#include <iostream>

#include <vector>

using namespace std;

class board{

public:

    static int n;

    vector<vector<int> >  b;

    board() {

        int i;

        b.resize(n);

        for(i=0; i<n; i++) b[i].resize(n);

    }

};

int main()

{

    cin >> board::n;  // error 발생 undefined reference

    board bb;

    cout << bb.b.size();

    return 0;

}

이게 왜 에러죠? undefined reference 라는데 왜 undefined  인지;;