2.cc

#include <iostream>
#include <iterator>
#include <list>

template<typename It>
size_t DistanceImpl(It begin, It end, std::forward_iterator_tag) {
  size_t result = 0;
  while (begin++ != end)
    ++result;
  return result;
}

template<typename It>
size_t DistanceImpl(It begin, It end, std::random_access_iterator_tag) {
  return end - begin;
}

template<typename It>
size_t Distance(It begin, It end) {
  using Tag = typename std::iterator_traits<It>::iterator_category;
  return DistanceImpl<It>(begin, end, Tag());
}

int main(void) {
  int x[] = {1, 2, 3};
  std::list<int> l(x, x+3);
  return Distance(l.begin(), l.end());
}