Visitor模式详解--设计模式（19）

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Visitor模式来源：

       在面向对象系统的开发和设计过程，经常会遇到一种情况就是需求变更（RequirementChanging），经常我们做好的一个设计、实现了一个系统原型，咱们的客户又会有了新的需求。我们又因此不得不去修改已有的设计，最常见就是解决方案就是给已经设计、实现好的类添加新的方法去实现客户新的需求，这样就陷入了设计变更的梦魇：不停地打补丁，其带来的后果就是设计根本就不可能封闭、编译永远都是整个系统代码。Visitor模式则提供了一种解决方案。

Visitor模式作用：

      将更新（变更）封装到一个类中（访问操作），并由待更改类提供一个接收接口，则可达到效果。

Visitor模式UML结构图如图1所示：

Visitor模式构成：

抽象访问者(Visitor)：抽象出访问元素的动作

具体访问者(ConcreteVisitor)：实现访问元素的动作

抽象元素(Element)：定义一个接受访问的操作，其参数为访问者

具体元素(ConcreteElement)：实现接受访问操作

对象结构类(ObjectStructure)：可以枚举元素，并且管理元素

客户端(Client) ：定义元素集合，然后接受不同访问者的访问

Visitor模式源码示例：

Visitor.h

#pragma once
class CCommonEmployee;
class CManager;
class IVisitor
{
public:

    IVisitor(void)
    {
    }

    virtual ~IVisitor(void)
    {
    }
    virtual void Visit(CCommonEmployee commonEmployee) = 0;
    virtual void Visit(CManager manager) = 0;
    virtual int GetTotalSalary() = 0;
};

//BaseVisitor.h
#pragma once
#include "ivisitor.h"
#include "CommonEmployee.h"
#include "Manager.h"
#include <iostream>
using std::string;
class CBaseVisitor :
    public IVisitor
{
public:
    CBaseVisitor(void);
    ~CBaseVisitor(void);
    void Visit(CCommonEmployee commonEmployee);
    void Visit(CManager manager);
    int GetTotalSalary();
private:
    string GetBasicInfo(CEmployee *pemployee);
    string GetManagerInfo(CManager manager);
    string GetCommonEmployee(CCommonEmployee employee);
    static const int MANAGER_COEFFICENT = 5;
    static const int COMMONEMPLOYEE_COEFFICENT = 2;
    int m_commonTotal;
    int m_managerTotal;
    void CalCommonSalary(int salary);
    void CalManagerSalary(int salary);
};

//BaseVisitor.cpp
#include "StdAfx.h"
#include "..\CommonDeclare\Convert.h"
#include "BaseVisitor.h"
#include <iostream>
using std::string;
using std::cout;
using std::endl;

CBaseVisitor::CBaseVisitor(void)
{
    m_commonTotal = 0;
    m_managerTotal = 0;
}

CBaseVisitor::~CBaseVisitor(void)
{
}

void CBaseVisitor::Visit(CCommonEmployee commonEmployee)
{
    cout << this->GetCommonEmployee(commonEmployee).c_str() << endl;
    this->CalCommonSalary(commonEmployee.GetSalary());
}

void CBaseVisitor::Visit(CManager manager)
{
    cout << this->GetManagerInfo(manager).c_str() << endl;
    this->CalManagerSalary(manager.GetSalary());
}

string CBaseVisitor::GetBasicInfo(CEmployee *pemployee)
{
    string info = "";
    info.append("姓名：");
    info.append(pemployee->GetName());
    info.append("\t");
    info.append("性别：");
    info.append(CConvert::ToString(pemployee->GetSex()));
    info.append("\t");
    info.append("薪水：");
    info.append(CConvert::ToString(pemployee->GetSalary()));
    info.append("\t");
    return info;
}

string CBaseVisitor::GetManagerInfo(CManager manager)
{
    string basicInfo = this->GetBasicInfo(&manager);
    string otherInfo = "";
    otherInfo.append("业绩：");
    otherInfo.append(manager.GetPerformance());
    otherInfo.append("\t");
    basicInfo.append(otherInfo);
    return basicInfo;
}

string CBaseVisitor::GetCommonEmployee(CCommonEmployee employee)
{
    string basicInfo = this->GetBasicInfo(&employee);
    string otherInfo = "";
    otherInfo.append("工作：");
    otherInfo.append(employee.GetJob());
    otherInfo.append("\t");
    basicInfo.append(otherInfo);
    return basicInfo;
}

int CBaseVisitor::GetTotalSalary()
{
    return this->m_commonTotal + this->m_managerTotal;
}

void CBaseVisitor::CalCommonSalary(int salary)
{
    this->m_commonTotal += salary;
}

void CBaseVisitor::CalManagerSalary(int salary)
{
    this->m_managerTotal += salary;
}

