Trait std::boxed::FnBox
[−]
[src]
pub trait FnBox<A> { type Output; fn call_box(self: Box<Self>, args: A) -> Self::Output; }
Unstable (
fnbox
): Newly introduced
FnBox
is a version of the FnOnce
intended for use with boxed
closure objects. The idea is that where one would normally store a
Box<FnOnce()>
in a data structure, you should use
Box<FnBox()>
. The two traits behave essentially the same, except
that a FnBox
closure can only be called if it is boxed. (Note
that FnBox
may be deprecated in the future if Box<FnOnce()>
closures become directly usable.)
Example
Here is a snippet of code which creates a hashmap full of boxed
once closures and then removes them one by one, calling each
closure as it is removed. Note that the type of the closures
stored in the map is Box<FnBox() -> i32>
and not Box<FnOnce() -> i32>
.
#![feature(fnbox)] use std::boxed::FnBox; use std::collections::HashMap; fn make_map() -> HashMap<i32, Box<FnBox() -> i32>> { let mut map: HashMap<i32, Box<FnBox() -> i32>> = HashMap::new(); map.insert(1, Box::new(|| 22)); map.insert(2, Box::new(|| 44)); map } fn main() { let mut map = make_map(); for i in &[1, 2] { let f = map.remove(&i).unwrap(); assert_eq!(f(), i * 22); } }
Associated Types
type Output
Unstable (
fnbox
): Newly introduced