commit	e5467b447a4d727acf74b475e2fcd403ee3766f0	[log] [tgz]
author	Treehugger Robot <treehugger-gerrit@google.com>	Wed Jun 15 21:00:27 2022 +0000
committer	Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>	Wed Jun 15 21:00:27 2022 +0000
tree	b733df8e10490ca94f221f649a725cd943c525c8
parent	54b8294da47f58c6953413d43fba291ccdc9f3ee [diff]
parent	1a69b4470ca7cade882d1f815f3b16bd6e60b35b [diff]

tree: b733df8e10490ca94f221f649a725cd943c525c8

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.