add State pattern

state/README.md

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+## State
+
+The pattern allows an object to alter its behavior when its internal state changes.
+The object will appear to change its class. It has behavioral purpose and applies 
+to the objects.
+
+### When to use
+
+* when an object's behavior depends on its state, and it must change its behavior at run-time depending on that state
+* operations have large, multipart conditional statements that depend on the object's state

state/State.cpp

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+/*
+ * C++ Design Patterns: State
+ * Author: Jakub Vojvoda [github.com/JakubVojvoda]
+ * 2016
+ *
+ * Source code is licensed under MIT License
+ * (for more details see LICENSE)
+ *
+ */
+
+#include <iostream>
+
+/*
+ * State
+ * defines an interface for encapsulating the behavior associated
+ * with a particular state of the Context
+ */
+class State {
+public:
+  virtual ~State() { /* ... */ }
+  virtual void handle() = 0;
+  // ...
+};
+
+/*
+ * Concrete States
+ * each subclass implements a behavior associated with a state
+ * of the Context
+ */
+class ConcreteStateA : public State {
+public:
+  ~ConcreteStateA() { /* ... */ }
+
+  void handle() {
+    std::cout << "State A handled." << std::endl;
+  }
+  // ...
+};
+
+class ConcreteStateB : public State {
+public:
+  ~ConcreteStateB() { /* ... */ }
+
+  void handle() {
+    std::cout << "State B handled." << std::endl;
+  }
+  // ...
+};
+
+/*
+ * Context
+ * defines the interface of interest to clients
+ */
+class Context {
+public:
+  Context() : state() { /* ... */ }
+
+  ~Context() {
+    delete state;
+  }
+
+  void setState(State *s) {
+    if (state) {
+      delete state;
+    }
+    state = s;
+  }
+
+  void request() {
+    state->handle();
+  }
+  // ...
+
+private:
+  State *state;
+  // ...
+};
+
+
+int main()
+{
+  State *stateA = new ConcreteStateA;
+  State *stateB = new ConcreteStateB;
+
+  Context *context = new Context;
+
+  context->setState(stateA);
+  context->request();
+
+  context->setState(stateB);
+  context->request();
+
+  delete context;
+  return 0;
+}