Struct OnceCell

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

Cell which can be lazy instantiated with an asynchronous block and is safely share-able between threads.

Implementations

impl<T> OnceCell<T>

pub fn new() -> Self

Creates a new empty OnceCell. Currently this function is not const due to Mutex limitations, so to share between multiple threads an Arc needs to be used.

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

where F: Future<Output = T>,

Get or initialize this cell with the given asynchronous block. If the cell is already initialized, the current value will be returned. Otherwise the asynchronous block is used to initialize the OnceCell. This function will always return a value.

Example

let cell = OnceCell::new();
cell.get_or_init(async {
    0 // expensive calculation
}).await;
assert_eq!(cell.get(), Some(&0));

pub async fn get_or_try_init<F, E>(&self, f: F) -> Result<&T, E>

where F: Future<Output = Result<T, E>>,

Get or initialize this cell with the given asynchronous block. If the cell is already initialized, the current value will be returned. Otherwise the asynchronous block is used to initialize the OnceCell. This function will always return a value.

Example

let cell = OnceCell::new();
cell.get_or_try_init(async {
    Ok::<_, Infallible>(0) // expensive calculation
}).await;
assert_eq!(cell.get(), Some(&0));

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

Returns the value of the OnceCell or None if the OnceCell has not been initialized yet.

pub fn initialized(&self) -> bool

If the OnceCell is already initialized, either by having have called get_or_init() or get_or_try_init().

Trait Implementations

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: 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: Send> Send for OnceCell<T>

impl<T: Sync + Send> Sync for OnceCell<T>