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  26 <div class="titlepage"><div><div><h4 class="title">
  27 <a name="fiber.when_any.when_all_functionality.when_all__return_values"></a><a class="link" href="when_all__return_values.html" title="when_all, return values">when_all,
  28         return values</a>
  29 </h4></div></div></div>
  30 <p>
  31           As soon as we want to collect return values from all the task functions,
  32           we can see right away how to reuse <a class="link" href="../when_any/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
  33           queue&lt;T&gt; for the purpose. All we have to do is avoid closing it after
  34           the first value!
  35         </p>
  36 <p>
  37           But in fact, collecting multiple values raises an interesting question:
  38           do we <span class="emphasis"><em>really</em></span> want to wait until the slowest of them
  39           has arrived? Wouldn't we rather process each result as soon as it becomes
  40           available?
  41         </p>
  42 <p>
  43           Fortunately we can present both APIs. Let's define <code class="computeroutput"><span class="identifier">wait_all_values_source</span><span class="special">()</span></code> to return <code class="computeroutput"><span class="identifier">shared_ptr</span><span class="special">&lt;</span><span class="identifier">buffered_channel</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;&gt;</span></code>.
  44         </p>
  45 <p>
  46           <a name="wait_all_values"></a>Given <code class="computeroutput"><span class="identifier">wait_all_values_source</span><span class="special">()</span></code>, it's straightforward to implement <code class="computeroutput"><span class="identifier">wait_all_values</span><span class="special">()</span></code>:
  47         </p>
  48 <p>
  49 </p>
  50 <pre class="programlisting"><span class="keyword">template</span><span class="special">&lt;</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">&gt;</span>
  51 <span class="identifier">std</span><span class="special">::</span><span class="identifier">vector</span><span class="special">&lt;</span> <span class="keyword">typename</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">result_of</span><span class="special">&lt;</span> <span class="identifier">Fn</span><span class="special">()</span> <span class="special">&gt;::</span><span class="identifier">type</span> <span class="special">&gt;</span>
  52 <span class="identifier">wait_all_values</span><span class="special">(</span> <span class="identifier">Fn</span> <span class="special">&amp;&amp;</span> <span class="identifier">function</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>
  53     <span class="identifier">std</span><span class="special">::</span><span class="identifier">size_t</span> <span class="identifier">count</span><span class="special">(</span> <span class="number">1</span> <span class="special">+</span> <span class="keyword">sizeof</span> <span class="special">...</span> <span class="special">(</span> <span class="identifier">functions</span><span class="special">)</span> <span class="special">);</span>
  54     <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">&lt;</span> <span class="identifier">Fn</span><span class="special">()</span> <span class="special">&gt;::</span><span class="identifier">type</span> <span class="identifier">return_t</span><span class="special">;</span>
  55     <span class="keyword">typedef</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">vector</span><span class="special">&lt;</span> <span class="identifier">return_t</span> <span class="special">&gt;</span> <span class="identifier">vector_t</span><span class="special">;</span>
  56     <span class="identifier">vector_t</span> <span class="identifier">results</span><span class="special">;</span>
  57     <span class="identifier">results</span><span class="special">.</span><span class="identifier">reserve</span><span class="special">(</span> <span class="identifier">count</span><span class="special">);</span>
  58
  59     <span class="comment">// get channel</span>
  60     <span class="identifier">std</span><span class="special">::</span><span class="identifier">shared_ptr</span><span class="special">&lt;</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">fibers</span><span class="special">::</span><span class="identifier">buffered_channel</span><span class="special">&lt;</span> <span class="identifier">return_t</span> <span class="special">&gt;</span> <span class="special">&gt;</span> <span class="identifier">chan</span> <span class="special">=</span>
  61         <span class="identifier">wait_all_values_source</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">Fn</span> <span class="special">&gt;(</span> <span class="identifier">function</span><span class="special">),</span>
  62                                 <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>
  63     <span class="comment">// fill results vector</span>
  64     <span class="identifier">return_t</span> <span class="identifier">value</span><span class="special">;</span>
  65     <span class="keyword">while</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">channel_op_status</span><span class="special">::</span><span class="identifier">success</span> <span class="special">==</span> <span class="identifier">chan</span><span class="special">-&gt;</span><span class="identifier">pop</span><span class="special">(</span><span class="identifier">value</span><span class="special">)</span> <span class="special">)</span> <span class="special">{</span>
  66         <span class="identifier">results</span><span class="special">.</span><span class="identifier">push_back</span><span class="special">(</span> <span class="identifier">value</span><span class="special">);</span>
  67     <span class="special">}</span>
  68     <span class="comment">// return vector to caller</span>
  69     <span class="keyword">return</span> <span class="identifier">results</span><span class="special">;</span>
  70 <span class="special">}</span>
  71 </pre>
  72 <p>
  73         </p>
  74 <p>
  75           It might be called like this:
  76         </p>
  77 <p>
  78 </p>
  79 <pre class="programlisting"><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="identifier">values</span> <span class="special">=</span>
  80     <span class="identifier">wait_all_values</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="string">"wav_late"</span><span class="special">,</span>   <span class="number">150</span><span class="special">);</span> <span class="special">},</span>
  82             <span class="special">[](){</span> <span class="keyword">return</span> <span class="identifier">sleeper</span><span class="special">(</span><span class="string">"wav_middle"</span><span class="special">,</span> <span class="number">100</span><span class="special">);</span> <span class="special">},</span>
  83             <span class="special">[](){</span> <span class="keyword">return</span> <span class="identifier">sleeper</span><span class="special">(</span><span class="string">"wav_early"</span><span class="special">,</span>   <span class="number">50</span><span class="special">);</span> <span class="special">});</span>
  84 </pre>
  85 <p>
  86         </p>
  87 <p>
  88           As you can see from the loop in <code class="computeroutput"><span class="identifier">wait_all_values</span><span class="special">()</span></code>, instead of requiring its caller to count
  89           values, we define <code class="computeroutput"><span class="identifier">wait_all_values_source</span><span class="special">()</span></code> to <a class="link" href="../../synchronization/channels/buffered_channel.html#buffered_channel_close"><code class="computeroutput">buffered_channel::close()</code></a> the
  90           queue when done. But how do we do that? Each producer fiber is independent.
