With the associated_consts
`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
`Foowill have to define
` will have to define ID
`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); }#![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
`Foo, you can leave it unimplemented, as with
`, you can leave it unimplemented, as
with i32
`i32. It will then use the default value. But, as in
`. It will then use the default value. But, as in i64
`i64`, we can also
add our own definition.
Associated constants don’t have to be associated with a trait. An impl
`implblock for a
` block
for a struct
`struct` works fine too:
#![feature(associated_consts)] struct Foo; impl Foo { pub const FOO: u32 = 3; }