A nested C++ class "using private members of its parent"

nested_class.cpp

#include <iostream>

class Foo {
    public:
        class Bar {
            public:
                Bar(Foo* owner) : owner(owner) {}
                int getter() { return owner->a; }
            private:
                Foo* const owner;
        };

        Bar b;
        Foo() : b(this), a(42) {}

    protected:
        int a;
};

int main(int argc, char* argv[]) {
    Foo f;
    std::cout << f.b.getter() << std::endl;
    return 0;
}