How to access C++ enum from QML?

c++ qt qml qtquick2

55,266

Solution 1

You can wrap the enum in a class which derives from QObject (and that you expose to QML):

style.hpp :

#ifndef STYLE_HPP
#define STYLE_HPP

#include <QtGlobal>
#if QT_VERSION < QT_VERSION_CHECK(5,0,0)
    // Qt 4
    #include <QDeclarativeEngine>
#else
    // Qt 5
    #include <QQmlEngine>
#endif

// Required derivation from QObject
class StyleClass : public QObject
{
    Q_OBJECT

    public:
        // Default constructor, required for classes you expose to QML.
        StyleClass() : QObject() {}

        enum EnStyle
        {
            STYLE_RADIAL,
            STYLE_ENVELOPE,
            STYLE_FILLED
        };
        Q_ENUMS(EnStyle)

        // Do not forget to declare your class to the QML system.
        static void declareQML() {
            qmlRegisterType<StyleClass>("MyQMLEnums", 13, 37, "Style");
        }
};

#endif    // STYLE_HPP

main.cpp:

#include <QApplication>
#include "style.hpp"

int main (int argc, char ** argv) {
    QApplication a(argc, argv);

    //...

    StyleClass::declareQML();

    //...

    return a.exec();
}

QML Code:

import MyQMLEnums 13.37
import QtQuick 2.0    // Or 1.1 depending on your Qt version

Item {
    id: myitem

    //...

    property int item_style: Style.STYLE_RADIAL

    //...
}

Solution 2

As of Qt 5.8 you can expose enums from a namespace:

Define the namespace and enum:

#include <QObject>

namespace MyNamespace
{
    Q_NAMESPACE         // required for meta object creation
    enum EnStyle {
        STYLE_RADIAL,
        STYLE_ENVELOPE,
        STYLE_FILLED
    };
    Q_ENUM_NS(EnStyle)  // register the enum in meta object data
}

qmlRegisterUncreatableMetaObject(
  MyNamespace::staticMetaObject, // meta object created by Q_NAMESPACE macro
  "my.namespace",                // import statement (can be any string)
  1, 0,                          // major and minor version of the import
  "MyNamespace",                 // name in QML (does not have to match C++ name)
  "Error: only enums"            // error in case someone tries to create a MyNamespace object
);

Use it in a QML file:

import QtQuick 2.0
import my.namespace 1.0

Item {
    Component.onCompleted: console.log(MyNamespace.STYLE_RADIAL)
}

References:

https://www.kdab.com/new-qt-5-8-meta-object-support-namespaces/

http://doc.qt.io/qt-5/qqmlengine.html#qmlRegisterUncreatableMetaObject

http://doc.qt.io/qt-5/qobject.html#Q_ENUM_NS

Solution 3

Additional information (not documented prior to Qt 5.5):

Your enum value names must start with a Capital letter.

This will work:

enum EnStyle
{
    STYLE_RADIAL,
    STYLE_ENVELOPE,
    STYLE_FILLED
};
Q_ENUMS(EnStyle)

This does not:

enum EnStyle
{
    styleRADIAL,
    styleENVELOPE,
    styleFILLED
};
Q_ENUMS(EnStyle)

You won't get any kind of error at compile time, they are just ignored by the QML engine.

Solution 4

I found a very nice solution for using ENUMs from C++ class in QML, here: Enums in Qt QML - qml.guide. The post was so good, I felt obliged to share it here with the SO community. And IMHO attribution should always be done, hence added the link to the post.

The post basically describes:

1) How to create an ENUM type in Qt/C++:

// statusclass.h

#include <QObject>

class StatusClass
{
    Q_GADGET
public:
    explicit StatusClass();

    enum Value {
        Null,
        Ready,
        Loading,
        Error
    };
    Q_ENUM(Value)
};

2) How to register the class with QML engine as an "Uncreatable Type":
(This is the part which makes this solution nice and distinct.)

// main.cpp

...
QQmlApplicationEngine engine;
qmlRegisterUncreatableType<StatusClass>("qml.guide", 1, 0, "StatusClass",
                                        "Not creatable as it is an enum type.");
...

Use of qmlRegisterUncreatableType prevents instantiation of StatusClass in QML. A warning will be logged if a user tries to instantiate this class:

qrc:/main.qml:16 Not creatable as it is an enum type.

3) Finally, how to use the ENUM in a QML file:

// main.qml

import QtQuick 2.9
import QtQuick.Window 2.2

import qml.guide 1.0

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

    Component.onCompleted: {
        console.log(StatusClass.Ready); // <--- Here's how to use the ENUM.
    }
}

Important note:
ENUM is supposed to be used by referencing it with the class name, like this StatusClass.Ready. If the same class is also being used in QML as a context property...

