设计模式 - State

意图：允许一个对象在其内部状态改变时改变它的行为。对象看起来似乎修改了它的类。

class Context;

class State
{
public:
    State()
    {}
    virtual ~State()
    {}
    virtual void Operation(Context* context) = 0;
};

class ConcreteStateA: public State
{
public:
    ConcreteStateA()
    {}
    ~ConcreteStateA()
    {}
    void Operation(Context* context);
};

class ConcreteStateB: public State
{
public:
    ConcreteStateB()
    {}
    ~ConcreteStateB()
    {}
    void Operation(Context* context);
};

class Context
{
public:
    Context()
    {
        this->stateA = new ConcreteStateA();
        this->stateB = new ConcreteStateB();
        this->current_state = this->stateA;
    }
    ~Context()
    {
        delete this->stateA;
        delete this->stateB;
    }
    void Request()
    {
        current_state->Operation(this);
    }

private:
    friend class State;
    friend class ConcreteStateA;
    friend class ConcreteStateB;
    void ChangeState(State* state)
    {
        this->current_state = state;
    }
    State* stateA;
    State* stateB;
    State* current_state;
};

void ConcreteStateA::Operation(Context* context)
{
    cout<<"ConcreteStateA::Operation()"<<endl;
    context->ChangeState(context->stateB);
}

void ConcreteStateB::Operation(Context* context)
{
    cout<<"ConcreteStateB::Operation()"<<endl;
    context->ChangeState(context->stateA);
}