QML: "Cannot assign Derived to Base"

[alexcrist]

Hello,

I've been trying for 2 days to find an answer to this, but either I've been using the wrong (key)words, either there's no answer.

I have in C++ a class 'Point' derived from QGeoCoordinate and marked with Q_GADGET (just like QGeoCoordinate). Metatype is also declared as "Q_DECLARE_METATYPE(Point)"

When an instance of this class reaches QML (either by Q_PROPERTY of another class, or an argument of a signal), everything can be read correctly from the object (latitude, longitude from QGeoCoordinate, and other members from Point). But if I try to assign it to something that needs a QGeoCoordinate (for instance, a map.center, or a polyline point), I get a QML error on runtime: "Error: Cannot assign Point to QGeoCoordinate"

Since Point derives QGeoCoordinate, I expected this to work. Am I doing something wrong, or this is not possible?

Here's my sample code:
point.h:

#pragma once
#include <QGeoCoordinate>

class Point : public QGeoCoordinate
{
    Q_GADGET
    Q_PROPERTY(float azimuth READ getAzimuth CONSTANT)

public:
    Point()
    {
        setLatitude(25);
        setLongitude(25);
    }

    float getAzimuth() const { return m_azimuth; }

private:
    float m_azimuth = 13.15;
};

Q_DECLARE_METATYPE(Point)

controller.h

#pragma once
#include "point.h"
#include <QObject>

class Controller : public QObject
{
    Q_OBJECT
    Q_PROPERTY(Point point READ getPoint CONSTANT)

public:
    Controller() { }
    Point getPoint() const { return point; }

private:
    Point point;
};

main.cpp:

#include "controller.h"
#include <QGuiApplication>
#include <QQmlApplicationEngine>
#include <QQmlContext>

int main(int argc, char *argv[])
{
    QGuiApplication a(argc, argv);
    QQmlApplicationEngine engine;
    Controller controller;

    engine.rootContext()->setContextProperty("controller", &controller);
    engine.load(QUrl(QStringLiteral("qrc:/main.qml")));

    return a.exec();
}

main.qml:

import QtQuick 2.15
import QtQuick.Controls 1.4
import QtLocation 5.15
import QtQuick.Window 2.2

Window {
    visible: true
    visibility: "Maximized"

    Plugin {
        id: mapPlugin
        name: "esri"
    }

    Map {
        id: map
        anchors.fill: parent
        plugin: mapPlugin
    }

    Button {
        text: "clic"
        onClicked: {
            console.log(controller.point.azimuth)
            console.log(controller.point.latitude)
            console.log(controller.point.longitude)


            map.center = controller.point // => qrc:/main.qml:28: Error: Cannot assign Point to QGeoCoordinate
        }
    }
}

Thanks!

[raven-worx]

@alexcrist
i dont see that you are registering your derived type to QML?!
registering a extending type should be enough i think:
https://doc.qt.io/qt-5/qtqml-cppintegration-definetypes.html#registering-extension-objects

[alexcrist]

@raven-worx Indeed I didn't. But the documentation states that ”An extension class is a regular QObject, with a constructor that takes a QObject pointer.” and ”This takes effect if the type is exposed to QML using a QML_ELEMENT or QML_NAMED_ELEMENT() macro.”, neither of which applies here.

Both QGeoCoordinate and Point are Q_GADGETs, not Q_OBJECTs. Am I mistaking that QML_EXTENDED doesn't work with Q_GADGETs?

And, as I said, the Point instance can be accessed correctly in QML, the 3 console.logs work just fine, so the type itself seems to be registered in QML. The issue is with assigning it to other QGeoCoordinate properties of other objects.

[raven-worx]

@alexcrist said in QML: "Cannot assign Derived to Base":

And, as I said, the Point instance can be accessed correctly in QML, the 3 console.logs work just fine

i assume it works because the properties are taken from the known QGeoCoordinate qml type.

Does it work when you simply change the type of the qproperty definition to QGeoCoordinate?

Q_PROPERTY(QGeoCoordinate point READ getPoint CONSTANT)

[alexcrist]

There are 3 logs there:

console.log(controller.point.azimuth) // =>prints correctly, from Point
console.log(controller.point.latitude) // =>prints correctly, from QGeoCoordinate (base)
console.log(controller.point.longitude) // =>prints correctly, from QGeoCoordinate (base)

map.center = controller.point // => fails with: qrc:/main.qml:28: Error: Cannot assign Point to QGeoCoordinate

The first one, azimuth, belongs to the Point class, the other 2 are from QGeoCoordinate. Map.center fails

If I change the Q_PROPERTY as you suggested (and NO other changes anywhere), the code builds properly, and the behavior changes:

console.log(controller.point.azimuth) // => fails: prints ”undefined” (makes sense, can't access Point properties)
console.log(controller.point.latitude) // =>prints correctly, from QGeoCoordinate
console.log(controller.point.longitude) // =>prints correctly, from QGeoCoordinate

map.center = controller.point // => works correctly!

So, not quite there yet... :(

[raven-worx]

@alexcrist
i looked into the QML internals and you cannot assign a value with a custom type to a property (QObject pointers are a special case - and some others)

For your case i suggest you either subclass Point from QObject wich returns a QGeoCoordinate in a property or invokable method (e.g. toGeoCoordinate()), or the controller returns a copy of your Point as a QGeoCoordinate.

Either way you cannot subclass all value types and assing them to a property expecting a specific type, since the registered meta-type id is used for comparison. Thus the Point meta-type cannot be "casted" to the QGeoCoordinate meta type.

[GrecKo]

@raven-worx said in QML: "Cannot assign Derived to Base":

i looked into the QML internals and you cannot assign a value with a custom type to a property (QObject pointers are a special case - and some others)

What does that refer to? Assigning a to a custom Q_GADGET property?
Because you can do that.

[raven-worx]

@GrecKo
depending on the type of declared property.
the meta type system doesnt check if a metatype derives from another metatype.

in this case the type of the property is QGeoCoordinate, and only accepts values of this type and no others.

https://code.qt.io/cgit/qt/qtdeclarative.git/tree/src/qml/jsruntime/qv4qobjectwrapper.cpp?h=v5.14.0#n505

[alexcrist]

Well, that's too bad. :(

Thanks for your help. I've applied a workaround like this in Point:

Q_PROPERTY(QGeoCoordinate coordinates READ coordinates CONSTANT)
const QGeoCoordinate &coordinates() const { return *this; }

And seems to do the job. It's not ideal, since I also have a QList<Point> (used in a Repeater in QML), and there I need to make a copy of the list with casted objects. But if that's the only way it can work now... so be it. :)