<![CDATA[Qobject_cast triggers an assertion on QML components [SOLVED]]]>I'm using this code to iterate through the children of my custom object (C++, derived from QObject) and check for a certain type:

@
foreach (QObject *o, children()) {
Foo f = qobject_cast<Foo>(o);
if (f) {
// do foo stuff with f
}
}
@

This works great as long as there are only C++ objects in children(). However, when I make a component that extends a "Foo", like this:

@import FooLib 1.0

Foo {
...
}
@

qobject_cast<Foo *> triggers an assertion failure:

@ASSERT: "!dp->isFunction()" in file qml/qqmlpropertycache.cpp, line 1509@

Here's a stack trace:

@148 raise() 0x0000003562a32885
147 abort() 0x0000003562a34065
146 qt_message_fatal() qlogging.cpp:806 0x00007ffff6758da9
145 QMessageLogger::fatal() qlogging.cpp:355 0x00007ffff6756ffa
144 qt_assert() qglobal.cpp:1951 0x00007ffff675233c
143 QQmlPropertyCache::toMetaObjectBuilder() qqmlpropertycache.cpp:1509 0x00007ffff7c21db5
142 QQmlPropertyCache::createMetaObject() qqmlpropertycache.cpp:528 0x00007ffff7c1dc69
141 QQmlVMEMetaObject::toDynamicMetaObject() qqmlvmemetaobject.cpp:539 0x00007ffff7b66944
140 QObjectData::dynamicMetaObject() qobject.cpp:189 0x00007ffff69732f4
139 ShotgunEntity::metaObject() moc_shotgunentity.cpp:100 0x00007fffec20e16b
138 QMetaObject::cast() qmetaobject.cpp:338 0x00007ffff6941eda
137 qobject_cast<Property*>() qobject.h:451 0x00007fffec1d03b6
136 NodeBase::inputs() nodebase.cpp:364 0x00007fffec1cf758 @

Am I doing something wrong?

thanks,
-Mark

]]>https://forum.qt.io/topic/21176/qobject_cast-triggers-an-assertion-on-qml-components-solvedRSS for NodeSun, 09 Aug 2026 01:30:46 GMTTue, 06 Nov 2012 22:54:02 GMT60<![CDATA[Reply to Qobject_cast triggers an assertion on QML components [SOLVED] on Wed, 07 Nov 2012 19:32:35 GMT]]>Solved!

I figured it out after being unable to reproduce it in a simplified test case.

The base class of Foo has a default property called "data", patterned after QQmlItem. I had forgotten this and added an invokable method, also called "data", to Foo. In concrete instances of Foo, this wasn't a problem because they had different method signatures, but as soon as I started using QML instances, the invokable clobbered the default property.

]]>https://forum.qt.io/post/156328https://forum.qt.io/post/156328Wed, 07 Nov 2012 19:32:35 GMT<![CDATA[Reply to Qobject_cast triggers an assertion on QML components [SOLVED] on Wed, 07 Nov 2012 14:39:10 GMT]]>bq. My guess is that in QML, inheritance works differently, for example through composition.

Good point, I think they're all internally instances of QQmlComponent or some such. I'll have to figure out a different way to check if a QObject conforms to a particular interface.

bq. The QMetaObject is then created on demand (In this case the qobject_cast).

Well, it tries to create the metaobject on demand, anyway. I'm not sure why, but QML component objects do not have an associated QMetaObject. I discovered this when trying to see how hard it would be to add support for native QML attached properties objects (answer: hard enough that I gave up).

I'll whip up a small test case when I have a chance and file a bug report.

So bug aside, anyone know how to check if a QML instance conforms to an interface?

edit: I tried guarding the qobject_cast with
@if (o->inherits("Foo"))@

The qobject_cast still triggers an assertion for QML instances "derived" from Foo.

]]>https://forum.qt.io/post/156302https://forum.qt.io/post/156302Wed, 07 Nov 2012 14:39:10 GMT<![CDATA[Reply to Qobject_cast triggers an assertion on QML components [SOLVED] on Wed, 07 Nov 2012 14:18:27 GMT]]>This seems to be a bug in QML. The meta data from QML Components is stored in a structure called QQmlPropertyCache. The QMetaObject is then created on demand (In this case the qobject_cast).

The creation of the QMetaObject seems to fail/assert in your case.

]]>https://forum.qt.io/post/156299https://forum.qt.io/post/156299Wed, 07 Nov 2012 14:18:27 GMT<![CDATA[Reply to Qobject_cast triggers an assertion on QML components [SOLVED] on Wed, 07 Nov 2012 05:42:47 GMT]]>This is because your QML type is not "Foo". It's some internal type like QMLTYPE_123. You can check

@
Foo {
id: object
Component.onCompleted: console.log(object)
}
@

My guess is that in QML, inheritance works differently, for example through composition.

]]>https://forum.qt.io/post/156243https://forum.qt.io/post/156243Wed, 07 Nov 2012 05:42:47 GMT