C++primer（第五版）Sales_item.h头文件

C++primer（第五版）1.51练习章节需要有一个Sales_item类，但是给的网站找不到，直接复制下面就好咯：

#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