#cow #arcow


Atomically Reference-counted Copy-On-Write shared pointer

1 stable release

Uses new Rust 2021

1.0.0 Jul 19, 2022

#875 in Rust patterns


166 lines


This crate provides a thread-safe, reference counted pointer that uses copy-on-write semantics to allow mutability.


Make an Arcow<T>. This acts like a cheaply-clonable T that can be mutated normally, even if T is actually expensive to clone. You can freely send clones of Arcow<T> to multiple threads.

Note that atomic operations aren't free; this is why Arcow doesn't implement Copy. Cloning Arcow<T> and sending that to another thread will only be cheaper than cloning T if:

  1. T is big (expensive to clone), AND
  2. A significant proportion of Arcow<T> clones will be used and consumed without mutation.

Why not

You should consider using Cow<T> if possible. And if you want mutations to propagate to other references to the same underlying T, then you don't want copy-on-write at all, you actually want to do something like wrap a Mutex<T> in an Arc.


Say you're implementing a game server. You keep track of several iterations of game state, potentially a lot of them, to allow for saving, client prediction, and other issues. Most of your game state is trivial, so each iteration just has a separate copy. But your game takes place on a Map, and that Map is typically around 64KiB. That's not very much in the grand scheme of things, but it quickly adds up, and the overhead of copying the map into every new iteration wastes a lot of CPU time. Your automated tests are particularly affected, as they spend almost all their time copying instead of actually testing your logic.

So you wrap the Map in an Arc. Now you've replaced a 64KiB copy with a single atomic operation. You pat yourself on the back and proceed.

Except that the Map can change.

In the vast majority of iterations, the Map isn't changed. But every so often, somebody will chop down a tree, or build a wall, or pave a road, and when they do, those changes are reflected in the Map. If you remove the Arc, this is no problem, since each iteration has its own copy of the map to mutate. But now you're copying a whole 64KiB for every iteration, even though less than 1% of iterations will subsequently mutate the map.

Cow doesn't work for this case, because there's no long-lived "master copy" of the map that all the other game states can borrow from. No particular iteration can be trusted for that. Old iterations will be pruned as they stop being relevant, and new iterations aren't necessarily going to be kept (e.g. abandoned prediction timelines).

Cloning the Arc<Map> every time there's a change is a working solution, but what if two players chop down a tree in the same tick? That's a wasted copy. And then there's the case where you're an internal server of a client in a singleplayer mode, and there's only ever one relevant iteration... now you're making a copy of the Map every change even though there's never more than one reference!

Thus, Arcow. It acts very much like a simplified Arc, with one additional feature: it can be mutated. A living reference-counted pointer is either unique or shared. If you have a unique Arcow, mutation is simple. DerefMut will return a mutable reference to the inner type. As long as that mutable reference is alive, you can't make another clone of the Arcow*, so uniqueness is preserved. If you have a shared Arcow, DerefMut will first "split" your Arcow into a unique clone of the original shared value, and then you're all set.

(*Not quite accurate. You can clone an Arcow that you've currently mutably borrowed, you just can't mutate through that borrow anymore afterwards. See:)

# use arcow::Arcow;
// error[E0502]: cannot borrow `a` as immutable because it is also borrowed as mutable
   let mut a = Arcow::new(456);
   let borrowed = &mut *a;
//                      - mutable borrow occurs here
   *borrowed = 42;
   let b = a.clone();
//         ^^^^^^^^^ immutable borrow occurs here
   *borrowed = 47;
// --------- mutable borrow later used here


Arcow is copyright 2022, Solra Bizna, and licensed under either of:

at your option.

Unless you explicitly state otherwise, any contribution intentionally submitted for inclusion in the Arcow crate by you, as defined in the Apache-2.0 license, shall be dual licensed as above, without any additional terms or conditions.

No runtime deps