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.

Example

use once_cell::sync::OnceCell;

static CELL: OnceCell<String> = OnceCell::new();
assert!(CELL.get().is_none());

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

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

Methods

impl<T> OnceCell<T>

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

Creates a new empty cell.

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

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

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

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

Example

use once_cell::sync::OnceCell;

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

fn main() {
    assert!(CELL.get().is_none());

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

    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

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.

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. The exact outcome is unspecified. Current implementation deadlocks, but this may be changed to a panic in the future.

Example

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);

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

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.

Note that this method requires parking_lot Cargo feature.

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. The exact outcome is unspecified. Current implementation deadlocks, but this may be changed to a panic in the future.

Example

use once_cell::sync::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))

Trait Implementations

impl<T: PartialEq> PartialEq<OnceCell<T>> for OnceCell<T>

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

This method tests for self and other values to be equal, and is used by ==.

#[must_use]

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

This method tests for !=.

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

fn from(value: T) -> Self

Performs the conversion.

impl<T> Default for OnceCell<T>

fn default() -> OnceCell<T>

Returns the "default value" for a type.

impl<T: Debug> Debug for OnceCell<T>

fn fmt(&self, f: &mut Formatter) -> Result

Formats the value using the given formatter.