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C++ Primer阅读笔记

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 typeSales_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 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 = 0; // explicitly initializeddouble 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_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 << 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;
}