On 09/09/2020 10:55, Andrei Golubev wrote:
Small update on QList::erase: the iterator invalidation model will be aligned with std::vector's erase (in progress of merging the change to dev and it got approved already). This means that, when erasing, only the iterators that correspond to erased elements and elements after them (until end()) will be invalidated*. The corner case is erasing from the beginning where formal definition seems to allow right-shifting erasure (feel free to prove me wrong) and the only concern I have is when someone holds QList::constData()/data() pointer, since this will be invalidated in the corner case.
data() is invalidated if one erases begin(), so no problem.
*- except when detach is necessary or shrinking happens
This is another against automatic shrinking; it may invalidate everything rather than just the erased area. I don't understand the "detach" part, is it about holding references across a detach, so they refer to the original container, not the detached one? That's not different from we always had, isn't it?
As to whether we should or should not shrink when erasing too many elements, there are clearly two opposite opinions (see the "replied to" conversation). My assumption would be that shrinking in erase has some justification (otherwise, why do it in the first place?). Thus, there are already Qt-relevant cases that benefit from having smaller memory footprint. On the other hand, "please do not free memory, I still need it" use-case is also justified. However, chances are that when you really need a certain memory to be allocated/preserved, there is a call to QList::reserve() prior to insertions/deletions. And since we do not shrink in case of reserved memory, users should be safe in this case, in general.
Are the new behaviors autotested for all the leaf classes, incl. things like detaching a reserve()d container? The usual problem is that the code duplication in QVector/QString/QByteArray gave them completely different behaviors (there's a number of bug reports open).
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