# Struct std::sync::LazyLock

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    pub struct LazyLock<T, F = fn() -> T> { /* private fields */ }

🔬This is a nightly-only experimental API. (`lazy_cell` #109736)
Expand description

A value which is initialized on the first access.

This type is a thread-safe `LazyCell`, and can be used in statics.

## Examples

Initialize static variables with `LazyLock`.

    #![feature(lazy_cell)]

    use std::collections::HashMap;

    use std::sync::LazyLock;

    static HASHMAP: LazyLock<HashMap<i32, String>> = LazyLock::new(|| {
        println!("initializing");
        let mut m = HashMap::new();
        m.insert(13, "Spica".to_string());
        m.insert(74, "Hoyten".to_string());
        m
    });

    fn main() {
        println!("ready");
        std::thread::spawn(|| {
            println!("{:?}", HASHMAP.get(&13));
        }).join().unwrap();
        println!("{:?}", HASHMAP.get(&74));

        // Prints:
        //   ready
        //   initializing
        //   Some("Spica")
        //   Some("Hoyten")
    }

Run

Initialize fields with `LazyLock`.

    #![feature(lazy_cell)]

    use std::sync::LazyLock;

    #[derive(Debug)]
    struct UseCellLock {
        number: LazyLock<u32>,
    }
    fn main() {
        let lock: LazyLock<u32> = LazyLock::new(|| 0u32);

        let data = UseCellLock { number: lock };
        println!("{}", *data.number);
    }

Run

## Implementations§

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### impl<T, F: FnOnce() -> T> LazyLock<T, F>

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#### pub const fn new(f: F) -> LazyLock<T, F>

🔬This is a nightly-only experimental API. (`lazy_cell` #109736)

Creates a new lazy value with the given initializing function.

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#### pub fn into_inner(this: Self) -> Result<T, F>

🔬This is a nightly-only experimental API. (`lazy_cell_consume` #109736)

Consumes this `LazyLock` returning the stored value.

Returns `Ok(value)` if `Lazy` is initialized and `Err(f)` otherwise.

##### Examples

    #![feature(lazy_cell)]
    #![feature(lazy_cell_consume)]

    use std::sync::LazyLock;

    let hello = "Hello, World!".to_string();

    let lazy = LazyLock::new(|| hello.to_uppercase());

    assert_eq!(&*lazy, "HELLO, WORLD!");
    assert_eq!(LazyLock::into_inner(lazy).ok(), Some("HELLO, WORLD!".to_string()));

Run
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#### pub fn force(this: &LazyLock<T, F>) -> &T

🔬This is a nightly-only experimental API. (`lazy_cell` #109736)

Forces the evaluation of this lazy value and returns a reference to result. This is equivalent to the `Deref` impl, but is explicit.

##### Examples

    #![feature(lazy_cell)]

    use std::sync::LazyLock;

    let lazy = LazyLock::new(|| 92);

    assert_eq!(LazyLock::force(&lazy), &92);
    assert_eq!(&*lazy, &92);

Run

## Trait Implementations§

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### impl<T: Debug, F> Debug for LazyLock<T, F>

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

Formats the value using the given formatter. Read more
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### impl<T: Default> Default for LazyLock<T>

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#### fn default() -> LazyLock<T>

Creates a new lazy value using `Default` as the initializing function.

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### impl<T, F: FnOnce() -> T> Deref for LazyLock<T, F>

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#### type Target = T

The resulting type after dereferencing.
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#### fn deref(&self) -> &T

Dereferences the value.
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### impl<T, F> Drop for LazyLock<T, F>

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#### fn drop(&mut self)

Executes the destructor for this type. Read more
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### impl<T: RefUnwindSafe + UnwindSafe, F: UnwindSafe> RefUnwindSafe for LazyLock<T, F>

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### impl<T: Sync + Send, F: Send> Sync for LazyLock<T, F>

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### impl<T: UnwindSafe, F: UnwindSafe> UnwindSafe for LazyLock<T, F>

## Auto Trait Implementations§

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### impl<T, F> Send for LazyLock<T, F>where F: Send, T: Send,

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### impl<T, F> Unpin for LazyLock<T, F>where F: Unpin, T: Unpin,

## 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,

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#### fn borrow(&self) -> &T

Immutably borrows from an owned value. Read more
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### impl<T> BorrowMut<T> for Twhere T: ?Sized,

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

Mutably borrows from an owned value. Read more
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### impl<T> From<T> for T

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#### fn from(t: T) -> T

Returns the argument unchanged.

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

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#### 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, 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.
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#### 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.