rust - 创建特征对象的副本

Question

如何创建特征对象的副本以进行本地计算？

问题是我想要一个函数，它接受对特征对象的可变引用，以便进行一些计算并根据这些计算对其进行修改，但在计算过程中，该函数必须临时修改特征对象并在本地使用其修改后的版本进行计算。

我尝试了以下方法(及其变体):

struct Data {
    fields: Vec<f32>
}

trait CustomCollection {
    fn get_mut(&mut self) -> &mut [f32];
}

impl CustomCollection for Data {
    fn get_mut(&mut self) -> &mut [f32] {
    return &mut self.fields;
    }
}

fn copy(s: &mut dyn CustomCollection) {
    let c: Box::<dyn CustomCollection> = Box::new(*s);
}

fn main() {
    let mut d = Data{fields: vec![1f32, 2.0, 3.0]};
    copy(&mut d);
}

但我收到错误:

error[E0277]: the size for values of type `dyn CustomCollection` cannot be known at compilation time
  --> src/test.rs:16:51
   |
16 |     let c: Box::<dyn CustomCollection> = Box::new(*s);
   |                                          -------- ^^ doesn't have a size known at compile-time
   |                                          |
   |                                          required by a bound introduced by this call
   |
   = help: the trait `Sized` is not implemented for `dyn CustomCollection`
note: required by a bound in `Box::<T>::new`
  --> /rustc/fc594f15669680fa70d255faec3ca3fb507c3405/library/alloc/src/boxed.rs:204:6
   |
   = note: required by this bound in `Box::<T>::new`

error: aborting due to previous error

For more information about this error, try `rustc --explain E0277`.

Answer 1

您可以使用clone来扩展您想要构建特征对象的特征。或copy返回 Box<dyn CustomCollection> 的方法。例如这样:

#[derive(Clone)]
struct Data {
    fields: Vec<f32>
}

trait CustomCollection {
    fn get_mut(&mut self) -> &mut [f32];
    
    fn clone_box(&self) -> Box<dyn CustomCollection>;
}

impl CustomCollection for Data {
    fn get_mut(&mut self) -> &mut [f32] {
        &mut self.fields
    }
    
    fn clone_box(&self) -> Box<dyn CustomCollection> {
        Box::new(<Self as Clone>::clone(self))
    }
}

fn copy(s: &mut dyn CustomCollection) -> Box<dyn CustomCollection> {
    s.clone_box()
}

fn main() {
    let mut d = Data{fields: vec![1f32, 2.0, 3.0]};
    copy(&mut d);
}

Playground.

---

@prog-fh 非常友善和尖锐 this answer out，其中包含我在示例中尝试完成的任务的更灵活的实现。还有 dyn-clone @dtolnay 的 crate 随时可供您使用，因此您不必自己实现克隆特征对象。