[Development] QIntrusiveSharedPointer

Philippe philwave at gmail.com
Fri Aug 9 08:58:39 CEST 2019


Interesting and subtle indeed.

This also relates to constant-ness AFAIU...
When a method is non-const, then we can assume the object can't be concurrently
accessed.
Therefore  we can assume the state can't pass from detached to shared
while the method is executed.

Philippe


On Thu, 8 Aug 2019 15:32:00 -0700
Thiago Macieira <thiago.macieira at intel.com> wrote:

> On Thursday, 8 August 2019 02:43:07 PDT Mutz, Marc via Development wrote:
> > If there _was_ a QIntrusiveSharedPointer::isDetached(), _then_ it would
> > have the same problem as shared_ptr::unique() in that the result becomes
> > meaningless upon return to the caller. Such a method might still useful,
> > but it's not currently proposed; qIntrusiveDetached() is _not_ that
> > function.
> 
> Not really, because of different semantics.
> 
> isDetached() = false can have changed before you make use of the result.
> 
> But isDetached() = true is useful. If an object is detached, it *stays* 
> detached. This allows you to optimise for that case, while still supporting 
> the non-detached case albeit slower.
> 
> The examples are the in-place helper functions in QtCore that are called from 
> rvalue-ref members. Like:
> 
> QByteArray QString::toLatin1_helper_inplace(QString &s)
> {
>     if (!s.isDetached())
>         return qt_convert_to_latin1(s);
>    [... modify s ...]
> }
> 
> -- 
> Thiago Macieira - thiago.macieira (AT) intel.com
>   Software Architect - Intel System Software Products
> 
> 
> 
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