Struct std::sync::OnceLock

source ·
pub struct OnceLock<T> { /* private fields */ }
🔬This is a nightly-only experimental API. (once_cell #74465)
Expand description

A synchronization primitive which can be written to only once.

This type is a thread-safe OnceCell, and can be used in statics.

Examples

#![feature(once_cell)]

use std::sync::OnceLock;

static CELL: OnceLock<String> = OnceLock::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!");
Run

Implementations

🔬This is a nightly-only experimental API. (once_cell #74465)

Creates a new empty cell.

🔬This is a nightly-only experimental API. (once_cell #74465)

Gets the reference to the underlying value.

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

🔬This is a nightly-only experimental API. (once_cell #74465)

Gets the mutable reference to the underlying value.

Returns None if the cell is empty. This method never blocks.

🔬This is a nightly-only experimental API. (once_cell #74465)

Sets the contents of this 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’s value was set by this call.

Examples
#![feature(once_cell)]

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));
}
Run
🔬This is a nightly-only experimental API. (once_cell #74465)

Gets the contents of the cell, initializing it with f if the cell was empty.

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
#![feature(once_cell)]

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);
Run
🔬This is a nightly-only experimental API. (once_cell #74465)

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.

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)]

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))
Run
🔬This is a nightly-only experimental API. (once_cell #74465)

Consumes the OnceLock, returning the wrapped value. Returns None if the cell was empty.

Examples
#![feature(once_cell)]

use std::sync::OnceLock;

let cell: OnceLock<String> = OnceLock::new();
assert_eq!(cell.into_inner(), None);

let cell = OnceLock::new();
cell.set("hello".to_string()).unwrap();
assert_eq!(cell.into_inner(), Some("hello".to_string()));
Run
🔬This is a nightly-only experimental API. (once_cell #74465)

Takes the value out of this OnceLock, moving it back to an uninitialized state.

Has no effect and returns None if the OnceLock hasn’t been initialized.

Safety is guaranteed by requiring a mutable reference.

Examples
#![feature(once_cell)]

use std::sync::OnceLock;

let mut cell: OnceLock<String> = OnceLock::new();
assert_eq!(cell.take(), None);

let mut cell = OnceLock::new();
cell.set("hello".to_string()).unwrap();
assert_eq!(cell.take(), Some("hello".to_string()));
assert_eq!(cell.get(), None);
Run

Trait Implementations

Returns a copy of the value. Read more
Performs copy-assignment from source. Read more
Formats the value using the given formatter. Read more

Creates a new empty cell.

Example
#![feature(once_cell)]

use std::sync::OnceLock;

fn main() {
    assert_eq!(OnceLock::<()>::new(), OnceLock::default());
}
Run
Executes the destructor for this type. Read more

Create a new cell with its contents set to value.

Example
#![feature(once_cell)]

use std::sync::OnceLock;

let a = OnceLock::from(3);
let b = OnceLock::new();
b.set(3)?;
assert_eq!(a, b);
Ok(())
Run
This method tests for self and other values to be equal, and is used by ==. Read more
This method tests for !=. The default implementation is almost always sufficient, and should not be overridden without very good reason. Read more

Auto Trait Implementations

Blanket Implementations

Gets the TypeId of self. Read more
Immutably borrows from an owned value. Read more
Mutably borrows from an owned value. Read more
Converts to this type from the input type.

Returns the argument unchanged.

Calls U::from(self).

That is, this conversion is whatever the implementation of From<T> for U chooses to do.

The resulting type after obtaining ownership.
Creates owned data from borrowed data, usually by cloning. Read more
Uses borrowed data to replace owned data, usually by cloning. Read more
The type returned in the event of a conversion error.
Performs the conversion.
The type returned in the event of a conversion error.
Performs the conversion.