copy constructor not called during return from a function

Question

why is copy constructor not being called for the return call of function func in the last lines of main function ..it is being called when i send a parameter by value but not when i am returning a value

class A
    {
        public:
        int x , y , z;
        A(int x=4 , int y=2 , int z=1)
        {
            this->x = x;
            this->y = y;
            this->z = z;
        }

        A(A& a)
        {
            x = a.x;
            y = a.y;
            z = a.z;
            printf("Copy Constructor called\n");
            a.x++;
        }

        //not a copy constructor
        A(A *a)
        {
            x = a->x;
            y = a->y;
            z = a->z;
            printf("Some Constructor called\n");
            (a->x)++;
        }
        void tell() { printf("x=%d y=%d z=%d\n" , x , y , z);}
    };

    A func()
    {
    A a;

    return a;
    }

    int main()
    {
        A a1;

        a1=func(); //why is copy constructor not called while returning
        a1.tell();
        return 0;
    }

Read about [copy elision](http://en.wikipedia.org/wiki/Copy_elision) ([also here](http://en.cppreference.com/w/cpp/language/copy_elision)). — Some programmer dude, Oct 19 '13 at 15:41

score 3 · Answer 1 · edited May 23 '17 at 10:25

3

This is because of copy-elision. The compiler is allowed to omit the copy and store the result directly in the object. You can turn off copy-elision with the compiler option -fno-elide-constructors (I wouldn't recommend it though).

Related: What are copy elision and return value optimization?

edited May 23 '17 at 10:25

Community

1
1

answered Oct 19 '13 at 15:41

David G

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copy constructor not called during return from a function

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