Q_DECLARE_SMART_POINTER_METATYPE type restriction

[Joel Bodenmann]

I was investigating the idea of storing an std::shared_ptr<T> in a QVariant. This would allow me to put a shared pointer to my custom data int a QComboBox's data field.
I'm aware of the existence of QSharedPointer. The data I am working with is only available wrapped into an std::shared_ptr (data provided by a 3rd party component). This leaves me with the choice of either storing raw pointers or registering my own type for the use with QVariant.

For this, I've been looking at Q_DECLARE_SMART_POINTER_METATYPE. The documentation has the following to say about this macro:

This macro makes the smart pointer SmartPointer known to QMetaType as a smart pointer. This makes it possible to put an instance of SmartPointer<T> into a QVariant, if T is a type which inherits QObject.

I'd be interested to know why T would be restricted to a type which inherits QObject.
Can anybody enlighten me?

[SGaist]

Hi,

Pretty good question. I think it something you should bring to the interest mailing list where the Qt developers/maintainers can be found.

[Christian Ehrlicher]

From a short look at the code without understanding the whole stuff I would guess that internally there is only a QObject pointer is stored. So it must be a QObject to track the lifetime.