FnOnce in std::ops - Rust
Description: The version of the call operator that takes a by-value receiver.

## FnOnce

# Trait FnOnce

1.0.0 (const: unstable) · Source

    pub trait FnOnce<Args>where
        Args: Tuple,{
        type Output;

        // Required method
        extern "rust-call" fn call_once(self, args: Args) -> Self::Output;
    }

Expand description

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 closures 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 point

## Required Associated Types§

1.12.0 · Source

#### type Output

The returned type after the call operator is used.

## Required Methods§

Source

#### 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.

## Implementors§

1.0.0 (const: unstable) · Source§

### impl<A, F> FnOnce<A> for &F
where A: Tuple, F: Fn<A> + ?Sized,

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

1.0.0 (const: unstable) · Source§

### impl<A, F> FnOnce<A> for &mut F
where A: Tuple, F: FnMut<A> + ?Sized,

Source§

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

1.35.0 · Source§

### impl<Args, F, A> FnOnce<Args> for Box<F, A>
where Args: Tuple, F: FnOnce<Args> + ?Sized, A: Allocator,

Source§

#### type Output = <F as FnOnce<Args>>::Output

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### impl<F, Args> FnOnce<Args> for Exclusive<F>
where F: FnOnce<Args>, Args: Tuple,

Source§

#### type Output = <F as FnOnce<Args>>::Output

1.9.0 · Source§

### impl<R, F> FnOnce() for AssertUnwindSafe<F>
where F: FnOnce() -> R,

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#### type Output = R