Struct LazyCell

struct LazyCell<T, F = fn() -> T> { ... }

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

Implementations

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

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

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

Examples

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);

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

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

Examples

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));

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

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!");

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

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()));

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);

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

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

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<P, T> Receiver for LazyCell<T, F>

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

fn type_id(self: &Self) -> TypeId

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

fn borrow(self: &Self) -> &T

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

fn borrow_mut(self: &mut Self) -> &mut T

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

fn from(t: T) -> T

Returns the argument unchanged.

impl<T, F = fn() -> T> Freeze for LazyCell<T, F>

impl<T, F = fn() -> T> RefUnwindSafe for LazyCell<T, F>

impl<T, F = fn() -> T> Sync for LazyCell<T, F>

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

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

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

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

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

fn deref(self: &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.

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

fn deref_mut(self: &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.

impl<T, U> Into for LazyCell<T, F>

fn into(self: Self) -> U

Calls U::from(self).

That is, this conversion is whatever the implementation of [From]<T> for U chooses to do.

impl<T, U> TryFrom for LazyCell<T, F>

fn try_from(value: U) -> Result<T, <T as TryFrom<U>>::Error>

impl<T, U> TryInto for LazyCell<T, F>

fn try_into(self: Self) -> Result<U, <U as TryFrom<T>>::Error>

impl<T: Default> Default for LazyCell<T>

fn default() -> LazyCell<T>

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

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

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