iterator trait

struct input_iterator_tag {};

template<class T>
struct Iterator
{
    typedef T Tag;
};

template <class T>
class InputIterator:public Iterator<input_iterator_tag>
{

};


template <typename Iterator, typename Distance>
void Advance(Iterator& i, Distance di, input_iterator_tag)
{

}

template <typename Iterator>
struct IteratorTrait
{
    typedef typename Iterator::Tag IteratorTag;

};

template <typename Iterator, typename Distance>
void Advance(Iterator& i, Distance di )
{
    Advance(i,di,typename IteratorTrait<Iterator>::IteratorTag());
}

  InputIterator<int> it;
    Advance(it,10);