Allocating memory for an empty array

[moonzy]

fn main() {
    // 1
    let empty_arr1: [i32; 0] = [];
    // 2
    let empty_arr2: [i32; 0];
    // 3
    let empty_arr3 = [0u8; 0];
}

For what cases does the allocation occur?

1. Yes
2. No
3. Yes

Am I right?

[quinedot]

None of them allocate. They don't allocate on the heap because arrays are inline values. They don't allocate on the stack either because they're zero-sized.

[moonzy]

Thanks) Are there any other ways to create an empty array ?

[quinedot]

There's a ton of ways in some sense or another...

// e.g.
let _: [u8; 0] = Vec::new().try_into().unwrap();

...but why?

[moonzy]

To demonstrate how to create an empty array. I don’t really know where this will come in handy, but still

[quinedot]

Generally that's just [] when the type can be inferred (like #1) or [dummy_value ; 0] where it can't be inferred but a dummy value is readily available (ala #3).

If neither applied, there are various workarounds that aren't really empty-array specific, like using an annotated temporary or calling a function that returns the value, etc.

All arrays of size 0 implement Default for example.

let _ = <[SomeType; 0]>::default();

[moonzy]

Thanks)