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());
Implementations§
Source§impl<T> OnceCell<T>
impl<T> OnceCell<T>
1.70.0 · Sourcepub fn get(&self) -> Option<&T>
pub fn get(&self) -> Option<&T>
Gets the reference to the underlying value.
Returns None
if the cell is uninitialized.
1.70.0 · Sourcepub fn get_mut(&mut self) -> Option<&mut T>
pub fn get_mut(&mut self) -> Option<&mut T>
Gets the mutable reference to the underlying value.
Returns None
if the cell is uninitialized.
1.70.0 · Sourcepub fn set(&self, value: T) -> Result<(), T>
pub fn set(&self, value: T) -> Result<(), T>
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());
Sourcepub fn try_insert(&self, value: T) -> Result<&T, (&T, T)>
🔬This is a nightly-only experimental API. (once_cell_try_insert
)
pub fn try_insert(&self, value: T) -> Result<&T, (&T, T)>
once_cell_try_insert
)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 · Sourcepub fn get_or_init<F>(&self, f: F) -> &Twhere
F: FnOnce() -> T,
pub fn get_or_init<F>(&self, f: F) -> &Twhere
F: FnOnce() -> T,
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);
Sourcepub fn get_mut_or_init<F>(&mut self, f: F) -> &mut Twhere
F: FnOnce() -> T,
🔬This is a nightly-only experimental API. (once_cell_get_mut
)
pub fn get_mut_or_init<F>(&mut self, f: F) -> &mut Twhere
F: FnOnce() -> T,
once_cell_get_mut
)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);
Sourcepub fn get_or_try_init<F, E>(&self, f: F) -> Result<&T, E>
🔬This is a nightly-only experimental API. (once_cell_try
)
pub fn get_or_try_init<F, E>(&self, f: F) -> Result<&T, E>
once_cell_try
)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))
Sourcepub fn get_mut_or_try_init<F, E>(&mut self, f: F) -> Result<&mut T, E>
🔬This is a nightly-only experimental API. (once_cell_get_mut
)
pub fn get_mut_or_try_init<F, E>(&mut self, f: F) -> Result<&mut T, E>
once_cell_get_mut
)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) · Sourcepub const fn into_inner(self) -> Option<T>
pub const fn into_inner(self) -> Option<T>
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 · Sourcepub fn take(&mut self) -> Option<T>
pub fn take(&mut self) -> Option<T>
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);