How do I return an iterator that has a reference to something inside a RefCell?

Question

I\'m trying to create a method that returns an iterator over the values of HashMap that is boxed inside a RefCell, but i\'m having an error where <

Answer 1

You cannot do this.

The ultimate problem is that std::collections::hash_map::Values holds a reference, but you don't have "just" a reference. You have the smart pointer Ref.

The easiest solution I know of is to invert the code:

impl Foo {
    fn with_iter<F, T>(&self, f: F) -> T
    where
        F: FnOnce(Values<i32, i32>) -> T,
    {
        f(self.map.borrow().values())
    }
}

fn main() {
    let foo = Foo {
        map: Rc::new(RefCell::new(HashMap::new())),
    };

    foo.with_iter(|i| {
        for v in i {
            println!("{}", v)
        }
    })
}

Here, the Values iterator no longer needs to outlive the result of borrow, so there's no additional complexity.

If you are OK with leaking your implementation, you can return the Ref:

impl Foo {
    fn iter(&self) -> Ref<'_, HashMap<i32, i32>> {
        self.map.borrow()
    }
}

for v in foo.iter().values() {
    println!("{}", v)
}

In newer versions of Rust, you can return an unnamed type that implements Deref:

use std::ops::Deref;

impl Foo {
    fn iter(&self) -> impl Deref<Target = HashMap<i32, i32>> + '_ {
        self.map.borrow()
    }
}

See also:

• How do I return a reference to something inside a RefCell without breaking encapsulation?