[Development] unique_ptr and Qt, Take 2

Matthew Woehlke mwoehlke.floss at gmail.com
Thu Jun 13 23:36:48 CEST 2019

On 13/06/2019 16.09, Ville Voutilainen wrote:
> The minor problem being that to not so small audiences,
> new Whoosh(something);
> just looks like a bug, and then you need to look three times to
> realize that something is a parent.

Sure... but `something.createChild<Whoosh>()` can help with that.

OTOH, I just realized a problem with that... when the new child needs to
take its parent in the ctor for other reasons. I don't know if there is
an easy solution to that. (Of course, you can still pass the parent with
createChild, but then you've violated DRY.)

>> That's one of the things I love about Qt; object hierarchies give me
>> working dynamic memory management without needing even smart pointers.
> That's the one thing that makes me queasy about using Qt in large 
> applications; I always need to worry about ownership relationships,
> because I can't program with smart pointers.
I guess your mileage varies. My experience has been the opposite.

To be fair, I expect a lot of that is "instinctive"... that is, when you
come from a background where not using smart pointers is scary, the Qt
way is probably going to rub you the wrong way, even though it's nowhere
near as dangerous as your experience causes you to believe. Conversely,
being comfortable with Qt and its minimal need for smart pointers likely
produces an aversion to sprinkling them everywhere "because it's safer".

> [...] the largeish application was leaking like a sieve and doing
> use-after-free in all too many places.

QPointer is great for avoiding this :-). (Also, properly "owned"
signal/slot connections.)

> On Thu, 13 Jun 2019 at 22:54, Matthew Woehlke wrote:
>> My "concern", and possibly why everyone fixated on the unique_ptr stuff,
>> is how wide-sweeping the necessary API changes will be. Modifying every
>> method, everywhere, that reparents a QObject is a little intimidating.
>> (Maybe you have some WIP that can put this into perspective?)
> Like https://codereview.qt-project.org/c/qt/qtbase/+/260618 ?

Uh... yeah, I suppose. And... yeah. Yike. That's a scary (or at least
intimidating) amount of API churn. In particular, there are enough files
being touched to strongly discourage me even looking at the diffs...


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