LazyCell in core::cell - Rust
Description: A value which is initialized on the first access.

# Struct LazyCell

1.80.0 · Source

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

Expand description

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§

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

1.80.0 (const: 1.80.0) · Source

#### 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!");

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#### pub const fn into_inner(this: Self) -> 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.

##### §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()));

1.80.0 · Source

#### 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);

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#### 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.

##### §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);

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

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#### 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, 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);

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#### 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, 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§

1.80.0 · Source§

### 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
1.80.0 · Source§

### 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.

1.80.0 · Source§

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

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#### type Target = T

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

Dereferences the value.

## Auto Trait Implementations§

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

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

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

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

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

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

## Blanket Implementations§

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### impl<T> Any for T
where 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 T
where T: ?Sized,

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

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

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#### 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

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

Returns the argument unchanged.

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

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#### 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<P, T> Receiver for P
where P: Deref<Target = T> + ?Sized, T: ?Sized,

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#### type Target = T

🔬This is a nightly-only experimental API. (`arbitrary_self_types` #44874)
The target type on which the method may be called.
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### impl<T, U> TryFrom<U> for T
where U: Into<T>,

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#### type Error = Infallible

The type returned in the event of a conversion error.
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#### 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 T
where U: TryFrom<T>,

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

The type returned in the event of a conversion error.
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#### fn try_into(self) -> Result<U, <U as TryFrom<T>>::Error>

Performs the conversion.