# Associated Constants

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); }

    #![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)] fn main() { trait Foo { const ID: i32; } impl Foo for i32 { } }

    #![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`, 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:

#![feature(associated_consts)] fn main() { struct Foo; impl Foo { const FOO: u32 = 3; } }

    #![feature(associated_consts)]

    struct Foo;

    impl Foo {
        const FOO: u32 = 3;
    }