C++primer（第五版）1.51練習章節需要有一個Sales_item類，但是給的網站有點蛋疼，直接複製下面就好咯：

#ifndef SALESITEM_H
#define SALESITEM_H
#include <iostream>
#include <string>

class Sales_item
{
public:
	Sales_item(const std::string &book):isbn(book),units_sold(0),revenue(0.0){}
	Sales_item(std::istream &is){ is >> *this;}
	friend std::istream& operator>>(std::istream &,Sales_item &);
	friend std::ostream& operator<<(std::ostream &,const Sales_item &);
public:
	Sales_item & operator+=(const Sales_item&);
public:
	double avg_price() const;
	bool same_isbn(const Sales_item &rhs)const
	{
		return isbn == rhs.isbn;
	}
	Sales_item():units_sold(0),revenue(0.0){}
public:
	std::string isbn;
	unsigned units_sold;
	double revenue;
};

using std::istream;
using std::ostream;
Sales_item operator+(const Sales_item &,const Sales_item &);
inline bool operator==(const Sales_item &lhs,const Sales_item &rhs)
{
	return lhs.units_sold == rhs.units_sold && lhs.revenue == rhs.revenue && lhs.same_isbn(rhs);
}
inline bool operator!=(const Sales_item &lhs,const Sales_item &rhs)
{
	return !(lhs == rhs);
}

inline Sales_item & Sales_item::operator +=(const Sales_item &rhs)
{
	units_sold += rhs.units_sold;
	revenue += rhs.revenue;
	return *this;
}
inline Sales_item operator+(const Sales_item &lhs,const Sales_item &rhs)
{
	Sales_item ret(lhs);
	ret += rhs;
	return ret;
}
inline istream& operator>>(istream &in,Sales_item &s)
{
	double price;
	in >> s.isbn >> s.units_sold >> price;
	if(in)
		s.revenue = s.units_sold * price;
	else
		s = Sales_item();
	return in;
}
inline ostream& operator<<(ostream &out,const Sales_item &s)
{
	out << s.isbn << "\t" <<s.units_sold << "\t" << s.revenue << "\t" << s.avg_price();
	return out;
}
inline double Sales_item::avg_price() const
{
	if(units_sold)
		return revenue/units_sold;
	else
		return 0;
}
#endif