Should try_recv on a Reciver take &mut self or &self


I’ve implemented a queue which internally uses a vector as a buffer, where writers and readers have their own indexes, which they increment upon each send/recv operation. I’ve modeled the API after the std::sync::mpsc, and in it the try_recv method takes a immutable reference to self. To make this work in my queue I had to use AtomicUsize.

But now that I think about it there is no reason that I know of for the try_recv function to be immutable.
So the question is: Is there a reason for try_recv to be immutable, and if so what would you suggest I do since the receiver increments it’s index on each try_recv call


It’s better to think of &/&mut not as immutable vs mutable, but whether it allows shared or requires exclusive access.

Do you require that there has to be only one call to try_recv at a time? If not, it’s fine to allow shared access.

In general, if you can, it’s fine to make things take a shared reference, since these are much more flexible and easier to work with by users of your library.


in my implementation neither Receiver nor the Sender should be concurrently accessed by multiple threads. They themselves provide an interface to a “channel”.
But that also applies to std::sync::mpsp and crossbeam::channel, but in both cases the send and recv methods accept immutable references to self.

If it’s just for convenience’s sake, then I am fine with my implementation having a stricter API.


If you believe taking &self is safe, then do it. No reason to arbitrarily require &mut self if it isn’t actually required.

I think mpsc::Receiver is protected from multiple calls to try_recv by Receiver not implementing Sync. (impl<T> !Sync for Receiver<T> in

The only reasons you would want to take &mut self instead of &self are

  1. if your code is unsound if it were implemented using &self. In particular, interior mutability on anything outside an UnsafeCell or atomic is unsound, so if taking &self requires you to do that then that’s a good reason to take &mut self instead.
  2. if &self allows users to use it in a way you believe will be incorrect the majority of the time.

I don’t see either of these being true, so I would just keep &self. But I haven’t looked at the code in depth so my assessment could be incorrect.

Edit: It looks like BareSubscriber is sync (impl<T> Sync for BareSubscriber<T> is in the documentation), so your current method taking &self will allow two threads to call try_recv at the same time. If you think this will cause problems, even if it doesn’t cause unsoundness, I would recommend taking &mut self. If it will do the right thing no matter how many threads are accessing it concurrently, then no matter. Since Subscriber is Clone, I assume that the latter is true.