请大家帮我看看这个程序的问题出在哪里，在下谢过了！

catbert

1. #include <cstring>
   
2. #include <iostream>
   
3. using namespace std;
   
4. 
   
5. const int MAXCHAR = 80;
   
6. 
   
7. class xstr {
   
8. public:
   
9.   xstr();
   
10.   xstr(const char *);
    
11.   xstr(const xstr& xstrobj);
    
12.   xstr& operator = (const xstr& rhsobj);
    
13.   ~xstr();
    
14.   friend ostream& operator << (ostream& outputStream, const xstr & str_a);
    
15.   friend istream& operator >> (istream& inputStream, xstr& str_a);
    
16.   friend bool operator < (const xstr& sa, const xstr& sb);
    
17. private:
    
18.   char *_str;
    
19.   int len;
    
20. };
    
21. 
    
22. xstr::xstr() :len(0) {
    
23.   _str = NULL;
    
24. };
    
25. 
    
26. xstr::xstr(const char *stra) {
    
27.   len = strlen(stra);
    
28.   _str = new char[len+1];
    
29.   strncpy(_str, stra, MAXCHAR);
    
30. }
    
31. 
    
32. xstr::xstr(const xstr &xstrobj):len(xstrobj.len) {
    
33.   _str = new char[len+1];
    
34.   strncpy(_str, xstrobj._str, MAXCHAR);
    
35. }
    
36. 
    
37. xstr& xstr::operator = (const xstr & rhs) {
    
38.   if (len < rhs.len) {
    
39.     delete [] _str;
    
40.     _str = new char[rhs.len+1];
    
41.   }
    
42.   strncpy(_str, rhs._str, MAXCHAR);
    
43.   return *this;
    
44. }
    
45. 
    
46. xstr::~xstr() {
    
47.   delete [] _str;
    
48. }
    
49. 
    
50. ostream& operator << (ostream &outputStream, const xstr & str_a) {
    
51.   outputStream << str_a._str;
    
52.   return outputStream;
    
53. }
    
54. 
    
55. istream& operator >> (istream &inputStream, xstr & str_a) {
    
56.   char ch;
    
57.   if (str_a.len < MAXCHAR) {
    
58.     if (str_a._str != NULL)
    
59.       delete [] str_a._str;
    
60.     str_a._str = new char[MAXCHAR+1];
    
61.   }
    
62.   int ct =0;
    
63.   inputStream >> ch;
    
64.   while ((isalnum(ch))&&(ct<MAXCHAR)) {
    
65.     str_a._str[ct++] = ch;
    
66.     inputStream >> ch;
    
67.   }
    
68.   str_a._str[ct]='\0';
    
69.   return inputStream;
    
70. }
    
71. 
    
72. bool operator < (const xstr& sa, const xstr& sb) {
    
73.   return (strncmp(sa._str, sb._str, MAXCHAR)<0);
    
74. }
    
75. 
    
76. int main() {
    
77. 
    
78.   xstr sa("abc");
    
79. 
    
80.   cin >> sa;
    
81. 
    
82.   exit(0);
    
83. }

复制代码

namtso

xstr:str(const char *stra) {
  len = strlen(stra);
  _str = new char[len+1];
  strncpy(_str, stra, MAXCHAR);
}

这里溢出了。。。。。

tyc611

xstr& xstr:perator = (const xstr & rhs) {
  if (len < rhs.len) {
    delete [] _str;
    _str = new char[rhs.len+1];
  }
  strncpy(_str, rhs._str, MAXCHAR);
  return *this;
}

istream& operator >> (istream &inputStream, xstr & str_a) {
  char ch;
  if (str_a.len < MAXCHAR) {
    if (str_a._str != NULL)
      delete [] str_a._str;
    str_a._str = new char[MAXCHAR+1];
  }
  int ct =0;
  inputStream >> ch;
  while ((isalnum(ch))&&(ct<MAXCHAR)) {
    str_a._str[ct++] = ch;
    inputStream >> ch;
  }
  str_a._str[ct]='\0';
  return inputStream;
}

没有修改len成员的值

catbert

原帖由 tyc611 于 2006-10-4 10:34 发表


没有修改len成员的值

去掉了len，仍然在同样的地方crash。

catbert

原帖由 namtso 于 2006-10-4 03:33 发表
xstr:str(const char *stra) {
  len = strlen(stra);
  _str = new char[len+1];
  strncpy(_str, stra, MAXCHAR);
}

这里溢出了。。。。。

还是不行：

1. #include <iostream>
   
2. 
   
