Question

I.e if within a class definition I overload operator+ or operator= does this have any effect on the operator+=? and vice versa.

Or are these operators completely independent unless otherwise defined?

Answer 1

No, these operators are completely independent.

You can of course implement one using the others, but by default they’re independent.

struct X
{
    X& operator = (const X&);
    X operator + (const X&) const;
    //X& operator += (const X& other) 
    //        { operator=(operator+(other)); return *this; }
};

X x, y;
x += y; //doesn't compile unless you uncomment that line