CODEWITHSUNDEEP
rust
Frank C.
Frank C.

Reputation: 8096

Struggling with Clone on Trait

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/

Upvotes: 0

Views: 318

Answers (1)

Chayim Friedman
Chayim Friedman

Reputation: 71605

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?.

Upvotes: 2

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