[英]Implementing Borrow trait for a type with a lifetime
I'm trying to use.我正在尝试使用。 strongly typed wrapper for a "keys" in my program, so that I don't mistake arbitrary strings for a Key.我的程序中的“键”的强类型包装器,这样我就不会将任意字符串误认为是键。 I have:我有:
#[derive(Debug, Clone, PartialEq, Eq, Hash)]
struct Key(String);
I have a HashMap<Key, _>
, and I want to lookup values with a reference to a key type (ie not having to own the string).我有一个HashMap<Key, _>
,我想通过引用键类型来查找值(即不必拥有字符串)。 It seems like what I need to do is:看来我需要做的是:
#[derive(Debug, Clone, PartialEq, Eq, Hash)]
struct KeyRef<'a>(&'a String);
(in reality I'd want KeyRef<'a>(&'a str)
, but using String
makes for a clearer example) (实际上我想要KeyRef<'a>(&'a str)
,但使用String
可以提供更清晰的示例)
Borrow<KeyRef<'_>> for Key
Borrow<KeyRef<'_>> for Key
I've tried my best, here's a playground link我已经尽力了,这是一个游乐场链接
My most explicit attempt (annotating all lifetimes) is:我最明确的尝试(注释所有生命周期)是:
impl<'a> Borrow<KeyRef<'a>> for Key {
fn borrow<'b>(&'b self) -> &'b KeyRef<'a> where 'b: 'a {
let string_ref : &'a String = &self.0;
let key_ref : &'a KeyRef<'a> = &KeyRef(string_ref);
key_ref
}
}
Which gives me the error: "lifetime parameters or bounds on method borrow
do not match the trait declaration".这给了我错误:“方法borrow
的生命周期参数或边界与特征声明不匹配”。
Intuitively it feels like this should be possible:直觉上感觉这应该是可能的:
'a
, so any value of KeyRef cannot outlive 'a
. KeyRef 持有生命周期'a
的引用,因此 KeyRef 的任何值都不能超过'a
。fn borrow<'b>(&'b self)
, 'b
can't be greater than 'a
due to the above在fn borrow<'b>(&'b self)
中,由于上述原因, 'b
不能大于'a
But the compiler doesn't seem to like my explicit attempt to demonstrate that (with where 'b: 'a
), and leaving it off I get "cannot infer an appropriate lifetime for borrow expression due to conflicting requirements"但是编译器似乎不喜欢我明确尝试证明这一点(使用where 'b: 'a
),并且将其关闭我得到“由于要求冲突,无法推断借用表达式的适当生命周期”
As far as I understand your situation, you are needlessly overcomplicating things.据我了解你的情况,你不必要地把事情复杂化了。 A straightforward implementation:一个简单的实现:
use std::collections::HashMap;
use std::borrow::Borrow;
#[derive(Debug, Clone, PartialEq, Eq, Hash)]
struct Key(String);
impl Borrow<str> for Key {
fn borrow(&self) -> &str {
&self.0
}
}
impl Borrow<String> for Key {
fn borrow(&self) -> &String {
&self.0
}
}
fn main() {
let mut map = HashMap::new();
map.insert(Key("one".to_owned()), 1);
// Because Key is Borrow<String>
println!("{:?}", map.get("one".to_owned()));
// Because Key is Borrow<str>
println!("{:?}", map.get("one"));
}
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