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.
For a thread-safe version of this struct, see std::sync::OnceLock
.
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());
RunImplementations§
source§impl<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());
Runsourcepub 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);
Runsourcepub 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))
Runsourcepub 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()));
Runsourcepub 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);
Run