Sales_data.h

class Sales_data
{
public:
    Sales_data() = default;
    Sales_data(const std::string &s, unsigned n, double p):
            bookNo(s), units_sold(n), revenue(p*n) {}
    Sales_data(const std::string &s) : bookNo(s) {}
    Sales_data(std::istream&);
    std::string isbn() const { return bookNo; }
    Sales_data &combine(const Sales_data&);
private:
    double avg_price() const
    {
        return units_sold ? revenue/units_sold : 0;
    }
    std::string bookNo;
    unsigned units_sold = 0;
    double revenue = 0.0;
};