[Development] Changed enum property behaviour in Qt v6.8

[Fabian Kosmale]

Hi,

QMetaProperty::isEnumType's documentation probably should be adjusted to 
mention that it really is about having an enum marked with Q_ENUM, not 
about generic C++ enums.

For your use-case: I suspect you need to create a dynamic meta-type for 
your enum (or rather, the QtPrivate::QMetaTypeInterface), and pass that 
to `QMetaEnumBuilder::setMetaType`. You can probably get away by copying 
most information from the QMetaTypeInterface of the underlying type, and 
just adjusting name and setting the flags.

I suspect we don't have this kind of code in Qt, becuse QML currently 
doesn't allow QML defined enum properites, which would be the place 
where it's needed.

Kind regards,
Fabian Kosmale


On 04.10.24 14:43, Phil Thompson wrote:
> Hi Fabian,
> 
> The change was https://codereview.qt-project.org/c/qt/qtbase/+/559265
> 
> Reverting this change means I get the same behaviour as I get with v6.7.
> 
> I am (and always have been) using QMetaObjectBuilder and addEnumerator() 
> (the overload that takes a QByteArray argument).
> 
> I've compared the following scenarios...
> 
> a moc-generated meta-object where Q_ENUM is not used
> a moc-generated meta-object where Q_ENUM is used
> a dynamically created meta-object.
> 
> ...for v6.7.1, v6.8.0 and v6.8.0 with the change reverted. In each case 
> I looked at the results of...
> 
> QMetaEnum.metaType().isValid()
> QMetaProperty.metaType().isValid()
> QMetaProperty.isEnumType()
> 
> The results in all cases for v6.7.1 and v6.8.0 with the change reverted 
> were identical.
> 
> For moc-generated meta-objects...
> 
> - the results were consistent across all versions of Qt
> - QMetaProperty.metaType().isValid() always returned true (even if 
> QMetaEnum.metaType().isValid() returned false)
> - QMetaProperty.isEnumType() returned true if Q_ENUM was used and false 
> if it wasn't
> 
> For dynamically created meta-objects...
> 
> - the results were dependent on the version of Qt
> - QMetaProperty.metaType().isValid() always returned false
> - QMetaProperty.isEnumType() returned true for v6.7.1 and false for v6.8.0
> 
> I was surprised by the result of QMetaProperty.isEnumType() for a 
> moc-generated meta-object without Q_ENUM. The docs suggest this should 
> be true for all enums whether or not Q_ENUM is used.
> 
> This actually means that the v6.8 dynamic behaviour is closer to the 
> moc-generated behaviour but means it seems to be impossible to 
> dynamically create an enum property where QMetaProperty.isEnumType() 
> returns true.
> 
> Some questions...
> 
> Why does QMetaProperty.metaType().isValid() always return true for 
> moc-generated meta-objects and false for dynamically created meta-objects?
> 
> Is there any way to dynamically register an enum with the meta-type 
> system (so that QMetaEnum.metaType().isValid() returns true)?
> 
> Regards,
> Phil
> 
> On 04/10/2024 08:02, Fabian Kosmale via Development wrote:
>> Hi Phil,
>>
>> I'm not quite sure which change caused the issue, and I believe a
>> bugreport with a reproducer might shed some light on the issue.
>>
>> Nevertheless, if you're already using the private QMetaObjectBuilder
>> API, I would expect that it should work if you build the enum also
>> dynamically via addEnumerator and then working with the returned
>> QMetaEnumBuilder.
>>
>> Kind regards,
>> Fabian
>>
>> On 03.10.24 17:52, Phil Thompson via Development wrote:
>>> On 03/10/2024 16:31, Volker Hilsheimer wrote:
>>>>> On 3 Oct 2024, at 17:07, Phil Thompson via Development 
>>>>> <development at qt-project.org> wrote:
>>>>>
>>>>> On 03/10/2024 15:44, Thiago Macieira wrote:
>>>>>> On Thursday 3 October 2024 05:06:08 GMT-7 Phil Thompson via 
>>>>>> Development wrote:
>>>>>>> Yes, that's the change - commenting out the new test fixes the 
>>>>>>> problem.
>>>>>>> I am dynamically building the meta-object. You don't really build an
>>>>>>> enum, you just provide a description of it to the meta-object. It 
>>>>>>> would
>>>>>>> be nice to be able to register an enum with the meta-type system
>>>>>>> dynamically.
>>>>>> Or you can set the EnumOrFlag flag in the QMetaProperty flags 
>>>>>> field to
>>>>>> force the
>>>>>> constructor to search.
>>>>>
>>>>> Sorry, can you be more specific? I can't see how to do it given the 
>>>>> private nature of the flags.
>>>>>
>>>>> Phil
>>>>
>>>>
>>>> Hey Phil,
>>>>
>>>> How do you generate a dynamic meta object without using private APIs?
>>>> Do you use QMetaObjectBuilder (which is private), or do you generate
>>>> the tables and arrays by hand?
>>>>
>>>> If you use QMetaObjectBuilder, then
>>>> QMetaPropertyBuilder::setEnumOrFlag would be the way to go.
>>>>
>>>> Volker
>>>
>>> I'm using QMetaObjectBuilder and already calling setEnumOrFlag, so 
>>> that doesn't seem to solve the problem.
>>>
>>> An alternative that would work for me would be to bypass the new test 
>>> if the meta-type was invalid (ie. the enum wasn't registered) but I 
>>> don't know if that would nullify the purpose of the new test.
>>>
>>> Phil
>>
>> -- 
>> Fabian Kosmale
>> Manager R&D
>>
>> The Qt Company GmbH
>> Erich-Thilo-Str. 10
>> D-12489 Berlin
>> fabian.kosmale at qt.io
>> +49 1638686070

-- 
Fabian Kosmale
Manager R&D

The Qt Company GmbH
Erich-Thilo-Str. 10
D-12489 Berlin
fabian.kosmale at qt.io
+49 1638686070