C 範囲ベースの For ループで現在のオブジェクトのインデックスを取得するにはどうすればよいですか?

template <typename T>
class Indexer {
public:
    class iterator {
        typedef typename iterator_extractor<T>::type inner_iterator;
        typedef typename std::iterator_traits<inner_iterator>::reference inner_reference;
    public:
        typedef std::pair<size_t, inner_reference> reference;

        iterator(inner_iterator it): _pos(0), _it(it) {}

        reference operator*() const { return reference(_pos, *_it); }

        iterator& operator++() { ++_pos; ++_it; return *this; }
        iterator operator++(int) { iterator tmp(*this); ++*this; return tmp; }

        bool operator==(iterator const& it) const { return _it == it._it; }
        bool operator!=(iterator const& it) const { return !(*this == it); }

    private:
        size_t _pos;
        inner_iterator _it;
    };

    Indexer(T& t): _container(t) {}

    iterator begin() const { return iterator(_container.begin()); }
    iterator end() const { return iterator(_container.end()); }

private:
    T& _container;
}; // class Indexer

template <typename T>
Indexer<T> index(T& t) { return Indexer<T>(t); }

std::vector<int> v{1, 2, 3, 4, 5, 6, 7, 8, 9};

for (auto p: index(v)) {
    std::cout << p.first << ": " << p.second << "\n";
}

0: 1
1: 2
2: 3
3: 4
4: 5
5: 6
6: 7
7: 8
8: 9