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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>)
 158       </p>
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