Overloading operators in and out of classes

• Suppose you have a class Complex with two pieces of private data, double real; and double imag;
• You wish to overload the + operator so that it works on objects of type Complex
• There are two ways: (a) declare the operator outside the class as a friend of the Complex class; (b) declare the operator to be a member of the Complex class
• (a) declaration:
  

  class Complex {
  public:
  Complex(double re, double im) : real(re), imag(im) {}
  ...
  friend Complex operator+(Complex& a, Complex& b);
  private:
  double real;
  double imag;
  }

  
  definition (out of the class):
  

  Complex operator+(Complex& a, Complex& b) {
  Complex ret(a.real + b.real, a.imag + b.imag);
  return ret;
  }

• (b) declaration:
  

  class Complex {
  public:
  Complex(double re, double im) : real(re), imag(im) {}
  ...
  Complex operator+(Complex& b);
  private:
  double real;
  double imag;
  }

  
  definition (in the class):
  

  Complex Complex::operator+(Complex& b) {
  Complex ret(this->real + b.real, this->imag + b.imag);
  return ret;
  }

• this-> is not strictly required, but it makes it clear that the left operand is now the object calling the operator+ method