Question

For example,

Number operator+(Number a, Number b) {

    return Number(a.x + b.x);


}

Would this cause some kind of “unable to access private member error”. I understand that if I don’t pass by reference, the Number a, and Number b are copied on the stack and used in the body of the function. However, I don’t see why they do not allow access to the originals’ private members. How am I misunderstanding the concept of an object? Also how come friend and member function don’t need to pass by reference?

Answer 1

operator+ is an unbound function, i.e. it is not a member of Number so, this is the rule, it has no implicit access to private members.

The rule is not affected by you passing Number objects by value or by reference. Access protection is applied to every access to a Number object, even if it is your private, stack-based copy.

There are at least three ways out:

• declare Number operator+(Number, Number) a friend of Number
• add a public getter for x so the variable is accessible.
• implement the += operator as a member of the class and implement the free operator in terms of it: return Number(a) += b;