src/doc/book/associated-constants.md

   1 % Associated Constants
   2
   3 With the `associated_consts` feature, you can define constants like this:
   4
   5 ```rust
   6 #![feature(associated_consts)]
   7
   8 trait Foo {
   9     const ID: i32;
  10 }
  11
  12 impl Foo for i32 {
  13     const ID: i32 = 1;
  14 }
  15
  16 fn main() {
  17     assert_eq!(1, i32::ID);
  18 }
  19 ```
  20
  21 Any implementor of `Foo` will have to define `ID`. Without the definition:
  22
  23 ```rust,ignore
  24 #![feature(associated_consts)]
  25
  26 trait Foo {
  27     const ID: i32;
  28 }
  29
  30 impl Foo for i32 {
  31 }
  32 ```
  33
  34 gives
  35
  36 ```text
  37 error: not all trait items implemented, missing: `ID` [E0046]
  38      impl Foo for i32 {
  39      }
  40 ```
  41
  42 A default value can be implemented as well:
  43
  44 ```rust
  45 #![feature(associated_consts)]
  46
  47 trait Foo {
  48     const ID: i32 = 1;
  49 }
  50
  51 impl Foo for i32 {
  52 }
  53
  54 impl Foo for i64 {
  55     const ID: i32 = 5;
  56 }
  57
  58 fn main() {
  59     assert_eq!(1, i32::ID);
  60     assert_eq!(5, i64::ID);
  61 }
  62 ```
  63
  64 As you can see, when implementing `Foo`, you can leave it unimplemented, as
  65 with `i32`. It will then use the default value. But, as in `i64`, we can also
  66 add our own definition.
  67
  68 Associated constants don’t have to be associated with a trait. An `impl` block
  69 for a `struct` or an `enum` works fine too:
  70
  71 ```rust
  72 #![feature(associated_consts)]
  73
  74 struct Foo;
  75
  76 impl Foo {
  77     const FOO: u32 = 3;
  78 }
  79 ```