    const_iterator operator+(difference_type rhs) const {
      if (charPtr_) {
        return const_iterator(charPtr_ + rhs, char16Ptr_);
      }
      return const_iterator(charPtr_, char16Ptr_ + rhs);
    }
    const_iterator operator+(difference_type rhs) const {
      if (charPtr_) {
        return const_iterator(charPtr_ + rhs, char16Ptr_);
      }
      return const_iterator(charPtr_, char16Ptr_ + rhs);
    }