Order of multiple C++ QML_ELEMENT instantiation

stasioz

I am learning Qt 6 Quick for about a month now. I have the following classes:

class Class1 : public QObject {
Q_OBJECT
Q_SINGLETON
Q_ELEMENT
....
};

class Class2 : public QObject {
Q_OBJECT
Q_SINGLETON
Q_ELEMENT
....
};

I understand that when using qmlRegisterSingletonInstance or other methods specified in main.cpp

// main.cpp
...
Class1 instance1;
Class2 instance2;
qmlRegisterSingletonInstance("MyObject1", 1, 0, "MyApi1", instance1.get());
qmlRegisterSingletonInstance("MyObject2", 1, 0, "MyApi2", instance2.get());
...

I can specify the order of the instantiations.

How about when using macro QML_ELEMENT and QML_SINGLETON? Are they instantiated in the order of files inclusion in CMakeLists.txt qt_add_qml_module ? or does it happen during import statements in qml file?

FKosmale

Consider the order to be undefined, and something you shouldn't rely on. To be honest, that's the same as if you were to use qmlRegisterSingletonType using the imperative API; there's no instantiation at the point of the registration, that will happen later when the engine actually needs the singleton (and "needs" is intentionally vague here).

If you have an order-dependent construction, you should (in that order of preferability)
a) consider if you can move away from that pattern, or
b) use https://doc-snapshots.qt.io/qt6-dev/qqmlengine.html#setExternalSingletonInstance once 6.12 is released or
c) keep using imperative registration.