Enum in Qt property

Question

I have a code, which works with Qt 5.5 and doesn't with Qt 5.2 . Problem is with this enum :

#include <QtCore/QMetaType>
enum Area
{
    Area_A,
    Area_B,
    Area_C
};

Q_DECLARE_METATYPE(Area)

Then I have an object, which exposes this area property:

class MyClass : public QObject
{
    Q_OBJECT
    Q_PROPERTY(Area area READ area WRITE setArea NOTIFY areaChanged)
public:    
    explicit MyClass(QObject *parent = 0)
        : QObject(parent), m_area(Area_A){}

    Area area() const { return m_area; }    
    void setArea(Area area) {
        m_area = area;
        emit areaChanged(area);
    }

signals:
    void areaChanged(Area area);    
private:
    Area m_area;
};

And main.cpp:

#include <QtGui/QGuiApplication>
#include <QtQml/QQmlApplicationEngine>
#include <QtQml/QQmlContext>
#include <QtQml/QtQml>
#include "MyClass.h"

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

    qmlRegisterType<MyClass>("GLib", 1, 0, "MyClass");

    MyClass controller;
    QQmlApplicationEngine engine;
    engine.rootContext()->setContextProperty("controller", &controller);
    engine.load("./main.qml");

    controller.setArea(Area_B);
    return app.exec();
}

It compiles, everything is OK. But when I tried to use area property in qml :

import QtQuick 2.0
import QtQuick.Window 2.0
import GLib 1.0

Window {
    visible: true
    id: root

    property int area: controller.area 

    Text {
        id: name
        text: "Test"
        x: area * 30
        y: area * 30
    }
}

I have run-time errors, if Qt 5.2 is used (Linux, x64):

QMetaProperty::read: Unable to handle unregistered datatype 'Area' for property 'MyClass::area' file:///home/yech844/devel/test_qml/main.qml:10:24: Unable to assign [undefined] to int QMetaProperty::read: Unable to handle unregistered datatype 'Area' for property 'MyClass::area' file:///home/yech844/devel/test_qml/main.qml:10:24: Unable to assign [undefined] to int

Is it a bug in Qt? Why I can't use Enum, which is declared out of Class scope?

Answer 1

Qt 5.5 introduced Q_ENUM macro which removed the need to use Q_DECLARE_METATYPE. Read more about it here: https://woboq.com/blog/q_enum.html

Answer 2

I don't know why the code works in Qt 5.5, but I know why it doesn't in Qt 5.2.

Q_DECLARE_METATYPE(...) only makes the type available in static (compiled) context. For example, if you want to use the type in QVariant::fromValue(...) . Here, the type you pass to the function can be processed during compile time, and for this, Q_DECLARE_METATYPE is enough.

However, if you want to use a type in a pure runtime context, for example in a QML document, the Qt runtime doesn't know the type declared with Q_DECLARE_METATYPE . For this, a function call (evaluated during runtime) needs to be made, and qRegisterMetatype is the tool for this:

qRegisterMetaType<Area>("Area");

My guess for Qt 5.5 not needing that line is that qmlRegisterType might detect the use of the meta type in the property and automatically calls the above function for you.