alloc::boxed::FnBox - Rust
Description: API documentation for the Rust `FnBox` trait in crate `alloc`.
Keywords: rust, rustlang, rust-lang, FnBox

# Trait alloc::boxed::FnBox [−] [src]

    pub trait FnBox<A> {
        type Output;
        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<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`

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

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

## 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

## Implementors

- `impl<A, F> FnBox<A> for F where F: FnOnce<A>`

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