How to initialize an array of Vec? [duplicate] - rust

Question

I’m trying to initialize a fixed-size array of some nullable, non-copyable type, like an Option<Box<Thing>> for some kind of Thing. I’d like to pack two of them into a struct without any extra indirection. I’d like to write something like this:
let array: [Option<Box<Thing>>; SIZE] = [None; SIZE];
But it doesn’t work because the [e; n] syntax requires that e implements Copy. Of course, I could expand it into SIZE Nones, but that can be unwieldy when SIZE is large. I don’t believe this can be done with a macro without an unnatural encoding of SIZE. Is there a good way to do it?
Yes, this is easy with unsafe; is there a way to do it without unsafe?

Answer 1

As of Rust 1.38 (released in September 2019), a cleaner alternative to previously posted answers is possible using an intermediate const initializer. This approach works for arrays of any size:
const SIZE: usize = 100;
const INIT: Option<Box<Thing>> = None;
let array: [Option<Box<Thing>>; SIZE] = [INIT; SIZE];
(It works with or without the Box; the example uses Box because it was used in the question.)
One limitation is that the array item must have a default representation that can be evaluated at compile time - a constant, enum variant, or a primitive container composed of those. None or a tuple of numbers will work, but a non-empty Vec or String won't.

Answer 2

You could use the Default trait to initialize the array with default values:
let array: [Option<Box<Thing>>; SIZE] = Default::default();
See this playground for a working example.
Note that this will only work for arrays with up to 32 elements, because Default::default is only implemented for up to [T; 32]. See https://doc.rust-lang.org/std/default/trait.Default.html#impl-Default-for-%5BT%3B%2032%5D.

Answer 3

As of Rust 1.55.0 (which introduced [T]::map()), the following will work:
const SIZE: usize = 100;
#[derive(Debug)]
struct THING { data: i64 }
let array = [(); SIZE].map(|_| Option::<THING>::default());
for x in array {
println!("x: {:?}", x);
}
Rust Playground

Answer 4

I'm copying the answer by chris-morgan and adapting it to match the question better, to follow the recommendation by dbaupp downthread, and to match recent syntax changes:
use std::mem;
use std::ptr;
#[derive(Debug)]
struct Thing {
number: usize,
}
macro_rules! make_array {
($n:expr, $constructor:expr) => {{
let mut items: [_; $n] = mem::uninitialized();
for (i, place) in items.iter_mut().enumerate() {
ptr::write(place, $constructor(i));
}
items
}}
}
const SIZE: usize = 50;
fn main() {
let items = unsafe { make_array!(SIZE, |i| Box::new(Some(Thing { number: i }))) };
println!("{:?}", &items[..]);
}
Note the need to use unsafe here: The problem is that if the constructor function panic!s, this would lead to undefined behavior.

Answer 5

Go through the heap
If you can create a Vec of your type, you can convert it into an array:
use std::convert::TryInto;
#[derive(Clone)]
struct Thing;
const SIZE: usize = 100;
fn main() {
let v: Vec<Option<Thing>> = vec![None; SIZE];
let v: Box<[Option<Thing>; SIZE]> = match v.into_boxed_slice().try_into() {
Ok(v) => v,
Err(_) => unreachable!(),
};
let v: [Option<Thing>; SIZE] = *v;
}
In many cases, you actually want to leave it as a Vec<T>, Box<[T]>, or Box<[T; N]> as these types all put the data in the heap. Large arrays tend to be... large... and you don't want all that data on the stack.
See also:
What is the use of into_boxed_slice() methods?
How to get a slice as an array in Rust?
How do I get an owned value out of a `Box`?
Keep it simple
Type out all the values:
struct Thing;
const SIZE: usize = 5;
fn main() {
let array: [Option<Box<Thing>>; SIZE] = [None, None, None, None, None];
}
You could use a build script to generate this code for you. For an example of this, see:
How to create a static string at compile time

Answer 6

An alternative approach using the arrayvec crate that generalizes easily to situations other than initializing everything with a fixed value:
use arrayvec::ArrayVec;
let array = std::iter::repeat(None)
.take(SIZE)
.collect::<ArrayVec<Option<Box<Thing>>, SIZE>>()
.into_inner()
.unwrap();
(playground)

Answer 7

On nightly Rust, you can use inline const. This is a variant of the answer by #user4815162342, but one that doesn't require you to declare a separate constant and repeat the type:
#![feature(inline_const)]
let array: [Option<Box<Thing>>; SIZE] = [const { None }; SIZE];
Until this is stabilized (hopefully soon), you can also use the inline-const crate, but this does require you to repeat the type.

Answer 8

let stackoverflow: [Option<&mut ()>;0xDEADBEEF] = std::array::from_fn(|_| None);
dbg!(stackoverflow);
playground