Simulate dynamic dispatch of generic functions

1

I'm trying to do something like:

pub trait RegisterHook {
    fn register<V>(&mut self);
}

struct Register {
    hooks: Vec<Box<dyn RegisterHook>>,
}

impl Register {
    fn add_hook<R: RegisterHook>(&mut self, hook: H) {
        self.hooks.push(Box::new(hook))
    }

    fn register<V>(&mut self) {
        for hook in &mut self.hooks {
            hook.register::<V>()
        }
    }
}

Of course this can't be done like that cause a trait with generic functions not marked as Self: Sized can't be used as a trait object but was wondering if there's a workaround to simulate that behavior in some other way.

2

You can often define a new trait that isn't generic but whose implementation delegates to the RegisterHook trait. However, I don't think it's possible for this particular use-case when you don't actually take any arguments of the generic type.

3

Could you post an example of how would you do that when passing an argument of the type?

4

Sure, you could do this:

use std::fmt::Display;

trait MyTrait {
    fn takes_display<T: Display>(&self, arg: &T);
}

trait MyTraitDyn {
    fn takes_display(&self, arg: &dyn Display);
}
impl<T> MyTraitDyn for T
where
    T: MyTrait,
    T: ?Sized,
{
    fn takes_display(&self, arg: &dyn Display) {
        MyTrait::takes_display(self, &arg);
    }
}

Then store an Box<dyn MyTraitDyn>.

1 Like
5

I see. I was just trying something like that. The problem is my parameter trait has associated types which makes this even more difficult.

As in:

trait Parameter {
    type A;
}

trait MyTrait {
    fn takes_parameter<P: Parameter>(&self, param: P);
}

Cause in the dynamic version you need to specify all the associated types which makes it pretty impractical
.
Thanks!

6

If it has associated types, you can sometimes do one of these:

  1. Give the associated type a fixed value.
  2. Define another helper trait that doesn't have associated types.

But sometimes you can't, and then you're out of luck.

2 Likes
7

Actually I'm trying this and it doesn't work. &dyn Trait`` doesn't necessarily implement Trait but also I need to preserve type information which is lost when using the object trait.

8

Sure, this technique only works in some cases. For the specific problem of &dyn Trait not implementing Trait, you can define a wrapper struct around the reference and implement the trait for your wrapper.

9

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