《Primer C++ 第五版》第一章1.5节练习 头文件Sales_item.h

 #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 Sale