Trait

Often a Rust program needs the concept of shared features or an interface. Multiple structs have the same feature, and need to be used in a unified way.

In Rust, we can have interfaces that are shared between multiple structs by using the trait-keyword. With impl, we indicate how those traits are implemented.

Example

Consider this example Rust program—we have an Animal trait, and it is shared between the Bird and Snake structs. On the Animal trait, we get the get_weight function.

Note To implement the Animal trait for Bird structs, we specify a get_weight function that is based on the Bird's feather count.

Note 2 For the snake implementation of Animal, we base the get_weight function on the number of scales the snake has.

Info The print_weight function receives an Animal trait instance, and we must use the "dyn" keyword to specify this is a trait.

Finally The program instantiates the Bird and Snake, and uses the Animal trait to access their get_weight functions in print_weight.

trait Animal {
    fn get_weight(&self) -> u32;
}

struct Bird {
    feathers: u32
}

impl Animal for Bird {
    fn get_weight(&self) -> u32 {
        self.feathers * 2
    }
}

struct Snake {
    scales: u32
}

impl Animal for Snake {
    fn get_weight(&self) -> u32 {
        self.scales * 15
    }
}

fn print_weight(arg: &dyn Animal) {
    // Use Animal trait.
    println!("WEIGHT: {}", arg.get_weight());
}

fn main() {
    // Create Bird and Snake, use Animal trait.
    let bird = Bird { feathers: 100 };
    print_weight(&bird);

    let snake = Snake { scales: 50 };
    print_weight(&snake);
}WEIGHT: 200
WEIGHT: 750

Language notes

In other programming languages (like C#) the trait-keyword is most similar to an interface. Traits can be used to implement features of object-oriented programming.

With traits, we create rules about how structs can be used in a unified way. So we can call a specific function on multiple struct types—this can make code clearer and more correct.