Binary operator member function one argument

A binary operator has two operands, for example a + b. When it is overloaded as a member function, the left operand is the object that calls the function, and it is passed implicitly through the this pointer. So only the right operand needs to be passed explicitly as an argument.

class Complex {
    float re, im;
public:
    Complex operator+(Complex b) {       // one argument: right operand
        Complex r;
        r.re = re + b.re;                // re  is this->re  (left operand a)
        r.im = im + b.im;
        return r;
    }
};
// c = a + b;   is interpreted as   c = a.operator+(b);

Here a is the invoking object (available as this), and b is the only explicit argument. In contrast, a friend (non-member) function has no this pointer, so it needs two arguments for a binary operator.