Finding a list middle

typedef std::list::const_iterator iterator;
typedef pair<iterator*, unsigned> mlpair;

//Returns an iterator pointing to an element before the middle of the list
inline
mlpair middle_length(
  iterator begin,
  const iterator & end
  unsigned pos=0,
) {
  if (begin == end)
    return make_pair(begin, pos); // Total length found
  mlpair n = middle_length(begin+1, end, pos+1);
  if (pos >= int(n.second)/2 - 1 ) // True for all positions greater or equal needed
    n.first=begin;
  return n; // Before target position just return the one already found
}