Dynamic object creation in Qt6

[Christian Ehrlicher]

Not looked into the details but why do you now need QMetaType::create() instead QMetaObject::newInstance()?

[aliks-os]

@Christian-Ehrlicher
Because newInstance is marked as depricated

[Christian Ehrlicher]

Where?
https://doc.qt.io/qt-6/qmetaobject.html#newInstance-1

[aliks-os]

here
https://doc.qt.io/qt-6/qmetaobject-obsolete.html

[aliks-os]

and compilator give warning as well

[VRonin]

That method was just replaced with a variadic. It's an implementation detail, once the deprecated function gets removed the new version of the method steps in seamlessly so you don't need to worry about it. The compiler gives warning because a non-template method is always preferred to a template one.
You can try newInstance<QObject*>(nullptr); to force the template method to be used

[aliks-os]

@VRonin Oh, thanks for pointing. I will try the new implementation

[aliks-os]

I re-coded, but in my case newInstance returns nullptr

QMetaType metaType = QMetaType::fromName(QString("TestDevice*").toLatin1().constData());
        if (metaType.isValid() && metaType.flags().testFlag(QMetaType::PointerToQObject))
        {
            const QMetaObject *mo = metaType.metaObject();   // Not sure about it - is that correct?
            if (mo)
            {
                QObject *objectPtr = mo->newInstance<TestDevice*>(nullptr);   // Returns nullptr
                auto device = static_cast<TestDevice*>(objectPtr);
                device->setObjectName("ttt");
            }
        }

[VRonin]

QObject *objectPtr = mo->newInstance<TestDevice*>(nullptr); // Returns nullptr

The template argument(s) should be the type of the argument(s) of the constructor.
if the constructor is TestDevice(QObject *parent) then the template should be mo->newInstance<QObject*>(nullptr);

[aliks-os]

@VRonin
yes constructor is TestDevice(QObject *parent)

not clear re mo->newInstance<QObject*>(nullptr);
Should here I indicate a class of argument? Not a creating class?

[VRonin]

Just to make it clearer, imagine newInstance is:

template <class T>
QObject* newInstance(T arg){
return new TestDevice(arg);
}

In your code you are calling newInstance<TestDevice*> so T is TestDevice*. Since TestDevice doesn't have a constructor TestDevice(TestDevice*) the compiler fails. T should be a QObject* because that's what the TestDevice constructor expects so the template argument should be QObject*

[aliks-os]

@VRonin Many thanks! Now all works as should be. Latest version is

    QMetaType metaType = QMetaType::fromName("TestDevice*");
    if (metaType.isValid() && metaType.flags().testFlag(QMetaType::PointerToQObject))
    {
        const QMetaObject* metaObject = metaType.metaObject();
        if (metaObject)
        {
            // Correctly use the type TestDevice* with newInstance
            QObject* objectPtr = metaObject->newInstance<QObject*>(nullptr); // Pass nullptr for the parent argument
            if (objectPtr)
            {
                auto device = static_cast<TestDevice*>(objectPtr);
                device->setObjectName("ttt");
                qDebug() << "Device created and object name set to:" << device->objectName() << device->metaObject()->className();
            }
            else
            {
                qWarning() << "Failed to create an instance using newInstance for type: TestDevice*";
            }
        }
        else
        {
            qWarning() << "MetaObject is null for type: TestDevice*";
        }
    }
    else
    {
        qWarning() << "Invalid or unsupported type: TestDevice*";
    }