Associated Types

Associated types are placeholder types which are supplied by the trait implementation.

#[derive(Debug)]
struct Meters(i32);
#[derive(Debug)]
struct MetersSquared(i32);
trait Multiply {
    type Output;
    fn multiply(&self, other: &Self) -> Self::Output;
}
impl Multiply for Meters {
    type Output = MetersSquared;
    fn multiply(&self, other: &Self) -> Self::Output {
        MetersSquared(self.0 * other.0)
    }
}
fn main() {
    println!("{:?}", Meters(10).multiply(&Meters(20)));
}

• Associated types are sometimes also called “output types”. The key observation is that the implementer, not the caller, chooses this type.

• Many standard library traits have associated types, including arithmetic operators and Iterator.