[Development] C++20 comparisons @ Qt (was: Re: C++20 @ Qt)

[Marc Mutz]

Hi Ulf,

On 04.11.22 12:13, Ulf Hermann via Development wrote:
> One thing I haven't understood about the ordering problem is why we 
> cannot just define our "invalid" values to always be < any valid one and 
> equal to other invalid ones. This way we get at least weak ordering for 
> all our types and we're done.

That's what we're currently doing, yes. At least for QDateTime. For 
QString, we have that isNull() compares equal to empty. Each class has 
it's own semantics. I'll have a look at QTypeRevision, but that prompted 
me to remember QOperatingSystemVersion, where we really, genuinely have 
several sets of values where elements from different sets are not 
ordered (Win10 <? OSX13), and Win op OSX is always false, for all op. 
There, partial_ordering is consistent with the current implementation.

We don't _need_ to make invalid values unordered, but it would be the 
mathematically correct thing to do. We removed QVariant::op< because of 
these problems. partial_ordering would allow to bring it back.

> There may be types where existing operator< work differently (*cough* 
> QTypeRevision), but that just means we need to emulate that same 
> behavior with the new operators.

Without having looked at QTypeRevision, I agree in general.

> Indeed the NaN behavior has always been a pain to deal with every time 
> I've encountered it. If we have a chance to avoid it, we should.
> 
> What is the downside of such an approach?

The (potential) downside is that it's arbitrary and classes might behave 
inconsistently (one sorts invalid before valid ones, the other vice 
versa). It may also be a bit more work to document (unless we choose not 
to mention that little detail). Or we can't reap any future tool support 
that might be developed for partial_ordering (sanitizers, etc).

Overall, I don't feel strongly about it (no pun intended). From the 
ongoing discussion more and more examples pop up, though, where 
partial_ordering would be a really good idea (QVariant, QOSVersion, 
QTypeRevision?), and if those definite use-cases make people more 
familiar with partial_ordering, maybe invalid-as-unorderable would also 
come to be felt as more natural.

Thanks,
Marc