訪問者模式（Visitor）是一種分離對象數據結構與行為的方法，通過這種分離，可以為一個已存在的類或類群增加新的操作而無需為它們作任何修改。 結構圖為：
公司的人事評估需要人事部訪問每一個員工列表，逐個對員工作出評估，人事部有兩個訪問者，一個訪問者評估員工假期，另一個評估員工薪資。
實現代碼：
//Element.h
class Visitor;
class Element
{
public:
Element();
virtual ~Element();
virtual void Accept(Visitor*) = 0;
friend class Visitor;
};
//Element.cpp
#include "stdafx.h"
#include "Element.h"
Element::Element()
{
}
Element::~Element()
{
}
//Employee.h
#include "Element.h"
class Employee : public Element
{
public:
Employee(char*, double, int);
virtual ~Employee();
void Accept(Visitor*);
char* GetName();
double GetSalary();
int GetVacationDays();
void SetSalary(double);
void SetVacationDays(int);
private:
char* m_pName;
double m_dSalary;
int m_nVacationDays;
};
//Employee.cpp
#include "stdafx.h"
#include "Employee.h"
#include "Visitor.h"
Employee::Employee(char* pName, double dSalary, int nVacationDays)
{
m_pName = pName;
m_dSalary = dSalary;
m_nVacationDays = nVacationDays;
}
Employee::~Employee()
{
}
void Employee::Accept(Visitor* pVisitor)
{
pVisitor->Visit(this);
}
char* Employee::GetName()
{
return m_pName;
}
double Employee::GetSalary()
{
return m_dSalary;
}
int Employee::GetVacationDays()
{
return m_nVacationDays;
}
void Employee::SetSalary(double dSalary)
{
m_dSalary = dSalary;
}
void Employee::SetVacationDays(int nVacationDays)
{
m_nVacationDays = nVacationDays;
}
//Visitor.h
class Element;
class Visitor
{
public:
virtual ~Visitor() = 0;
virtual void Visit(Element*) = 0;
protected:
Visitor();
};
//Visitor.cpp
#include "stdafx.h"
#include "Visitor.h"
Visitor::Visitor()
{
}
Visitor::~Visitor()
{
}
//IncomeVisitor.h
#include "Visitor.h"
class IncomeVisitor : public Visitor
{
public:
IncomeVisitor();
virtual ~IncomeVisitor();
void Visit(Element*);
};
//IncomeVisitor.cpp
#include "stdafx.h"
#include "IncomeVisitor.h"
#include "Employee.h"
#include <iostream>
using namespace std;
IncomeVisitor::IncomeVisitor()
{
}
IncomeVisitor::~IncomeVisitor()
{
}
void IncomeVisitor::Visit(Element* pElement)
{
//訪問者提高員工10%的薪資
Employee* pEmployee = static_cast<Employee*>(pElement);
double dSalary = pEmployee->GetSalary();
pEmployee->SetSalary(dSalary * 1.10);
cout << "員工 " << pEmployee->GetName()
<< " 的新工資是：" << pEmployee->GetSalary() << "元" << endl;
}
//VacationVisitor.h
#include "Visitor.h"
class VacationVisitor : public Visitor
{
public:
VacationVisitor();
virtual ~VacationVisitor();
void Visit(Element*);
};
//VacationVisitor.cpp
#include "stdafx.h"
#include "VacationVisitor.h"
#include "Employee.h"
#include <iostream>
using namespace std;
VacationVisitor::VacationVisitor()
{
}
VacationVisitor::~VacationVisitor()
{
}
void VacationVisitor::Visit(Element* pElement)
{
//訪問者為員工增加3天假期
Employee* pEmployee = static_cast<Employee*>(pElement);
int nVacationDays = pEmployee->GetVacationDays();
pEmployee->SetVacationDays(nVacationDays + 3);
cout << "員工 " << pEmployee->GetName()
<< " 的新假期是：" << pEmployee->GetVacationDays() << "天" << endl;
}
//main.cpp
#include "stdafx.h"
#include "Employee.h"
#include "IncomeVisitor.h"
#include "VacationVisitor.h"
int main(int argc, char* argv[])
{
Element* pEmployeeA = new Employee("張三", 10000.00, 10);
Element* pEmployeeB = new Employee("李四", 15000.00, 20);
IncomeVisitor incomeV;
VacationVisitor vacationV;
incomeV.Visit(pEmployeeA);
vacationV.Visit(pEmployeeB);
return 0;
}
代碼中，我們通過訪問者（IncomeVisitor、VacationVisitor）對員工張三與李四進行評估，并給張三增加10%的工資，給李四增加3天假期。
最后輸出為：
員工 張三 的新工資是：11000元
員工 李四 的新假期是：23天