类向前声明小问题

#include <iostream>

using namespace std;
class Visitor;//，，，，，，，
//被访问者
class Library{
private :
    int x,y;
public :
    Library(){
        x=0;
        y=0;
    }
    void Accept(Visitor *v);
    void Do();
    ~Library();
};
//访问者
class Visitor{
public :
    Visitor(){
        cout<<"efas"<<endl;
    }
    void visit(Library *l);

};

void Library::Accept(Visitor *v){
    v->visit(this);
}

void Library::Do(){
    cout<<"x="<<x<<endl;
    cout<<"y="<<y<<endl;
}

void Visitor::visit(Library *l){
    l->Do();
}

int main()
{
   Visitor *pv=new Visitor();
   Library  *pl=new Library();
   pl->Accept(pv);
   delete pl;
   delete pv;
}