复制构造函数和运算符重载：C ++

Question

Consider Below example:

#include <iostream>

using namespace std;

class Test{

public:
    int a;
    Test(int a=0):a(a){ cout << "Ctor " << a << endl;}
    Test(const Test& A):a(A.a){ cout << "Cpy Ctor " << a << endl;}
    Test operator+ ( Test A){
    Test temp;
    temp.a=this->a + A.a;
    return temp;
    }
};

int main()
{
Test b(10);
Test a = b ;
cout << a.a << endl;
return 0;
}

The Output is:

$ ./Test
Ctor 10
Cpy Ctor 10
10

It calls Copy constructor. Now, suppose If we modify the code as:

int main()
{
Test b(10);
Test a = b + Test(5);
cout << a.a << endl;
return 0;
}

The Output becomes:

$ ./Test
Ctor 10
Ctor 5
Ctor 0
15

The expression Test a = b + Test(5); does not call copy constructor. What I thought that that b+ Test(5) should be used to instantiate a new object a of type Test , so this should call copy constructor. Can someone explain the output?

Thanks

Answer 1

See copy elision: http://en.wikipedia.org/wiki/Copy_elision

Basically, no temp is constructed and result is put directly into Test a object.