[Development] QIntrusiveSharedPointer

[Allan Sandfeld Jensen]

On Thursday, 8 August 2019 11:33:04 CEST Philippe wrote:
> > No. When you call detach(), you're in a mutating function of the object
> > and you can assume that you're the only thread executing on a member
> > function of that thread, because if that's not the case, you're in UB
> > land. Only const methods are allowed to execute concurrently on the same
> > object. Mutators must get exclusive access.
> 
> Absolutly right, but a  method "isDetached()" which is const
> has no usefulness on its own, independently from a detach() procedure.
> This is the purpose of my orginal remark.
> 
It has uses when combined with move-like semantics, when it is detached it 
makes sense to perform in-place transforms on the "shared" data, but it if 
isn't that would trigger two iterations over the data, first the detach 
triggering a copy, and then the iteration over the data transforming it. If it 
is shared, you can just iterate over the data read-only putting the result 
into a new object. This is used many places with QImage.

Note though that QImage has a second use of isDetached() since QImage detach() 
also copies user provided data even when not shared.

'Allan