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POExtra copy constructors when std::function is used
 

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  1. POExtra copy constructors when std::function is used
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    <p>In my project, there are two components: producer and consumer. Producer is responsible for processing some information and giving the result to consumer. The result is passed with help of function object. </p> <p>I am using a function for passing this information. You could see how it could look in the code below. </p> <pre><code>#include &lt;iostream&gt; #include &lt;vector&gt; #include &lt;functional&gt; using namespace std; class Data { std::vector&lt;int&gt; vec; public: Data() { cout &lt;&lt; "Data()" &lt;&lt; endl; } Data(const Data&amp; rr) : vec(rr.vec) { cout &lt;&lt; "Data(Data&amp;)" &lt;&lt; endl; } ~Data() { cout &lt;&lt; "~Data()" &lt;&lt; endl; } Data(Data&amp;&amp; rr) : vec(move(rr.vec)) { cout &lt;&lt; "Data(Data&amp;&amp;)" &lt;&lt; endl; } void get() { } }; class Producer { public: void process(function&lt;void(Data)&gt; f) { Data data; f(move(data)); } void process2(void(&amp;pf)(Data)) { Data c; pf(move(c)); } }; void Consume(Data a) { cout &lt;&lt; "Consume(data)" &lt;&lt; endl; } int main() { { cout &lt;&lt; "use function() " &lt;&lt; endl; Producer p; p.process([](Data a) { cout &lt;&lt; "Consume(data)" &lt;&lt; endl; }); } { cout &lt;&lt; endl; cout &lt;&lt; "use function pointer" &lt;&lt; endl; Producer p;; p.process2(Consume); } return 0; } </code></pre> <p>it has the following output</p> <pre><code> use function() Data() Data(Data&amp;&amp;) Data(Data&amp;) Data(Data&amp;) Consume(data) ~Data() ~Data() ~Data() ~Data() use function pointer Data() Data(Data&amp;&amp;) Consume(data) ~Data() ~Data() </code></pre> <p>There are additional copy constructors when function object is used. </p> <p>Did I do something wrong? Is it possible to get rid of these extra constructors? </p> <p>Thank you in advance.</p> <p>I am using VC10 SP1.</p>
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    1. USSerik
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    1. COWith g++ it just calls the move constructor 3 times.
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      1. USkennytm
    2. CO`std::function` is big and expensive... hope you have good compiler optimizations, but there's a limit to how much can be optimized, given the vast flexibility that `std::function` offers you. In a crunch, prefer function pointers or `auto`/lambda over an explicit conversion to `std::function` when performance matters.
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      1. POExtra copy constructors when std::function is used
      1. USKerrek SB
    3. COIt will be fixed in MSVC11. See my answer here : http://stackoverflow.com/questions/8203211/how-can-i-make-the-storage-of-c-lambda-objects-more-efficient/8205037#8205037
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      1. POExtra copy constructors when std::function is used
      1. USThomas Petit
 

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