stl::string memory leak associated with polymorphism

From:

daniel@fin.de

Newsgroups:

microsoft.public.vc.stl

Date:

24 Sep 2006 03:58:13 -0700

Message-ID:

<1159095493.529250.306820@m7g2000cwm.googlegroups.com>

Hi,
I spent a week on tracking down some memory leak which apparently comes
from the stl or the c++-Implementation itself. I tried Visual Studio
2002 and Visual C++ 2005 which both created the problem. Since I do not
have anything sophisticated as for example Bounds Checker, I used the
debug heap. Here is the code, (compile an run in debug mode in a
default "Windows Console Application" project):

-----------------------------------------------------------------------
-----------------------------------------------------------------------
#include <crtdbg.h>
#include <string>

using namespace std;

class Base
{
public:
    virtual void test() =0;
};

class Final : public Base
{
public:
    string str;
    void test() { str ="0123456789012345";}
};

int main(int argc,char **argv)
{
    _CrtMemState state;
    _CrtMemCheckpoint(&state);

    Base *t =new Final;
    t->test();
    delete t;

    _CrtMemState state2, stateDiff;
    _CrtMemCheckpoint(&state2);
    _CrtMemDumpAllObjectsSince(&state);
    _CrtMemDifference(&stateDiff, &state, &state2);
    _CrtMemDumpStatistics(&stateDiff);
}

-----------------------------------------------------------------------
-----------------------------------------------------------------------
This results in the following debug output:
-----------------------------------------------------------------------
-----------------------------------------------------------------------

Dumping objects ->
{45} normal block at 0x00323E70, 32 bytes long.
Data: <0123456789012345> 30 31 32 33 34 35 36 37 38 39 30 31 32 33 34
35
Object dump complete.
0 bytes in 0 Free Blocks.
32 bytes in 1 Normal Blocks.
0 bytes in 0 CRT Blocks.
0 bytes in 0 Ignore Blocks.
0 bytes in 0 Client Blocks.
Largest number used: 0 bytes.
Total allocations: 64 bytes.

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

Thus I have a 32 bytes leak per call. Two more observations:
1. When changing the code from
    Base *t =new Final;
to
    Final *t =new Final;
the problem is resolved - no leak anymore.
2. When the string "0123456789012345" is less than 16 bytes long, the
leak disappears and the longer it becomes, the larger the leak.

I would like to know if anyone can reproduce this problem and if there
is a known workaround. I already had a short look at the MSKB but did
not find anything at once.

Thanks in advance for any help,
Daniel