commit	e116d736ebc312725ee4df730d1c86123a6fcd11	[log] [tgz]
author	Elliott Hughes <enh@google.com>	Thu Apr 01 16:26:32 2021 -0700
committer	Elliott Hughes <enh@google.com>	Thu Apr 01 16:26:32 2021 -0700
tree	a6da12188f6aae0cc4180117c2c4a704e93829c2
parent	4985b53f0d9def947e1aae390d5fdef1d490d394 [diff]

tree: a6da12188f6aae0cc4180117c2c4a704e93829c2

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

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