Nullpointer passed when using pointer to const object as parameter for c++ method called from qml

[AlaNan]

Consider the following minimal example:
main.cpp

#include <QApplication>
#include <QQmlApplicationEngine>
#include <QtQml>
#include "test.h"

int main(int argc, char *argv[])
{
    QApplication app(argc, argv);

    qmlRegisterType<Test>("nanotron.test.test", 1, 0, "Test");
    QQmlApplicationEngine engine;
    engine.load(QUrl(QStringLiteral("qrc:/main.qml")));

    return app.exec();
}

test.h

#ifndef TEST_H
#define TEST_H

#include <QObject>

class Test : public QObject
{
    Q_OBJECT
    Q_PROPERTY(int member MEMBER m_member NOTIFY memberChanged)
public:
    explicit Test(QObject *parent = 0);
    Q_INVOKABLE void print(const Test* other);

signals:
    void memberChanged();

public slots:

private:
    int m_member;
};

Q_DECLARE_METATYPE(const Test*)
#endif // TEST_H

test.cpp

#include "test.h"
#include <QDebug>

Test::Test(QObject *parent) : QObject(parent)
{
}

void Test::print(const Test *other)
{
    qWarning() << "Value of test object's meber:" << other->m_member;
}

main.qml

import QtQuick 2.5
import QtQuick.Controls 1.4
import nanotron.test.test 1.0

ApplicationWindow {
    visible: true
    width: 640
    height: 480
    title: qsTr("Hello World")

    Component {
        id: testComponent
        Test {
        }
    }

    menuBar: MenuBar {
        Menu {
            title: qsTr("File")
            MenuItem {
                text: qsTr("&Open")
                onTriggered: {
                    var test1 = testComponent.createObject()
                    var test2 = testComponent.createObject()
                    test1.member = 4711
                    test2.print(test1)
                }
            }
            MenuItem {
                text: qsTr("Exit")
                onTriggered: Qt.quit();
            }
        }
    }

    Label {
        text: qsTr("Hello World")
        anchors.centerIn: parent
    }
}

When I run this and click on File -> Open the program crashes. In the debugger I can see that NULL is passed to the print method of the test class. I can trace it back to the qt_metacall method of the moc class that is generated for test.
When I remove the const from the parameter in print, i.e change

Q_INVOKABLE void print(const Test* other);

to

Q_INVOKABLE void print(Test* other);

and

void Test::print(const Test *other)

to

void Test::print(Test *other)

and

Q_DECLARE_METATYPE(const Test*)

to

Q_DECLARE_METATYPE(Test*)

it works fine.

Can someone explain this to me? Is it a bug? Is it expected? Is it possible to use pointers to const objects when passing parameters from QML to c++?

[AlaNan]

Come on!
Nobody any idea?
What could be the reason for changing a valid pointer to a null pointer on the callers side when the signature of the parameter is const ptr??
Is it just me?
Can anybody reproduce this???

[SGaist]

Hi and welcome to devnet,

You should use qmlRegisterType<const Test>("nanotron.test.test", 1, 0, "Test");

[AlaNan]

Thank you very much!
I guessed it must be something trivial. Of course I have to register the exact type that I want to use with the meta type system.
I'm not sure if like the fact that registering the wrong type leads to a null pointer being passed. Do you happen to know what is the reason for that? Is it the unchecked result of a dynamic cast? I certainly would prefer, if an exception could be raised in that case. Do you think this would be possible to add?

[SGaist]

You're welcome !

No I don't, sorry, I haven't touched the internals of the QtQuick/QML modules.

That's a question you should bring to the interest mailing list. You'll find there Qt's developers/maintainers(this forum is more user oriented)