Add subdirectory for practicing design patterns in C++

+ Add example for singleton

cpp/CMakeLists.txt

@@ -20,4 +20,5 @@ project(
 add_subdirectory(algorithms)
 add_subdirectory(cmake)
 add_subdirectory(datastructs)
+add_subdirectory(patterns)
 add_subdirectory(sdl-cmake)

cpp/patterns/CMakeLists.txt

@@ -0,0 +1,18 @@
+###############################################################################
+## Author: Shaun Reed                                                        ##
+## Legal: All Content (c) 2021 Shaun Reed, all rights reserved               ##
+## About: A project for practicing C++ design patterns                       ##
+##                                                                           ##
+## Contact: shaunrd0@gmail.com  | URL: www.shaunreed.com | GitHub: shaunrd0  ##
+##############################################################################
+#
+cmake_minimum_required(VERSION 3.15)
+
+project(
+    #[[NAME]]   DesignPatterns
+    VERSION     1.0
+    DESCRIPTION "A project for practicing various design patterns in C++"
+    LANGUAGES   CXX
+)
+
+add_subdirectory(singleton)

cpp/patterns/singleton/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 singleton C++ design pattern          ##
+##                                                                           ##
+## Contact: shaunrd0@gmail.com  | URL: www.shaunreed.com | GitHub: shaunrd0  ##
+##############################################################################
+#
+cmake_minimum_required(VERSION 3.15)
+
+project(
+    #[[NAME]]   Singleton
+    VERSION     1.0
+    DESCRIPTION "An example of the singleton design pattern in C++"
+    LANGUAGES   CXX
+)
+
+add_library(singleton "singleton.cpp")
+add_executable(singleton-test "main.cpp")
+target_link_libraries(singleton-test singleton)

cpp/patterns/singleton/main.cpp

@@ -0,0 +1,15 @@
+
+#include "singleton.h"
+
+int main(const int argc, const char *argv[])
+{
+  // Creates a singleton, initializes message in ctor
+  Singleton &s = Singleton::getInstance();
+  s.showMessage();
+  // Update already existing Singleton message, show it
+  s.updateMessage("First update\n");
+  Singleton::getInstance().showMessage();
+  // Update already existing Singleton message, show it
+  Singleton::getInstance().updateMessage("Second update\n");
+  s.showMessage();
+}

cpp/patterns/singleton/singleton.cpp

@@ -0,0 +1,15 @@
+
+#include "singleton.h"
+
+Singleton::~Singleton()
+{
+  // Delete any allocated data, close any opened files, etc..
+}
+
+Singleton &Singleton::getInstance()
+{
+  // Construct a new singleton if it doesnt exist, return it
+  // + If a static Singleton exists already, just return it
+  static Singleton l;
+  return l;
+}

cpp/patterns/singleton/singleton.h

@@ -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