# Struct core::cell::LazyCell

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

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

A value which is initialized on the first access.

## 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

Run

## Implementations

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

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#### 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!");

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

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#### 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);

Run

## Trait Implementations

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

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

Formats the value using the given formatter. Read more
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### impl<T: Default> Default for LazyCell<T>

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

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

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

#### type Target = T

The resulting type after dereferencing.
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#### fn deref(&self) -> &T

Dereferences the value.

## Auto Trait Implementations

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

### impl<T, F> Send for LazyCell<T, F>where
F: Send,
T: Send,

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

### impl<T, F> Unpin for LazyCell<T, F>where
F: Unpin,
T: Unpin,

### impl<T, F> UnwindSafe for LazyCell<T, F>where
F: UnwindSafe,
T: UnwindSafe,

## Blanket Implementations

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### impl<T> Any for Twhere
T: 'static + ?Sized,

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#### fn type_id(&self) -> TypeId

Gets the `TypeId` of `self`. Read more
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### impl<T> Borrow<T> for Twhere
T: ?Sized,

const: unstable · source

#### fn borrow(&self) -> &T

Immutably borrows from an owned value. Read more
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### impl<T> BorrowMut<T> for Twhere
T: ?Sized,

const: unstable · source

#### fn borrow_mut(&mut self) -> &mut T

Mutably borrows from an owned value. Read more
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### impl<T> From<T> for T

const: unstable · source

#### fn from(t: T) -> T

Returns the argument unchanged.

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### impl<T, U> Into<U> for Twhere
U: From<T>,

const: unstable · source

#### fn into(self) -> U

Calls `U::from(self)`.

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

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### impl<T, U> TryFrom<U> for Twhere
U: Into<T>,

#### type Error = Infallible

The type returned in the event of a conversion error.
const: unstable · source

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

Performs the conversion.
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### impl<T, U> TryInto<U> for Twhere
U: TryFrom<T>,

#### type Error = <U as TryFrom<T>>::Error

The type returned in the event of a conversion error.
const: unstable · source

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

Performs the conversion.