CODEWITHSUNDEEP
rust
boundless-forest
boundless-forest

Reputation: 427

When(How) to use const generic in rust?

I am learning how to use const generics in rust. An easy demo like this: Playground

Can somebody tell me how to use a const generic array as a return type?

fn foo<const C: usize>(n: u32) -> [i32; C] {
    // failed
    // [1; 3]
    // failed
    [1; n]
}

fn hoo<const N: usize>(arr: [i32; N]) {
    println!("the array is {:?}", arr);
}

fn main() {
    // Give a number, create a array, this fails
    let f = foo(3);

    // const array as input
    hoo([1]);
    hoo([1, 2, 3]);
}

The compile message as following:

  Compiling playground v0.0.1 (/playground)
error[E0435]: attempt to use a non-constant value in a constant
 --> src/main.rs:2:9
  |
1 | fn foo<const C: usize>(n: u32) -> [i32; C] {
  |                        - this would need to be a `const`
2 |     [1; n]
  |         ^

error: aborting due to previous error

For more information about this error, try `rustc --explain E0435`.
error: could not compile `playground`

To learn more, run the command again with --verbose.

Upvotes: 1

Views: 4127

Answers (1)

Silvio Mayolo
Silvio Mayolo

Reputation: 70297

The point of const generics is that the size of the array is known at compile-time. If you want a runtime-known size, then that's what Vec<T> is for. Consider

fn foo<const C: usize>() -> [i32; C] {
    [1; C]
}

Then call it as

foo::<3>();

Note that, by its nature, this forces you to only use constant values; if you try to put a local variable into the generic argument position, you'll get an error.

Upvotes: 7

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