【设计模式】备忘录

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mob604756fd7a56 2013-06-08 20:18:00

文章标签 #include ios 构造函数 c++ 外部类 文章分类 代码人生

备忘录，其实名字也比较形象。

我们经常遇到将一个类的状态恢复到历史版本的需求。比如一个记事本，想保存一个上N个状态，通过Ctrl+Z可以恢复此前编辑内容。备忘录就非常适合这种场景。此时发起者类，希望能够将自身状态保留在某处，而且不希望过多的暴露细节。为了上述目的，发起者会在某个时刻通过new出一个备忘类对象，并将该对象托管给管理者类。然后需要恢复状态时，又从管理者类中获取具体状态，将自身恢复到某个状态。

为了保持封装性，备忘录一般除了将自己暴露给发起者外(或者可以认为是发起者将自己状态暴露给了备忘类)，不为外部类提供任何访问状态相关接口。C++中这个通常是通过将自己所有接口都改成private(包括构造函数)，然后将发起者设为自己的友元类来实现。

写了个demo，可以支持多个版本:

/***************************************************************************
 * 
 * Copyright (c) 2013 itegel.com, Inc. All Rights Reserved
 * 
 **************************************************************************/



/**
 * @file test_memento.cpp
 * @author itegel
 * @date 2013/06/08 11:29:08
 * @brief 
 *  
 **/

#include <string>
#include <map>
#include <iostream>
using namespace std;

class State{
    public:
        int version;
        string content;
};

class Memento{
    public:
    private:
        friend class NoteBook;
        Memento(State state) : _state(state){}
        ~Memento(){}

        State * GetState(){
            return &_state;
        }
        void SetState(State state){
            _state = state;
        }
        State _state;
};

//originator
class NoteBook{
    public:
        NoteBook(int version, string content){
            _state.version = version;
            _state.content = content;            
        }

        NoteBook(State & state):_state(state){

        }

        void SetMemento(Memento * mem){
            _state = mem->_state;
        }

        Memento * GetMemento(){
            return new Memento(_state);
        }

        void SetState(int version, string content){
            _state.version = version;
            _state.content = content;
        }

        int GetVersion(){
            return _state.version;
        }

        void PrintState(){
            cout<<"Note:\n\tversion:\t"<<_state.version<<endl<<"\tcontent:\t"<<_state.content<<endl;
        }

    private:
        State _state;
};


class CareTaker{
    public:
        CareTaker(){}
        void Save(int id, Memento * mem){
            _memento_map[id] = mem;
        }
        Memento * GetMemento(int id){
            return _memento_map[id];
        }
    private:
        map<int, Memento *> _memento_map;
};

int main(){
    NoteBook my_note(0, "");
    CareTaker care_taker;

    my_note.PrintState();
    cout<<"Save State 0!"<<endl;
    care_taker.Save(my_note.GetVersion(), my_note.GetMemento());

    cout<<endl<<"Edit for 1st time!"<<endl;
    my_note.SetState(1, "Hello world!");
    my_note.PrintState();
    care_taker.Save(my_note.GetVersion(), my_note.GetMemento());

    cout<<endl<<"Edit for 2nd time!"<<endl;
    my_note.SetState(2, "Hello World! It is the 2nd version!");
    my_note.PrintState();
    care_taker.Save(my_note.GetVersion(), my_note.GetMemento());

    cout<<endl<<"Edit for 3rd time!"<<endl;
    my_note.SetState(3, "Hello Itegel! It is the 3rd version!");
    my_note.PrintState();
    care_taker.Save(my_note.GetVersion(), my_note.GetMemento());


    cout<<endl<<"Restore 2nd version:"<<endl;
    Memento * mem = care_taker.GetMemento(2);
    my_note.SetMemento(mem);
    my_note.PrintState();

    cout<<endl<<"Restore 0 version:"<<endl;
    mem = care_taker.GetMemento(0);
    my_note.SetMemento(mem);
    my_note.PrintState();

    return 0;
}

执行结果：

Note:
  version:  0
  content:  
Save State 0!

Edit for 1st time!
Note:
  version:  1
  content:  Hello world!

Edit for 2nd time!
Note:
  version:  2
  content:  Hello World! It is the 2nd version!

Edit for 3rd time!
Note:
  version:  3
  content:  Hello Itegel! It is the 3rd version!

Restore 2nd version:
Note:
  version:  2
  content:  Hello World! It is the 2nd version!

Restore 0 version:
Note:
  version:  0
  content: