rust - 来自引用的特征对象

Question

下面是(大致)取自 rust 书第 17.2 章的特征对象示例。对于我的用例，我想在创建 screen 后继续使用 button 和 select_box (请参阅注释掉的 println!() 在声明 screen 之后)，但是我不能，因为 button 和 select_box 被移动到 screen .在我看来，解决方案似乎是让 screen 借用 select_box 和 screen，而不是取得所有权。但是，我不知道该怎么做。我试过从以下引用创建框:

let screen = Screen {
    components: vec![Box::new(&select_box), Box::new(&button)],
};

但这会产生如下错误:

the trait `Draw` is not implemented for `&SelectBox`

fn main() {
    let select_box = SelectBox {
        width: 75,
        height: 10,
        options: vec![
            String::from("Yes"),
            String::from("Maybe"),
            String::from("No"),
        ],
    };
    let button = Button {
        width: 50,
        height: 10,
        label: String::from("OK"),
    };
    let screen = Screen {
        components: vec![Box::new(select_box), Box::new(button)],
    };
    // println!("button width: {}", button.width);
    screen.run();
}

trait Draw {
    fn draw(&self);
}

struct Screen {
    components: Vec<Box<dyn Draw>>,
}

impl Screen {
    fn run(&self) {
        for component in self.components.iter() {
            component.draw();
        }
    }
}

struct Button {
    width: u32,
    height: u32,
    label: String,
}

impl Draw for Button {
    fn draw(&self) {
        println!("Button({}, {}, {})", self.width, self.height, self.label)
    }
}

struct SelectBox {
    width: u32,
    height: u32,
    options: Vec<String>,
}

impl Draw for SelectBox {
    fn draw(&self) {
        println!(
            "SelectBox({}, {}, {})",
            self.width,
            self.height,
            self.options.join(";")
        )
    }
}

Answer 1

通常的解决方案是对 &T 和 &mut T 进行全面实现，其中 T: Draw:

impl<T: ?Sized + Draw> Draw for &'_ T {
    fn draw(&self) {
        <T as Draw>::draw(&**self)
    }
}
impl<T: ?Sized + Draw> Draw for &'_ mut T {
    fn draw(&self) {
        <T as Draw>::draw(&**self)
    }
}

但是，你会得到另一个错误:

error[E0597]: `select_box` does not live long enough
  --> src/main.rs:17:35
   |
17 |         components: vec![Box::new(&select_box), Box::new(&button)],
   |                          ---------^^^^^^^^^^^-
   |                          |        |
   |                          |        borrowed value does not live long enough
   |                          cast requires that `select_box` is borrowed for `'static`
...
21 | }
   | - `select_box` dropped here while still borrowed

error[E0597]: `button` does not live long enough
  --> src/main.rs:17:58
   |
17 |         components: vec![Box::new(&select_box), Box::new(&button)],
   |                                                 ---------^^^^^^^-
   |                                                 |        |
   |                                                 |        borrowed value does not live long enough
   |                                                 cast requires that `button` is borrowed for `'static`
...
21 | }
   | - `button` dropped here while still borrowed

这是因为dyn Trait实际上是dyn Trait + 'static。您需要添加生命周期参数:

struct Screen<'a> {
    components: Vec<Box<dyn Draw + 'a>>,
}

impl Screen<'_> {

Playground .