Add example of observer pattern in C++

cpp/patterns/observer/observer.cpp

@@ -0,0 +1,39 @@
+
+#include "observer.hpp"
+
+void Subject::Notify()
+{
+  for (const auto &o : observerList) { o->Update(message);}
+}
+
+void Subject::SendMessage(std::string message_)
+{
+  message = message_;
+  Notify();
+}
+
+
+/******************************************************************************/
+// Observer
+
+int Observer::totalObservers = 0;
+
+Observer::Observer(Subject &s) : subject(s) {
+  std::cout << "Observer " << ++totalObservers << " is subscribed to "
+            << s.getName() << "\n";
+  s.Attach(this);
+  number = totalObservers;
+}
+
+void Observer::Update(std::string &message)
+{
+  std::cout << "Observer " << number << " has message from "
+            << subject.getName() << ": " << message;
+}
+
+void Observer::Detach()
+{
+  std::cout << "Observer " << number << " has detached from "
+            << subject.getName() << std::endl;
+  subject.Detach(this);
+}