C++ Example

#include <iostream>
#include <typeinfo>
/**
 * State Design Pattern
 *
 * Intent: Lets an object alter its behavior when its internal state changes. It
 * appears as if the object changed its class.
 */

/**
 * The base State class declares methods that all Concrete State should
 * implement and also provides a backreference to the Context object, associated
 * with the State. This backreference can be used by States to transition the
 * Context to another State.
 */

class Context;

class State {
  /**
   * @var Context
   */
 protected:
  Context *context_;

 public:
  virtual ~State() {
  }

  void set_context(Context *context) {
    this->context_ = context;
  }

  virtual void Handle1() = 0;
  virtual void Handle2() = 0;
};

/**
 * The Context defines the interface of interest to clients. It also maintains a
 * reference to an instance of a State subclass, which represents the current
 * state of the Context.
 */
class Context {
  /**
   * @var State A reference to the current state of the Context.
   */
 private:
  State *state_;

 public:
  Context(State *state) : state_(nullptr) {
    this->TransitionTo(state);
  }
  ~Context() {
    delete state_;
  }
  /**
   * The Context allows changing the State object at runtime.
   */
  void TransitionTo(State *state) {
    std::cout << "Context: Transition to " << typeid(*state).name() << ".\n";
    if (this->state_ != nullptr)
      delete this->state_;
    this->state_ = state;
    this->state_->set_context(this);
  }
  /**
   * The Context delegates part of its behavior to the current State object.
   */
  void Request1() {
    this->state_->Handle1();
  }
  void Request2() {
    this->state_->Handle2();
  }
};

/**
 * Concrete States implement various behaviors, associated with a state of the
 * Context.
 */

class ConcreteStateA : public State {
 public:
  void Handle1() override;

  void Handle2() override {
    std::cout << "ConcreteStateA handles request2.\n";
  }
};

class ConcreteStateB : public State {
 public:
  void Handle1() override {
    std::cout << "ConcreteStateB handles request1.\n";
  }
  void Handle2() override {
    std::cout << "ConcreteStateB handles request2.\n";
    std::cout << "ConcreteStateB wants to change the state of the context.\n";
    this->context_->TransitionTo(new ConcreteStateA);
  }
};

void ConcreteStateA::Handle1() {
  {
    std::cout << "ConcreteStateA handles request1.\n";
    std::cout << "ConcreteStateA wants to change the state of the context.\n";

    this->context_->TransitionTo(new ConcreteStateB);
  }
}

/**
 * The client code.
 */
void ClientCode() {
  Context *context = new Context(new ConcreteStateA);
  context->Request1();
  context->Request2();
  delete context;
}

int main() {
  ClientCode();
  return 0;
}

Context: Transition to 14ConcreteStateA.
ConcreteStateA handles request1.
ConcreteStateA wants to change the state of the context.
Context: Transition to 14ConcreteStateB.
ConcreteStateB handles request2.
ConcreteStateB wants to change the state of the context.
Context: Transition to 14ConcreteStateA.