Observer design pattern in C++

Question

Is the observer design pattern already defined in STL (Like the java.util.Observer and java.util.Observable in Java) ?

Answer 1

#include <iostream>
#include <string>
#include <set>
using namespace std;
class Subject;
class Observer {
public:
  virtual void update(Subject & subject) = 0;
};
// also knows as Observable in literature
class Subject
{
  string state;
  set<Observer*> observers;
public:
  void attachObserver(Observer *o) { observers.insert(o); }
  void detachObserver(Observer *o) { observers.erase(o); }
  void notifyObservers()
  {
    for (auto &o : observers)
    {
      o->update(*this);
    }
  }
  string getState() { return state; }
  void changeState(const string & s)
  {
    state = s;
    notifyObservers();
  }
};
class ObserverImpl : public Observer
{
  string state;
public:
  void update(Subject & sbj) override
  {
    state = sbj.getState();
  }
  string getState() { return state; }
};
int main()
{
  ObserverImpl a, b, c;
  Subject subject;
  subject.attachObserver(&a);
  subject.attachObserver(&b);
  subject.attachObserver(&c);
  subject.changeState("Observer pattern");
  cout << a.getState() << endl;
  cout << b.getState() << endl;
  cout << c.getState() << endl;
  return 0;
}

please also see UML/flow diagrams http://codepatterns.ddns.net/