OnceLock in std::sync - Rust
Description: A synchronization primitive which can nominally be written to only once.

## OnceLock

# Struct OnceLock

1.70.0 · Source

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

Expand description

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.

Unlike `Mutex`, `OnceLock` is never poisoned on panic.

## §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§

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### impl<T> OnceLock<T>

1.70.0 (const: 1.70.0) · Source

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

Creates a new uninitialized cell.

1.70.0 · Source

#### 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.

1.70.0 · Source

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

Gets the mutable reference to the underlying value.

Returns `None` if the cell is uninitialized.

This method never blocks. Since it borrows the `OnceLock` mutably, it is statically guaranteed that no active borrows to the `OnceLock` exist, including from other threads.

1.86.0 · Source

#### 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);
    })

1.70.0 · Source

#### 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));
    }

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

🔬This is a nightly-only experimental API. (`once_cell_try_insert` #116693)

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));
    }

1.70.0 · Source

#### 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 if the function doesn’t panic.

##### §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_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` #121641)

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

This method never blocks. Since it borrows the `OnceLock` mutably, it is statically guaranteed that no active borrows to the `OnceLock` exist, including from other threads.

##### §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);

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` #109737)

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))

Source

#### 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` #121641)

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. Since it borrows the `OnceLock` mutably, it is statically guaranteed that no active borrows to the `OnceLock` exist, including from other threads.

##### §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))

1.70.0 · Source

#### 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()));

1.70.0 · 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` was uninitialized.

Since this method borrows the `OnceLock` mutably, it is statically guaranteed that no active borrows to the `OnceLock` exist, including from other threads.

##### §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§

1.70.0 · Source§

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

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

Returns a duplicate of the value. Read more
1.0.0 · Source§

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

Performs copy-assignment from `source`. Read more
1.70.0 · Source§

### impl<T: Debug> Debug for OnceLock<T>

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

Formats the value using the given formatter. Read more
1.70.0 (const: unstable) · Source§

### impl<T> Default for OnceLock<T>

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#### fn default() -> OnceLock<T>

Creates a new uninitialized cell.

##### §Example

    use std::sync::OnceLock;

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

1.70.0 · Source§

### impl<T> Drop for OnceLock<T>

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#### fn drop(&mut self)

Executes the destructor for this type. Read more
1.70.0 · Source§

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

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#### fn from(value: T) -> Self

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(())

1.70.0 · Source§

### impl<T: PartialEq> PartialEq for OnceLock<T>

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

Equality for two `OnceLock`s.

Two `OnceLock`s 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());

1.0.0 · Source§

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

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

### impl<T: Eq> Eq for OnceLock<T>

1.70.0 · Source§

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

1.70.0 · Source§

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

1.70.0 · Source§

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

1.70.0 · Source§

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

## Auto Trait Implementations§

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### impl<T> !Freeze for OnceLock<T>

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

## Blanket Implementations§

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

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#### fn type_id(&self) -> TypeId

Gets the `TypeId` of `self`. Read more
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### impl<T> Borrow<T> for T
where T: ?Sized,

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#### fn borrow(&self) -> &T

Immutably borrows from an owned value. Read more
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### impl<T> BorrowMut<T> for T
where T: ?Sized,

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

Mutably borrows from an owned value. Read more
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### impl<T> CloneToUninit for T
where T: Clone,

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

🔬This is a nightly-only experimental API. (`clone_to_uninit` #126799)
Performs copy-assignment from `self` to `dest`. Read more
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### impl<T> From<!> for T

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#### fn from(t: !) -> T

Converts to this type from the input type.
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### impl<T> From<T> for T

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#### fn from(t: T) -> T

Returns the argument unchanged.

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

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#### fn into(self) -> U

Calls `U::from(self)`.

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

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

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#### type Owned = T

The resulting type after obtaining ownership.
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#### fn to_owned(&self) -> T

Creates owned data from borrowed data, usually by cloning. Read more
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#### fn clone_into(&self, target: &mut T)

Uses borrowed data to replace owned data, usually by cloning. Read more
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### impl<T, U> TryFrom<U> for T
where U: Into<T>,

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#### type Error = Infallible

The type returned in the event of a conversion error.
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#### fn try_from(value: U) -> Result<T, <T as TryFrom<U>>::Error>

Performs the conversion.
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### impl<T, U> TryInto<U> for T
where U: TryFrom<T>,

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

The type returned in the event of a conversion error.
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#### fn try_into(self) -> Result<U, <U as TryFrom<T>>::Error>

Performs the conversion.