Sales_data.h

#ifndef SALES_DATA_H
#define SALES_DATA_H

#include <string>
#include <iostream>

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

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

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

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

Sales_data add(const Sales_data&, const Sales_data&);
std::ostream &print(std::ostream&, const Sales_data&);

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

#endif