C++primer 第一章网站上的代码

<pre name="code" class="cpp">C++primer 第一章网站上的代码：

#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;elses = 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;elsereturn 0;}#endif