[Development] QHash iteration vs std::unordered_map

[Corentin]

Qt wasn't initially designed with STL compatibility in mind ( in the early
2000, STL support was poor on most platforms and Qt actually predates c++98
- though QMap do not, I think). - I would argue that STL compatibility is
still an issue more often than not, but it's another discussion :)
As for a change in Qt6... It would result in *a lot* of hard-to-track
source breakages, so I hope not ( one solution would be to offer a powerful
refactoring tool).



Le dim. 16 avr. 2017 à 17:06, Mark Gaiser <markg85 at gmail.com> a écrit :

> On Sun, Apr 16, 2017 at 4:44 PM, Corentin <corentin.jabot at gmail.com>
> wrote:
> > Funny, a friend at kdab asked me about that exact question a few minutes
> > ago.
> > The reason for the difference is most certainly an historical one ( and
> > can't be changed because it would break quite a bit of code ).
> >
> > If you want the same behavior, you can create a proxy for your
> associative
> > container instance, with a custom iterator whose operator*() returns a
> std
> > pair ( or a QPair ) - quite a bit of boilterplate code.
>
> It probably does come down to historic reasons, but that would be
> strange as well.
> I doubt that the C++ committee changed what is returned by operator*()
> (correct me if i'm wrong) so i'm inclined to think that the
> std::<container> stuff is returning it as an std::pair for quite a
> while now.
> That again makes me wonder, why did Qt diverge from that?
>
> And... if Qt plans to change it in Qt6?
>
> >
> >
> >
> >
> > Le dim. 16 avr. 2017 à 15:57, Mark Gaiser <markg85 at gmail.com> a écrit :
> >>
> >> Hi,
> >>
> >> Take this simple example:
> >>
> >>   QHash<int, QString> test = {
> >>     {10, "aaa"},
> >>     {20, "bbb"},
> >>     {30, "ccc"}
> >>   };
> >>
> >>   for (const auto &entry: qAsConst(test)) {
> >>     qDebug() << entry;
> >>   }
> >>
> >> It returns:
> >> "aaa"
> >> "ccc"
> >> "bbb"
> >>
> >> and the std::unordered_map version:
> >>   std::unordered_map<int, QString> test = {
> >>     {10, "aaa"},
> >>     {20, "bbb"},
> >>     {30, "ccc"}
> >>   };
> >>
> >>   for (const auto &entry: test) {
> >>     qDebug() << entry;
> >>   }
> >>
> >> it returns:
> >> std::pair(30,"ccc")
> >> std::pair(10,"aaa")
> >> std::pair(20,"bbb")
> >>
> >> As you can see, the QHash iteration directly returns the value. The
> >> std::unordered_map returns a std::par with the key and value for the
> >> current iteration (in it's respective first and second members). Both
> >> approaches probably have arguments in favor and against. What i'm
> >> curious about is why there is a difference at all?
> >>
> >> I'm curious because the behavior is a bit unexpected when compared to
> >> std::unordered_map. I would have guessed QHash to follow the same
> >> logic as std::unordered_map, only with a Qt syntax. So for instance a
> >> return of QPair(...), not the value directly.
> >>
> >> Thanks,
> >> Mark
> >> _______________________________________________
> >> Development mailing list
> >> Development at qt-project.org
> >> http://lists.qt-project.org/mailman/listinfo/development
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