[Interest] sharing singleton instance between C++ and QML

[Ulf Hermann]

> For the record it is actually possible (I proved it) to create multiple 
> instances of the same singleton with only one engine using a singleton 
> published from C++ if you concoct your C++ side, um,  poorly. But that's 
> another matter for people stuck before C++11...

I've fixed QTBUG-116432 if that's what you're referring to. See 
https://codereview.qt-project.org/c/qt/qtdeclarative/+/499804

> But for the lay person, what is wrong with ensuring the single singleton 
> approach and why is it so important to you that QML engines MUST own 
> their own singletons?

There are two major reasons:

1. Object ownership: Objects returned from methods (also the create() 
method) are supposed to be owned by the engine. People don't realize and 
don't setObjectOwnership() and then their program crashes when the 
engine terminates and tries to delete the singleton.

2. Thread affinity: The singleton the engine creates lives in the 
engine's thread. If the singleton returned from create() lives in a 
different thread and you're not very careful, you get a horrible mess. 
This can easily happen if your singleton comes from "somewhere" you 
don't control.

Indeed the same problems surface if you return an object from an 
invokable method, but singletons are a much more prominent feature in 
the language, especially since they can often replace context 
properties. Therefore, I'm spending more effort on recommending the 
safest possible method to create singletons.

> The create() method does require setting C++ ownership (but as we 
> discussed, this breaks down with QML_FORIEGN registrations.

I guess this is about https://bugreports.qt.io/browse/QTBUG-111169 ? If 
so, the problem is not with any and all QML_FOREIGN registrations. It's 
just that the auto-generated registrations assume the engine owns the 
object.

best regards,
Ulf