How to access a Q_ENUM declared in C++ class from QML?

Question

I have a Q_ENUM declared in MyClass like below:

class MyClass {
public:
    enum Enum_Test {
        eTestA,
        eTestB
    }
    Q_ENUM(Enum_Test)

    Q_OBJECT
    Q_PROPERTY(MyClass::Enum_Test enumTest READ GetEnumTest WRITE SetEnumTest )
}

I have MyClass registered on the QML side like below and able to access it.

auto my_class = std::make_shared<MyClass>();
qmlRegisterUncreatableType<MyClass>("MyClass", 1,0, "MyClass","Cannot create type MyClass in QML");
rootContext()->setContextProperty("my_class", my_class.get());

How do I access Q_ENUM Enum_Test from QML?

The shown piece of code is not how you _register_ a class, this is setting a context property. You need `qmlRegisterType` after which you can do `MyClass.TestA`. Also the enum _must_ start with a capital — Amfasis, Mar 29 '21 at 13:34
I updated my question. If I get an example of how to access the Q_ENUM which is the primary point of my question, I can take care of registering the class properly. — TheWaterProgrammer, Mar 29 '21 at 13:37

Amfasis · Accepted Answer · 2022-12-19T09:00:06.967

6

Your class needs two adjustments.

as pointed out by JarMan, it needs a metaObject, which can be obtained by deriving from QObject and adding Q_OBJECT:
```
class MyClass : public QObject
{
    Q_OBJECT
    ...
};
```
Actually, it would also be possible to use Q_GADGET but you already seem to lean towards Q_OBJECT. But, as requested, here we go:
```
class MyClass
{
  Q_GADGET

public:
  enum Enum_Test {
      ETestA,
      ETestB
  };
  Q_ENUM(Enum_Test)
};
```
Keep in mind that Q_GADGET cannot have signals, so I left out the property and only have this class as "enum-placeholder".

The enum value names need to be capitalized:

enum Enum_Test {
    ETestA,
    ETestB
};
Q_ENUM(Enum_Test)

Then you can use it in QML as:

     QtObject {
         property int myEnumVal: MyClass.ETestA
     }

Note that support for enums is somewhat limited because the mix with JavaScript. The values will be converted to integers. Also, when used in a JavaScript switch-statement, typo's will not be warned about by QtCreator (assuming version 4.14)

edited Dec 19 '22 at 09:00

answered Mar 29 '21 at 14:16

Amfasis

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1

Thanks. I am accepting this. It would be nice to show the `Q_GADGET` based example and I can accept it. Do you mind editing your answer using `Q_GADGET`? – TheWaterProgrammer Mar 29 '21 at 14:27
1

@TheWaterProgrammer No problem, I have added it – Amfasis Mar 29 '21 at 17:21
In my case, "constexpr is not valid here" error in this example because there is no semicolon for definition ```Enum_Test```. I used Visual Studio 2019 and Qt 5.15. What's the difference? – WOOSEOK CHOI Dec 19 '22 at 04:13
1

@WOOSEOKCHOI That is probably an error in my post, will edit – Amfasis Dec 19 '22 at 08:59

Luca Carlon · Answer 2 · 2021-03-30T11:48:26.790

The answer provided by @Amfasis is already good. As I tend to do this pretty often, I wrote a little shortcut for defining enums with all the features Qt offers, including the availability in QML here: https://github.com/carlonluca/lqtutils#lqtutils_enumh.

You only need to include the header, define your enum values like:

L_DECLARE_ENUM(Enum_Test,
    ETestA,
    ETestB
)

and register it where you prefer with:

Enum_Test::qmlRegisterMySharedEnum("some.uri", 1, 0);

Also this uses simple namespaces, which is lighter than using QObjects or gadgets.

Actually, I got used to always declare my enums like this when I'm using Qt, because I can also benefit from QMetaEnum. For instance, I really like to be able to log like this:

qDebug() << "Value:" << Enum_Test::ETestA;

getting:

Value: Enum_Test::ETestA

instead of a simple integer.

This is excellent! Thanks for posting this answer even after there was an accepted one. — TheWaterProgrammer, Mar 30 '21 at 12:21

How to access a Q_ENUM declared in C++ class from QML?

2 Answers2