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

[Thiago Macieira]

On Monday, 13 November 2023 09:15:10 PST Marc Mutz via Development wrote:
> On 13.11.23 17:15, Thiago Macieira wrote:
> > On Monday, 13 November 2023 01:34:08 PST Ivan Solovev via Development 
wrote:
> > I don't think this minor thing is worth the hassle. It uglifies our API
> > for
> > little gain.
> 
> What, specifically, is ugly? That we have two Qt partial_ordering types?

Yes.

> Anyway, that's subjective. Objectively, it breaks the impasse between
> full efficiency in future-looking C++20 builds and Qt BC guarantees.

We're not losing anything either because the only case where this would be 
necessary is if we needed to return a std::partial_order result from a non-
inline function that is, in turn, calling the non-inline QMetaType and/or 
QVariant functions. We have zero of those today and aren't likely to get any 
of them, probably ever.

Some user code might, but I find it unlikely too. It could happen if the user 
decided to wrap a QVariant in a larger type that did have a three-way out-of-
line comparison function.

Everywhere else, the optimiser will do the right thing.

> Given that Qt 7.0 is many years away, I don't think it's too much to ask
> to go the extra mile and remove the overhead. We seem to agree how to do
> it technically, and it won't be on you to implement all this, but on
> Ivan and me. I think I speak for Ivan, too, if I say that we'd rather
> today than tomorrow see this stuff merged. The next FF is already
> approaching again.

It could be done, but I just don't see the value.

If we do it, please come up with proper Qt-style class names for it. No 
snake_case.

-- 
Thiago Macieira - thiago.macieira (AT) intel.com
  Cloud Software Architect - Intel DCAI Cloud Engineering
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