What is a trait?
- Rust course
- Intermediate
- Lesson 1/115
- 4 min
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}"); // 1-2If 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.
Practice 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().
Starter code
fn main() {
// buraya yaz
}Sample 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());
}Expected output
Miyav
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 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) Correct answer
Explanation: Every method without a default body must be written in the implementation.
2 Implement the trait
impl Ses ____ Kedi {
fn ses(&self) -> String {
String::from("Miyav")
}
}impl Ses for Kedi {
fn ses(&self) -> String {
String::from("Miyav")
}
}Explanation: A trait implementation has the form impl Trait for Type.
3 Syntax and meaning
| Term | 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 |