# 1.0.0[−][src]Trait std::ops::FnOnce

    #[lang = "fn_once"]
    #[must_use = "closures are lazy and do nothing unless called"]
    pub trait FnOnce<Args> {
        type Output;
        extern "rust-call" fn call_once(self, args: Args) -> Self::Output;
    }

The version of the call operator that takes a by-value receiver.

Instances of `FnOnce` can be called, but might not be callable multiple times. Because of this, if the only thing known about a type is that it implements `FnOnce`, it can only be called once.

`FnOnce` is implemented automatically by closure that might consume captured variables, as well as all types that implement `FnMut`, e.g., (safe) function pointers (since `FnOnce` is a supertrait of `FnMut`).

Since both `Fn` and `FnMut` are subtraits of `FnOnce`, any instance of `Fn` or `FnMut` can be used where a `FnOnce` is expected.

Use `FnOnce` as a bound when you want to accept a parameter of function-like type and only need to call it once. If you need to call the parameter repeatedly, use `FnMut` as a bound; if you also need it to not mutate state, use `Fn`.

See the chapter on closures in The Rust Programming Language for some more information on this topic.

Also of note is the special syntax for `Fn` traits (e.g. `Fn(usize, bool) -> usize`). Those interested in the technical details of this can refer to the relevant section in the Rustonomicon.

# Examples

## Using a `FnOnce` parameter

    fn consume_with_relish<F>(func: F)
        where F: FnOnce() -> String
    {
        // `func` consumes its captured variables, so it cannot be run more
        // than once.
        println!("Consumed: {}", func());

        println!("Delicious!");

        // Attempting to invoke `func()` again will throw a `use of moved
        // value` error for `func`.
    }

    let x = String::from("x");
    let consume_and_return_x = move || x;
    consume_with_relish(consume_and_return_x);

    // `consume_and_return_x` can no longer be invoked at this pointRun

## Associated Types

### `type Output` 1.12.0

The returned type after the call operator is used.

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## Required methods

### `extern "rust-call" fn call_once(self, args: Args) -> Self::Output`

🔬 This is a nightly-only experimental API. (`fn_traits` #29625)

Performs the call operation.

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## Implementors

### `impl<'_, A, F> FnOnce<A> for &'_ F where
F: Fn<A> + ?Sized,` [src]

#### `type Output = <F as FnOnce<A>>::Output`

#### `extern "rust-call" fn call_once(self, args: A) -> <F as FnOnce<A>>::Output`[src]

🔬 This is a nightly-only experimental API. (`fn_traits` #29625)

### `impl<'_, A, F> FnOnce<A> for &'_ mut F where
F: FnMut<A> + ?Sized,` [src]

#### `type Output = <F as FnOnce<A>>::Output`

#### `extern "rust-call" fn call_once(self, args: A) -> <F as FnOnce<A>>::Output`[src]

🔬 This is a nightly-only experimental API. (`fn_traits` #29625)

### `impl<A, F> FnOnce<A> for Box<F> where
F: FnOnce<A> + ?Sized,` [src]

#### `type Output = <F as FnOnce<A>>::Output`

####
ⓘImportant traits for Box<I>

### Important traits for Box<I>

`impl<I> Iterator for Box<I> where
I: Iterator + ?Sized, type Item = <I as Iterator>::Item;impl<F> Future for Box<F> where
F: Unpin + Future + ?Sized, type Output = <F as Future>::Output;impl<R: Read + ?Sized> Read for Box<R>impl<W: Write + ?Sized> Write for Box<W>`
`extern "rust-call" fn call_once(self, args: A) -> <Box<F> as FnOnce<A>>::Output`[src]

🔬 This is a nightly-only experimental API. (`fn_traits` #29625)

### `impl<R, F: FnOnce() -> R> FnOnce() for AssertUnwindSafe<F>`[src]

#### `type Output = R`

#### `extern "rust-call" fn call_once(self, _args: ()) -> R`[src]

🔬 This is a nightly-only experimental API. (`fn_traits` #29625)
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