Method AppendToString

extern/re2/re2/stringpiece.h:130–132

  }

  void AppendToString(std::string* target) const {
    target->append(data_, size_);
  }

  size_type copy(char* buf, size_type n, size_type pos = 0) const;
  StringPiece substr(size_type pos = 0, size_type n = npos) const;