来自引用的特征对象

Question

下面是（大致）取自 rust book chapter 17.2 的 trait object 示例。对于我的用例，我想在创建 screen 后继续使用 button 和 select_box（请参阅声明 screen 后注释掉的 println!()），但是我我不能，因为 button 和 select_box 移到了 screen。在我看来，解决方案似乎是 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。我试过从以下参考创建框：

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 进行全面实施，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)
    }
}

然而，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.

Answer 2

您可以使用 Rc 而不是 Box 以使 screen 和 main 函数可以引用两个组件 select_box 和 button同时

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

trait Draw {
    fn draw(&self);
}

struct Screen {
    components: Vec<Rc<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(";")
        )
    }
}

Playground

这是输出：

button width: 50
SelectBox(75, 10, Yes;Maybe;No)
Button(50, 10, OK)

Answer 3

Trait 对象可以通过 any 指针类型（如引用、Rc、Arc 等）使用，而不仅仅是 Box.所以，如果你想 Screen 借用组件，你可以简单地存储 references 到代表组件的 Draw 特征对象：

struct Screen<'c> {
    components: Vec<&'c dyn Draw>,
}

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

Playground

来自引用的特征对象

Trait objects from references

rust

trait-objects