# Struct std::sync::OnceLock

1.70.0 · source ·

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

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

    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!");

Run

## Implementations§

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### impl<T> OnceLock<T>

const: 1.70.0 · source

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

Creates a new empty cell.

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#### 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.

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#### 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.

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#### 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));
    }

Run
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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)

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));
    }

Run
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#### 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);

Run
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#### 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))

Run
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#### 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()));

Run
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#### 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);

Run

## Trait Implementations§

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

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

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

Creates a new empty cell.

##### Example

    use std::sync::OnceLock;

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

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

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#### fn drop(&mut self)

Executes the destructor for this type. Read more
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### impl<T> From<T> for OnceLock<T>

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#### 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(())

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

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

This method tests for `self` and `other` values to be equal, and is used by `==`.
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#### 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.
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### impl<T: Eq> Eq for OnceLock<T>

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

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

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

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

## Auto Trait Implementations§

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

## Blanket Implementations§

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### impl<T> Any for Twhere 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 Twhere 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 Twhere 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> 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 Twhere 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 Twhere 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 Twhere 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 Twhere 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.