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Something strange was happening to the class info. Your valiant author was assigned the task of figuring out what was happening. After a little playing around, I decided that what was probably happening is that someone had gotten a hold of a bad pointer and was stepping all over the class.
To try and find out where the class was being overwritten, I put a couple of magic numbers at the beginning and end of the data for the class. I expected these magic numbers to get changed when things went wrong. But I was surprised to learn that things went wrong much sooner than expected.
So why does the magic go out of the class?
1 #include <stdlib.h>
2 #include <iostream>
3 #include <cstring>
4
5 /************************************************
6 * info -- A class to hold information. *
7 * *
8 * Note: *
9 * Because someone is walking all over our *
10 * memory and destroying our data, we *
11 * have put two guards at the beginning *
12 * and end of our class. If someone *
13 * messes with us these numbers will *
14 * be destroyed. *
15 * *
16 * Member functions: *
17 * set_data -- Store a string in our data. *
18 * get_data -- Get the data string. *
19 * check_magic -- Check the magic numbers. *
20 ************************************************/
21 // Magic numbers for the start and end of the
22 // data in the class info
23 const int START_MAGIC = 0x11223344;
24 const int END_MAGIC = 0x5567788;
25 class info
26 {
27 private:
28 // Magic protection constant
29 const int start_magic;
30
31 // String to be stored
32 char data[30];
33
34 // Magic protection constant
35 const int end_magic;
36 public:
37 info(void):
38 start_magic(START_MAGIC),
39 end_magic(END_MAGIC)
40 {}
41
42 // Copy constructor defaults
43 // Assignment operator defaults
44 // Destructor defaults
45
46 // Store some data in the class
47 void set_data(
48 // Data to be stored
49 const char what[]
50 )
51 {
52 strcpy(data, what);
53 }
54
55 // Get the data from the class
56 char *get_data(void)
57 {
58 return (data);
59 }
60
61 // Verify that the magic
62 // numbers are correct
63 void check_magic(void)
64 {
65 if ((start_magic != START_MAGIC) ||
66 (end_magic != END_MAGIC))
67 {
68 std::cout <<
69 "Info has lost its magic\n";
70 }
71 }
72 };
73
74 /************************************************
75 * new_info -- Create a new version of the *
76 * info class. *
77 ************************************************/
78 struct info *new_info(void)
79 {
80 struct info *result; // Newly created result.
81
82 result = (struct info *)
83 malloc(sizeof(struct info));
84
85 // Make sure the structure is clear
86 memset(result, '\0', sizeof(result));
87
88 return (result);
89 }
90 int main()
91 {
92 // An info class to play with
93 class info *a_info = new_info();
94
95 a_info->set_data("Data");
96 a_info->check_magic();
97 return (0);
98 }
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Profanity is the one language that all programmers understand.
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