面试：C++实现访问者模式

参考：深入应用C++11，访问者模式

 #include <iostream>

 class ConcreteElement1;
 class ConcreteElement2;

 class Visitor{
 public:
     virtual ~Visitor(){};
     virtual void visit(ConcreteElement1&) = ;
     virtual void visit(ConcreteElement2&) = ;
 };

 class ConcreteVisitor : public Visitor{
 public:
     virtual ~ConcreteVisitor(){};
     void visit(ConcreteElement1& e){
         std::cout << "ConcreteElement1::visit" << std::endl;
     }
     void visit(ConcreteElement2& e){
         std::cout << "ConcreteElement2::visit" << std::endl;
     }
 };

 class Element{
 public:
     virtual ~Element(){};
     virtual void accept(Visitor&) = ;
 };

 class ConcreteElement1 : public Element{
 public:
     virtual ~ConcreteElement1(){}
     void accept(Visitor& v){
         v.visit(*this);
     }
 };
 class ConcreteElement2 : public Element{
 public:
     virtual ~ConcreteElement2(){};
     void accept(Visitor& v){
         v.visit(*this);
     }

 };

 int main(){
     ConcreteVisitor visitor;
     ConcreteElement1 emt1 = ConcreteElement1();
     emt1.accept(visitor);
     ConcreteElement2 emt2 = ConcreteElement2();
     emt2.accept(visitor);
     return ;
 }