An example of private member access without friendship in C++

access_to_private_members_1.cpp

// This example is a variation on the code posted by Dave Abrahams at:
// https://gist.github.com/1528856.

#include <iostream>

template <typename Tag>
typename Tag::type saved_private_v;

template <typename Tag, typename Tag::type x>
bool save_private_v = (saved_private_v<Tag> = x);

class Widget {
public:
    Widget(int i) : i_(i) {}
private:
    int i_;
    int f_() const {return i_;}
};

struct Widget_i_ {using type = int Widget::*;};
struct Widget_f_ {using type = int (Widget::*)() const;};

template bool save_private_v<Widget_i_, &Widget::i_>;
template bool save_private_v<Widget_f_, &Widget::f_>;

int main() {
    Widget w(42);
    std::cout << w.*saved_private_v<Widget_i_> << '\n';
    std::cout << (w.*saved_private_v<Widget_f_>)() << '\n';
}