Exception on setStyleSheet(). Can't figure why.

nikitablack

Hello.

I have pretty big project and unfortunately I can't provide small reproducible piece of code. But I glad to hear ideas. I have a 2 main windows. When I open a new one I delete previously opened. And at some point I'm calling:

qApp->setStyleSheet(styleSheetStr);

And this line fails with exception read access violation at: 0x0.
The call stack after calling of the method looks like this:

QJsonObject::value	Qt5Cored		0x66f535ca	
QUndoGroup::cleanChanged	Qt5Widgetsd		0x6427bb1c	
QUndoGroup::cleanChanged	Qt5Widgetsd		0x643cfcfd	
QUndoGroup::cleanChanged	Qt5Widgetsd		0x643c49ac	
QUndoGroup::cleanChanged	Qt5Widgetsd		0x6429fc8b	
SkinManager::setSkin	SkinManager.cpp	30	0x13f410965

So the actual fail happens in QJsonObject::value. What can cause this behavior or how can I debug a problem?

mrjj

'''
With no code, it is just random guesses/questions, but sometimes that does work too :)

where does styleSheetStr lives?
Can it run out of scope.

if you remove the line, it will not fail ?

the qApp remains valid and non zero at all times?

if you do not delete the previously opened main window (for test) does it still fail ?

ambershark

Since QApplication knows about all the top level widgets, my guess here is that you are deleting one of the QMainWindow objects and your application doesn't realize it happened by the time you try to set a style sheet.

If you are doing something like delete mainWindow1 then qApp->setStyleSheet() that may be your issue. Try changing to a mainWindow1->deleteLater() and see if the crash goes away.

Also definitely check into @mrjj's suggestions, especially making sure qApp is a legit pointer.

You can also try calling the change to stylesheets later, after the event loop has had a chance to clean up the main window that was destroyed. You can do this with a QTimer::singleShot. This of course assumes the deleteLater() doesn't fix the issue.

nikitablack

Thank you guys. The problem exactly related to deleting a window. Without deleting it everything works ok. Unfortunately nothing from the above helped and with minimal example the issue does not reproduces. But now I know where to dig. Thanks.

nikitablack

An update. After investigating I found the problem. But I have a lack of knowledge in the field.
For my project I need borderless windows. I'm using a QWinWidgets from qt-solutions . It uses some undocumented functions to obtain window handle (on Windows). And my problem arises when I'm destroying this window with DestroyWindow(). I don't know what's happens inside, but somehow QApplication, HWND and stylesheets relates to each other.

mrjj

@nikitablack

This is pure speculation:

In the QwinWidgets code for its event handler it has

  case QEvent::Close:
    	::SetActiveWindow(hParent);
	if (w->testAttribute(Qt::WA_DeleteOnClose))
	    deleteLater();
	break;

so if you set that flag on your window, and do not delete it yourself. will it still crash ?

window->setAttribute(Qt::WA_DeleteOnClose);

That way the QwinWidget controls the deletion since it seems to manage it.

Also, if you are only using QwinWidget for a borderless window, you could maybe just use a normal window and

window->setWindowFlags ( Qt::Popup |  Qt::Dialog | Qt::FramelessWindowHint );

(might need to adjust flags)

nikitablack

@mrjj, thank you for reply. I can't use just flags with a normal window for several reasons. One of them - I have to keep standard window functionality like minimize/maximize anumations, aero shake, aero snap. The QEvent::Close also not works - the program simply never reaches this line.

mrjj

@nikitablack

Ok, so QWinWidgets does provide more than borderless. Just checking.

Looking In qapplication.cpp file, at
setStyleSheet, i see nothing that looks like anything from the call stack dump. ?

It does call QApplication::setStyle(QStyle *style) which will
iterate over all Widgets.

So if you deleted QWinWidgets still to be found, that would explain it.
But why should it be there still.

you could check it fast with

 foreach (QWidget *widget, QApplication::allWidgets())
      qDebug() <<  widget->objectName();

nikitablack

@mrjj
Yes, with QWinWidget I'm able to grab native handle and do whatever I want.
I did the check and didn't found anything suspicious.
One thing that helped me - is a call to qApp->style()->unpolish(qApp);. After this line I'm able to call qApp->setStyleSheet() without an exception.

mrjj

@nikitablack
ok, so its not due to it sticking around in widget list.

so

 qApp->style()->unpolish(qApp)

is kinda a "fix" even we never know the real reason.