# [−][src]Struct std::lazy::Lazy

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

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

A value which is initialized on the first access.

# Examples

    #![feature(once_cell)]

    use std::lazy::Lazy;

    let lazy: Lazy<i32> = Lazy::new(|| {
        println!("initializing");
        92
    });
    println!("ready");
    println!("{}", *lazy);
    println!("{}", *lazy);

    // Prints:
    //   ready
    //   initializing
    //   92
    //   92

Run

## Implementations

### `impl<T, F> Lazy<T, F>`[src]

#### `pub fn new(init: F) -> Lazy<T, F>`[src]

🔬 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::lazy::Lazy;

    let hello = "Hello, World!".to_string();

    let lazy = Lazy::new(|| hello.to_uppercase());

    assert_eq!(&*lazy, "HELLO, WORLD!");

Run

### `impl<T, F> Lazy<T, F> where
F: FnOnce() -> T,` [src]

#### `pub fn force(this: &Lazy<T, F>) -> &Tⓘ

### Notable traits for &'_ mut F

`impl<'_, F> Future for &'_ mut F where
F: Unpin + Future + ?Sized, type Output = <F as Future>::Output;impl<'_, I> Iterator for &'_ mut I where
I: Iterator + ?Sized, type Item = <I as Iterator>::Item;impl<R: Read + ?Sized, '_> Read for &'_ mut Rimpl<W: Write + ?Sized, '_> Write for &'_ mut W`
`[src]

🔬 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::lazy::Lazy;

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

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

Run

## Trait Implementations

### `impl<T, F> Debug for Lazy<T, F> where
T: Debug,` [src]

#### `fn fmt(&self, f: &mut Formatter<'_>) -> Result<(), Error>`[src]

Formats the value using the given formatter. Read more

### `impl<T> Default for Lazy<T, fn() -> T> where
T: Default,` [src]

#### `fn default() -> Lazy<T, fn() -> T>`[src]

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

### `impl<T, F> Deref for Lazy<T, F> where
F: FnOnce() -> T,` [src]

#### `type Target = T`

The resulting type after dereferencing.

#### `fn deref(&self) -> &Tⓘ

### Notable traits for &'_ mut F

`impl<'_, F> Future for &'_ mut F where
F: Unpin + Future + ?Sized, type Output = <F as Future>::Output;impl<'_, I> Iterator for &'_ mut I where
I: Iterator + ?Sized, type Item = <I as Iterator>::Item;impl<R: Read + ?Sized, '_> Read for &'_ mut Rimpl<W: Write + ?Sized, '_> Write for &'_ mut W`
`[src]

Dereferences the value.

## Auto Trait Implementations

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

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

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

### `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

### `impl<T> Any for T where
T: 'static + ?Sized,` [src]

#### `fn type_id(&self) -> TypeId`[src]

Gets the `TypeId` of `self`. Read more

### `impl<T> Borrow<T> for T where
T: ?Sized,` [src]

#### `fn borrow(&self) -> &Tⓘ

### Notable traits for &'_ mut F

`impl<'_, F> Future for &'_ mut F where
F: Unpin + Future + ?Sized, type Output = <F as Future>::Output;impl<'_, I> Iterator for &'_ mut I where
I: Iterator + ?Sized, type Item = <I as Iterator>::Item;impl<R: Read + ?Sized, '_> Read for &'_ mut Rimpl<W: Write + ?Sized, '_> Write for &'_ mut W`
`[src]

Immutably borrows from an owned value. Read more

### `impl<T> BorrowMut<T> for T where
T: ?Sized,` [src]

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

### Notable traits for &'_ mut F

`impl<'_, F> Future for &'_ mut F where
F: Unpin + Future + ?Sized, type Output = <F as Future>::Output;impl<'_, I> Iterator for &'_ mut I where
I: Iterator + ?Sized, type Item = <I as Iterator>::Item;impl<R: Read + ?Sized, '_> Read for &'_ mut Rimpl<W: Write + ?Sized, '_> Write for &'_ mut W`
`[src]

Mutably borrows from an owned value. Read more

### `impl<T> From<T> for T`[src]

#### `fn from(t: T) -> T`[src]

Performs the conversion.

### `impl<T, U> Into<U> for T where
U: From<T>,` [src]

#### `fn into(self) -> U`[src]

Performs the conversion.

### `impl<T, U> TryFrom<U> for T where
U: Into<T>,` [src]

#### `type Error = Infallible`

The type returned in the event of a conversion error.

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

Performs the conversion.

### `impl<T, U> TryInto<U> for T where
U: TryFrom<T>,` [src]

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

The type returned in the event of a conversion error.

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

Performs the conversion.