pub struct OnceCell<T>(_);
Expand description
A thread-safe cell which can be written to only once.
OnceCell
provides &
references to the contents without RAII guards.
Reading a non-None
value out of OnceCell
establishes a
happens-before relationship with a corresponding write. For example, if
thread A initializes the cell with get_or_init(f)
, and thread B
subsequently reads the result of this call, B also observes all the side
effects of f
.
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!");
Implementations§
source§impl<T> OnceCell<T>
impl<T> OnceCell<T>
sourcepub const fn with_value(value: T) -> OnceCell<T>
pub const fn with_value(value: T) -> OnceCell<T>
Creates a new initialized cell.
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.
sourcepub fn wait(&self) -> &T
pub fn wait(&self) -> &T
Gets the reference to the underlying value, blocking the current thread until it is set.
use once_cell::sync::OnceCell;
let mut cell = std::sync::Arc::new(OnceCell::new());
let t = std::thread::spawn({
let cell = std::sync::Arc::clone(&cell);
move || cell.set(92).unwrap()
});
// Returns immediately, but might return None.
let _value_or_none = cell.get();
// Will return 92, but might block until the other thread does `.set`.
let value: &u32 = cell.wait();
assert_eq!(*value, 92);
t.join().unwrap();
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 is allowed to violate the invariant of writing to a OnceCell
at most once because it requires &mut
access to self
. As with all
interior mutability, &mut
access permits arbitrary modification:
use once_cell::sync::OnceCell;
let mut cell: OnceCell<u32> = OnceCell::new();
cell.set(92).unwrap();
cell = OnceCell::new();
sourcepub unsafe fn get_unchecked(&self) -> &T
pub unsafe fn get_unchecked(&self) -> &T
Get the reference to the underlying value, without checking if the cell is initialized.
Safety
Caller must ensure that the cell is in initialized state, and that the contents are acquired by (synchronized to) this thread.
sourcepub fn set(&self, value: T) -> Result<(), T>
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));
}
sourcepub fn try_insert(&self, value: T) -> Result<&T, (&T, T)>
pub fn try_insert(&self, value: T) -> Result<&T, (&T, T)>
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.
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);
sourcepub fn get_or_try_init<F, E>(&self, f: F) -> Result<&T, E>where
F: FnOnce() -> Result<T, E>,
pub fn get_or_try_init<F, E>(&self, f: F) -> Result<&T, E>where F: FnOnce() -> 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.
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))
sourcepub fn take(&mut self) -> Option<T>
pub fn take(&mut self) -> Option<T>
Takes the value out of this OnceCell
, moving it back to an uninitialized state.
Has no effect and returns None
if the OnceCell
hasn’t been initialized.
Examples
use once_cell::sync::OnceCell;
let mut cell: OnceCell<String> = OnceCell::new();
assert_eq!(cell.take(), None);
let mut cell = OnceCell::new();
cell.set("hello".to_string()).unwrap();
assert_eq!(cell.take(), Some("hello".to_string()));
assert_eq!(cell.get(), None);
This method is allowed to violate the invariant of writing to a OnceCell
at most once because it requires &mut
access to self
. As with all
interior mutability, &mut
access permits arbitrary modification:
use once_cell::sync::OnceCell;
let mut cell: OnceCell<u32> = OnceCell::new();
cell.set(92).unwrap();
cell = OnceCell::new();
sourcepub fn into_inner(self) -> Option<T>
pub fn into_inner(self) -> Option<T>
Consumes the OnceCell
, returning the wrapped value. Returns
None
if the cell was empty.
Examples
use once_cell::sync::OnceCell;
let cell: OnceCell<String> = OnceCell::new();
assert_eq!(cell.into_inner(), None);
let cell = OnceCell::new();
cell.set("hello".to_string()).unwrap();
assert_eq!(cell.into_inner(), Some("hello".to_string()));