[Development] Circular dependencies for Q_PROPERTY

Simon Hausmann Simon.Hausmann at qt.io
Mon Apr 27 12:08:48 CEST 2020


I think the solution may involve keeping forward-declarations but adding visibility to the "other" types in moc generated code by using Q_MOC_INCLUDE in A to include B and vice versa. Does that make sense?

From: Development <development-bounces at qt-project.org> on behalf of Mike Krus via Development <development at qt-project.org>
Sent: Monday, April 27, 2020 11:52
To: Qt Development Group <development at qt-project.org>
Subject: [Development] Circular dependencies for Q_PROPERTY


I have 2 classes, A and B, derived from QObject, each have a property
of type pointer-to-other-class. So

class A : public QObject {
Q_PROPERTY(B *foo …)


class B : public QObject {
Q_PROPERTY(A *foo …)

Because of the circular dependency, I can’t #include the full class
definition, just do forward declaration.

Now this fails in Qt 6, seems the moc generated code needs the
full class declaration.

Looking at generated moc code, looks like creating the meta object
requires a qt_metaTypeArray<A*, bool> which checks that A derives
from QObject by calling  IsPointerToTypeDerivedFromQObject
which uses sizeof() which requires the full type.

Any way around this? Seems like a rather big regression compared
to Qt 5…


Mike Krus | mike.krus at kdab.com | Senior Software Engineer
KDAB (UK) Ltd., a KDAB Group company
Tel: UK Office +44 1625 809908   Mobile +44 7833 491941
KDAB - The Qt Experts, C++, OpenGL Experts

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