C++ primer中的sales_item代码中为啥可以直接访问私有的成员？
没见到友元的声明。



#ifndef SALESITEM_H
#define SALESITEM_H

#include <iostream>
#include <string>

class Sales_item {
friend bool operator==(const Sales_item&, const Sales_item&);
public:
Sales_item(const std::string &book):
isbn(book), units_sold(0), revenue(0.0) { }
Sales_item(std::istream &is) { is >> *this; }
friend std::istream& operator>>(std::istream&, Sales_item&);
friend std::ostream& operator<<(std::ostream&, const Sales_item&);
public:
Sales_item& operator+=(const Sales_item&);
public:
double avg_price() const;
bool same_isbn(const Sales_item &rhs) const
{ return isbn == rhs.isbn; }
Sales_item(): units_sold(0), revenue(0.0) { }
private:
std::string isbn;
unsigned units_sold;
double revenue;

};

Sales_item operator+(const Sales_item&, const Sales_item&);

inline bool
operator==(const Sales_item &lhs, const Sales_item &rhs)
{
return lhs.units_sold == rhs.units_sold &&
lhs.revenue == rhs.revenue &&
lhs.same_isbn(rhs);
}

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

using std::istream; using std::ostream;

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

inline
Sales_item
operator+(const Sales_item& lhs, const Sales_item& rhs)
{
Sales_item ret(lhs); // copy lhs into a local object that we'll return

ret += rhs; // add in the contents of rhs
return ret; // return ret by value
}

inline
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(); // input failed: reset object to default state
return in;
}

inline
ostream&
operator<<(ostream& out, const Sales_item& s)
{
out << s.isbn << "\t" << s.units_sold << "\t"
<< s.revenue << "\t" << s.avg_price();
return out;
}

inline
double Sales_item::avg_price() const
{
if (units_sold)
return revenue/units_sold;
else
return 0;
}


#endif

[解决办法]
类的封装性，就是私有成员只有类本身才可以调用的！
申明友元也可以调用，其实破坏了它的这样封装性的！