[Qt-interest] Maintaining strong references (shared pointer) to Qt objects

[Anatoli Steinmark]

Hi Andreas,

I understand your point as to why it makes little
sense to use reference counting for widgets.
Shows my inexperience in how GUI toolkits work.

The strategy I'm going to use is keeping a weak reference
to the widgets through QWeakPointer and doing the following
in the destructor of the script objects:

//...
if weak reference is not null
    if qt object does not have a parent
        delete qt object
    endif
endif
// ...

If a script tries to use a weak reference to a deleted object
I will do what you suggested: throw an exception.

This looks ok to me.


Regards,
Anatoli


-------- Original-Nachricht --------
> Datum: Sat, 24 Jul 2010 10:55:29 +0200
> Von: Andreas Pakulat <apaku at gmx.de>
> An: qt-interest at trolltech.com
> Betreff: Re: [Qt-interest] Maintaining strong references (shared pointer) to Qt	objects

> On 24.07.10 08:51:56, Anatoli Steinmark wrote:
> > -------- Original-Nachricht --------
> > Datum: Sat, 24 Jul 2010 08:45:22 +0200
> > Von: "Anatoli Steinmark" <derletztemohikaner at gmx.de>
> > An: "Malyushytsky, Alex" <alex at wai.com>
> > Betreff: Re: [Qt-interest] Maintaining strong references (shared
> pointer) to Qt	objects
> > 
> > Hi Alex,
> > 
> > > I don't understand a reason widget should not be deleted.
> > > What is going to happen to it? Persist until application exits?
> > 
> > I'm writing a scripting language binding for Qt. This requires Qt
> objects to be exposed as objects of the scripting language. Thus the need to
> maintain a strong reference, since if a parent destroys a child, the reference
> in the script object becomes a dangling pointer.
> > 
> > Until the script object is collected (destroyed) by the script engine,
> the Qt object it exposes should not be deleted.
> 
> Thats not how (existing) script bindings work. You simply cannot do
> this, what you can do though is getting notified when the wrapped Qt
> object is being destroyed and then make sure that you throw a proper
> exception when a user tries to access the script object. 
> 
> > > It does not mean you can't prevent widget to be deleted by parent.
> > > For example you can subclass parent and keep the least of the widget
> you
> > > don't want to be deleted.
> > > Re-parenting them to NULL in the parent destructor should do it.
> > 
> > So I would need to subclass every Qt class I want to expose to script?
> > 
> > I see this is a solution, but if there is a way to avoid this
> mass-subclassing, I would prefer it.
> 
> Most script-language bindings are generated these days, so the number of
> classes to generate is not that much of a problem. Of course there's
> going to be cases where auto-generation fails or you'll have to tweak
> it.
> 
> > I always read that Qt does reference counting everywhere, so if that's
> > true there has to be a way to acquire and release references.
> 
> It does reference counting on data-classes like QString, QList, QMap
> etc. But it never did and cannot do this for the widgets.
> 
> There is a SharedPointer class in Qt4.6 and later which might or might
> not help (I'm not 100% sure I understand its semantics). Thiago once
> blogged on the Qt blog about the various smart-pointer classes in Qt4.6
> and later, maybe have a look at that.
> 
> > > If it still keeps the references to external objects in the widget,
> > > release them when widget is deleted.
> > > If you reference this widget in external code, notify such code that
> it is
> > > going to be deleted.
> > 
> > Hm, that would be very counter-intuitive and surprising to programmers
> > if the script object suddenly looses the Qt object it represents just
> > because a parent is destroyed.
> > 
> > Or to put it another way: If there is a valid script object, it should
> > have a valid reference to the Qt object it represents.
> 
> As I said before, thats not how existing script bindings for Qt work
> (afaik).
> 
> Andreas
> 
> -- 
> Beware of low-flying butterflies.
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