参数类型可能不够长? [英] Parameter type may not live long enough?
问题描述
以下代码段给了我一个错误:
The following code segment gives me an error:
use std::rc::Rc;
// Definition of Cat, Dog, and Animal (see the last code block)
// ...
type RcAnimal = Rc<Box<Animal>>;
fn new_rc_animal<T>(animal: T) -> RcAnimal
where
T: Animal /* + 'static */ // works fine if uncommented
{
Rc::new(Box::new(animal) as Box<Animal>)
}
fn main() {
let dog: RcAnimal = new_rc_animal(Dog);
let cat: RcAnimal = new_rc_animal(Cat);
let mut v: Vec<RcAnimal> = Vec::new();
v.push(cat.clone());
v.push(dog.clone());
for animal in v.iter() {
println!("{}", (**animal).make_sound());
}
}
error[E0310]: the parameter type `T` may not live long enough
--> src/main.rs:8:13
|
4 | fn new_rc_animal<T>(animal: T) -> RcAnimal
| - help: consider adding an explicit lifetime bound `T: 'static`...
...
8 | Rc::new(Box::new(animal) as Box<Animal>)
| ^^^^^^^^^^^^^^^^
|
note: ...so that the type `T` will meet its required lifetime bounds
--> src/main.rs:8:13
|
8 | Rc::new(Box::new(animal) as Box<Animal>)
| ^^^^^^^^^^^^^^^^
但这编译得很好:
use std::rc::Rc;
// Definition of Cat, Dog, and Animal (see the last code block)
// ...
fn new_rc_animal<T>(animal: T) -> Rc<Box<T>>
where
T: Animal,
{
Rc::new(Box::new(animal))
}
fn main() {
let dog = new_rc_animal(Dog);
let cat = new_rc_animal(Cat);
}
错误的原因是什么?唯一真正的区别似乎是运算符 as
的使用.类型怎么可能活得不够久?(游乐场)
What is the cause of the error? The only real difference seems to be the use of operator as
. How can a type not live long enough? (playground)
// Definition of Cat, Dog, and Animal
trait Animal {
fn make_sound(&self) -> String;
}
struct Cat;
impl Animal for Cat {
fn make_sound(&self) -> String {
"meow".to_string()
}
}
struct Dog;
impl Animal for Dog {
fn make_sound(&self) -> String {
"woof".to_string()
}
}
<小时>
附录
为了澄清,我有两个问题:
Just to clarify, I had two questions:
- 为什么这不起作用?...在接受的答案中得到了解决.
- 相对于值或引用,类型如何是短暂的?......这在评论中得到了解决.剧透:类型只是存在,因为它是一个编译时概念.
- Why doesn't this work? ... which is addressed in the accepted answer.
- How can a type, as opposed to a value or reference, be shortlived? ... which was addressed in the comments. Spoiler: a type simply exists since it's a compile-time concept.
推荐答案
实际上有很多类型可以活得不够久":所有类型都有生命周期参数.
There are actually plenty of types that can "not live long enough": all the ones that have a lifetime parameter.
如果我要介绍这种类型:
If I were to introduce this type:
struct ShortLivedBee<'a>;
impl<'a> Animal for ShortLivedBee<'a> {}
ShortLivedBee
对任何生命周期都无效,但仅对 'a
有效.
ShortLivedBee
is not valid for any lifetime, but only the ones that are valid for 'a
as well.
所以在你的情况下
where T: Animal + 'static
我可以输入您的函数的唯一 ShortLivedBee
是 ShortLivedBee<'static>
.
the only ShortLivedBee
I could feed into your function is ShortLivedBee<'static>
.
造成这种情况的原因是,在创建 Box
时,您正在创建一个 trait 对象,该对象需要具有关联的生命周期.如果不指定,则默认为'static
.所以你定义的类型实际上是:
What causes this is that when creating a Box<Animal>
, you are creating a trait object, which need to have an associated lifetime. If you do not specify it, it defaults to 'static
. So the type you defined is actually:
type RcAnimal = Rc<Box<Animal + 'static>>;
这就是为什么您的函数要求将 'static
绑定添加到 T
:不可能存储 ShortLivedBee<'a>
在 Box
除非 'a = 'static
.
That's why your function require that a 'static
bound is added to T
: It is not possible to store a ShortLivedBee<'a>
in a Box<Animal + 'static>
unless 'a = 'static
.
另一种方法是为您的 RcAnimal
添加生命周期注释,如下所示:
An other approach would be to add a lifetime annotation to your RcAnimal
, like this:
type RcAnimal<'a> = Rc<Box<Animal + 'a>>;
并更改您的函数以明确生命周期关系:
And change your function to explicit the lifetime relations:
fn new_rc_animal<'a, T>(animal: T) -> RcAnimal<'a>
where T: Animal + 'a {
Rc::new(Box::new(animal) as Box<Animal>)
}
这篇关于参数类型可能不够长?的文章就介绍到这了,希望我们推荐的答案对大家有所帮助,也希望大家多多支持IT屋!