设计模式-Visitor(行为模式) 一个类在不修改自己的同时增加了新的操作，存在问题是 1：破坏了封装性 2：扩展困难

//以下代码来源： 设计模式精解－GoF 23种设计模式解析附C++实现源码

//Visitor.h

#pragma once

class ConcreateElementA;
class ConcreateElementB;
class Element;

class Visitor
{
public:
    virtual ~Visitor();
    virtual void VisitConcreateElementA(Element* elm) = 0;
    virtual void VisitConcreateElementB(Element* elm) = 0;
protected:
    Visitor();
private:
};

class ConcreateVisitorA :public Visitor
{
public:
    ConcreateVisitorA();
    virtual ~ConcreateVisitorA();
    virtual void VisitConcreateElementA(Element* elm);
    virtual void VisitConcreateElementB(Element* elm);
protected:
private:
};

class ConcreateVisitorB :public Visitor
{
public:
    ConcreateVisitorB();
    virtual ~ConcreateVisitorB();
    virtual void VisitConcreateElementA(Element* elm);
    virtual void VisitConcreateElementB(Element* elm);
protected:
private:
};

//Element.h

#pragma once

class Visitor;

class Element
{
public:
    virtual ~Element();
    virtual void Accept(Visitor* vis) = 0;
protected:
    Element();
private:
};

class ConcreateElementA :public Element
{
public:
    ConcreateElementA();
    ~ConcreateElementA();
    void Accept(Visitor* vis);
protected:
private:
};

class ConcreateElementB :public Element
{
public:
    ConcreateElementB();
    ~ConcreateElementB();
    void Accept(Visitor* vis);
protected:
private:
};

//Visitor.cpp

#include"Element.h"
#include"Visitor.h"

#include<iostream>

Visitor::Visitor(){}
Visitor::~Visitor(){}
ConcreateVisitorA::ConcreateVisitorA(){}
ConcreateVisitorA::~ConcreateVisitorA(){}
void ConcreateVisitorA::VisitConcreateElementA(Element* elm)
{
    std::cout << "i will visit concreateElementA ... " << std::endl;
}

void ConcreateVisitorA::VisitConcreateElementB(Element* elm)
{
    std::cout << "i will visit concreateElementB ..." << std::endl;
}

ConcreateVisitorB::ConcreateVisitorB(){}
ConcreateVisitorB::~ConcreateVisitorB(){}

void ConcreateVisitorB::VisitConcreateElementA(Element* elm)
{
    std::cout << "i will visit ConcreateElementA ..." << std::endl;
}

void ConcreateVisitorB::VisitConcreateElementB(Element* elm)
{
    std::cout << "i will visit ConcreateElementB ..." << std::endl;
}

//Element.cpp

#include "Element.h"
#include "Visitor.h"

#include<iostream>

Element::Element(){}
Element::~Element(){}
void Element::Accept(Visitor* vis){}

ConcreateElementA::ConcreateElementA(){}
ConcreateElementA::~ConcreateElementA(){}
void ConcreateElementA::Accept(Visitor* vis)
{
    vis->VisitConcreateElementA(this);
    std::cout << "Visitting ConcreateElementA ... " << std::endl;
}

ConcreateElementB::ConcreateElementB(){}
ConcreateElementB::~ConcreateElementB(){}
void ConcreateElementB::Accept(Visitor* vis)
{
    vis->VisitConcreateElementB(this);
    std::cout << "Visitting ConcreateElementB ... " << std::endl;
}

//main.cpp

#include"Element.h"
#include"Visitor.h"
#include<string>

int main(int args, char* argv)
{
    Visitor* vis = new ConcreateVisitorA();
    Element* elm = new ConcreateElementA();
    elm->Accept(vis);
    getchar();
    return 0;
}