copy constructor, pass by value and return by value, chain operation, compiler

Question

I wrote this program in two different compilers and I got two different results:

#include <iostream>
using namespace std;

class Point {
public:
  int n;
  Point() { n = 0; }
  Point operator= (Point p) { return *this; }
  Point(const Point& p) { cout<<"copy\n"; }
  ~Point() { cout<<"Destruct\n"; }
};

int main() {
  Point p1, p2, p3;
  p1 = p2 = p3;
  return 0;
}

Compiler 1:

copy
copy
copy
Destruct
Destruct
Destruct
Destruct
Destruct
Destruct

Compiler 2:

copy
copy
Destruct
copy
Destruct
Destruct
Destruct
Destruct
Destruct

I know that some compilers optimize the pass/return by value of an object from a function by not calling the copy constructor. Is this the reason for the difference between the two results?

More importantly, why is the copy constructor called twice for the p2 = p3 part of the code but only once for p1 = ... ?

I don't do much OO programming using C++, that is why I am confused with this simple question. I really appreciate some hints

Answer 1

Your assignment operator should return Point & not Point, and it should take the parameter as a reference, too :

Point &operator = (const Point &p) { return *this; }

Otherwise, unnecessary copying may occur. Most likely, one copy is created going in to the assignment operator, then the returned value is copied to both p2 and p1.