[][src]Struct once_cell::sync::OnceCell

pub struct OnceCell<T>(_);

A thread-safe cell which can be written to only once.

Unlike ::std::sync::Mutex, a OnceCell provides simple & references to the contents.


use once_cell::sync::OnceCell;

static CELL: OnceCell<String> = OnceCell::new();

std::thread::spawn(|| {
    let value: &String = CELL.get_or_init(|| {
        "Hello, World!".to_string()
    assert_eq!(value, "Hello, World!");

let value: Option<&String> = CELL.get();
assert_eq!(value.unwrap().as_str(), "Hello, World!");


impl<T> OnceCell<T>[src]

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

Creates a new empty cell.

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

Gets the reference to the underlying value. Returns None if the cell is empty.

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

Sets the contents of this cell to value. Returns Ok(()) if the cell was empty and Err(value) if it was full.


use once_cell::sync::OnceCell;

static CELL: OnceCell<i32> = OnceCell::new();

fn main() {

    std::thread::spawn(|| {
        assert_eq!(CELL.set(92), Ok(()));

    assert_eq!(CELL.set(62), Err(62));
    assert_eq!(CELL.get(), Some(&92));

pub fn get_or_init<F: FnOnce() -> T>(&self, f: F) -> &T[src]

Gets the contents of the cell, initializing it with f if the cell was empty. May threads may call get_or_init concurrently with different initializing functions, but it is guaranteed that only one function will be executed.


use once_cell::sync::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);

Trait Implementations

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

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

This method tests for !=.

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

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

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

Auto Trait Implementations

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

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

Blanket Implementations

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

impl<T, U> Into<U> for T where
    U: From<T>, 

impl<T, U> TryFrom<U> for T where
    U: Into<T>, 

type Error = Infallible

The type returned in the event of a conversion error.

impl<T, U> TryInto<U> for T where
    U: TryFrom<T>, 

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

The type returned in the event of a conversion error.

impl<T> BorrowMut<T> for T where
    T: ?Sized

impl<T> Borrow<T> for T where
    T: ?Sized

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