c++访问者模式

1.访问者模式简介

   在访问者模式（Visitor Pattern）中，我们使用了一个访问者类，它改变了元素类的执行算法。通过这种方式，元素的执行算法可以随着访问者改变而改变。

2.源码实现

#include <iostream>
#include <string>
#include <list>

using namespace std;

class Element;

class Visitor
{
public:
    virtual void Visit(Element *element){};
};

class Element
{
public:
    virtual void Accept(Visitor *visitor){};
};

class Employee : public Element
{
public:
    string name;
    double income;
    int vacationDays;

    Employee(string name, double income, int vacationDays)
    {
        this->name = name;
        this->income = income;
        this->vacationDays = vacationDays;
    }
    
    void Accept(Visitor *visitor)
    {
        visitor->Visit(this);
    }
};

class IncomeVisitor : public Visitor
{
public:
    void Visit(Element *element)
    {
        Employee *employee = ((Employee *)element);
        employee->income *= 1.10;
        cout << employee->name << "'s new income: " << employee->income << endl;
    }
};

class VacationVisitor : public Visitor
{
public:
    void Visit(Element *element)
    {
        Employee *employee = ((Employee *)element);
        employee->vacationDays += 3;
        cout << employee->name << "'s new vacation days: " << employee->vacationDays << endl;
    }
};

class Employees
{
private:
    list<Employee *> employees;

public:
    void Attach(Employee *employee)
    {
        employees.push_back(employee);
    }

    void Detach(Employee *employee)
    {
        employees.remove(employee);
    }

    void Accept(Visitor *visitor)
    {
        for(list<Employee *>::iterator it=employees.begin(); it!=employees.end(); it++)
        {
            (*it)->Accept(visitor);
        }
    }
};

int main(int argc, char **argv)
{
    Employees *e = new Employees();

    e->Attach(new Employee("Tom", 25000.0, 14));
    e->Attach(new Employee("Thomas", 35000.0, 16));
    e->Attach(new Employee("Roy", 45000.0, 21));

    IncomeVisitor *v1 = new IncomeVisitor();
    VacationVisitor *v2 = new VacationVisitor();

    e->Accept(v1);
    e->Accept(v2);

    return 0;
}

3.编译源码

$ g++ -o example example.cpp

4.运行及其结果

$ ./example
Tom's new income: 27500
Thomas's new income: 38500
Roy's new income: 49500
Tom's new vacation days: 17
Thomas's new vacation days: 19
Roy's new vacation days: 24