issue with Custom type to use it as a value property and to receive it from a C++ method...

mbruel

Hi,
I've created ModelId to be used in both my C++ and QML.

My goal was to use it in models QAbstractListModel cf VarietyModel2::data, using this method.

It is working but I've two issues in my Delegate:
My Delegate is getting a ModelId in seed_company_id from the model.

1.: I'd like to use it as a property of a Combobox and I'm getting this error: qrc:/qml/SettingsVarietyDelegate.qml:73:21: Unable to assign ModelId to ModelId*
So a property on a custom type has to be a pointer? can't we use it as a value? What solution would I have?
In this particular example I've used previousIndex int variable but I guess this trick won't always work...

2.: it seems QML gets QVariant<ModelId> from the return of a C++ method instead of directly a ModelId
In this piece of code:

                    onCurrentIndexChanged: {
                                            if (!initialized)
                                                return;
                                            let newCompanyId = cppFamily.seedCompanyIdFromProxyRow(currentIndex);
                                            let currentCompanyId = cppFamily.seedCompanyIdFromProxyRow(previousIndex)
                                            print("[MB_TRACE][seedCompanyField] newCompanyId: " + newCompanyId);
                                            print("[MB_TRACE][seedCompanyField] currentCompanyId: " + currentCompanyId)
                                            if (currentCompanyId !== newCompanyId)
                                                cppFamily.updateVarietyCompanySeed(varietyModel.sourceRow(index), crop_id, variety_id,
                                                                                 currentCompanyId, newCompanyId);
                                            previousIndex = currentIndex;
                                        }

seedCompanyIdFromProxyRow returns a ModelId (from C++) but here is the print result:

qml: [MB_TRACE][seedCompanyField] newCompanyId: QVariant(ModelId)
qml: [MB_TRACE][seedCompanyField] currentCompanyId: QVariant(ModelId)

And I'm not able to use any ModelId methods like ModelId::str() to be able to print the value.
If I update the log to be print("[MB_TRACE][seedCompanyField] newCompanyId: " + newCompanyId.str());
Then I'm getting this error: qrc:/qml/SettingsVarietyDelegate.qml:95: TypeError: Property 'str' of object QVariant(ModelId) is not a function
Why is that? How can I access the methods of my object?

IntruderExcluder

You need to pass yor ModelId object as Qbject*, but your method objId() returns copy of ModelId which is inherited from QObject. You can change your function to return pointer to your member and then use something like return QVariant::fromValue(qobject_cast<QObject*>(ur_ptr)); in your model.
But keep in mind that you need proper memory management in this case: QML may delete your QObject* with gc call which can lead to double free of same pointer. To make it proper see data ownership in Qt.

mbruel

@IntruderExcluder
Yep I came to the same conclusion, we need to use pointers to share custom types... That's quite boring, gonna have to update plenty of methods... I'll give it a try next week.
I know for the ownership. That's quite boring that Qt doesn't provide a Macro (or a way) to use on Custom Types in order to prevent QML to take ownership of any of their instances... This would be really convenient!
But all right, I'll set a C++ fake parent for all my ModelIds as I want to keep them on the stack and don't want to make their ModelObject real owner derived from QObject...

mbruel

so it seems we need to use pointers on custom types to use them as QML property.
it is kind of working to exchange custom types by value between C++ and QML but we can't use their methods in QML...
good to know...

GrecKo

If you want to use value semantics, don't inherit from QObject and use Q_GADGET instead of Q_OBJECT.

mbruel

@GrecKo hum are you sure about that?
I spent the whole day updating my code to use pointers... :'( anyway I will get some advantages doing so, my code will be faster but still...
I kind of doubt that even using Q_GADGET I'd be able to use a custom type by value in QML.
Are you sure I'd be able to affect a value to a QML property?

Edit: just to check I've made my ModelId not inherit from QObject and use Q_GADGET instead.
I'm getting a compilation error on the QML type registration: qmlRegisterType<ModelId>(packageURI, ver.maj, ver.min, "ModelId");:

/opt/Qt/5.12.6/gcc_64/include/QtQml/qqmlprivate.h:106: error: ‘QQmlPrivate::QQmlElement<T>::~QQmlElement() [with T = ModelId]’ marked ‘override’, but does not override
In file included from /opt/Qt/5.12.6/gcc_64/include/QtQml/qqml.h:43,
                 from /opt/Qt/5.12.6/gcc_64/include/QtQml/qqmlengine.h:47,
                 from /opt/Qt/5.12.6/gcc_64/include/QtQuick/qquickimageprovider.h:46,
                 from /opt/Qt/5.12.6/gcc_64/include/QtQuick/QQuickImageProvider:1,
                 from /home/bruel/Documents/tieum/myDev/c++/QT/github/qrop/core/qrpimageprovider.h:6,
                 from /home/bruel/Documents/tieum/myDev/c++/QT/github/qrop/desktop/main.cpp:24:
/opt/Qt/5.12.6/gcc_64/include/QtQml/qqmlprivate.h: In instantiation of ‘class QQmlPrivate::QQmlElement<ModelId>’:
/opt/Qt/5.12.6/gcc_64/include/QtQml/qqmlprivate.h:124:37:   required from ‘void QQmlPrivate::createInto(void*) [with T = ModelId]’
/opt/Qt/5.12.6/gcc_64/include/QtQml/qqml.h:287:31:   required from ‘int qmlRegisterType(const char*, int, int, const char*) [with T = ModelId]’
/home/bruel/Documents/tieum/myDev/c++/QT/github/qrop/desktop/main.cpp:108:69:   required from here
/opt/Qt/5.12.6/gcc_64/include/QtQml/qqmlprivate.h:106:9: error: ‘QQmlPrivate::QQmlElement<T>::~QQmlElement() [with T = ModelId]’ marked ‘override’, but does not override
         ~QQmlElement() override {
         ^

I believe we need QObjects for QML no?