#ifndef SALESITEM_H
#define SALESITEM_H
// Definition of Sales_item class and related functions goes here

#include <iostream>
#include <string>
class Sales_item {
friend bool operator==(const Sales_item&, const Sales_item&);
// other members as before
public:
    // added constructors to initialize from a string or an istream
    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:
    // operations on Sales_item objects
    // member binary operator: left-hand operand bound to implicit this pointer
    Sales_item& operator+=(const Sales_item&);
    // other members as before
   
public:
    // operations on Sales_item objects
    double avg_price() const;
    bool same_isbn(const Sales_item &rhs) const
        { return isbn == rhs.isbn; }
    // default constructor needed to initialize members of built-in type
    Sales_item(): units_sold(0), revenue(0.0) { }
// private members as before
private:
    std::string isbn;
    unsigned units_sold;
    double revenue;
};

// nonmember binary operator: must declare a parameter for each operand
Sales_item operator+(const Sales_item&, const Sales_item&);
inline bool
operator==(const Sales_item &lhs, const Sales_item &rhs)
{
    // must be made a friend of Sales_item
    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); // != defined in terms of operator==
}
using std::istream; using std::ostream;
// assumes that both objects refer to the same isbn
inline
Sales_item& Sales_item::operator+=(const Sales_item& rhs)
{
    units_sold += rhs.units_sold;
    revenue += rhs.revenue;
    return *this;
}
// assumes that both objects refer to the same isbn
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;
    // check that the inputs succeeded
    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
看c++ primer第一章就出现这个类，谁能帮忙翻译一下下啊！
以前看老谭的c++电子书，看到类就没有再看了。，所有买了本这个书
结果看起来第一章就这个，谁能帮忙加一下下注释啊！！
谢谢