什么是访问者模式？

      Definition：Represent an operation to be performed on the elements of an object structure. Visitor lets you define a new operation without changing the classes of the elements on which it operates.

      访问者模式最合适的使用情况是需要对一个家族的对象逐个访问，并根据具体的对象做出不同的操作，而且不希望改变原来的对象。当然在设计的时候需要让家族成员定义一个支持访问者模式的接口，即accept函数。以后需要添加新功能的时候只要增加相应的具体访问者类。

      当然访问者模式也有缺点，比如家族类里面新加了成员，每个具体访问者类也需要相应改变；类的状态对访问者完全透明，违背数据隐藏原则。等等

      访问者模式有用到双分配（double-dispatch）技术，由于Client使用的是抽象类Element和Visitor，无法达到对具体的Element调用相应的visit函数，需要在Element类的函数accept中转发visitor.visit(this)，此时this是一个具体的类，能够调用到正确的函数。

 

类图结构：

Example：

      例子模式对班级活动收费，但是男生收费是女生费用的2倍。在不能向Student中添加函数的情况下，可以使用访问者模式。

people.h代码如下：

#ifndef __PEOPLE_H__#define __PEOPLE_H__#include 
    
     class IVisitor;class People{public:	People(std::string strName, int nMoney) : m_strName(strName), m_nMoney(nMoney) {}	void pay(int nMoneyToPay);	virtual void accept(IVisitor* pVisitor) = 0;	const std::string& getName() const;private:	int m_nMoney;	std::string m_strName;};class Man : public People{public:	Man(std::string strName, int nMoney) : People(strName, nMoney) {}	virtual void accept(IVisitor* pVisitor);};class Woman : public People{public:	Woman(std::string strName, int nMoney) : People(strName, nMoney) {}	virtual void accept(IVisitor* pVisitor);};#endif
    

 

people.cpp代码如下：

#include "people.h"#include "visitor.h"#include 
    
     void People::pay( int nMoneyToPay ){	m_nMoney -= nMoneyToPay;}const std::string& People::getName() const{	return m_strName;}void Man::accept( IVisitor* pVisitor ){	pVisitor->visit(this);}void Woman::accept( IVisitor* pVisitor ){	pVisitor->visit(this);}
    

 

visitor.h代码如下：

#ifndef __VISITOR_H__#define __VISITOR_H__class Man;class Woman;class IVisitor{public:	virtual void visit(Man* pMan) = 0;	virtual void visit(Woman* pWoman) = 0;};class ChargeMoneyVisitor : public IVisitor{public:	ChargeMoneyVisitor() : m_nSum(0) {}	virtual void visit(Man* pMan);	virtual void visit(Woman* pWoman);	int getSum() const;private:	int m_nSum;	static const int PAY = 10;};#endif

 

visitor.cpp代码如下：

#include "visitor.h"#include "people.h"#include 
    
     void ChargeMoneyVisitor::visit( Man* pMan ){	int nMoneyToCharge = 2*PAY;	pMan->pay(nMoneyToCharge);	m_nSum += nMoneyToCharge;	std::cout << pMan->getName() << " is charged " << nMoneyToCharge << " RMB\n";}void ChargeMoneyVisitor::visit( Woman* pWoman ){	int nMoneyToCharge = PAY;	pWoman->pay(nMoneyToCharge);	m_nSum += nMoneyToCharge;	std::cout << pWoman->getName() << " is charged " << nMoneyToCharge << " RMB\n";}int ChargeMoneyVisitor::getSum() const{	return m_nSum;}
    

 

main.cpp代码如下：

#include "visitor.h"#include "people.h"#include 
    
     #include 
     
      int main(){	std::vector
      
        students;	students.push_back(new Man("Bob", 100));	students.push_back(new Woman("Lily", 100));	ChargeMoneyVisitor chargeMoneyVisitor;	std::vector
       
        ::const_iterator iter = students.begin();	for (; iter != students.end(); ++iter)	{		People* pPeople = *iter;		pPeople->accept(&chargeMoneyVisitor);	}	std::cout << "Total charge money: " << chargeMoneyVisitor.getSum() << " RMB\n";	iter = students.begin();	for (; iter != students.end(); ++iter)	{		delete *iter;	}	return 0;}
       
      
     
    

 

运行结果如下：