1 stable release
1.20.2 | Nov 3, 2024 |
---|
#1731 in Rust patterns
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Overview
once_cell
provides two new cell-like types, unsync::OnceCell
and sync::OnceCell
. OnceCell
might store arbitrary non-Copy
types, can be assigned to at most once and provide direct access
to the stored contents. In a nutshell, API looks roughly like this:
impl OnceCell<T> {
fn new() -> OnceCell<T> { ... }
fn set(&self, value: T) -> Result<(), T> { ... }
fn get(&self) -> Option<&T> { ... }
}
Note that, like with RefCell
and Mutex
, the set
method requires only a shared reference.
Because of the single assignment restriction get
can return an &T
instead of Ref<T>
or MutexGuard<T>
.
once_cell
also has a Lazy<T>
type, build on top of OnceCell
which provides the same API as
the lazy_static!
macro, but without using any macros:
use std::{sync::Mutex, collections::HashMap};
use once_cell::sync::Lazy;
static GLOBAL_DATA: Lazy<Mutex<HashMap<i32, String>>> = Lazy::new(|| {
let mut m = HashMap::new();
m.insert(13, "Spica".to_string());
m.insert(74, "Hoyten".to_string());
Mutex::new(m)
});
fn main() {
println!("{:?}", GLOBAL_DATA.lock().unwrap());
}
More patterns and use-cases are in the docs!
Related crates
- double-checked-cell
- lazy-init
- lazycell
- mitochondria
- lazy_static
- async_once_cell
- generic_once_cell (bring your own mutex)
Parts of once_cell
API are included into std
as of Rust 1.70.0.
Dependencies
~0–4.5MB