[Development] QVariant comparison in Qt6

Thiago Macieira thiago.macieira at intel.com
Sun Sep 20 08:03:35 CEST 2020

On Saturday, 19 September 2020 11:41:08 PDT Ville Voutilainen wrote:
> On Fri, 18 Sep 2020 at 21:58, Thiago Macieira <thiago.macieira at intel.com> 
> > Anyway, the int suffices because we only need four values:
> > equal/equivalent,> 
> > less than, greater than, unordered. We can even adopt the same values:
> >     // less=0xff, equiv=0x00, greater=0x01, unordered=0x02
> > 
> > or we can use -127 for unordered like libc++ does or -128 like Microsoft
> > STL.
> Well, if we envision making it convertible to something later on, then
> *PLEASE* use a class type.
> I have recent experience on a particular codebase that wanted to
> migrate from custom allocators
> to standard allocators, and one significant part of it was converting
> standard allocators to the custom
> ones. But that didn't work because the API vocabulary type was
> CustomAllocator*.
> You can't write a conversion from a type you don't own to an int, or
> vice versa. Don't fall into that trap, it's impossible
> to dig yourself out of it.

Sorry, I don't understand the issue.

I'm thinking of an internal function that is used to implement operator<=> 
(and if pre-C++20 support is desired, operator== and operator<).

That's how I implemented QCborValue, though compare() is public and it does 
not need unordered comparisons. If it works for memcmp and QString::compare, I 
don't see why it couldn't work here too.

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

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