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% Associated Constants
With the `associated_consts` feature, you can define constants like this:
```rust
#![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:
```rust,ignore
#![feature(associated_consts)]
trait Foo {
const ID: i32;
}
impl Foo for i32 {
}
```
gives
```text
error: not all trait items implemented, missing: `ID` [E0046]
impl Foo for i32 {
}
```
A default value can be implemented as well:
```rust
#![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` works fine too:
```rust
#![feature(associated_consts)]
struct Foo;
impl Foo {
pub const FOO: u32 = 3;
}
```
|
