Revise the program from the previous exercise to append its output to its given file. Run the program on the same output file at least twice to ensure that the data are preserved

#include <string>
#include <iostream>
#include <fstream>

class Sales_data;
std::istream &read(std::istream &, Sales_data &);

class Sales_data {
  friend Sales_data add(const Sales_data &, const Sales_data &);
  friend std::istream &read(std::istream &, Sales_data &);
  friend std::ostream &print(std::ostream &, const Sales_data &);

public:
  //Sales_data() : bookNo(""), units_sold(0), revenue(0.0) {}
  Sales_data() : Sales_data("", 0, 0.0) {}
  //Sales_data(const std::string &no) : bookNo(no) {}
  explicit Sales_data(const std::string &no) : Sales_data(no, 0, 0.0) {}
  //Sales_data(const std::string &no = "") : bookNo(no) {}
  Sales_data(const std::string &no, unsigned us, double price)
      : bookNo(no), units_sold(us), revenue(price * us) {}
  explicit Sales_data(std::istream &is) : Sales_data() { read(is, *this); }
  //Sales_data(std::istream &is = std::cin) { read(is, *this); }

  std::string isbn() const { return bookNo; }
  Sales_data &combine(const Sales_data &);

private:
  double avg_price() const;
  std::string bookNo;
  unsigned units_sold = 0;
  double revenue = 0.0;
};

Sales_data &Sales_data::combine(const Sales_data &rhs) {
  units_sold += rhs.units_sold;
  revenue += rhs.revenue;
  return *this;
}

Sales_data add(const Sales_data &lhs, const Sales_data &rhs) {
  Sales_data sum = lhs;  // Use default copy constructor
  sum.combine(rhs);
  return sum;
}

std::istream &read(std::istream &is, Sales_data &item) {
  double price;
  is >> item.bookNo >> item.units_sold >> price;
  item.revenue = item.units_sold * price;
  return is;
}

std::ostream &print(std::ostream &os, const Sales_data &item) {
  os << item.isbn() << " " << item.units_sold << " "
     << item.revenue << " " << item.avg_price();
  return os;
}

inline
double Sales_data::avg_price() const {
  return units_sold ? revenue / units_sold : 0;
}

int process_transaction(std::istream &in, std::ostream &out) {
  Sales_data total;
  if (read(in, total)) {
    Sales_data trans;
    while (read(in, trans)) {
      if (total.isbn() == trans.isbn()) {
        total.combine(trans);
      } else {
        print(out, total) << std::endl;
        total = trans;  // Use default copy constructor
      }
    }
    print(out, total) << std::endl;
  } else {
    std::cerr << "No data!" << std::endl;
    return -1;
  }
  return 0;
}

int main(int argc, char **argv) {
  if (argc < 2) {
    std::cerr << "Usage: 8.8 <input filename> [<output filename>]" << std::endl;
    return -3;
  }
  std::ifstream in(argv[1]);
  if (!in) {
    std::cerr << "Fail to open file: " << argv[1] << std::endl;
    return -2;
  }
  if (argc >= 3) {
    std::ofstream out(argv[2], std::ofstream::app);
    if (!out) {
      std::cerr << "Fail to open file: " << argv[1] << std::endl;
      return -2;
    }
    process_transaction(in, out);
  } else {
    process_transaction(in, std::cout);
  }

  return 0;
}