Rust traits

Traits are similar to interfaces except they can have a default implementation.

Basics

Define and implement trait

struct Cat { name: String }
struct Dog { name: String, breed: String }

// all implementors of 'Pet' must have method 'play' with this method signature
trait Pet {
    fn play(&self) -> bool;
}


impl Pet for Cat {
    fn play(&self) -> bool {
        println!("you give a ball of yarn to {}", self.name);
        true
    }
}

Use trait implementors in method signature

// use trait as param type
fn play_with_pet(pet: &impl Pet) -> bool {
    pet.play();
}

let cat = Cat{name: "maize".to_string()};
let dog = Dog{name: "midnight".to_string(), breed: "?".to_string()};
play_with_pet(&cat);
play_with_pet(&dog);

In Function Signatures

As Param

// with impl
fn play_with_pet(pet: &impl Pet) -> bool {}

// trait-bound-syntax (generic)
fn play_with_pet<P: Pet>(pet: P) -> bool {}

Requires object implements multiple interfaces

// with impl
fn play_with_pet(pet: &(impl Pet + Display)) -> bool {}

// with where clause
fn play_with_pet<P>(p: P) -> bool
where
    P: Pet + Display
{}

As Return

If you're only ever returning one possible concretion, you can do this:

fn gimme_pet() -> impl Pet {
    // ...
}

But if you may return different types of implementors of Pet, you'll need to do this:

// TODO

Default Method Implementation

Unlike interfaces in most other languages, you can define a default method implementation on the trait.

TODO:

can you require fields as well? (in this case, name)

struct Cat { name: String }
struct Dog { name: String, breed: String }

trait Pet {
    fn play(&self) {
        println!("rub")
    }
}