  91           It has no idea whether it is the last one to <a class="link" href="../../synchronization/channels/buffered_channel.html#buffered_channel_push"><code class="computeroutput">buffered_channel::push()</code></a> a
  92           value.
  93         </p>
  94 <p>
  95           <a name="wait_nqueue"></a>We can address that problem with a counting fa&#231;ade
  96           for the <code class="computeroutput"><span class="identifier">queue</span><span class="special">&lt;&gt;</span></code>.
  97           In fact, our fa&#231;ade need only support the producer end of the queue.
  98         </p>
  99 <p>
 100           [wait_nqueue]
 101         </p>
 102 <p>
 103           <a name="wait_all_values_source"></a>Armed with <code class="computeroutput"><span class="identifier">nqueue</span><span class="special">&lt;&gt;</span></code>, we can implement <code class="computeroutput"><span class="identifier">wait_all_values_source</span><span class="special">()</span></code>.
 104           It starts just like <a class="link" href="../when_any/when_any__return_value.html#wait_first_value"><code class="computeroutput"><span class="identifier">wait_first_value</span><span class="special">()</span></code></a>. The difference is that we wrap
 105           the <code class="computeroutput"><span class="identifier">queue</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;</span></code>
 106           with an <code class="computeroutput"><span class="identifier">nqueue</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;</span></code>
 107           to pass to the producer fibers.
 108         </p>
 109 <p>
 110           Then, of course, instead of popping the first value, closing the queue
 111           and returning it, we simply return the <code class="computeroutput"><span class="identifier">shared_ptr</span><span class="special">&lt;</span><span class="identifier">queue</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;&gt;</span></code>.
 112         </p>
 113 <p>
 114 </p>
 115 <pre class="programlisting"><span class="comment">// Return a shared_ptr&lt;buffered_channel&lt;T&gt;&gt; from which the caller can</span>
 116 <span class="comment">// retrieve each new result as it arrives, until 'closed'.</span>
 117 <span class="keyword">template</span><span class="special">&lt;</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">&gt;</span>
 118 <span class="identifier">std</span><span class="special">::</span><span class="identifier">shared_ptr</span><span class="special">&lt;</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">fibers</span><span class="special">::</span><span class="identifier">buffered_channel</span><span class="special">&lt;</span> <span class="keyword">typename</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">result_of</span><span class="special">&lt;</span> <span class="identifier">Fn</span><span class="special">()</span> <span class="special">&gt;::</span><span class="identifier">type</span> <span class="special">&gt;</span> <span class="special">&gt;</span>
 119 <span class="identifier">wait_all_values_source</span><span class="special">(</span> <span class="identifier">Fn</span> <span class="special">&amp;&amp;</span> <span class="identifier">function</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>
 120     <span class="identifier">std</span><span class="special">::</span><span class="identifier">size_t</span> <span class="identifier">count</span><span class="special">(</span> <span class="number">1</span> <span class="special">+</span> <span class="keyword">sizeof</span> <span class="special">...</span> <span class="special">(</span> <span class="identifier">functions</span><span class="special">)</span> <span class="special">);</span>
 121     <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">&lt;</span> <span class="identifier">Fn</span><span class="special">()</span> <span class="special">&gt;::</span><span class="identifier">type</span> <span class="identifier">return_t</span><span class="special">;</span>
 122     <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">buffered_channel</span><span class="special">&lt;</span> <span class="identifier">return_t</span> <span class="special">&gt;</span> <span class="identifier">channel_t</span><span class="special">;</span>
 123     <span class="comment">// make the channel</span>
 124     <span class="keyword">auto</span> <span class="identifier">chanp</span><span class="special">(</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">make_shared</span><span class="special">&lt;</span> <span class="identifier">channel_t</span> <span class="special">&gt;(</span> <span class="number">64</span><span class="special">)</span> <span class="special">);</span>
 125     <span class="comment">// and make an nchannel facade to close it after 'count' items</span>
 126     <span class="keyword">auto</span> <span class="identifier">ncp</span><span class="special">(</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">make_shared</span><span class="special">&lt;</span> <span class="identifier">nchannel</span><span class="special">&lt;</span> <span class="identifier">return_t</span> <span class="special">&gt;</span> <span class="special">&gt;(</span> <span class="identifier">chanp</span><span class="special">,</span> <span class="identifier">count</span><span class="special">)</span> <span class="special">);</span>
 127     <span class="comment">// pass that nchannel facade to all the relevant fibers</span>
 128     <span class="identifier">wait_all_values_impl</span><span class="special">&lt;</span> <span class="identifier">return_t</span> <span class="special">&gt;(</span> <span class="identifier">ncp</span><span class="special">,</span>
