Copy

Like Clone, but indicates the type is can be bitwise copied.

Derivable: ✅

When to implement: If possible, but with caveats.

#![allow(unused)]
fn main() {
// Copy is just a marker trait with Clone as a supertrait.
// pub trait Copy: Clone { }

#[derive(Clone, Copy)]
pub struct Copyable(u8, u16, u32, u64);
}
- Clone represents a deep clone, and so does copy, but copy suggests to the compiler that a value can be copied bitwise.

  • When not to implement/derive: If you do not want to implicitly create copies when dereferencing values of a type, do not implement this trait.

    Copy enables implicit duplication, so be careful about what types you’re implementing this on.

  • Ask the class: Can a type with heap data (Vec, BTreeMap, Rc, etc.) be copy? Should it be?

    It both cannot and should not, this is a misuse of this trait.

    Bitwise copying on these types would mean types with heap data would no longer have exclusive ownership of a pointer, breaking the invariants usually upheld by Rust and its ecosystem.

    Multiple Vecs would point to the same data in memory. Adding and removing data would only update individual Vecs length and capacity values. The same for BTreeMap.

    Bitwise copying of Rcs would not update the reference counting value within the pointers, meaning there could be two instances of a Rc value that believe themselves to be the only Rc for that pointer. Once one of them is destroyed, the reference count will become 0 on one of them and the inner value dropped despite there being another Rc still alive.