#ifndef SALES_ITEM_H#define SALES_ITEM_H#include<iostream>#include<string>using namespace std;class Sales_item{    friend ostream& operator<<(ostream &os,const Sales_item &);    friend istream& operator>>(istream &is,Sales_item &);   private:        string bookno;        unsigned units_sold=0;        double revenue=0.0;   public:    string isbn()const    {        return this->bookno;    }    double avg_price() const;    Sales_item& operator=(const Sales_item &);    Sales_item& operator+(const Sales_item &);    Sales_item& operator+=(const Sales_item &);};#endif // SALES_ITEM_H

#include "Sales_item.h"istream& operator>>(istream &in,Sales_item &s){    double price;    in>>s.bookno>>s.units_sold>>price;    if(in)        s.revenue=s.units_sold*price;    else        s=Sales_item();    return in;}ostream& operator<<(ostream &out,const Sales_item &s){    out<<s.isbn()<<" "<<s.units_sold<<" "<<s.revenue<<" "<<s.avg_price()<<endl;    return out;}double Sales_item::avg_price() const{    if(units_sold)        return revenue/units_sold;    else        return 0;}Sales_item& Sales_item::operator=(const Sales_item &s){    units_sold=s.units_sold;    revenue=s.revenue;    isbn()=s.isbn();    return *this;}Sales_item& Sales_item::operator+(const Sales_item &s){    if(this->isbn()==s.isbn())    {        units_sold+=s.units_sold;        revenue+=s.revenue;    }    return *this;}Sales_item& Sales_item::operator+=(const Sales_item &s){    units_sold+=s.units_sold;    revenue+=s.revenue;    return *this;}

#include"Sales_item.h"int main(){    Sales_item book;    cin>>book;    cout<<book<<endl;    Sales_item item1,item2;    cin>>item1>>item2;    cout<<item1+item2<<endl;    return 0;}