Is it possible to expose a C++ Class that doesn't have a default constructor to QML ?

[c64zottel]

@Curtwagner1984 I just tried to pass the id as "this" pointer and it worked.

So, you register all your derived classes as usual and use them with a proper id.
The generic function takes a pointer of the base class, and you call that function with the id you issued in Qml.

[Curtwagner1984]

@c64zottel said in Is it possible to expose a C++ Class that doesn't have a default constructor to QML ?:

@Curtwagner1984 I just tried to pass the id as "this" pointer and it worked.

So, you register all your derived classes as usual and use them with a proper id.
The generic function takes a pointer of the base class, and you call that function with the id you issued in Qml.

Can you please elaborate on what exactly you did? I'm not sure I follow. An example would be best.

[c64zottel]

@Curtwagner1984 Something like this:

class Base : public QObject
{
...
  Q_INVOKABLE virtual void func();

signals:
  void activated();
}

class D1 : public Base
{
    void func() {
        do stuff;
    }
}

class D2 : public Base
{
    void func() {
        do other stuff;
    }
}

class SomeClientUsingBase : public QObject
{
    Q_INVOKABLE void functionDealingWithSomethingDerivedFromBase( Base * b ) {
        b->func();  // uses either D1 or D2
    }
}

In Qml:

qmlRegisterType<SomeClientUsingBase>("com.me.qmlcomponents", 1, 0, "Client");
qmlRegisterType<Base>("com.me.qmlcomponents", 1, 0, "Base");
qmlRegisterType<D1>("com.me.qmlcomponents", 1, 0, "D1");
qmlRegisterType<D2>("com.me.qmlcomponents", 1, 0, "D2");

Client {
    id: waitingToUseBase
}

D1 {
    id: d1
    onActivated: waitingToUseBase.functionDealingWithSomethingDerivedFromBase( d1 )
}

D2 {
    id: d2
    onActivated: waitingToUseBase.functionDealingWithSomethingDerivedFromBase( d1 )
}

[sierdzio]

@sierdzio said in Is it possible to expose a C++ Class that doesn't have a default constructor to QML ?:

You can, however, add an init() function and pass your pointer there, after QML engine creates the object.

How would I go about doing this? How would I know that the object was created in QML in order to trigger the init() function?
I suppose I could connect it to a signal that is triggered by the default constructor, is that what you are suggesting?

Ugh, as always with QML, there are many ways to do it and I never know which solution to recommend :D Let's try this:

// C++
// add DbManager to QML context somewhere. Example:
DbManager manager;
QQmlApplicationEngine engine;
engine.rootContext()->setContextProperty("Manager", &manager);
[...]
class BasicListModel
{
  Q_INVOKABLE void init(DbManager *manager);
}

// QML
[...]
model: BasicListModel {
  Component.onCompleted: {
    init(manager);
  }
}

[Curtwagner1984]

This post is deleted!

[Curtwagner1984]

@sierdzio Thank you! It works exactly as expected.

Now I wonder, can I create an instance of BasicListModel dynamically? Something along the lines of :

function generateDynamicInstance()
{
    // returns a new instance of BasicListModel
}

Button {
      onClicked:{
        var newInstance = generateDynamicInstance();
        myListView.model = newInstance;
      }
  
}

ListView{
    id:myListView
}

[sierdzio]

@Curtwagner1984 said in Is it possible to expose a C++ Class that doesn't have a default constructor to QML ?:

Now I wonder, can I create an instance of BasicListModel dynamically? Something along the lines of :

Yes, this page in docs describes it: link. Or you can use a Loader.

[Curtwagner1984]

@sierdzio
I've already looked at the page Dynamic QML Object Creation from JavaScript ,but I thought the function Qt.createComponent() is only for .qml files , and the createObject object method only works on components created by the Qt.createComponent() method.

I've tried to create a component manually like so:

Component {
        id: myTestComponent
        ActorModel {
            id: actModel
            Component.onCompleted: {
                console.log("ACTORMODEL:ON COMPLETE TRIGGERED")
                init(dbManager)                
            }
        }
    }

and then use the createObject method on it like this myTestComponent.createObject , but it didn't complile. (Clearly, I'm doing it wrong, but I couldn't find how to use this method on a C++ registered Object in the documentation.)

I did manage to use a Loader like this:

Component {
        id: myTestComponent
        ActorModel {
            id: actModel
            Component.onCompleted: {
                console.log("ACTORMODEL:ON COMPLETE TRIGGERED")
                init(dbManager)                
            }
        }
    }
        
    Loader{
        id: myTestLoader
        sourceComponent: myTestComponent       
    }


ListView{
  id:myList
  model:myTestLoader.item
}

But I'm unsure as to how to trigger a creation of a new instance.

[Curtwagner1984]

@sierdzio I managed to get it to work with createObject like so:

Component {
        id: myTestComponent
        ActorModel {
            id: actModel
            Component.onCompleted: {
                console.log("ACTORMODEL:ON COMPLETE TRIGGERED")
                init(dbManager)                
            }
        }
    }

Button {
  onClicked: {
    // creates a new instance of ActorModel
     var x = myTestComponent.createObject();     
     myList.model = x;
  }
}

ListView{
  id:myList
}

Thank you very much for your help!

[sierdzio]

You're welcome, happy coding :-)

And thanks for sharing the solution, maybe it will help somebody else, too.