Читал книги. Видел много однотипных примеров. Везде написано делать так:

#include <iostream>
using namespace std;

class Complex
{
    double re;
    double im;
public:
    Complex (double re, double im);
    friend ostream& operator<< (ostream& s, const Complex& z);
};

Complex::Complex (double re, double im)
{
    this->re = re;
    this->im = im;
}

ostream& operator<< (ostream& s, Complex& z)
{
    return s << '(' << z.re << ',' << z.im << ')';
}


void main(void)
{
    Complex a(1.2,0.2);
    cout << "a=" << a;
}

Однако,

Z:\Coding\Cpp\iotest\main.cpp(23) : error C2248: 're' : cannot access private member declared in class 'Complex'
Z:\Coding\Cpp\iotest\main.cpp(6) : see declaration of 're'
Z:\Coding\Cpp\iotest\main.cpp(23) : error C2248: 'im' : cannot access private member declared in class 'Complex'
Z:\Coding\Cpp\iotest\main.cpp(7) : see declaration of 'im'

Почему? Ведь friend же??