[Development] QMetaType and non-const references

[Volker Hilsheimer]

> On 19 Jul 2022, at 21:34, Fabian Kosmale <fabian.kosmale at qt.io> wrote:
> 
> Hi,
> 
> besides treating T& and T (which I think is fine for 6.4), anything else should probably only be done in 6.5.
> I don't currently see much interdependence in the changes you proposed, however we should carefully
> check that DBus, Qt Remote Objects and QML don't get accidentally broken.
> 
> Regarding the string example:
> 
> you need something to actually register the string metatype (e.g. calling id or qRegisterMetaTypeId)
> #include <string>
> #include <QDebug>
> #include <QMetaType>
> 
> Q_DECLARE_METATYPE(std::string) // without this line, the example will still not work
> 
> int main() {
>   qDebug() << QMetaType::fromType<std::string>().name();
>   qDebug() << QMetaType::fromName("std::string");
>   QMetaType::fromType<std::string>().id();
>   qDebug() << QMetaType::fromName("std::string");
> }
> 
> Only when you register the metatype will it actually be registered under the name
> "std::string" which is necessary for the name to metatype lookup. If you end up with a
> std::string parameter in a meta-method of a meta-object, there's a good chance that
> the registration will happen via QMetaObject::RegisterMethodArgumentMetaType
> when it would be needed (assuming that std::string was complete when moc run on
> the file containing it).


Ok, so QMetaType::name() will not necessarily return the name that was used in QMetaType::fromName, and the name returned might be compiler and C++ library specific.

That is good to know and should perhaps be mentioned in the documentation. It means that QMetaType objects cannot be serialized, I suppose.

Volker


> ________________________________________
> Von: Development <development-bounces at qt-project.org> im Auftrag von Volker Hilsheimer <volker.hilsheimer at qt.io>
> Gesendet: Dienstag, 19. Juli 2022 20:45
> An: Macieira, Thiago
> Cc: development at qt-project.org
> Betreff: Re: [Development] QMetaType and non-const references
> 
> 
>> On 19 Jul 2022, at 18:08, Thiago Macieira <thiago.macieira at intel.com> wrote:
>> 
>>> Lastly, an aside: Q_DECLARE_METATYPE being effectively unnecessary was the
>>> goal for Qt 6, but is not the case yet. I consider that a bug that should
>>> finally be fixed in 6.5.
>> 
>> Right. There should be exactly one use for Q_DECLARE_METATYPE: when the name
>> you want to register is not the name that the C++ language recognises for your
>> type, and this should only be done for legacy reasons.
>> 
>>      typedef QMap<int, int> IntMap;
>>      Q_DECLARE_METATYPE(IntMap);
>> 
>> And you may have the string "IntMap" somewhere in your meta objects.
> 
> Interestingly,
> 
>    qDe0bug() << QMetaType::fromType<std::string>().name();
>    qDebug() << QMetaType::fromName("std::string");
> 
> prints, with Apple clang:
> 
> std::__1::basic_string<char>
> QMetaType()
> 
> but with msvc 22:
> 
> std::basic_string<char,std::char_traits<char>,std::allocator<char>>
> QMetaType()
> 
> and then "std::string" in your meta object won’t work either, I suppose?
> 
> But interestingly, neither does
> 
> Q_DECLARE_METATYPE(std::string).
> 
> With
> 
> qRegisterMetaType<std::string>(“std::string”);
> 
> I get
> 
> std::__1::basic_string<char>
> QMetaType(std::__1::basic_string<char>)
> 
> with clang and
> 
> td::basic_string<char,std::char_traits<char>,std::allocator<char>>
> QMetaType(std::basic_string<char,std::char_traits<char>,std::allocator<char>>)
> 
> with msvc 22. What does that mean if e.g. "std::string" is present in a meta object?
> 
> 
>> Any opinion on what must happen together or not at all?
> 
> 
> I agree with Fabian, it seems uncontroversial to treat T& and T as different types in the QMetaType system. Code that relies on them being the same needs to be fixed anyway. Doing that doesn’t seem to prevent anything else discussed.
> 
> 
> Volker
> 
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