Important: Please read the Qt Code of Conduct -

Unable to assign GenericDataModel to GenericDataModel

  • I'm developing a large Qt-based framework, so I'll try to give watered-down minimal examples where possible.
    I have a plugin system for defining new data models for viewing though custom QML view components.

    These plugins are dynamically loaded using QPluginLoader, as with the Plug 'n' Paint example. In my plugin interface, I can call a function that supplies me with a pointer to a newly allocated base class:

    GenericDataModel *mdl = mod->CreateModel("frt");

    where mod is a pointer to the loaded plugin interface. GenericDataModel is a derived class of QAbstractItemModel, already confirmed to work with other models. GenericDataModel is also registered as a type with QML:

    qmlRegisterType<GenericDataModel>("plugin.genericmodel", 1,  0, "GenericDataModel");

    This works fine, and I then load the data into the model without any problems. I confirm with the debugger that the data is there and laid out correctly. I then register the model into QML like so:

    QQmlApplicationEngine engine;
    QQmlContext *ctxt = engine.rootContext();
    ctxt->setContextProperty("MDLFileTest", QVariant::fromValue(mdl));

    I then apply this to a custom QML view, CustomList.qml:

    import QtQuick 2.7
    import QtQml.Models 2.2
    import QtQuick.Controls 2.2
    import plugin.genericmodel 1.0
    ListView {
        id: listView
        property GenericDataModel dataModel
        model: dataModel
        delegate { ... }

    Which I then apply to a page:

    CustomList {
        id: customList
        dataModel: MDLFileTest

    When loading this from a plugin (i.e. the plugin defines a class that extends GenericDataModel, and allocates one and returns it as a GenericDataModel* to the main application), I get the QML error at runtime:

    Unable to assign GenericDataModel to GenericDataModel

    At this point, as far as the QML layer is concerned, I've assigned a GenericDataModel for a property expecting a GenericDataModel. However, when I do exactly the same thing, but move the header and source file for the derived class into the main application, so that it is compiled in, it works perfectly...

    Can somebody explain this error to me? I've seen it before with two different types that cannot be casted, but this is an error trying to assign type X to type X! Quite strange. I have played around with the Q_OBJECT macro and gotten it to read:

    Unable to assign FRTFileModel to GenericDataModel

    The strange thing is, FRTFileModel is a direct descendant of GenericDataModel!

    I tried to keep this question as minimal as possible, but if anyone wants me to try to create a minimal example project I can give it a go as well.

  • I feel like I have somehow created a situation where there are 2 class types with the name "GenericDataModel"... The GenericDataModel class is declared and defined inside a seperate library that is statically linked to both the application and the plugin. Calling


    returns true, yet calling


    returns null.

    Am I correct in thinking that this means that I have 2 separate class types that have the same name in the Qt meta object system? Obviously this is not what I am intending... How can I fix this? Both the plugin and the application include the same genericdatamodel.h from a static library, and both link to the same static library? Is this not the way that I should be doing this?

    Please give your input, thanks.

  • Interestingly, if I make the plugin static, it works fine. Only when the plugin is dynamic does this error happen... Very strange.