C++设计模式——访问者模式

  1 #include <iostream>
  2 #include <vector>
  3 using namespace std;
  4 
  5 class ConcreteElementA;
  6 class ConcreteElementB;
  7 
  8 class Visitor
  9 {
 10 public:
 11      virtual void VisitConcreteElementA(ConcreteElementA *pElementA) = 0;
 12      virtual void VisitConcreteElementB(ConcreteElementB *pElementB) = 0;
 13 };
 14 
 15 class ConcreteVisitor1 : public Visitor
 16 {
 17 public:
 18      void VisitConcreteElementA(ConcreteElementA *pElementA);
 19      void VisitConcreteElementB(ConcreteElementB *pElementB);
 20 };
 21 
 22 void ConcreteVisitor1::VisitConcreteElementA(ConcreteElementA *pElementA)
 23 {
 24      // 现在根据传进来的pElementA，可以对ConcreteElementA中的element进行操作
 25 }
 26 
 27 void ConcreteVisitor1::VisitConcreteElementB(ConcreteElementB *pElementB)
 28 {
 29      // 现在根据传进来的pElementB，可以对ConcreteElementB中的element进行操作
 30 }
 31 
 32 class ConcreteVisitor2 : public Visitor
 33 {
 34 public:
 35      void VisitConcreteElementA(ConcreteElementA *pElementA);
 36      void VisitConcreteElementB(ConcreteElementB *pElementB);
 37 };
 38 
 39 void ConcreteVisitor2::VisitConcreteElementA(ConcreteElementA *pElementA)
 40 {
 41      // ...
 42 }
 43 
 44 void ConcreteVisitor2::VisitConcreteElementB(ConcreteElementB *pElementB)
 45 {
 46      // ...
 47 }
 48 
 49 // Element object
 50 class Element
 51 {
 52 public:
 53      virtual void Accept(Visitor *pVisitor) = 0;
 54 };
 55 
 56 class ConcreteElementA : public Element
 57 {
 58 public:
 59      void Accept(Visitor *pVisitor);
 60 };
 61 
 62 void ConcreteElementA::Accept(Visitor *pVisitor)
 63 {
 64      pVisitor->VisitConcreteElementA(this);
 65 }
 66 
 67 class ConcreteElementB : public Element
 68 {
 69 public:
 70      void Accept(Visitor *pVisitor);
 71 };
 72 
 73 void ConcreteElementB::Accept(Visitor *pVisitor)
 74 {
 75      pVisitor->VisitConcreteElementB(this);
 76 }
 77 
 78 // ObjectStructure类，能枚举它的元素，可以提供一个高层的接口以允许访问者访问它的元素
 79 class ObjectStructure
 80 {
 81 public:
 82      void Attach(Element *pElement);
 83      void Detach(Element *pElement);
 84      void Accept(Visitor *pVisitor);
 85 
 86 private:
 87      vector<Element *> elements;
 88 };
 89 
 90 void ObjectStructure::Attach(Element *pElement)
 91 {
 92      elements.push_back(pElement);
 93 }
 94 
 95 void ObjectStructure::Detach(Element *pElement)
 96 {
 97      vector<Element *>::iterator it = find(elements.begin(), elements.end(), pElement);
 98      if (it != elements.end())
 99      {
100           elements.erase(it);
101      }
102 }
103 
104 void ObjectStructure::Accept(Visitor *pVisitor)
105 {
106      // 为每一个element设置visitor，进行对应的操作
107      for (vector<Element *>::const_iterator it = elements.begin(); it != elements.end(); ++it)
108      {
109           (*it)->Accept(pVisitor);
110      }
111 }
112 
113 int main()
114 {
115      ObjectStructure *pObject = new ObjectStructure;
116 
117      ConcreteElementA *pElementA = new ConcreteElementA;
118      ConcreteElementB *pElementB = new ConcreteElementB;
119 
120      pObject->Attach(pElementA);
121      pObject->Attach(pElementB);
122 
123      ConcreteVisitor1 *pVisitor1 = new ConcreteVisitor1;
124      ConcreteVisitor2 *pVisitor2 = new ConcreteVisitor2;
125 
126      pObject->Accept(pVisitor1);
127      pObject->Accept(pVisitor2);
128 
129      if (pVisitor2) delete pVisitor2;
130      if (pVisitor1) delete pVisitor1;
131      if (pElementB) delete pElementB;
132      if (pElementA) delete pElementA;
133      if (pObject) delete pObject;
134 
135      return 0;
136 }