[Development] Why does QLabel::pixmap() return `const QPixmap*`?

[Konstantin Ritt]

Null pixmap actually means "no pixmap" (though there is a QPixmap instance,
with null pixmap data). Clearly, the API should be changed to QPixmap
QLabel::pixmap().

Regards,
Konstantin

2014-11-30 22:21 GMT+04:00 Yves Bailly <yves.bailly at laposte.net>:

> On 30/11/2014 09:35, Olivier Goffart wrote:
> > On Sunday 30 November 2014 06:37:11 Kevin Kofler wrote:
> >> Sze Howe Koh wrote:
> >>> I'm curious about the rationale behind this API design. I can't think
> >>> of any other Qt function that returns an implicitly-shared object by
> >>> pointer, so this seems inconsistent. e.g. QWidget::font() returns a
> >>> QFont by const-ref.
> >>
> >> Probably because the pointer can be null. C++ does not allow null
> >> references. (The compiler might let you get away with it, but in
> principle,
> >> it is not allowed.)
> >
> > That's not the reason because you can return a null pixmap
> (QPixmap::isNull).
>
> However a null pixmap and no pixmap at all are two different things :-)
>
> QPixmap const* pix = label->pixmap();
> if ( pix == nullptr )
> {
>    // this label has no pixmap
> }
> else if ( pix->isNull() )
> {
>    // this label does have a pixmap, which is null/empty
> }
> else
> {
>    // etc.
> }
>
> Because of the different meanings, the API seems sensible to me.
>
> Regards,
>
> --
> (o< | Yves Bailly                          | -o)
> //\ | Linux Dijon  : http://www.coagul.org | //\
> \_/ |                                      | \_/`
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