【c++ primer, 5e】

#include <iostream>
#include <string>
using namespace std;

// Sales_data.h
struct Sales_data {
    // public部分，对象看起来是怎么样的。
    std::string isbn() const { return bookNo; } // inline function
    Sales_data& combine(const Sales_data&);
    double avg_price() const;
    // private部分，数据成员。
    std::string bookNo;
    unsigned units_sold = 0;
    double revenue = 0.0;
};
// Sales_data的非成员接口函数
Sales_data add(const Sales_data&, const Sales_data&);
std::ostream &print(std::ostream&, const Sales_data&);
std::istream &read(std::iostream&, Sales_data&);


// Sales_data.cpp
Sales_data& Sales_data::combine(const Sales_data &rhs)
{
    units_sold += rhs.units_sold;
    revenue += rhs.revenue;
    return *this;
}
istream &read(istream &is, Sales_data &item)
{
    double price = 0;
    is >> item.bookNo >> item.units_sold >> price;
    item.revenue = price * item.units_sold;
    return is;
}
ostream &print(ostream &os, const Sales_data &item)
{
    os << item.isbn() << " " << item.units_sold << " " << item.revenue << " " << item.avg_price();
    return os;
}
double Sales_data::avg_price() const {
    if (units_sold)
        return revenue/units_sold;
    else
        return 0;
}
Sales_data add(const Sales_data &lhs, const Sales_data &rhs)
{
    Sales_data sum = lhs;
    sum.combine(rhs);
    return sum;
}


// main.cpp
int main()
{
    Sales_data total;
    if (read(cin, total)) {
        Sales_data trans;
        while (read(cin, trans)) {
            if (total.isbn() == trans.isbn())
                total.combine(trans);
            else {
                print(cout, total) << endl;
                total = trans;
            }
        }
        print(cout, total) << endl;
    } else {
        cerr << "No data?!" << endl; 
    }
    return 0;
}