C++ constructor

 1 struct Sales_data {
 2      // constructors added
 3     Sales_data() = default;
 4     Sales_data(const std::string &s): bookNo(s) { }
 5     Sales_data(const std::string &s, unsigned n, double p):
 6     bookNo(s), units_sold(n), revenue(p*n) { }
 7     Sales_data(std::istream &);
 8     //other members as before
 9     std::string isbn() const { return bookNo; }
10     Sales_data& combine(const Sales_data&);
11     double avg_price() const;
12     std::string bookNo;
13     unsigned units_sold = 0;
14     double revenue = 0.0;
15 };