 129                                       <span class="identifier">std</span><span class="special">::</span><span class="identifier">forward</span><span class="special">&lt;</span> <span class="identifier">Fn</span> <span class="special">&gt;(</span> <span class="identifier">function</span><span class="special">),</span>
 130                                       <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>
 131     <span class="comment">// then return the channel for consumer</span>
 132     <span class="keyword">return</span> <span class="identifier">chanp</span><span class="special">;</span>
 133 <span class="special">}</span>
 134 </pre>
 135 <p>
 136         </p>
 137 <p>
 138           For example:
 139         </p>
 140 <p>
 141 </p>
 142 <pre class="programlisting"><span class="identifier">std</span><span class="special">::</span><span class="identifier">shared_ptr</span><span class="special">&lt;</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">fibers</span><span class="special">::</span><span class="identifier">buffered_channel</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> <span class="identifier">chan</span> <span class="special">=</span>
 143     <span class="identifier">wait_all_values_source</span><span class="special">(</span>
 144             <span class="special">[](){</span> <span class="keyword">return</span> <span class="identifier">sleeper</span><span class="special">(</span><span class="string">"wavs_third"</span><span class="special">,</span>  <span class="number">150</span><span class="special">);</span> <span class="special">},</span>
 145             <span class="special">[](){</span> <span class="keyword">return</span> <span class="identifier">sleeper</span><span class="special">(</span><span class="string">"wavs_second"</span><span class="special">,</span> <span class="number">100</span><span class="special">);</span> <span class="special">},</span>
 146             <span class="special">[](){</span> <span class="keyword">return</span> <span class="identifier">sleeper</span><span class="special">(</span><span class="string">"wavs_first"</span><span class="special">,</span>   <span class="number">50</span><span class="special">);</span> <span class="special">});</span>
 147 <span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span> <span class="identifier">value</span><span class="special">;</span>
 148 <span class="keyword">while</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">channel_op_status</span><span class="special">::</span><span class="identifier">success</span> <span class="special">==</span> <span class="identifier">chan</span><span class="special">-&gt;</span><span class="identifier">pop</span><span class="special">(</span><span class="identifier">value</span><span class="special">)</span> <span class="special">)</span> <span class="special">{</span>
 149     <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_values_source() =&gt; '"</span> <span class="special">&lt;&lt;</span> <span class="identifier">value</span>
 150               <span class="special">&lt;&lt;</span> <span class="string">"'"</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>
 151 <span class="special">}</span>
 152 </pre>
 153 <p>
 154         </p>
 155 <p>
 156           <a name="wait_all_values_impl"></a><code class="computeroutput"><span class="identifier">wait_all_values_impl</span><span class="special">()</span></code> really is just like <a class="link" href="../when_any/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>
 157           except for the use of <code class="computeroutput"><span class="identifier">nqueue</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;</span></code> rather than <code class="computeroutput"><span class="identifier">queue</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;</span></code>:
 158         </p>
 159 <p>
 160 </p>
 161 <pre class="programlisting"><span class="keyword">template</span><span class="special">&lt;</span> <span class="keyword">typename</span> <span class="identifier">T</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">Fn</span> <span class="special">&gt;</span>
 162 <span class="keyword">void</span> <span class="identifier">wait_all_values_impl</span><span class="special">(</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">shared_ptr</span><span class="special">&lt;</span> <span class="identifier">nchannel</span><span class="special">&lt;</span> <span class="identifier">T</span> <span class="special">&gt;</span> <span class="special">&gt;</span> <span class="identifier">chan</span><span class="special">,</span>
 163                            <span class="identifier">Fn</span> <span class="special">&amp;&amp;</span> <span class="identifier">function</span><span class="special">)</span> <span class="special">{</span>
 164     <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">chan</span><span class="special">,</span> <span class="identifier">function</span><span class="special">](){</span>
 165                               <span class="identifier">chan</span><span class="special">-&gt;</span><span class="identifier">push</span><span class="special">(</span><span class="identifier">function</span><span class="special">());</span>
 166                           <span class="special">}).</span><span class="identifier">detach</span><span class="special">();</span>
 167 <span class="special">}</span>
 168 </pre>
 169 <p>
 170         </p>
 171 </div>
 172 <table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
 173 <td align="left"></td>
 174 <td align="right"><div class="copyright-footer">Copyright &#169; 2013 Oliver Kowalke<p>
 175         Distributed under the Boost Software License, Version 1.0. (See accompanying
 176         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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