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List:       kde-devel
Subject:    Re: Doubt about STL hash_map
From:       David Kamphausen <david.kamphausen () web ! de>
Date:       2003-04-24 7:21:02
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Hi!

The STL works :-)
Your local variable s is used to store the information of the key in the 
hash_map. So every time you store something the old key gets overwritten. 
Mind that hash_map doesn't make any copies of your pointers. Try using string 
instead of const char* - it is easier to use and your example there's no real 
overhead with this.

David Kamphausen

Am Mittwoch, 23. April 2003 23:34 schrieb andre-kde:
> The first program.
> In this program i just change the value of the variable 's' and
> the program doesn't works. See the code bellow:
>
> #include <hash_map>
> #include <iostream>
> #include <cstring>
> #include <stdio.h>
>
> struct eqstr
> {
>   bool operator()(const char* s1, const char* s2) const
>   {
>     return strcmp(s1, s2) == 0;
>   }
> };
>
> int main()
> {
>   hash_map<const char*, int, hash<const char*>, eqstr> h;
>
>   char s[16];
>   for (int i = 0; i < 2; i++) {
>     for (int j = 0; j < 1000; j++) {
>         sprintf(s, "%d", j);
with j>0: old values of the keys are overwritten
>         if (i == 0)
>             h[s] = j;
the pointer to s[0] is used as key. 
>         else {
>             if (h[s] != j)
>                 cout << "h[" << s << "] -> " << h[s] << endl;
>         }
>     }
>   }
> }
>
> The second program.
> In this program i change the address of the variable 's' and the
> program works well. See the code bellow:
>
> #include <hash_map>
> #include <iostream>
> #include <cstring>
> #include <stdio.h>
>
> struct eqstr
> {
>   bool operator()(const char* s1, const char* s2) const
>   {
>     return strcmp(s1, s2) == 0;
>   }
> };
>
> int main()
> {
>   hash_map<const char*, int, hash<const char*>, eqstr> h;
>
>   char *s;
>   for (int i = 0; i < 2; i++) {
>     for (int j = 0; j < 1000; j++) {
>         s = new char[16];
>         sprintf(s, "%d", j);
// nothing gets overwritten because s points to a new memory position
>         if (i == 0)
>             h[s] = j;
>         else {
>             if (h[s] != j)
>                 cout << "h[" << s << "] -> " << h[s] << endl;
>         }
>     }
>   }
> }

-- 
David Kamphausen
david.kamphausen@web.de

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