How to default-initialize a struct containing an array in Rust?

Question

What is the recommended way to declare a struct that contains an array, and then create a zero-initialized instance?

Here is the struct:

#[derive(De         


        

Answer 1

If you're sure to initialize every field with zero, this would work:

impl Default for Histogram {
    fn default() -> Histogram {
        unsafe { std::mem::zeroed() }
    }
}

Answer 2

Rust does not implement Default for all arrays because it does not have non-type polymorphism. As such, Default is only implemented for a handful of sizes.

You can, however, implement a default for your type:

impl Default for Histogram {
    fn default() -> Histogram {
        Histogram {
            sum: 0,
            bins: [0; 256],
        }
    }
}

Note: I would contend that implementing Default for u32 is fishy to start with; why 0 and not 1? or 42? There is no good answer, so no obvious default really.

Answer 3

Indeed, at the time of writing, support for fixed-length arrays is still being hashed out in the standard library:

https://github.com/rust-lang/rust/issues/7622

Answer 4

I'm afraid you can't do this, you will need to implement Default for your structure yourself:

struct Histogram {
    sum: u32,
    bins: [u32; 256],
}

impl Default for Histogram {
    #[inline]
    fn default() -> Histogram {
        Histogram {
            sum: 0,
            bins: [0; 256],
        }
    }
}

Numeric types have nothing to do with this case, it's more like problems with fixed-size arrays. They still need generic numerical literals to support this kind of things natively.