#include <iostream>

struct foo {
    int a = 5;
    int b = 2;
    void print() {
        std::cout << a + b << std::endl;
    }
};

template <typename T>
struct bar : T {
    void print2() {
        std::cout << T::a * T::b << std::endl;
        T::b++;
    }
};

template <typename Base>
bar<Base>* extend_with_bar(Base& base) {
    return static_cast<bar<Base>*>(&base);
}

int main() {
    foo f;

    f.print();

    auto b = extend_with_bar(f);

    b->print2();
    b->print2();
    b->print();
    f.print();
}