[−][src]Trait std::boxed::FnBox

pub trait FnBox<A>: FnOnce<A> {
    fn call_box(self: Box<Self>, args: A) -> Self::Output;
}
🔬 This is a nightly-only experimental API. (fnbox #28796)

will be deprecated if and when Box<FnOnce> becomes usable

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<dyn FnOnce()> in a data structure, you should use Box<dyn 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<dyn FnOnce()> closures become directly usable.)

Examples

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<dyn FnBox() -> i32> and not Box<dyn FnOnce() -> i32>.

#![feature(fnbox)]

use std::boxed::FnBox;
use std::collections::HashMap;

fn make_map() -> HashMap<i32, Box<dyn FnBox() -> i32>> {
    let mut map: HashMap<i32, Box<dyn 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);
    }
}Run

Required methods

fn call_box(self: Box<Self>, args: A) -> Self::Output

🔬 This is a nightly-only experimental API. (fnbox #28796)

will be deprecated if and when Box<FnOnce> becomes usable

Performs the call operation.

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Implementors

impl<A, F> FnBox<A> for F where
    F: FnOnce<A>, [src]

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