Add example of observer pattern in C++

cpp/patterns/observer/main.cpp

@@ -0,0 +1,33 @@
+
+#include <iostream>
+
+#include "observer.hpp"
+
+int main(const int argc, const char * argv[])
+{
+  Subject subject("Radio");
+  Observer *john = new Observer(subject);
+
+  // Test alerting john as the only subscribed observer
+  std::cout << "\nAlerting one observer...\n\n";
+  subject.SendMessage("Alert 1\n");
+
+
+  // Attach a new observer
+  Observer joe(subject);
+  // Alert all subscribed observers
+  std::cout << "\nAlerting all observers...\n\n";
+  subject.SendMessage("Alert 2\n");
+
+  std::cout << std::endl;
+  // Detach joe from the subject
+  joe.Detach();
+  // Send another alert to john
+  std::cout << "Alerting one observer...\n\n";
+  subject.SendMessage("Alert 3\n");
+  john->Detach();
+
+  // This alert will not be heard by anyone
+  std::cout << "\nAlerting no observers...\n\n";
+  subject.SendMessage("Alert 4\n");
+}