ex7_26_sales_data.h

#ifndef CP5_ex7_26_h
#define CP5_ex7_26_h

#include <iostream>
#include <string>

// define Sales_data::avg_price as an inline function.
class Sales_data {
    friend std::istream& read(std::istream& is, Sales_data& item);
    friend std::ostream& print(std::ostream& os, const Sales_data& item);
    friend Sales_data add(const Sales_data& lhs, const Sales_data& rhs);

public:
    Sales_data() = default;
    Sales_data(const std::string& s) : bookNo(s) {}
    Sales_data(const std::string& s, unsigned n, double p)
        : bookNo(s), units_sold(n), revenue(n * p)
    {
    }
    Sales_data(std::istream& is) { read(is, *this); }

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

private:
    inline double avg_price() const;

private:
    std::string bookNo;
    unsigned units_sold = 0;
    double revenue = 0.0;
};

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

// declarations for nonmember parts of the Sales_data interface.
std::istream& read(std::istream& is, Sales_data& item);
std::ostream& print(std::ostream& os, const Sales_data& item);
Sales_data add(const Sales_data& lhs, const Sales_data& rhs);

#endif