[Development] QHash iteration vs std::unordered_map

[Mark Gaiser]

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
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