  const_iterator begin() const {
    if (isASCII()) {
      return const_iterator(castToCharPtr());
    }
    return const_iterator(castToChar16Ptr());
  }
  const_iterator begin() const {
    if (isASCII()) {
      return const_iterator(castToCharPtr());
    }
    return const_iterator(castToChar16Ptr());
  }