std::lazy::SyncOnceCell - Rust
Description: API documentation for the Rust `SyncOnceCell` struct in crate `std`.
Keywords: rust, rustlang, rust-lang, SyncOnceCell

# [−][src]Struct std::lazy::SyncOnceCell

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

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

A synchronization primitive which can be written to only once.

This type is a thread-safe `OnceCell`.

# Examples

    #![feature(once_cell)]

    use std::lazy::SyncOnceCell;

    static CELL: SyncOnceCell<String> = SyncOnceCell::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!");

Run

## Implementations

### `impl<T> SyncOnceCell<T>`[src]

#### `pub const fn new() -> SyncOnceCell<T>`[src]

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

Creates a new empty cell.

#### `pub fn get(&self) -> Option<&T>`[src]

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

Gets the reference to the underlying value.

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

#### `pub fn get_mut(&mut self) -> Option<&mut T>`[src]

🔬 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 method never blocks.

#### `pub fn set(&self, value: T) -> Result<(), T>`[src]

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

Sets the contents of this cell to `value`.

Returns `Ok(())` if the cell's value was updated.

# Examples

    #![feature(once_cell)]

    use std::lazy::SyncOnceCell;

    static CELL: SyncOnceCell<i32> = SyncOnceCell::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));
    }

Run

#### `pub fn get_or_init<F>(&self, f: F) -> &Tⓘ

### Notable traits for &'_ mut I

`impl<'_, I> Iterator for &'_ mut I where
I: Iterator + ?Sized, type Item = <I as Iterator>::Item;impl<'_, F> Future for &'_ mut F where
F: Unpin + Future + ?Sized, type Output = <F as Future>::Output;impl<R: Read + ?Sized, '_> Read for &'_ mut Rimpl<W: Write + ?Sized, '_> Write for &'_ mut W`
where
F: FnOnce() -> T,` [src]

🔬 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.

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.

# Examples

    #![feature(once_cell)]

    use std::lazy::SyncOnceCell;

    let cell = SyncOnceCell::new();
    let value = cell.get_or_init(|| 92);
    assert_eq!(value, &92);
    let value = cell.get_or_init(|| unreachable!());
    assert_eq!(value, &92);

Run

#### `pub fn get_or_try_init<F, E>(&self, f: F) -> Result<&T, E> where
F: FnOnce() -> Result<T, E>,` [src]

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

# Examples

    #![feature(once_cell)]

    use std::lazy::SyncOnceCell;

    let cell = SyncOnceCell::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

#### `pub fn into_inner(self) -> Option<T>`[src]

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

Consumes the `SyncOnceCell`, returning the wrapped value. Returns `None` if the cell was empty.

# Examples

    #![feature(once_cell)]

    use std::lazy::SyncOnceCell;

    let cell: SyncOnceCell<String> = SyncOnceCell::new();
    assert_eq!(cell.into_inner(), None);

    let cell = SyncOnceCell::new();
    cell.set("hello".to_string()).unwrap();
    assert_eq!(cell.into_inner(), Some("hello".to_string()));

Run

#### `pub fn take(&mut self) -> Option<T>`[src]

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

Takes the value out of this `SyncOnceCell`, moving it back to an uninitialized state.

Has no effect and returns `None` if the `SyncOnceCell` hasn't been initialized.

Safety is guaranteed by requiring a mutable reference.

# Examples

    #![feature(once_cell)]

    use std::lazy::SyncOnceCell;

    let mut cell: SyncOnceCell<String> = SyncOnceCell::new();
    assert_eq!(cell.take(), None);

    let mut cell = SyncOnceCell::new();
    cell.set("hello".to_string()).unwrap();
    assert_eq!(cell.take(), Some("hello".to_string()));
    assert_eq!(cell.get(), None);

Run

## Trait Implementations

### `impl<T: Clone> Clone for SyncOnceCell<T>`[src]

#### `fn clone(&self) -> SyncOnceCell<T>`[src]

Returns a copy of the value. Read more

#### `fn clone_from(&mut self, source: &Self)`1.0.0[src]

Performs copy-assignment from `source`. Read more

### `impl<T: Debug> Debug for SyncOnceCell<T>`[src]

#### `fn fmt(&self, f: &mut Formatter<'_>) -> Result`[src]

Formats the value using the given formatter. Read more

### `impl<T> Default for SyncOnceCell<T>`[src]

#### `fn default() -> SyncOnceCell<T>`[src]

Returns the "default value" for a type. Read more

### `impl<T> Drop for SyncOnceCell<T>`[src]

#### `fn drop(&mut self)`[src]

Executes the destructor for this type. Read more

### `impl<T: Eq> Eq for SyncOnceCell<T>`[src]

### `impl<T> From<T> for SyncOnceCell<T>`[src]

#### `fn from(value: T) -> Self`[src]

Performs the conversion.

