data-structuresrustinterior-mutability

Having Rc<T>s in one module and Rc<RefCell<T>>s in another referencing the same data

I have a data structure like this:

mod nodes {
    struct Node {
        predecessors: Vec<Rc<Node>>
    }
}

I obviously don't need mutability of the nodes in that module. I also don't have cyclic references here.

But I have another module, which contains nodes, where I need mutability:

pub mod network {
    mod nodes {...}

    struct Network {
        nodes: Vec<Vec<Rc<RefCell<Node>>>>
    }
}

I just can't figure out a way to create the Rc<Node>s for the nodes themselves together with the Rc<RefCell<Node>>s for the Network.

Do you have any idea on how to implement the immutability in the nodes module next to the mutable nodes in the network module?

Or is there no other way than to declare the references to the predecessor nodes inside the Node struct as Rc<RefCell<Node>>, even though I don't need mutability here (I would like to avoid that)?

Solution

  • Rust has single ownership. If a Node is owned by a RefCell, it cannot also be owned by a different Rc at the same time.

    When a Node can be modified in one place via RefCell, these changes cannot be allowed to show up elsewhere in Rc<Node>. This would violate immutability of Rc<Node>.

    Also note that Node is a struct value, not a pointer to an object. There's no extra layer of indirection that would even make it possible to share it between RefCell and other Rc. Its data is copied and inlined in its container.