Template Q_GADGET with Q_ENUM

St.Stanislav

Hello everyone!

I'm having deal with the enums that should be accessible in QML, so I create one class for a one enum in the following way:

class DisplayInterfaceType {
    Q_GADGET
public:
    enum Value {
        UNDEFINED = -1,
        DIGITAL,
        ANALOG
    };
    Q_ENUM(Value);

    static void registerQML() {
        qmlRegisterUncreatableType<DisplayInterfaceType>("App.Enums", 1, 0, "DisplayInterfaceType", "DisplayInterfaceType");
        qRegisterMetaType<DisplayInterfaceType::Value>("DisplayInterfaceType::Value");
    }

    static DisplayInterfaceType::Value fromInt(int value) {
        QMetaEnum metaEnum = QMetaEnum::fromType<DisplayInterfaceType::Value>();

        const char* ptr = metaEnum.valueToKey(value);
            if (ptr != nullptr) {
            bool result = false;
            metaEnum.keyToValue(ptr, &result);
            if (result) {
                return static_cast<DisplayInterfaceType::Value>(value);
            }
        }
        return DisplayInterfaceType::Value::UNDEFINED;
    }
};

But now I have a lot of enums created in this way. So every time I need to create one more class, copy already implemented, change class name, change strings in registration method and add necessary enums. Can this process be simplified? I thought about a template inheritance, but Q_GADGET cannot be derived. So my question is: can adding these enums be simplified?

VRonin

Seems to me what you need is actually a macro:

#define REGISTERED_ENUM(ClassName, EnumName) \
Q_ENUM(EnumName); \
static void registerQML() { \
    qmlRegisterUncreatableType<ClassName>("App.Enums", 1, 0, #ClassName, #ClassName); \
    qRegisterMetaType<ClassName::EnumName>(#ClassName"::"#EnumName); \
}

class DisplayInterfaceType {
    Q_GADGET
public:
    enum Value {
        UNDEFINED = -1,
        DIGITAL,
        ANALOG
    };
    REGISTERED_ENUM(DisplayInterfaceType,Value);
    static DisplayInterfaceType::Value fromInt(int value) {
        QMetaEnum metaEnum = QMetaEnum::fromType<DisplayInterfaceType::Value>();

        const char* ptr = metaEnum.valueToKey(value);
            if (ptr != nullptr) {
            bool result = false;
            metaEnum.keyToValue(ptr, &result);
            if (result) {
                return static_cast<DisplayInterfaceType::Value>(value);
            }
        }
        return DisplayInterfaceType::Value::UNDEFINED;
    }
};

@St-Stanislav said in Template Q_GADGET with Q_ENUM:

Q_GADGET cannot be derived.

I don't think this is correct.

KroMignon

@St-Stanislav said in Template Q_GADGET with Q_ENUM:

But now I have a lot of enums created in this way. So every time I need to create one more class, copy already implemented, change class name, change strings in registration method and add necessary enums. Can this process be simplified? I thought about a template inheritance, but Q_GADGET cannot be derived. So my question is: can adding these enums be simplified?

You could create on class who centralize all enums.
Then you only have to register this one.

raven-worx

@St-Stanislav
you can place the enums in a namespace and register the namespace in QML:
https://doc.qt.io/qt-5/qqmlengine.html#qmlRegisterUncreatableMetaObject

St.Stanislav

@raven-worx Thank you for the answer! Can the conversion method (fromInt in my example) be implemented within this approach? I mean it looks like I cannot implement one conversion method to serve every enum in the defined namespace. Moreover, can this approach be used for registering every enum as a meta type? I have read somewhere that Q_ENUM and Q_ENUMS (maybe Q_ENUM_NS as well) should register it as a meta type. But I remember that I have faced some cases when I needed to implement it directly, because there were some errors (don't remember details, though).

raven-worx

@St-Stanislav
see https://doc.qt.io/qt-5/qobject.html#Q_NAMESPACE
For slots / invokable methods you need a QObject

St.Stanislav

@raven-worx Yes, I know about QObject, but can not get, how can it help in this case.

Anyway, I have found some kind of a workaround with making one base Q_GADGET class with Q_ENUM and template method to register every derived class.

VRonin

Seems to me what you need is actually a macro:

#define REGISTERED_ENUM(ClassName, EnumName) \
Q_ENUM(EnumName); \
static void registerQML() { \
    qmlRegisterUncreatableType<ClassName>("App.Enums", 1, 0, #ClassName, #ClassName); \
    qRegisterMetaType<ClassName::EnumName>(#ClassName"::"#EnumName); \
}

class DisplayInterfaceType {
    Q_GADGET
public:
    enum Value {
        UNDEFINED = -1,
        DIGITAL,
        ANALOG
    };
    REGISTERED_ENUM(DisplayInterfaceType,Value);
    static DisplayInterfaceType::Value fromInt(int value) {
        QMetaEnum metaEnum = QMetaEnum::fromType<DisplayInterfaceType::Value>();

        const char* ptr = metaEnum.valueToKey(value);
            if (ptr != nullptr) {
            bool result = false;
            metaEnum.keyToValue(ptr, &result);
            if (result) {
                return static_cast<DisplayInterfaceType::Value>(value);
            }
        }
        return DisplayInterfaceType::Value::UNDEFINED;
    }
};

@St-Stanislav said in Template Q_GADGET with Q_ENUM:

Q_GADGET cannot be derived.

I don't think this is correct.

St.Stanislav

@VRonin Thank you for your MACROS version, I thought about it, but firstly have tried templates. I will check your suggestion, thanks again. My template solution is:

template<typename T>
static void registerQML() {
	QString typeName = QString(typeid (T).name()).split(" ").last();

	qmlRegisterUncreatableType<T>("App.Enums", 1, 0, typeName.toLocal8Bit().data(), typeName);
	qRegisterMetaType<T::Value>(QString("%1::Value").arg(typeName).toLocal8Bit().data());
}

template<typename T>
static auto fromInt(int value) {
	QMetaEnum metaEnum = QMetaEnum::fromType<T::Value>();

	const char* ptr = metaEnum.valueToKey(value);
	if (ptr != nullptr) {
		bool result = false;
		metaEnum.keyToValue(ptr, &result);

		if (result) {
			return static_cast<typename T::Value>(value);
		}
	}
	return T::UNDEFINED;
}

@VRonin said in Template Q_GADGET with Q_ENUM:

I don't think this is correct.

Yes, I found my typo, I meant "templated" :)

VRonin

@St-Stanislav said in Template Q_GADGET with Q_ENUM:

Yes, I found my typo, I meant "templated" :)

There is a workaround for this: https://github.com/woboq/verdigris but I don't think it's worth it in your case