Construct an array of objects in Rust

Question

This is my very first Rust program that actually has a purpose and I'm still lost with the syntax. So I have trait objects like these:

trait HC<'a> {
    fn info(&self) -> TestInfo<'static>;
    fn test(&self) -> HCResult<'a>;
}

#[allow(non_camel_case_types)]
struct Test_NTP;
impl<'a> HC<'a> for Test_NTP {
    fn info(&self) -> TestInfo<'static> {
        TestInfo { ... }
    }

    fn test(&self) -> HCResult<'a> {
        ...
    }
}

#[allow(non_camel_case_types)]
struct Test_Last;
impl<'a> HC<'a> for Test_Last {
    fn info(&self) -> TestInfo<'static> {
        TestInfo { ... }
    }

    fn test(&self) -> HCResult<'a> {
        ...
    }
}

I want to construct an array of these objects those define the HC trait, tupled with an ID, so I can iterate through them and call their test() functions.

const HC_TESTS: [(i16, &dyn HC)] = [
    (1, &Test_NTP),
    (2, &Test_Last)
];

I tried several variations, but I always get a "doesn't have a size known at compile-time" error. I thought putting the objects in Boxes would solve the problem, but I get the same error:

error[E0277]: the size for values of type `[(i16, Box<&'static (dyn HC<'static> + 'static)>)]` cannot be known at compilation time
   --> src/hc.rs:125:17
    |
125 | const HC_TESTS: [(i16, Box<&dyn HC>)] = [
    |                 ^^^^^^^^^^^^^^^^^^^^^ doesn't have a size known at compile-time
    |
    = help: the trait `Sized` is not implemented for `[(i16, Box<&'static (dyn HC<'static> + 'static)>)]`

The structs don't have any fields, the only reason I use them is to bound the info() and test() functions together, and I don't even use &self, they're only needed to dispatch virtual methods. What am I doing wrong?

Works for me so the problem must be somewhere else, but we will need a minimal reproducible example to diagnose further. — Jmb
– Jmb, Commented May 8, 2022 at 6:15

user2722968 · Accepted Answer · 2022-05-08 11:23:43Z

2

Your syntax is off just slightly. The error

the trait `Sized` is not implemented for `[(i16, Box<&'static (dyn HC<'static> + 'static)>)]

refers to the most outer [...], which is a slice.

In

const HC_TESTS: [(i16, Box<&dyn HC>)] =

you explicitly set the type of HC_TESTS to be a slice, not an array. A slice is always unsized, so the compiler complains. What you want is simply

const HC_TESTS: [(i16, Box<&dyn HC>); 2] =

which sets the type of HC_TESTS to be an array of exactly two elements.

Notice that the other answers and comments here ("works for me") use this syntax as well.

answered May 8, 2022 at 11:23

user2722968

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1 Comment

MegaBrutal Over a year ago

Thank you, this was the problem. I thought Rust would know the size of the array by the amount of items listed. Reminds me of my Pascal days! But of course it needs something to differentiate between array and slice.

Netwave · Accepted Answer · 2022-05-09 07:07:33Z

1

~~Your problem is that you cannot use &~~ (you actually can, the problem is that you didn't actually set the array length [(i16, &dyn HC); 2]).

Also you could have use Box. But since Box::new is not const, you would need to wrap it in some other lazy wrapper, std::lazy::Lazy is not stable jet but would work:

const HC_TESTS: Lazy<[(i16, Box<dyn HC>); 2]> = Lazy::new(|| [
    (1, Box::new(Test_NTP)),
    (2, Box::new(Test_Last))
]);

Playground

Meanwhile you can use once_cell Lazy, which works the same.

Another option is to wrap all of them in a enum, so you can construct them, and impl the trait for that enum so you can use it later:

enum Tests {
    NTP(Test_NTP),
    Last(Test_Last)
}

impl<'a> HC<'a> for Tests {
    fn info(&self) -> TestInfo<'static> {
        match self {
            Self::NTP(e) => e.info(),
            Self::Last(e) => e.info()
        }
    }

    fn test(&self) -> HCResult<'a> {
        match self {
            Self::NTP(e) => e.test(),
            Self::Last(e) => e.test()
        }
    }
}


const HC_TESTS: [(i16, Tests); 2] = [
    (1, Tests::NTP(Test_NTP)),
    (2, Tests::Last(Test_Last))
];

Playground

edited May 9, 2022 at 7:07

answered May 7, 2022 at 21:19

Netwave

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6 Comments

Chayim Friedman Over a year ago

You can create the entire array inside Lazy.

Jmb Over a year ago

Why do you say he can't use &? Playground

Netwave Over a year ago

@Jmb, oh, how does that work?. Does it allocate an static object?

Jmb Over a year ago

@Netwave yes, so long as the object can be static allocated (e.g. no Box, Vec, maps or similar).

Netwave Over a year ago

@Jmb, well that makes sense. I never had the need so didn't knew about it. Thanks for the tip!!

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