[英]Returning a reference to a Box<dyn Trait>
Started learning rust this weekend, and I'm trying to write a little private helper method on my class to look up some internal values, but it's proving quite complicated.这个周末开始学习 rust,我试图在我的 class 上写一个小的私有辅助方法来查找一些内部值,但事实证明它相当复杂。 I have a few structs implementing a trait:我有一些实现特征的结构:
trait Animal {}
struct Dog {}
impl Dog for Animal{}
struct Cat{}
impl Cat for Animal{}
along with a container which holds multiple variants of animal:以及一个装有多种动物变体的容器:
struct Person {
my_dog: Dog,
my_cat: Cat
}
What I want to do is write a helper which looks up the appropriate Animal, like this:我想要做的是编写一个帮助程序来查找合适的动物,如下所示:
impl Person {
fn look_up_animal(&self, animal_name: &str) -> Box<&dyn Animal> {
match animal_name {
"dog" => Box::from(&self.my_dog),
"cat" => Box::from(&self.my_cat)
}
}
}
Unfortunately, I can't quite figure out how to do the types for this.不幸的是,我不太清楚如何为此做类型。 The version above using references and Box
gives an error like "expected struct Box<&dyn Animal>
but got struct Box<&Dog>
, so I'm not really sure how to convert a regular value into a dyn
value.上面使用引用和Box
的版本给出了类似“expected struct Box<&dyn Animal>
but got struct Box<&Dog>
的错误,所以我不太确定如何将常规值转换为dyn
值。
The fully explicit version of what you want would be你想要的完全明确的版本是
impl Person {
fn look_up_animal(&self, animal_name: &str) -> Option<Box<&dyn Animal>> {
match animal_name {
"dog" => Some(Box::from(&self.my_dog as &dyn Animal)),
"cat" => Some(Box::from(&self.my_cat as &dyn Animal)),
_ => None,
}
}
}
Rust is able to coerce one layer of references to trait objects, but two layers of references (both Box
and &
) trips it up. Rust 能够强制对特征对象的一层引用,但是两层引用( Box
和&
)将其绊倒。 (Having a reference inside a Box
also triggers a clippy
warning.) That suggests another solution. (在Box
中有一个引用也会触发一个clippy
警告。)这提出了另一种解决方案。
impl Person {
fn look_up_animal(&self, animal_name: &str) -> Option<&dyn Animal> {
match animal_name {
"dog" => Some(&self.my_dog as &dyn Animal),
"cat" => Some(&self.my_cat as &dyn Animal),
_ => None,
}
}
}
This time, the explicit cast as &dyn Animal
is unnecessary since there's just a single layer of references.这一次,显式转换as &dyn Animal
是不必要的,因为只有一层引用。
impl Person {
fn look_up_animal(&self, animal_name: &str) -> Option<&dyn Animal> {
match animal_name {
"dog" => Some(&self.my_dog),
"cat" => Some(&self.my_cat),
_ => None,
}
}
}
Note: I added in the Option
so that there's something to return when animal_name
isn't "dog"
or "cat"
.注意:我在Option
中添加了一些东西,以便在animal_name
不是"dog"
或"cat"
时返回一些东西。 You could also panic or default to something else that implements Animal
.您也可能会恐慌或默认使用其他实现Animal
的东西。
Started learning rust this weekend, and I'm trying to write a little private helper method on my class to look up some internal values, but it's proving quite complicated.这个周末开始学习 rust,我正在尝试在我的 class 上编写一个小的私有帮助方法来查找一些内部值,但事实证明它相当复杂。 I have a few structs implementing a trait:我有一些实现特征的结构:
trait Animal {}
struct Dog {}
impl Dog for Animal{}
struct Cat{}
impl Cat for Animal{}
along with a container which holds multiple variants of animal:以及一个装有多种动物变体的容器:
struct Person {
my_dog: Dog,
my_cat: Cat
}
What I want to do is write a helper which looks up the appropriate Animal, like this:我想要做的是编写一个查找适当动物的助手,如下所示:
impl Person {
fn look_up_animal(&self, animal_name: &str) -> Box<&dyn Animal> {
match animal_name {
"dog" => Box::from(&self.my_dog),
"cat" => Box::from(&self.my_cat)
}
}
}
Unfortunately, I can't quite figure out how to do the types for this.不幸的是,我不太清楚如何为此做类型。 The version above using references and Box
gives an error like "expected struct Box<&dyn Animal>
but got struct Box<&Dog>
, so I'm not really sure how to convert a regular value into a dyn
value.上面使用引用和Box
的版本给出了一个错误,如 "expected struct Box<&dyn Animal>
but got struct Box<&Dog>
,所以我不确定如何将常规值转换为dyn
值。
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