Struct std::cell::LazyCell

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

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

A value which is initialized on the first access.

For a thread-safe version of this struct, see std::sync::LazyLock.

Examples

#![feature(once_cell)]

use std::cell::LazyCell;

let lazy: LazyCell<i32> = LazyCell::new(|| {
    println!("initializing");
    92
});
println!("ready");
println!("{}", *lazy);
println!("{}", *lazy);

// Prints:
//   ready
//   initializing
//   92
//   92

Implementations

impl<T, F> LazyCell<T, F>where F: FnOnce() -> T,

pub const fn new(init: F) -> LazyCell<T, F>

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

Creates a new lazy value with the given initializing function.

Examples

#![feature(once_cell)]

use std::cell::LazyCell;

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

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

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

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

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

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

This is equivalent to the Deref impl, but is explicit.

Examples

#![feature(once_cell)]

use std::cell::LazyCell;

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

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

Trait Implementations

impl<T, F> Debug for LazyCell<T, F>where T: Debug,

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

Formats the value using the given formatter.

impl<T> Default for LazyCell<T, fn() -> T>where T: Default,

fn default() -> LazyCell<T, fn() -> T>

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

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

type Target = T

The resulting type after dereferencing.

fn deref(&self) -> &T

Dereferences the value.