Struct once_cell::unsync::OnceCell [−][src]
pub struct OnceCell<T> { /* fields omitted */ }
A cell which can be written to only once. Not thread safe.
Unlike ::std::cell::RefCell
, a OnceCell
provides simple &
references to the contents.
Example
use once_cell::unsync::OnceCell; let cell = OnceCell::new(); assert!(cell.get().is_none()); let value: &String = cell.get_or_init(|| { "Hello, World!".to_string() }); assert_eq!(value, "Hello, World!"); assert!(cell.get().is_some());
Methods
impl<T> OnceCell<T>
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impl<T> OnceCell<T>
pub const INIT: OnceCell<T>
INIT: OnceCell<T> = OnceCell{inner: <UnsafeCell>::new(None),}
An empty cell, for initialization in a const
context.
pub fn new() -> OnceCell<T>
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pub fn new() -> OnceCell<T>
Creates a new empty cell.
pub fn get(&self) -> Option<&T>
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pub fn get(&self) -> Option<&T>
Gets the reference to the underlying value. Returns None
if the cell is empty.
pub fn set(&self, value: T) -> Result<(), T>
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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::unsync::OnceCell; let cell = OnceCell::new(); assert!(cell.get().is_none()); assert_eq!(cell.set(92), Ok(())); assert_eq!(cell.set(62), Err(62)); assert!(cell.get().is_some());
pub fn get_or_init<F: FnOnce() -> T>(&self, f: F) -> &T
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pub fn get_or_init<F: FnOnce() -> T>(&self, f: F) -> &T
Gets the contents of the cell, initializing it with f
if the cell was empty.
Example
use once_cell::unsync::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: FnOnce() -> Result<T, E>, E>(
&self,
f: F
) -> Result<&T, E>
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pub fn get_or_try_init<F: FnOnce() -> Result<T, E>, E>(
&self,
f: F
) -> 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.
Example
use once_cell::unsync::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))
Trait Implementations
impl<T: Debug> Debug for OnceCell<T>
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impl<T: Debug> Debug for OnceCell<T>
fn fmt(&self, f: &mut Formatter) -> Result
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fn fmt(&self, f: &mut Formatter) -> Result
Formats the value using the given formatter. Read more
impl<T: Default> Default for OnceCell<T>
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impl<T: Default> Default for OnceCell<T>