Struct OnceCell

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pub struct OnceCell<T> { /* private fields */ }

Expand description

A cell which can be written to only once. It is not thread safe.

Unlike std::cell::RefCell, a OnceCell provides simple & references to the contents.

§Example

use once_cell::unsync::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§

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impl<T> OnceCell<T>

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pub const fn new() -> OnceCell<T>

Creates a new empty cell.

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pub const fn with_value(value: T) -> OnceCell<T>

Creates a new initialized cell.

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pub fn get(&self) -> Option<&T>

Gets a reference to the underlying value.

Returns None if the cell is empty.

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pub fn get_mut(&mut self) -> Option<&mut T>

Gets a mutable reference to the underlying value.

Returns None if the cell is empty.

This method is allowed to violate the invariant of writing to a OnceCell at most once because it requires &mut access to self. As with all interior mutability, &mut access permits arbitrary modification:

use once_cell::unsync::OnceCell;

let mut cell: OnceCell<u32> = OnceCell::new();
cell.set(92).unwrap();
*cell.get_mut().unwrap() = 93;
assert_eq!(cell.get(), Some(&93));

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pub fn set(&self, value: T) -> Result<(), T>

Sets the contents of this cell to value.

Returns Ok(()) if the cell was empty and Err(value) if it was full.

§Example

use once_cell::unsync::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());

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pub fn try_insert(&self, value: T) -> Result<&T, (&T, T)>

Like set, but also returns a reference to the final cell value.

§Example

use once_cell::unsync::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());

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pub fn get_or_init<F>(&self, f: F) -> &T
where 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.

§Example

use once_cell::unsync::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);

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pub fn get_or_try_init<F, E>(&self, f: F) -> Result<&T, E>
where F: FnOnce() -> Result<T, E>,

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.

§Example

use once_cell::unsync::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))

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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.

§Examples

use once_cell::unsync::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);

This method is allowed to violate the invariant of writing to a OnceCell at most once because it requires &mut access to self. As with all interior mutability, &mut access permits arbitrary modification:

use once_cell::unsync::OnceCell;

let mut cell: OnceCell<u32> = OnceCell::new();
cell.set(92).unwrap();
cell = OnceCell::new();

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pub fn into_inner(self) -> Option<T>

Consumes the OnceCell, returning the wrapped value.

Returns None if the cell was empty.

§Examples

use once_cell::unsync::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()));