#include <iostream>

struct Base {
    virtual void f() {
        std::cout << 1;
    }
};

struct Derived: Base {
    virtual void f() {
        std::cout << 2;
    }
};

struct Subderived: Derived {
    void f() {
        std::cout << 3;
    }
};

int main() {
    Derived d;
    Base& b = d;
    b.f();

    return 0;
}