3. using namespace std;
   
4. 
   
5. const int MAXCHAR = 80;
   
6. 
   
7. class xstr {
   
8. public:
   
9.   xstr();
   
10.   xstr(const char *);
    
11.   xstr(const xstr& xstrobj);
    
12.   xstr& operator = (const xstr& rhsobj);
    
13.   int len() const;
    
14.   ~xstr();
    
15.   friend ostream& operator << (ostream& outputStream, const xstr & str_a);
    
16.   friend istream& operator >> (istream& inputStream, xstr& str_a);
    
17.   friend bool operator < (const xstr& sa, const xstr& sb);
    
18. private:
    
19.   char *_str;
    
20. };
    
21. 
    
22. xstr::xstr() {
    
23.   _str = NULL;
    
24. };
    
25. 
    
26. xstr::xstr(const char *stra) {
    
27.   _str = new char[strlen(stra)+1];
    
28.   strncpy(_str, stra, MAXCHAR);
    
29. }
    
30. 
    
31. xstr::xstr(const xstr &xstrobj) {
    
32.   _str = new char[xstrobj.len()+1];
    
33.   strncpy(_str, xstrobj._str, MAXCHAR);
    
34. }
    
35. 
    
36. xstr& xstr::operator = (const xstr & rhs) {
    
37.   if (len() < rhs.len()) {
    
38.     delete [] _str;
    
39.     _str = new char[rhs.len()+1];
    
40.   }
    
41.   strncpy(_str, rhs._str, MAXCHAR);
    
42.   return *this;
    
43. }
    
44. 
    
45. int xstr::len() const {
    
46.   return strlen(_str);
    
47. }
    
48. 
    
49. xstr::~xstr() {
    
50.   delete [] _str;
    
51. }
    
52. 
    
53. ostream& operator << (ostream &outputStream, const xstr & str_a) {
    
54.   outputStream << str_a._str;
    
55.   return outputStream;
    
56. }
    
57. 
    
58. istream& operator >> (istream &inputStream, xstr & str_a) {
    
59.   char ch;
    
60.   if (str_a.len() < MAXCHAR) {
    
61.     if (str_a._str != NULL)
    
62.       delete [] str_a._str;  //这里有问题，但是不知道为什么
    
63.     str_a._str = new char[MAXCHAR+1];
    
64.   }
    
65.   int ct =0;
    
66.   inputStream >> ch;
    
67.   while ((isalnum(ch))&&(ct<MAXCHAR)) {
    
68.     str_a._str[ct++] = ch;
    
69.     inputStream >> ch;
    
70.   }
    
71.   str_a._str[ct]='\0';
    
72.   return inputStream;
    
73. }
    
74. 
    
75. bool operator < (const xstr& sa, const xstr& sb) {
    
76.   return (strncmp(sa._str, sb._str, MAXCHAR)<0);
    
77. }
    
78. 
    
79. int main() {
    
80. 
    
81.   xstr sa("abc");
    
82. 
    
83.   cin >> sa;
    
84. 
    
85.   exit(0);
    
86. }

复制代码

一运行就crash：

1. $ ./a.out
   
2. *** glibc detected *** ./a.out: free(): invalid next size (fast): 0x09a15008 ***
   
3. ======= Backtrace: =========
   
4. /lib/libc.so.6[0x4488ca68]
   
5. /lib/libc.so.6(__libc_free+0x78)[0x4488ff6f]
   
6. /usr/lib/libstdc++.so.6(_ZdlPv+0x21)[0x44cc56c1]
   
7. /usr/lib/libstdc++.so.6(_ZdaPv+0x1d)[0x44cc571d]
   
8. ./a.out(__gxx_personality_v0+0x25a)[0x804894e]
   
