默认方法

trait Speak {
    fn speak(&self) -> String {
        String::from("Hi.")
    }
}

默认情况下将调用该方法，除非它在 impl 块中被覆盖。

struct Human;
struct Cat;

impl Speak for Human {}

impl Speak for Cat {
    fn speak(&self) -> String {
        String::from("Meow.")
    }
}

fn main() {
    let human = Human {};
    let cat = Cat {};
    println!("The human says {}", human.speak());
    println!("The cat says {}", cat.speak());
}

输出：

人类说嗨。

猫说喵。