Trait std::ops::FnOnce

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

    // Required method
    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 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

type Output

Required Methods

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

Implementors

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

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

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

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

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

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

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

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

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

type Output = R