  void move_field_offset(my_ptrdiff_t ptr_diff)
  {
    Field::move_field_offset(ptr_diff);
    bit_ptr= ADD_TO_PTR(bit_ptr, ptr_diff, uchar*);
  }
  void move_field_offset(my_ptrdiff_t ptr_diff)
  {
    Field::move_field_offset(ptr_diff);
    bit_ptr= ADD_TO_PTR(bit_ptr, ptr_diff, uchar*);
  }