Is there a way to use QMetaEnum with Q_ENUMS belonging to non Q_OBJECT or Q_GADGET class?

Question

For example I have the following class:

namespace someName
{
    class someClass
    {
        Q_ENUMS(ESomeEnum)

        public:
        enum ESomeEnum {ENUM_A, ENUM_B, ENUM_C};

        // ... some other things ..
    }
}

Q_DECLARE_METATYPE(someName::someClass)

Is there a way to use QMetaEnum::valueToKey or QMetaEnum::keyToValue ?

Tried the method in this answer but got the following error:

error: static assertion failed: QMetaEnum::fromType only works with enums declared as Q_ENUM or Q_FLAG
#define Q_STATIC_ASSERT_X(Condition, Message) static_assert(bool(Condition), Message)

I can employ X-Macros to get my desired output but it would also be nice to learn more tricks in Qt.

You are using `Q_ENUMS`, I think you have to use `Q_ENUM` in this case. — thuga, Jun 07 '16 at 10:59
i am maintaining code written in this manner and changing Q_ENUMS to Q_ENUM would flag unnecessary "panic" to the project manager as this affects a lot of files. — misilita, Jun 08 '16 at 02:00
As of right now, the accepted answer should be different. @DrumM is correct, since Qt 5.8 you could use `Q_NAMESPACE` + `Q_ENUM_NS` combo. — sthlm58, Jan 22 '20 at 10:41

score 9 · Accepted Answer · answered Jun 07 '16 at 13:04

No, there isn't, because Q_ENUM's functionality is implemented in code generated by moc, and moc ignores classes that are neither Q_OBJECT nor Q_GADGET. There's no reason for not using a Q_GADGET since it has no effect on object size: adds no virtual methods nor data fields.

The following demonstrates this:

#include <QtCore>

namespace Ns {
class Class {
   Q_GADGET
public:
   enum ESomeEnum {ENUM_A, ENUM_B, ENUM_C};
   Q_ENUM(ESomeEnum)
};
}

int main() {
   auto metaEnum = QMetaEnum::fromType<Ns::Class::ESomeEnum>();
   qDebug() << sizeof(Ns::Class) << metaEnum.valueToKey(Ns::Class::ENUM_A);
}
#include "main.moc"

Output:

1 ENUM_A

On this particular platform (and many others), empty classes are of size 1.

score 5 · Answer 2 · answered Mar 01 '19 at 10:48

5

Yes, since 5.8 you can do:

namespace MyLibrary
{ 
Q_NAMESPACE 

enum class MYLIBRARYSHARED_EXPORT MyEnumClass
{
...
}; 

Q_ENUM_NS(MyEnumClass)

...
} // namespace MyLibrary

answered Mar 01 '19 at 10:48

jaques-sam

2,578
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score 4 · Answer 3 · edited Jun 24 '17 at 12:56

Q_ENUM is like the old Q_ENUMS but with these differences:

It needs to be placed after the enum in the source code.
Only one enum can be put in the macro.
It enables QMetaEnum::fromType<T>().
These enums are automatically declared as a QMetaTypes (no need to add them in Q_DECLARE_METATYPE anymore).
enums passed to qDebug will print the name of the value rather than the number.
When put in a QVariant, toString() gives the value name. The value name is printed by QCOMPARE (since Qt 5.6)

Taken from WOBOQ blog post on the topic, read it for additional information regarding Q_ENUM vs Q_ENUMS.

Is there a way to use QMetaEnum with Q_ENUMS belonging to non Q_OBJECT or Q_GADGET class?

3 Answers3