[Development] Converting types in Qt

[Olivier Goffart]

On Tuesday 15 July 2014 10:38:52 Poenitz Andre wrote:
> Olivier Goffart wrote:
> > Jędrzej Nowacki wrote:
> > >   1. Are we allowed to add new conversions?
> > >   
> > >      The question is tricky because adding a new conversion is a
> > >      behavior
> > >      change, as this code:
> > >      if (variant.canConvert(QMetaType::int)) ...
> > >      may work differently. If we add custom types into the mix,
> > >      everything
> > > 
> > > is even more complex.
> > 
> > I'd say yes, for sensible conversion
> > You are right that it is a behaviour change, but i think it is worth
> > changing it.
> Why?
> 
> On one hand you promise binary compatibility. On the other hand
> behaviour changes are proposed to be done on an "nice to have"
> base?
> 
> Do we really think that's ok to disallow changing some int foo() { return
> 42; } to some int bar() { return 42; } that's easy to discover at build
> time, but it's fine to change int foo() { return 42; } to int foo() {
> return -1; } ?

It's always a dilemma. We have to look at how likely we are to break 
applications and I don't think adding a conversion is likely to cause 
breakages.

> > so Qt can know it and use it. For certain types we can do much better,
> > 
> > because we can automatically convert some types. For example:
> >          QVector<char> -> QLinkedList<int>
> 
> QVector<char> v; v.append(130);
> QLinkedList<int> l = /*some"Sensible"AutomatedConversion*/(v);
> 
> assert(l.first() == 130) ?  "Depends" ?

QVariant(char(130)).toInt() already exists. You may argue that it is not 
sensible but that's another issue.
I guess the example is more between QVector<T1> -> QLinkedList<T2> when there 
exist a conversion from T1 to T2

> > >          A conversion may be arbitrary. For example, most of the
> > >          conversions
> > > 
> > > to QString. Should "bool(false) -> QString" return "False", "0",
> > > "false"?
> > > What about precision of, for example, "double -> QString" ?
> > 
> > We use common sense on a case by case basic.
> 
> I tend to believe the common sense in type conversion land is pretty
> close to "avoid to do it automatically, prefer to make it explicit".
> 
> Already now it's far too easy to make mistakes based on the "nice
> and easy" QByteArray -> QVariant -> QString conversions when
> accidentally writing QByteArrays into QSettings and reading QStrings
> back. There shouldn't be more of that.

What's the mistake here? Wrong encoding? Bad performances?

When one use QVariant, it is because we want to enjoy dynamic typing and nice 
conversions.

-- 
Olivier 

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