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POimplementing future::then() equivalent for asynchronous execution in c++11
 

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  1. POimplementing future::then() equivalent for asynchronous execution in c++11
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    <p>I have a few questions about the implementation of the function <code>then()</code> in <a href="http://channel9.msdn.com/Shows/Going+Deep/C-and-Beyond-2012-Herb-Sutter-Concurrency-and-Parallelism" rel="nofollow noreferrer">Herb Sutter's talk</a>. This function is used to chain asynchronous operations, the parameter <code>f</code> is a future from one operation and the parameter <code>w</code> is the 'work' for this operation (lambda).</p> <pre><code>template &lt;typename Fut, typename Work&gt; auto then(Fut f, Work w) -&gt; future&lt;decltype(w(f.get()))&gt; { return async([=]{ w(f.get()); }); } </code></pre> <p>An example of application would be:</p> <pre><code> std::future&lt;int&gt; f = std::async([]{ std::this_thread::sleep_for(std::chrono::microseconds(200)); return 10; }); auto f2 = then(std::move(f), [](int i){ return 2 * i; }); </code></pre> <p>The main thread spawns the tasks but does not wait for any of them to finish.</p> <p>Firstly, <code>future&lt;T&gt;</code> does not have a copy constructor. This means, that the suggested implementation can be only used with <code>shared_future&lt;T&gt;</code> unless we change the call to <code>async()</code> to move the future into the lambda. <a href="https://stackoverflow.com/questions/14200678/c11-async-continuations-or-attempt-at-then-semantics/14200861#comment20089049_14200861">This SO question</a> suggested a way of doing it but it seems too complicated. I re-implemented the function and I am wondering whether my code is correct or whether I missed something...</p> <p>Secondly, the future that is passed to the <code>then()</code> function might be <code>void</code> so we actually need 2 implementations of <code>then()</code>, right? One for futures returning <code>T</code> and one for futures returning <code>void</code>.</p> <p>Lastly, should the lambda inside the body of <code>then()</code> not have a return statement so that we can actually return the value back? Without the return statement, then returns <code>future&lt;void&gt;</code>, right?</p> <p>I tried to address the above points and this is what I came up with. Is it correct?</p> <pre><code>template &lt;typename T, typename Work&gt; auto then(future&lt;T&gt; f, Work w) -&gt; future&lt;decltype(w(f.get()))&gt; { return async([](future&lt;T&gt; f, Work w) { return w(f.get()); }, move(f), move(w)); } template &lt;typename Work&gt; auto then(future&lt;void&gt; f, Work w) -&gt; future&lt;decltype(w())&gt; { return async([](future&lt;void&gt; f, Work w) { f.wait(); return w(); }, move(f), move(w)); } </code></pre>
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    1. USRoman Kutlak
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      1. USMatthew Johnson
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    1. COWhat do you mean by "correct?"
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      1. USRobert Harvey
    2. COBy "correct" I mean logically correct. The code compiles fine (clang 3.3 + libc++) but that does not mean it does what it is supposed to do.
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      1. USRoman Kutlak
    3. CONo, you do not need another version because of void - it is perfectly fine to do something like this: `void f(); void g() { return f(); }`.
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      1. USlego
 

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