9. ./a.out[0x8048b68]
   
10. /lib/libc.so.6(__libc_start_main+0xdc)[0x4483e4e4]
    
11. ./a.out(__gxx_personality_v0+0x5d)[0x8048751]
    
12. ======= Memory map: ========
    
13. 08048000-08049000 r-xp 00000000 08:02 4120826    /home/xliu/courses/419/62/me/a.out
    
14. 08049000-0804a000 rw-p 00001000 08:02 4120826    /home/xliu/courses/419/62/me/a.out
    
15. 09a15000-09a36000 rw-p 09a15000 00:00 0          [heap]
    
16. 4480b000-4480c000 r-xp 4480b000 00:00 0          [vdso]
    
17. 4480c000-44825000 r-xp 00000000 08:02 4840232    /lib/ld-2.4.so
    
18. 44825000-44826000 r--p 00018000 08:02 4840232    /lib/ld-2.4.so
    
19. 44826000-44827000 rw-p 00019000 08:02 4840232    /lib/ld-2.4.so
    
20. 44829000-44956000 r-xp 00000000 08:02 4840233    /lib/libc-2.4.so
    
21. 44956000-44958000 r--p 0012d000 08:02 4840233    /lib/libc-2.4.so
    
22. 44958000-44959000 rw-p 0012f000 08:02 4840233    /lib/libc-2.4.so
    
23. 44959000-4495c000 rw-p 44959000 00:00 0
    
24. 4495e000-44981000 r-xp 00000000 08:02 4842240    /lib/libm-2.4.so
    
25. 44981000-44982000 r--p 00022000 08:02 4842240    /lib/libm-2.4.so
    
26. 44982000-44983000 rw-p 00023000 08:02 4842240    /lib/libm-2.4.so
    
27. 44bb8000-44bc3000 r-xp 00000000 08:02 4842242    /lib/libgcc_s-4.1.1-20060525.so.1
    
28. 44bc3000-44bc4000 rw-p 0000a000 08:02 4842242    /lib/libgcc_s-4.1.1-20060525.so.1
    
29. 44c10000-44cf2000 r-xp 00000000 08:02 1190795    /usr/lib/libstdc++.so.6.0.8
    
30. 44cf2000-44cf6000 r--p 000e1000 08:02 1190795    /usr/lib/libstdc++.so.6.0.8
    
31. 44cf6000-44cf7000 rw-p 000e5000 08:02 1190795    /usr/lib/libstdc++.so.6.0.8
    
32. 44cf7000-44cfd000 rw-p 44cf7000 00:00 0
    
33. b7e00000-b7e21000 rw-p b7e00000 00:00 0
    
34. b7e21000-b7f00000 ---p b7e21000 00:00 0
    
35. b7f19000-b7f1b000 rw-p b7f19000 00:00 0
    
36. b7f35000-b7f36000 rw-p b7f35000 00:00 0
    
37. bfe4e000-bfe63000 rw-p bfe4e000 00:00 0          [stack]
    
38. Aborted
    
39. $

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是delete这行出错：

1. (gdb) where
   
2. #0  0x4480b410 in __kernel_vsyscall ()
   
3. #1  0x44850ee9 in raise () from /lib/libc.so.6
   
4. #2  0x448524f1 in abort () from /lib/libc.so.6
   
5. #3  0x4488553b in __libc_message () from /lib/libc.so.6
   
6. #4  0x4488ca68 in _int_free () from /lib/libc.so.6
   
7. #5  0x4488ff6f in free () from /lib/libc.so.6
   
8. #6  0x44cc56c1 in operator delete () from /usr/lib/libstdc++.so.6
   
9. #7  0x44cc571d in operator delete[] () from /usr/lib/libstdc++.so.6
   
10. #8  0x0804894e in operator>> ()
    
11. #9  0x08048b68 in main ()
    
12. (gdb)   

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[ 本帖最后由 catbert 于 2006-10-4 13:10 编辑 ]

namtso

都跟你说了，是 strncpy(_str, stra, MAXCHAR);的时候溢出了。。。。。。
回去查一下strncpy手册就明白了。