//Employee.h
#pragma once
#include "Visitor.h"
#include <iostream>
using std::string;
class CEmployee
{
public:
static int MALE;
static int FEMALE;
CEmployee(void);
virtual ~CEmployee(void);
string GetName();
void SetName(string name);
int GetSalary();
void SetSalary(int v);
int GetSex();
void SetSex(int v);
virtual void Accept(IVisitor *pVisitor) = 0;
private:
string m_name;
int m_salary;
int m_sex;
};

//Employee.cpp
#include "StdAfx.h"
#include "Employee.h"
int CEmployee::MALE = 0;
int CEmployee::FEMALE = 1;

CEmployee::CEmployee(void)
{
}

CEmployee::~CEmployee(void)
{
}

string CEmployee::GetName()
{
return m_name;
}

void CEmployee::SetName( string name )
{
m_name = name;
}

int CEmployee::GetSalary()
{
return m_salary;
}

void CEmployee::SetSalary( int v )
{
m_salary = v;
}

int CEmployee::GetSex()
{
return m_sex;
}

void CEmployee::SetSex( int v )
{
m_sex = v;
}

//Manager.h
#pragma once
#include "employee.h"
#include "IVisitor.h"
#include <iostream>
using std::string;
class CManager :
public CEmployee
{
public:
CManager(void);
~CManager(void);
string GetPerformance();
void SetPerformance(string per);
void Accept(IVisitor *pVisitor);
protected:
string GetOtherInfo();
private:
string m_performance;
};

//Manager.cpp
#include "StdAfx.h"
#include "Manager.h"
#include <iostream>
using std::string;

CManager::CManager(void)
{
this->m_performance = "";
}

CManager::~CManager(void)
{
}

string CManager::GetPerformance()
{
return m_performance;
}

void CManager::SetPerformance(string per)
{
this->m_performance = per;
}

string CManager::GetOtherInfo()
{
string info = "";
info.append("业绩：");
info.append(this->m_performance);
info.append("\t");
return info;
}

void CManager::Accept(IVisitor *pVisitor)
{
pVisitor->Visit(*this);
}

// Visitor.cpp : 定义控制台应用程序的入口点。
//
#include "stdafx.h"
#include "Employee.h"
#include "CommonEmployee.h"
#include "Manager.h"
#include "BaseVisitor.h"
#include "..\CommonDeclare\Convert.h"
#include <vector>
#include <iostream>
using std::vector;
using std::cout;
using std::endl;

void MockEmployee(vector<CEmployee*> *pvce)
{
CCommonEmployee *pZhangSan = new CCommonEmployee();
pZhangSan->SetJob("编写Java程序，绝对的蓝领、苦工加搬运工");
pZhangSan->SetName("张三");
pZhangSan->SetSalary(1800);
pZhangSan->SetSex(CEmployee::MALE);
pvce->push_back(pZhangSan);

CCommonEmployee *pLiSi = new CCommonEmployee();
pLiSi->SetJob("页面美工，审美素质太不流行了！");
pLiSi->SetName("李四");
pLiSi->SetSalary(1900);
pLiSi->SetSex(CEmployee::FEMALE);
pvce->push_back(pLiSi);

CManager *pWangWu = new CManager();
pWangWu->SetPerformance("基本上是负值，但是我会拍马屁呀");
pWangWu->SetName("王五");
pWangWu->SetSalary(1900);
pWangWu->SetSex(CEmployee::FEMALE);
pvce->push_back(pWangWu);
}

void DoIt()
{
vector<CEmployee*> vce;
MockEmployee(&vce);
vector<CEmployee*>::const_iterator readIt = vce.begin();

CBaseVisitor visitor;
for (; readIt != vce.end(); readIt ++)
{
(*readIt)->Accept(&visitor);
}
cout << "本公司的月工资总额是：" << CConvert::ToString(visitor.GetTotalSalary()).c_str() << endl;

vector<CEmployee*>::reverse_iterator delIt = vce.rbegin();
for (; delIt != vce.rend(); delIt++)
{
delete (*delIt);
(*delIt) = NULL;
}
vce.clear();
}

Main.cpp

main(int argc, _TCHAR* argv[])
{
DoIt();
return 0;
}

Visitor模式优点：

(1).实现简单

Visitor模式缺点：

(1).破坏了封装性。Visitor模式要求Visitor可以从外部修改Element对象的状态，这一般通过两个方式来实现：a）Element提供足够的public接口，使得Visitor可以通过调用这些接口达到修改Element状态的目的；b）Element暴露更多的细节给Visitor，或者让Element提供public的实现给Visitor（当然也给了系统中其他的对象），或者将Visitor声明为Element的friend类，仅将细节暴露给Visitor。但是无论那种情况，特别是后者都将是破坏了封装性原则（实际上就是C++的friend机制得到了很多的面向对象专家的诟病）。

Visitor模式详解--设计模式（19）

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原文地址：http://blog.csdn.net/fanyun_01/article/details/51841355