observer pattern in cpp

#include <vector>

class ; 
class Observer { 
public:
    virtual ~Observer() = default;
    virtual void Update(Subject&) = 0;
};

class  { 
public: 
    virtual ~Subject() = default;
    void Attach(Observer& o) { observers.push_back(&o); }
    void Detach(Observer& o) {
        observers.erase(std::remove(observers.begin(), observers.end(), &o));
    }
    void Notify() {
        for (auto* o : observers) {
            o->Update(*this);
        }
    }
private:
    std::vector<Observer*> observers; 
};

class ClockTimer : public Subject { 
public:
    void SetTime(int hour, int minute, int second) {
        this->hour = hour; 
        this->minute = minute;
        this->second = second;

        Notify(); 
    }

    int GetHour() const { return hour; }
    int GetMinute() const { return minute; }
    int GetSecond() const { return second; }

private: 
    int hour;
    int minute;
    int second;
}; 

class DigitalClock: public Observer { 
public: 
    explicit DigitalClock(ClockTimer& s) : subject(s) { subject.Attach(*this); }
    ~DigitalClock() { subject.Detach(*this); }
    void Update(Subject& theChangedSubject) override {
        if (&theChangedSubject == &subject) {
            Draw();
        }
    }

    void Draw() {
        int hour = subject.GetHour(); 
        int minute = subject.GetMinute(); 
        int second = subject.GetSecond(); 

        std::cout << "Digital time is " << hour << ":" 
                << minute << ":" 
                << second << std::endl;           
    }

private:
    ClockTimer& subject;
};

class AnalogClock: public Observer { 
public: 
    explicit AnalogClock(ClockTimer& s) : subject(s) { subject.Attach(*this); }
    ~AnalogClock() { subject.Detach(*this); }
    void Update(Subject& theChangedSubject) override {
        if (&theChangedSubject == &subject) {
            Draw();
        }
    }
    void Draw() {
        int hour = subject.GetHour(); 
        int minute = subject.GetMinute(); 
        int second = subject.GetSecond(); 

        std::cout << "Analog time is " << hour << ":" 
                << minute << ":" 
                << second << std::endl; 
    }
private:
    ClockTimer& subject;
};

int main() { 
    ClockTimer timer; 
    DigitalClock digitalClock(timer); 
    AnalogClock analogClock(timer); 

    timer.SetTime(14, 41, 36);
}