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

[Giuseppe D'Angelo]

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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