private_member_of_same_class.cpp

#include <iostream>

class Foo {
public:
    Foo() {}
    Foo(const int x) : x(x) {}

    int other_x(const Foo& other) {
        // Accessing private member variable
        return other.x;
    }

private:
    int x;
};

class Derived : public Foo {
public:
    Derived() {}
    Derived(const int x) : Foo(x) {}
};

int main() {
    Foo f1, f2(4);
    Derived d(42);

    // Same class
    std::cout << f1.other_x(f2) << std::endl;

    // Inherited
    std::cout << d.other_x(f2) << std::endl;
    std::cout << f1.other_x(d) << std::endl;

    // Error
    //f2.x;

    return 0;
}