Default trait implementation for primitive array data type [duplicate]

Question

How can I initialize an integer array [i32; 100] with the default value of i32 (0) by deriving the Default trait? I wrote this code:

#[derive(Default)]
struct A {
    a: i32,
    arr: [i32; 100],
}

But the compiler rejects it with this error:

error[E0277]: the trait bound `[i32; 100]: Default` is not satisfied
 --> src/main.rs:4:5
  |
1 | #[derive(Default)]
  |          ------- in this derive macro expansion
...
4 |     arr: [i32; 100],
  |     ^^^^^^^^^^^^^^^ the trait `Default` is not implemented for `[i32; 100]`
  |
  = help: the following other types implement trait `Default`:
            [T; 0]
            [T; 1]
            [T; 2]
            [T; 3]
            [T; 4]
            [T; 5]
            [T; 6]
            [T; 7]
          and 27 others
  = note: this error originates in the derive macro `Default` (in Nightly builds, run with -Z macro-backtrace for more info)

Mac O'Brien · Accepted Answer · 2024-01-03 05:00:28Z

2

Default is currently only implemented for arrays of T: Default up to a length of 32. This is a historical restriction from before Rust had const generics, so the compiler could not cover multiple array lengths with a single implementation.

There is an open issue in the Rust repo (Use const generics for array Default impl) to impl [T; N]: Default for any N, but as of 2024 this is a work in progress.

You will have to manually implement Default for your struct:

struct A {
    a: i32,
    arr: [i32; 100],
}

impl Default for A {
    fn default() -> Self {
        A {
            a: Default::default(),
            arr: [Default::default(); 100],
        }
    }
}

answered Jan 3, 2024 at 5:00

Mac O'Brien

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