    bool operator<(const const_iterator &rhs) const {
      if (charPtr_) {
        return charPtr_ < rhs.charPtr_;
      }
      return char16Ptr_ < rhs.char16Ptr_;
    }
    bool operator<(const const_iterator &rhs) const {
      if (charPtr_) {
        return charPtr_ < rhs.charPtr_;
      }
      return char16Ptr_ < rhs.char16Ptr_;
    }