Re: Templates, Structs and Invalid Pointers - where did it go wrong

From:
anon <anon@no.no>
Newsgroups:
comp.lang.c++
Date:
Thu, 28 Aug 2008 11:36:48 +0200
Message-ID:
<g95rin$p26$1@news01.versatel.de>
DaveJ wrote:
[...]

Another class defines a struct, that contains this template class as a
member. In one function the struct is created, but when it goes out
of scope the program cores.


It doesn't core for me (unfortunately)

Examining the core it appears a call to free in the template classes
destructor caused the core. When I traced through the code with the


Here is even smaller example producing exactly what you are doing:

int main()
{
    int a = 5;
    int *b = &a;
    free( b );
}

debugger I found that after the template class constructor exits, the
address that one of its member's points to changes.

Anyway I've included a cut down and simplified version of the code
below. I found the resolution was to modify the struct so that it
takes a reference to the template class.


I guess you are out of luck. What you are doing is not allowed.

------ VarBuf Class <VarBuf.h> ------
#ifndef __TVARBUF_H__
#define __TVARBUF_H__

#include <stddef.h>
#include <string.h>
#include <stdlib.h>


These are c headers. Should be:
#include <cstddef>
#include <cstring>
#include <cstdlib>

template<size_t N> class TVarBuf
{
public:
            TVarBuf (const char *initStr);
            ~TVarBuf ();
private:
    char m_Buf[N + 1];
    char* m_Str;
    size_t m_Capacity;
    size_t m_Length;
};
#endif

template<size_t N>
TVarBuf<N>::TVarBuf(const char *initStr) :


initStr is not used

    m_Str(NULL),
    m_Capacity(N),
    m_Length(0)
{
    m_Str = m_Buf;
    m_Str[0] = '\0';
}

template<size_t N>
TVarBuf<N>::~TVarBuf()
{
    if(m_Str != m_Buf)


should be
if (m_Str != &m_Buf[0])

    {
        free(m_Str);


Where do you allocate this memory?

    }
}

---------------------------------

---- Main App <main.cpp> -----

1 #include "VarBuf.h"
2 #include "stdio.h"
3 #include "time.h"


Do not use c headers in a c++ program

4 typedef TVarBuf<6> ProfKey; // Create a typedef for our template
class


This typedef looks very C-ish

5 typedef struct
6 {
7 time_t m_CurrTime;
8 int m_Docs;
9 int m_OldDocs;
10 ProfKey m_OldestProfUrl; // this is the template class
11 } Prof;
12
13 int main(int argc, char** argv)
14 {
15 {
16 time_t now = time(NULL);
17 Prof profile = {now, 0, 0, NULL};
18 } // Application cores after it leaves here and the profile
object goes out of scope
19 return 0;
20 }

------------------------------------------

Compiling the above code with the following command
g++ -g -Wall -omain main.cpp

and running the main binary results in:
free(): invalid pointer 0xbfffd66c!

I modified the code slightly so that line 10 of main reads:
-----------------------------------------------
ProfKey& m_OldestProfUrl; //take a reference to ProfKey
-----------------------------------------------
and line 17 now reads
------------------------------------------------
ProfKey key(NULL);
Prof profile = {now, 0, 0, key}
------------------------------------------------


This doesn't solve your problem, which is in the TVarBuf class

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