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
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>
sourcepub fn try_insert(&self, value: T) -> Result<&T, (&T, T)>
🔬This is a nightly-only experimental API. (once_cell_try_insert
#116693)
pub fn try_insert(&self, value: T) -> Result<&T, (&T, T)>
once_cell_try_insert
#116693)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,
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
#121641)
pub fn get_mut_or_init<F>(&mut self, f: F) -> &mut Twhere
F: FnOnce() -> T,
once_cell_get_mut
#121641)sourcepub fn get_or_try_init<F, E>(&self, f: F) -> Result<&T, E>
🔬This is a nightly-only experimental API. (once_cell_try
#109737)
pub fn get_or_try_init<F, E>(&self, f: F) -> Result<&T, E>
once_cell_try
#109737)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
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
#121641)
pub fn get_mut_or_try_init<F, E>(&mut self, f: F) -> Result<&mut T, E>
once_cell_get_mut
#121641)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))