Rewrite the transaction-processing program you wrote for the exercises in § 7.1.2 (p. 260) to use these new functions

#include <string>
#include <iostream>

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

  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;
  return os;
}

int main() {
  Sales_data total;
  if (read(std::cin, total)) {
  //double price;
  //if (std::cin >> total.bookNo >> total.units_sold >> price) {
  //  total.revenue = total.units_sold * price;
    Sales_data trans;
    while (read(std::cin, trans)) {
    //while (std::cin >> trans.bookNo >> trans.units_sold >> price) {
    //  trans.revenue = trans.units_sold * price;
      if (total.isbn() == trans.isbn()) {
        total.combine(trans);
      } else {
        print(std::cout, total) << std::endl;
        //std::cout << total.bookNo << " "
        //          << total.units_sold << " "
        //          << total.revenue << std::endl;
        total = trans;  // Use default copy constructor
      }
    }
    print(std::cout, total) << std::endl;
    //std::cout << total.bookNo << " "
    //          << total.units_sold << " "
    //          << total.revenue << std::endl;
  } else {
    std::cerr << "No data!" << std::endl;
    return -1;
  }

  return 0;
}