On Thu, 10 Sep 2020 at 11:46, Andrei Golubev <andrei.golu...@qt.io> wrote: > > That's interesting. So the container remembers what sort of a reserve > request I made on it, and uses > that as the preferred size whenever the element count of the container > changes? > > Yes. Calling reserve typically means that you do not want to have extra > reallocations while updating the container's content (instead you allocate > sufficient space upfront). In case of QList, we make sure there are no > reallocations happening unless absolutely necessary, e.g. appending 1 element > when size == capacity would allocate bigger memory chunk - reservation is a > hint, but not an obligation. Consequently, QList's shrinking erase also does > not shrink when capacity is reserved.
Well, yeah - calling reserve before growing a container avoids extra allocations. Calling reserve before removing elements doesn't have such a "typical" effect. Should I expect to do a reserve call with a *smaller* value than my current size is before removing elements from a container, in order to set a "target size"? For any standard container, such a reserve is a complete noop because it can't be responded to by shrinking a capacity lower than the size, but with QList it apparently records a preferred size? _______________________________________________ Development mailing list Development@qt-project.org https://lists.qt-project.org/listinfo/development
