[Development] Should QVariant be doing fuzzy comparisons on doubles?

Mathias Hasselmann mathias at taschenorakel.de
Wed Sep 21 09:23:35 CEST 2016

Am 21.09.2016 um 08:57 schrieb Olivier Goffart:
> On Mittwoch, 21. September 2016 08:01:22 CEST Mathias Hasselmann wrote:
>> As much as I'd like to debug this code now to prove me right, I sadly
>> have deadlines to meet this week. So I have to behave myself to not dig
>> up the code right now. Maybe later. Or someone else.
> Turns out you are right. If there is no registered comparison function, we do
> a memcmp in 'customCompare' in qvariant.cpp:
> https://code.woboq.org/qt5/qtbase/src/corelib/kernel/qvariant.cpp.html#1079
> That is indeed an undefined behavior!

Cool. Or rather not. Anyway: Thank you for looking this up!

>>>> It is easy to forget registering comparator functions and currently Qt
>>>> doesn't help in debugging such issues. So I wonder if QVariant should
>>>> forbid comparison of custom types without having a comparator function
>>>> registered.
>>> That's a source incompatible change.
>> No, it's not. It's changing undefined behavior into defined behavior.
> But in many case, we want to put something in a QVariant, and we never compare
> this variant.
> Forbidding types that do not have an operator== to be in a QVariant might be
> to strict.

I fear balance has changed dramatically since we switched from 32 bit to 
64 bit. In the old 32 bit days our structs tended to be densely packed 
with our enums and ints. Many structs were well defined as they 
contained no padding wholes. Memcmp was working. World was good.

These days with compilers often defaulting to sizeof(int) == 4 while 
using a memory alignment of 8 we usually end up with horrible 
inefficient, very sparse structs where often half of the bytes are 
undefined. Actually the compiler gurus' decision to use stay with 32 
bits for ints feels slightly stupid now that I think about it: No 
improvement in memory efficiency, but lots of undefined behavior.

Maybe some clever use of type traits can tell us if your structs contain 
alignment wholes?


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