[Development] The future of smart pointers in Qt API

[André Pönitz]

On Tue, Feb 04, 2020 at 07:17:13PM +0200, Ville Voutilainen wrote:
> On Tue, 4 Feb 2020 at 18:48, Volker Hilsheimer <volker.hilsheimer at qt.io> wrote:
> > >>> But that just means that QBoxLayout::addWidget(std::unique_ptr<>) has to
> > >>> behave imho saner in that edge case and actually delete the widgets that
> > >>> were added but never parented. (And I should really implement that to show
> > >>> how it would work.)
> > >>
> > >>
> > >> That seems like a terrible idea. One of the layout’s jobs is to set up the
> > >> parent/child hierarchy so that you as a developer don’t have to.
> > >
> > > I don't see how that idea changes that.
> >
> >
> > Iif the layout deletes widgets that were created without parent, instead of
> > adding it to the correct parent, then that’s a change. You have to create
> > widgets with parents, and add them to the layout (which might then add it to
> > another parent, which at the very least introduces unnecessary overhead of
> > ChildAdded/Removed event processing).
> 
> Right; the layout shouldn't insta-kill parentless children without trying to
> reparent them first.
> 
> > >> Do we really want to make it harder for people to write UIs just because
> > >> the resulting code doesn’t satisfy a C++ idiom?
> > > It's not a question of satisfying idioms for the sake of satisfying idioms,
> > > it's a question of avoiding bugs that we know based on decades of experience
> > > to be difficult to avoid with raw pointers.
> > That’s my point: I do not believe that the decades of experience we have with
> > QObject’s parent/child model support the claim that it is inferior to using
> > smart-pointers. It’s not something people constantly struggle with.
> 
> I have a completely polar opposite experience, it's the main source of memory
> leaks and double deletes in Qt application code. 

> [...]

> Your experience suggests that this is not something people constantly
> struggle with, my experience suggests that it's the #1 top-priority bug in Qt,
> hands down, no competition.

*shrug*

I am with Volker here.

Seriously, *double deletion*? In real, proper Qt-style code?

The only problem is with people passing out when seeing different numbers of 'new'
and 'delete'. And it's only a *real* problem when do not they take "Relax, it's ok"
as an answer, and start to actively interfere. And that's when double deletion
comes into play.

> > If you understand Qt (and don’t assume that by understanding some basic C++
> > concepts you can use any framework), then you see that widgets are added to a
> > layout that is used, and are therefore managed objects.
> 
> Managed by what? Also, it seems downright detrimental if understanding basic C++
> concepts is useless with Qt.

I don't think that's received as much of an insult as you meant it to be
by parts of your audience. Not requiring the verbosity of "basic C++" in
user code is surely a benefit of Qt for me, and if people are appalled by
not getting that with Qt, well, not my problem.
   
Andre'