共享类型
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
.