[Development] Metatype system in Qt6

[Stottlemyer, Brett (B.S.)]

Apologies for reviving an old thread, but this just came up in a code review (https://codereview.qt-project.org/c/qt/qtremoteobjects/+/287828 if anyone is curious).

On 12/5/19, 11:56 AM, "Development on behalf of Olivier Goffart" <development-bounces at qt-project.org on behalf of olivier at woboq.com> wrote:
    
    That's a source incompatible change, and the question is if we really want it.
    
...
    
    Do we want to automatically register the operators?
    Do we want to automatically register the types used in signal/slots, or just 
    the properties?
    
I've got a bit of a different take on this question.  For QtRO (and likely also for QML), I don't believe the problem of registration can be solved at compile time and we will always need a good/easy way to register at runtime.  I'm all for better compile time capability, but it will be problematic if that limits what can be registered at runtime.

The problem I've run in to is this: how are container types of runtime types are handled?  As Olivier mentioned, marshalling a custom type via QDataStream first puts the type's name in the stream, then the serialization of the instance.  That means common use-cases such as 'QVector<MyType>' and 'QMap<int, MyType>' are likely to be seen in the output.  What I'd like to see is container types for all basic types (i.e., statically registered by Qt) registered automatically (i.e., QVector<int>).  In addition, we need a good way to register relevant container types when types are added at runtime.  We are dynamically registering gadget and enum types without compile time help, but that is insufficient to allow containers of those types to be used.

The problem here is different, as the type will be fully defined and there won't be issues of forward declarations.  Does the template magic allow the containers to be registered as well?  Or is there a way to serialize container types as the type itself, as well as a potentially empty list of flags, where flags could include that the type is used in a container?  Otherwise handling `QHash<int, MyType>`, `QHash<QString, MyType>`, etc, become problematic...

Regards,
Brett