The following Rust code tries to store a closure of zero arguments in an array and call the function.

fn main() {
    println!("The answer is: {}", solution_fns[0]());
}

const solution_fns: [fn() -> isize] =
    [|| (1..=999).filter(|e| divides(3, e) || divides(5, e)).sum()];

fn divides(d: usize, n: usize) -> bool {
    n % d == 0
}

isize] = [|| (1..=999).filter(|e| divides(3, e) || divides(5, e)).sum()]; fn divides(d: usize, n: usize) -> bool { n % d == 0 } " rel="nofollow noreferrer">Link to Rust playground. Unfortunately it does not compile:

error[E0277]: the size for values of type [fn() -> isize] cannot be known at compilation time --> src/main.rs:5:21 | 5 | const solution_fns: [fn() -> isize] = |
^^^^^^^^^^^^^^^ doesn't have a size known at compile-time | = help: the trait Sized is not implemented for [fn() -> isize]

I understand that you cannot constuct an array (or Vec) of things of which the size is not known at compile-time. However I understand the type [fn() -> isize] to be array of function pointers, and I don't see why a function pointer should not have a known size. Sticking the closure in a Box does not seem to help:

const solution_fns: [Box<fn() -> isize>] = [Box::new(|| {
    (1..=999).filter(|e| divides(3, e) || divides(5, e)).sum()
})];

How then can I store an array of closures?

CodePudding user response：

The problem is not in the fn pointer, but that your array doesnt have a size. It is simply solved by adding the expected size to the array declaration:

const solution_fns: [fn() -> usize; 1] =
    [|| (1..=999usize).filter(|&e| divides(3, e) || divides(5, e)).sum()];

Playground