Struct OnceLock

pub struct OnceLock<T> { /* private fields */ }

A synchronization primitive which can nominally be written to only once.

This type is a thread-safe OnceCell, and can be used in statics. In many simple cases, you can use LazyLock<T, F> instead to get the benefits of this type with less effort: LazyLock<T, F> “looks like” &T because it initializes with F on deref! Where OnceLock shines is when LazyLock is too simple to support a given case, as LazyLock doesn’t allow additional inputs to its function after you call LazyLock::new(|| ...).

A OnceLock can be thought of as a safe abstraction over uninitialized data that becomes initialized once written.

Examples

Writing to a OnceLock from a separate thread:

use std::sync::OnceLock;

static CELL: OnceLock<usize> = OnceLock::new();

// `OnceLock` has not been written to yet.
assert!(CELL.get().is_none());

// Spawn a thread and write to `OnceLock`.
std::thread::spawn(|| {
    let value = CELL.get_or_init(|| 12345);
    assert_eq!(value, &12345);
})
.join()
.unwrap();

// `OnceLock` now contains the value.
assert_eq!(
    CELL.get(),
    Some(&12345),
);

You can use OnceLock to implement a type that requires “append-only” logic:

use std::sync::{OnceLock, atomic::{AtomicU32, Ordering}};
use std::thread;

struct OnceList<T> {
    data: OnceLock<T>,
    next: OnceLock<Box<OnceList<T>>>,
}
impl<T> OnceList<T> {
    const fn new() -> OnceList<T> {
        OnceList { data: OnceLock::new(), next: OnceLock::new() }
    }
    fn push(&self, value: T) {
        // FIXME: this impl is concise, but is also slow for long lists or many threads.
        // as an exercise, consider how you might improve on it while preserving the behavior
        if let Err(value) = self.data.set(value) {
            let next = self.next.get_or_init(|| Box::new(OnceList::new()));
            next.push(value)
        };
    }
    fn contains(&self, example: &T) -> bool
    where
        T: PartialEq,
    {
        self.data.get().map(|item| item == example).filter(|v| *v).unwrap_or_else(|| {
            self.next.get().map(|next| next.contains(example)).unwrap_or(false)
        })
    }
}

// Let's exercise this new Sync append-only list by doing a little counting
static LIST: OnceList<u32> = OnceList::new();
static COUNTER: AtomicU32 = AtomicU32::new(0);

const LEN: u32 = 1000;
thread::scope(|s| {
    for _ in 0..thread::available_parallelism().unwrap().get() {
        s.spawn(|| {
            while let i @ 0..LEN = COUNTER.fetch_add(1, Ordering::Relaxed) {
                LIST.push(i);
            }
        });
    }
});

for i in 0..LEN {
    assert!(LIST.contains(&i));
}

Implementations

impl<T> OnceLock<T>

pub const fn new() -> OnceLock<T>

Creates a new uninitialized cell.

pub fn get(&self) -> Option<&T>

Gets the reference to the underlying value.

Returns None if the cell is uninitialized, or being initialized. This method never blocks.

pub fn get_mut(&mut self) -> Option<&mut T>

Gets the mutable reference to the underlying value.

Returns None if the cell is uninitialized, or being initialized. This method never blocks.

pub fn wait(&self) -> &T

Blocks the current thread until the cell is initialized.

Example

Waiting for a computation on another thread to finish:

use std::thread;
use std::sync::OnceLock;

let value = OnceLock::new();

thread::scope(|s| {
    s.spawn(|| value.set(1 + 1));

    let result = value.wait();
    assert_eq!(result, &2);
})

pub fn set(&self, value: T) -> Result<(), T>

Initializes the contents of the cell to value.

May block if another thread is currently attempting to initialize the cell. The cell is guaranteed to contain a value when set returns, though not necessarily the one provided.

Returns Ok(()) if the cell was uninitialized and Err(value) if the cell was already initialized.

