Struct once_cell::unsync::OnceCell

source ·
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§

source§

impl<T> OnceCell<T>

source

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

Creates a new empty cell.

source

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

Creates a new initialized cell.

source

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

Gets a reference to the underlying value.

Returns None if the cell is empty.

source

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));
source

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());
source

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());
source

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.

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);
source

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))
source

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();
source

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()));

Trait Implementations§

source§

impl<T: Clone> Clone for OnceCell<T>

source§

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

Returns a copy of the value. Read more
source§

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

Performs copy-assignment from source. Read more
source§

impl<T: Debug> Debug for OnceCell<T>

source§

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

Formats the value using the given formatter. Read more
source§

impl<T> Default for OnceCell<T>

source§

fn default() -> Self

Returns the “default value” for a type. Read more
source§

impl<T> From<T> for OnceCell<T>

source§

fn from(value: T) -> Self

Converts to this type from the input type.
source§

impl<T: PartialEq> PartialEq<OnceCell<T>> for OnceCell<T>

source§

fn eq(&self, other: &Self) -> bool

This method tests for self and other values to be equal, and is used by ==.
1.0.0 · source§

fn ne(&self, other: &Rhs) -> bool

This method tests for !=. The default implementation is almost always sufficient, and should not be overridden without very good reason.
source§

impl<T: Eq> Eq for OnceCell<T>

source§

impl<T: RefUnwindSafe + UnwindSafe> RefUnwindSafe for OnceCell<T>

source§

impl<T: UnwindSafe> UnwindSafe for OnceCell<T>

Auto Trait Implementations§

§

impl<T> Send for OnceCell<T>where
    T: Send,

§

impl<T> !Sync for OnceCell<T>

§

impl<T> Unpin for OnceCell<T>where
    T: Unpin,

Blanket Implementations§

source§

impl<T> Any for Twhere
    T: 'static + ?Sized,

source§

fn type_id(&self) -> TypeId

Gets the TypeId of self. Read more
source§

impl<T> Borrow<T> for Twhere
    T: ?Sized,

const: unstable · source§

fn borrow(&self) -> &T

Immutably borrows from an owned value. Read more
source§

impl<T> BorrowMut<T> for Twhere
    T: ?Sized,

const: unstable · source§

fn borrow_mut(&mut self) -> &mut T

Mutably borrows from an owned value. Read more
source§

impl<T> From<!> for T

const: unstable · source§

fn from(t: !) -> T

Converts to this type from the input type.
source§

impl<T> From<T> for T

const: unstable · source§

fn from(t: T) -> T

Returns the argument unchanged.

source§

impl<T, U> Into<U> for Twhere
    U: From<T>,

const: unstable · source§

fn into(self) -> U

Calls U::from(self).

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

source§

impl<T> ToOwned for Twhere
    T: Clone,

§

type Owned = T

The resulting type after obtaining ownership.
source§

fn to_owned(&self) -> T

Creates owned data from borrowed data, usually by cloning. Read more
source§

fn clone_into(&self, target: &mut T)

Uses borrowed data to replace owned data, usually by cloning. Read more
source§

impl<T, U> TryFrom<U> for Twhere
    U: Into<T>,

§

type Error = Infallible

The type returned in the event of a conversion error.
const: unstable · source§

fn try_from(value: U) -> Result<T, <T as TryFrom<U>>::Error>

Performs the conversion.
source§

impl<T, U> TryInto<U> for Twhere
    U: TryFrom<T>,

§

type Error = <U as TryFrom<T>>::Error

The type returned in the event of a conversion error.
const: unstable · source§

fn try_into(self) -> Result<U, <U as TryFrom<T>>::Error>

Performs the conversion.