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POGCC 3.42 and VC2008 treat std::transform differently, take a reference or a copy of the functor?
 

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  1. POGCC 3.42 and VC2008 treat std::transform differently, take a reference or a copy of the functor?
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    <p>I have Googled this functor-related topic, and it seems that people would generally say that the Standard Library will take a functor as a copy by default. Many examples have been found, and this is true for <code>std::for_each</code>. GCC and VC2008 compilers have yielded the same result: both take a copy of the functor.</p> <p>But for <code>std::transform</code> I have found a difference in the testing code below:</p> <pre><code>#include &lt;iostream&gt; #include &lt;vector&gt; using namespace std; #define MSG(msg) cout &lt;&lt; msg &lt;&lt; endl; struct MyFunctor{ MyFunctor() { MSG("MyFunctor constructor"); } MyFunctor(const MyFunctor&amp; myf) { MSG("MyFunctor copy constructor"); } ~MyFunctor() { MSG("MyFunctor destructor"); } int operator()(int i) { i = -(i+1); return i; } }; int main() { vector&lt;int&gt; myvec; myvec.push_back(1); myvec.push_back(1); myvec.push_back(1); myvec.push_back(1); std::transform(myvec.begin(),myvec.end(),myvec.begin(),MyFunctor()); system("pause"); } /* gcc result: MyFunctor constructor MyFunctor destructor vc2008 result: MyFunctor constructor MyFunctor copy constructor MyFunctor copy constructor MyFunctor destructor MyFunctor destructor MyFunctor destructor */ </code></pre> <p>The result seems to suggest that GCC takes <code>MyFunctor</code> as a reference while vc2008 takes it as a copy (just as both of them treat <code>for_each</code>). </p> <p>Why do GCC and vc2008 behave differently in this matter, and which one should be the right practice?</p>
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    1. USAndriy Tylychko
    1. USGob00st
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      1. USHossein
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    1. CODid you compile in release mode with optimisations enabled? This merely looks as though GCC inlines the call to `transform` and elides the copy.
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      1. POGCC 3.42 and VC2008 treat std::transform differently, take a reference or a copy of the functor?
      1. USKonrad Rudolph
    2. COYes, it's just a copy elision for one of the copies. But VS2008 generated 3 extra copy apart from the necessary. And I have been using release mode with optimization enabled.
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      1. POGCC 3.42 and VC2008 treat std::transform differently, take a reference or a copy of the functor?
      1. USGob00st
 

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