问题

代码

Sales_item.h

#include<iostream>
#include<string>
class Sales_item
{
    public:
        Sales_item(){};
        Sales_item(std::string insid, int num, double price);
        friend std::istream& operator>>(std::istream& is, Sales_item& item);
        friend std::ostream& operator<<(std::ostream& os, const Sales_item& item);
        friend Sales_item operator+(const Sales_item& item1,const Sales_item& item2);
    private:
        std::string insid_;
        int num_;
        double price_;
};

Sales_item.cpp

#include"Sales_item.h"

Sales_item::Sales_item(std::string insid, int num, double price)
:insid_(insid)
,num_(num)
,price_(price)
{
}

std::istream& operator>>(std::istream& is, Sales_item& item)
{
    is>>item.insid_>>item.num_>>item.price_;
    return is;
}

std::ostream& operator<<(std::ostream& os,const Sales_item& item)
{
    os<< item.insid_<<" "<<item.num_<<" "<<item.num_*item.price_<<" "<<item.price_;
    return os;
}

Sales_item operator+(const Sales_item& item1,const Sales_item& item2)
{
    if(item1.insid_==item2.insid_)
    {
        return Sales_item(item1.insid_, item1.num_+item2.num_, (item1.num_*item1.price_+item2.num_*item2.price_)/(item1.num_+item2.num_));
    }
    else
    {
        return item1;
    }
}

prog1.cc

#include<iostream>
#include "Sales_item.h"
int main()
{
    Sales_item item1,item2;
    std::cin>>item1>>item2;
    std::cout<<"相加结果："<<std::endl;
    std::cout<<item1+item2<<std::endl;
    return 0;
}

运行

g++ prog1.cc Sales_item.cpp -o prog

[root@localhost myPractice]# ./prog 
0-201-78345-X 3 20.00
0-201-78345-X 2 25.00
相加结果：
0-201-78345-X 5 110 22

知识点

友元函数可以访问其所属类的私有成员，但不是类的成员函数。友元函数没有隐含的this指针，因此不能直接访问类的非静态成员变量.

思考：
1.不使用友元函数怎么实现？
2.哪些运算符不能重载？