associated_consts
The tracking issue for this feature is: #29646
With the associated_consts
feature, you can define constants like this:
#![feature(associated_consts)] trait Foo { const ID: i32; } impl Foo for i32 { const ID: i32 = 1; } fn main() { assert_eq!(1, i32::ID); }
Any implementor of Foo
will have to define ID
. Without the definition:
#![feature(associated_consts)]
trait Foo {
const ID: i32;
}
impl Foo for i32 {
}
gives
error: not all trait items implemented, missing: `ID` [E0046]
impl Foo for i32 {
}
A default value can be implemented as well:
#![feature(associated_consts)] trait Foo { const ID: i32 = 1; } impl Foo for i32 { } impl Foo for i64 { const ID: i32 = 5; } fn main() { assert_eq!(1, i32::ID); assert_eq!(5, i64::ID); }
As you can see, when implementing Foo
, you can leave it unimplemented, as
with i32
. It will then use the default value. But, as in i64
, we can also
add our own definition.
Associated constants don’t have to be associated with a trait. An impl
block
for a struct
or an enum
works fine too:
# #![allow(unused_variables)] # #fn main() { #![feature(associated_consts)] struct Foo; impl Foo { const FOO: u32 = 3; } #}