome"></a> | |
| 51 | </p> | |
| 52 | <pre class="programlisting"><span class="keyword">template</span><span class="special"><</span> <span class="keyword">typename</span> <span class="identifier">Fn</span><span class="special">,</span> <span class="keyword">typename</span> <span class="special">...</span> <span class="identifier">Fns</span> <span class="special">></span> | |
| 53 | <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">Fn</span><span class="special">()</span> <span class="special">>::</span><span class="identifier">type</span> | |
| 54 | <span class="identifier">wait_first_outcome</span><span class="special">(</span> <span class="identifier">Fn</span> <span class="special">&&</span> <span class="identifier">function</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> | |
| 55 | <span class="comment">// In this case, the value we pass through the channel is actually a</span> | |
| 56 | <span class="comment">// future -- which is already ready. future can carry either a value or an</span> | |
| 57 | <span class="comment">// exception.</span> | |
| 58 | <span class="keyword">typedef</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">Fn</span><span class="special">()</span> <span class="special">>::</span><span class="identifier">type</span> <span class="identifier">return_t</span><span class="special">;</span> | |
| 59 | <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">future</span><span class="special"><</span> <span class="identifier">return_t</span> <span class="special">></span> <span class="identifier">future_t</span><span class="special">;</span> | |
| 60 | <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">future_t</span> <span class="special">></span> <span class="identifier">channel_t</span><span class="special">;</span> | |
| 61 | <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> | |
| 62 | <span class="comment">// launch all the relevant fibers</span> | |
| 63 | <span class="identifier">wait_first_outcome_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> | |
| 64 | <span class="identifier">std</span><span class="special">::</span><span class="identifier">forward</span><span class="special"><</span> <span class="identifier">Fn</span> <span class="special">>(</span> <span class="identifier">function</span><span class="special">),</span> | |
| 65 | <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> | |
| 66 | <span class="comment">// retrieve the first future</span> | |
| 67 | <span class="identifier">future_t</span> <span class="identifier">future</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> | |
| 68 | <span class="comment">// close the channel: no subsequent push() has to succeed</span> | |
| 69 | <span class="identifier">channelp</span><span class="special">-></span><span class="identifier">close</span><span class="special">();</span> | |
| 70 | <span class="comment">// either return value or throw exception</span> | |
| 71 | <span class="keyword">return</span> <span class="identifier">future</span><span class="special">.</span><span class="identifier">get</span><span class="special">();</span> | |
| 72 | <span class="special">}</span> | |
| 73 | </pre> | |
| 74 | <p> | |
| 75 | </p> | |
| 76 | <p> | |
| 77 | So far so good — but there's a timing issue. How should we obtain the <code class="computeroutput"><span class="identifier">future</span><span class="special"><></span></code> | |
| 78 | to <a class="link" href="../../synchronization/channels.html#unbounded_channel_push"><code class="computeroutput">unbounded_channel::push()</code></a> on the channel? | |
| 79 | </p> | |
| 80 | <p> | |
| 81 | We could call <a class="link" href="../../synchronization/futures/future.html#fibers_async"><code class="computeroutput">fibers::async()</code></a>. That would certainly produce | |
| 82 | a <code class="computeroutput"><span class="identifier">future</span><span class="special"><></span></code> | |
| 83 | for the task function. The trouble is that it would return too quickly! | |
| 84 | We only want <code class="computeroutput"><span class="identifier">future</span><span class="special"><></span></code> | |
| 85 | items for <span class="emphasis"><em>completed</em></span> tasks on our <code class="computeroutput"><span class="identifier">unbounded_channel</span><span class="special"><></span></code>. In fact, we only want the <code class="computeroutput"><span class="identifier">future</span><span class="special"><></span></code> | |
| 86 | for the one that completes first. If each fiber launched by <code class="computeroutput"><span class="identifier">wait_first_outcome</span><span class="special">()</span></code> | |
| 87 | were to <code class="computeroutput"><span class="identifier">push</span><span class="special">()</span></code> | |
| 88 | the result of calling <code class="computeroutput"><span class="identifier">async</span><span class="special">()</span></code>, the channel would only ever report the | |
| 89 | result of the leftmost task item — <span class="emphasis"><em>not</em></span> the one that | |
| 90 | completes most quickly. | |
| 91 | </p> | |
| 92 | <p> | |
| 93 | Calling <a class="link" href="../../synchronization/futures/future.html#future_get"><code class="computeroutput">future::get()</code></a> on the future returned by <code class="computeroutput"><span class="identifier">async</span><span class="special">()</span></code> | |
| 94 | wouldn't be right. You can only call <code class="computeroutput"><span class="identifier">get</span><span class="special">()</span></code> once per <code class="computeroutput"><span class="identifier">future</span><span class="special"><></span></code> instance! And if there were an | |
