Struct std::cell::OnceCell

source ·
pub struct OnceCell<T> {
    inner: UnsafeCell<Option<T>>,
}
🔬This is a nightly-only experimental API. (once_cell #74465)
Expand description

A cell which can be written to only once.

Unlike RefCell, a OnceCell only provides shared &T references to its value. Unlike Cell, a OnceCell doesn’t require copying or replacing the value to access it.

For a thread-safe version of this struct, see std::sync::OnceLock.

Examples

#![feature(once_cell)]

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());
Run

Fields§

§inner: UnsafeCell<Option<T>>
🔬This is a nightly-only experimental API. (once_cell #74465)

Implementations§

🔬This is a nightly-only experimental API. (once_cell #74465)

Creates a new empty cell.

🔬This is a nightly-only experimental API. (once_cell #74465)

Gets the reference to the underlying value.

Returns None if the cell is empty.

🔬This is a nightly-only experimental API. (once_cell #74465)

Gets the mutable reference to the underlying value.

Returns None if the cell is empty.

🔬This is a nightly-only experimental API. (once_cell #74465)

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
#![feature(once_cell)]

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());
Run
🔬This is a nightly-only experimental API. (once_cell #74465)

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
#![feature(once_cell)]

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);
Run
🔬This is a nightly-only experimental API. (once_cell #74465)

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)]

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))
Run
🔬This is a nightly-only experimental API. (once_cell #74465)

Consumes the cell, returning the wrapped value.

Returns None if the cell was empty.

Examples
#![feature(once_cell)]

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()));
Run
🔬This is a nightly-only experimental API. (once_cell #74465)

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
#![feature(once_cell)]

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);
Run

Trait Implementations§

Returns a copy of the value. Read more
Performs copy-assignment from source. Read more
Formats the value using the given formatter. Read more
Returns the “default value” for a type. Read more

Creates a new OnceCell<T> which already contains the given value.

This method tests for self and other values to be equal, and is used by ==.
This method tests for !=. The default implementation is almost always sufficient, and should not be overridden without very good reason.

Auto Trait Implementations§

Blanket Implementations§

Gets the TypeId of self. Read more
Immutably borrows from an owned value. Read more
Mutably borrows from an owned value. Read more
Converts to this type from the input type.

Returns the argument unchanged.

Calls U::from(self).

That is, this conversion is whatever the implementation of From<T> for U chooses to do.

The resulting type after obtaining ownership.
Creates owned data from borrowed data, usually by cloning. Read more
Uses borrowed data to replace owned data, usually by cloning. Read more
The type returned in the event of a conversion error.
Performs the conversion.
The type returned in the event of a conversion error.
Performs the conversion.