pub trait FnOnce<Args> {
type Output;
const 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