copy constructor not called in template class

Question

#include <iostream>
using namespace std;

template <typename T> class tt
{
    public :
    int data;
    tt()
    {
        std::cout << std::endl << "   CONSTRUCTOR" << std::endl;
    }
    tt(const tt & that)
    {
        std::cout << std::endl << "    COPY CONSTRUCTOR" << std::endl;
    }
};


tt<int> test(void)
{
    std::cout << std::endl << "      INSIDE " << std::endl; tt<int> a; a.data =10 ;return a;
}

int main() {
    // your code goes her
    //tt<int> b;
    tt<int> a =test();
    cout<<a.data; //so that return value optimisation doesn't take place
    return 0;
}

Why is the copy constructor not getting called in this case?

It gets called in the following case though

tt<int> b;
tt<int> a =b;

code link : http://ideone.com/e9lq3C

edit : This is not duplicate of What are copy elision and return value optimization?, because returned value is being referenced inside code.

@πάντα ῥεῖ please see the edit, why it is not duplicate — q126y, Jan 08 '16 at 12:12
Why do you think that the returned value being referenced inside the code affects this? — jcoder, Jan 08 '16 at 12:15
@JBB why not? `a` has different location in memory than the temporary `tt` returned by `test()`. So if we are referencing memory location of `a`, then first it need to be constructed, in this case copy constructed. — q126y, Jan 08 '16 at 12:18
@q126y no it doesn't, print the pointers and see. That's the whole point of RVO, you construct the object in the stack of the calling function to avoid copying it. — PeterT, Jan 08 '16 at 12:20

score 2 · Accepted Answer · edited Jan 08 '16 at 12:23

2

a does not have a different location to the temporary. That is what copy elision and return value optimization do... they make it so that you don't get a temporary. Where you say a in test(), the optimization means you are referring to the same location as a in main. So no copy is required.

edited Jan 08 '16 at 12:23

jpo38

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answered Jan 08 '16 at 12:21

Scott Langham

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score 1 · Answer 2 · answered Jan 08 '16 at 12:18

1

This is all due to compiler optimization (at least RVO). Compilers ends up creating one and only one object her (the one you asked to be created locally in test() becomes the same object as your a variable).

answered Jan 08 '16 at 12:18

jpo38

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copy constructor not called in template class

2 Answers2