Q_ENUM undefined value in qml when enum is defined in base class

Question

Q_ENUM does not work as (I) expected when used on enum that is inherited.

For example there is base struct A (legacy class) and B that inherits from A and from QObject. A has an enum and B has Q_ENUM of this enum.

b.h

#pragma once

#include <QObject>
#include <QDebug>

struct A
{
    A() = default;
    virtual ~A() = default;

    enum E_FOO{
        FOO = 1000,
        FOOO,
        FOOOO
    };
};

class B
        : public QObject
        , public A
{
    Q_OBJECT
    Q_ENUMS(E_FOO)  // OPTION 1 doesn't work

public:
    B() {
        qDebug() << "enums from A" << B::FOO << B::FOOO<<B::FOOOO;
        qDebug() << "enums from B" << B::BAR  << B::BARR  << B::BARRR;
    }
//    Q_ENUM(E_FOO)  // OPTION 2 doesn't work

//    using E_FOO_FORWARDED = A::E_FOO;  // OPTION 3 doesn't work
//    Q_ENUM(E_FOO_FORWARDED)


   // additional enum (for testing a "normal" case)
    enum E_BAR {
        BAR = 1111,
        BARR,
        BARRR
    };
    Q_ENUM(E_BAR)
};

here is some qml file to load that just prints values of enums when created.

MainWindow.qml

import QtQuick 2.7
import QtQuick.Layouts  1.0
import QtQuick.Controls 2.3

import mmm 1.0


ApplicationWindow { 
    Component.onCompleted:{
        console.log("checking if values are correct:")
        console.log("B.FOO should be = 1000", B.FOO)
        console.log("B.BAR should be = 1111", B.BAR)
    }
}

And here is a main file to run it all:

main.cpp

#include <QGuiApplication>

#include <QtQml/QQmlApplicationEngine>
#include <QtQuick/QQuickWindow>

#include <QObject>
#include <QDebug>
#include "b.h"

int main(int argc, char *argv[])
{
    qmlRegisterUncreatableType<B>("mmm",1, 0, "B", "err");

    // (check qDebug capabilities that are coded inside the 
    B b; consturctor)

    // checking enum inside qml
    QGuiApplication a(argc, argv);
    QQmlApplicationEngine eng(QUrl("qrc:/qml/MainWindow.qml"));
    QQuickWindow* window = qobject_cast<QQuickWindow*>(eng.rootObjects().first());
    window->show();

    return a.exec();
}

and the result is

enums from A 1000 1001 1002
enums from B B::E_BAR(BAR) B::E_BAR(BARR) B::E_BAR(BARRR)
qml: checking if values are correct:
qml: B.FOO should be = 1000 undefined
qml: B.BAR should be = 1111 1111

So:

new feature of Q_ENUM (name printing in qDebug) does not work
old Q_ENUMS does not work too
value in qml is undefined
normal enum (that is not from a base class) is working as expected

I tried making an alias type (using E_FOO_FORWARDED = A::E_FOO) with same results. Is there a solution for this problem?

Additional files for the project

inherited_enum.pro

QT += gui core qml quick

TEMPLATE = app
CONFIG += c++14 console qml
SOURCES += main.cpp 
RESOURCES += qml.qrc

qml.qrc

<RCC>
    <qresource prefix="/qml">
        <file>MainWindow.qml</file>
    </qresource>
</RCC>

I guess the problem could be solved be deriving the base class from QObject and defining the enum there. — folibis
– folibis, Commented Jul 24, 2018 at 10:15
Yes, but I wanted the base class to be as lean as possible. I ended up declaring "forwarded" enum like enum E_FOO_FORWARDED{F_FOO = A::FOO, ...} — BPiek
– BPiek, Commented Jul 24, 2018 at 10:23

king_nak · Accepted Answer · 2018-07-24 08:50:03Z

1

Qt extensions like Q_ENUM are handled by macros and moc, which only handle a required subset of C++. It does not really handle inheritance, and only works on the current class definition.

As a consequence, you have to put your Q_ENUMS in the defining class of the underlying enum.

answered Jul 24, 2018 at 8:50

king_nak

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1 Comment

BPiek Over a year ago

I feel like there should be some kind of error message or at least warning. B::FOO is visible from the class so I suppose it's moc where the problem is. I tried to overcome this by alias type but it's probably not supported either (as you mentioned).

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