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line4 class Visitor;

line19         visitor.visitElementA(*this);

#include <iostream>
using namespace std;

class Visitor{
public:
    virtual void visitElementA(ElementA& element) = 0;
    virtual void visitElementB(ElementB& element) = 0;
    
    virtual ~Visitor(){}
};

class Element
{
public:
    virtual void accept(Visitor& visitor) = 0; //第一次多态辨析

    virtual ~Element(){}
};

...