Struct OnceCell 

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

A cell that’s initialized at most once, asynchronously.

Concurrent callers of OnceCell::get_or_init that race to be first all wait for the same single initialization to complete rather than each running their own initializer.

Implementations

impl<T> OnceCell<T>

pub const fn new() -> Self

Create an empty, uninitialized cell.

pub const fn new_with(value: T) -> Self

Create a cell already initialized with value.

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

The cell’s value, if it’s already been initialized.

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

Set the cell’s value if it isn’t already initialized.

Errors

Returns value back if the cell was already initialized (or is concurrently being initialized by Self::get_or_init).

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

where F: FnOnce() -> Fut, Fut: Future<Output = T>, T: Send + Sync,

Return the cell’s value, initializing it with (the result of awaiting) init first if necessary. Concurrent callers racing to initialize the same cell all observe the same initialization — only the winner’s init future actually runs.

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

Consume the cell, returning its value if initialized.

Trait Implementations

impl<T> Default for OnceCell<T>

fn default() -> Self

Returns the “default value” for a type.

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

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