Why are operators sometimes stand-alone and sometimes class methods?

Question

Why is that sometimes an operator override is defined as a method in the class, like

MyClass& MyClass::operatorFoo(MyClass& other) { .... return this; };

and sometimes it's a separate function, like

MyClass& operatorFoo(MyClass& first, MyClass& bar)

Are they equivalent? What rules govern when you do it one way and when you do it the other?

Answer 1

If you want to be able to do something like 3 + obj you have to define a free (non-member) operator.

If you want to make your operators protected or private, you have to make them methods.

Some operators cannot be free functions, e.g., operator->.

This is already answered here:

difference between global operator and member operator