How to declare an operator overload outside of a C++ class?

Question

OOP

How to declare an addition operator for two classes, provided that the operator itself must be declared OUTSIDE the class?
Tried like this, gives an error that I can not access the fields a and b. With this played for a long time, hands fall.

class Zespolona {
private:
    double a, b;
public:
    int getA () {
        return a;
    }
    int getB () {
        return b;
    }
    int setA (int i) {
        a = i;
    }
    int setB (int i) {
        b = i;
    }
    Zespolona operator+(Zespolona q);
    Zespolona operator-(Zespolona q);
};

Zespolona operator +(Zespolona q) {
    double aa = q.getA();
    aa += a;
    double bb = q.getB();
    bb += this->b;
    return q;
}
Zespolona operator -(Zespolona q) {
    double aa = q.getA();
    aa -= this->a;
    double bb = q.getB();
    bb -= this->b;
    return q;
}

If something is not clear in the question, please clarify. Now I live in Poland, I forget Russian already. Dziękuję z gory :)

Answer 1

Zespolona operator+(const Zespolona& left, const Zespolona& right) { ... }
Zespolona operator-(const Zespolona& left, const Zespolona& right) { ... }

Inside the class itself, you need to add friend declarations:

friend Zespolona operator+(const Zespolona& left, const Zespolona& right);
friend Zespolona operator-(const Zespolona& left, const Zespolona& right);

Answer 2

#include <iostream>
using namespace std;

class Zespolona 
{
public:
  Zespolona(double _a = 0, double _b = 0) :a(_a), b(_b) {}
  void print() {cout << a << " " << b << endl;}
private:
  double a, b;
  friend Zespolona operator+(const Zespolona& l, const Zespolona& r);
};

Zespolona operator+(const Zespolona& l, const Zespolona& r)
{
  return Zespolona(l.a+r.a, l.b+r.b);
}

int main()
{
  Zespolona one(1,2), two(3,4);
  Zespolona three = one + two;
  three.print();
  system("pause");
  return 0;
}