Polymorphic Q_GADGETs

[arietto]

Hello! Do you have any hints concerning (de-)serialization of polymorphic Q_GADGETs? They can also have Q_CLASSINFO. I suppose that I have to create a "switch" for property getter/setter which checks some flag in JSON. It will choose, in fact, among variant.value<Class1>(), variant.value<Class2>() and so on for deserialization.

[Skycoder42]

With the current implementation, this is not directly possible. The reason is that gadgets are value types and c++ can simply not handle polymorphism for value types correctly. Also, gadgets do not have a non-static metaobject associated, which means it's impossible to dynamically detect their type.

However, if you are fine with a workaround, I guess the following would work:

Assuming you have the following (simplified) classes:

class GadgetBase {
    //...
};

class Gadget1 : public GadgetBase {
    // ...
};

class Gadget2 : public GadgetBase {
    // ...
};

class SomeOtherGadget {
    GadgetBase *theGadget; //should be either 1 or 2
};

You could implement SomeOtherGadget as follows:

class SomeOtherGadget {
    Q_GADGET

    Q_PROPERTY(QJsonObject theGadget READ getGad WRITE setGad)

    GadgetBase *theGadget;
    QJsonSerializer *serializer;

    SomeOtherGadget() {
        serializer = // get the serializer somehow or just create a new one
    }

    QJsonObject getGad() const {
        QJsonObject obj;
        QString type = getGadgetClassName(theGadget);
        if(type == "Gadget1")
            obj = serializer->serialize(*static_cast<Gadget1*>(theGadget));
        else if(type == "Gadget2")
            obj = serializer->serialize(*static_cast<Gadget2*>(theGadget));
        // ...
        obj["@class"] = type;
        return obj;
    }

    void setGad(const QJsonObject &obj) {
        if(obj["@class"] == "Gadget1")
            theGadget = new Gadget1(serializer->deserialize<Gadget1>(obj)); //copy-construct to the heap
        else if(obj["@class"] == "Gadget2")
            theGadget = new Gadget2(serializer->deserialize<Gadget2>(obj)); //copy-construct to the heap
        // ...
    }
};

It's a little complicated and verbose, but more or less the only way to make gadget serialization "polymorphic" (I used a special property name @class here to keep the same syntax in JSON as the QObject based classes. But you can of course store that information however you like). I hope this can help you

[Skycoder42]

Another alternative would be to implement a custom QJsonTypeConverter that does more or less the same thing. If you have many classes that need this polymorphism, using a custom converter would be much more convenient I guess.

[arietto]

Thank you for your huge reply. I suppose that you code sample will be helpful for other users too.