How do I implement Observer pattern?

#include 
#include 
#include 

// Forward declaration of Observer class
class Observer;

// Subject class
class Subject {
public:
    virtual void attach(Observer* observer) = 0;
    virtual void detach(Observer* observer) = 0;
    virtual void notify() = 0;
};

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

// Concrete Subject class
class ConcreteSubject : public Subject {
private:
    std::vector observers;
    int state;

public:
    void attach(Observer* observer) override {
        observers.push_back(observer);
    }

    void detach(Observer* observer) override {
        observers.erase(std::remove(observers.begin(), observers.end(), observer), observers.end());
    }

    void notify() override {
        for (Observer* observer : observers) {
            observer->update(state);
        }
    }

    void setState(int value) {
        state = value;
        notify();
    }
};

// Concrete Observer class
class ConcreteObserver : public Observer {
private:
    int observerState;

public:
    void update(int value) override {
        observerState = value;
        std::cout << "Observer state updated to: " << observerState << std::endl;
    }
};

int main() {
    ConcreteSubject subject;
    ConcreteObserver observer1;
    ConcreteObserver observer2;

    subject.attach(&observer1);
    subject.attach(&observer2);

    subject.setState(1); // Both observers' state will be updated
    subject.setState(2); // Both observers' state will be updated

    return 0;
}