Add example of observer pattern in C++

cpp/patterns/CMakeLists.txt

@@ -22,3 +22,4 @@ add_subdirectory(factory)
 add_subdirectory(prototype)
 add_subdirectory(abstract-factory)
 add_subdirectory(state)
+add_subdirectory(observer)

cpp/patterns/observer/CMakeLists.txt

@@ -0,0 +1,20 @@
+###############################################################################
+## Author: Shaun Reed                                                        ##
+## Legal: All Content (c) 2021 Shaun Reed, all rights reserved               ##
+## About: A project for practicing the observer C++ design pattern           ##
+##                                                                           ##
+## Contact: shaunrd0@gmail.com  | URL: www.shaunreed.com | GitHub: shaunrd0  ##
+##############################################################################
+#
+cmake_minimum_required(VERSION 3.15)
+project(
+    #[[NAME]]   Observer
+    VERSION     1.0
+    DESCRIPTION "An example of the state design pattern in C++"
+    LANGUAGES   CXX
+)
+add_compile_options("-Wall")
+
+add_library(observer "observer.cpp")
+add_executable(observer-test "main.cpp")
+target_link_libraries(observer-test observer)

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");
+}

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);
+}

cpp/patterns/observer/observer.hpp

@@ -0,0 +1,47 @@
+
+#ifndef OBSERVER_HPP
+#define OBSERVER_HPP
+
+#include <string>
+#include <list>
+#include <iostream>
+
+#include "observer.hpp"
+
+class Observer;
+
+class Subject {
+public:
+  Subject(std::string name_) : name(name_) {}
+  virtual ~Subject() { std::cout << name << " subject destroyed\n";}
+
+  void Attach(Observer * o) { observerList.push_back(o);}
+  void Detach(Observer * o) { observerList.remove(o);}
+  void Notify();
+
+  void SendMessage(std::string message_);
+  inline std::string getName() const { return name;}
+
+private:
+  std::list<Observer *> observerList;
+  std::string message, name;
+};
+
+
+/******************************************************************************/
+// Observer
+
+class Observer {
+public:
+  Observer(Subject &s);
+
+  void Update(std::string &message);
+  void Detach();
+
+private:
+  Subject &subject;
+  int number;
+  static int totalObservers;
+};
+
+#endif // OBSERVER_HPP