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26<div class="titlepage"><div><div><h4 class="title">
27<a name="fiber.when_any.when_all_functionality.when_all__heterogeneous_types"></a><a class="link" href="when_all__heterogeneous_types.html" title="when_all, heterogeneous types">when_all,
28 heterogeneous types</a>
29</h4></div></div></div>
30<p>
31 But what about the case when we must wait for all results of different
32 types?
33 </p>
34<p>
35 We can present an API that is frankly quite cool. Consider a sample struct:
36 </p>
37<p>
38</p>
39<pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">Data</span> <span class="special">{</span>
40 <span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span> <span class="identifier">str</span><span class="special">;</span>
41 <span class="keyword">double</span> <span class="identifier">inexact</span><span class="special">;</span>
42 <span class="keyword">int</span> <span class="identifier">exact</span><span class="special">;</span>
43
44 <span class="keyword">friend</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">ostream</span><span class="special">&amp;</span> <span class="keyword">operator</span><span class="special">&lt;&lt;(</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">ostream</span><span class="special">&amp;</span> <span class="identifier">out</span><span class="special">,</span> <span class="identifier">Data</span> <span class="keyword">const</span><span class="special">&amp;</span> <span class="identifier">data</span><span class="special">);</span>
45 <span class="special">...</span>
46<span class="special">};</span>
47</pre>
48<p>
49 </p>
50<p>
51 Let's fill its members from task functions all running concurrently:
52 </p>
53<p>
54</p>
55<pre class="programlisting"><span class="identifier">Data</span> <span class="identifier">data</span> <span class="special">=</span> <span class="identifier">wait_all_members</span><span class="special">&lt;</span> <span class="identifier">Data</span> <span class="special">&gt;(</span>
56 <span class="special">[](){</span> <span class="keyword">return</span> <span class="identifier">sleeper</span><span class="special">(</span><span class="string">"wams_left"</span><span class="special">,</span> <span class="number">100</span><span class="special">);</span> <span class="special">},</span>
57 <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">150</span><span class="special">);</span> <span class="special">},</span>
58 <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>
59<span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="string">"wait_all_members&lt;Data&gt;(success) =&gt; "</span> <span class="special">&lt;&lt;</span> <span class="identifier">data</span> <span class="special">&lt;&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span>
60</pre>
61<p>
62 </p>
63<p>
64 Note that for this case, we abandon the notion of capturing the earliest
65 result first, and so on: we must fill exactly the passed struct in left-to-right
66 order.
67 </p>
68<p>
69 That permits a beautifully simple implementation:
70 </p>
71<p>
72</p>
73<pre class="programlisting"><span class="comment">// Explicitly pass Result. This can be any type capable of being initialized</span>
74<span class="comment">// from the results of the passed functions, such as a struct.</span>
75<span class="keyword">template</span><span class="special">&lt;</span> <span class="keyword">typename</span> <span class="identifier">Result</span><span class="special">,</span> <span class="keyword">typename</span> <span class="special">...</span> <span class="identifier">Fns</span> <span class="special">&gt;</span>
76<span class="identifier">Result</span> <span class="identifier">wait_all_members</span><span class="special">(</span> <span class="identifier">Fns</span> <span class="special">&amp;&amp;</span> <span class="special">...</span> <span class="identifier">functions</span><span class="special">)</span> <span class="special">{</span>
77 <span class="comment">// Run each of the passed functions on a separate fiber, passing all their</span>
78 <span class="comment">// futures to helper function for processing.</span>
79 <span class="keyword">return</span> <span class="identifier">wait_all_members_get</span><span class="special">&lt;</span> <span class="identifier">Result</span> <span class="special">&gt;(</span>
80 <span class="identifier">boost</span><span class="special">::</span><span class="identifier">fibers</span><span class="special">::</span><span class="identifier">async</span><span class="special">(</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">forward</span><span class="special">&lt;</span> <span class="identifier">Fns</span> <span class="special">&gt;(</span> <span class="identifier">functions</span><span class="special">)</span> <span class="special">)</span> <span class="special">...</span> <span class="special">);</span>
81<span class="special">}</span>
82</pre>
83<p>
84 </p>
85<p>
86</p>
87<pre class="programlisting"><span class="keyword">template</span><span class="special">&lt;</span> <span class="keyword">typename</span> <span class="identifier">Result</span><span class="special">,</span> <span class="keyword">typename</span> <span class="special">...</span> <span class="identifier">Futures</span> <span class="special">&gt;</span>
88<span class="identifier">Result</span> <span class="identifier">wait_all_members_get</span><span class="special">(</span> <span class="identifier">Futures</span> <span class="special">&amp;&amp;</span> <span class="special">...</span> <span class="identifier">futures</span><span class="special">)</span> <span class="special">{</span>
89 <span class="comment">// Fetch the results from the passed futures into Result's initializer</span>
90 <span class="comment">// list. It's true that the get() calls here will block the implicit</span>
91 <span class="comment">// iteration over futures -- but that doesn't matter because we won't be</span>
92 <span class="comment">// done until the slowest of them finishes anyway. As results are</span>
93 <span class="comment">// processed in argument-list order rather than order of completion, the</span>
94 <span class="comment">// leftmost get() to throw an exception will cause that exception to</span>
95 <span class="comment">// propagate to the caller.</span>
96 <span class="keyword">return</span> <span class="identifier">Result</span><span class="special">{</span> <span class="identifier">futures</span><span class="special">.</span><span class="identifier">get</span><span class="special">()</span> <span class="special">...</span> <span class="special">};</span>
97<span class="special">}</span>
98</pre>
99<p>
100 </p>
101<p>
102 It is tempting to try to implement <code class="computeroutput"><span class="identifier">wait_all_members</span><span class="special">()</span></code> as a one-liner like this:
103 </p>
104<pre class="programlisting"><span class="keyword">return</span> <span class="identifier">Result</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">async</span><span class="special">(</span><span class="identifier">functions</span><span class="special">).</span><span class="identifier">get</span><span class="special">()...</span> <span class="special">};</span>
