How do I return a reference to something inside a RefCell without breaking encapsulation?
I have a struct that has inner mutability.
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();
}
Produces the error:
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 | | }
| |_____^
The problem is that I can't have a function on Foo
that returns a borrowed vec
, because the borrowed vec
is only valid for the lifetime of the Ref
, but the Ref
goes out of scope immediately.
I think the Ref
must stick around 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.
Now I could instead write a function like this that returns the entire interior:
pub fn get_mutable_interior(&self) -> std::cell::Ref<MutableInterior>;
However this potentially exposes fields (MutableInterior.hide_me
in this example) that are really private implementation details to Foo
.
Ideally I just want to expose the vec
itself, potentially with a guard to implement the dynamic borrowing behavior. Then callers do not have to find out about hide_me
.
You can create a new struct similar to the Ref<'a,T>
guard returned by RefCell::borrow()
, in order to wrap this Ref
and avoid having it going out of scope, like this:
use std::cell::Ref;
struct FooGuard<'a> {
guard: Ref<'a, MutableInterior>,
}
then, you can implement the Deref
trait for it, so that it can be used as if it was a &Vec<i32>
:
use std::ops::Deref;
impl<'b> Deref for FooGuard<'b> {
type Target = Vec<i32>;
fn deref(&self) -> &Vec<i32> {
&self.guard.vec
}
}
after that, update your get_items()
method to return a FooGuard
instance:
impl Foo {
pub fn get_items(&self) -> FooGuard {
FooGuard {
guard: self.interior.borrow(),
}
}
}
and Deref
does the magic:
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();
let v: &Vec<i32> = &items;
}
Instead of creating a brand new type, you can use Ref::map
(since Rust 1.8). This has the same result as Levans' existing answer:
use std::cell::Ref;
impl Foo {
pub fn get_items(&self) -> Ref<'_, Vec<i32>> {
Ref::map(self.interior.borrow(), |mi| &mi.vec)
}
}
You can also use new features like impl Trait
to hide the Ref
from the API:
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)
}
}
You can wrap the Vec
in an Rc
.
use std::cell::RefCell;
use std::rc::Rc;
struct MutableInterior {
hide_me: i32,
vec: Rc<Vec<i32>>,
}
struct Foo {
interior: RefCell<MutableInterior>,
}
impl Foo {
pub fn get_items(&self) -> Rc<Vec<i32>> {
self.interior.borrow().vec.clone() // clones the Rc, not the Vec
}
}
fn main() {
let f = Foo {
interior: RefCell::new(MutableInterior {
vec: Rc::new(Vec::new()),
hide_me: 2,
}),
};
let borrowed_f = &f;
let items = borrowed_f.get_items();
}
When you need to mutate the Vec
, use Rc::make_mut
to obtain a mutable reference to the Vec
. If there are still other Rc
s referring to the Vec
, make_mut
will dissociate the Rc
from the other Rc
s, clone the Vec
and update itself to refer to that new Vec
, then give you a mutable reference to it. This ensures that the value in the other Rc
s doesn't suddenly change (because Rc
by itself doesn't provide interior mutability).