OnceCell in std::cell - Rust
Description: A cell which can nominally be written to only once.

## OnceCell

# Struct OnceCell

1.70.0 · Source

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

Expand description

A cell which can nominally be written to only once.

This allows obtaining a shared `&T` reference to its inner value without copying or replacing it (unlike `Cell`), and without runtime borrow checks (unlike `RefCell`). However, only immutable references can be obtained unless one has a mutable reference to the cell itself. In the same vein, the cell can only be re-initialized with such a mutable reference.

A `OnceCell` can be thought of as a safe abstraction over uninitialized data that becomes initialized once written.

For a thread-safe version of this struct, see `std::sync::OnceLock`.

## §Examples

    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());

## Implementations§

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### impl<T> OnceCell<T>

1.70.0 (const: 1.70.0) · Source

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

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.

1.70.0 · Source

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

Initializes the contents of the cell to `value`.

##### §Errors

This method returns `Ok(())` if the cell was uninitialized and `Err(value)` if it was already initialized.

##### §Examples

    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());

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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.

##### §Errors

This method 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::cell::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());

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.

##### §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

    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);

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#### 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.

##### §Panics

If `f()` panics, the panic is propagated to the caller, and the cell remains uninitialized.

##### §Examples

    #![feature(once_cell_get_mut)]

    use std::cell::OnceCell;

    let mut cell = OnceCell::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);

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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 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`. Doing so results in a panic.

##### §Examples

    #![feature(once_cell_try)]

    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))

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#### 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.

##### §Panics

If `f()` panics, the panic is propagated to the caller, and the cell remains uninitialized.

##### §Examples

    #![feature(once_cell_get_mut)]

    use std::cell::OnceCell;

    let mut cell: OnceCell<u32> = OnceCell::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));

    let Ok(value) = value else { return; };
    *value += 2;
    assert_eq!(cell.get(), Some(&1236))

1.70.0 (const: 1.83.0) · Source

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

Consumes the cell, returning the wrapped value.

Returns `None` if the cell was uninitialized.

##### §Examples

    use std::cell::OnceCell;

    let cell: OnceCell<String> = OnceCell::new();
    assert_eq!(cell.into_inner(), None);

    let cell = OnceCell::new();
    let _ = cell.set("hello".to_owned());
    assert_eq!(cell.into_inner(), Some("hello".to_owned()));

1.70.0 · Source

#### 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` is uninitialized.

Safety is guaranteed by requiring a mutable reference.

##### §Examples

    use std::cell::OnceCell;

    let mut cell: OnceCell<String> = OnceCell::new();
    assert_eq!(cell.take(), None);

    let mut cell = OnceCell::new();
    let _ = cell.set("hello".to_owned());
    assert_eq!(cell.take(), Some("hello".to_owned()));
    assert_eq!(cell.get(), None);

## Trait Implementations§

1.70.0 · Source§

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

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#### fn clone(&self) -> OnceCell<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
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### impl<T> Debug for OnceCell<T>
where T: Debug,

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

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

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

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

Returns the “default value” for a type. Read more
1.70.0 (const: unstable) · Source§

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

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#### fn from(value: T) -> OnceCell<T>

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

1.70.0 · Source§

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

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

Tests for `self` and `other` values to be equal, and is used by `==`.
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 for OnceCell<T>
where T: Eq,

1.70.0 · Source§

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

## Auto Trait Implementations§

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

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

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

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

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### 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,

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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.