On Monday, 24 February 2025 11:37:21 Brasilia Standard Time Sebastian
Wittmeier via Std-Proposals wrote:
> what is the relevance of the inheritance from QObject for QPointer?

There are two advantages. One, sizeof(QPointer<T>) == sizeof(T*), so you can
replace an existing plain pointer member with a QPointer and not change the
size or layout of your structure. Not something that the standard cares much
about (it's an ODR violation), but for those of us who maintain libraries with
ABI compatibility, it's important.

Second, it allows a plain QObject to be tracked by a QPointer. It doesn't
require the object being held by a special class à la std::unique_ptr that
effects the deletion, thus allowing tracking of any QObject that may exist and
however the lifetime of that object is implemented. Remember that QObject has
a parent-child tree relationship, where the parent object deletes the
children.

Admittedly, an external holder class would be a superior solution because it
would guarantee clearing of the QPointer trackers *before* object destruction
started, thus it would either be the exact type in question or it would be
clear; right now QPointer can only guarantee it still was a QObject at that
time. But given that we can't make any guarantees of what the state of the
object will be on the next instant, that's not a big deal.

-- 
Thiago Macieira - thiago (AT) macieira.info - thiago (AT) kde.org
  Principal Engineer - Intel DCAI Platform & System Engineering