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25 | <div class="section"> | |
26 | <div class="titlepage"><div><div><h4 class="title"> | |
27 | <a name="fiber.when_any.when_any.when_any__simple_completion"></a><a name="wait_first_simple_section"></a><a class="link" href="when_any__simple_completion.html" title="when_any, simple completion">when_any, | |
28 | simple completion</a> | |
29 | </h4></div></div></div> | |
30 | <p> | |
31 | The simplest case is when you only need to know that the first of a set | |
32 | of asynchronous tasks has completed — but you don't need to obtain a return | |
33 | value, and you're confident that they will not throw exceptions. | |
34 | </p> | |
35 | <p> | |
36 | <a name="wait_done"></a>For this we introduce a <code class="computeroutput"><span class="identifier">Done</span></code> | |
37 | class to wrap a <code class="computeroutput"><span class="keyword">bool</span></code> variable | |
38 | with a <a class="link" href="../../synchronization/conditions.html#class_condition_variable"><code class="computeroutput">condition_variable</code></a> and a <a class="link" href="../../synchronization/mutex_types.html#class_mutex"><code class="computeroutput">mutex</code></a>: | |
39 | </p> | |
40 | <p> | |
41 | </p> | |
42 | <pre class="programlisting"><span class="comment">// Wrap canonical pattern for condition_variable + bool flag</span> | |
43 | <span class="keyword">struct</span> <span class="identifier">Done</span> <span class="special">{</span> | |
44 | <span class="keyword">private</span><span class="special">:</span> | |
45 | <span class="identifier">boost</span><span class="special">::</span><span class="identifier">fibers</span><span class="special">::</span><span class="identifier">condition_variable</span> <span class="identifier">cond</span><span class="special">;</span> | |
46 | <span class="identifier">boost</span><span class="special">::</span><span class="identifier">fibers</span><span class="special">::</span><span class="identifier">mutex</span> <span class="identifier">mutex</span><span class="special">;</span> | |
47 | <span class="keyword">bool</span> <span class="identifier">ready</span> <span class="special">=</span> <span class="keyword">false</span><span class="special">;</span> | |
48 | ||
49 | <span class="keyword">public</span><span class="special">:</span> | |
50 | <span class="keyword">typedef</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">shared_ptr</span><span class="special"><</span> <span class="identifier">Done</span> <span class="special">></span> <span class="identifier">ptr</span><span class="special">;</span> | |
51 | ||
52 | <span class="keyword">void</span> <span class="identifier">wait</span><span class="special">()</span> <span class="special">{</span> | |
53 | <span class="identifier">std</span><span class="special">::</span><span class="identifier">unique_lock</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">mutex</span> <span class="special">></span> <span class="identifier">lock</span><span class="special">(</span> <span class="identifier">mutex</span><span class="special">);</span> | |
54 | <span class="identifier">cond</span><span class="special">.</span><span class="identifier">wait</span><span class="special">(</span> <span class="identifier">lock</span><span class="special">,</span> <span class="special">[</span><span class="keyword">this</span><span class="special">](){</span> <span class="keyword">return</span> <span class="identifier">ready</span><span class="special">;</span> <span class="special">});</span> | |
55 | <span class="special">}</span> | |
56 | ||
57 | <span class="keyword">void</span> <span class="identifier">notify</span><span class="special">()</span> <span class="special">{</span> | |
58 | <span class="special">{</span> | |
59 | <span class="identifier">std</span><span class="special">::</span><span class="identifier">unique_lock</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">mutex</span> <span class="special">></span> <span class="identifier">lock</span><span class="special">(</span> <span class="identifier">mutex</span><span class="special">);</span> | |
60 | <span class="identifier">ready</span> <span class="special">=</span> <span class="keyword">true</span><span class="special">;</span> | |
61 | <span class="special">}</span> <span class="comment">// release mutex</span> | |
62 | <span class="identifier">cond</span><span class="special">.</span><span class="identifier">notify_one</span><span class="special">();</span> | |
63 | <span class="special">}</span> | |
64 | <span class="special">};</span> | |
65 | </pre> | |
66 | <p> | |
67 | </p> | |
68 | <p> | |
69 | The pattern we follow throughout this section is to pass a <a href="http://www.cplusplus.com/reference/memory/shared_ptr/" target="_top"><code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">shared_ptr</span><span class="special"><></span></code></a> | |
70 | to the relevant synchronization object to the various tasks' fiber functions. | |
71 | This eliminates nagging questions about the lifespan of the synchronization | |
72 | object relative to the last of the fibers. | |
73 | </p> | |
74 | <p> | |
75 | <a name="wait_first_simple"></a><code class="computeroutput"><span class="identifier">wait_first_simple</span><span class="special">()</span></code> uses that tactic for <a class="link" href="when_any__simple_completion.html#wait_done"><code class="computeroutput"><span class="identifier">Done</span></code></a>: | |
76 | </p> | |
77 | <p> | |
78 | </p> | |
79 | <pre class="programlisting"><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> | |
80 | <span class="keyword">void</span> <span class="identifier">wait_first_simple</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> | |
81 | <span class="comment">// Use shared_ptr because each function's fiber will bind it separately,</span> | |
82 | <span class="comment">// and we're going to return before the last of them completes.</span> | |
83 | <span class="keyword">auto</span> <span class="identifier">done</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">Done</span> <span class="special">>()</span> <span class="special">);</span> | |
