The iterator has index by pair.

index_iterator.hpp

#ifndef INDEX_IT_LIB
#define INDEX_IT_LIB

template<class Iterator>
class index_iterator {
  size_t _i = 0;
  Iterator _it;
public:
  index_iterator(Iterator it) : _it(it) {}

  auto operator*() { return std::make_pair(_i, *_it); }
  auto operator++() { ++_i; ++_it; return std::make_pair(_i, _it); }

  auto operator+(size_t amount) const {
    auto adv = index_iterator<Iterator>(_it + amount);
    adv._i = _i + amount;
    return adv;
  }
  bool operator!=(index_iterator<Iterator> const &r) const { return _it != r._it; }
};

#endif // INDEX_IT_LIB