Rust Associated Constants


  • #![feature(associated_consts)]
  • const ID: i32;


This feature is currently available only in nightly compiler. Tracking issue #29646

Using Associated Constants

// Must enable the feature to use associated constants

use std::mem;

// Associated constants can be used to add constant attributes to types
trait Foo {
    const ID: i32;

// All implementations of Foo must define associated constants
// unless a default value is supplied in the definition.
impl Foo for i32 {
    const ID: i32 = 1;

struct Bar;

// Associated constants don't have to be bound to a trait to be defined
impl Bar {
    const BAZ: u32 = 5;

fn main() {
    assert_eq!(1, i32::ID);

    // The defined constant value is only stored once, so the size of
    // instances of the defined types doesn't include the constants.
    assert_eq!(4, mem::size_of::<i32>());
    assert_eq!(0, mem::size_of::<Bar>());