设计模式系列4—–C++实现备忘录模式(Memento Pattern)

什么是备忘录模式？

      Without violating encapsulation, capture and externalize an object’s internal state so that the object can be returned to this state later.

      其实‘人’如其名，备忘模式就是为了对一个对象做备忘，允许做类似“重播”、“倒带”等操作。能用这种模式的前提是需要备忘的对象能够抽象出一个状态对象。而且备忘录模式也不会违背封装原则公开内部状态。

类图结构：

      Originator即是需要备忘的类，它里面包含内部状态state对象。Originator能够通过createMemento创建保存点，也可以通过setMemento从给定的备忘回复内部状态。Caretaker负责保存备忘。

Example：

      例子模拟游戏里面备忘人物的移动，当然这里为了方便人物的状态简化为只有坐标，当需要备忘的地方，可以备忘人物的状态，只有可以通过此来恢复。代码如下：

#include <iostream>
#include <vector>
using namespace std;

struct State
{
public:
	State(int xx, int yy) : x(xx), y(yy) {}

	friend ostream& operator << (ostream& out, const State& state)
	{
		out << state.x << ", " << state.y;

		return out;
	}

	int x;
	int y;
};

class Memento
{
public:
	Memento(State* pState) : m_pState(pState) {}

	State* getState() const
	{
		return m_pState;
	}

private:
	State* m_pState;
};

class Player
{
public:
	void setState(State* pState)
	{
		m_pState = pState;
		cout << "Player move to " << *pState << endl;
	}

	Memento* createMemo()
	{
		return new Memento(m_pState);
	}

	void setMemo(Memento* pMemo)
	{
		setState(pMemo->getState());
	}

private:
	State* m_pState;
};

class Recorder
{
public:
	Recorder(Player* pPlayer) : m_pPlayer(pPlayer) {}

	void record()
	{
		m_memoList.push_back(m_pPlayer->createMemo());
	}

	void replay()
	{
		cout << "nbegin replay..." << endl;
		vector<Memento*>::const_iterator iter = m_memoList.begin();
		for (; iter != m_memoList.end(); ++iter)
		{
			Memento* pMemo = *iter;
			m_pPlayer->setMemo(pMemo);
		}
		cout << endl;
	}

	void rewind()
	{
		cout << "nbegin rewind..." << endl;
		vector<Memento*>::const_reverse_iterator iter = m_memoList.rbegin();
		for (; iter != m_memoList.rend(); ++iter)
		{
			Memento* pMemo = *iter;
			m_pPlayer->setMemo(pMemo);
		}
	}

	void reset()
	{
		cout << "nreset recorder" << endl;
		vector<Memento*>::iterator iter = m_memoList.begin();
		for (; iter != m_memoList.end(); ++iter)
		{
			Memento* pMemo = *iter;
			delete pMemo;
		}
		m_memoList.clear();
	}

private:
	Player* m_pPlayer;
	vector<Memento*> m_memoList;
};

int main()
{
	State* pState1 = new State(1, 2);
	State* pState2 = new State(3, 4);
	Player* pPlayer = new Player();
	Recorder* pRecorder = new Recorder(pPlayer);
	
	pPlayer->setState(pState1);
	pRecorder->record();
	
	pPlayer->setState(pState2);
	pRecorder->record();

	pRecorder->replay();
	pRecorder->rewind();
	pRecorder->reset();

	delete pState1;
	delete pState2;
	delete pPlayer;
	delete pRecorder;

	return 0;
}

      运行结果如下：

参考资料：

PatternCraft – Memento Pattern

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