difference between global operator and member operator

Question

Is there a difference between defining a global operator that takes two references for a class and defining a member operator that takes only the right operand?

Glob

Answer 1

Your smartest option is to make it a friend function.

As JaredPar mentions, the global implementation cannot access protected and private class members, but there's a problem with the member function too.

C++ will allow implicit conversions of function parameters, but not an implicit conversion of this.

If types exist that can be converted to your X class:

class Y
{
public:
    operator X();  // Y objects may be converted to X
};


X x1, x2;
Y y1, y2;

Only some of the following expressions will compile with a member function.

x1 == x2;   // Compiles with both implementations
x1 == y1;   // Compiles with both implementations
y1 == x1;   // ERROR!  Member function can't convert this to type X
y1 == y2;   // ERROR!  Member function can't convert this to type X

The solution, to get the best of both worlds, is to implement this as a friend:

class X
{
    int value;

public:

    friend bool operator==( X& left, X& right ) 
    {
        return left.value == right.value;
    };
};