impl<T> OnceCell<T>

const fn new() -> OnceCell<T>

Creates a new empty cell.

const fn with_value(value: T) -> OnceCell<T>

Creates a new initialized cell.

fn get(self: &Self) -> Option<&T>

Gets the reference to the underlying value.

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

fn wait(self: &Self) -> &T

Gets the reference to the underlying value, blocking the current thread until it is set.

use once_cell::sync::OnceCell;

let mut cell = std::sync::Arc::new(OnceCell::new());
let t = std::thread::spawn({
    let cell = std::sync::Arc::clone(&cell);
    move || cell.set(92).unwrap()
});

// Returns immediately, but might return None.
let _value_or_none = cell.get();

// Will return 92, but might block until the other thread does `.set`.
let value: &u32 = cell.wait();
assert_eq!(*value, 92);
t.join().unwrap();

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

Gets the mutable reference to the underlying value.

Returns None if the cell is empty.

This method is allowed to violate the invariant of writing to a OnceCell at most once because it requires &mut access to self. As with all interior mutability, &mut access permits arbitrary modification:

use once_cell::sync::OnceCell;

let mut cell: OnceCell<u32> = OnceCell::new();
cell.set(92).unwrap();
cell = OnceCell::new();

unsafe fn get_unchecked(self: &Self) -> &T

Get the reference to the underlying value, without checking if the cell is initialized.

Safety

Caller must ensure that the cell is in initialized state, and that the contents are acquired by (synchronized to) this thread.

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

Sets the contents of this cell to value.

Returns Ok(()) if the cell was empty and Err(value) if it was full.

Example

use once_cell::sync::OnceCell;

static CELL: OnceCell<i32> = OnceCell::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));
}

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

Like set, but also returns a reference to the final cell value.

Example

use once_cell::unsync::OnceCell;

let cell = OnceCell::new();
assert!(cell.get().is_none());

assert_eq!(cell.try_insert(92), Ok(&92));
assert_eq!(cell.try_insert(62), Err((&92, 62)));

assert!(cell.get().is_some());

fn get_or_init<F>(self: &Self, f: F) -> &T
where
    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.

Example

use once_cell::sync::OnceCell;

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

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

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.

Example

use once_cell::sync::OnceCell;

let cell = OnceCell::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))

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

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

Has no effect and returns None if the OnceCell hasn't been initialized.

Examples

use once_cell::sync::OnceCell;

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

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

This method is allowed to violate the invariant of writing to a OnceCell at most once because it requires &mut access to self. As with all interior mutability, &mut access permits arbitrary modification:

use once_cell::sync::OnceCell;

let mut cell: OnceCell<u32> = OnceCell::new();
cell.set(92).unwrap();
cell = OnceCell::new();

fn into_inner(self: Self) -> Option<T>

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

Examples

use once_cell::sync::OnceCell;

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

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

impl<T> Any for OnceCell<T>

fn type_id(self: &Self) -> TypeId

impl<T> Borrow for OnceCell<T>

fn borrow(self: &Self) -> &T

impl<T> BorrowMut for OnceCell<T>

fn borrow_mut(self: &mut Self) -> &mut T

impl<T> CloneToUninit for OnceCell<T>

unsafe fn clone_to_uninit(self: &Self, dest: *mut u8)

impl<T> Default for OnceCell<T>

fn default() -> OnceCell<T>

impl<T> Freeze for OnceCell<T>

impl<T> From for OnceCell<T>

fn from(value: T) -> Self

impl<T> From for OnceCell<T>

fn from(t: never) -> T

impl<T> From for OnceCell<T>

fn from(t: T) -> T

Returns the argument unchanged.

impl<T> RefUnwindSafe for OnceCell<T>

impl<T> Send for OnceCell<T>

impl<T> Sync for OnceCell<T>

impl<T> ToOwned for OnceCell<T>

fn to_owned(self: &Self) -> T

fn clone_into(self: &Self, target: &mut T)

impl<T> Unpin for OnceCell<T>

impl<T> UnsafeUnpin for OnceCell<T>

impl<T> UnwindSafe for OnceCell<T>

impl<T, U> Into for OnceCell<T>

fn into(self: Self) -> U

Calls U::from(self).

That is, this conversion is whatever the implementation of [From]<T> for U chooses to do.

impl<T, U> TryFrom for OnceCell<T>

fn try_from(value: U) -> Result<T, <T as TryFrom<U>>::Error>

impl<T, U> TryInto for OnceCell<T>

fn try_into(self: Self) -> Result<U, <U as TryFrom<T>>::Error>

impl<T: Clone> Clone for OnceCell<T>

fn clone(self: &Self) -> OnceCell<T>

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

impl<T: Eq> Eq for OnceCell<T>

impl<T: PartialEq> PartialEq for OnceCell<T>

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

impl<T: fmt::Debug> Debug for OnceCell<T>

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