[][src]Struct std::lazy::OnceCell

pub struct OnceCell<T> { /* fields omitted */ }
🔬 This is a nightly-only experimental API. (once_cell #74465)

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::lazy::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

impl<T> OnceCell<T>[src]

pub fn new() -> OnceCell<T>[src]

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

Creates a new empty cell.

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

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

Gets the reference to the underlying value.

Returns None if the cell is empty.

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

🔬 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.

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

🔬 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::lazy::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

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

Notable traits for &'_ mut F

impl<'_, F> Future for &'_ mut F where
    F: Unpin + Future + ?Sized
type Output = <F as Future>::Output;impl<'_, I> Iterator for &'_ mut I where
    I: Iterator + ?Sized
type Item = <I as Iterator>::Item;impl<R: Read + ?Sized, '_> Read for &'_ mut Rimpl<W: Write + ?Sized, '_> Write for &'_ mut W
where
    F: FnOnce() -> T, 
[src]

🔬 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::lazy::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

pub fn get_or_try_init<F, E>(&self, f: F) -> Result<&T, E> where
    F: FnOnce() -> Result<T, E>, 
[src]

🔬 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::lazy::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

pub fn into_inner(self) -> Option<T>[src]

🔬 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::lazy::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

pub fn take(&mut self) -> Option<T>[src]

🔬 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::lazy::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

impl<T> Clone for OnceCell<T> where
    T: Clone
[src]

impl<T> Debug for OnceCell<T> where
    T: Debug
[src]

impl<T> Default for OnceCell<T>[src]

impl<T> Eq for OnceCell<T> where
    T: Eq
[src]

impl<T> From<T> for OnceCell<T>[src]

impl<T> PartialEq<OnceCell<T>> for OnceCell<T> where
    T: PartialEq<T>, 
[src]

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

impl<T> Any for T where
    T: 'static + ?Sized
[src]

impl<T> Borrow<T> for T where
    T: ?Sized
[src]

impl<T> BorrowMut<T> for T where
    T: ?Sized
[src]

impl<T> From<!> for T[src]

impl<T> From<T> for T[src]

impl<T, U> Into<U> for T where
    U: From<T>, 
[src]

impl<T> ToOwned for T where
    T: Clone
[src]

type Owned = T

The resulting type after obtaining ownership.

impl<T, U> TryFrom<U> for T where
    U: Into<T>, 
[src]

type Error = Infallible

The type returned in the event of a conversion error.

impl<T, U> TryInto<U> for T where
    U: TryFrom<T>, 
[src]

type Error = <U as TryFrom<T>>::Error

The type returned in the event of a conversion error.