如何将盒装特征转换为特征引用? [英] How to convert a boxed trait into a trait reference?

问题描述

我有以下代码试图从装箱的特征中获取对特征对象的引用:

I have the following code that tries to take a reference to a trait object from a boxed trait:

trait T {}

struct S {}

impl T for S {}

fn main() {
    let struct_box: Box<S> = Box::new(S {});
    let struct_ref: &S = &struct_box;

    let trait_box: Box<T> = Box::new(S {});
    let trait_ref: &T = &trait_box;
}

编译器返回以下错误:

error[E0277]: the trait bound `std::boxed::Box<T>: T` is not satisfied
  --> src/main.rs:12:25
   |
12 |     let trait_ref: &T = &trait_box;
   |                         ^^^^^^^^^^ the trait `T` is not implemented for `std::boxed::Box<T>`
   |
   = note: required for the cast to the object type `T`

如何正确地从Box借用&T?

推荐答案

Box 实现 AsRef trait，它提供了方法as_ref()，所以你可以以这种方式将其转换为参考:

Box<T> implements the AsRef<T> trait, which provides the method as_ref(), so you can turn it into a reference that way:

let trait_ref: &T = trait_box.as_ref();

通常，deref 强制 意味着你通常不需要明确地写出来.如果您将 Box 类型的值传递给采用 &T 的函数，编译器将为您插入转换.如果您想调用 T 上采用 &self 的方法之一，编译器将为您插入转换.然而，解引用强制不适用于特征，因此这不会发生在特征对象上.

Normally, deref coercions mean that you don't usually need to write this out explicitly. If you pass a value of type Box<T> to a function that takes &T, the compiler will insert the conversion for you. If you want to call one of the methods on T that takes &self, the compiler will insert the conversion for you. However, deref coercions don't apply to traits, so this won't happen for trait objects.

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