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

# [−][src]Struct std::lazy::OnceCell

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

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

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.

# Examples

    #![feature(once_cell)]

    use std::lazy::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

## Implementations

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

#### `pub fn new() -> OnceCell<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.

#### `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.

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

🔬 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::lazy::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

#### `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.

# 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::lazy::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

#### `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`. Doing so results in a panic.

# Examples

    #![feature(once_cell)]

    use std::lazy::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

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

🔬 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::lazy::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

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

🔬 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::lazy::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

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

#### `pub fn clone(&self) -> OnceCell<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 for OnceCell<T> where
T: Debug,` [src]

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

Formats the value using the given formatter. Read more

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

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

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

### `impl<T> Eq for OnceCell<T> where
T: Eq,` [src]

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

#### `pub fn from(value: T) -> OnceCell<T>`[src]

Performs the conversion.

### `impl<T> PartialEq<OnceCell<T>> for OnceCell<T> where
T: PartialEq<T>,` [src]

#### `pub fn eq(&self, other: &OnceCell<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 `!=`.

## Auto Trait Implementations

### `impl<T> !RefUnwindSafe for OnceCell<T>`

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

### `impl<T> !Sync for OnceCell<T>`

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

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

## 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.