[Interest] QSharedDataPointer detach too much ( don't call const )

[Michal Lazo]

But for example
http://code.qt.io/cgit/qt/qtbase.git/tree/src/gui/text/qtextcursor.cpp#n1487
in this code I found at least 4 x detach
void QTextCursor::deleteChar()
{
if (!d || !d->priv)    1x detach
return;

if (d->position != d->anchor)            2x detach removeSelectedText();
return; }
{
if (!d->canDelete(d->position))       only 1x detach
return;
d->adjusted_anchor = d->anchor = d->priv->nextCursorPosition(d->anchor,
QTextLayout::SkipCharacters);    1x detach
d->remove();
d->setX();
}

On Thu, Sep 20, 2018 at 1:01 PM Giuseppe D'Angelo via Interest <
interest at qt-project.org> wrote:

> On 20/09/18 12:43, Michal Lazo wrote:
> > "So my whole C++ world just changed."
> > Can you point me to right article in C++ documentation ?
>
> http://eel.is/c++draft/over.match
>
> See in particular [over.match.funcs] §11.3.1.4 , and then you need to
> reason about the implications for the overload resolution.
>
>
> > So Qt devs I found some code for example
> >
> http://code.qt.io/cgit/qt/qtbase.git/tree/src/gui/text/qtextcursor.cpp#n1151
> > And it looks like it should be better to call const variant
> > what do you think ??
>
> Possibly, but what's the big drama about that line?
>
>
> It's not a mistery that QSharedDataPointer is very prone to "accidental"
> detaches, so its usage is frowned upon in new code, in favour of
> QExplictlySharedDataPointer (which however requires manual calls to
> detach(), and one needs to remember about them).
>
> For instance, in a setter such as
>
> > void MyClass::setFoo(int newFoo) {
> >     if (d->foo == newFoo)
> >         return;
> >     d->foo = newFoo;
> > }
>
> (assuming d is a QSharedDataPointer<MyClassPrivate>) the d->foo access
> in the if is supposed to be "readonly", yet it happily detaches the
> private, even if the new value passed to the setter is identical to the
> old value and thus one might expect that nothing needs to be done.
>
> (No, noone is porting QTextCursor away from QSharedDataPointer at this
> moment in time.)
>
>
> My 2 c,
> --
> 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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