我有一个具有内部可变性的结构。
use std::cell::RefCell;
struct MutableInterior {
hide_me: i32,
vec: Vec<i32>,
}
struct Foo {
//although not used in this particular snippet,
//the motivating problem uses interior mutability
//via RefCell.
interior: RefCell<MutableInterior>,
}
impl Foo {
pub fn get_items(&self) -> &Vec<i32> {
&self.interior.borrow().vec
}
}
fn main() {
let f = Foo {
interior: RefCell::new(MutableInterior {
vec: Vec::new(),
hide_me: 2,
}),
};
let borrowed_f = &f;
let items = borrowed_f.get_items();
}
产生错误:
error[E0597]: borrowed value does not live long enough
--> src/main.rs:16:10
|
16 | &self.interior.borrow().vec
| ^^^^^^^^^^^^^^^^^^^^^^ temporary value does not live long enough
17 | }
| - temporary value only lives until here
|
note: borrowed value must be valid for the anonymous lifetime #1 defined on the method body at 15:5...
--> src/main.rs:15:5
|
15 | / pub fn get_items(&self) -> &Vec<i32> {
16 | | &self.interior.borrow().vec
17 | | }
| |_____^
问题是我无法在 Foo
上使用函数返回借来的 vec
, 因为借了vec
仅在 Ref
的生命周期内有效,但是 Ref
立即超出范围。
我认为 Ref
必须坚持 because :
RefCell<T>
uses Rust's lifetimes to implement 'dynamic borrowing', a process whereby one can claim temporary, exclusive, mutable access to the inner value. Borrows forRefCell<T>
s are tracked 'at runtime', unlike Rust's native reference types which are entirely tracked statically, at compile time. BecauseRefCell<T>
borrows are dynamic it is possible to attempt to borrow a value that is already mutably borrowed; when this happens it results in task panic.
现在我可以改为编写一个返回整个内部的函数:
pub fn get_mutable_interior(&self) -> std::cell::Ref<MutableInterior>;
然而,这可能会向 MutableInterior.hide_me
公开字段(在本例中为 Foo
),这些字段是真正私有(private)的实现细节。 .
理想情况下,我只想公开 vec
本身,可能有一个守卫来实现动态借用行为。这样调用者就不必了解 hide_me
了。 .
最佳答案
您可以使用 Ref::map
而不是创建一个全新的类型(自 Rust 1.8 起)。这与 Levans' existing answer 具有相同的结果:
use std::cell::Ref;
impl Foo {
pub fn get_items(&self) -> Ref<'_, Vec<i32>> {
Ref::map(self.interior.borrow(), |mi| &mi.vec)
}
}
您还可以使用 impl Trait
等新功能从 API 中隐藏 Ref
:
use std::cell::Ref;
use std::ops::Deref;
impl Foo {
pub fn get_items(&self) -> impl Deref<Target = Vec<i32>> + '_ {
Ref::map(self.interior.borrow(), |mi| &mi.vec)
}
}
关于rust - 如何在不破坏封装的情况下返回对 RefCell 中某些内容的引用?,我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/58838253/