pub struct OnceLock<T> { /* private fields */ }Expand description
A synchronization primitive which can nominally be written to only once.
This type is a thread-safe OnceCell, and can be used in statics.
In many simple cases, you can use LazyLock<T, F> instead to get the benefits of this type
with less effort: LazyLock<T, F> “looks like” &T because it initializes with F on deref!
Where OnceLock shines is when LazyLock is too simple to support a given case, as LazyLock
doesn’t allow additional inputs to its function after you call LazyLock::new(|| ...).
§Examples
Writing to a OnceLock from a separate thread:
use std::sync::OnceLock;
static CELL: OnceLock<usize> = OnceLock::new();
// `OnceLock` has not been written to yet.
assert!(CELL.get().is_none());
// Spawn a thread and write to `OnceLock`.
std::thread::spawn(|| {
let value = CELL.get_or_init(|| 12345);
assert_eq!(value, &12345);
})
.join()
.unwrap();
// `OnceLock` now contains the value.
assert_eq!(
CELL.get(),
Some(&12345),
);RunYou can use OnceLock to implement a type that requires “append-only” logic:
use std::sync::{OnceLock, atomic::{AtomicU32, Ordering}};
use std::thread;
struct OnceList<T> {
data: OnceLock<T>,
next: OnceLock<Box<OnceList<T>>>,
}
impl<T> OnceList<T> {
const fn new() -> OnceList<T> {
OnceList { data: OnceLock::new(), next: OnceLock::new() }
}
fn push(&self, value: T) {
// FIXME: this impl is concise, but is also slow for long lists or many threads.
// as an exercise, consider how you might improve on it while preserving the behavior
if let Err(value) = self.data.set(value) {
let next = self.next.get_or_init(|| Box::new(OnceList::new()));
next.push(value)
};
}
fn contains(&self, example: &T) -> bool
where
T: PartialEq,
{
self.data.get().map(|item| item == example).filter(|v| *v).unwrap_or_else(|| {
self.next.get().map(|next| next.contains(example)).unwrap_or(false)
})
}
}
// Let's exercise this new Sync append-only list by doing a little counting
static LIST: OnceList<u32> = OnceList::new();
static COUNTER: AtomicU32 = AtomicU32::new(0);
let vec = (0..thread::available_parallelism().unwrap().get()).map(|_| thread::spawn(|| {
while let i @ 0..=1000 = COUNTER.fetch_add(1, Ordering::Relaxed) {
LIST.push(i);
}
})).collect::<Vec<thread::JoinHandle<_>>>();
vec.into_iter().for_each(|handle| handle.join().unwrap());
for i in 0..=1000 {
assert!(LIST.contains(&i));
}
RunImplementations§
source§impl<T> OnceLock<T>
impl<T> OnceLock<T>
1.70.0 · sourcepub fn get(&self) -> Option<&T>
pub fn get(&self) -> Option<&T>
Gets the reference to the underlying value.
Returns None if the cell is empty, or being initialized. This
method never blocks.
1.70.0 · sourcepub fn get_mut(&mut self) -> Option<&mut T>
pub fn get_mut(&mut self) -> Option<&mut T>
Gets the mutable reference to the underlying value.
Returns None if the cell is empty. This method never blocks.
1.70.0 · sourcepub fn set(&self, value: T) -> Result<(), T>
pub fn set(&self, value: T) -> Result<(), T>
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
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));
}Runsourcepub fn try_insert(&self, value: T) -> Result<&T, (&T, T)>
🔬This is a nightly-only experimental API. (once_cell_try_insert #116693)
pub fn try_insert(&self, value: T) -> Result<&T, (&T, T)>
once_cell_try_insert #116693)Sets the contents of this cell to value if the cell was empty, 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 set returns, though not necessarily the one provided.
Returns Ok(&value) if the cell was empty and Err(¤t_value, value) if it was full.
§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));
}Run1.70.0 · sourcepub fn get_or_init<F>(&self, f: F) -> &Twhere
F: FnOnce() -> T,
pub fn get_or_init<F>(&self, f: F) -> &Twhere
F: FnOnce() -> T,
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
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);Runsourcepub fn get_mut_or_init<F>(&mut self, f: F) -> &mut Twhere
F: FnOnce() -> T,
🔬This is a nightly-only experimental API. (once_cell_get_mut #121641)
pub fn get_mut_or_init<F>(&mut self, f: F) -> &mut Twhere
F: FnOnce() -> T,
once_cell_get_mut #121641)Gets the mutable reference of the contents of the cell, initializing
it with f if the cell was empty.
Many threads may call get_mut_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.
§Examples
#![feature(once_cell_get_mut)]
use std::sync::OnceLock;
let mut cell = OnceLock::new();
let value = cell.get_mut_or_init(|| 92);
assert_eq!(*value, 92);
*value += 2;
assert_eq!(*value, 94);
let value = cell.get_mut_or_init(|| unreachable!());
assert_eq!(*value, 94);Runsourcepub fn get_or_try_init<F, E>(&self, f: F) -> Result<&T, E>
🔬This is a nightly-only experimental API. (once_cell_try #109737)
pub fn get_or_try_init<F, E>(&self, f: F) -> Result<&T, E>
once_cell_try #109737)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_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))Runsourcepub fn get_mut_or_try_init<F, E>(&mut self, f: F) -> Result<&mut T, E>
🔬This is a nightly-only experimental API. (once_cell_get_mut #121641)
pub fn get_mut_or_try_init<F, E>(&mut self, f: F) -> Result<&mut T, E>
once_cell_get_mut #121641)Gets the mutable reference of 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.
§Examples
#![feature(once_cell_get_mut)]
use std::sync::OnceLock;
let mut cell: OnceLock<u32> = OnceLock::new();
// Failed initializers do not change the value
assert!(cell.get_mut_or_try_init(|| "not a number!".parse()).is_err());
assert!(cell.get().is_none());
let value = cell.get_mut_or_try_init(|| "1234".parse());
assert_eq!(value, Ok(&mut 1234));
*value.unwrap() += 2;
assert_eq!(cell.get(), Some(&1236))Run1.70.0 · sourcepub fn into_inner(self) -> Option<T>
pub fn into_inner(self) -> Option<T>
Consumes the OnceLock, returning the wrapped value. Returns
None if the cell was empty.
§Examples
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()));Run1.70.0 · sourcepub fn take(&mut self) -> Option<T>
pub fn take(&mut self) -> Option<T>
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
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