餐馆那些事之：Visitor Pattern

 

餐馆那些事之：Visitor Pattern

分类： Design Pattern 2012-02-21 21:02  274人阅读  评论(0)  收藏  举报

1. 概述
Visitor Pattern本质上是通过方法和数据分离，从而实现：
1）在不影响现有数据结构的情况下，添加新的方法
2）双分派（在选择一个方法的时候，不仅仅要根据消息接收者的运行时类型，还要根据参数的运行时类型）
3）Recovering lost type information（一个很好的例子： http://sourcemaking.com/design_patterns/visitor/cpp/3）
Visitor Pattern不适用于那些类结构不稳定的case，因为，Object增加一个子类，Visitor及其子类都有增加一个方法。
类图：

2. 实例
餐馆里厨师做菜，服务员负责提供服务，给他们新加一个责任，每次干完活后，做下大扫除
代码：

 #include <iostream>
 using namespace std;

 //Object
 class Employee
 {
 public:
         virtual ~Employee()
         {
         }

         virtual void accept(class Visitor*) = 0;
 };

 //Concrete_object_1
 class Waiter : public Employee
 {
 public:
         void accept(Visitor*);
         void provide_service()
         {
                 cout << "Waiter provide service" << endl;
         }
 };

 //Concrete_object_2
 class Cook : public Employee
 {
 public:
         void accept(Visitor*);
         void do_the_cooking()
         {
                 cout << "Cook do the cooking" << endl;
         }
 };

 //Visitor
 class Visitor
 {
 public:
         virtual void visit(Waiter*) = 0;
         virtual void visit(Cook*) = 0;
 };

 //Concrete_visitor_1
 class Clean_visitor : public Visitor
 {
 public:
         void visit(Waiter* waiter)
         {
                 waiter->provide_service();
                 cout << "Waiter do some cleaning" << endl;
         }

         void visit(Cook* cook)
         {
                 cook->do_the_cooking();
                 cout << "Cook do some cleaning" << endl;
         }
 };

 void Waiter::accept(Visitor* visitor)
 {
         visitor->visit(this);
 }

 void Cook::accept(Visitor* visitor)
 {
         visitor->visit(this);
 }

 int main()
 {
         Employee* cook = new Cook;
         Employee* waiter = new Waiter;

         Clean_visitor clean;
         cook->accept(&clean);
         waiter->accept(&clean);

         delete cook;
         delete waiter;

         return 0;
 }

运行输出：
Cook do the cooking
Cook do some cleaning
Waiter provide service
Waiter do some cleaning