Struct std::cell::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.

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());
Run

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 empty cell.

1.70.0 · source

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

Gets the reference to the underlying value.

Returns None if the cell is empty.

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 empty.

1.70.0 · source

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

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
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
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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 the cell to value if the cell was empty, then returns a reference to it.

§Errors

This method 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::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());
Run
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 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
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
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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 with f if the cell was empty.

§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);
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. 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))
Run
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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 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.

§Examples
#![feature(once_cell_get_mut)]

use std::cell::OnceCell;

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

// Failed initializers do not change the value
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))
Run
1.70.0 · source

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

Consumes the cell, returning the wrapped value.

Returns None if the cell was empty.

§Examples
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
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 hasn’t been initialized.

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();
cell.set("hello".to_string()).unwrap();
assert_eq!(cell.take(), Some("hello".to_string()));
assert_eq!(cell.get(), None);
Run

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 copy 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 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 · 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 · 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

This method tests for self and other values to be equal, and is used by ==.
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.
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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default unsafe fn clone_to_uninit(&self, dst: *mut T)

🔬This is a nightly-only experimental API. (clone_to_uninit #126799)
Performs copy-assignment from self to dst. 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.