search

rust-lang / wg-allocators

Home of the Allocators working group: Paving a path for a standard set of allocator traits to be used in collections!
http://bit.ly/hello-wg-allocators
203 stars 9 forks source link

Question: Should Collection::clone_from clone the allocator or preserve its own? #115

Closed yanchith closed 10 months ago

yanchith commented 10 months ago

Should Collection::clone_from(&mut self, other: &Self) keep the original allocator, or should it clone it from other? The way the trait is defined allows for cloning (A: Clone), but on some deeper level this makes me uneasy. I have always interpreted clone_from as "reuse memory from self, but clone the data from other", and I wouldn't expect a collection's allocator to ever change once it is initialized.

Had a cursory look at liballoc and hashbrown, and neither seems to clone the allocator? On the other hand, there's nothing that formally prevents them from doing so?

The more I think about this, the more I think that it should not clone. If nothing else, if the two allocators are actually different, they wouldn't know what to do with each other's pointers for resizing/deallocating. Maybe this is worth documenting?

Context: I am writing a BucketArray style collection (sometimes also called a Slab) to help with growing behavior on bump allocators.

zakarumych commented 10 months ago

If memory gets reused, the allocator must be kept. No questions here.

Amanieu commented 10 months ago

I would say that it is "implementation defined": the collection can either reuse the existing allocator, or clone the allocator from the source. The latter can happen when deriving Clone (#98374) since that will use the default implementation of clone_from which simply calls clone and replaces the existing value.

yanchith commented 10 months ago

Thank you for both of your answers. This totally makes sense, and was an interesting exercise to think about. I ended up not cloning the allocator in clone_from, as that would almost certainly be incorrect in my case.

Closing this. Hopefully it will be useful to someone in the future.