Struct once_cell::sync::OnceCell [−][src]
pub struct OnceCell<T> { /* fields omitted */ }
A thread-safe cell which can be written to only once.
Unlike ::std::sync::Mutex
, a OnceCell
provides simple &
references to the contents.
Example
use once_cell::sync::OnceCell; static CELL: OnceCell<String> = OnceCell::INIT; 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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impl<T> OnceCell<T>
pub const INIT: OnceCell<T>
INIT: OnceCell<T> = OnceCell{once: ONCE_INIT, value: <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::sync::OnceCell; static CELL: OnceCell<i32> = OnceCell::INIT; 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: 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. May threads may call get_or_init
concurrently with different initializing functions, but
it is guaranteed that only one function will be executed.
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);