Same thing, re-add my copyright line.

There's a sizeable code just being moved, you're supposed to keep my copyright line too. ;-)

Missing space before ":", but I don't see why it would need to inherit from QObject in the first place.

Also I'm not sure I like the name since a) it is not an interface and b) it is not only for caching. :-)

MemoryStore?

Try to avoid changing the signature of those and avoid making them virtual. It seems to be only about change notification, so I would propose something like:

• Introduce private virtuals like onCollectionCreated(col) and friends which do nothing;
• Have them called at the end of the public methods;
• In the sub-class just overload the onCollectionCreated() and friends.

ditto.

ditto.

The virtual isn't needed here.

Space before & note after.

No need to move that to MemoryStore. I think those method can (and should) stay in the fake data. Their implementation might need to be adjusted a tiny bit of course.

Same for those, they shall not be in MemoryStore.

Space before & not after.