QMetaType alias in Qt6

[crueegg]

I'm migrating a Qt5.15 client/server to Qt6.5. QVariant::fromValue() returns a different type/name for aliased types. The appended sample code returns no longer the typedef name MyClassList in Qt6 with

using MyClassList = QList<MyClass>;
Q_DECLARE_METATYPE(MyClassList)

Output:

Qt 5.15.2
QVariant::fromValue(MyClassList()).typeName=MyClassList
QMetaType(qMetaTypeId<MyClassList>()).name=MyClassList

Qt 6.5.0
QVariant::fromValue(MyClassList()).typeName=QList<MyClass>
QMetaType(qMetaTypeId<MyClassList>()).name=QList<MyClass>

Code:

#include <QCoreApplication>
#include <QList>
#include <QMetaProperty>
#include <QVariant>

class MyClass {};
Q_DECLARE_METATYPE(MyClass);

using MyClassList = QList<MyClass>;
Q_DECLARE_METATYPE(MyClassList);

int main(int argc, char *argv[])
{
  QCoreApplication a(argc, argv);
  qInfo("Qt %s", QT_VERSION_STR);

  QVariant v = QVariant::fromValue(MyClassList());
  qInfo("QVariant::fromValue(MyClassList()).typeName=%s", v.typeName());

  QMetaType mtMcl = QMetaType(qMetaTypeId<MyClassList>());
  qInfo("QMetaType(qMetaTypeId<MyClassList>()).name=%s", qPrintable(mtMcl.name()));

  return 0;
}

• Is there a way QVariant uses the aliased name MyClassList?

• How do I get the Aliased QMetaType?
  These two methods still get the wrong type:
  QMetaType(qMetaTypeId<MyClassList>())
QMetaType(qRegisterMetaType<MyClassList>("MyClassList"))

Thanks in advance

[Asperamanca]

@crueegg
If memory serves, metatypes for any QList<RegisteredType> and similar are now automatically registered by Qt. I suspect that your Q_DECLARE_METATYPE is now simply ignored, since the type is already registered.

[crueegg]

@Asperamanca Q_DECLARE_METATYPE adds an additional QMetaType object with the aliased name to the global metasystem. But the compiler (clang) uses QList<MyClass> for T in QVariant::fromValue(), which results with the wrong QMetaType.

[Asperamanca]

@crueegg
What exactly do you use the typename for?

[crueegg]

@Asperamanca The customized typenames are used in the serialization. Qt5 client doesn't understand QList<MyClass> from the Qt6 server (vs). It only recognises the custom types (MyClassList).

My prototype solution is to customize QVariant::save(QDataStream &s) where I map the typenames.

I still didn't find a method where I can get the MyClassList QMetaType object. The debugger shows me the list with all registered names, but how can I iterate/access a specific one?

[Asperamanca]

@crueegg Maybe qRegisterMetaType can help.

"This function is useful only for registering an alias (typedef) for every other use case Q_DECLARE_METATYPE and qMetaTypeId() should be used instead."

[crueegg]

@Asperamanca The qRegisterMetaTypeof the Q_DECLARE_METATYPE macro is implicitely called in QVariant::fromValue().
I'm wondering when/how this alias is/can be retrieved.

This is the only possible way I found, but I don't like to have strings as types:

QMetaType mtFn = QMetaType::fromName("MyClassList");
qInfo("QMetaType::fromName(\"MyClassList\").name=%s", qPrintable(mtFn.name()));

Output: QMetaType::fromName("MyClassList").name=QList<MyClass>

[T. Kloczko]

@crueegg
Hi, unfortunately, we had to cope with the same issue. The only way to circumvent it was to create a singleton class that handles a mapping between the id of the QMetaType and the custom alias.
See https://gitlab.inria.fr/dtk/dtk-core/-/issues/30 for more details.
Thib.