无法克隆Vec <Box <Trait >>因为Trait无法成为对象

Question

我正在尝试克隆盒装特征的向量.自然地简单地派生Clone所有实现我的特征的结构是不够的,因为编译器在编译时不知道实现特征的所有结构都有Clone.

好的,所以我接着尝试使用Clonesupertrait,但这只会导致标题中的错误.我对解决方案感到茫然.

这是最小工作实现(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`

这里是游乐场的链接,可以轻松运行代码.

我也尝试elements在Sequencea中制作矢量Vec<Box<MusicElement + Clone>>,但这也不起作用.

我无法在网上找到任何有用的解决方案,所以这是我的问题:如何使代码可以克隆？

Answer 1

解决方案在于结合迄今为止评论中的建议 - @Lukas Kalbertodt的评论中的答案告诉您必须为所有compatible('static + MusicElement + Clone)类型创建一个全面的特征实现.需要为您执行的唯一后续步骤正在改变Note.name从字段的类型&'a str来String,通过@Boiethios的文件档案化管理:

#![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);
}

编译,所以它应该足够了!