[英]Implementing Deref on a struct that owns a boxed trait
I'd like to implement Deref
and DefrefMut
on a struct that owns a boxed trait, eg: 我想在拥有盒装特征的结构上实现
Deref
和DefrefMut
,例如:
use std::ops::{Deref, DerefMut};
trait Quack {
fn quack(&self);
}
struct QuackWrap {
value: Box<Quack>
}
impl Deref for QuackWrap {
type Target = Box<Quack>;
fn deref<'a>(&'a self) -> &'a Box<Quack> {
&self.value
}
}
impl DerefMut for QuackWrap {
fn deref_mut<'a>(&'a mut self) -> &'a mut Box<Quack> {
&mut self.value
}
}
This fails to compile with the following error: 无法编译时出现以下错误:
src/main.rs:14:5: 16:6 error: method `deref` has an incompatible type for trait: expected bound lifetime parameter 'a, found concrete lifetime [E0053]
src/main.rs:14 fn deref<'a>(&'a self) -> &'a Box<Quack> {
src/main.rs:15 &self.value
src/main.rs:16 }
src/main.rs:20:5: 22:6 error: method `deref_mut` has an incompatible type for trait: expected bound lifetime parameter 'a, found concrete lifetime [E0053]
src/main.rs:20 fn deref_mut<'a>(&'a mut self) -> &'a mut Box<Quack> {
src/main.rs:21 &mut self.value
src/main.rs:22 }
If I replace Box<Quack>
with Box<String>
(or a similar type), it works. 如果我用
Box<String>
(或类似的类型)替换Box<Quack>
,它可以工作。 The problem is that Quack
is a trait. 问题是
Quack
是一个特质。 But I'm not sure why that generated the error message it did. 但我不确定为什么会产生错误消息。 Any ideas?
有任何想法吗?
My question is similar to another SO question , but not quite the same. 我的问题类似于另一个SO问题 ,但不完全一样。 In that question, the struct has a type parameter with the trait as a constraint.
在该问题中,struct具有类型参数,其中trait为约束。 Whereas in my question, there is no type parameter.
而在我的问题中,没有类型参数。
I don't want to confuse the issues, but there's a good reason that I need Box<Quack>
in my application. 我不想混淆这些问题,但我有充分的理由在我的应用程序中需要
Box<Quack>
。 Ie I can't replace Quack
with a type parameter. 即我不能用类型参数替换
Quack
。 In case you care, the reason is discussed further in another SO question . 如果您关心,原因将在另一个SO问题中进一步讨论 。
When in doubt, add more lifetime annotations: 如有疑问,请添加更多生命周期注释:
use std::ops::{Deref, DerefMut};
trait Quack {
fn quack(&self);
}
struct QuackWrap<'b> {
value: Box<Quack + 'b>
}
impl<'b> Deref for QuackWrap<'b>{
type Target = Box<Quack + 'b>;
fn deref<'a>(&'a self) -> &'a Box<Quack + 'b> {
&self.value
}
}
impl<'b> DerefMut for QuackWrap<'b> {
fn deref_mut<'a>(&'a mut self) -> &'a mut Box<Quack + 'b> {
&mut self.value
}
}
Based on Brian's answer and Shepmaster's explanation, I updated my code as follow. 根据Brian的回答和Shepmaster的解释,我更新了我的代码如下。 I also simplified the
QuackWrap
struct. 我还简化了
QuackWrap
结构。 (That wasn't strictly necessary, but it's arguably better style than what I was doing before.) (这不是绝对必要的,但它的风格可能比以前更好。)
use std::ops::{Deref, DerefMut};
trait Quack {
fn quack(&self);
}
struct QuackWrap(Box<Quack>);
impl Deref for QuackWrap {
type Target = Box<Quack + 'static>;
fn deref<'a>(&'a self) -> &'a Box<Quack + 'static> {
let QuackWrap(ref v) = *self;
v
}
}
impl DerefMut for QuackWrap {
fn deref_mut<'a>(&'a mut self) -> &'a mut Box<Quack + 'static> {
let QuackWrap(ref mut v) = *self;
v
}
}
There may be a more concise way to destructure QuackWrap
in the deref
and deref_mut
implementations. 有可能是解构一个更简洁的方式
QuackWrap
在deref
和deref_mut
实现。 Some of those more obscure syntax rules elude me. 其中一些比较模糊的语法规则让我望而却步。 But for now this is fine.
但是现在这很好。
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