Examples

use std::sync::OnceLock;

static CELL: OnceLock<i32> = OnceLock::new();

fn main() {
    assert!(CELL.get().is_none());

    std::thread::spawn(|| {
        assert_eq!(CELL.set(92), Ok(()));
    }).join().unwrap();

    assert_eq!(CELL.set(62), Err(62));
    assert_eq!(CELL.get(), Some(&92));
}

pub fn try_insert(&self, value: T) -> Result<&T, (&T, T)>

🔬This is a nightly-only experimental API. (once_cell_try_insert)

Initializes the contents of the cell to value if the cell was uninitialized, then returns a reference to it.

May block if another thread is currently attempting to initialize the cell. The cell is guaranteed to contain a value when try_insert returns, though not necessarily the one provided.

Returns Ok(&value) if the cell was uninitialized and Err((&current_value, value)) if it was already initialized.

Examples

#![feature(once_cell_try_insert)]

use std::sync::OnceLock;

static CELL: OnceLock<i32> = OnceLock::new();

fn main() {
    assert!(CELL.get().is_none());

    std::thread::spawn(|| {
        assert_eq!(CELL.try_insert(92), Ok(&92));
    }).join().unwrap();

    assert_eq!(CELL.try_insert(62), Err((&92, 62)));
    assert_eq!(CELL.get(), Some(&92));
}

pub fn get_or_init<F>(&self, f: F) -> &T

where F: FnOnce() -> T,

Gets the contents of the cell, initializing it to f() if the cell was uninitialized.

Many threads may call get_or_init concurrently with different initializing functions, but it is guaranteed that only one function will be executed.

Panics

If f() panics, the panic is propagated to the caller, and the cell remains uninitialized.

It is an error to reentrantly initialize the cell from f. The exact outcome is unspecified. Current implementation deadlocks, but this may be changed to a panic in the future.

Examples

use std::sync::OnceLock;

let cell = OnceLock::new();
let value = cell.get_or_init(|| 92);
assert_eq!(value, &92);
let value = cell.get_or_init(|| unreachable!());
assert_eq!(value, &92);

pub fn get_mut_or_init<F>(&mut self, f: F) -> &mut T

where F: FnOnce() -> T,

🔬This is a nightly-only experimental API. (once_cell_get_mut)

Gets the mutable reference of the contents of the cell, initializing it to f() if the cell was uninitialized.

This method never blocks.

Panics

If f() panics, the panic is propagated to the caller, and the cell remains uninitialized.

Examples

#![feature(once_cell_get_mut)]

use std::sync::OnceLock;

let mut cell = OnceLock::new();
let value = cell.get_mut_or_init(|| 92);
assert_eq!(*value, 92);

*value += 2;
assert_eq!(*value, 94);

let value = cell.get_mut_or_init(|| unreachable!());
assert_eq!(*value, 94);

pub fn get_or_try_init<F, E>(&self, f: F) -> Result<&T, E>

where F: FnOnce() -> Result<T, E>,

🔬This is a nightly-only experimental API. (once_cell_try)

Gets the contents of the cell, initializing it to f() if the cell was uninitialized. If the cell was uninitialized and f() failed, an error is returned.

Panics

If f() panics, the panic is propagated to the caller, and the cell remains uninitialized.

It is an error to reentrantly initialize the cell from f. The exact outcome is unspecified. Current implementation deadlocks, but this may be changed to a panic in the future.

Examples

#![feature(once_cell_try)]

use std::sync::OnceLock;

let cell = OnceLock::new();
assert_eq!(cell.get_or_try_init(|| Err(())), Err(()));
assert!(cell.get().is_none());
let value = cell.get_or_try_init(|| -> Result<i32, ()> {
    Ok(92)
});
assert_eq!(value, Ok(&92));
assert_eq!(cell.get(), Some(&92))

pub fn get_mut_or_try_init<F, E>(&mut self, f: F) -> Result<&mut T, E>

where F: FnOnce() -> Result<T, E>,

🔬This is a nightly-only experimental API. (once_cell_get_mut)

Gets the mutable reference of the contents of the cell, initializing it to f() if the cell was uninitialized. If the cell was uninitialized and f() failed, an error is returned.

