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

A synchronization primitive which can be written to only once.

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

§Examples

Using OnceCell to store a function’s previously computed value (a.k.a. ‘lazy static’ or ‘memoizing’):

use std::sync::OnceLock;

struct DeepThought {
    answer: String,
}

impl DeepThought {
    fn new() -> Self {
        Self {
            // M3 Ultra takes about 16 million years in --release config
            answer: Self::great_question(),
        }
    }
}

fn computation() -> &'static DeepThought {
    // n.b. static items do not call [`Drop`] on program termination, so if
    // [`DeepThought`] impls Drop, that will not be used for this instance.
    static COMPUTATION: OnceLock<DeepThought> = OnceLock::new();
    COMPUTATION.get_or_init(|| DeepThought::new())
}

// The `DeepThought` is built, stored in the `OnceLock`, and returned.
let _ = computation().answer;
// The `DeepThought` is retrieved from the `OnceLock` and returned.
let _ = computation().answer;

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),
);

Fields§

§once: Once§value: UnsafeCell<MaybeUninit<T>>§_marker: PhantomData<T>

Implementations§

source§

impl<T> OnceLock<T>

const: 1.70.0 · source

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

Creates a new empty cell.

source

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.

source

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.

source

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));
}
source

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

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

Sets the contents of this cell to value if the cell was empty, 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 set returns, though not necessarily the one provided.

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

§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));
}
source

pub fn get_or_init<F>(&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.

§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);
source

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 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))
source

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()));
source

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);

Trait Implementations§

source§

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

source§

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

Returns a copy of the value. Read more
1.0.0 · source§

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

Performs copy-assignment from source. Read more
source§

impl<T> Debug for OnceLock<T>
where T: Debug,

source§

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

Formats the value using the given formatter. Read more
source§

impl<T> Default for OnceLock<T>

source§

fn default() -> OnceLock<T>

Creates a new empty cell.

§Example
use std::sync::OnceLock;

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

impl<T> Drop for OnceLock<T>

source§

fn drop(&mut self)

Executes the destructor for this type. Read more
source§

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

source§

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

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(())
source§

impl<T> PartialEq for OnceLock<T>
where T: PartialEq,

source§

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

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

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.
source§

impl<T> Eq for OnceLock<T>
where T: Eq,

source§

impl<T> RefUnwindSafe for OnceLock<T>

source§

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

source§

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

source§

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

Auto Trait Implementations§

§

impl<T> Unpin for OnceLock<T>
where T: Unpin,

Blanket Implementations§

source§

impl<T> Any for T
where T: 'static + ?Sized,

source§

fn type_id(&self) -> TypeId

Gets the TypeId of self. Read more
§

impl<U> As for U

§

fn as_<T>(self) -> T
where T: CastFrom<U>,

Casts self to type T. The semantics of numeric casting with the as operator are followed, so <T as As>::as_::<U> can be used in the same way as T as U for numeric conversions. Read more
source§

impl<T> Borrow<T> for T
where T: ?Sized,

source§

fn borrow(&self) -> &T

Immutably borrows from an owned value. Read more
source§

impl<T> BorrowMut<T> for T
where T: ?Sized,

source§

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

Mutably borrows from an owned value. Read more
§

impl<T1> DecodeUntypedSlice for T1
where T1: From<UntypedValue>,

§

fn decode_untyped_slice(results: &[UntypedValue]) -> Result<T1, UntypedError>

Decodes the slice of [UntypedValue] as a value of type Self. Read more
§

impl<T> Downcast for T
where T: Any,

§

fn into_any(self: Box<T>) -> Box<dyn Any>

Convert Box<dyn Trait> (where Trait: Downcast) to Box<dyn Any>. Box<dyn Any> can then be further downcast into Box<ConcreteType> where ConcreteType implements Trait.
§

fn into_any_rc(self: Rc<T>) -> Rc<dyn Any>

Convert Rc<Trait> (where Trait: Downcast) to Rc<Any>. Rc<Any> can then be further downcast into Rc<ConcreteType> where ConcreteType implements Trait.
§

fn as_any(&self) -> &(dyn Any + 'static)

Convert &Trait (where Trait: Downcast) to &Any. This is needed since Rust cannot generate &Any’s vtable from &Trait’s.
§

fn as_any_mut(&mut self) -> &mut (dyn Any + 'static)

Convert &mut Trait (where Trait: Downcast) to &Any. This is needed since Rust cannot generate &mut Any’s vtable from &mut Trait’s.
§

impl<T> DowncastSync for T
where T: Any + Send + Sync,

§

fn into_any_arc(self: Arc<T>) -> Arc<dyn Any + Send + Sync>

Convert Arc<Trait> (where Trait: Downcast) to Arc<Any>. Arc<Any> can then be further downcast into Arc<ConcreteType> where ConcreteType implements Trait.
source§

impl<Q, K> Equivalent<K> for Q
where Q: Eq + ?Sized, K: Borrow<Q> + ?Sized,

source§

fn equivalent(&self, key: &K) -> bool

Compare self to key and return true if they are equal.
§

impl<Q, K> Equivalent<K> for Q
where Q: Eq + ?Sized, K: Borrow<Q> + ?Sized,

§

fn equivalent(&self, key: &K) -> bool

Checks if this value is equivalent to the given key. Read more
source§

impl<Q, K> Equivalent<K> for Q
where Q: Eq + ?Sized, K: Borrow<Q> + ?Sized,

source§

fn equivalent(&self, key: &K) -> bool

Compare self to key and return true if they are equal.
source§

impl<T> From<!> for T

source§

fn from(t: !) -> T

Converts to this type from the input type.
source§

impl<T> From<T> for T

source§

fn from(t: T) -> T

Returns the argument unchanged.

source§

impl<T, U> Into<U> for T
where U: From<T>,

source§

fn into(self) -> U

Calls U::from(self).

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

§

impl<T> Pointable for T

§

const ALIGN: usize = _

The alignment of pointer.
§

type Init = T

The type for initializers.
§

unsafe fn init(init: <T as Pointable>::Init) -> usize

Initializes a with the given initializer. Read more
§

unsafe fn deref<'a>(ptr: usize) -> &'a T

Dereferences the given pointer. Read more
§

unsafe fn deref_mut<'a>(ptr: usize) -> &'a mut T

Mutably dereferences the given pointer. Read more
§

unsafe fn drop(ptr: usize)

Drops the object pointed to by the given pointer. Read more
source§

impl<T> Same for T

§

type Output = T

Should always be Self
source§

impl<T> ToOwned for T
where T: Clone,

§

type Owned = T

The resulting type after obtaining ownership.
source§

fn to_owned(&self) -> T

Creates owned data from borrowed data, usually by cloning. Read more
source§

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

Uses borrowed data to replace owned data, usually by cloning. Read more
source§

impl<T, U> TryFrom<U> for T
where U: Into<T>,

§

type Error = Infallible

The type returned in the event of a conversion error.
source§

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

Performs the conversion.
source§

impl<T, U> TryInto<U> for T
where U: TryFrom<T>,

§

type Error = <U as TryFrom<T>>::Error

The type returned in the event of a conversion error.
source§

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

Performs the conversion.
§

impl<V, T> VZip<V> for T
where V: MultiLane<T>,

§

fn vzip(self) -> V