1.0.0[−][src]Trait core::ops::FnOnce
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.
Required methods
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impl<'_, A, F: ?Sized> FnOnce<A> for &'_ F where
F: Fn<A>,
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F: Fn<A>,
impl<'_, A, F: ?Sized> FnOnce<A> for &'_ mut F where
F: FnMut<A>,
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F: FnMut<A>,