#include <iostream>
using namespace std;

class State;
class ConcreteStateB;
class ConcreteStateA;

class State {
public:
 virtual void OperationChangeState(Context*) = 0;
 virtual void OperationInterface(Context*) = 0;
 virtual ~State() { }
protected:
 bool ChangeState(Context* con, State *st) {
 con->ChangeState(st);
 }
};

class ConcreteStateA :public State {
public:
 void OperationChangeState(Context* con) {
 OperationInterface(con);
 this->ChangeState(con, new ConcreteStateB());
 }

 void OperationInterface(Context* con) {
 cout << "ConcreteStateA::OperationInterface..." << endl;
 }
};

class ConcreteStateB :public State {
public:
 void OperationChangeState(Context* con) {
 OperationInterface(con);
 this->ChangeState(con, new ConcreteStateA());
 }

 void OperationInterface(Context* con) {
 cout << "ConcreteStateB::OperationInterface..." << endl;
 }
};

class Context {
public:
 Context(State* st) {
 _st = st;
 }

 void OperationInterface() {
 _st->OperationInterface(this);
 }

 void OperationChangeState() {
 _st->OperationInterface(this);
 }
private:
 friend class State;
 bool ChangeState(State* st) {
 _st = st;
 return true;
 }

 State *_st;
};

int main() {
 State *st = new ConcreteStateA();
 Context *con = new Context(st);
 con->OperationInterface();
 con->OperationInterface();
 con->OperationInterface();

 delete con;
 delete st;

 return 0;
}