tag	70a101eee82635867559d732fcca18653efd3fe9
tagger	The Android Open Source Project <initial-contribution@android.com>	Mon Sep 21 17:00:09 2020 -0700
object	936c278fac220e3c0a1090d12572dfd1247e0a5c

commit	936c278fac220e3c0a1090d12572dfd1247e0a5c	[log] [tgz]
author	android-build-prod (mdb) <android-build-team-robot@google.com>	Thu Jul 16 21:24:04 2020 +0000
committer	android-build-prod (mdb) <android-build-team-robot@google.com>	Thu Jul 16 21:24:04 2020 +0000
tree	c2f75907aea00b012f8ef54c261ccd7aa241ee61
parent	9a0cb5e3a76267fa3fd991980a4bfa6afe4c45e9 [diff]
parent	517fe9d8bb8400e7514ff3feb6fe8f451c7768a4 [diff]

tree: c2f75907aea00b012f8ef54c261ccd7aa241ee61

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.