Final answer:
The leftmost operand must be the one on the left side of the operator when an operator function is implemented as a member function, as it implicitly becomes the 'this' argument for the function.
Step-by-step explanation:
The one on the left side of the operator
When an operator function is implemented as a member function in a class in object-oriented programming, specifically in C++ or similar languages, the leftmost operand, which is the object on which the operator is being invoked, implicitly becomes the 'this' argument for the member function. Therefore, when you define an operator as a member function, the leftmost operand, meaning the one before the operator, must be an instance of the class to which the operator function belongs. This is because member functions are called with respect to an object of the class. For example, in the expression obj1 + obj2, if the operator '+' is a member of the class of obj1, then obj1 implicitly becomes the 'this' and the operation is essentially processed as obj1.operator+(obj2).
When the operator function is implemented as a member function, the leftmost operand must be the one on the left side of the operator.
For example, in the code objectA + objectB, if the operator function is a member function of objectA, then objectA will be the leftmost operand.