如何克隆存储盒装特征对象的结构？

Question

我编写了一个程序,它具有特征Animal和Dog实现特征的结构.它还有一个AnimalHouse存储动物作为特征对象的结构Box<Animal>.

trait Animal {
    fn speak(&self);
}

struct Dog {
    name: String,
}

impl Dog {
    fn new(name: &str) -> Dog {
        return Dog {
            name: name.to_string(),
        };
    }
}

impl Animal for Dog {
    fn speak(&self) {
        println!{"{}: ruff, ruff!", self.name};
    }
}

struct AnimalHouse {
    animal: Box<Animal>,
}

fn main() {
    let house = AnimalHouse {
        animal: Box::new(Dog::new("Bobby")),
    };
    house.animal.speak();
}

它返回"Bobby:ruff,ruff!" 正如所料,但如果我尝试克隆house编译器返回错误:

fn main() {
    let house = AnimalHouse {
        animal: Box::new(Dog::new("Bobby")),
    };
    let house2 = house.clone();
    house2.animal.speak();
}

error[E0599]: no method named `clone` found for type `AnimalHouse` in the current scope
  --> src/main.rs:31:24
   |
23 | struct AnimalHouse {
   | ------------------ method `clone` not found for this
...
31 |     let house2 = house.clone();
   |                        ^^^^^
   |
   = help: items from traits can only be used if the trait is implemented and in scope
   = note: the following trait defines an item `clone`, perhaps you need to implement it:
           candidate #1: `std::clone::Clone`

我#[derive(Clone)]之前试图添加struct AnimalHouse并得到另一个错误:

error[E0277]: the trait bound `Animal: std::clone::Clone` is not satisfied
  --> src/main.rs:25:5
   |
25 |     animal: Box<Animal>,
   |     ^^^^^^^^^^^^^^^^^^^ the trait `std::clone::Clone` is not implemented for `Animal`
   |
   = note: required because of the requirements on the impl of `std::clone::Clone` for `std::boxed::Box<Animal>`
   = note: required by `std::clone::Clone::clone`

如何使结构可AnimalHouse克隆？一般来说,积极使用特质对象是惯用的吗？

Answer 1

有一些问题.首先,没有什么要求Animal也可以实现Clone.您可以通过更改特征定义来解决此问题:

trait Animal: Clone {
    /* ... */
}

这将导致Animal不再是对象安全,这意味着Box<Animal>它将变得无效,因此不是很好.

您可以做的是插入一个额外的步骤.要惠特(加上@ChrisMorgan的评论).

trait Animal: AnimalClone {
    fn speak(&self);
}

// Splitting AnimalClone into its own trait allows us to provide a blanket
// implementation for all compatible types, without having to implement the
// rest of Animal.  In this case, we implement it for all types that have
// 'static lifetime (*i.e.* they don't contain non-'static pointers), and
// implement both Animal and Clone.  Don't ask me how the compiler resolves
// implementing AnimalClone for Animal when Animal requires AnimalClone; I
// have *no* idea why this works.
trait AnimalClone {
    fn clone_box(&self) -> Box<Animal>;
}

impl<T> AnimalClone for T
where
    T: 'static + Animal + Clone,
{
    fn clone_box(&self) -> Box<Animal> {
        Box::new(self.clone())
    }
}

// We can now implement Clone manually by forwarding to clone_box.
impl Clone for Box<Animal> {
    fn clone(&self) -> Box<Animal> {
        self.clone_box()
    }
}

#[derive(Clone)]
struct Dog {
    name: String,
}

impl Dog {
    fn new(name: &str) -> Dog {
        Dog {
            name: name.to_string(),
        }
    }
}

impl Animal for Dog {
    fn speak(&self) {
        println!("{}: ruff, ruff!", self.name);
    }
}

#[derive(Clone)]
struct AnimalHouse {
    animal: Box<Animal>,
}

fn main() {
    let house = AnimalHouse {
        animal: Box::new(Dog::new("Bobby")),
    };
    let house2 = house.clone();
    house2.animal.speak();
}

通过介绍clone_box,我们可以解决尝试克隆特征对象的问题.

Answer 2

我的objekt箱子实现的可重复使用的版本DK。的回答。有了它，您只需很少的更改就可以使原始代码正常工作。

• 一行导入板条箱。
• 要添加的一行objekt::Clone作为的特征Animal，要求每个动物实现都是可克隆的。
• 一行为生成标准库的Clone实现Box<Animal>。

#[macro_use] extern crate objekt;

trait Animal: objekt::Clone {
    fn speak(&self);
}

clone_trait_object!(Animal);

#[derive(Clone)]
struct Dog {
    name: String,
}

impl Dog {
    fn new(name: &str) -> Dog {
        Dog { name: name.to_owned() }
    }
}

impl Animal for Dog {
    fn speak(&self) {
        println!{"{}: ruff, ruff!", self.name};
    }
}

#[derive(Clone)]
struct AnimalHouse {
    animal: Box<Animal>,
}

fn main() {
    let house = AnimalHouse {
        animal: Box::new(Dog::new("Bobby")),
    };
    let house2 = house.clone();
    house2.animal.speak();
}

Answer 3

在以前的答案正确地回答了关于存储盒装特质对象的问题。

关于标题，而不是使用特质对象的惯用方式，可以采用另一种解决方案，即使用Rc智能指针代替a Box：这避免了解决对象安全性的变通方法：

#[derive(Clone)]
struct AnimalHouse {
    animal: Rc<Animal>,
}

fn main() {
    let house = AnimalHouse { animal: Rc::new(Dog::new("Bobby")) };
    let house2 = house.clone();
    house2.animal.speak();
}

注意：Rc<T>仅用于单线程方案；还有Arc<T>。