The stand-alone function operator+ is a friend of the Complex class
class Complex
{
...
friend Complex operator+ (const Complex& a, const Complex& b);
...
};
so it can read (and manipulate) the private data parts \( re \) and \( im \) via
inline Complex operator+ (const Complex& a, const Complex& b)
{ return Complex (a.re + b.re, a.im + b.im); }