Struggling with Clone on Trait

Question

Somewhat new to Trait implementation/usage:

If I declare a trait thus:

pub trait A: Debug + Clone {
    fn as_base58_string(&self) -> String;
    fn as_bytes(&self) -> [u8; 32];
}

With a concrete implementation:

    impl A for P {
        fn as_base58_string(&self) -> String {
            self.to_base58_string()
        }

        fn as_bytes(&self) -> [u8; 32] {
            self.to_bytes()
        }
    }

And have a function like:

pub fn print_akey(akey: &dyn A) {
    println!("A{:?}", akey);
}

I am getting this error

the trait `A` cannot be made into an object`A` cannot be made into an object

Even though the concrete type P is cloneable?

If I remove Clone from the trait declaration the warning goes away/

Please always post the full error from `cargo check`. – Chayim Friedman Jun 15 '22 at 10:22 — Chayim Friedman, Jun 15 '22 at 10:22

score 1 · Accepted Answer · answered Jun 15 '22 at 10:22

Clone is not object safe. That means you cannot create a dyn Clone, and thus you cannot create dyn Trait for any Trait that has Clone as a supertrait.

The reason for that is that Clone::clone() returns Self, and one of the rules for object safe traits is that they must never mention the Self type (outside of &self or &mut self).

If you want to clone a trait object (dyn Trait), see How to clone a struct storing a boxed trait object?.

Struggling with Clone on Trait

1 Answers1