[Qt-interest] Is there any way to dynamically rearrange a dialog'slayout?

[Ellen Kestrel]

Thanks for the suggestions.  Would explicitly setting the parent to NULL
also remove the reference to the child from a parent object (if, for
example, the dialog or the parent layout still think that they layout or
widget is their child...) as well, or might I have to do that manually
also?  Or would that not matter?  In all cases I've tried to be careful to
delete the former children of a layout before deleting the layout itself,
after removing them from the layout, of course.  If I do it the other way,
will the parent layout take care of deleting all of its children, or will it
leave them with an invalid pointer to the now-deleted layout?

EK

On Wed, May 19, 2010 at 6:46 PM, Ross Bencina <rossb-lists at audiomulch.com>wrote:

> Hi Ellen
>
> Just a guess, but it sounds like some of your widgets are getting deleted
> when you delete the layouts -- ie, somewhere you have something which is the
> child of something you're deleting which shouldn't be.
>
> The documentation is a bit confusing regarding transfer of ownership in
> different cases (see Layout::addChildWidget, QLayout::addItem,
> QLayout::removeWidget etc). In any case, it's worth keeping in mind that the
> ownership graph(parent/child relationship) is not always the same as the
> layout nesting graph. One thing you could try is to make sure, after
> removing something from a layout, you reset its parent to your Dialog (or to
> NULL), that way it's lifetime won't be tied to anything you care about.
>
> QObject::dumpObjectTree() might be helpful for debugging this.
>
> You could also consider keeping your pointer lists up to date by connecting
> to QObject::destroyed() on each object. Keep in mind that objects may be
> deleted using deleteLater() so this signal isn't always synchronous.
>
> HTH
>
> Ross.
>
> ----- Original Message ----- From: Ellen Kestrel
> To: qt-interest at trolltech.com
> Sent: Thursday, May 20, 2010 10:49 AM
> Subject: [Qt-interest] Is there any way to dynamically rearrange a
> dialog'slayout?
>
>
>
> I have a dialog whose main layout is a QVBoxLayout that contains mostly a
> series of identically structured QHBoxLayouts.  The QHBoxLayouts, and the
> widgets in them, are all stored in lists of QPointers as members of the
> dialog.  I want to make it so that more of these layouts can be added to the
> dialog dynamically, so I made slots that added or removed widgets and
> layouts to and from these lists of pointers (without actually deleting
> anything) and then called an updating function that first removes everything
> from the main layout and deletes any layout items that are not in one of the
> lists of pointers, and then adds everything that is in a list back into the
> main layout.  All of these functions check and recheck for the lists having
> the wrong number of items, having null pointers, etc., and print error
> messages if anything is wrong.  Yet, in addition to the main layout getting
> screwed up and laying out sublayouts on top of other sublayouts, it crashes
> at seemingly random times without printing any of my error messages.  What
> it does print to the terminal begins thusly:
>
> *** glibc detected *** ./ExecutableName: free(): invalid next size (fast):
> 0x09778378 ***
> ======= Backtrace: =========
> ...
>
> It also constantly prints this message, whether it crashes or not:
>
> QLayout::addChildLayout: layout "" already has a parent
>
> It looks like QLayout::removeItem is not properly removing layouts from
> other layouts.  I did not create any of these layouts with parent widgets,
> since setting the layouts takes care of that.
>
> Again, at every single point where there could be a null pointer to cause a
> crash, *there is error checking*, none of my error messages are ever
> printed, and neither is there a standard segfault message.  Is there any way
> to do this, or should I just give up?
>
> EK
>
>
>
>
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