Type information in observer pattern?

[Lord-Turmoil]

Int the current implementation of observer pattern, observer can only get abstract Subject*. In this case, the observer cannot tell what subject it is.

So, I wonder if there is any way to support typing in this pattern? If template is used, can it avoid another embarrassing matter that the implementation must be placed in the header file?

For example, I want to do things like this:

class ConcreteObserver : public Observer
{
public:
    void Update(ConcreteSubject& subject);
}

[Lord-Turmoil]

// abstract

template <typename TObserver, typename TSubject>
class Observer;

template <typename TObserver, typename TSubject>
class Subject
{
public:
    void attach(Observer<TObserver, TSubject>* observer)
    {
        _observers.push_back(observer);
    }

    void notify()
    {
        for (auto observer : _observers)
        {
            observer->genericUpdate(this);
        }
    }
private:
    std::vector<Observer<TObserver, TSubject>*> _observers;
};

template<typename TObserver, typename TSubject>
class Observer
{
    friend class Subject<TObserver, TSubject>;
private:
    // don't want to be inherited, but only want Subject to access it
    void genericUpdate(Subject<TObserver, TSubject>* subject)
    {
        static_cast<TObserver*>(this)->update(*static_cast<TSubject*>(subject));
    }
};

// concrete

class ConcreteObserver;

class ConcreteSubject : public Subject<ConcreteObserver, ConcreteSubject>
{
};

class ConcreteObserver : public Observer<ConcreteObserver, ConcreteSubject>
{
public:
    void update(ConcreteSubject& subject)
    {
        std::cout << "Now we have type information." << std::endl;
    }
};

I can only come up with this ugly one with many template arguments and forward declarations. Maybe you can give a better solution to this.

[Lord-Turmoil]

Interestingly, I found some pretty good answers. One is class-oriented solution, and the other is function oriented.

• https://www.sobyte.net/post/2021-10/cplusplus-17-observer-pattern/
• https://github.com/Ansoulom/cpp-observe