// interface

class Subject
{
public:
    virtual void registerObserver() = 0;
    virtual void removeObserver() = 0;
    virtual void notifyObserver() = 0;
};

class Observer
{
public:
    virtual void update() = 0;
};

// classes

class ConcreteSubject : public Subject
{
public:
    void getState();
    void setState();
};

class ConcreteObserver : public Observer
{
public:
};