    difference_type operator-(const const_iterator &rhs) const {
      if (charPtr_) {
        return charPtr_ - rhs.charPtr_;
      }
      return char16Ptr_ - rhs.char16Ptr_;
    }
    difference_type operator-(const const_iterator &rhs) const {
      if (charPtr_) {
        return charPtr_ - rhs.charPtr_;
      }
      return char16Ptr_ - rhs.char16Ptr_;
    }