#include <iostream>

class A
{
public:

  const A& foo(int x) const  { return *this; }
  const A& operator<<(int x) const  { return *this; }
};

int f(int x) 
{ 
  std::cout << x << std::endl; 
  return x;
}

int main()
{
  A a;

  a.foo(f(1)).foo(f(2)).foo(f(3));
  a << f(4) << f(5) << f(6);
}

3
2
1
6
5
4