# Struct core::cell::OnceCell

source · [−]

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

🔬This is a nightly-only experimental API. (`once_cell` #74465)
Expand description

A cell which can be written to only once.

Unlike `RefCell`, a `OnceCell` only provides shared `&T` references to its value. Unlike `Cell`, a `OnceCell` doesn’t require copying or replacing the value to access it.

## Examples

    #![feature(once_cell)]

    use std::cell::OnceCell;

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

    let value: &String = cell.get_or_init(|| {
        "Hello, World!".to_string()
    });
    assert_eq!(value, "Hello, World!");
    assert!(cell.get().is_some());

Run

## Implementations

source

### impl<T> OnceCell<T>

source

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

🔬This is a nightly-only experimental API. (`once_cell` #74465)

Creates a new empty cell.

source

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

🔬This is a nightly-only experimental API. (`once_cell` #74465)

Gets the reference to the underlying value.

Returns `None` if the cell is empty.

source

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

🔬This is a nightly-only experimental API. (`once_cell` #74465)

Gets the mutable reference to the underlying value.

Returns `None` if the cell is empty.

source

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

🔬This is a nightly-only experimental API. (`once_cell` #74465)

Sets the contents of the cell to `value`.

##### Errors

This method returns `Ok(())` if the cell was empty and `Err(value)` if it was full.

##### Examples

    #![feature(once_cell)]

    use std::cell::OnceCell;

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

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

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

Run
source

#### pub fn get_or_init<F>(&self, f: F) -> &Twhere
F: FnOnce() -> T,

🔬This is a nightly-only experimental API. (`once_cell` #74465)

Gets the contents of the cell, initializing it with `f` if the cell was empty.

##### 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`. Doing so results in a panic.

##### Examples

    #![feature(once_cell)]

    use std::cell::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);

Run
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` #74465)

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`. Doing so results in a panic.

##### Examples

    #![feature(once_cell)]

    use std::cell::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))

Run
source

#### pub fn into_inner(self) -> Option<T>

🔬This is a nightly-only experimental API. (`once_cell` #74465)

Consumes the cell, returning the wrapped value.

Returns `None` if the cell was empty.

##### Examples

    #![feature(once_cell)]

    use std::cell::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()));

Run
source

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

🔬This is a nightly-only experimental API. (`once_cell` #74465)

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.

Safety is guaranteed by requiring a mutable reference.

##### Examples

    #![feature(once_cell)]

    use std::cell::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);

Run

## Trait Implementations

source

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

source

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

source

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

Formats the value using the given formatter. Read more
source

### impl<T> Default for OnceCell<T>

source

#### fn default() -> Self

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

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

source

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

Creates a new `OnceCell<T>` which already contains the given `value`.

source

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

source

#### fn eq(&self, other: &Self) -> bool

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

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

## Auto Trait Implementations

### impl<T> !RefUnwindSafe for OnceCell<T>

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

### impl<T> !Sync for OnceCell<T>

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

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

## Blanket Implementations

source

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

source

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

Gets the `TypeId` of `self`. Read more
source

### impl<T> Borrow<T> for Twhere
T: ?Sized,

const: unstable · source

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

Immutably borrows from an owned value. Read more
source

### impl<T> BorrowMut<T> for Twhere
T: ?Sized,

const: unstable · source

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

Mutably borrows from an owned value. Read more
source

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

const: unstable · source

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

Converts to this type from the input type.
source

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

const: unstable · source

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

Returns the argument unchanged.

source

### impl<T, U> Into<U> for Twhere
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.

source

### impl<T, U> TryFrom<U> for Twhere
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.
source

### impl<T, U> TryInto<U> for Twhere
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.