不能克隆 Vec<Box<Trait>> 因为 Trait 不能变成 object

Question

我正在尝试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推导 Clone 是不够的，因为编译器在编译时不知道所有实现特征的结构都具有 Clone.

好的，所以我随后尝试使用 Clone 作为超级特征，但这只会导致标题中的错误。我不知所措。

这是最小的工作实现（或者不工作，因为我不能克隆）

#![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`

下面是一个 link to the playground 的简单代码运行。

我也试过将 Sequence 中的 elements 向量变成 Vec<Box<MusicElement + Clone>>，但那也没用。

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：如何使代码可克隆？

Answer 1

解决方案在于结合目前评论中的建议 - tells you that you must create a blanket trait implementation for all compatible ('static + MusicElement + Clone) types. The only subsequent step necessary for your implementation is changing the Note.name field's type from &'a str to String, 中的答案：

#![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);
}

编译通过，所以应该足够了！

Answer 2

我的objekt crate provides a reusable implementation of 。有了它，您只需进行最少的更改即可使您的原始代码正常工作。

之前：

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

之后：

#[macro_use]
extern crate objekt;

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

clone_trait_object!(MusicElement);

// Everything else as you wrote it.

不能克隆 Vec<Box<Trait>> 因为 Trait 不能变成 object

Can't clone Vec<Box<Trait>> because Trait cannot be made into an object

clone

rust

trait-objects

之前：

之后：