84 | <span class="identifier">wait_first_simple_impl</span><span class="special">(</span> <span class="identifier">done</span><span class="special">,</span> <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> | |
85 | <span class="identifier">done</span><span class="special">-></span><span class="identifier">wait</span><span class="special">();</span> | |
86 | <span class="special">}</span> | |
87 | </pre> | |
88 | <p> | |
89 | </p> | |
90 | <p> | |
91 | <a name="wait_first_simple_impl"></a><code class="computeroutput"><span class="identifier">wait_first_simple_impl</span><span class="special">()</span></code> is an ordinary recursion over the argument | |
92 | pack, capturing <code class="computeroutput"><span class="identifier">Done</span><span class="special">::</span><span class="identifier">ptr</span></code> for each new fiber: | |
93 | </p> | |
94 | <p> | |
95 | </p> | |
96 | <pre class="programlisting"><span class="comment">// Degenerate case: when there are no functions to wait for, return</span> | |
97 | <span class="comment">// immediately.</span> | |
98 | <span class="keyword">void</span> <span class="identifier">wait_first_simple_impl</span><span class="special">(</span> <span class="identifier">Done</span><span class="special">::</span><span class="identifier">ptr</span><span class="special">)</span> <span class="special">{</span> | |
99 | <span class="special">}</span> | |
100 | ||
101 | <span class="comment">// When there's at least one function to wait for, launch it and recur to</span> | |
102 | <span class="comment">// process the rest.</span> | |
103 | <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> | |
104 | <span class="keyword">void</span> <span class="identifier">wait_first_simple_impl</span><span class="special">(</span> <span class="identifier">Done</span><span class="special">::</span><span class="identifier">ptr</span> <span class="identifier">done</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> | |
105 | <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> <span class="special">[</span><span class="identifier">done</span><span class="special">,</span> <span class="identifier">function</span><span class="special">](){</span> | |
106 | <span class="identifier">function</span><span class="special">();</span> | |
107 | <span class="identifier">done</span><span class="special">-></span><span class="identifier">notify</span><span class="special">();</span> | |
108 | <span class="special">}).</span><span class="identifier">detach</span><span class="special">();</span> | |
109 | <span class="identifier">wait_first_simple_impl</span><span class="special">(</span> <span class="identifier">done</span><span class="special">,</span> <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> | |
110 | <span class="special">}</span> | |
111 | </pre> | |
112 | <p> | |
113 | </p> | |
114 | <p> | |
115 | The body of the fiber's lambda is extremely simple, as promised: call the | |
116 | function, notify <a class="link" href="when_any__simple_completion.html#wait_done"><code class="computeroutput"><span class="identifier">Done</span></code></a> | |
117 | when it returns. The first fiber to do so allows <code class="computeroutput"><span class="identifier">wait_first_simple</span><span class="special">()</span></code> to return — which is why it's useful to | |
118 | have <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">shared_ptr</span><span class="special"><</span><span class="identifier">Done</span><span class="special">></span></code> | |
119 | manage the lifespan of our <code class="computeroutput"><span class="identifier">Done</span></code> | |
120 | object rather than declaring it as a stack variable in <code class="computeroutput"><span class="identifier">wait_first_simple</span><span class="special">()</span></code>. | |
121 | </p> | |
122 | <p> | |
123 | This is how you might call it: | |
124 | </p> | |
125 | <p> | |
126 | </p> | |
127 | <pre class="programlisting"><span class="identifier">wait_first_simple</span><span class="special">(</span> | |
128 | <span class="special">[](){</span> <span class="identifier">sleeper</span><span class="special">(</span><span class="string">"wfs_long"</span><span class="special">,</span> <span class="number">150</span><span class="special">);</span> <span class="special">},</span> | |
129 | <span class="special">[](){</span> <span class="identifier">sleeper</span><span class="special">(</span><span class="string">"wfs_medium"</span><span class="special">,</span> <span class="number">100</span><span class="special">);</span> <span class="special">},</span> | |
130 | <span class="special">[](){</span> <span class="identifier">sleeper</span><span class="special">(</span><span class="string">"wfs_short"</span><span class="special">,</span> <span class="number">50</span><span class="special">);</span> <span class="special">});</span> | |
131 | </pre> | |
132 | <p> | |
133 | </p> | |
134 | <p> | |
135 | In this example, control resumes after <code class="computeroutput"><span class="identifier">wait_first_simple</span><span class="special">()</span></code> when <a class="link" href="../../when_any.html#wait_sleeper"><code class="computeroutput"><span class="identifier">sleeper</span><span class="special">(</span><span class="string">"wfs_short"</span><span class="special">,</span> | |
136 | <span class="number">50</span><span class="special">)</span></code></a> | |
137 | completes — even though the other two <code class="computeroutput"><span class="identifier">sleeper</span><span class="special">()</span></code> fibers are still running. | |
138 | </p> | |
139 | </div> | |
140 | <table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr> | |
141 | <td align="left"></td> | |
142 | <td align="right"><div class="copyright-footer">Copyright © 2013 Oliver Kowalke<p> | |
143 | Distributed under the Boost Software License, Version 1.0. (See accompanying | |
144 | 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>) | |
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