Struct std::sync::LazyLock

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

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

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")
}

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);
}

Implementations

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

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.

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()));

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);

Trait Implementations

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

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

Formats the value using the given formatter.

impl<T: Default> Default for LazyLock<T>

fn default() -> LazyLock<T>

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

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

type Target = T

The resulting type after dereferencing.

fn deref(&self) -> &T

Dereferences the value.

impl<T, F> Drop for LazyLock<T, F>

fn drop(&mut self)

Executes the destructor for this type.

impl<T: RefUnwindSafe + UnwindSafe, F: UnwindSafe> RefUnwindSafe for LazyLock<T, F>

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

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