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POstd::function vs template
 

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  1. POstd::function vs template
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    <p>Thanks to C++11 we received the <code>std::function</code> family of functor wrappers. Unfortunately, I keep hearing only bad things about these new additions. The most popular is that they are horribly slow. I tested it and they truly suck in comparison with templates.</p> <pre><code>#include &lt;iostream&gt; #include &lt;functional&gt; #include &lt;string&gt; #include &lt;chrono&gt; template &lt;typename F&gt; float calc1(F f) { return -1.0f * f(3.3f) + 666.0f; } float calc2(std::function&lt;float(float)&gt; f) { return -1.0f * f(3.3f) + 666.0f; } int main() { using namespace std::chrono; const auto tp1 = system_clock::now(); for (int i = 0; i &lt; 1e8; ++i) { calc1([](float arg){ return arg * 0.5f; }); } const auto tp2 = high_resolution_clock::now(); const auto d = duration_cast&lt;milliseconds&gt;(tp2 - tp1); std::cout &lt;&lt; d.count() &lt;&lt; std::endl; return 0; } </code></pre> <p>111 ms vs 1241 ms. I assume this is because templates can be nicely inlined, while <code>function</code>s cover the internals via virtual calls.</p> <p>Obviously templates have their issues as I see them:</p> <ul> <li>they have to be provided as headers which is not something you might not wish to do when releasing your library as a closed code,</li> <li>they may make the compilation time much longer unless <code>extern template</code>-like policy is introduced,</li> <li>there is no (at least known to me) clean way of representing requirements (concepts, anyone?) of a template, bar a comment describing what kind of functor is expected.</li> </ul> <p>Can I thus assume that <code>function</code>s can be used as <em>de facto</em> standard of passing functors, and in places where high performance is expected templates should be used?</p> <hr> <p>Edit:</p> <p>My compiler is the Visual Studio 2012 <em>without</em> CTP.</p>
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    1. USAndy Prowl
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    1. COUse `std::function` if and only if you actually *need* a heterogeneous collection of callable objects (i.e no further discriminating information is available at runtime).
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      1. USKerrek SB
    2. COYou're comparing the wrong things. Templates are used in both cases - it's not "`std::function` or templates". I think here the issue is simply wrapping a lambda in `std::function` vs not wrapping a lambda in `std::function`. At the moment your question is like asking "should I prefer an apple, or a bowl?"
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    3. COWhether 1ns or 10ns, both is nothing.
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