# Struct once_cell::sync::OnceCell

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    pub struct OnceCell<T>(_);

Expand description

A thread-safe cell which can be written to only once.

`OnceCell` provides `&` references to the contents without RAII guards.

Reading a non-`None` value out of `OnceCell` establishes a happens-before relationship with a corresponding write. For example, if thread A initializes the cell with `get_or_init(f)`, and thread B subsequently reads the result of this call, B also observes all the side effects of `f`.

## Example

    use once_cell::sync::OnceCell;

    static CELL: OnceCell<String> = OnceCell::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!");

## Implementations

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### impl<T> OnceCell<T>

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

Creates a new empty cell.

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

Creates a new initialized 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 wait(&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();

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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 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();

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#### pub unsafe fn get_unchecked(&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.

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#### pub fn set(&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));
    }

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#### pub fn try_insert(&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());

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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.

##### 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);

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#### pub fn get_or_try_init<F, E>(&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))

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#### pub fn take(&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();

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#### pub fn into_inner(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()));

## Trait Implementations

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

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

Returns a copy of the value. Read more

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

Performs copy-assignment from `source`. Read more

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

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

Returns the “default value” for a type. Read more

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### impl<T> From<T> for OnceCell<T>

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

Converts to this type from the input type.

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

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

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

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

This method tests for `!=`.

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

## Auto Trait Implementations

### impl<T> RefUnwindSafe for OnceCell<T> where
T: UnwindSafe + RefUnwindSafe,

### impl<T> Send for OnceCell<T> where
T: Send,

### impl<T> Sync for OnceCell<T> where
T: Send + Sync,

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

### impl<T> UnwindSafe for OnceCell<T> where
T: UnwindSafe,

## 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,

const: unstable · source

#### 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,

const: unstable · source

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

Mutably borrows from an owned value. Read more

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### impl<T> From<!> for T

const: unstable · source

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

Converts to this type from the input type.

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

const: unstable · source

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

Returns the argument unchanged.

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

const: unstable · 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.

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

#### 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>,

#### type Error = Infallible

The type returned in the event of a conversion error.

const: unstable · source

#### 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>,

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

The type returned in the event of a conversion error.

const: unstable · source

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

Performs the conversion.