#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 typeSales_item(): units_sold(0), revenue(0.0) { }Sales_item(const std::string &book): bookNo(book), units_sold(0), revenue(0.0) { }Sales_item(std::istream &is) { is >> *this; }
public:// operations on Sales_item objects// member binary operator: left-hand operand bound to implicit this pointerSales_item& operator+=(const Sales_item&);// operations on Sales_item objectsstd::string isbn() const { return bookNo; }double avg_price() const;
// private members as before
private:std::string bookNo;      // implicitly initialized to the empty stringunsigned units_sold;double revenue;
};// 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_itemreturn 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 returnret += 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 succeededif (in)s.revenue = s.units_sold * price;else s = Sales_item();  // input failed: reset object to default statereturn in;
}std::ostream&
operator<<(std::ostream& out, const Sales_item& s)
{out << " bookNo "<< s.isbn() << " soldNum " << s.units_sold << " totalSoldMoney "<< s.revenue << " price " << s.avg_price();return out;
}double Sales_item::avg_price() const
{if (units_sold) return revenue/units_sold; else return 0;
}
#endif复制代码

下面开始编写代码：

hello.cc

#include <iostream>
#include "Sales_item.h"
int main()
{Sales_item totalBook;//统计对象if(std::cin>>totalBook){//第一次读取的值赋值给totalBookSales_item  currBook;//当前对象while(std::cin>>currBook)//从第二次开始，读取的值赋给currBook{if(totalBook.isbn()==currBook.isbn())//上一次读取的对象与当前读取的对象书本编号相等，就叠加{totalBook += currBook;}else{//否则就打印上次书本统计的结果，接着又把当前读取的对象赋值给统计对象std::cout<<totalBook<<std::endl;totalBook = currBook;}}std::cout<<totalBook<<std::endl;//打印最后统计的值}else{std::cerr<<"no data!!!"<<std::endl;return -1;}return 0;}复制代码

逻辑类似于上一个练习，就不一一分析了。

接下来就是编译阶段

1：生成可执行程序

在window下会生成book.exe可执行程序

ps：上述的所有文件都在统一目录下，这点很重要，不然会报错

2：读取销售记录文件以及程序输出到文件中