Trait collections::boxed::FnBox

pub trait FnBox<A> {
    type Output;
    fn call_box(self: Box<Self>, args: A) -> Self::Output;
}

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

Required Methods

Implementors

• impl<A, F> FnBox<A> for F where F: FnOnce<A>