Struggling with signal which has a default parameter.

[JPNaude]

Ok, I've made the slot public and rebuilt. Still I get a null everytime.

From a brief look I see you are emitting layoutChanged() from your slot, and that one is with a default value, which would explain why in the debugger you see first the one with a non NULL object, and then one with NULL.
You have a recursion there.

I did mention it but probably not clear enough, the signal and slot are on different classes. Thus the layoutChanged() I emit in the slot is not the same, it is layoutChanged() on QAbstractItemModel.