Struct OnceCell

pub struct OnceCell<T> {
    inner: UnsafeCell<Option<T>>,
}

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());

Fields

inner: UnsafeCell<Option<T>>

Implementations

impl<T> OnceCell<T>

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

Creates a new uninitialized cell.

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

Gets the reference to the underlying value.

Returns None if the cell is uninitialized.

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

Gets the mutable reference to the underlying value.

Returns None if the cell is uninitialized.

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());

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

🔬This is a nightly-only experimental API. (once_cell_try_insert)

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());

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);

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)

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);

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)

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))

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)

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))

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()));

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

impl<T> Clone for OnceCell<T>

where T: Clone,

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

Returns a duplicate of the value.

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

Performs copy-assignment from source.

impl<T> Debug for OnceCell<T>

where T: Debug,

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

Formats the value using the given formatter.

impl<T> Default for OnceCell<T>

fn default() -> OnceCell<T>

Returns the “default value” for a type.

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

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

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

impl<T> PartialEq for OnceCell<T>

where T: PartialEq,

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

where T: Eq,

impl<T> !Sync for OnceCell<T>