How to call generic trait method in Rust

Question

Having these rather contrived type definitions

trait Generic<T> {
    fn some(&self) -> T;
}

impl<T> Generic<T> for i32
where
    T: Default,
{
    fn some(&self) -> T {
        T::default()
    }
}

I would like to call some method explicitly specifying type T. Below code apparently does not work because the method itself is not generic.

fn main() {
    let int: i32 = 45;
    println!( "some: {}", int.some<bool>() );
}

What's the right way of calling some?

Compiler is not able to infer since println! macro accepts generic type. Here is some ideas : play.rust-lang.org/… — Ömer Erden
– Ömer Erden, Commented Dec 25, 2019 at 6:33
@ÖmerErden Great answer! But why is that &int.some() as &bool works while int.some() as bool does not? — Mooh
– Mooh, Commented Jan 1, 2020 at 9:02
You can't directly cast T into a target type, please see: doc.rust-lang.org/reference/expressions/… — Ömer Erden
– Ömer Erden, Commented Jan 1, 2020 at 11:05

Optimistic Peach · Accepted Answer · 2019-12-25 18:18:35Z

7

You must specify the exact type, as you tried. Unfortunately, your function is not generic, and instead your implementation is generic, so you'd have to do the following:

fn main() {
    let int: i32 = 45;
    println!("some: {}", <i32 as Generic<bool>>::some(&int));
    // Or,
    println!("some: {}", Generic::<bool>::some(&int));
}

Alternatively you could define a helper trait:

trait HasSome {
    fn other_some<T>(&self) -> T where Self: Generic<T> {
        <Self as Generic<T>>::some(self)
    }
}
impl<T> HasSome for T {} // Blanket impl.

Playground.

On a side note, please know that when specifying the generics of a type or function you need to use the "turbofish" ::<> operator:

let foo = Vec::<i32>::new(); // Vec<i32>
let foo = my_generic_function::<usize>(); // Calls my_generic_function with usize
let foo = Option::<usize>::None;
let foo = None::<usize>;

edited Dec 25, 2019 at 18:18

answered Dec 25, 2019 at 18:11

Optimistic Peach

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