add Singleton pattern

singleton/README.md

@@ -0,0 +1,15 @@
+## Singleton
+
+Ensure a class only has one instance, and provide a global point of access to it.
+Pattern has creational purpose and deals with object relationships, which are more 
+dynamic. The Singleton is often used as a part another design patterns (see [Fa<46>ade] 
+and [Flyweight]). 
+
+### When to use
+
+* there must be exactly one instance of a class, and it must be accessible to clients from a well-known access point
+* when the sole instance should be extensible by subclassing, and clients should be able to use an extended instance without modifying their code
+
+[Fa<46>ade]: https://github.com/JakubVojvoda/design-patterns-cpp/tree/master/facade
+[Flyweight]: https://github.com/JakubVojvoda/design-patterns-cpp/tree/master/flyweight
+ 

singleton/Singleton.cpp

@@ -0,0 +1,46 @@
+/*
+ * C++ Design Patterns: Singleton
+ * Author: Jakub Vojvoda [github.com/JakubVojvoda]
+ * 2016
+ *
+ * Source code is licensed under MIT licence
+ * (for more details see LICENCE)
+ *
+ */
+
+#include <iostream>
+
+/*
+ * Singleton
+ * has private static variable to hold one instance of the class
+ * and method which gives us a way to instantiate the class
+ */
+class Singleton {
+public:
+  static Singleton *get() {
+    if (instance == NULL) {
+      instance = new Singleton();
+    }
+    return instance;
+  }
+
+  void tell() {
+    std::cout << "This is Singleton." << std::endl;
+    // ...
+  }
+
+  // ...
+
+private:
+  static Singleton *instance;
+  // ...
+};
+
+
+int main()
+{
+  Singleton singleton;
+  singleton.tell();
+
+  return 0;
+}