[Development] parented_ptr

[Daniel Schürmann]

On Wed, Nov 5, 2025 at 9:10 AM Arno Rehn <a.rehn at menlosystems.com> wrote:

> On 05.11.2025 08:22, Daniel Schürmann wrote:
> > However I am not confident that this bullet proved enough for being
> > integrated in the upstream library.
> > * It works by guessing the parent argument which can fail at runtime
> > without even a compiler message, in users code.
> > * It does not yet (<x++23) support constructors requiring a sub
> > class parent like QWidget or any other user defined sub class.
> > * We can make it more robust by constructing without parent and then
> > call setParent() causing some extra calls and bookkeeping. I
> > don't want that.
>
> Honestly I think you're over-engineering this somewhat. "parent comes
> last" is an established convention when dealing with QObject-derived
> classes. QWidgets are the same, see for example QPushButton:
>
>      QPushButton(const QIcon &icon, const QString &text, QWidget *parent)
>
> Just forward the args and append "this" as the last parameter to pass
> the parent along.
>
> Re subclass parents: You don't actually need C++23 for that. We can
> (ab-)use a templated implicit conversion operator to auto-cast a type to
> the one required by the constructor:
>
> template<class Q>
> struct autocast
> {
>      Q *v;
>      template<class T>
>      operator T*() const { return static_cast<T*>(v); }
> };
>
> template<class T, class... Args>
> QParentedPtr<T> createChild(Args&&... args) {
>      return new T(std::forward<Args>(args)..., autocast<QObject>{this});
> }
>
> Full example on godbolt: https://godbolt.org/z/T8h6hWWzP
>
> Granted, this would allow someone to do
>
> QObject o;
> o.createChild<QWidget>();
>
> which would likely crash and burn at runtime, so there are tradeoffs..
>
> Cheers,
> Arno
>
> --
> Arno Rehn
> Principal Software Engineer
> Tel +49 89 189 166 0
> Fax +49 89 189 166 111
> a.rehn at menlosystems.com
> www.menlosystems.com
>
> Menlo Systems GmbH
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>

Hi Arno

"parent comes last" seems to be only a rule of thumb unfortunately. See
here:

https://github.com/search?q=repo%3Aqt%2Fqtbase+%22parent+%3D+nullptr%2C%22&type=code

Your autocast code is a nice Idea. It has hower no safety net for
incompatible casts:
See https://godbolt.org/z/f6fdrKddh

I have messed around with some static asserts, but I struggle with getting
the original type of "this"
qobject_cast() works at runtime.
Do you have an idea to do it at compile time?

Best regards,

Daniel
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