View difference between Paste ID: pwxyzFFd and Ad4xKQcr

 class Observable
 {
 public:
   Observable() : obs_list() {}
 
   void add_observer(Observer* o) { obs_list.push_back(o); }
   void remove_observer(Observer* o) { ... }
 
 protected:
   void notify_all()
   {
     for (Observer* o : obs_list)
       o->notify();
   }
 
 private:
   vector<Observer*> obs_list;
 };
 
 class Data : public Observable
 {
   ...
 };
 
 
 class Observer
 {
   virtual Observer();
   virtual void notify() = 0;
   ...
 };
 
 class Bar_Chart : public Observer
 {
   Bar_Chart(){...}
 
   void notify() override
   {
     ...
   }
 };
 
 int main()
 {
   Data& observable{};
   Bar_chart* bar_chart{new Bar_Chart{}};
   observable.add_observer(bar_chart);
   ...
 }