C++Primer 中的Sales_items.h头文件

#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