On 16/01/2023 12:21, Johannes Schaub via Std-Discussion wrote:
> For example, can it be used in Qt for the following?
>
> |gsl::owner<QWidget*> w{new QWidget{parent}} |
>
> In this example, the ownership is shared by the new-site and the
> |parent|, because the code who has |new|-ed the object can |delete| |w|,
> and |W|'s destructor will take itself out of |parent| children list.
> However, if |w| is not deleted at the new-site, then |parent| will
> delete it in its destructor. It is not enforced by the type system that
> there are no new-site users left, so I'm not sure about the ownership
> kind here.
>
> 1. Is this an example of shared ownership, and
> 2. can |gsl::owner| be used for it?
>

We had quite some debates in Qt about how to properly mark these
pointers and I don't think we ended up with a decent outcome.


But gsl::owner is the wrong choice here. From the guidelines:

> https://isocpp.github.io/CppCoreGuidelines/CppCoreGuidelines#SS-views

> owner<T*> // a T* that owns the object pointed/referred

> owner is used to mark owning pointers in code

> An owner<T> differs from a resource handle for a T by still requiring an explicit delete.

The problem is that you're not owning `w`. If a static analysis tool
doesn't see a further ownership transfer of `w` (but you just drop it on
the floor) nor a delete, then the tool is allowed to emit a diagnostic
-- you're leaking it! But that's false.

In my experience with big Qt codebases, I'd even claim that you could
use a raw pointer there, with the intent of express _lack_ of ownership.
While `delete w` is perfectly legal and does the right thing (creating
the interesting ownership model that you mention), in practice no one
does it.

I'm not sure if I'd call it "shared ownership", as normally one uses
that term to refer to scenarios (shared_ptr) where as long as there is
at least one owner, a resource can't be destroyed. This looks more like
a "weak shared ownership" model, where one owner can destroy a resource
at any time, but the other owners will know about it and don't use it
any more / don't double delete.


To compound the effect, the Qt APIs give developers tremendous amounts
of leeway but ultimately end up "doing the right thing". I don't think
there can be a universal approach here without also bringing in some
strict coding guidelines for your own project. Examples:


1)

   QWidget *parent = new QWidget;
   QWidget *w = new QWidget;
   w->setParent(parent); // "ownership transfer" of `w`, after
construction. What do you decorate with gsl::owner here?


2)

   QWidget *parent = new QWidget;
   QWidget *w = new QWidget;

   QLayout *l = new QVBoxLayout;
   parent->setLayout(l); // "ownership transfer"

   l->addWidget(w); // `w`'s ownership is transferred to `parent`,
                    // not to `l`!

   delete l; // OK. `w` still owned by `parent`

3)

   QWidget *parent = new QWidget;
   QWidget *w = new QWidget;

   QLayout *l = new QVBoxLayout;

   l->addWidget(w); // `w` still without an owner

   parent->setLayout(l); // `l` and `w`'s ownerships get both transferred



My 2 c,

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Giuseppe D'Angelo | giuseppe.dangelo_at_[hidden] | Senior Software Engineer
KDAB (France) S.A.S., a KDAB Group company
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