C++Primer之编写含类及其成员函数的头文件并使用

头文件：

#ifndef SALES_DATA_H
#define SALES_DATA_H
#include<string>
using namespace std;
struct Sales_data
{
	string isbn() const {return bookNo;}
	Sales_data& combine(const Sales_data&);
	double avg_price() const;
	string bookNo;
	unsigned units_sold = 0;
	double revenue = 0.0;
};
Sales_data add(const Sales_data& lns, const Sales_data& rhs)
{
	Sales_data sum = lns;
	sum.combine(rhs);
	return sum;
}
istream& read(istream& is, Sales_data& item)
{
	double price = 0;
	is >> item.bookNo >> item.units_sold >> price;
	item.revenue = price * item.units_sold;
	return is;
}
ostream& print(ostream& os, const Sales_data& item)
{
	os << item.isbn() << " " << item.units_sold << " " << item.revenue << " " << item.avg_price();
	return os;
}
double Sales_data::avg_price()const {
	if (units_sold)
		return revenue / units_sold;
	else
		return 0;
}
Sales_data& Sales_data::combine(const Sales_data& rhs)
{
	units_sold += rhs.units_sold;
	revenue += rhs.revenue;
	return *this;
}
#endif // !SALES_DATA_H

源文件：

#include<iostream>
#include<string>
#include"Sale_data.h"
using namespace std;
int main()
{
	Sales_data total;
	if (read(cin, total))
	{
		Sales_data trans;
		while (read(cin, trans))
		{
			if (trans.isbn() == total.isbn())
			{
				total.combine(trans);
			}
			else
			{
				print(cout, total) << endl;
				total = trans;
			}
		}
		print(cout, total) << endl;
	}
	else
		cout << "No Data." << endl;
}