5.11 访问者模式 visitor

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1．意图：

封装一些作用于某种数据结构中的各元素的操作，它可以在不改变这个数据结构的前提下定义作用于这些元素的新的操作。

2.结构图

3.源码

//Element.h#ifndef _ELEMENT_H_#define _ELEMENT_H_class Visitor;class Element{ public: virtual ~Element(); virtual void  Accept(Visitor* vis) = 0; protected: Element() ;  private: }; class ConcreteElementA:public Element { public: ConcreteElementA(); ~ConcreteElementA(); void Accept(Visitor*  vis); protected: private: }; class ConcreteElementB:public  Element { public: ConcreteElementB(); ~ConcreteElementB(); void Accept(Visitor*  vis); protected: private: }; #endif //~_ELEMENT_H_

//Visitor.h#ifndef _VISITOR_H_#define _VISITOR_H_class ConcreteElementA;class ConcreteElementB;class Element;class Visitor{public :virtual ~Visitor();virtual voidVisitConcreteElementA(Element*elm)=0;virtual voidVisitConcreteElementB(Element*elm)=0;protected:Visitor();private:};class ConcreteVisitorA:public  Visitor{public :ConcreteVisitorA();virtual ~ConcreteVisitorA();virtual voidVisitConcreteElementA(Element*elm);virtual voidVisitConcreteElementB(Element*elm);protected:private:};class ConcreteVisitorB:public Visitor{public :ConcreteVisitorB();virtual ~ConcreteVisitorB();virtual voidVisitConcreteElementA(Element*elm);virtual voidVisitConcreteElementB(Element*elm);protected:private:};#endif//~_VISITOR_H_

//Element.cpp#include "Element.h"#include "Visitor.h"#include <iostream>using namespace std;Element::Element(){}Element::~Element(){}void Element::Accept(Visitor*vis){}ConcreteElementA::ConcreteElementA(){}ConcreteElementA::~ConcreteElementA(){}void ConcreteElementA::Accept(Visitor*vis){vis->VisitConcreteElementA(this);cout<<"visiting ConcreteElementA..."<<endl;}ConcreteElementB::ConcreteElementB(){}ConcreteElementB::~ConcreteElementB(){}void ConcreteElementB::Accept(Visitor*vis){vis->VisitConcreteElementB(this);cout<<"visiting ConcreteElementB..."<<endl;}

//Visitor. cpp #include "Visitor.h"#include "Element.h" #include <iostream> using namespace std; Visitor::Visitor() { } Visitor::~Visitor () { } ConcreteVisitorA::ConcreteVisitorA(){ } ConcreteVisitorA::~ConcreteVisitorA(){ } void ConcreteVisitorA::VisitConcreteElementA(Element*elm){ cout<< "i will visit ConcreteElementA..."<<endl; } void ConcreteVisitorA::VisitConcreteElementB(Element*elm){ cout<< "i will visit ConcreteElementB..."<<endl;} ConcreteVisitorB::ConcreteVisitorB(){ } ConcreteVisitorB::~ConcreteVisitorB(){ } void ConcreteVisitorB::VisitConcreteElementA(Element*elm){ cout<< "i will visitConcreteElementA..."<<endl;} void ConcreteVisitorB::VisitConcreteElementB(Element*elm){ cout<< "i will visit ConcreteElementB..."<< endl; }

#include "Element.h"#include "Visitor.h"#include <iostream>using namespace std;int main(int argc,char*argv[]){Visitor*vis=new ConcreteVisitorA();Element*elm=new ConcreteElementA();elm->Accept(vis);getchar();return 0;}