[英]How do I implement FromStr with a concrete lifetime?
我想为具有生命周期参数的结构实现FromStr
:
use std::str::FromStr;
struct Foo<'a> {
bar: &'a str,
}
impl<'a> FromStr for Foo<'a> {
type Err = ();
fn from_str(s: &str) -> Result<Foo<'a>, ()> {
Ok(Foo { bar: s })
}
}
pub fn main() {
let foo: Foo = "foobar".parse().unwrap();
}
但是,编译器抱怨:
error[E0495]: cannot infer an appropriate lifetime due to conflicting requirements
--> src/main.rs:11:12
|
11 | Ok(Foo { bar: s })
| ^^^
|
help: consider using an explicit lifetime parameter as shown: fn from_str(s: &'a str) -> Result<Foo<'a>, ()>
--> src/main.rs:9:5
|
9 | fn from_str(s: &str) -> Result<Foo<'a>, ()> {
| ^
将 impl 更改为
impl<'a> FromStr for Foo<'a> {
type Err = ();
fn from_str(s: &'a str) -> Result<Foo<'a>, ()> {
Ok(Foo { bar: s })
}
}
给出这个错误
error[E0308]: method not compatible with trait
--> src/main.rs:9:5
|
9 | fn from_str(s: &'a str) -> Result<Foo<'a>, ()> {
| ^ lifetime mismatch
|
= note: expected type `fn(&str) -> std::result::Result<Foo<'a>, ()>`
= note: found type `fn(&'a str) -> std::result::Result<Foo<'a>, ()>`
note: the anonymous lifetime #1 defined on the block at 9:51...
--> src/main.rs:9:52
|
9 | fn from_str(s: &'a str) -> Result<Foo<'a>, ()> {
| ^
note: ...does not necessarily outlive the lifetime 'a as defined on the block at 9:51
--> src/main.rs:9:52
|
9 | fn from_str(s: &'a str) -> Result<Foo<'a>, ()> {
| ^
help: consider using an explicit lifetime parameter as shown: fn from_str(s: &'a str) -> Result<Foo<'a>, ()>
--> src/main.rs:9:5
|
9 | fn from_str(s: &'a str) -> Result<Foo<'a>, ()> {
| ^
我不相信你可以在这种情况下实现FromStr
。
fn from_str(s: &str) -> Result<Self, <Self as FromStr>::Err>;
特征定义中没有任何内容将输入的生命周期与输出的生命周期联系起来。
不是直接回答,但我只是建议制作一个接受参考的构造函数:
struct Foo<'a> {
bar: &'a str
}
impl<'a> Foo<'a> {
fn new(s: &str) -> Foo {
Foo { bar: s }
}
}
pub fn main() {
let foo = Foo::new("foobar");
}
这样做的附带好处是没有任何故障模式 - 无需unwrap
。
你也可以只实现From
:
struct Foo<'a> {
bar: &'a str,
}
impl<'a> From<&'a str> for Foo<'a> {
fn from(s: &'a str) -> Foo<'a> {
Foo { bar: s }
}
}
pub fn main() {
let foo: Foo = "foobar".into();
}
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