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

[Thiago Macieira]

On terça-feira, 20 de setembro de 2016 11:29:32 PDT Olivier Goffart wrote:
> > Is such a behavior change really acceptable for 5.8? I think it can break
> > applications that were relying on the current behavior in pretty bad, hard
> > to debug ways (random bugs based on rounding errors).
> 
> I tend to agree with that.
> This is a behavior change, and i see no reason to do it. Comparing double
> for equality with operator== is a bad idea.
> I know QVariant::operator== has problems, but i don't think this is
> something that should be changed.

I did add a changelog for important behaviour change and I do think it should 
be changed. Fuzzy comparisons only operate on certain values. We know there's  
a problem with them when both numbers are close to zero: for example, fuzzy 
comparing 1.2e-12 to 0 is false, comparing it to -1.2e-12 is false, even 
though they could be the result from the same operation.

I also think that if you want a fuzzy comparison in variants, you should call 
qFuzzyCompare(QVariant, QVariant) (which doesn't exist, I can add it).

> (Correct me if i'm wrong, but this might actually have quite some bad
> performance impact on QML where lot of bindings are done on a real value and
> they are compared for equality before emiting the changed signal. (or does
> QML takes the value out of qvariant before comparing?))

Question for Simon.

> On the other hand, the fact that qFuzzyCompare(inf, -inf) returns true looks
> like a bug within qFuzzyCompare.

And I will reject any patch that adds more complexity in qFuzzyCompare for a 
0.01% corner-case. Instead, we should document that it only works for finite 
numbers far enough away from zero. If your number can be infinite or NaN or 
zero, you have to find something else.

-- 
Thiago Macieira - thiago.macieira (AT) intel.com
  Software Architect - Intel Open Source Technology Center