Struct OnceCell

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

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());

Implementations

impl<T> OnceCell<T>

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

Creates a new empty cell.

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

Gets the reference to the underlying value.

Returns None if the cell is empty.

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

Gets the mutable reference to the underlying value.

Returns None if the cell is empty.

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());

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());

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);

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);

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))

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))

pub const 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()));

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);

Trait Implementations

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

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

Returns a copy of the value.

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

Performs copy-assignment from source.

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

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

Formats the value using the given formatter.

impl<T> Default for OnceCell<T>

fn default() -> Self

Returns the “default value” for a type.

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

fn from(value: T) -> Self

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

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

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

Tests for self and other values to be equal, and is used by ==.

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

Tests for !=. The default implementation is almost always sufficient, and should not be overridden without very good reason.

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

impl<T> !Sync for OnceCell<T>