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

Edward Welbourne edward.welbourne at qt.io
Fri Nov 4 11:43:34 CET 2022

On Thursday, 3 November 2022 09:48:49 PDT Marc Mutz via Development wrote:
>>> Here, too, I feel lost. I'm struggling to see what a NIH
>>> std::partial_ordering w/o the weak and strong counterparts and w/o
>>> op<=> language support could achieve, except another vocabulary type
>>> mismatch.  Can you elaborate?

On 03.11.22 18:38, Thiago Macieira wrote:
>> We can use it until we can depend on C++20. Like QSpan, that's why it's there.

Marc Mutz (4 November 2022 11:22)
> The difference is that QSpan vs. std::span doesn't create an impedance
> mismatch. QPartialOrdering vs. std::partial_ordering does, esp. since
> QPartialOrdering lacks an implicit conversion to/from
> std::partial_ordering, and the member names are different.

Since my comments on QDateTime are implicated here, I should note that -
since it would be a behaviour change - an initial transition to a weak
ordering that preserves invalid < valid could perfectly well suffice
until we *do* transition to C++20 and *can* use std::partial_order, at
which point I do think it would make sense to change our types with
invalid (but supported) null-forms to make them incomparable, ideally
all at the same time.

> With the exception of qfloat32, I'm not aware of any Qt type that
> would exhibit partial ordering semantics atm.

I take it you mean qfloat16, whose NaN is incomparable,


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