c++ 컴파일 에러 문제

using namespace std;
 
template <typename E>
class VectorCompleteTree {
private:
    vector<E> V;
public:
    typedef typename vector<E>::iterator Position;
protected:
    Position pos(int i) const
    {
        return V.begin() + i; // error detected!!
    }
    int idx(const Position& p) const
    {
        return p - V.begin();
    }
public:
    VectorCompleteTree() : V(1) { }
    int size() const
    {
        return V.size() - 1;
    }
    Position left(Position& p)
    {
        return pos(2*idx(p));
    }
    Position right(Position& p)
    {
        return pos(2*idx(p)+1);
    }
    Position parent(Position& p)
    {
        return pos(idx(p)/2);
    }
    bool hasLeft(const Position& p) const
    {
        return 2*idx(p) <= size();
    }
    bool hasRight(const Position& p) const
    {
        return 2*idx(p) + 1 <= size();
    }
    bool isRoot(const Position& p) const
    {
        return idx(p) == 1;
    }
    Position root()
    {
        return pos(1);
    }
    Position last()
    {
        return pos(size());
    }
    void addLast(const E& e)
    {
        V.push_back(e);
    }
    void removeLast()
    {
        V.pop_back();
    }
    void swap(const Position& p, const Position& q)
    {
        E temp = *p;
        *p = *q;
        *q = temp;
    }
};