interface.cpp

class Player {
public:
    Player();
    virtual void who() = 0;
};

class HumanPlayer: public Player {
    void who() { ... };
};

class CpuPlayer: public Player {
    void who() { ... };
};


Player *p1 = new HumanPlayer();
Player *p2 = new CpuPlayer();
p1->who();
p1->who();