pub struct OnceCell<T> { /* private fields */ }once_cell #74465)Expand description
A cell which can be written to only once.
Unlike RefCell, a OnceCell only provides shared &T references to its value.
Unlike Cell, a OnceCell doesn’t require copying or replacing the value to access it.
Examples
#![feature(once_cell)]
use std::cell::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());Implementations
sourceimpl<T> OnceCell<T>
impl<T> OnceCell<T>
sourcepub const fn new() -> OnceCell<T>
🔬This is a nightly-only experimental API. (once_cell #74465)
pub const fn new() -> OnceCell<T>
once_cell #74465)Creates a new empty cell.
sourcepub fn get(&self) -> Option<&T>
🔬This is a nightly-only experimental API. (once_cell #74465)
pub fn get(&self) -> Option<&T>
once_cell #74465)Gets the reference to the underlying value.
Returns None if the cell is empty.
sourcepub fn get_mut(&mut self) -> Option<&mut T>
🔬This is a nightly-only experimental API. (once_cell #74465)
pub fn get_mut(&mut self) -> Option<&mut T>
once_cell #74465)Gets the mutable reference to the underlying value.
Returns None if the cell is empty.
sourcepub fn set(&self, value: T) -> Result<(), T>
🔬This is a nightly-only experimental API. (once_cell #74465)
pub fn set(&self, value: T) -> Result<(), T>
once_cell #74465)Sets the contents of the cell to value.
Errors
This method returns Ok(()) if the cell was empty and Err(value) if
it was full.
Examples
#![feature(once_cell)]
use std::cell::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());sourcepub fn get_or_init<F>(&self, f: F) -> &Twhere
F: FnOnce() -> T,
🔬This is a nightly-only experimental API. (once_cell #74465)
pub fn get_or_init<F>(&self, f: F) -> &Twhere
F: FnOnce() -> T,
once_cell #74465)Gets the contents of the cell, initializing it with f
if the cell was empty.
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. Doing
so results in a panic.
Examples
#![feature(once_cell)]
use std::cell::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>,
🔬This is a nightly-only experimental API. (once_cell #74465)
pub fn get_or_try_init<F, E>(&self, f: F) -> Result<&T, E>where
F: FnOnce() -> Result<T, E>,
once_cell #74465)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. Doing
so results in a panic.
Examples
#![feature(once_cell)]
use std::cell::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 into_inner(self) -> Option<T>
🔬This is a nightly-only experimental API. (once_cell #74465)
pub fn into_inner(self) -> Option<T>
once_cell #74465)Consumes the cell, returning the wrapped value.
Returns None if the cell was empty.
Examples
#![feature(once_cell)]
use std::cell::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()));sourcepub fn take(&mut self) -> Option<T>
🔬This is a nightly-only experimental API. (once_cell #74465)
pub fn take(&mut self) -> Option<T>
once_cell #74465)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.
Safety is guaranteed by requiring a mutable reference.
Examples
#![feature(once_cell)]
use std::cell::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);