Struct LazyCell

pub struct LazyCell<T, F = fn() -> T> {
    state: UnsafeCell<State<T, F>>,
}

A value which is initialized on the first access.

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

Poisoning

If the initialization closure passed to LazyCell::new panics, the cell will be poisoned. Once the cell is poisoned, any threads that attempt to access this cell (via a dereference or via an explicit call to force()) will panic.

This concept is similar to that of poisoning in the std::sync::poison module. A key difference, however, is that poisoning in LazyCell is unrecoverable. All future accesses of the cell from other threads will panic, whereas a type in std::sync::poison like std::sync::poison::Mutex allows recovery via PoisonError::into_inner().

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

Fields

state: UnsafeCell<State<T, F>>

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 #125623)

Consumes this LazyCell returning the stored value.

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

Panics

Panics if the cell is poisoned.

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.

Panics

If the initialization closure panics (the one that is passed to the new() method), the panic is propagated to the caller, and the cell becomes poisoned. This will cause all future accesses of the cell (via force() or a dereference) to panic.

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 #129333)

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

Panics

If the initialization closure panics (the one that is passed to the new() method), the panic is propagated to the caller, and the cell becomes poisoned. This will cause all future accesses of the cell (via force() or a dereference) to panic.

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 #129333)

Returns a mutable reference to the value if initialized. Otherwise (if uninitialized or poisoned), returns None.

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 #129333)

Returns a reference to the value if initialized. Otherwise (if uninitialized or poisoned), returns None.

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, 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,

fn deref(&self) -> &T

Panics

If the initialization closure panics (the one that is passed to the new() method), the panic is propagated to the caller, and the cell becomes poisoned. This will cause all future accesses of the cell (via force() or a dereference) to panic.

type Target = T

The resulting type after dereferencing.

impl<T, F> DerefMut for LazyCell<T, F>

where F: FnOnce() -> T,

fn deref_mut(&mut self) -> &mut T

Panics

If the initialization closure panics (the one that is passed to the new() method), the panic is propagated to the caller, and the cell becomes poisoned. This will cause all future accesses of the cell (via force() or a dereference) to panic.