访问者【设计模式】

知识点

访问者（visitor）

知识内容

表示一个作用于某对象结构中的各元素操作。它使你可以在不改变各元素的类的前提下定义作用于这些元素的操作。

示例

#include<iostream>
#include<typeinfo>
#include<string>

using namespace std;

class ConcreteElementA;
class ConcreteElementB;

class Visitor
{
  public:
    virtual void VisitConcreteElementA(ConcreteElementA*){} ;
    virtual void VisitConcreteElementB(ConcreteElementB*){};
};

class ConcreteVistor1 : public Visitor
{
  private:
    string visitResult;
  public:
    void Vistor1Format(ConcreteElementA*,ConcreteElementB*);
    virtual void VisitConcreteElementA(ConcreteElementA*);
    virtual void VisitConcreteElementB(ConcreteElementB*);
    void GetVisitor1Result()
    {
      cout << visitResult << endl;
    }
};
class ConcreteVistor2 : public Visitor
{
  private:
    string visitResult;
  public:
    void Vistor2Format(ConcreteElementA*,ConcreteElementB*);
    virtual void VisitConcreteElementA(ConcreteElementA*);
    virtual void VisitConcreteElementB(ConcreteElementB*);
    void GetVisitor2Result()
    {
      cout << visitResult << endl;
    }
};
class Element
{
  public:
    virtual void Accept(Visitor&){};
};
class ConcreteElementA : public Element
{
  public:
    virtual void Accept(Visitor& v); 
    void OperationA()
    {
      cout << "concrete element a is processing..." << endl;
    }
};

class ConcreteElementB : public Element
{
  public:
    virtual void Accept(Visitor& v);
    void OperationB()
    {
      cout << "concrete element b is processing..." << endl;
    }
};

void ConcreteElementA::Accept(Visitor& v)
{
  v.VisitConcreteElementA(this);
}
void ConcreteElementB::Accept(Visitor& v)
{
  v.VisitConcreteElementB(this);
}
void ConcreteVistor1::Vistor1Format(
    ConcreteElementA* concreteA, ConcreteElementB* concreteB)
{
   VisitConcreteElementA(concreteA); 
   cout << "visitor1 format result." << endl;
   VisitConcreteElementB(concreteB);
}
void ConcreteVistor1::VisitConcreteElementA(ConcreteElementA* concreteA)
{
  concreteA ->  OperationA();
  cout << "visitor1 adding: wwe we we" << endl;
  visitResult = visitResult + typeid(concreteA).name();
}
void ConcreteVistor1::VisitConcreteElementB(ConcreteElementB* concreteB)
{
  concreteB -> OperationB();
  cout << "visitor1 adding: mmo mo mo" << endl;
  visitResult = visitResult + typeid(concreteB).name();
}
void ConcreteVistor2::Vistor2Format(
    ConcreteElementA* concreteA, ConcreteElementB* concreteB)
{
  VisitConcreteElementA(concreteA);
  cout << "visitor2 format result." << endl;
  VisitConcreteElementB(concreteB);
}
void ConcreteVistor2::VisitConcreteElementA(ConcreteElementA* concreteA)
{
  concreteA ->  OperationA();
  cout << "visitor2 adding: wwewewe" << endl;
}
void ConcreteVistor2::VisitConcreteElementB(ConcreteElementB* concreteB)
{
   concreteB -> OperationB();
   cout << "visitor2 adding: mmomomo" << endl;
}

int main()
{
  ConcreteElementA* concreteElementA = new ConcreteElementA();
  concreteElementA -> OperationA();  //没有访问器的原始结果
  cout << "=====" << endl;
  ConcreteElementB* concreteElementB1 = new ConcreteElementB();
  concreteElementB1 -> OperationB();  //没有访问器的原始结果 
  cout << "=====" << endl;
  Element* elementA = concreteElementA;
  Element* elementB1 = concreteElementB1;

  ConcreteVistor1 visitorA;
  elementB1 -> Accept(visitorA);  //访问器变更的结果
  cout << "=====" << endl;
  elementA -> Accept(visitorA);
  cout << "=====" << endl;
  visitorA.GetVisitor1Result(); //访问器的组合结果
   cout << "=====" << endl;

  ConcreteElementB* concreteElementB = new ConcreteElementB();
  concreteElementB -> OperationB();  //没有访问器的原始结果
  cout << "=====" << endl;
  Element* elementB = concreteElementB;

  ConcreteVistor2 visitorB;
  elementB -> Accept(visitorB);  //访问器变更的结果
  cout << "=====" << endl;
  visitorB.GetVisitor2Result(); //访问器的组合结果
   cout << "=====" << endl;
  return 0;
}

链接

https://github.com/xierensong/learngit/blob/master/DPattern/22/1.cpp