1--该章节新知识点

① 在 UNIX 和 Windows 系统中，执行完一个程序后，可以通过 echo 命令获得其返回值；

# UNIX系统中，通过如下命令获得状态
echo $?

② 在标准库中，定义了两个输出流 ostream 对象：cerr 和 clog；

cerr 通常用于输出警告和错误信息，称为标准错误；

clog 通常用于输出程序时的一般性信息；

③ std::endl 的作用：用于结束当前行，并将与设备关联的缓冲区（buffer）中的内容刷到设备中；

缓冲刷新操作可以保证程序所产生的所有输出都写入输入流中，而不是仅停留在内存中等待写入流；

④ 当遇到文件结束符或遇到一个无效输入时，istream对象的状态会变为无效；处于无效状态的istream对象会使条件（while循环）变为假；

在 UNIX 系统中，输入文件结束符的命令为：Ctrl + D；

⑤ 编程习惯问题：

标准库头文件通常不带后缀；

包含来自标准库的头文件时，应该用尖括号（< >）包含头文件名；

对于不属于标准库的头文件，习惯用双引号（" "）包围；

2--书包程序完整代码

① Sales_item.h

#ifndef SALESITEM_H
// we're here only if SALESITEM_H has not yet been defined 
#define SALESITEM_H

// Definition of Sales_item class and related functions goes here
#include <iostream>
#include <string>

class Sales_item {
// these declarations are explained section 7.2.1, p. 270 
// and in chapter 14, pages 557, 558, 561
friend std::istream& operator>>(std::istream&, Sales_item&);
friend std::ostream& operator<<(std::ostream&, const Sales_item&);
friend bool operator<(const Sales_item&, const Sales_item&);
friend bool 
operator==(const Sales_item&, const Sales_item&);
public:
    // constructors are explained in section 7.1.4, pages 262 - 265
    // default constructor needed to initialize members of built-in type
    Sales_item() = default;
    Sales_item(const std::string &book): bookNo(book) { }
    Sales_item(std::istream &is) { is >> *this; }
public:
    // operations on Sales_item objects
    // member binary operator: left-hand operand bound to implicit this pointer
    Sales_item& operator+=(const Sales_item&);
    
    // operations on Sales_item objects
    std::string isbn() const { return bookNo; }
    double avg_price() const;
// private members as before
private:
    std::string bookNo;      // implicitly initialized to the empty string
    unsigned units_sold = 0; // explicitly initialized
    double revenue = 0.0;
};

// used in chapter 10
inline
bool compareIsbn(const Sales_item &lhs, const Sales_item &rhs) 
{ return lhs.isbn() == rhs.isbn(); }

// 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.isbn() == rhs.isbn();
}

inline bool 
operator!=(const Sales_item &lhs, const Sales_item &rhs)
{
    return !(lhs == rhs); // != defined in terms of operator==
}

// assumes that both objects refer to the same ISBN
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
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
}

std::istream& 
operator>>(std::istream& in, Sales_item& s)
{
    double price;
    in >> s.bookNo >> 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;
}

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

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

② 测试程序

#include <iostream>
#include "Sales_item.h"

int main(){
    Sales_item total;

    if(std::cin >> total){
        Sales_item trans;
        while(std::cin >> trans){
            if(total.isbn() == trans.isbn()){
                total += trans;
            }
            else{
                std::cout << total << std::endl;
                total = trans;
            }
        }
        std::cout << total << std::endl;
    }
    else{
        std::cerr << "No data?!" << std::endl;
        return -1;
    }

    return 0;
}