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How can I implement Deref for a struct that holds an Rc<Refcell<Trait>>?

My goal is to delegate method calls against my struct to a Trait's methods, where the Trait object is inside an Rc of RefCell.

I tried to follow the advice from this question: How can I obtain an &A reference from a Rc<RefCell<A>>?

I get a compile error.

use std::rc::Rc;
use std::cell::RefCell;
use std::fmt::*;
use std::ops::Deref;

pub struct ShyObject {
    pub association: Rc<RefCell<dyn Display>>
}

impl Deref for ShyObject {
    type Target = dyn Display;
    fn deref<'a>(&'a self) -> &(dyn Display + 'static) {
        &*self.association.borrow()
    }
}

fn main() {}

Here is the error:

error[E0515]: cannot return value referencing temporary value
  --> src/main.rs:13:9
   |
13 |         &*self.association.borrow()
   |         ^^-------------------------
   |         | |
   |         | temporary value created here
   |         returns a value referencing data owned by the current function

My example uses Display as the trait; in reality I have a Trait with a dozen methods. I am trying to avoid the boilerplate of having to implement all those methods and just burrow down to the Trait object in each call.

like image 766
Paul Chernoch Avatar asked Mar 03 '23 11:03

Paul Chernoch


1 Answers

You can't. RefCell::borrow returns a Ref<T>, not a &T. If you try to do this in a method then you will need to first borrow the Ref<T> but it will go out of scope.

Instead of implementing Deref, you could have a method that returns something that does:

impl ShyObject {
    fn as_deref(&self) -> impl Deref<Target = dyn Display> {
        self.association.borrow()
    }
}

Otherwise, since you only want to expose the Display implementation of the inner data anyway, you can workaround it by actually dereferencing a different type which delegates:

pub struct ShyObject {
    association: Assocation<dyn Display>,
}

struct Assocation<T: ?Sized>(Rc<RefCell<T>>);

impl<T: Display + ?Sized> fmt::Display for Assocation<T> {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        write!(f, "{}", self.0.borrow())
    }
}

impl Deref for ShyObject {
    type Target = dyn Display + 'static;
    fn deref(&self) -> &Self::Target {
        &self.association
    }
}
like image 113
Peter Hall Avatar answered Apr 28 '23 11:04

Peter Hall