| 95 | exception, it would be rethrown inside the helper fiber at the producer | |
| 96 | end of the channel, rather than propagated to the consumer end. | |
| 97 | </p> | |
| 98 | <p> | |
| 99 | We could call <a class="link" href="../../synchronization/futures/future.html#future_wait"><code class="computeroutput">future::wait()</code></a>. That would block the helper fiber | |
| 100 | until the <code class="computeroutput"><span class="identifier">future</span><span class="special"><></span></code> | |
| 101 | became ready, at which point we could <code class="computeroutput"><span class="identifier">push</span><span class="special">()</span></code> it to be retrieved by <code class="computeroutput"><span class="identifier">wait_first_outcome</span><span class="special">()</span></code>. | |
| 102 | </p> | |
| 103 | <p> | |
| 104 | That would work — but there's a simpler tactic that avoids creating an extra | |
| 105 | fiber. We can wrap the task function in a <a class="link" href="../../synchronization/futures/packaged_task.html#class_packaged_task"><code class="computeroutput">packaged_task<></code></a>. | |
| 106 | While one naturally thinks of passing a <code class="computeroutput"><span class="identifier">packaged_task</span><span class="special"><></span></code> to a new fiber — that is, in fact, | |
| 107 | what <code class="computeroutput"><span class="identifier">async</span><span class="special">()</span></code> | |
| 108 | does — in this case, we're already running in the helper fiber at the producer | |
| 109 | end of the channel! We can simply <span class="emphasis"><em>call</em></span> the <code class="computeroutput"><span class="identifier">packaged_task</span><span class="special"><></span></code>. | |
| 110 | On return from that call, the task function has completed, meaning that | |
| 111 | the <code class="computeroutput"><span class="identifier">future</span><span class="special"><></span></code> | |
| 112 | obtained from the <code class="computeroutput"><span class="identifier">packaged_task</span><span class="special"><></span></code> is certain to be ready. At that | |
| 113 | point we can simply <code class="computeroutput"><span class="identifier">push</span><span class="special">()</span></code> it to the channel. | |
| 114 | </p> | |
| 115 | <p> | |
| 116 | <a name="wait_first_outcome_impl"></a> | |
| 117 | </p> | |
| 118 | <pre class="programlisting"><span class="keyword">template</span><span class="special"><</span> <span class="keyword">typename</span> <span class="identifier">T</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">CHANNELP</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">Fn</span> <span class="special">></span> | |
| 119 | <span class="keyword">void</span> <span class="identifier">wait_first_outcome_impl</span><span class="special">(</span> <span class="identifier">CHANNELP</span> <span class="identifier">channel</span><span class="special">,</span> <span class="identifier">Fn</span> <span class="special">&&</span> <span class="identifier">function</span><span class="special">)</span> <span class="special">{</span> | |
| 120 | <span class="identifier">boost</span><span class="special">::</span><span class="identifier">fibers</span><span class="special">::</span><span class="identifier">fiber</span><span class="special">(</span> | |
| 121 | <span class="comment">// Use std::bind() here for C++11 compatibility. C++11 lambda capture</span> | |
| 122 | <span class="comment">// can't move a move-only Fn type, but bind() can. Let bind() move the</span> | |
| 123 | <span class="comment">// channel pointer and the function into the bound object, passing</span> | |
| 124 | <span class="comment">// references into the lambda.</span> | |
| 125 | <span class="identifier">std</span><span class="special">::</span><span class="identifier">bind</span><span class="special">(</span> | |
| 126 | <span class="special">[](</span> <span class="identifier">CHANNELP</span> <span class="special">&</span> <span class="identifier">channel</span><span class="special">,</span> | |
| 127 | <span class="keyword">typename</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">decay</span><span class="special"><</span> <span class="identifier">Fn</span> <span class="special">>::</span><span class="identifier">type</span> <span class="special">&</span> <span class="identifier">function</span><span class="special">)</span> <span class="special">{</span> | |
| 128 | <span class="comment">// Instantiate a packaged_task to capture any exception thrown by</span> | |
| 129 | <span class="comment">// function.</span> | |
| 130 | <span class="identifier">boost</span><span class="special">::</span><span class="identifier">fibers</span><span class="special">::</span><span class="identifier">packaged_task</span><span class="special"><</span> <span class="identifier">T</span><span class="special">()</span> <span class="special">></span> <span class="identifier">task</span><span class="special">(</span> <span class="identifier">function</span><span class="special">);</span> | |
| 131 | <span class="comment">// Immediately run this packaged_task on same fiber. We want</span> | |
| 132 | <span class="comment">// function() to have completed BEFORE we push the future.</span> | |
| 133 | <span class="identifier">task</span><span class="special">();</span> | |
| 134 | <span class="comment">// Pass the corresponding future to consumer. Ignore</span> | |
| 135 | <span class="comment">// channel_op_status returned by push(): might be closed; we</span> | |
| 136 | <span class="comment">// simply don't care.</span> | |
