Struct ImmutableCell

Source

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

Implementations§

Source §

pub fn get_or<E>(&self, error: E) -> Result<&T, E>

Methods from Deref<Target = OnceLock<T>>§

1.70.0 · Source

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

Gets the reference to the underlying value.

Returns None if the cell is uninitialized, or being initialized. This method never blocks.

1.86.0 · Source

pub fn wait(&self) -> &T

Blocks the current thread until the cell is initialized.

§Example

Waiting for a computation on another thread to finish:

use std::thread;
use std::sync::OnceLock;

let value = OnceLock::new();

thread::scope(|s| {
    s.spawn(|| value.set(1 + 1));

    let result = value.wait();
    assert_eq!(result, &2);
})

1.70.0 · Source

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

Initializes the contents of the cell to value.

May block if another thread is currently attempting to initialize the cell. The cell is guaranteed to contain a value when set returns, though not necessarily the one provided.

Returns Ok(()) if the cell was uninitialized and Err(value) if the cell was already initialized.

§Examples

use std::sync::OnceLock;

static CELL: OnceLock<i32> = OnceLock::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));
}

Source

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

🔬This is a nightly-only experimental API. (once_cell_try_insert)

Initializes the contents of the cell to value if the cell was uninitialized, then returns a reference to it.

May block if another thread is currently attempting to initialize the cell. The cell is guaranteed to contain a value when try_insert returns, though not necessarily the one provided.

Returns Ok(&value) if the cell was uninitialized and Err((&current_value, value)) if it was already initialized.

§Examples

#![feature(once_cell_try_insert)]

use std::sync::OnceLock;

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

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

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

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

1.70.0 · Source

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

Gets the contents of the cell, initializing it to f() if the cell was uninitialized.

Many 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.

§Examples

use std::sync::OnceLock;

let cell = OnceLock::new();
let value = cell.get_or_init(|| 92);
assert_eq!(value, &92);
let value = cell.get_or_init(|| unreachable!());
assert_eq!(value, &92);

Source

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

🔬This is a nightly-only experimental API. (once_cell_try)

Gets the contents of the cell, initializing it to f() if the cell was uninitialized. If the cell was uninitialized 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. The exact outcome is unspecified. Current implementation deadlocks, but this may be changed to a panic in the future.

§Examples

#![feature(once_cell_try)]

use std::sync::OnceLock;

let cell = OnceLock::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))