10-12.md

#ifndef UNTITLED1_SALES_DATA_H
#define UNTITLED1_SALES_DATA_H

#include <iostream>
#include <string>

class Sales_data;
std::istream &read(std::istream &, Sales_data &);

class Sales_data {
    friend Sales_data add(const Sales_data &, const Sales_data &);
    friend std::istream &read(std::istream &, Sales_data &);
    friend std::ostream &print(std::ostream &, const Sales_data &);
public:
    Sales_data(): Sales_data(" ", 0, 0){}
    Sales_data(double p):bookNo(" "), units_sold(0), revenue(0){};
    Sales_data(const std::string &s):bookNo(s){};
    Sales_data(const std::string &s, unsigned long n, double p):
                bookNo(s), units_sold(n),revenue(p*n){};
    //提供cin作为接受istream&参数的构造函数的默认实参
    Sales_data(std::istream &is) {read(is, *this);}
    std::string isbn() const {
        return bookNo;
    }
    Sales_data & combine(const Sales_data &a);
private:
    double avg_price() const;
    std::string bookNo;
    unsigned long units_sold = 0;
    long double revenue = 0.0;
};

Sales_data& Sales_data::combine(const Sales_data &a) {
    units_sold += a.units_sold;
    revenue += a.revenue;
    return *this;
}

Sales_data add(const Sales_data &asd, const Sales_data &bsd){
    Sales_data tmpsum = asd;
    tmpsum.combine(bsd);
    return tmpsum;
}

std::istream &read(std::istream &is, Sales_data &item){
    double price;
    is >> item.bookNo >> item.units_sold >> price;
    item.revenue = item.units_sold * price;
    return is;
}

std::ostream &print(std::ostream &os, const Sales_data &item){
    os << item.isbn() << " " << item.units_sold << " " << item.revenue;
    return os;
}

inline
double Sales_data::avg_price() const {
    if (units_sold)
        return revenue / units_sold;
    else
        return 0;
}
#endif //UNTITLED1_SALES_DATA_H

#include <iostream>
using std::cin; using std::cout; using std::endl; using std::cerr;
#include <fstream>
using std::ifstream;
#include <string>
using std::string;
#include <vector>
using std::vector;
#include <algorithm>
using std::stable_sort;
#include "Sales_data.h"

bool compareIsbn(const Sales_data &sd1, const Sales_data &sd2){
    return sd1.isbn() < sd2.isbn();
}

int main() {
    vector<Sales_data> vS;
    Sales_data sd;
    string pt("/home/raymain/CLionProjects/CPPLv1/book_sales.txt");
    ifstream input(pt);
    if (input.is_open()){
        while (read(input, sd)){
            vS.push_back(sd);
        }
    } else {
        cerr << "Failed to open file!" << endl;
        return EXIT_FAILURE;
    }
    cout << "Original:\n";
    for (const auto &a : vS)
        print(cout, a) << endl;
    stable_sort(vS.begin(), vS.end(), compareIsbn);
    cout << "After stable_sort:\n";
    for (const auto &a : vS)
        print(cout, a) << endl;
}