How do I create an array of unboxed functions / closures?

Question

Editor\'s note: This question was asked before Rust 1.0 and some of the syntax has changed since then, but the underlying concepts remain. Some answers have b

Answer 1

It is possible to collect multiple closures from an iterator into a Vec:

fn main() {
    let a: Vec<_> = (0..10).map(|i| move || i).collect();

    println!("{} {} {}", a[1](), a[5](), a[9]());
}

The move keyword causes ownership of i to be transferred to the closure.

See also:

• How do I collect into an array?

Answer 2

You need to use move || i.

move implies that this closure will take i by value rather than by reference. By default, these closures would only take references to i. In your case it is forbidden as the lifetime of i is limited to the body of the loop.

Also, Rust will complain that your array may not be fully initialized. To avoid it, you can either use a Vec<_>, or use std::mem::uninitialized:

fn main() {
    let mut a: [_; 10] = unsafe { std::mem::uninitialized() };
    for i in 0..10 {
        a[i] = move || i;
    }
    println!("{} {} {}", a[1](), a[5](), a[9]())
}