[Development] QVariant comparison in Qt6

[Albert Astals Cid]

El divendres, 18 de setembre de 2020, a les 13:25:45 CEST, Lars Knoll va 
escriure:
> > On 18 Sep 2020, at 11:23, Albert Astals Cid via Development
> > <development at qt-project.org> wrote:
 
> > El divendres, 18 de setembre de 2020, a les 11:00:23 CEST, André Somers va
> > 
 escriure:
> > 
> >> On 18-09-2020 09:12, Albert Astals Cid via Development wrote:
> >> 
> >>> El divendres, 18 de setembre de 2020, a les 2:54:53 CEST, Thiago
> >>> Macieira
> >>> va> 
> >>> escriure:
> >>> 
> >>>> On Thursday, 17 September 2020 16:15:47 PDT Bernhard Lindner wrote:
> >>>> 
> >>>>> Hi!
> >>>>> 
> >>>>> There was a discussion about the decision to deprecate (remove?)
> >>>>> QVariant
> >>>>> comparison (<,>) in Qt6 completely.
> >>>>> 
> >>>>> Has anything changed for Qt6 (especially regarding comparison of
> >>>>> equal
> >>>>> and/or convertible types)? Is the complete deprecation still the
> >>>>> latest
> >>>>> decision?
> >>>> 
> >>>> 
> >>>> Yes, it's changed; no, it's not deprecation.
> >>>> 
> >>>> What's been removed is the conversion. Aside from the numeric types,
> >>>> comparing two variants of different types will always result in false.
> >>>> If
> >>>> you want to compare across types, convert one to the other's type or to
> >>>> a
> >>>> common third type. You know what data you put in there and you should
> >>>> know
> >>>> what conversions can be lossy or not.
> >>> 
> >>> 
> >>> But i don't what data someone else put it in a QVariant.
> >>> 
> >>> The fact that QAbstractItemModelPrivate::isVariantLessThan exists
> >>> should
> >>> be
> >>> proof enough that you can't expect my class to know what someone else
> >>> put
> >>> inside a QVariant, i mean that's basically the defining feature of
> >>> QVariant "it can hold random things for you".
> >>> 
> >>> 
> >>>  Is the official Qt position that we should all "copy&paste"
> >>> 
> >>> 
> >>> QAbstractItemModelPrivate::isVariantLessThan into our item model
> >>> classes
> >>> that need sorting?
> >> 
> >> 
> >> How often do you really have items of different QVariant types in a
> >> single column and and role in a model?
> > 
> > 
> > Probably not often, but that means adding new API to classes forcing users
> > of 
 those classes to specify either a single type for each role/column
> > (so the comparison can be done internally in the class) or a comparer
> > function for each role/column and offloading the comparison back to the
> > user, makes things a bit annoying.
> > 
> > 
> >> 
> >> From what I understand, in Qt 6 you can still compare QVariants if they
> >> contain the same type - be that a user type or not - for both equality
> >> and lessThan provided the type in QVariant supports such comparisons.
> >> Right?
> 
> > 
> > There's no lessThan in Qt6 for QVariant, so no, you can't do that [unless
> > you 
 unbox the data from the QVariant, but then you're not comparing
> > QVariants anymore :)].
> 
> 
> I’m not really in favour of adding an operator<() to QVariant again, as it
> would need to provide some decent ordering, and that would probably cause
> conflicts with operator=() that does somewhat loose comparisons for numeric
> types.
 
> But we could provide a compare() method similar to QMetaType. I don’t think
> we can provide total ordering here as some types stored in a QVariant do
> not provide comparison operators, but we could basically provide the same
> as QMetaType:
 
> std::optional<int> QVariant::compare(const QVariant &other);
> 
> Would that be good enough?

I think that would solve some (probably even most) of the pain, yes :)

Cheers,
  Albert

> 
> Cheers,
> Lars
> 
> 
> 
> > Cheers,
> > 
> >  Albert
> > 
> > 
> > 
> >> 
> >> André
> >> 
> >> 
> >> _______________________________________________
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> >> Development at qt-project.org
> >> https://lists.qt-project.org/listinfo/development
> > 
> > 
> > 
> > -- 
> > Albert Astals Cid | albert.astals.cid at kdab.com | Senior Software Engineer
> > Klarälvdalens Datakonsult AB, a KDAB Group company
> > Tel: Sweden (HQ) +46-563-540090, USA +1-866-777-KDAB(5322)
> > KDAB - The Qt, C++ and OpenGL Experts
> > 
> > _______________________________________________
> > Development mailing list
> > Development at qt-project.org
> > https://lists.qt-project.org/listinfo/development
> 
> 


-- 
Albert Astals Cid | albert.astals.cid at kdab.com | Senior Software Engineer
Klarälvdalens Datakonsult AB, a KDAB Group company
Tel: Sweden (HQ) +46-563-540090, USA +1-866-777-KDAB(5322)
KDAB - The Qt, C++ and OpenGL Experts