How to use Rust OnceLock get_mut() method?

Question

The Rust documentation has a OnceLock::get_mut method that returns a mutable reference. But the following code does not compile. What am I missing?

use std::sync::OnceLock;
#[derive(Clone)]
struct System {
    comp: Vec<i32>,
}

static GLOB: OnceLock<System> = OnceLock::new();

impl System {
    fn new() -> Self {
        Self { comp: Vec::new() }
    }
}
fn modify(global: &mut System) {
    global.comp.push(42);
}

fn main() {
    let sys: Box<System> = Box::new(System::new());
    GLOB.get_or_init(|| *sys);
    let global: &mut System = GLOB.get_mut().unwrap();
    println!("global size is {}", global.comp.len());
    modify(global);
    println!("global size is {}", global.comp.len());
}

Is it really possible to get a mutable reference? How am I supposed to use get_mut()?

cafce25 · Accepted Answer · 2024-05-23 13:41:11Z

3

You can't get a mutable reference to any binding that's not marked as mutable, that includes statics, though a static mut is discouraged and very likely not what you want. But you don't have to use OnceLock with statics:

let mut glob: OnceLock<System> = OnceLock::new();
let global: &mut System = glob.get_mut().unwrap();

Though if you need mutable access after initialization you should use a Mutex or RwLock instead.

edited May 23, 2024 at 13:41

answered May 23, 2024 at 9:33

cafce25

29.9k5 gold badges49 silver badges68 bronze badges

Sign up to request clarification or add additional context in comments.

3 Comments

Chayim Friedman Over a year ago

At this point, why use OnceLock?

cafce25 Over a year ago

@ChayimFriedman I would imagine it's useful in similar situations where Mutex::get_mut is. There might be part of the program that only has a shared reference, but another part might have exclusive access and may want to mutate the inner value. Honestly I don't have a concrete usecase, but I can imagine someone might have one.

Zhou Lebo Over a year ago

Exactly. I have a data model and there is one single thread that can modify the model. All of the other threads must act accordingly. I guess I will use RwLock...

Collectives™ on Stack Overflow

How to use Rust OnceLock get_mut() method?

1 Answer 1

3 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

3 Comments

Your Answer

Sign up or log in

Post as a guest

Related