Struct LazyCell

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

core A value which is initialized on the first access.

Re-exported from core::cell:: .

---

A value which is initialized on the first access.

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

Examples

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(f: F) -> LazyCell<T, F>

Creates a new lazy value with the given initializing function.

Examples

use std::cell::LazyCell;

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

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

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

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

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

Consumes this LazyCell returning the stored value.

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

Examples

#![feature(lazy_cell_into_inner)]

use std::cell::LazyCell;

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

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

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

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

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

use std::cell::LazyCell;

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

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

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

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

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

Examples

#![feature(lazy_get)]
use std::cell::LazyCell;

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

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

impl<T, F> LazyCell<T, F>

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

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

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

Examples

#![feature(lazy_get)]

use std::cell::LazyCell;

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

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

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

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

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

Examples

#![feature(lazy_get)]

use std::cell::LazyCell;

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

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

Trait Implementations

impl<T: ConstDefault> ConstDefault for LazyCell<T>

Available on crate feature std only.

const DEFAULT: Self

Returns the compile-time “default value” for a type.

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>

where T: Default,

fn default() -> LazyCell<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.