Questions about Q_PROPERTY and migration (Qt5 -> Qt6)

[mipr]

Hi,

I am migrating the Qt version of my application (from 5.15 to 6.4).
And I have compilation errors and one in particular :

Type argument of Q_PROPERTY or Q_DECLARE_METATYPE(T*) must be fully defined

I found some links that explain why this error appears :

• What's new in QMetaType + QVariant
• Qt6 regression with QAbstractListModel? Type argument of Q_PROPERTY or Q_DECLARE_METATYPE(T*) must be fully defined (Qt Forum)

But I didn't understand everything... So I have many questions

1. What is an opaque pointer ?
My hypothesis is that it is a pointer to an object whose behaviour is unknown (like a class in a library where we only have access to the header). Therefore, a pointer to a known class (that we have created and that is in the same module for example) is not an opaque pointer.
Am I right ?

2. If I have a custom object like in this code :

Q_PROPERTY(Camera activeCamera READ getActiveCamera NOTIFY activeCamerasChanged)

I need to replace include "Camera.h" by Q_MOC_INCLUDE("Camera.h"). Is it correct ?

3. If I have a pointer of a custom object

Q_PROPERTY(Camera* activeCamera READ getActiveCamera NOTIFY activeCamerasChanged)

Should I need to replace the Q_MOC_INCLUDE by a Q_DECLARE_OPAQUE_POINTER ?

#ifndef OPAQUE_Camera
#define OPAQUE_Camera
Q_DECLARE_OPAQUE_POINTER(Camera*)
#endif // OPAQUE_Camera

4. If I have a QList of pointer

Q_PROPERTY(QList<Camera*> cameras READ getCameras NOTIFY camerasChanged)

Is the previous Q_DECLARE_OPAQUE_POINTER correct ?

5. If I have a Camera object and a QList<Camera*>, what should I use ?

Thanks for your help !

[sierdzio]

@mipr said in Questions about Q_PROPERTY and migration (Qt5 -> Qt6):

[...]

1. What is an opaque pointer ?
My hypothesis is that it is a pointer to an object whose behaviour is unknown (like a class in a library where we only have access to the header). Therefore, a pointer to a known class (that we have created and that is in the same module for example) is not an opaque pointer.
Am I right ?

No. Opaque pointer is a pointer about which compiler knows nothing apart from name. In other words: a pointer to forward-declared type.

For example:

class QString;

QString *something;

In this example, QString is forward-declared. The compiler knows only it's type (it's a class) and name ("QString"), but knows nothing else (not even methods inside of it like QString::isEmpty()).

Typically you will have forward-declared types in headers (to speed up compilation time) and only actually include the full data (#include <QString>;) in the source file.

In Qt 6, types of Qt properties must be fully defined. So, in simplest terms:

class QString; // BAD in Qt6!
#include <QString>; // GOOD in Qt6!
Q_PROPERTY(QString something ...)

2. If I have a custom object like in this code :

Q_PROPERTY(Camera activeCamera READ getActiveCamera NOTIFY activeCamerasChanged)

I need to replace include "Camera.h" by Q_MOC_INCLUDE("Camera.h"). Is it correct ?

No need to replace anything. A non-pointer has to be fully declared anyway so no change is necessary.

3. If I have a pointer of a custom object

Q_PROPERTY(Camera* activeCamera READ getActiveCamera NOTIFY activeCamerasChanged)

Should I need to replace the Q_MOC_INCLUDE by a Q_DECLARE_OPAQUE_POINTER ?

#ifndef OPAQUE_Camera
#define OPAQUE_Camera
Q_DECLARE_OPAQUE_POINTER(Camera*)
#endif // OPAQUE_Camera

No, it should be enough to use either #include (if you don't care about forward declarations) or Q_MOC_INCLUDE.

4. If I have a QList of pointer

Q_PROPERTY(QList<Camera*> cameras READ getCameras NOTIFY camerasChanged)

Is the previous Q_DECLARE_OPAQUE_POINTER correct ?

I don't know.

5. If I have a Camera object and a QList<Camera*>, what should I use ?

I don't understand the question, sorry.

[mipr]

@sierdzio Thank you a lot for your answer !

Your explaination on the opaque pointer is clear, I didn't have that notion before.

With this knowledge, most of my questions are wrong, like the 5th.