| 137 | <span class="identifier">channel</span><span class="special">-></span><span class="identifier">push</span><span class="special">(</span> <span class="identifier">task</span><span class="special">.</span><span class="identifier">get_future</span><span class="special">()</span> <span class="special">);</span> | |
| 138 | <span class="special">},</span> | |
| 139 | <span class="identifier">channel</span><span class="special">,</span> | |
| 140 | <span class="identifier">std</span><span class="special">::</span><span class="identifier">forward</span><span class="special"><</span> <span class="identifier">Fn</span> <span class="special">>(</span> <span class="identifier">function</span><span class="special">)</span> | |
| 141 | <span class="special">)).</span><span class="identifier">detach</span><span class="special">();</span> | |
| 142 | <span class="special">}</span> | |
| 143 | </pre> | |
| 144 | <p> | |
| 145 | </p> | |
| 146 | <p> | |
| 147 | Calling it might look like this: | |
| 148 | </p> | |
| 149 | <p> | |
| 150 | </p> | |
| 151 | <pre class="programlisting"><span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span> <span class="identifier">result</span> <span class="special">=</span> <span class="identifier">wait_first_outcome</span><span class="special">(</span> | |
| 152 | <span class="special">[](){</span> <span class="keyword">return</span> <span class="identifier">sleeper</span><span class="special">(</span><span class="string">"wfos_first"</span><span class="special">,</span> <span class="number">50</span><span class="special">);</span> <span class="special">},</span> | |
| 153 | <span class="special">[](){</span> <span class="keyword">return</span> <span class="identifier">sleeper</span><span class="special">(</span><span class="string">"wfos_second"</span><span class="special">,</span> <span class="number">100</span><span class="special">);</span> <span class="special">},</span> | |
| 154 | <span class="special">[](){</span> <span class="keyword">return</span> <span class="identifier">sleeper</span><span class="special">(</span><span class="string">"wfos_third"</span><span class="special">,</span> <span class="number">150</span><span class="special">);</span> <span class="special">});</span> | |
| 155 | <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special"><<</span> <span class="string">"wait_first_outcome(success) => "</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> | |
| 156 | <span class="identifier">assert</span><span class="special">(</span><span class="identifier">result</span> <span class="special">==</span> <span class="string">"wfos_first"</span><span class="special">);</span> | |
| 157 | ||
| 158 | <span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span> <span class="identifier">thrown</span><span class="special">;</span> | |
| 159 | <span class="keyword">try</span> <span class="special">{</span> | |
| 160 | <span class="identifier">result</span> <span class="special">=</span> <span class="identifier">wait_first_outcome</span><span class="special">(</span> | |
| 161 | <span class="special">[](){</span> <span class="keyword">return</span> <span class="identifier">sleeper</span><span class="special">(</span><span class="string">"wfof_first"</span><span class="special">,</span> <span class="number">50</span><span class="special">,</span> <span class="keyword">true</span><span class="special">);</span> <span class="special">},</span> | |
| 162 | <span class="special">[](){</span> <span class="keyword">return</span> <span class="identifier">sleeper</span><span class="special">(</span><span class="string">"wfof_second"</span><span class="special">,</span> <span class="number">100</span><span class="special">);</span> <span class="special">},</span> | |
| 163 | <span class="special">[](){</span> <span class="keyword">return</span> <span class="identifier">sleeper</span><span class="special">(</span><span class="string">"wfof_third"</span><span class="special">,</span> <span class="number">150</span><span class="special">);</span> <span class="special">});</span> | |
| 164 | <span class="special">}</span> <span class="keyword">catch</span> <span class="special">(</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">exception</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">e</span><span class="special">)</span> <span class="special">{</span> | |
| 165 | <span class="identifier">thrown</span> <span class="special">=</span> <span class="identifier">e</span><span class="special">.</span><span class="identifier">what</span><span class="special">();</span> | |
| 166 | <span class="special">}</span> | |
| 167 | <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special"><<</span> <span class="string">"wait_first_outcome(fail) threw '"</span> <span class="special"><<</span> <span class="identifier">thrown</span> | |
| 168 | <span class="special"><<</span> <span class="string">"'"</span> <span class="special"><<</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span> | |
| 169 | <span class="identifier">assert</span><span class="special">(</span><span class="identifier">thrown</span> <span class="special">==</span> <span class="string">"wfof_first"</span><span class="special">);</span> | |
| 170 | </pre> | |
| 171 | <p> | |
| 172 | </p> | |
| 173 | </div> | |
| 174 | <table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr> | |
| 175 | <td align="left"></td> | |
| 176 | <td align="right"><div class="copyright-footer">Copyright © 2013 Oliver Kowalke<p> | |
| 177 | Distributed under the Boost Software License, Version 1.0. (See accompanying | |
| 178 | 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>) | |
| 179 | </p> | |
| 180 | </div></td> | |
| 181 | </tr></table> | |
| 182 | <hr> | |
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