Traits

Define a trait

A trait lists method signatures that types can share. It says what behavior looks like without saying how any one type works. Any type that implements every listed method is said to implement the trait.

trait Greeter { fn greet(&self) -> String; }

The greet method borrows the value and returns owned text. No body appears here, so each implementor writes its own version. The next section gives User its version.

Implement it

An impl block ties the trait to one concrete type. The header names both the trait and the type, and the body fills in each method. User stores a name, so greeting formats that name into a sentence.

struct User { name: String, } impl Greeter for User { fn greet(&self) -> String { format!("hello, {}", self.name) } }

Other types can implement Greeter too, each with different text. Code that takes a Greeter works with all of them at once. Shared behavior without shared fields is the whole point of traits.

Default methods

Traits may ship a body for methods that most types want unchanged. Implementors inherit the default and override only what differs. Loud builds on greet, so it converts whatever greet returns.

trait Greeter { fn greet(&self) -> String; fn loud(&self) -> String { self.greet().to_uppercase() } }

A type that implements greet gets loud for free. Types with special needs write their own loud and the default steps aside. Defaults keep traits small while staying flexible.

Derive cheap traits

Some standard traits are so mechanical that the compiler writes them on request. Debug prints values for developers, and Clone duplicates them on demand. One attribute line above the struct enables both at once.

#[derive(Debug, Clone)] struct User { name: String, } fn main() { let user = User { name: String::from("ada") }; println!("{:?}", user.clone()); }

Deriving works only for traits that opt in, which the standard library marks clearly. Debug output uses the {:?} placeholder seen above. Reach for derive before hand writing any obvious impl.

Next: Generics

Article author: Arthur Isaev