// Created by Ketan Lalcheta // Improved by me as response to q/a question #include #include using namespace std; // the publish-subscribe infrastructure class Observable; class Observer { public: virtual void Update(Observable *observable) = 0; }; class Observable { private: list _observers; public: virtual void Attach(Observer* observer) { _observers.push_back(observer); } virtual void Detach(Observer* observer) { auto it = _observers.begin(); for(; it != _observers.end(); ++it) { if(*it == observer) _observers.erase(it); break; } } virtual void Notify() { for(auto it = _observers.begin(); it != _observers.end(); ++it) { (*it)->Update(this); } } }; // Application: Family Ties .... // Note that there is no publish-subscribe // infrastructure code here any more // a child is an observable thing for parents // but observable for anybody (aunt and uncle, baby sitter, ...) // it notifies to whoever is interested ... class Child : public Observable { private: string name; public: Child(string name) { this->name = name; } string Name() { return name; } void GotInjured() { cout << name << ": 'I got injured'\n"; Notify(); } }; // parents have a dependency on child; it is specifically // a child - their child - they want to be notified about // thus, child is injected into the parents class Parent : public Observer { public: virtual ~Parent() { cout << "Parent de-allocated\n"; } virtual void Update(Observable*) = 0; }; class Father : public Parent { public: ~Father() { cout << "Father de-allocated\n"; } void run() { cout << "Father runs and reaches to son : "; } // overrides Update from Observer virtual void Update(Observable *subject) { Child * child = dynamic_cast(subject); run(); cout << "Oops, " << child->Name() << "! Your father is here to help you." << endl; } }; class Mother : public Parent { public: ~Mother() { cout << "Mother de-allocated\n"; } void run() { cout << "Mother runs and reaches to son : "; } // overrides Update from Observer virtual void Update(Observable* subject) { Child * child = dynamic_cast(subject); run(); cout << "Oops, " << child->Name() << "Your mother is here to help you." << endl; } }; // driver int main() { Child peter("Peter"); Child bob("Bob"); Parent* papa = new Father(); Parent* mom = new Mother(); peter.GotInjured(); cout << endl << endl; cout << "-------->Now adding mom and papa as observers" << endl; peter.Attach(papa); peter.Attach(mom); bob.Attach(mom); bob.GotInjured(); cout << endl << endl; peter.GotInjured(); cout << endl << endl; cout << "-------->Now removing papa as observers" << endl; peter.Detach(papa); peter.GotInjured(); cout << endl << endl; delete mom; delete papa; return 0; }

Observer Design Pattern