### `impl<T: PartialEq> PartialEq<SyncOnceCell<T>> for SyncOnceCell<T>`[src]

#### `fn eq(&self, other: &SyncOnceCell<T>) -> bool`[src]

This method tests for `self` and `other` values to be equal, and is used by `==`. Read more

#### `#[must_use]fn ne(&self, other: &Rhs) -> bool`1.0.0[src]

This method tests for `!=`.

### `impl<T: RefUnwindSafe + UnwindSafe> RefUnwindSafe for SyncOnceCell<T>`[src]

### `impl<T: Send> Send for SyncOnceCell<T>`[src]

### `impl<T: Sync + Send> Sync for SyncOnceCell<T>`[src]

### `impl<T: UnwindSafe> UnwindSafe for SyncOnceCell<T>`[src]

## Auto Trait Implementations

### `impl<T> Unpin for SyncOnceCell<T> where
T: Unpin,`

## Blanket Implementations

### `impl<T> Any for T where
T: 'static + ?Sized,` [src]

#### `pub fn type_id(&self) -> TypeId`[src]

Gets the `TypeId` of `self`. Read more

### `impl<T> Borrow<T> for T where
T: ?Sized,` [src]

#### `pub fn borrow(&self) -> &Tⓘ

### Notable traits for &'_ mut I

`impl<'_, I> Iterator for &'_ mut I where
I: Iterator + ?Sized, type Item = <I as Iterator>::Item;impl<'_, F> Future for &'_ mut F where
F: Unpin + Future + ?Sized, type Output = <F as Future>::Output;impl<R: Read + ?Sized, '_> Read for &'_ mut Rimpl<W: Write + ?Sized, '_> Write for &'_ mut W`
`[src]

Immutably borrows from an owned value. Read more

### `impl<T> BorrowMut<T> for T where
T: ?Sized,` [src]

#### `pub fn borrow_mut(&mut self) -> &mut Tⓘ

### Notable traits for &'_ mut I

`impl<'_, I> Iterator for &'_ mut I where
I: Iterator + ?Sized, type Item = <I as Iterator>::Item;impl<'_, F> Future for &'_ mut F where
F: Unpin + Future + ?Sized, type Output = <F as Future>::Output;impl<R: Read + ?Sized, '_> Read for &'_ mut Rimpl<W: Write + ?Sized, '_> Write for &'_ mut W`
`[src]

Mutably borrows from an owned value. Read more

### `impl<T> From<!> for T`[src]

#### `pub fn from(t: !) -> T`[src]

Performs the conversion.

### `impl<T> From<T> for T`[src]

#### `pub fn from(t: T) -> T`[src]

Performs the conversion.

### `impl<T, U> Into<U> for T where
U: From<T>,` [src]

#### `pub fn into(self) -> U`[src]

Performs the conversion.

### `impl<T> ToOwned for T where
T: Clone,` [src]

#### `type Owned = T`

The resulting type after obtaining ownership.

#### `pub fn to_owned(&self) -> T`[src]

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

#### `pub fn clone_into(&self, target: &mut T)`[src]

🔬 This is a nightly-only experimental API. (`toowned_clone_into` #41263)

recently added

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

### `impl<T, U> TryFrom<U> for T where
U: Into<T>,` [src]

#### `type Error = Infallible`

The type returned in the event of a conversion error.

#### `pub fn try_from(value: U) -> Result<T, <T as TryFrom<U>>::Error>`[src]

Performs the conversion.

### `impl<T, U> TryInto<U> for T where
U: TryFrom<T>,` [src]

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

The type returned in the event of a conversion error.

#### `pub fn try_into(self) -> Result<U, <U as TryFrom<T>>::Error>`[src]

Performs the conversion.