commit	a7bb59bc1a3677cb84ed6eefa91eb555ffe25ba1	[log] [tgz]
author	Matthew Maurer <mmaurer@google.com>	Tue Mar 07 17:24:22 2023 -0800
committer	Matthew Maurer <mmaurer@google.com>	Tue Mar 07 17:24:22 2023 -0800
tree	12c59a21225d1e0948cd3b4c7db5e10c2aec25e6
parent	1da3db55c968861a587abbe8b29659537a7bbaee [diff]

tree: 12c59a21225d1e0948cd3b4c7db5e10c2aec25e6

README.md

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)

The API of once_cell is being proposed for inclusion in std.