Snippet [22] was unused and the example using snippet [21] neglected
to show how its oldCategoryFilter got initialized, which is what [22]
does. But it turns out the example code was crashy in any case, as it
left the oldCategoryFilter uninitialized (albeit probably null) until
the call to installFilter() had returned, and installFilter() calls
the new filter, so the new filter needs to check oldCategoryFilter
before calling it. The doc also failed to explain why it's OK to not
defer to the prior filter in these calls during installFilter(), so
revise the doc and example so that the latter behaves sensibly and
readers of the former are likely to use the function safely.
This amends commit 8f0654ceb8 which
added the snippets and the use of one of them, but not the other.
Fixes: QTBUG-49704
Pick-to: 6.4 6.3 6.2 5.15
Change-Id: Iddb8d97b0bef417d8f16e7910730cfa59ea3e715
Reviewed-by: Kai Koehne <kai.koehne@qt.io>
6ae2bfad41