[−][src]Struct once_cell::sync::OnceCell
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!");
Methods
impl<T> OnceCell<T>
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pub const fn new() -> OnceCell<T>
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Creates a new empty cell.
pub fn get(&self) -> Option<&T>
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Gets the reference to the underlying value.
Returns None
if the cell is empty, or being initialized. This
method never blocks.
pub fn get_mut(&mut self) -> Option<&mut T>
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Gets the mutable reference to the underlying value.
Returns None
if the cell is empty.
pub unsafe fn get_unchecked(&self) -> &T
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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.
pub fn set(&self, value: T) -> Result<(), T>
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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>(&self, f: F) -> &T where
F: FnOnce() -> T,
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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);
pub fn get_or_try_init<F, E>(&self, f: F) -> Result<&T, E> where
F: FnOnce() -> Result<T, E>,
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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))
pub fn into_inner(self) -> Option<T>
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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()));
Trait Implementations
impl<T: Clone> Clone for OnceCell<T>
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impl<T: Debug> Debug for OnceCell<T>
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impl<T> Default for OnceCell<T>
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impl<T: Eq> Eq for OnceCell<T>
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impl<T> From<T> for OnceCell<T>
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impl<T: PartialEq> PartialEq<OnceCell<T>> for OnceCell<T>
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Auto Trait Implementations
impl<T> RefUnwindSafe for OnceCell<T> where
T: RefUnwindSafe + UnwindSafe,
T: RefUnwindSafe + UnwindSafe,
impl<T> Send for OnceCell<T> where
T: Send,
T: Send,
impl<T> Sync for OnceCell<T> where
T: Send + Sync,
T: Send + Sync,
impl<T> Unpin for OnceCell<T> where
T: Unpin,
T: Unpin,
impl<T> UnwindSafe for OnceCell<T> where
T: UnwindSafe,
T: UnwindSafe,
Blanket Implementations
impl<T> Any for T where
T: 'static + ?Sized,
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T: 'static + ?Sized,
impl<T> Borrow<T> for T where
T: ?Sized,
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T: ?Sized,
impl<T> BorrowMut<T> for T where
T: ?Sized,
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T: ?Sized,
fn borrow_mut(&mut self) -> &mut T
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impl<T> From<!> for T
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impl<T> From<T> for T
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impl<T, U> Into<U> for T where
U: From<T>,
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U: From<T>,
impl<T> ToOwned for T where
T: Clone,
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T: Clone,
type Owned = T
The resulting type after obtaining ownership.
fn to_owned(&self) -> T
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fn clone_into(&self, target: &mut T)
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impl<T, U> TryFrom<U> for T where
U: Into<T>,
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U: Into<T>,
type Error = Infallible
The type returned in the event of a conversion error.
fn try_from(value: U) -> Result<T, <T as TryFrom<U>>::Error>
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impl<T, U> TryInto<U> for T where
U: TryFrom<T>,
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U: TryFrom<T>,