c++第三天

1、复习了一下昨天的内容 while(std::cin >> value) 扫描【标准输入】

2、在网上下载Sales_item.h 代码如下

#ifndef SALESITEM_H
#define SALESITEM_H
#include <iostream>
#include <string>


class Sales_item
{
public:
 Sales_item(const std::string &book):isbn(book),units_sold(0),revenue(0.0){}
 Sales_item(std::istream &is){ is >> *this;}
 friend std::istream& operator>>(std::istream &,Sales_item &);
 friend std::ostream& operator<<(std::ostream &,const Sales_item &);
public:
 Sales_item & operator+=(const Sales_item&);
public:
 double avg_price() const;
 bool same_isbn(const Sales_item &rhs)const
 {
  return isbn == rhs.isbn;
 }
 Sales_item():units_sold(0),revenue(0.0){}
public:
 std::string isbn;
 unsigned units_sold;
 double revenue;
};

using std::istream;
using std::ostream;
Sales_item operator+(const Sales_item &,const Sales_item &);
inline bool operator==(const Sales_item &lhs,const Sales_item &rhs)
{
 return lhs.units_sold == rhs.units_sold && lhs.revenue == rhs.revenue && lhs.same_isbn(rhs);
}
inline bool operator!=(const Sales_item &lhs,const Sales_item &rhs)
{
 return !(lhs == rhs);
}

inline Sales_item & Sales_item::operator +=(const Sales_item &rhs)
{
 units_sold += rhs.units_sold;
 revenue += rhs.revenue;
 return *this;
}
inline Sales_item operator+(const Sales_item &lhs,const Sales_item &rhs)
{
 Sales_item ret(lhs);
 ret += rhs;
 return ret;
}
inline istream& operator>>(istream &in,Sales_item &s)
{
 double price;
 in >> s.isbn >> s.units_sold >> price;
 if(in)
  s.revenue = s.units_sold * price;
 else
  s = Sales_item();
 return in;
}
inline ostream& operator<<(ostream &out,const Sales_item &s)
{
 out << s.isbn << "	" <<s.units_sold << "	" << s.revenue << "	" << s.avg_price();
 return out;
}
inline double Sales_item::avg_price() const
{
 if(units_sold)
  return revenue/units_sold;
 else
  return 0;
}
#endif

3、使用文件重定向, 例如：$ prog1<input.txt>output.txt

遇到的问题：

 1、在c++primer e5里， 访问isbn用的是 item1.isbn()，

而在我下载的头文件里 isbn直接就被定义为public 而没有写isdn()这个函数。因此在下面的代码里就直接用item1.isdn访问对象的isdn。

2、另外， std::cerr << "xxx" 不受重定向影响，xxx 直接输出在 cmd 界面

练习1.19

#include<iostream>

int main()
{
    std::cout << "Enter two numbers:" << std::endl;
    int v1 = 0, v2 = 0;
    std::cin >> v1 >> v2;
    int temp = 0;
    if(v1 > v2){
        temp = v1;
        v1 = v2;
        v2 = temp;
    }
    for(;v1 <= v2; ++v1){
        std::cout << v1 << std::endl;
    }
    return 0;
}