Struct std::sync::OnceLock

pub struct OnceLock<T> {
    once: Once,
    value: UnsafeCell<MaybeUninit<T>>,
    _marker: PhantomData<T>,
}

A synchronization primitive which can be written to only once.

This type is a thread-safe OnceCell, and can be used in statics.

Examples

use std::sync::OnceLock;

static CELL: OnceLock<String> = OnceLock::new();
assert!(CELL.get().is_none());

std::thread::spawn(|| {
    let value: &String = CELL.get_or_init(|| {
        "Hello, World!".to_string()
    });
    assert_eq!(value, "Hello, World!");
}).join().unwrap();

let value: Option<&String> = CELL.get();
assert!(value.is_some());
assert_eq!(value.unwrap().as_str(), "Hello, World!");

Fields

once: Once

value: UnsafeCell<MaybeUninit<T>>

_marker: PhantomData<T>

PhantomData to make sure dropck understands we’re dropping T in our Drop impl.

use std::sync::OnceLock;

struct A<'a>(&'a str);

impl<'a> Drop for A<'a> {
    fn drop(&mut self) {}
}

let cell = OnceLock::new();
{
    let s = String::new();
    let _ = cell.set(A(&s));
}

Implementations

impl<T> OnceLock<T>

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

Creates a new empty cell.

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

Gets the reference to the underlying value.

Returns None if the cell is empty, 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 empty. This method never blocks.

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

Sets the contents of this 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’s value was set by this call.

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 get_or_init<F>(&self, f: F) -> &Twhere F: FnOnce() -> T,

Gets the contents of the cell, initializing it with f if the cell was empty.

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_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 #109737)

Gets the contents of the cell, initializing it with f if the cell was empty. If the cell was empty 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 into_inner(self) -> Option<T>

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

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 hasn’t been initialized.

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);

fn is_initialized(&self) -> bool

fn initialize<F, E>(&self, f: F) -> Result<(), E>where F: FnOnce() -> Result<T, E>,

unsafe fn get_unchecked(&self) -> &T

Safety

The value must be initialized

unsafe fn get_unchecked_mut(&mut self) -> &mut T

Safety

The value must be initialized

Trait Implementations

impl<T: Clone> Clone for OnceLock<T>

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> Debug for OnceLock<T>

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

Formats the value using the given formatter.

impl<T> Default for OnceLock<T>

fn default() -> OnceLock<T>

Creates a new empty 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: Eq> Eq for OnceLock<T>

fn assert_receiver_is_total_eq(&self)

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

fn from(value: T) -> Self

Create 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> PartialEq<OnceLock<T>> for OnceLock<T>

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

This method tests for self and other values to be equal, and is used by ==.

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

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

impl<T: RefUnwindSafe + UnwindSafe> RefUnwindSafe for OnceLock<T>

impl<T: Send> Send for OnceLock<T>

impl<T: Sync + Send> Sync for OnceLock<T>

impl<T: UnwindSafe> UnwindSafe for OnceLock<T>