QList shared class behaviour question

[JonB]

I do not have Qt/compiler at hand, so excuse if I am wrong or could just have tested. This is a quick "brainstorm" thought....

As one of Qt's implicitly shared classes I believe something like either of the following is allowed and causes sharing/no copy at least initially:

const QList a = { ... } ;
const QList b = a;

QList a = { ... } ;
QList b = a;

I want a list from the outside world which is read-only to me, I want to create a list from it which I may modify but may not. There will be many of these lists and operations so speed is important. What about:

const QList a = { ... } ;
QList b = a;

Assuming (untested) the second line compiles, does it still start out with a shared-no-copy until I modify it or does copying a const to a non-const do a copy right from the assignment time?

P.S.
While I am here. There isn't a very quick/easy/dirty way to see if a QList is or isn't shared with another instance is there? Like showing the ref count or which list they really each are? (This would be for debugging/performance.) I guess I have to go == to find out, I believe QList== does a "are these the same shared list thing" rather than comparing each item?

[Christian Ehrlicher]

What are you afraid of here? That the 'copy from' list is const? Doesn't matter - the internal ref count is increased no matter if the container is const or not.

No, you can't see the refcount in simple way.

[JonB]

@Christian-Ehrlicher said in QList shared class behaviour question:

That the 'copy from' list is const? Doesn't matter - the internal ref count is increased no matter if the container is const or not.

Yep, just that. That it might have done an actual copy "in advance" when assigning a const to a non-const. So it will indeed still start out shared, I am happy for my purpose where there will be many copies but few modifications.