rust - How to abstract over a reference to a value or a value itself?

Question

I have a trait that defines an interface for objects that can hold a value. The trait has a way of getting the current value:

pub trait HasValue<T> {
    fn get_current_value(&self) -> &T;
}

This is fine, but I realized that depending on the actual implementation, sometimes it's convenient to return a reference if T is stored in a field, and sometimes it's convenient to return a clone of T if the backing field was being shared across threads (for example). I'm struggling to figure out how to represent this in the trait. I could have something like this:

pub enum BorrowedOrOwned<'a, T: 'a> {
    Borrowed(&'a T),
    Owned(T)
}

impl<'a, T: 'a> Deref for BorrowedOrOwned<'a, T> {
    type Target = T;

    fn deref(&self) -> &T {
        use self::BorrowedOrOwned::*;

        match self {
            &Borrowed(b) => b,
            &Owned(ref o) => o,
        }
    }
}

And change get_current_value() to return a BorrowedOrOwned<T> but I'm not sure that this is idiomatic. BorrowedOrOwned<T> kind of reminds me of Cow<T> but since the point of Cow is to copy-on-write and I will be discarding any writes, that seems semantically wrong.

Is Cow<T> the correct way to abstract over a reference or an owned value? Is there a better way than BorrowedOrOwned<T>?

Answer 1

I suggest you use a Cow, since your BorrowedOrOwned has no difference to Cow except that it has fewer convenience methods. Anybody that gets a hold of a BorrowedOrOwned object could match on it and get the owned value or a mutable reference to it. If you want to prevent the confusion of being able to get a mutable reference or the object itself, the solution below applies, too.

For your use case i'd simply stay with &T, since there's no reason to make the API more complex. If a user wants a usize, when T is usize, they can simply dereference the reference.

An owned object only makes sense, if you expect the user to actually process it in an owned fashion. And even then, Cow is meant to abstract over big/heavy objects that you pass by ownership for the purpose of not requiring anyone to clone it. Your use case is the opposite, you want to pass small objects by ownership to prevent users from needing to copy small objects, and instead you are copying it.