Add example for classic singleton

cpp/patterns/singleton/CMakeLists.txt

@@ -16,5 +16,6 @@ project(
 )
 
 add_library(singleton "singleton.cpp")
+add_library(singleton-pointer "singleton-pointer.cpp")
 add_executable(singleton-test "main.cpp")
-target_link_libraries(singleton-test singleton)
+target_link_libraries(singleton-test singleton singleton-pointer)

cpp/patterns/singleton/main.cpp

@@ -1,8 +1,13 @@
 
-#include "singleton.h"
+#include "singleton.hpp"
+#include "singleton-pointer.hpp"
 
 int main(const int argc, const char *argv[])
 {
+
+  /****************************************************************************/
+  // Meyers Singleton - static getInstance
+
   // Creates a singleton, initializes message in ctor
   Singleton &s = Singleton::getInstance();
   s.showMessage();
@@ -12,4 +17,20 @@ int main(const int argc, const char *argv[])
   // Update already existing Singleton message, show it
   Singleton::getInstance().updateMessage("Second update\n");
   s.showMessage();
+
+  /****************************************************************************/
+  // Classic Singleton - pointer getInstance
+
+
+  std::cout << "\n##### ClassicSingleton #####\n\n";
+
+  // Creates a classic singleton, initializes message in ctor
+  ClassicSingleton *classicSingleton = ClassicSingleton::getInstance();
+  classicSingleton->showMessage();
+  // Update already existing ClassicSingleton message, show it
+  classicSingleton->updateMessage("First update\n");
+  ClassicSingleton::getInstance()->showMessage();
+  // Update already existing ClassicSingleton message, show it
+  ClassicSingleton::getInstance()->updateMessage("Second update\n");
+  classicSingleton->showMessage();
 }

cpp/patterns/singleton/singleton-pointer.cpp

@@ -0,0 +1,11 @@
+
+#include "singleton-pointer.hpp"
+
+// Initialize pointer to instance to null; Allocate on first call to getInstance
+ClassicSingleton* ClassicSingleton::instance = nullptr;
+
+ClassicSingleton* ClassicSingleton::getInstance(){
+  // If the instance is null, return a new one; Otherwise return instance
+  if(instance == nullptr) instance = new ClassicSingleton;
+  return instance;
+}

cpp/patterns/singleton/singleton-pointer.hpp

@@ -0,0 +1,29 @@
+
+#ifndef SINGLETON_POINTER_HPP
+#define SINGLETON_POINTER_HPP
+
+#include <iostream>
+#include <string>
+
+class ClassicSingleton{
+public:
+  // returns pointer to single getInstance
+  static ClassicSingleton* getInstance();
+
+  // example functions manipulating singleton object
+  inline void showMessage() const { std::cout << message;}
+  void updateMessage(const std::string &m) { message=m;}
+
+private:
+  // Do not allow construction of this object; Use getInstance() instead
+  ClassicSingleton(){ message = "New ClassicSingleton\n";}
+  // Do not allow copying of this object
+  ClassicSingleton(const ClassicSingleton&){}
+  ClassicSingleton& operator=(const ClassicSingleton&){}
+
+  // Static pointer to instance of this singleton
+  static ClassicSingleton* instance;
+  std::string message;
+};
+
+#endif // SINGLETON_POINTER_HPP

cpp/patterns/singleton/singleton.cpp

@@ -1,5 +1,5 @@
 
-#include "singleton.h"
+#include "singleton.hpp"
 
 Singleton::~Singleton()
 {