Compile error when using Q_GADGET macro

[Vinoth Rajendran4]

Hi all,
I opened a new Qt Quick Application, just to learn the behavior of Q_GADGET macro

below is the code,

main.cpp

#include <QGuiApplication>
#include <QQmlApplicationEngine>

#include "base.h"

int main(int argc, char *argv[])
{
    QCoreApplication::setAttribute(Qt::AA_EnableHighDpiScaling);

    QGuiApplication app(argc, argv);

    qmlRegisterType<Base>("Base",1,0,"Base");  // register here

    QQmlApplicationEngine engine;
    const QUrl url(QStringLiteral("qrc:/main.qml"));
    QObject::connect(&engine, &QQmlApplicationEngine::objectCreated,
                     &app, [url](QObject *obj, const QUrl &objUrl) {
        if (!obj && url == objUrl)
            QCoreApplication::exit(-1);
    }, Qt::QueuedConnection);
    engine.load(url);

    return app.exec();
}

base.h

#ifndef BASE_H
#define BASE_H

#include <QObject>

class Base
{
    Q_GADGET

public:
    Base();

};
Q_DECLARE_METATYPE(Base)


#endif // BASE_H

base.cpp

#include "base.h"


Base::Base()
{

}

When building my application i am getting the error,
D:\Qt\5.15.2\msvc2019\include\QtQml\qqmlprivate.h:142: error: C3668: 'QQmlPrivate::QQmlElement<T>::~QQmlElement': method with override specifier 'override' did not override any base class methods
with
[
T=Base
]

Compiler used: MSVC 2019 32 bit

Can someone please help me, what is the issue.

Thanks in advance

[sierdzio]

@Vinoth-Rajendran4 said in Compile error when using Q_GADGET macro:

qmlRegisterType<Base>("Base",1,0,"Base"); // register here

Q_GADGETs cannot be registered as QML types.

[Vinoth Rajendran4]

@sierdzio : Thanks for the reply.

In the documentation, it is mentioned Q_GADGET macro supports Q_PROPERTY, Q_ENUM. How do we expose them to QML ?

[sierdzio]

Yes, QML understands gadgets, can modify properties etc. But it cannot instantiate a gadget, so you cannot register it as type. It's a pretty annoying limitation... and not very well documented.

So, to use a gadget in QML, make sure some object creates the gadget on C++ side, then returns it. Something like (pseudo code):

class MyController : public QObject
{
  Q_OBJECT

  Q_INVOKABLE MyGadget getGadget() const;
};

struct MyGadget
{
  Q_GADGET
  Q_PROPERTY(QString title MEMBER title)
  QString title = "Hello world!";
};

// QML:
console.log("Title is:", myController.getGadget().title)

You can also use gadget in a property instead of Q_INVOKABLE.

[Vinoth Rajendran4]

@sierdzio , Thanks for your clear explanation with code snippet. I got the point.