[Development] Removing the global static QObject from QPixmapCache

Giuseppe D'Angelo giuseppe.dangelo at kdab.com
Sun May 30 22:13:51 CEST 2021

On 30/05/2021 17:47, Konstantin Shegunov wrote:
> I was simply teasing. I was referring to this thread about 
> QTBUG-71545[1], where I wanted the application to destroy its children 
> explicitly so they see it as QCA, but I wasn't convincing enough. 
> Whereas QWidget(s) are doing it in their destructors for their children 
> for exactly the same reason, which was the "exception(s)" I was 
> referring to.

Well, it's roughly in the same area...

But while the "rule" that a parent gets destroyed before the children do 
is well-established, and changing it is extremely invasive and 
error-prone (you'd force every single class to override their destructor 
and clean their children up), we don't quite have hard-established rules 

1) if it's OK to create QObjects before creating a QCoreApplication┬╣
2) symmetrically, if it's OK for QObjects to outlive QCoreApplication
3) if it's OK for QObjects living in a QThread to outlive that thread's 
4) if it's OK for QObjects living in a QThread to outlive the QThread 
object itself
5) if it's OK for a QObject living in a thread A to be destroyed in thread B
6) if it's OK to create a QObject/QCoreApplication in a non-main thread
7) if it's OK to destroy and recreate a QCoreApplication in the same process
7b) like 7, but recreate the QCoreApplication in a different thread

The answer to most of these is "it depends" / "yes, but..." / "no, 
unless...", and I don't like that kind of answer at all. They also imply 
the whether it's a good idea to use QObjects into Q_GLOBAL_STATIC.

┬╣ (... generalizing: if it's OK to touch ANY Qt API whatsoever outside a 
QCoreApplication lifetime; but let's stay on topic, and stick to QObjects.)

My finger-in-the-wind opinion would be:

1) no QObjects must be created before creating QCoreApplication (and the 
generalization: almost NO Qt APIs are OK to use before QCA)
2) no
3) QObjects living in a thread must be destroyed before the thread 
terminates (so, for QThread, before run() returns)
4) no
5) no (4+5= to me, QObjects aren't re-entrant)
6) yes, but for GUI apps, it may depend on the platform
7) ditto
7b) ditto

=> so I'd be against allowing QObjects into Q_GLOBAL_STATIC (violates 2 
4 and 5, and possibly 1).

Giuseppe D'Angelo | giuseppe.dangelo at kdab.com | Senior Software Engineer
KDAB (France) S.A.S., a KDAB Group company
Tel. France +33 (0)4 90 84 08 53, http://www.kdab.com
KDAB - The Qt, C++ and OpenGL Experts

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