Qt-interest Archive, November 2006
crash in destructor of QBoxLayout
Message 1 in thread
Hi. I have an object which is a subclass of QDialog which has five
QHBoxLayout objects and one QVBoxLayout object as members. Note that
these are NOT pointers. I also have two QPushButton objects, a
QCheckBox, two QLineEdit objects, and three QLabel objects. None are
pointers.
In the QDialog's constructor, these objects are put together
programatically into a layout. The various widgets are added to the
QHBoxLayouts and then these are added to the QVBoxLayout. Finally, I
call setLayout making the QVBoxLayout the main layout for the dialog.
When my app quits, the QDialog object gets destroyed. Its constructor
causes an access violation. (I happen to be developing this on the
Mac and I get a EXC_BAD_ACCESS). It tells me that this is caused by
deallocation of a pointer that was not malloced.
I noticed that at one point, I was getting one of these error
messages about freeing non-malloced object. Then I rearranged the
order of the layouts in the class declaration, putting the
QVBoxLayout after the five QHBoxLayouts. This caused me to get five
of these messages before the crash instead of one.
The stack shows it to be crashed inside of
QBoxLayoutPrivate::deleteAll which is called from
QBoxLayout::~QBoxLayout which is called from the destructor of my
QDialog subclass.
From looking at deleteAll, it appears that it is the case that this
method removes layout items and deletes them. Obviously my objects
should not be deleted because they were not created with new.
Therefore, I guessed that the problem was that the QBoxLayout
destructor calls deleteAll which deletes any items that have been
added with addWidget or addLayout. So, I thought maybe I could remove
these things in the destructor so that they wouldn't be there when
delete all gets called which might cause this problem to be avoided.
So, I added code:
for (i=mHLayout0.count()-1;i>=0;--i) {
qDebug("Remove %d\n", i);
mHLayout0.removeItem(mHLayout0.takeAt(i));
}
for each one of my layouts. The qDebug is there so that I could
verify that removeItem was being called the correct number of times
and it does appear to be the case that it is called the correct
number of times.
I expected this to eliminate the access violation. However, it did
not do so.
I also tried calling setLayout(NULL) on the QDialog subclass object
inside its destructor. Still, no effect on the problem.
Any ideas on how to solve this? I could probably fix this by creating
everything with new, but I'd prefer to understand what is going on
and not to have to do that.
Thanks.
Brant Sears
--
[ signature omitted ]
