Struct LazyLock

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

A value which is initialized on the first access.

This type is a thread-safe LazyCell, and can be used in statics. Since initialization may be called from multiple threads, any dereferencing call will block the calling thread if another initialization routine is currently running.

Examples

Initialize static variables with LazyLock.

use std::sync::LazyLock;

// n.b. static items do not call [`Drop`] on program termination, so this won't be deallocated.
// this is fine, as the OS can deallocate the terminated program faster than we can free memory
// but tools like valgrind might report "memory leaks" as it isn't obvious this is intentional.
static DEEP_THOUGHT: LazyLock<String> = LazyLock::new(|| {
    // M3 Ultra takes about 16 million years in --release config
    another_crate::great_question()
});

// The `String` is built, stored in the `LazyLock`, and returned as `&String`.
let _ = &*DEEP_THOUGHT;

Initialize fields with LazyLock.

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> LazyLock<T, F>

where F: FnOnce() -> T,

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

Creates a new lazy value with the given initializing function.

Examples

use std::sync::LazyLock;

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

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

assert_eq!(&*lazy, "HELLO, WORLD!");

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

🔬This is a nightly-only experimental API. (lazy_cell_into_inner #125623)

Consumes this LazyLock returning the stored value.

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

Examples

#![feature(lazy_cell_into_inner)]

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_mut(this: &mut LazyLock<T, F>) -> &mut T

🔬This is a nightly-only experimental API. (lazy_get #129333)

Forces the evaluation of this lazy value and returns a mutable reference to the result.

Examples

#![feature(lazy_get)]
use std::sync::LazyLock;

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

let p = LazyLock::force_mut(&mut lazy);
assert_eq!(*p, 92);
*p = 44;
assert_eq!(*lazy, 44);

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

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

This method will block the calling thread if another initialization routine is currently running.

Examples

use std::sync::LazyLock;

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

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

impl<T, F> LazyLock<T, F>

pub fn get_mut(this: &mut LazyLock<T, F>) -> Option<&mut T>

🔬This is a nightly-only experimental API. (lazy_get #129333)

Returns a mutable reference to the value if initialized, or None if not.

Examples

#![feature(lazy_get)]

use std::sync::LazyLock;

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

assert_eq!(LazyLock::get_mut(&mut lazy), None);
let _ = LazyLock::force(&lazy);
*LazyLock::get_mut(&mut lazy).unwrap() = 44;
assert_eq!(*lazy, 44);

pub fn get(this: &LazyLock<T, F>) -> Option<&T>

🔬This is a nightly-only experimental API. (lazy_get #129333)

Returns a reference to the value if initialized, or None if not.

Examples

#![feature(lazy_get)]

use std::sync::LazyLock;

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

assert_eq!(LazyLock::get(&lazy), None);
let _ = LazyLock::force(&lazy);
assert_eq!(LazyLock::get(&lazy), Some(&92));

Trait Implementations

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

where T: Debug,

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

Formats the value using the given formatter.

impl<T> Default for LazyLock<T>

where T: Default,

fn default() -> LazyLock<T>

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

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

where F: FnOnce() -> T,

fn deref(&self) -> &T

Dereferences the value.

This method will block the calling thread if another initialization routine is currently running.

type Target = T

The resulting type after dereferencing.

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

fn drop(&mut self)

Executes the destructor for this type.

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

where T: RefUnwindSafe + UnwindSafe, F: UnwindSafe,

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

where T: Sync + Send, F: Send,

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

where T: UnwindSafe, F: UnwindSafe,

Layout

Note: Unable to compute type layout, possibly due to this type having generic parameters. Layout can only be computed for concrete, fully-instantiated types.