// main.cpp

...
QQmlApplicationEngine engine;
qmlRegisterUncreatableType<StatusClass>("qml.guide", 1, 0, "StatusClass",
                                        "Not creatable as it is an enum type.");

StatusClass statusClassObj; // Named such (perhaps poorly) for the sake of clarity in the example.
engine.rootContext()->setContextProperty("statusClassObj", &statusClassObj); // <--- like this
...

...then, sometimes people accidentally use the ENUM with the context property instead of the class name.

// main.qml

...
Component.onCompleted: {
    // Correct
    console.log(StatusClass.Ready);    // 1

    // Wrong
    console.log(statusClassObj.Ready); // undefined
}
...

The reason people tend to make this mistake is because Qt Creator's autocomplete feature lists ENUM as option, both when referencing using class name as well as the context property. So just exercise caution when in such a situation.

Solution 5

All this solutions can't enabled used this enum-class as parameter for signal/slot. This code compile, but not work in QML:

class DataEmitter : public QObject
{
    Q_OBJECT

public:
    ...
signals:
    void setStyle(StyleClass::EnStyle style);
}

...

emit setStyle(StyleClass.STYLE_RADIAL);

QML-part:

Connections {
    target: dataEmitter
    onSetStyle: {
         myObject.style=style
    }
}

And this code generate runtime error, as this:

IndicatorArea.qml:124: Error: Cannot assign [undefined] to int

For this code working, you must additional registry Qt metaobject type:

qRegisterMetaType<StyleClass::EnStyle>("StyleClass.EnStyle");

More details written here: https://webhamster.ru/mytetrashare/index/mtb0/1535044840rbtgvfmjys (rus)

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55,266

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Aquarius_Girl

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Updated on July 09, 2022

Comments

Aquarius_Girl almost 2 years

class StyleClass : public QObject {
public:
    typedef enum
        {
            STYLE_RADIAL,
            STYLE_ENVELOPE,
            STYLE_FILLED
        }  Style;

    Style m_style;
    //...
};

The .h file has the above code. How to access the above enum through QML?

Richard1403832 over 8 years

@big_gie I refer the honourable gentleman to the phrase "as of posting", and the Qt bug report I made at the time. It is now documented because I took some actions to add it to the documentation.
big_gie over 8 years

Thanks for reporting it to Qt! Thanks to you the docs is in better shape now ;) I simply wanted to add (and refer to the exact location) that it is now documented.
DavidJ about 7 years

Wondering if there is a way for enums not in a QObject derived class (e.g. in a software layer below the UI without Qt dependencies).
Fabien almost 7 years

Note: the class registered must provide a default constructor
air-dex almost 7 years

You're right. I have just edited my answer with additional details.
Alex Vallejo almost 7 years

@DavidJ we are wondering the same thing. As far as I can tell you can't do this. Right now we are doing this: enum MyEnum { Foo = MyOtherEnum::Foo }; Q_ENUMS(MyEnum)
Maxim Paperno over 6 years

@DavidJ As of Qt v5.8 you can do it for enums in namespaces. See answer below.
Marc Van Daele over 5 years

This now also seems to be documented here: doc.qt.io/qt-5/…
KernelPanic almost 5 years

This is very nice solution, however, how does one compares enum with its value?
air-dex almost 5 years

@KernelPanic Enums are integers in QML. So you can do something like myitem.item_style === Style.STYLE_RADIAL.
slayer over 4 years

@MaximPaperno How would you do this with a namespace enum in a header file that you are not able to modify? I tried to declare a new namespace and register the enum with a typedef like: typedef OtherEnumNamespace::OtherEnum NewEnum; Q_ENUM_NS(NewEnum) But it doesn't work
Maxim Paperno over 4 years

@slayer Did you mean to post this under my answer below? :) I think it's a good question, perhaps deserving a separate post. Does Q_ENUM_NS(OtherEnumNamespace::OtherEnum) in your new NS (with Q_NAMESPACE macro I presume) work? Maybe using instead of typedef for an alias? Just guessing! I think a new question might be the way to go.
ScottG about 4 years

A few years ago I spent the better part of a day trying to figure out why my enums weren't showing up in QML for this reason...
jhasse over 3 years

Any way to use those enums in C++?
pooya13 about 3 years

In case you are wondering like me, staticMetaObject is an object added to your namespace by the Q_NAMESPACE macro.
hl037_ almost 3 years

Couldn't it work with just a "using MyForeignEnum" in the class inheriting QObject ?
Matteo almost 3 years

Remember to re-run qmake after you add Q_NAMESPACE or the build process will not find the staticMetaObject
Curtwagner1984 about 2 years

The solution works,, but Is there some way the IDE (Qt Creator) can support this? As in the intelisense picking up the enums on the QML side? Because as I see, right now you need to manually write the enum names.