C++primer書店程序

#include 
#include 
#include 
#include 
#include 
using namespace std;

class Sales_data
{
    friend std::istream& operator>>(std::istream&, Sales_data&);
    friend std::ostream& operator<<(std::ostream&, const Sales_data&);
    friend Sales_data operator+(const Sales_data&,const Sales_data&);//非成員函數（對稱性）
    friend bool operator==(const Sales_data&,const Sales_data&);
    friend bool operator!=(const Sales_data&,const Sales_data&);
    
public:
    Sales_data():units_sold(0),revenue(0.0){}
    Sales_data(const std::string &s):bookNo(s),units_sold(0),revenue(0.0){}
    Sales_data(const std::string &s,unsigned n,double p):bookNo(s),units_sold(n),revenue(n*p){}
    Sales_data(std::istream &is);

    Sales_data& operator+=(const Sales_data&);//成員函數(復合賦值一般是成員函數)
    std::string isbn()const{return bookNo;}

    Sales_data& operator=(const std::string &s);
private:
    inline double avg_price()const;//inline
private:
    std::string bookNo; //isbn
    unsigned units_sold;//銷售量
    double revenue;     //總價
};

inline double Sales_data::avg_price() const
{
    return units_sold ? revenue/units_sold : 0;
}


Sales_data::Sales_data(std::istream &is)
{
    is>>*this;
}


std::istream& operator>>(std::istream& is,Sales_data& item)
{
    double price=0.0;
    is>>item.bookNo>>item.units_sold>>price;
    if(is)//檢測流的狀態，輸入易錯且改寫對象數據
        item.revenue=item.units_sold*price;
    else
        item=Sales_data();
    return is;
}

std::ostream& operator<<(std::ostream& os, const Sales_data& item)
{
    os<