Observer

Example

#include <iostream>
#include <vector>
#include <algorithm>
using namespace std;

typedef char* Message;

struct Observer { // abstract
    virtual void notify(Message m) = 0;
};

struct Concrete_observer : public Observer {
    Concrete_observer(char *name) : name(name) {}
    virtual void notify(Message m) {
        cout << name << " notified - message: " << m << endl;
    }

    char *name;
};

struct Subject { // abstract
    void register_observer(Observer *o) {
        observer_collection.push_back(o);
    }
    void unregister_observer(Observer *o) {
        remove(observer_collection.begin(), observer_collection.end(), o);
    }
    void notify_observers(Message m);

    vector<Observer *> observer_collection;
};

void Subject::notify_observers(Message m) {
    for(vector<Observer *>::iterator i = observer_collection.begin();
            i != observer_collection.end(); ++i)
            (*i)->notify(m);
}

class Concrete_subject : public Subject { };

main() {
    Concrete_subject cs;
    Concrete_observer obs1("obs1"), obs2("obs2");

    cs.register_observer(&obs1);
    cs.register_observer(&obs2);
    cs.notify_observers("first message");

    cs.unregister_observer(&obs1);
    cs.notify_observers("second message");
}

References

• Wikipedia:Observer pattern