Rust by Example

Traits

A trait is a collection of methods defined for an unknown type: Self. They can access other methods declared in the same trait.

Traits can be implemented for any data type. In the example below, we define Animal, a group of methods. The Animal trait is then implemented for the Sheep data type, allowing the use of methods from Animal with a Sheep.

struct Sheep { naked: bool, name: &'static str }

trait Animal {

// Static method signature; `Self` refers to the implementor type.

fn new(name: &'static str) -> Self;

// Instance method signatures; these will return a string.

fn name(&self) -> &'static str;

fn noise(&self) -> &'static str;

// Traits can provide default method definitions.

fn talk(&self) {

println!("{} says {}", self.name(), self.noise());

}

}

impl Sheep {

fn is_naked(&self) -> bool {

self.naked

}

fn shear(&mut self) {

if self.is_naked() {

// Implementor methods can use the implementor's trait methods.

println!("{} is already naked...", self.name());

} else {

println!("{} gets a haircut!", self.name);

self.naked = true;

}

}

}

// Implement the `Animal` trait for `Sheep`.

impl Animal for Sheep {

// `Self` is the implementor type: `Sheep`.

fn new(name: &'static str) -> Sheep {

Sheep { name: name, naked: false }

}

fn name(&self) -> &'static str {

self.name

}

fn noise(&self) -> &'static str {

if self.is_naked() {

"baaaaah?"

} else {

"baaaaah!"

}

}

// Default trait methods can be overridden.

fn talk(&self) {

// For example, we can add some quiet contemplation.

println!("{} pauses briefly... {}", self.name, self.noise());

}

}

fn main() {

// Type annotation is necessary in this case.

let mut dolly: Sheep = Animal::new("Dolly");

// TODO ^ Try removing the type annotations.

dolly.talk();

dolly.shear();

dolly.talk();

}

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