c++ primer学习指导（10）--1.5类简介

通过定义类可以定义自己的数据结构
重点了解三个问题：
1、类名是什么？（见下图）
2、它在哪里定义？（定义在Sales_item.h中）
3、它支持什么操作？（见下一节）
类的其他细节后面章节会将。
类定义代码贴在这里：
  1 #ifndef SALESITEM_H
  2 // we're here only if SALESITEM_H has not yet been defined 
  3 #define SALESITEM_H
  4 
  5 #include "Version_test.h" 
  6 
  7 // Definition of Sales_item class and related functions goes here
  8 #include <iostream>
  9 #include <string>
 10 
 11 class Sales_item {
 12 // these declarations are explained section 7.2.1, p. 270 
 13 // and in chapter 14, pages 557, 558, 561
 14 friend std::istream& operator>>(std::istream&, Sales_item&);
 15 friend std::ostream& operator<<(std::ostream&, const Sales_item&);
 16 friend bool operator<(const Sales_item&, const Sales_item&);
 17 friend bool 
 18 operator==(const Sales_item&, const Sales_item&);
 19 public:
 20     // constructors are explained in section 7.1.4, pages 262 - 265
 21     // default constructor needed to initialize members of built-in type
 22 #if defined(IN_CLASS_INITS) && defined(DEFAULT_FCNS)
 23     Sales_item() = default;
 24 #else
 25     Sales_item(): units_sold(0), revenue(0.0) { }
 26 #endif
 27     Sales_item(const std::string &book):
 28               bookNo(book), units_sold(0), revenue(0.0) { }
 29     Sales_item(std::istream &is) { is >> *this; }
 30 public:
 31     // operations on Sales_item objects
 32     // member binary operator: left-hand operand bound to implicit this pointer
 33     Sales_item& operator+=(const Sales_item&);
 34     
 35     // operations on Sales_item objects
 36     std::string isbn() const { return bookNo; }
 37     double avg_price() const;
 38 // private members as before
 39 private:
 40     std::string bookNo;      // implicitly initialized to the empty string
 41 #ifdef IN_CLASS_INITS
 42     unsigned units_sold = 0; // explicitly initialized
 43     double revenue = 0.0;
 44 #else
 45     unsigned units_sold;  
 46     double revenue;       
 47 #endif
 48 };
 49 
 50 // used in chapter 10
 51 inline
 52 bool compareIsbn(const Sales_item &lhs, const Sales_item &rhs) 
 53 { return lhs.isbn() == rhs.isbn(); }
 54 
 55 // nonmember binary operator: must declare a parameter for each operand
 56 Sales_item operator+(const Sales_item&, const Sales_item&);
 57 
 58 inline bool 
 59 operator==(const Sales_item &lhs, const Sales_item &rhs)
 60 {
 61     // must be made a friend of Sales_item
 62     return lhs.units_sold == rhs.units_sold &&
 63            lhs.revenue == rhs.revenue &&
 64            lhs.isbn() == rhs.isbn();
 65 }
 66 
 67 inline bool 
 68 operator!=(const Sales_item &lhs, const Sales_item &rhs)
 69 {
 70     return !(lhs == rhs); // != defined in terms of operator==
 71 }
 72 
 73 // assumes that both objects refer to the same ISBN
 74 Sales_item& Sales_item::operator+=(const Sales_item& rhs) 
 75 {
 76     units_sold += rhs.units_sold; 
 77     revenue += rhs.revenue; 
 78     return *this;
 79 }
 80 
 81 // assumes that both objects refer to the same ISBN
 82 Sales_item 
 83 operator+(const Sales_item& lhs, const Sales_item& rhs) 
 84 {
 85     Sales_item ret(lhs);  // copy (|lhs|) into a local object that we'll return
 86     ret += rhs;           // add in the contents of (|rhs|) 
 87     return ret;           // return (|ret|) by value
 88 }
 89 
 90 std::istream& 
 91 operator>>(std::istream& in, Sales_item& s)
 92 {
 93     double price;
 94     in >> s.bookNo >> s.units_sold >> price;
 95     // check that the inputs succeeded
 96     if (in)
 97         s.revenue = s.units_sold * price;
 98     else 
 99         s = Sales_item();  // input failed: reset object to default state
100     return in;
101 }
102 
103 std::ostream& 
104 operator<<(std::ostream& out, const Sales_item& s)
105 {
106     out << s.isbn() << " " << s.units_sold << " "
107         << s.revenue << " " << s.avg_price();
108     return out;
109 }
110 
111 double Sales_item::avg_price() const
112 {
113     if (units_sold) 
114         return revenue/units_sold; 
115     else 
116         return 0;
117 }
118 #endif