105</pre>
106<p>
107 The trouble with this tactic is that it would serialize all the task functions.
108 The runtime makes a single pass through <code class="computeroutput"><span class="identifier">functions</span></code>,
109 calling <a class="link" href="../../synchronization/futures/future.html#fibers_async"><code class="computeroutput">fibers::async()</code></a> for each and then immediately calling
110 <a class="link" href="../../synchronization/futures/future.html#future_get"><code class="computeroutput">future::get()</code></a> on its returned <code class="computeroutput"><span class="identifier">future</span><span class="special">&lt;&gt;</span></code>. That blocks the implicit loop.
111 The above is almost equivalent to writing:
112 </p>
113<pre class="programlisting"><span class="keyword">return</span> <span class="identifier">Result</span><span class="special">{</span> <span class="identifier">functions</span><span class="special">()...</span> <span class="special">};</span>
114</pre>
115<p>
116 in which, of course, there is no concurrency at all.
117 </p>
118<p>
119 Passing the argument pack through a function-call boundary (<code class="computeroutput"><span class="identifier">wait_all_members_get</span><span class="special">()</span></code>)
120 forces the runtime to make <span class="emphasis"><em>two</em></span> passes: one in <code class="computeroutput"><span class="identifier">wait_all_members</span><span class="special">()</span></code>
121 to collect the <code class="computeroutput"><span class="identifier">future</span><span class="special">&lt;&gt;</span></code>s
122 from all the <code class="computeroutput"><span class="identifier">async</span><span class="special">()</span></code>
123 calls, the second in <code class="computeroutput"><span class="identifier">wait_all_members_get</span><span class="special">()</span></code> to fetch each of the results.
124 </p>
125<p>
126 As noted in comments, within the <code class="computeroutput"><span class="identifier">wait_all_members_get</span><span class="special">()</span></code> parameter pack expansion pass, the blocking
127 behavior of <code class="computeroutput"><span class="identifier">get</span><span class="special">()</span></code>
128 becomes irrelevant. Along the way, we will hit the <code class="computeroutput"><span class="identifier">get</span><span class="special">()</span></code> for the slowest task function; after
129 that every subsequent <code class="computeroutput"><span class="identifier">get</span><span class="special">()</span></code> will complete in trivial time.
130 </p>
131<p>
132 By the way, we could also use this same API to fill a vector or other collection:
133 </p>
134<p>
135</p>
136<pre class="programlisting"><span class="comment">// If we don't care about obtaining results as soon as they arrive, and we</span>
137<span class="comment">// prefer a result vector in passed argument order rather than completion</span>
138<span class="comment">// order, wait_all_members() is another possible implementation of</span>
139<span class="comment">// wait_all_until_error().</span>
140<span class="keyword">auto</span> <span class="identifier">strings</span> <span class="special">=</span> <span class="identifier">wait_all_members</span><span class="special">&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">vector</span><span class="special">&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span> <span class="special">&gt;</span> <span class="special">&gt;(</span>
141 <span class="special">[](){</span> <span class="keyword">return</span> <span class="identifier">sleeper</span><span class="special">(</span><span class="string">"wamv_left"</span><span class="special">,</span> <span class="number">150</span><span class="special">);</span> <span class="special">},</span>
142 <span class="special">[](){</span> <span class="keyword">return</span> <span class="identifier">sleeper</span><span class="special">(</span><span class="string">"wamv_middle"</span><span class="special">,</span> <span class="number">100</span><span class="special">);</span> <span class="special">},</span>
143 <span class="special">[](){</span> <span class="keyword">return</span> <span class="identifier">sleeper</span><span class="special">(</span><span class="string">"wamv_right"</span><span class="special">,</span> <span class="number">50</span><span class="special">);</span> <span class="special">});</span>
144<span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="string">"wait_all_members&lt;vector&gt;() =&gt;"</span><span class="special">;</span>
145<span class="keyword">for</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">&amp;</span> <span class="identifier">str</span> <span class="special">:</span> <span class="identifier">strings</span><span class="special">)</span> <span class="special">{</span>
146 <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="string">" '"</span> <span class="special">&lt;&lt;</span> <span class="identifier">str</span> <span class="special">&lt;&lt;</span> <span class="string">"'"</span><span class="special">;</span>
147<span class="special">}</span>
148<span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span>
149</pre>
150<p>
151 </p>
152</div>
153<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
154<td align="left"></td>
155<td align="right"><div class="copyright-footer">Copyright &#169; 2013 Oliver Kowalke<p>
156 Distributed under the Boost Software License, Version 1.0. (See accompanying
157 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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