如何正确使用enable_if？

我需要学习如何使用enable_if.为此,我需要使用enable_if重新实现distance函数.我试过这个:

#include 
#include 
#include 
#include 
#include 

template
typename std::enable_if::value, std::iterator_traits::difference_type>::type  my_distance(In begin, In end, std::input_iterator_tag dummy){
  typename std::iterator_traits::difference_type n = 0;
  while(begin!=end){
    ++begin; ++n;
  }
  std::cout << "STEPPING" << std::endl;
  return n;
}

template
typename std::enable_if::value, std::iterator_traits::difference_type>::type my_distance(Ran begin, Ran end, std::random_access_iterator_tag dummy){
  std::cout << "RANDOM" << std::endl;
  return end - begin;
}

template  inline
typename std::iterator_traits::difference_type my_distance_wrapper(I begin, I end){
  typedef typename std::iterator_traits::iterator_category cat;
  return my_distance(begin, end, cat());
}

int main() {
  std::vector ve;
  std::list li;
  for(int i = 0; i < 3; i++){
    ve.push_back(i);
    li.push_back(i);
  }
  std::cout << my_distance_wrapper(ve.begin(), ve.end()) << std::endl;
  std::cout << my_distance_wrapper(li.begin(), li.end()) << std::endl;
  return 0;
}

template
bool f(){
  typedef typename std::iterator_traits::iterator_category cat;
  return std::random_access_iterator_tag == cat();
}

template
using is_random_access_iterator = 
  std::is_same::iterator_category,
               std::random_access_iterator_tag>;

template
std::enable_if_t::value,
                 typename std::iterator_traits::difference_type>
my_distance(iterator a, iterator b) { return a-b; }

template
std::enable_if_t::value,
                 typename std::iterator_traits::difference_type>
my_distance(iterator a, iterator b) { /* ... */ }

template
using enable_if_t = typename enable_if::type;