    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_;
    }