cpp

// "called" is an object (functor) of the class "callback_for_boxes"
  class callback_for_boxes {
public:
  std::vector<size_t> *_id_vector_a, *_id_vector_b;
  NA_Vector *_neighbors_a, *_neighbors_b;

  callback_for_boxes(std::vector<size_t> &id_vector_a,
                     std::vector<size_t> &id_vector_b, NA_Vector &neighbors_a,
                     NA_Vector &neighbors_b)
      : _id_vector_a(&id_vector_a), _id_vector_b(&id_vector_b),
        _neighbors_a(&neighbors_a), _neighbors_b(&neighbors_b) {}

  void operator()(const Box &a, const Box &b) {
    // New boxes. Store their indices and their cells.
    _id_vector_a->push_back(a.id());
    _id_vector_b->push_back(b.id());
    _neighbors_a->push_back(a.handle());
    _neighbors_b->push_back(b.handle());

    return;
  }
};