#include <boost/iterator/iterator_facade.hpp>

#include <vector>
#include <iterator>
#include <iostream>

class vector_iterator
  : public boost::iterator_facade<
      vector_iterator,
      int,
      boost::random_access_traversal_tag>
{
public:

  vector_iterator(char tag, std::vector<int>::iterator it) : _tag(tag), _it(it) {}

private:
  friend class boost::iterator_core_access;
  
  int& dereference() const { return *_it; }
  bool equal(vector_iterator const& other) const { return _it == other._it; }
  void increment() { ++_it; }
  void decrement() { --_it; }
  void advance(int n) { std::advance(_it, n); }

  std::vector<int>::iterator::difference_type
  distance_to(const vector_iterator& other) const
  {
    std::cerr << _tag << ".distance_to(" << other._tag << ")\n";
    std::vector<int>::iterator::difference_type result = std::distance(_it, other._it);
    std::cerr << "result = " << result << '\n';
    return result;
  }

private:
  const char _tag;
  std::vector<int>::iterator _it;
};

int main()
{
  std::vector<int> v;
  v.push_back(1);
  v.push_back(2);

  vector_iterator a('a', v.begin());
  vector_iterator b('b', v.end());

  std::cerr << std::distance(a, b) << '\n';
}