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.
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 empty.
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 empty.
1.70.0 · sourcepub fn set(&self, value: T) -> Result<(), T>
pub fn set(&self, value: T) -> Result<(), T>
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
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
)Sets the contents of the cell to value
if the cell was empty, then
returns a reference to it.
§Errors
This method returns Ok(&value)
if the cell was empty and
Err(¤t_value, value)
if it was full.
§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 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
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 with f
if the cell was empty.
§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 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_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 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.
§Examples
#![feature(once_cell_get_mut)]
use std::cell::OnceCell;
let mut cell: OnceCell<u32> = OnceCell::new();
// Failed initializers do not change the value
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));
*value.unwrap() += 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 empty.
§Examples
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()));
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
hasn’t been initialized.
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();
cell.set("hello".to_string()).unwrap();
assert_eq!(cell.take(), Some("hello".to_string()));
assert_eq!(cell.get(), None);
Trait Implementations§
impl<T> Eq for OnceCell<T>where
T: Eq,
impl<T> !Sync for OnceCell<T>
Auto Trait Implementations§
impl<T> !Freeze for OnceCell<T>
impl<T> !RefUnwindSafe for OnceCell<T>
impl<T> Send for OnceCell<T>where
T: Send,
impl<T> Unpin for OnceCell<T>where
T: Unpin,
impl<T> UnwindSafe for OnceCell<T>where
T: UnwindSafe,
Blanket Implementations§
source§impl<T> BorrowMut<T> for Twhere
T: ?Sized,
impl<T> BorrowMut<T> for Twhere
T: ?Sized,
source§fn borrow_mut(&mut self) -> &mut T
fn borrow_mut(&mut self) -> &mut T
source§impl<T> CloneToUninit for Twhere
T: Clone,
impl<T> CloneToUninit for Twhere
T: Clone,
source§unsafe fn clone_to_uninit(&self, dst: *mut T)
unsafe fn clone_to_uninit(&self, dst: *mut T)
clone_to_uninit
)§impl<Q, K> Equivalent<K> for Q
impl<Q, K> Equivalent<K> for Q
§fn equivalent(&self, key: &K) -> bool
fn equivalent(&self, key: &K) -> bool
§impl<Q, K> Equivalent<K> for Q
impl<Q, K> Equivalent<K> for Q
§fn equivalent(&self, key: &K) -> bool
fn equivalent(&self, key: &K) -> bool
key
and return true
if they are equal.