This method never blocks.

Panics

If f() panics, the panic is propagated to the caller, and the cell remains uninitialized.

Examples

#![feature(once_cell_get_mut)]

use std::sync::OnceLock;

let mut cell: OnceLock<u32> = OnceLock::new();

// Failed attempts to initialize the cell do not change its contents
assert!(cell.get_mut_or_try_init(|| "not a number!".parse()).is_err());
assert!(cell.get().is_none());

let value = cell.get_mut_or_try_init(|| "1234".parse());
assert_eq!(value, Ok(&mut 1234));
*value.unwrap() += 2;
assert_eq!(cell.get(), Some(&1236))

pub fn into_inner(self) -> Option<T>

Consumes the OnceLock, returning the wrapped value. Returns None if the cell was uninitialized.

Examples

use std::sync::OnceLock;

let cell: OnceLock<String> = OnceLock::new();
assert_eq!(cell.into_inner(), None);

let cell = OnceLock::new();
cell.set("hello".to_string()).unwrap();
assert_eq!(cell.into_inner(), Some("hello".to_string()));

pub fn take(&mut self) -> Option<T>

Takes the value out of this OnceLock, moving it back to an uninitialized state.

Has no effect and returns None if the OnceLock was uninitialized.

Safety is guaranteed by requiring a mutable reference.

Examples

use std::sync::OnceLock;

let mut cell: OnceLock<String> = OnceLock::new();
assert_eq!(cell.take(), None);

let mut cell = OnceLock::new();
cell.set("hello".to_string()).unwrap();
assert_eq!(cell.take(), Some("hello".to_string()));
assert_eq!(cell.get(), None);

Trait Implementations

impl<T> Clone for OnceLock<T>

where T: Clone,

fn clone(&self) -> OnceLock<T>

Returns a copy of the value.

fn clone_from(&mut self, source: &Self)

Performs copy-assignment from source.

impl<T> Debug for OnceLock<T>

where T: Debug,

fn fmt(&self, f: &mut Formatter<'_>) -> Result<(), Error>

Formats the value using the given formatter.

impl<T> Default for OnceLock<T>

fn default() -> OnceLock<T>

Creates a new uninitialized cell.

Example

use std::sync::OnceLock;

fn main() {
    assert_eq!(OnceLock::<()>::new(), OnceLock::default());
}

impl<T> Drop for OnceLock<T>

fn drop(&mut self)

Executes the destructor for this type.

impl<T> From<T> for OnceLock<T>

fn from(value: T) -> OnceLock<T>

Creates a new cell with its contents set to value.

Example

use std::sync::OnceLock;

let a = OnceLock::from(3);
let b = OnceLock::new();
b.set(3)?;
assert_eq!(a, b);
Ok(())

impl<T> PartialEq for OnceLock<T>

where T: PartialEq,

fn eq(&self, other: &OnceLock<T>) -> bool

Equality for two OnceLocks.

Two OnceLocks are equal if they either both contain values and their values are equal, or if neither contains a value.

Examples

use std::sync::OnceLock;

let five = OnceLock::new();
five.set(5).unwrap();

let also_five = OnceLock::new();
also_five.set(5).unwrap();

assert!(five == also_five);

assert!(OnceLock::<u32>::new() == OnceLock::<u32>::new());

fn ne(&self, other: &Rhs) -> bool

Tests for !=. The default implementation is almost always sufficient, and should not be overridden without very good reason.

impl<T> Eq for OnceLock<T>

where T: Eq,

impl<T> RefUnwindSafe for OnceLock<T>

where T: RefUnwindSafe + UnwindSafe,

impl<T> Send for OnceLock<T>

where T: Send,

impl<T> Sync for OnceLock<T>

where T: Sync + Send,

impl<T> UnwindSafe for OnceLock<T>

where T: UnwindSafe,