[Development] QVariant comparison in Qt6

[Albert Astals Cid]

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 :)].

Cheers,
  Albert

> 
> André
> 
> 
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-- 
Albert Astals Cid | albert.astals.cid at kdab.com | Senior Software Engineer
Klarälvdalens Datakonsult AB, a KDAB Group company
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