Is there a way to use Q_DECLARE_METATYPE() with enum class types? I know old enums work but what about these new, strongly typed, ones? Can't find anything regarding this problem elsewhere. I'm using the latest Qt version available.
Example:
enum Foo
{
A,
B,
C
};
Q_DECLARE_METATYPE(Foo)
QVariant var = Foo::A; // works fine
enum class Bar
{
X,
Y,
Z
};
Q_DECLARE_METATYPE(Bar)
QVariant var = Bar::X; // doesn't compile
This is because when you use a plain old enum:
enum Foo { A, B, C };
QVariant var = Foo::A;
the compiler in fact uses the following constructor to build up your var instance:
QVariant(const QVariant& other);
And going further, the other instance is created using the following non-explicit constructor:
QVariant(int val);
This is possible, because old enums can be treated as integral values.
To sum up, this is what the compiler sees and does behind the scene:
int temp = Foo::A;
QVariant var = QVariant(temp);
As you know, enum classes CAN NOT be treated as integral values without an explicit cast. Thus, the compiler cannot implicitly convert your type to int, and invoke a matching constructor (to be precise: the best candidate from all available constructors). That is, there is a predefined set of constructors QVariant offers. You cannot add a new one using Q_DECLARE_METATYPE macro.
To use QVariant with your own type, you should rather use QVariant::fromValue(const T& t) function:
enum class Foo { A, B, C };
QVariant var = QVariant::fromValue(Foo::A);
or alternatively:
enum class Foo { A, B, C };
QVariant var;
var.setValue(Foo::A);