Question

friend ostream& operator<<(ostream& os, MyClass& obj);

I have e few questions:
1. Why do I need to write ‘friend’?
2. Why do I need to write ‘&’ before ‘operator’, ‘os’ and ‘obj’?

Answer 1

The & in MyClass& makes the function take a reference to a MyClass object, not the object itself. (Similarly for the others.)

References are lightweight to pass around, and any change you make to the obj affects the original object. Without the & you would be instructing the compiler to construct a whole new MyClass in the call, destroy it on return and throw away any changes you might have made to its internal state.

The return of an ostream& is conventionally used to return the same ostream& that was passed in so you can write chains of shifts like cout << "hello " << 42 << endl; and have them behave the way you expect. (You could have it return something different – C++ makes it easy to completely mess with people’s expectations – but don’t do that.)