#include <vector>
#include <iostream>
#include <cassert>
using namespace std;

class Visitor{ public: virtual void visit(int &i)=0;
};

class MyVector:public vector<int>{
    public:
        void traverse(Visitor &v)
	{   for (int i=0; i<size(); i++) v.visit( at(i) );
	}
};//MyVector

class Adder:public Visitor {
private: int s;
public: Adder() :s(0){}
	void visit(int &i){ s = s + i; }
	int result(){return s;}
};//Adder

class Debugger:public Visitor {
public: void visit(int &i){ cerr<< i << " "; }
};//Debugger

int main()//test
{
	MyVector v;
	v.push_back(1);
	Debugger d;
	v.traverse(d);

	Adder s;
	assert(s.result() == 0);
	v.traverse(s);
	assert(s.result() == 1);
}