OnceCell in std::cell - Rust (original) (raw)
Struct OnceCell
1.70.0 · Source
pub struct OnceCell<T> { /* private fields */ }Expand description
A cell which can nominally be written to only once.
This allows obtaining a shared &T reference to its inner value without copying or replacing it (unlike Cell), and without runtime borrow checks (unlike RefCell). However, only immutable references can be obtained unless one has a mutable reference to the cell itself. In the same vein, the cell can only be re-initialized with such a mutable reference.
A OnceCell can be thought of as a safe abstraction over uninitialized data that becomes initialized once written.
For a thread-safe version of this struct, see std::sync::OnceLock.
§Examples
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());1.70.0 (const: 1.70.0) · Source
Creates a new uninitialized cell.
1.70.0 · Source
Gets the reference to the underlying value.
Returns None if the cell is uninitialized.
1.70.0 · Source
Gets the mutable reference to the underlying value.
Returns None if the cell is uninitialized.
1.70.0 · Source
Initializes the contents of the cell to value.
§Errors
This method returns Ok(()) if the cell was uninitialized and Err(value) if it was already initialized.
§Examples
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());🔬This is a nightly-only experimental API. (once_cell_try_insert #116693)
Initializes the contents of the cell to value if the cell was uninitialized, then returns a reference to it.
§Errors
This method returns Ok(&value) if the cell was uninitialized and Err((¤t_value, value)) if it was already initialized.
§Examples
#![feature(once_cell_try_insert)]
use std::cell::OnceCell;
let cell = OnceCell::new();
assert!(cell.get().is_none());
assert_eq!(cell.try_insert(92), Ok(&92));
assert_eq!(cell.try_insert(62), Err((&92, 62)));
assert!(cell.get().is_some());1.70.0 · Source
Gets the contents of the cell, initializing it to f()if the cell was uninitialized.
§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
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);🔬This is a nightly-only experimental API. (once_cell_get_mut #121641)
Gets the mutable reference of the contents of the cell, initializing it to f() if the cell was uninitialized.
§Panics
If f() panics, the panic is propagated to the caller, and the cell remains uninitialized.
§Examples
#![feature(once_cell_get_mut)]
use std::cell::OnceCell;
let mut cell = OnceCell::new();
let value = cell.get_mut_or_init(|| 92);
assert_eq!(*value, 92);
*value += 2;
assert_eq!(*value, 94);
let value = cell.get_mut_or_init(|| unreachable!());
assert_eq!(*value, 94);🔬This is a nightly-only experimental API. (once_cell_try #109737)
Gets the contents of the cell, initializing it to f() if the cell was uninitialized. If the cell was uninitialized 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_try)]
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))🔬This is a nightly-only experimental API. (once_cell_get_mut #121641)
Gets the mutable reference of the contents of the cell, initializing it to f() if the cell was uninitialized. If the cell was uninitialized and f() failed, an error is returned.
§Panics
If f() panics, the panic is propagated to the caller, and the cell remains uninitialized.
§Examples
#![feature(once_cell_get_mut)]
use std::cell::OnceCell;
let mut cell: OnceCell<u32> = OnceCell::new();
// Failed attempts to initialize the cell do not change its contents
assert!(cell.get_mut_or_try_init(|| "not a number!".parse()).is_err());
assert!(cell.get().is_none());
let value = cell.get_mut_or_try_init(|| "1234".parse());
assert_eq!(value, Ok(&mut 1234));
let Ok(value) = value else { return; };
*value += 2;
assert_eq!(cell.get(), Some(&1236))1.70.0 (const: 1.83.0) · Source
Consumes the cell, returning the wrapped value.
Returns None if the cell was uninitialized.
§Examples
use std::cell::OnceCell;
let cell: OnceCell<String> = OnceCell::new();
assert_eq!(cell.into_inner(), None);
let cell = OnceCell::new();
let _ = cell.set("hello".to_owned());
assert_eq!(cell.into_inner(), Some("hello".to_owned()));1.70.0 · Source
Takes the value out of this OnceCell, moving it back to an uninitialized state.
Has no effect and returns None if the OnceCell is uninitialized.
Safety is guaranteed by requiring a mutable reference.
§Examples
use std::cell::OnceCell;
let mut cell: OnceCell<String> = OnceCell::new();
assert_eq!(cell.take(), None);
let mut cell = OnceCell::new();
let _ = cell.set("hello".to_owned());
assert_eq!(cell.take(), Some("hello".to_owned()));
assert_eq!(cell.get(), None);Creates a new OnceCell<T> which already contains the given value.
Tests for self and other values to be equal, and is used by ==.
Tests for !=. The default implementation is almost always sufficient, and should not be overridden without very good reason.