I am trying to initialise an array of structs in Rust:
enum Direction {
North,
East,
South,
West,
}
struct RoadPoint {
direction: Direction,
index: i32,
}
// Initialise the array, but failed.
let data = [RoadPoint { direction: Direction::East, index: 1 }; 4];
When I try to compile, the compiler complains that the Copy trait is not implemented:
error[E0277]: the trait bound `main::RoadPoint: std::marker::Copy` is not satisfied
--> src/main.rs:15:16
|
15 | let data = [RoadPoint { direction: Direction::East, index: 1 }; 4];
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `std::marker::Copy` is not implemented for `main::RoadPoint`
|
= note: the `Copy` trait is required because the repeated element will be copied
How can the Copy trait be implemented?
#[derive(Clone, Copy)] is the right way, but for the record, it's not magical: It's easy to implement those traits manually, especially in easy cases such as yours: impl Copy for Direction {} impl Clone for Direction { fn clone(&self) -> Self { *self } }
You don't have to implement Copy yourself; the compiler can derive it for you:
#[derive(Copy, Clone)]
enum Direction {
North,
East,
South,
West,
}
#[derive(Copy, Clone)]
struct RoadPoint {
direction: Direction,
index: i32,
}
Note that every type that implements Copy must also implement Clone. Clone can also be derived.
Just prepend #[derive(Copy, Clone)] before your enum.
If you really want, you can also
impl Copy for MyEnum {}
The derive-attribute does the same thing under the hood.
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Cloneis a supertrait ofCopyso every type implementingCopyalso needs to implementClone.Clonewithout derivingCopy. It allows developers to do.clone()on the element explicitly, but it won't do it for you (that'sCopy's job). So at least there's a reason forCloneto exist separately fromCopy; I would go further and assumeCloneimplements the method, butCopymakes it automatic, without redundancy between the two.