[Interest] Automatic deletion of children of QObject

[Jochen Becher]

As children are stored in a simple list and new children will be
appended to its end it would be very simple to delete them in reverse
order: Simply traverse the list from count()-1 to 0. This would be a
one-line-change in method deleteChildren().

Of course if I say "delete it in reverse order of creation" I mean:
"delete it in reverse order of being added to its parent". So if you
remove an object and re-add it to the same or another object it will be
deleted earlier.

I changed my code to use its own list to maintain the children just for
deletion within the order I need.

Regards, Jochen


Am Dienstag, den 12.06.2012, 20:45 +0200 schrieb Andreas Pakulat:
> Hi,
> 
> On Tue, Jun 12, 2012 at 8:37 PM, Jochen Becher <jochen_becher at gmx.de>
> wrote:
>         Hi,
>         
>         Think of the following code:
>         
>         
>         QObject *parent = new QObject();
>         QObject *childA = new QObject(parent);
>         QObject *childB = new QObject)parent);
>         
>         delete parent;
>         
>         
>         In which order would you expect childA and childB gets
>         deleted? I would
>         expect childB first and childA second. This would be same
>         behavior as in
>         using normal variables:
>         
>         QObject parent;
>         QObject childA(&parent);
>         QObject childB(&parent);
>         
>         Here, on leaving the scope childB would be deleted first,
>         childA second
>         and parent last.
>         
>         Isn't it a bug in Qt to delete children in the same order
>         there were
>         added to a parent instead of the reverse order?
> 
> 
> No, Qt makes no such guarantees. In fact it doesn't guarantee any
> specific order of destruction for the children, if you rely on any
> order here, your code is buggy.
> 
> 
> Its also not possible to actually do this, since you can add/remove
> childs from the list during the runtime of the program and the QObject
> cannot know wether the last-added object is really the one to be
> destroyed first (in your ideal scenario) or wether its maybe the one
> created first but was removed and re-added later.
> 
> 
> Since QObject keeps an ordered list of its children, I think its
> reasonable that they simply run over it from beginning to end and
> destroy the childs instead of going through the extra hoop of running
> from end to beginning.
> 
> 
> Andreas