How do I borrow a RefCell, find a key, and return a reference to the result? [duplicate]

by

When you borrow from a RefCell, the reference you get has a shorter lifetime than the RefCell‘s. That’s because the reference’s lifetime is restricted by the guard returned by borrow(). That guard ensures that nobody else can take a mutable reference to the value until the guard is dropped.

However, you are trying to return a value without keeping a guard alive. If Frame had a method that took a &self argument but tried to mutate the map (which is possible with RefCell — if you don’t need to do that, then ditch the RefCell and write &mut self on the methods that mutate the map), you could accidentally destroy a String that somebody else has a reference to. That is exactly the kind of errors that the borrow checker was designed to report!

If the map values are effectively immutable (i.e. your type will not allow mutating the map’s values), you could also wrap them in an Rc in your map. You could therefore return a clone of the Rc<String> (this only clones the reference-counted pointer, not the underlying string), which would let you release the borrow on the map before returning from the function.

struct Frame {
    map: RefCell<HashMap<String, Rc<String>>>
}

impl Frame {
    fn lookup(&self, k: &String) -> Option<Rc<String>> {
        self.map.borrow().get(k).map(|x| x.clone())
    }
}

More Related Contents:

  1. How do I return a reference to something inside a RefCell without breaking encapsulation?
  2. Situations where Cell or RefCell is the best choice
  3. Traits and Rust
  4. Why do try!() and ? not compile when used in a function that doesn’t return Option or Result?
  5. Why does linking lifetimes matter only with mutable references?
  6. Why is adding a lifetime to a trait with the plus operator (Iterator + ‘a) needed?
  7. How do I collect into an array?
  8. Cannot borrow as mutable more than once at a time in one code – but can in another very similar
  9. Is it considered a bad practice to implement Deref for newtypes?
  10. Implementing a trait for multiple types at once
  11. How do I get an owned value out of a `Box`?
  12. Cannot infer an appropriate lifetime for autoref due to conflicting requirements
  13. What is lifetime elision in very simple terms?
  14. Why does this read input before printing?
  15. How to create a String directly?
  16. How to create an in-memory object that can be used as a Reader, Writer, or Seek in Rust?
  17. Why does my variable not live long enough?
  18. What is the syntax to match on a reference to an enum?
  19. How can I implement Rust’s Copy trait?
  20. Why can’t I return an &str value generated from a String?
  21. Why are recursive struct types illegal in Rust?
  22. How to implement a custom ‘fmt::Debug’ trait?
  23. What is the prelude?
  24. How to embed resources in Rust executable?
  25. Extend lifetime of variable
  26. What does Rust’s unary || (parallel pipe) mean?
  27. Downcast traits inside Rc for AST manipulation
  28. Why does Valgrind not detect a memory leak in a Rust program using nightly 1.29.0?
  29. How to check in Rust if architecture is 32 or 64 bit?
  30. What is the null pointer optimization in Rust?

Leave a Comment