Bug: 156525064

Clone this repo:
  1. f4b2fe4 Merge "Refresh Android.bp, cargo2android.json, TEST_MAPPING." by Joel Galenson · 6 weeks ago master
  2. aeca3d8 Refresh Android.bp, cargo2android.json, TEST_MAPPING. by Joel Galenson · 8 weeks ago
  3. 9836ee9 Enable tests am: 467ba80bf0 am: 734411c3a5 am: ebbeb81485 by Jeff Vander Stoep · 3 months ago
  4. ebbeb81 Enable tests am: 467ba80bf0 am: 734411c3a5 by Jeff Vander Stoep · 3 months ago
  5. 734411c Enable tests am: 467ba80bf0 by Jeff Vander Stoep · 3 months ago

Build Status Crates.io API reference

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.