[Development] unique_ptr and Qt, Take 2

Иван Комиссаров abbapoh at gmail.com
Sat May 4 14:54:57 CEST 2019


That’s exactly the problem we’re discussing here.

Consider this example (was writing without a compiler, might be typos)
class MyWidget : public QWidget
{
    QWidget container; // note, doesn’t require explicit parent, layout handles that
    QLabel label;
public:
    Widget(QWidget *parent) : QWidget(parent), childWidget(&container)
    {
       auto layout = new QMyLayout(this);
       layout->addWidget(&container); // is ownership transferred here?
       label.setText(«Hello»);    
    }
};

Who owns label? Is the ownership truly unique here? Is it shared? It’s something in-between - container thinks that it owns a label but in fact it does not, the real owner is MyWidget.

Note, that this worked for ages and people are using this idiom to save one extra allocation, plus it requires less typing. And this is intuitive. But that doesn’t exactly the «QObject deletes his children» idiom.

Maybe we need something like «maybe owner pointer» which has a ptr to an object and bool if it should delete it or not. This pointer can be constructed from reference (aka «on a stack») with own=false. However, the semantics and the API of such pointer is not clear for me, it’s easy to end up with another auto_ptr, so confider this as a crazy idea.

> 4 мая 2019 г., в 0:36, Giuseppe D'Angelo <giuseppe.dangelo at kdab.com> написал(а):
> 
> Il 04/05/19 00:33, Иван Комиссаров ha scritto:
>> Back to the topic - in case we really want to support the "stack case", I’d say shared_ptr is the only option (in combination with enable_shared_from_this which is already a kinda part of QObject, as mentioned above)
> 
> QObject ownership isn't shared, it's unique. A widget is in ONE hierarchy, which fixes its owning native window, its coordinate system, its Z order amongst its siblings; not N hierarchies.
> 
> -- 
> Giuseppe D'Angelo | giuseppe.dangelo at kdab.com | Senior Software Engineer
> KDAB (France) S.A.S., a KDAB Group company
> Tel. France +33 (0)4 90 84 08 53, http://www.kdab.com
> KDAB - The Qt, C++ and OpenGL Experts
> 



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