特徵物件

特徵物件可接受不同型別的值，舉例來說，在集合中會是這樣：

struct Dog {
    name: String,
    age: i8,
}
struct Cat {
    lives: i8,
}

trait Pet {
    fn talk(&self) -> String;
}

impl Pet for Dog {
    fn talk(&self) -> String {
        format!("Woof, my name is {}!", self.name)
    }
}

impl Pet for Cat {
    fn talk(&self) -> String {
        String::from("Miau!")
    }
}

fn main() {
    let pets: Vec<Box<dyn Pet>> = vec![
        Box::new(Cat { lives: 9 }),
        Box::new(Dog { name: String::from("Fido"), age: 5 }),
    ];
    for pet in pets {
        println!("Hello, who are you? {}", pet.talk());
    }
}

以下是配置 pets 後的記憶體配置：

<Dog as Pet>::talk<Cat as Pet>::talkStackHeapFidoptrlives9len2capacity2data:name,4,4age5vtablevtablepets: Vec<dyn Pet>data: CatDogProgram text
This slide should take about 10 minutes.
  • Types that implement a given trait may be of different sizes. This makes it impossible to have things like Vec<dyn Pet> in the example above.
  • 可透過 dyn Pet 這個方法向編譯器告知實作 Pet 的動態大小型別。
  • 在本例中，pets 和向量資料分別在堆疊和堆積上分配。這兩個向量元素都是「虛指標」：
    • A fat pointer is a double-width pointer. It has two components: a pointer to the actual object and a pointer to the virtual method table (vtable) for the Pet implementation of that particular object.
    • 名為 Fido 的 Dog 資料是 name 和 age 欄位。Cat 則有 lives 欄位。
  • 比較上述範例的輸出內容： 
    println!("{} {}", std::mem::size_of::<Dog>(), std::mem::size_of::<Cat>());
println!("{} {}", std::mem::size_of::<&Dog>(), std::mem::size_of::<&Cat>());
println!("{}", std::mem::size_of::<&dyn Pet>());
println!("{}", std::mem::size_of::<Box<dyn Pet>>());