实例：从一个文件中读取销售记录，并合并生成销售报告，实现销售册数，总销售额，平均售价

#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(): 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 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;
    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_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 << " 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

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totalBook
	   Sales_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;
   
}

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