[笔记] 《c++ primer》书店程序 Chapter7

Sales_data.h

 1 #ifndef SALES_DATA_H
 2 #define SALES_DATA_H
 3
 4 #include "Version_test.h"
 5
 6 #include <string>
 7 #include <iostream>
 8
 9 class Sales_data {
10 friend Sales_data add(const Sales_data&, const Sales_data&);
11 friend std::ostream &print(std::ostream&, const Sales_data&);
12 friend std::istream &read(std::istream&, Sales_data&);
13 public:
14     // constructors
15 // using the synthesized version is safe only
16 // if we can also use in-class initializers
17 #if defined(IN_CLASS_INITS) && defined(DEFAULT_FCNS)
18     Sales_data() = default;
19 #else
20     Sales_data(): units_sold(0), revenue(0.0) { }
21 #endif
22 #ifdef IN_CLASS_INITS
23     Sales_data(const std::string &s): bookNo(s) { }
24 #else
25     Sales_data(const std::string &s):
26                bookNo(s), units_sold(0), revenue(0.0) { }
27 #endif
28     Sales_data(const std::string &s, unsigned n, double p):
29                bookNo(s), units_sold(n), revenue(p*n) { }
30     Sales_data(std::istream &);
31
32     // operations on Sales_data objects
33     std::string isbn() const { return bookNo; }
34     Sales_data& combine(const Sales_data&);
35     double avg_price() const;
36 private:
37     std::string bookNo;
38 #ifdef IN_CLASS_INITS   // using the synthesized version is safe only
39     unsigned units_sold = 0;
40     double revenue = 0.0;
41 #else
42     unsigned units_sold;
43     double revenue;
44 #endif
45 };
46
47
48 // nonmember Sales_data interface functions
49 Sales_data add(const Sales_data&, const Sales_data&);
50 std::ostream &print(std::ostream&, const Sales_data&);
51 std::istream &read(std::istream&, Sales_data&);
52
53 // used in future chapters
54 inline
55 bool compareIsbn(const Sales_data &lhs, const Sales_data &rhs)
56 {
57     return lhs.isbn() < rhs.isbn();
58 }
59 #endif

• 成员函数通过隐式this指针访问调用它的对象
• 常量成员函数不可修改调用对象的内容
• 通过非成员函数访问类的私有成员，需在类内将此函数声明为友元（友元声明不是函数声明，函数声明在类外）
• 经常使用的短函数声明为内联函数并在.h文件中定义，类内定义的函数是隐式内联的
• 构造函数没有返回值，不能被声明为const

Sales_data.cpp

 1 #include <iostream>
 2 using std::istream; using std::ostream;
 3
 4 #include "Sales_data.h"
 5 Sales_data::Sales_data(std::istream &is)
 6 {
 7     // read will read a transaction from is into this object
 8     read(is, *this);
 9 }
10
11 double
12 Sales_data::avg_price() const {
13     if (units_sold)
14         return revenue/units_sold;
15     else
16         return 0;
17 }
18
19 // add the value of the given Sales_data into this object
20 Sales_data&
21 Sales_data::combine(const Sales_data &rhs)
22 {
23     units_sold += rhs.units_sold; // add the members of rhs into
24     revenue += rhs.revenue;       // the members of ``this'' object
25     return *this; // return the object on which the function was called
26 }
27
28 Sales_data
29 add(const Sales_data &lhs, const Sales_data &rhs)
30 {
31     Sales_data sum = lhs;  // copy data members from lhs into sum
32     sum.combine(rhs);      // add data members from rhs into sum
33     return sum;
34 }
35
36 // transactions contain ISBN, number of copies sold, and sales price
37 istream&
38 read(istream &is, Sales_data &item)
39 {
40     double price = 0;
41     is >> item.bookNo >> item.units_sold >> price;
42     item.revenue = price * item.units_sold;
43     return is;
44 }
45
46 ostream&
47 print(ostream &os, const Sales_data &item)
48 {
49     os << item.isbn() << " " << item.units_sold << " "
50        << item.revenue << " " << item.avg_price();
51     return os;
52 }

avg_price.cpp

 1 #include <iostream>
 2 using std::cerr; using std::cin; using std::cout; using std::endl;
 3
 4 #include "Sales_data.h"
 5
 6 int main()
 7 {
 8     Sales_data total;         // variable to hold the running sum
 9     if (read(cin, total))  {  // read the first transaction
10         Sales_data trans;     // variable to hold data for the next transaction
11         while(read(cin, trans)) {      // read the remaining transactions
12             if (total.isbn() == trans.isbn())   // check the isbns
13                 total.combine(trans);  // update the running total
14             else {
15                 print(cout, total) << endl;  // print the results
16                 total = trans;               // process the next book
17             }
18         }
19         print(cout, total) << endl;          // print the last transaction
20     } else {                                 // there was no input
21         cerr << "No data?!" << endl;         // notify the user
22     }
23
24     return 0;
25 }