add Decorator pattern

decorator/Decorator.cpp

@@ -0,0 +1,94 @@
+/*
+ * C++ Design Patterns: Decorator
+ * Author: Jakub Vojvoda [github.com/JakubVojvoda]
+ * 2016
+ *
+ * Source code is licensed under MIT licence
+ * (for more details see LICENCE)
+ *
+ */
+
+#include <iostream>
+
+/*
+ * Component
+ * defines an interface for objects that can have responsibilities
+ * added to them dynamically
+ */
+class Component {
+public:
+  virtual void operation() = 0;
+  // ...
+};
+
+/*
+ * Concrete Component
+ * defines an object to which additional responsibilities
+ * can be attached
+ */
+class ConcreteComponent : public Component {
+public:
+  void operation() {
+    std::cout << "Concrete Component operation" << std::endl;
+  }
+  // ...
+};
+
+/*
+ * Decorator
+ * maintains a reference to a Component object and defines an interface
+ * that conforms to Component's interface
+ */
+class Decorator : public Component {
+public:
+  Decorator(Component *c)
+    : component(c) {}
+
+  virtual void operation() {
+    component->operation();
+  }
+  // ...
+
+private:
+  Component *component;
+};
+
+/*
+ * Concrete Decorators
+ * add responsibilities to the component (can extend the state
+ * of the component)
+ */
+class ConcreteDecoratorA : public Decorator {
+public:
+  ConcreteDecoratorA(Component *c)
+    : Decorator(c) {}
+
+  void operation() {
+    Decorator::operation();
+    std::cout << "Decorator A" << std::endl;
+  }
+  // ...
+};
+
+class ConcreteDecoratorB : public Decorator {
+public:
+  ConcreteDecoratorB(Component *c)
+    : Decorator(c) {}
+
+  void operation() {
+    Decorator::operation();
+    std::cout << "Decorator B" << std::endl;
+  }
+  // ...
+};
+
+
+int main()
+{
+  Component *component = new ConcreteDecoratorA(
+              new ConcreteDecoratorB(new ConcreteComponent));
+
+  component->operation();
+
+  return 0;
+}