Add example for classic singleton

cpp/patterns/singleton/singleton.hpp

@@ -0,0 +1,25 @@
+
+#ifndef SINGLETON_H
+#define SINGLETON_H
+
+#include <iostream>
+#include <string>
+
+class Singleton {
+public:
+  static Singleton &getInstance();
+  ~Singleton();
+
+  inline void showMessage() const { std::cout << message;}
+  inline void updateMessage(const std::string &m) { message = m;}
+
+private:
+  std::string message;
+
+  // Don't allow copying of this class
+  Singleton() { message = "New singleton\n";}
+  Singleton(const Singleton &) {}
+  Singleton &operator=(const Singleton &) {}
+};
+
+#endif // SINGLETON_H