友元和重载的问题

#include "stdafx.h"

//#include "..\..\comm\Sales_item.h"



#include <iostream>

#include <string>

using namespace std;



class Sales_item

{

	friend istream& operator>>(istream &in, Sales_item &s);

public:

	Sales_item(): units_sold(0), revenue(0.0) {}

	Sales_item(const string &book):isbn(book), units_sold(0), revenue(0.0){}

	

public:

	Sales_item& operator+=(const Sales_item &);

  

public:

	double avg_price() const;

	inline bool same_isbn(const Sales_item &rhs) const { return isbn == rhs.isbn;}



private:

	string isbn;

	unsigned units_sold;

	double revenue;

};



istream& operator>>(istream &in, Sales_item &s)

{

	double price;

	in >> s.isbn >> s.units_sold >> price;

	if(in)

	{

		s.revenue = s.units_sold * price;

	}

	else

	{

		s = Sales_item();

	}



	return in;

}



inline Sales_item& Sales_item::operator+=(const Sales_item &rhs)

{

	units_sold += rhs.units_sold;

	revenue += rhs.revenue;

	return *this;

}



inline double Sales_item::avg_price() const

{

	if(units_sold)

	{

		return revenue / units_sold;

	}

	else

	{

		return 0;

	}

}



int main(int argc, char* argv[])

{



	Sales_item book;

	

	/*

	cout << "Enter transactions: " << endl;

	while(cin >> book)

	{

		cout << "ISBN, number of copies sold, "

			 << "total revenue, and average price are: "

			 << endl;

		cout << book << endl;

	}

	*/

	return 0;

}

error C2248: 'isbn' : cannot access private member declared in class 'Sales_item'
error C2248: 'units_sold' : cannot access private member declared in class 'Sales_item'
error C2248: 'revenue' : cannot access private member declared in class 'Sales_item'