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 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>.

Patterns

OnceCell might be useful for a variety of patterns.

Safe Initialization of global data

use std::{env, io};
use once_cell::sync::OnceCell;

#[derive(Debug)]
pub struct Logger {
    // ...
}
static INSTANCE: OnceCell<Logger> = OnceCell::INIT;

impl Logger {
    pub fn global() -> &'static Logger {
        INSTANCE.get().expect("logger is not initialized")
    }

    fn from_cli(args: env::Args) -> Result<Logger, io::Error> {
       // ...
    }
}

fn main() -> Result<(), io::Error> {
    let logger = Logger::from_cli(env::args())?;
    INSTANCE.set(logger).unwrap();
    // use `Logger::global()` from now on
    Ok(())
}

Lazy initialized global data

This is essentially lazy_static! macro, but without a macro.

use std::{sync::Mutex, collections::HashMap};
use once_cell::sync::OnceCell;

fn global_data() -> &'static Mutex<HashMap<i32, String>> {
    static INSTANCE: OnceCell<Mutex<HashMap<i32, String>>> = OnceCell::INIT;
    INSTANCE.get_or_init(|| {
        let mut m = HashMap::new();
        m.insert(13, "Spica".to_string());
        m.insert(74, "Hoyten".to_string());
        Mutex::new(m)
    })
}

There are also sync::Lazy and unsync::Lazy convenience types and macros to streamline this pattern:

#[macro_use]
extern crate once_cell;

use std::{sync::Mutex, collections::HashMap};
use once_cell::sync::Lazy;

static GLOBAL_DATA: Lazy<Mutex<HashMap<i32, String>>> = sync_lazy! {
    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());
}

General purpose lazy evaluation

Unlike lazy_static!, Lazy works with local variables.

use once_cell::unsync::Lazy;

fn main() {
    let ctx = vec![1, 2, 3];
    let thunk = Lazy::new(|| {
        ctx.iter().sum::<i32>()
    });
    assert_eq!(*thunk, 6);
}

If you need a lazy field in a struct, you probably should use OnceCell directly, because that will allow you to access self during initialization.

use std::{fs, io, path::PathBuf};
use once_cell::unsync::OnceCell;

struct Ctx {
    config_path: PathBuf,
    config: OnceCell<String>,
}

impl Ctx {
    pub fn get_config(&self) -> Result<&str, io::Error> {
        let cfg = self.config.get_or_try_init(|| {
            fs::read_to_string(&self.config_path)
        })?;
        Ok(cfg.as_str())
    }
}

Comparison with std

!Sync types	Access Mode	Drawbacks
Cell<T>	T	works only with Copy types
RefCel<T>	RefMut<T> / Ref<T>	may panic at runtime
unsync::OnceCell<T>	&T	assignable only once

Sync types	Access Mode	Drawbacks
AtomicT	T	works only with certain Copy types
Mutex<T>	MutexGuard<T>	may deadlock at runtime, may block the thread
sync::OnceCell<T>	&T	assignable only once, may block the thread

Technically, calling get_or_init will also cause a panic or a deadlock if it recursively calls itself. However, because the assignment can happen only once, such cases should be more rare than equivalents with RefCell and Mutex.

Implementation details

Implementation is based on lazy_static and lazy_cell crates and in some sense just streamlines and unifies the APIs of those crates.

To implement a sync flavor of OnceCell, this crates uses either ::std::sync::Once or ::parking_lot::Once. This is controlled by the parking_lot feature, which is enabled by default.

When using parking_lot, the crate is compatible with rustc 1.25.0, without parking_lot a minimum of 1.29.0 is required.

This crate uses unsafe.