design-patterns-cpp/state/State.cpp

/*
 * 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;
}