Creating a vector of mutable Trait objects (sometimes shared) in Rust

Question

I need to create a stack of pointers with the following constraints:

The pointers need to point to the same Trait object (so Box seems like a fit)
Those Trait objects may need to be modified (RefCell may need to be used?)
Two pointers in the stack may need to point to the same object (Rc seems like a fit)

Right now, the only way I've found to accommodate this is to use a Vec<Rc<RefCell<Box<dyn MyTrait>>>>. Is that the best solution though? It looks like a lot of pointer dereferences needed to access the objects.

*p conveniently goes through the whole pack of references and returns the trait object itself (as far as I recall) — Alexey S. Larionov
– Alexey S. Larionov, Commented Jun 12, 2021 at 17:43

Cryptjar · Accepted Answer · 2021-06-12 19:48:03Z

3

I'm not quite sure what you exactly mean with:

The pointers need to point to the same Trait object (so Box seems like a fit)

But if you are interested in storing objects of actually different types, then you need trait-objects and those need to be behind some sort of pointer such as a Box. And a Box is generally a good default (but there are alternatives).

Those Trait objects may need to be modified (RefCell may need to be used?)

Well, actually, that could still be done with a Box.

Two pointers in the stack may need to point to the same object (Rc seems like a fit)

Here, it gets difficult because in Rust sharable and mutable are kind of exclude each other. To be sharable, we need an Rc, which you can think of as a shared box. Then to make it mutable anyway, we can use interior mutability by using a RefCell. So, essentially a Rc<RefCell<_>>, which you can think of as a sharable & mutable Box.

Finally, if you put it all together into a Vec you get: Vec<Rc<RefCell<dyn MyTrait>>> (no Box).

This allows you to have different types in the Vec, having some instances even multiple times in it, and still allowing mutable access to each of them.

answered Jun 12, 2021 at 19:48

Cryptjar

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1 Comment

user7094 Over a year ago

For some reason when I tried RefCell<dyn MyTrait> it didn't work. But trying it now I manage to get the code to compile. Thanks!

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