80 lines
1.2 KiB
Markdown
80 lines
1.2 KiB
Markdown
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% Associated Constants
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With the `associated_consts` feature, you can define constants like this:
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```rust
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#![feature(associated_consts)]
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trait Foo {
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const ID: i32;
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}
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impl Foo for i32 {
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const ID: i32 = 1;
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}
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fn main() {
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assert_eq!(1, i32::ID);
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}
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```
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Any implementor of `Foo` will have to define `ID`. Without the definition:
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```rust,ignore
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#![feature(associated_consts)]
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trait Foo {
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const ID: i32;
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}
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impl Foo for i32 {
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}
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```
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gives
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```text
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error: not all trait items implemented, missing: `ID` [E0046]
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impl Foo for i32 {
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}
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```
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A default value can be implemented as well:
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```rust
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#![feature(associated_consts)]
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trait Foo {
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const ID: i32 = 1;
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}
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impl Foo for i32 {
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}
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impl Foo for i64 {
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const ID: i32 = 5;
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}
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fn main() {
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assert_eq!(1, i32::ID);
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assert_eq!(5, i64::ID);
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}
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```
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As you can see, when implementing `Foo`, you can leave it unimplemented, as
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with `i32`. It will then use the default value. But, as in `i64`, we can also
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add our own definition.
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Associated constants don’t have to be associated with a trait. An `impl` block
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for a `struct` works fine too:
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```rust
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#![feature(associated_consts)]
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struct Foo;
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impl Foo {
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pub const FOO: u32 = 3;
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}
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```
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