rust-by-practice/src/generics-traits/traits.md

Traits

A trait tells the Rust compiler about functionality a particular type has and can share with other types. We can use traits to define shared behavior in an abstract way. We can use trait bounds to specify that a generic type can be any type that has certain behavior.

Note: Traits are similar to a feature often called interfaces in other languages, although with some differences.

Examples

struct Sheep { naked: bool, name: String }

trait Animal {
    // Associated function signature; `Self` refers to the implementor type.
    fn new(name: String) -> Self;

    // Method signatures; these will return a string.
    fn name(&self) -> String;
    
    fn noise(&self) -> String;

    // 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: String) -> Sheep {
        Sheep { name: name, naked: false }
    }

    fn name(&self) -> String {
        self.name.clone()
    }

    fn noise(&self) -> String {
        if self.is_naked() {
            "baaaaah?".to_string()
        } else {
            "baaaaah!".to_string()
        }
    }
    
    // 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".to_string());
    // TODO ^ Try removing the type annotations.

    dolly.talk();
    dolly.shear();
    dolly.talk();
}

fn main() { assert_eq!(5, sum(2i8, 3u8)); assert_eq!(50, sum(20, 30.1)); }