在创建存储DST(例如,原始切片)的结构时,我可以使用普通的 #[derive(Eq, PartialEq, Ord, PartialOrd)]
工具来获取我的类型上此特征的实现:
#[derive(PartialEq, Eq, PartialOrd, Ord)]
struct A([u8]);
但是,如果我手动实现它们,那么编译器会抱怨我的类型没有实现 Sized
:
struct A([u8]);
impl AsRef<[u8]> for A {
fn as_ref(&self) -> &[u8] {
&self.0
}
}
impl<S: AsRef<[u8]>> PartialEq<S> for A {
fn eq(&self, other: &S) -> bool {
self.0.eq(other.as_ref())
}
}
impl Eq for A { }
impl<S: AsRef<[u8]>> PartialOrd<S> for A {
fn partial_cmp(&self, other: &S) -> Option<Ordering> {
let slice: &[u8] = &self.0;
slice.partial_cmp(other.as_ref())
}
}
impl Ord for A {
fn cmp(&self, other: &Self) -> Ordering {
self.partial_cmp(&other).unwrap()
}
}
编译结果:
rustc 1.12.0 (3191fbae9 2016-09-23)
error[E0277]: the trait bound `[u8]: std::marker::Sized` is not satisfied
--> <anon>:20:6
|
20 | impl Eq for A { }
| ^^
|
= note: `[u8]` does not have a constant size known at compile-time
= note: required because it appears within the type `A`
= note: required because of the requirements on the impl of `std::cmp::PartialEq` for `A`
= note: required by `std::cmp::Eq`
error[E0277]: the trait bound `[u8]: std::marker::Sized` is not satisfied
--> <anon>:29:6
|
29 | impl Ord for A {
| ^^^
|
= note: `[u8]` does not have a constant size known at compile-time
= note: required because it appears within the type `A`
= note: required because of the requirements on the impl of `std::cmp::PartialOrd` for `A`
= note: required by `std::cmp::Ord`
如果我创建一个具有固定大小的类型的变体(例如通过将其转换为固定大小的数组),那么我可以手动实现这些特征没有问题 .
struct B([u8; 5]);
impl AsRef<[u8]> for B {
fn as_ref(&self) -> &[u8] {
&self.0
}
}
impl<S: AsRef<[u8]>> PartialEq<S> for B {
fn eq(&self, other: &S) -> bool {
self.0.eq(other.as_ref())
}
}
impl Eq for B { }
impl<S: AsRef<[u8]>> PartialOrd<S> for B {
fn partial_cmp(&self, other: &S) -> Option<Ordering> {
let slice: &[u8] = &self.0;
slice.partial_cmp(other.as_ref())
}
}
impl Ord for B {
fn cmp(&self, other: &Self) -> Ordering {
self.partial_cmp(&other).unwrap()
}
}
Here is a playground link showing the issue .
我的问题是 - 如何在我的自定义DST上实现 Ord
和 Eq
,以便我可以利用任何 AsRef<[u8]>
的事实 partial_cmp/eq
,但也可以使用它来强制执行 Ord
/ Eq
trait实现的边界,如在我的例子中我用 struct B
做什么?
1 回答
问题是
S: AsRef<[u8]>
默认情况下也将S
限制为Sized
类型 . 您需要使用?Sized
来选择退出 .From the Rust Book .
以下编译对我来说很好:
Demo