pub struct OnceCell<T>(_);
Expand description

A thread-safe cell which can be written to only once.

OnceCell provides & references to the contents without RAII guards.

Reading a non-None value out of OnceCell establishes a happens-before relationship with a corresponding write. For example, if thread A initializes the cell with get_or_init(f), and thread B subsequently reads the result of this call, B also observes all the side effects of f.

Example

use once_cell::sync::OnceCell;

static CELL: OnceCell<String> = OnceCell::new();
assert!(CELL.get().is_none());

std::thread::spawn(|| {
    let value: &String = CELL.get_or_init(|| {
        "Hello, World!".to_string()
    });
    assert_eq!(value, "Hello, World!");
}).join().unwrap();

let value: Option<&String> = CELL.get();
assert!(value.is_some());
assert_eq!(value.unwrap().as_str(), "Hello, World!");

Implementations§

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impl<T> OnceCell<T>

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

Creates a new empty cell.

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

Creates a new initialized cell.

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

Gets the reference to the underlying value.

Returns None if the cell is empty, or being initialized. This method never blocks.

pub fn wait(&self) -> &T

Gets the reference to the underlying value, blocking the current thread until it is set.

use once_cell::sync::OnceCell;

let mut cell = std::sync::Arc::new(OnceCell::new());
let t = std::thread::spawn({
    let cell = std::sync::Arc::clone(&cell);
    move || cell.set(92).unwrap()
});

// Returns immediately, but might return None.
let _value_or_none = cell.get();

// Will return 92, but might block until the other thread does `.set`.
let value: &u32 = cell.wait();
assert_eq!(*value, 92);
t.join().unwrap();

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

Gets the 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::sync::OnceCell;

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

pub unsafe fn get_unchecked(&self) -> &T

Get the reference to the underlying value, without checking if the cell is initialized.

Safety

Caller must ensure that the cell is in initialized state, and that the contents are acquired by (synchronized to) this thread.

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::sync::OnceCell;

static CELL: OnceCell<i32> = OnceCell::new();

fn main() {
    assert!(CELL.get().is_none());

    std::thread::spawn(|| {
        assert_eq!(CELL.set(92), Ok(()));
    }).join().unwrap();

    assert_eq!(CELL.set(62), Err(62));
    assert_eq!(CELL.get(), Some(&92));
}

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());

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.

Many threads may call get_or_init concurrently with different initializing functions, but it is guaranteed that only one function will be executed.

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. The exact outcome is unspecified. Current implementation deadlocks, but this may be changed to a panic in the future.

Example
use once_cell::sync::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, 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. The exact outcome is unspecified. Current implementation deadlocks, but this may be changed to a panic in the future.

Example
use once_cell::sync::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))

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::sync::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::sync::OnceCell;

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

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::sync::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§

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

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

Returns a copy of the value. Read more
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fn clone_from(&mut self, source: &OnceCell<T>)

Performs copy-assignment from source. Read more
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impl<T> Debug for OnceCell<T>where T: Debug,

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

Formats the value using the given formatter. Read more
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impl<T> Default for OnceCell<T>

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fn default() -> OnceCell<T>

Returns the “default value” for a type. Read more
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impl<T> From<T> for OnceCell<T>

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fn from(value: T) -> OnceCell<T>

Converts to this type from the input type.
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impl<T> PartialEq<OnceCell<T>> for OnceCell<T>where T: PartialEq<T>,

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fn eq(&self, other: &OnceCell<T>) -> bool

This method tests for self and other values to be equal, and is used by ==.
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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.
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impl<T> Eq for OnceCell<T>where T: Eq,

Auto Trait Implementations§

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

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

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

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

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

Blanket Implementations§

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

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fn type_id(&self) -> TypeId

Gets the TypeId of self. Read more
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impl<T> Borrow<T> for Twhere T: ?Sized,

const: unstable · source§

fn borrow(&self) -> &T

Immutably borrows from an owned value. Read more
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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
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impl<Q, K> Equivalent<K> for Qwhere Q: Eq + ?Sized, K: Borrow<Q> + ?Sized,

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fn equivalent(&self, key: &K) -> bool

Compare self to key and return true if they are equal.
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impl<T> From<!> for T

const: unstable · source§

fn from(t: !) -> T

Converts to this type from the input type.
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impl<T> From<T> for T

const: unstable · source§

fn from(t: T) -> T

Returns the argument unchanged.

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impl<T> FromFd for Twhere T: From<OwnedFd>,

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fn from_fd(owned_fd: OwnedFd) -> T

👎Deprecated since 1.0.0: FromFd::from_fd is replaced by From<OwnedFd>::from
Constructs a new instance of Self from the given file descriptor. Read more
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fn from_into_fd<Owned>(into_owned: Owned) -> Selfwhere Owned: Into<OwnedFd>, Self: Sized + From<OwnedFd>,

Constructs a new instance of Self from the given file descriptor converted from into_owned. Read more
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impl<T> FromFilelike for Twhere T: From<OwnedFd>,

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fn from_filelike(owned: OwnedFd) -> T

Constructs a new instance of Self from the given filelike object. Read more
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fn from_into_filelike<Owned>(owned: Owned) -> Twhere Owned: IntoFilelike,

Constructs a new instance of Self from the given filelike object converted from into_owned. Read more
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impl<T> FromSocketlike for Twhere T: From<OwnedFd>,

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fn from_socketlike(owned: OwnedFd) -> T

Constructs a new instance of Self from the given socketlike object.
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fn from_into_socketlike<Owned>(owned: Owned) -> Twhere Owned: IntoSocketlike,

Constructs a new instance of Self from the given socketlike object converted from into_owned.
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impl<T> Instrument for T

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fn instrument(self, span: Span) -> Instrumented<Self>

Instruments this type with the provided Span, returning an Instrumented wrapper. Read more
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fn in_current_span(self) -> Instrumented<Self>

Instruments this type with the current Span, returning an Instrumented wrapper. Read more
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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.

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impl<T> Same<T> for T

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type Output = T

Should always be Self
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impl<T> ToOwned for Twhere T: Clone,

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type Owned = T

The resulting type after obtaining ownership.
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fn to_owned(&self) -> T

Creates owned data from borrowed data, usually by cloning. Read more
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fn clone_into(&self, target: &mut T)

Uses borrowed data to replace owned data, usually by cloning. Read more
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impl<T, U> TryFrom<U> for Twhere U: Into<T>,

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

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

The type returned in the event of a conversion error.
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fn try_into(self) -> Result<U, <U as TryFrom<T>>::Error>

Performs the conversion.
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impl<V, T> VZip<V> for Twhere V: MultiLane<T>,

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fn vzip(self) -> V

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impl<T> WithSubscriber for T

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fn with_subscriber<S>(self, subscriber: S) -> WithDispatch<Self>where S: Into<Dispatch>,

Attaches the provided Subscriber to this type, returning a WithDispatch wrapper. Read more
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fn with_current_subscriber(self) -> WithDispatch<Self>

Attaches the current default Subscriber to this type, returning a WithDispatch wrapper. Read more