无法克隆Vec <box <trait>&gt;因为Trait不能成为一个对象

时间:2018-04-25 08:54:52

标签: rust clone trait-objects

我试图克隆盒装特征的载体。 自然地简单地在实现我的特征的所有结构上导出Clone是不够的,因为编译器在编译时不知道实现该特征的所有结构都有Clone。 / p>

好的,所以我尝试使用Clone作为超级用户,但这只会导致标题中的错误。我为解决方案而感到茫然。

这里是最小工作实施(或不工作,因为我无法克隆)

#![allow(dead_code, unused_macros)]
use std::fmt::Debug;

trait MusicElement: Debug + Clone {
    fn duration(&self) -> f32;
}

#[derive(Debug, Clone)]
struct Note<'a> {
    name: &'a str,
    duration: f32,
}

impl<'a> MusicElement for Note<'a> {
    fn duration(&self) -> f32 {
        self.duration
    }
}

#[derive(Debug, Clone)]
struct Pause {
    duration: f32,
}

impl MusicElement for Pause {
    fn duration(&self) -> f32 {
        self.duration
    }
}

#[derive(Debug, Clone)]
struct Sequence {
    elements: Vec<Box<MusicElement>>,
}

impl MusicElement for Sequence {
    fn duration(&self) -> f32 {
        self.elements.iter().map(|e| e.duration()).sum()
    }
}

fn main() {
    let a4 = |dur| Box::new(Note { name: "a4", duration: dur });
    let seq = Sequence { elements: vec![a4(0.25), a4(0.25), a4(0.5)] };
    println!("{:?}", seq);
    let seq2 = seq.clone();
    println!("{:?}", seq2);
}

出现此错误:

error[E0038]: the trait `MusicElement` cannot be made into an object
  --> src/main.rs:33:5
   |
33 |     elements: Vec<Box<MusicElement>>,
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `MusicElement` cannot be made into an object
   |
   = note: the trait cannot require that `Self : Sized`

这是一个link to the playground,可以轻松运行代码。

我还试图在elementsSequence中制作Vec<Box<MusicElement + Clone>>向量,但这也不起作用。

我还没能在网上找到任何有用的解决方案,所以我的问题是:如何使代码可以克隆?

2 个答案:

答案 0 :(得分:9)

解决方案在于结合迄今为止评论中的建议 - @Lukas Kalbertodt's comment中的答案告诉您必须为所有兼容的('static + MusicElement + Clone)类型创建一个全面的特征实现。实施所需的唯一后续步骤是将Note.name字段的类型从&'a str更改为Stringas metioned by @Boiethios

#![allow(dead_code, unused_macros)]
use std::fmt::Debug;

trait MusicElement: MusicElementClone + Debug {
    fn duration(&self) -> f32;
}

trait MusicElementClone {
    fn clone_box(&self) -> Box<MusicElement>;
}

impl<T: 'static + MusicElement + Clone> MusicElementClone for T {
    fn clone_box(&self) -> Box<MusicElement> {
        Box::new(self.clone())
    }
}

impl Clone for Box<MusicElement> {
    fn clone(&self) -> Box<MusicElement> {
        self.clone_box()
    }
}

#[derive(Debug, Clone)]
struct Note {
    name: String,
    duration: f32,
}

impl MusicElement for Note {
    fn duration(&self) -> f32 {
        self.duration
    }
}

#[derive(Debug, Clone)]
struct Pause {
    duration: f32,
}

impl MusicElement for Pause {
    fn duration(&self) -> f32 {
        self.duration
    }
}

#[derive(Debug, Clone)]
struct Sequence {
    elements: Vec<Box<MusicElement>>,
}

impl MusicElement for Sequence {
    fn duration(&self) -> f32 {
        self.elements.iter().map(|e| e.duration()).sum()
    }
}

fn main() {
    let a4 = |dur| Box::new(Note { name: String::from("a4"), duration: dur });
    let seq = Sequence { elements: vec![a4(0.25), a4(0.25), a4(0.5)] };
    println!("{:?}", seq);
    let seq2 = seq.clone();
    println!("{:?}", seq2);
}

这个编译,所以它就足够了!

答案 1 :(得分:2)

我的objekt crate提供了jonny's answer的可重用实现。有了它,您可以使您的原始代码在最少的更改下工作。

之前:

trait MusicElement: Debug + Clone {
    fn duration(&self) -> f32;
}

后:

#[macro_use]
extern crate objekt;

trait MusicElement: Debug + objekt::Clone {
    fn duration(&self) -> f32;
}

clone_trait_object!(MusicElement);

// Everything else as you wrote it.