#include<iostream>
#include<list>
#include<tr1/memory> // for Smart Pointers
using namespace std;
using namespace tr1;

class Observer {
public:
  Observer() {}
  void update() {
    cout << "Signal Detected!" << endl;
  }
};

class Subject {
  typedef weak_ptr<Observer> WPtr;
  list<WPtr> obs;
  class Eq: unary_function<WPtr,bool> {
    WPtr lhs;
  public:
    Eq(WPtr wp): lhs(wp) {}
    bool operator()(WPtr rhs) {
      return lhs.lock() == rhs.lock();
    }
  };
public:
  Subject(): obs(0) {}
  void attach(WPtr o) { obs.push_back(o); }
  void detach(WPtr o) { obs.remove_if(Eq(o)); }
  void notify() {
    list<WPtr>::iterator oiter = obs.begin();
    while (oiter != obs.end()) {
      if (shared_ptr<Observer> tmp = oiter->lock())
        tmp->update();
      else
        cout << "No More Exist!" << endl;
      ++oiter;
    }
  }
};

int main()
{
  Subject s;
  shared_ptr<Observer> o0(new Observer()); s.attach(o0);
  shared_ptr<Observer> o1(new Observer()); s.attach(o1);
  shared_ptr<Observer> o2(new Observer()); s.attach(o2);
  o0.reset();   // destruct Observer -> No More Exist!
  s.detach(o1); // remove from notification list -> Just Unseen from Subject class
  s.notify();
  return 0;
}