QMap destructor bug

[Ruud]

Hi all,

We used a var yyyy of vollowing type:

typedef QMap<QString, QVariant> XXXX;
QMap<QString, XXXX> * yyyy;

This somewhat complex construction with a QMap containing a QMap works fine untill we call the destructor for yyyy . Then we get a crash!

We think that the problem is that the destructor of QMap calls another destructor of a QMap and we expect this destructor not to be reentrant.

Does anyone recognize this?
Does anyone think that the a QMap should not contain another QMap?

Thanks, Ruud.

[jacek]

Do you have something like this?
#include <qmap.h>
#include <qstring.h>
#include <qvariant.h>

typedef QMap< QString, QVariant > X;

int main()
{
QMap< QString, X > * y = new QMap< QString, X >();

X x;
x.insert( "AAA", 10 );

y->insert( "BBB", x );

delete y;

return 0;
}
It works OK on my system (PLD Linux, Qt 3.3.6). Even if QMap destructor wasn't re-entrant, it's still a class template, so these maps would have different destructors.

[Ruud]

this is the code I used:

(*yyyy)["stringA"]["stringB"].asMap()["styringC"].asStringList().append("stringD");

delete yyyy; //crash

remarkable is that the crash happens on most computers we use, but not all. We do not know what could be the difference between these computers...

[wysota]

The code you use doesn't make much sense. First of all there are no "as*" methods (I guess you call them from QVariant) but even if we assume these are "to*", they return a copy not a reference, so they create temporary objects which get deleted immediately, thus your '["styringC"].asStringList().append("stringD")' does exactly nothing. And probably it is causing the crash in the destructor.

[jacek]

#include <qmap.h>
#include <qstring.h>
#include <qvariant.h>

typedef QMap< QString, QVariant > X;

int main()
{
QMap< QString, X > * y = new QMap< QString, X >();

(*y)["stringA"]["stringB"].asMap()["styringC"].asStringList().append("stringD");

delete y;

return 0;
}

All seems to be OK:
(gdb) run
Starting program: /home/users/jacek/qtcentre/map/map_test

Program exited normally.

Does this crash happen always on a given computer? Maybe you create those maps in static or global objects? Did you try to obtain the stack trace?

[Ruud]

It happens on most computers we use. We use QT 3.3.1. On some computers we do not get the crtash but we use same QT version everywhere.
We are looking for a strange buggy here...

[dimitri]

There is probably a memory error somewhere else in the program. There's no issue with QMap "reentrancy" here since there are no threads involved. You need to run your program in a memory debugger such as Purify on Windows or Valgrind on Linux.

By the way, why allocate with new:

QMap<QString, XXXX> * yyyy
instead of allocating on the stack when possible:

QMap<QString, XXXX> yyyy