What is a trait?

Define shared behavior with traits.

What is a trait?

A trait is a contract of behaviour that types can share: "every type implementing this has these methods". It's similar to interfaces in other languages:

trait Ses {
    fn ses(&self) -> String;
}

struct Kedi;

impl Ses for Kedi {
    fn ses(&self) -> String {
        String::from("Miyav")
    }
}

A trait only defines the signature; each type writes the body in its own impl Trait for Type block.

Calling a trait method

Trait methods are called with a dot like normal methods, but the trait must be in scope. If it's defined in the same module it already is; if it comes from another crate, bring it in with use:

use std::fmt::Write;   // to apply the write! macro to a String

let mut s = String::new();
write!(s, "{}-{}", 1, 2).unwrap();
println!("{s}");   // 1-2

If the trait isn't in scope, the compiler says "method not found ... items from traits can only be used if the trait is in scope" and suggests the missing use line.

Exercise First trait

Define trait Ses with the method fn ses(&self) -> String. Implement it for struct Kedi; ("Miyav") and print the result of Kedi.ses().

Show example solution

trait Ses {
    fn ses(&self) -> String;
}

struct Kedi;

impl Ses for Kedi {
    fn ses(&self) -> String {
        String::from("Miyav")
    }
}

fn main() {
    println!("{}", Kedi.ses());
}

Test yourself

What happens if you forget to write the body of a trait method?
- The method runs empty
- Runtime panic
- It warns
- Compile error: missing trait item (E0046)
Show answer

Compile error: missing trait item (E0046) — Every method without a default body must be written in the implementation.

Implement the trait

impl Ses ____ Kedi {
    fn ses(&self) -> String {
        String::from("Miyav")
    }
}

Show answer

impl Ses for Kedi {
    fn ses(&self) -> String {
        String::from("Miyav")
    }
}

Syntax and meaning

trait Ses { .. }

Contract definition
impl Ses for Kedi

Implementation
fn ses(&self) -> String;

Signature without body
use std::fmt::Write

Brings the trait into scope

Next lesson: Implementing a trait

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