Limits of Trait Objects

use std::any::Any;

pub trait Trait: Any {}

impl Trait for i32 {}

fn main() {
    dbg!(size_of::<i32>()); // 4 bytes, owned value
    dbg!(size_of::<&i32>()); // 8 bytes, reference
    dbg!(size_of::<&dyn Trait>()); // 16 bytes, wide pointer
}

  • Trait objects are a limited way of solving problems.

  • If you want to downcast to a concrete type from a trait object, you will need to specify that the trait in question has Any as a supertrait or that the trait object is over the main trait and Any.

    Even then, you will still need to cast a dyn MyTrait to dyn Any

  • Trait objects have overhead in memory, they are “wide pointers” that need to hold not just the pointer to the data itself but another pointer for the vtable.

  • Trait objects, being dynamically sized types, can only be used practically via reference or pointer types.

    There is a baseline overhead of dereferencing the value and relevant trait methods when using trait objects.