# Struct once_cell::sync::Lazy

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

Expand description

A value which is initialized on the first access.

This type is thread-safe and can be used in statics.

## Example

    use std::collections::HashMap;

    use once_cell::sync::Lazy;

    static HASHMAP: Lazy<HashMap<i32, String>> = Lazy::new(|| {
        println!("initializing");
        let mut m = HashMap::new();
        m.insert(13, "Spica".to_string());
        m.insert(74, "Hoyten".to_string());
        m
    });

    fn main() {
        println!("ready");
        std::thread::spawn(|| {
            println!("{:?}", HASHMAP.get(&13));
        }).join().unwrap();
        println!("{:?}", HASHMAP.get(&74));

        // Prints:
        //   ready
        //   initializing
        //   Some("Spica")
        //   Some("Hoyten")
    }

## Implementations

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### impl<T, F> Lazy<T, F>

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#### pub const fn new(f: F) -> Lazy<T, F>

Creates a new lazy value with the given initializing function.

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#### pub fn into_value(this: Lazy<T, F>) -> Result<T, F>

Consumes this `Lazy` returning the stored value.

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

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

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#### pub fn force(this: &Lazy<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.

##### Example

    use once_cell::sync::Lazy;

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

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

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

Forces the evaluation of this lazy value and returns a mutable reference to the result. This is equivalent to the `Deref` impl, but is explicit.

##### Example

    use once_cell::sync::Lazy;

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

    assert_eq!(Lazy::force_mut(&mut lazy), &mut 92);

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

Gets the reference to the result of this lazy value if it was initialized, otherwise returns `None`.

##### Example

    use once_cell::sync::Lazy;

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

    assert_eq!(Lazy::get(&lazy), None);
    assert_eq!(&*lazy, &92);
    assert_eq!(Lazy::get(&lazy), Some(&92));

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

Gets the reference to the result of this lazy value if it was initialized, otherwise returns `None`.

##### Example

    use once_cell::sync::Lazy;

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

    assert_eq!(Lazy::get_mut(&mut lazy), None);
    assert_eq!(&*lazy, &92);
    assert_eq!(Lazy::get_mut(&mut lazy), Some(&mut 92));

## Trait Implementations

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

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

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

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

#### type Target = T

The resulting type after dereferencing.

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

Dereferences the value.

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

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

Mutably dereferences the value.

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### impl<T, F: RefUnwindSafe> RefUnwindSafe for Lazy<T, F> where
OnceCell<T>: RefUnwindSafe,

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### impl<T, F: Send> Sync for Lazy<T, F> where
OnceCell<T>: Sync,

## Auto Trait Implementations

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

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

### impl<T, F> UnwindSafe for Lazy<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,

const: unstable · source

#### 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,

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 T where
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 T where
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 T where
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.