Sales_item例子

#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;
}