    const_iterator &operator++() {
      if (charPtr_) {
        ++charPtr_;
      } else {
        ++char16Ptr_;
      }
      return *this;
    }
    const_iterator &operator++() {
      if (charPtr_) {
        ++charPtr_;
      } else {
        ++char16Ptr_;
      }
      return *this;
    }