A Rust crate for a generational map, handle map, whatever you want to
call it. Whatever it is, this is a random-access data structure that
stores an unordered bag of items and gives you a handle for each
specific item you insert. Looking things up by handle is
the backing storage is just a
Vec -- and items can be removed as
well, which is also
O(1). Handles are small (two
easy to copy, similar to a slice. The trick is that there is a
generation number stored with each item, so that a "dangling"
handle that refers to an item that has been removed is invalid.
Unlike array indices, if you remove an item from the array and a new
one gets put in its place, the old stale handle does not refer to the
new one and trying to use it will fail at runtime.
This is useful for various things: managing lots of uniform things with shared ownership (such as video game resources), interning strings, that sort of thing. Essentially by using handles to items you have runtime-checked memory safety: you can get an item from the map using the handle, since it's basically just an array index.
In practice this is not unrelated to
Rc, it's just that
the accounting of memory on cloning the
Rc, and this does it on
"dereferencing" by looking up an object's handle. Referencing
counting, threading garbage collection and this sort of map are all
different methods of achieving the same thing (checking for memory
safety at runtime) with different tradeoffs. With a reference count
you don't have to explicitly free items and can't have stale handles,
but loops require special handling and you pay for the accounting cost
on cloning a handle. With this you have to free items explicitly, but
stale handles can be safely detected and the accounting happens when
you look the item up. You can also use it as a slab allocator type
thing, where you pre-allocate a large amount of storage and free it
all at once.
GenMapis dropped (though it does reuse empty slots).
generational-arena-- I couldn't find this and so wrote
slotmap-- requires your items be
Copy, which I feel is too restrictive. Its
DenseSlotMapgets around this, but really at that point there's too many options and tradeoffs to consider. I just want something that Works.
handy-- Nice but I don't like some of the design decisions it makes: https://github.com/thomcc/handy/pull/1
slab-- Not really a generational map IMO, since it doesn't track generations.
specs-- A Storage in
specsis basically a type of generational map.
specsadds tons of other stuff though.