Question

Consider this:

#include <iostream>

struct A{
   A(){
       std::cout << "Create empty A" << std::endl;
   }

   A(const A& a){
       // Why is this never called??
       std::cout << "Never called" << std::endl;
   }
};

A genA() {
   A a;
   return a;
}

int main(int argc, const char *argv[])
{
   A a(genA()); // Expected to call copy constructor
   return 0;
}

Why is the copy constructor not called?

What should I do if I want to ensure that “Never called” is printed on the screen every time I copy A.

Answer 1

This is called as Return value optimization.
Compiler can optimize your code so that it bulds the object directly in the location where it would have been copied too. Thus there will be no reason to use the copy constructor.

Note: The standard explicitly allows it do so.