C++primer书店程序

#include <iostream>
#include <string>
#include <cassert>
#include <algorithm>
#include <vector>
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&);//非成员函数（对称性）
    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&);//成员函数(复合赋值通常是成员函数)
    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;//销售量
    double revenue;     //总价
};

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)//检測流的状态，输入易错且改写对象数据
        item.revenue=item.units_sold*price;
    else
        item=Sales_data();
    return is;
}

std::ostream& operator<<(std::ostream& os, const Sales_data& item)
{
    os<<item.isbn()<<" "<<item.units_sold<<" "
      <<item.revenue<<" "<<item.avg_price();
    return os;
}

Sales_data operator+(const Sales_data& item1,const Sales_data& item2)
{
    Sales_data s(item1);
    s+=item2;//调用复合赋值
    return s;
}

Sales_data& Sales_data::operator+=(const Sales_data& rhs)
{
    units_sold+=rhs.units_sold;
    revenue+=rhs.revenue;
    return *this;
}

bool operator==(const Sales_data& lhs,const Sales_data& rhs)
{
    return lhs.bookNo==rhs.bookNo &&
        lhs.units_sold==rhs.units_sold &&
        lhs.revenue==rhs.revenue;
}

bool operator!=(const Sales_data& lhs,const Sales_data& rhs)
{
    return !(lhs==rhs);
}


Sales_data& Sales_data::operator=(const std::string &s)
{
    *this=Sales_data(s);
    return *this;
}