Struct once_cell::unsync::OnceCell

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

A cell which can be written to only once. 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());

Methods

impl<T> OnceCell<T>

pub const INIT: OnceCell<T>

INIT: OnceCell<T> = OnceCell{inner: <UnsafeCell>::new(None),}

An empty cell, for initialization in a const context.

pub fn new() -> OnceCell<T>

Creates a new empty cell.

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

Gets the reference to the underlying value. Returns None if the cell is empty.

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());

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

Gets the contents of the cell, initializing it with f if the cell was empty.

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);

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

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))

Trait Implementations

impl<T: Debug> Debug for OnceCell<T>

fn fmt(&self, f: &mut Formatter) -> Result

Formats the value using the given formatter.

impl<T> Default for OnceCell<T>

fn default() -> OnceCell<T>

Returns the "default value" for a type.

impl<T: Clone> Clone for OnceCell<T>

fn clone(&self) -> OnceCell<T>

Returns a copy of the value.

fn clone_from(&mut self, source: &Self)

Performs copy-assignment from source.