struct Base {
    virtual ~Base() {}
    virtual Base *clone() = 0;

    virtual void hello() = 0;
};

struct DerivedA : public Base {
    ~DerivedA() = default;
    Base *clone() override;

    void hello() override;
};

struct DerivedB : public Base {
    ~DerivedB() = default;
    Base *clone() override;

    void hello() override;
};

struct GameState {
    GameState();

    GameState(const GameState &other);
    GameState &operator=(const GameState &other);
    ~GameState();

    Base *thing;
    void switchThing();
};