Expand description
Shareable mutable containers.
Rust memory safety is based on this rule: Given an object T
, it is only possible to
have one of the following:
- Having several immutable references (
&T
) to the object (also known as aliasing). - Having one mutable reference (
&mut T
) to the object (also known as mutability).
This is enforced by the Rust compiler. However, there are situations where this rule is not flexible enough. Sometimes it is required to have multiple references to an object and yet mutate it.
Shareable mutable containers exist to permit mutability in a controlled manner, even in the
presence of aliasing. Cell<T>
, RefCell<T>
, and OnceCell<T>
allow doing this in
a single-threaded way—they do not implement Sync
. (If you need to do aliasing and
mutation among multiple threads, Mutex<T>
, RwLock<T>
, OnceLock<T>
or atomic
types are the correct data structures to do so).
Values of the Cell<T>
, RefCell<T>
, and OnceCell<T>
types may be mutated through shared
references (i.e. the common &T
type), whereas most Rust types can only be mutated through
unique (&mut T
) references. We say these cell types provide ‘interior mutability’
(mutable via &T
), in contrast with typical Rust types that exhibit ‘inherited mutability’
(mutable only via &mut T
).
Cell types come in three flavors: Cell<T>
, RefCell<T>
, and OnceCell<T>
. Each provides
a different way of providing safe interior mutability.
§Cell<T>
Cell<T>
implements interior mutability by moving values in and out of the cell. That is, an
&mut T
to the inner value can never be obtained, and the value itself cannot be directly
obtained without replacing it with something else. Both of these rules ensure that there is
never more than one reference pointing to the inner value. This type provides the following
methods:
- For types that implement
Copy
, theget
method retrieves the current interior value by duplicating it. - For types that implement
Default
, thetake
method replaces the current interior value withDefault::default()
and returns the replaced value. - All types have:
replace
: replaces the current interior value and returns the replaced value.into_inner
: this method consumes theCell<T>
and returns the interior value.set
: this method replaces the interior value, dropping the replaced value.
Cell<T>
is typically used for more simple types where copying or moving values isn’t too
resource intensive (e.g. numbers), and should usually be preferred over other cell types when
possible. For larger and non-copy types, RefCell
provides some advantages.
§RefCell<T>
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 for RefCell<T>
s are
tracked at runtime, unlike Rust’s native reference types which are entirely tracked
statically, at compile time.
An immutable reference to a RefCell
’s inner value (&T
) can be obtained with
borrow
, and a mutable borrow (&mut T
) can be obtained with
borrow_mut
. When these functions are called, they first verify that
Rust’s borrow rules will be satisfied: any number of immutable borrows are allowed or a
single mutable borrow is allowed, but never both. If a borrow is attempted that would violate
these rules, the thread will panic.
The corresponding Sync
version of RefCell<T>
is RwLock<T>
.
§OnceCell<T>
OnceCell<T>
is somewhat of a hybrid of Cell
and RefCell
that works for values that
typically only need to be set once. This means that a reference &T
can be obtained without
moving or copying the inner value (unlike Cell
) but also without runtime checks (unlike
RefCell
). However, its value can also not be updated once set unless you have a mutable
reference to the OnceCell
.
OnceCell
provides the following methods:
get
: obtain a reference to the inner valueset
: set the inner value if it is unset (returns aResult
)get_or_init
: return the inner value, initializing it if neededget_mut
: provide a mutable reference to the inner value, only available if you have a mutable reference to the cell itself.
The corresponding Sync
version of OnceCell<T>
is OnceLock<T>
.
§When to choose interior mutability
The more common inherited mutability, where one must have unique access to mutate a value, is one of the key language elements that enables Rust to reason strongly about pointer aliasing, statically preventing crash bugs. Because of that, inherited mutability is preferred, and interior mutability is something of a last resort. Since cell types enable mutation where it would otherwise be disallowed though, there are occasions when interior mutability might be appropriate, or even must be used, e.g.
- Introducing mutability ‘inside’ of something immutable
- Implementation details of logically-immutable methods.
- Mutating implementations of
Clone
.
§Introducing mutability ‘inside’ of something immutable
Many shared smart pointer types, including Rc<T>
and Arc<T>
, provide containers that can
be cloned and shared between multiple parties. Because the contained values may be
multiply-aliased, they can only be borrowed with &
, not &mut
. Without cells it would be
impossible to mutate data inside of these smart pointers at all.
It’s very common then to put a RefCell<T>
inside shared pointer types to reintroduce
mutability:
use std::cell::{RefCell, RefMut};
use std::collections::HashMap;
use std::rc::Rc;
fn main() {
let shared_map: Rc<RefCell<_>> = Rc::new(RefCell::new(HashMap::new()));
// Create a new block to limit the scope of the dynamic borrow
{
let mut map: RefMut<'_, _> = shared_map.borrow_mut();
map.insert("africa", 92388);
map.insert("kyoto", 11837);
map.insert("piccadilly", 11826);
map.insert("marbles", 38);
}
// Note that if we had not let the previous borrow of the cache fall out
// of scope then the subsequent borrow would cause a dynamic thread panic.
// This is the major hazard of using `RefCell`.
let total: i32 = shared_map.borrow().values().sum();
println!("{total}");
}
RunNote that this example uses Rc<T>
and not Arc<T>
. RefCell<T>
s are for single-threaded
scenarios. Consider using RwLock<T>
or Mutex<T>
if you need shared mutability in a
multi-threaded situation.
§Implementation details of logically-immutable methods
Occasionally it may be desirable not to expose in an API that there is mutation happening
“under the hood”. This may be because logically the operation is immutable, but e.g., caching
forces the implementation to perform mutation; or because you must employ mutation to implement
a trait method that was originally defined to take &self
.
use std::cell::OnceCell;
struct Graph {
edges: Vec<(i32, i32)>,
span_tree_cache: OnceCell<Vec<(i32, i32)>>
}
impl Graph {
fn minimum_spanning_tree(&self) -> Vec<(i32, i32)> {
self.span_tree_cache
.get_or_init(|| self.calc_span_tree())
.clone()
}
fn calc_span_tree(&self) -> Vec<(i32, i32)> {
// Expensive computation goes here
vec![]
}
}
Run§Mutating implementations of Clone
This is simply a special - but common - case of the previous: hiding mutability for operations
that appear to be immutable. The clone
method is expected to not change the
source value, and is declared to take &self
, not &mut self
. Therefore, any mutation that
happens in the clone
method must use cell types. For example, Rc<T>
maintains its
reference counts within a Cell<T>
.
use std::cell::Cell;
use std::ptr::NonNull;
use std::process::abort;
use std::marker::PhantomData;
struct Rc<T: ?Sized> {
ptr: NonNull<RcBox<T>>,
phantom: PhantomData<RcBox<T>>,
}
struct RcBox<T: ?Sized> {
strong: Cell<usize>,
refcount: Cell<usize>,
value: T,
}
impl<T: ?Sized> Clone for Rc<T> {
fn clone(&self) -> Rc<T> {
self.inc_strong();
Rc {
ptr: self.ptr,
phantom: PhantomData,
}
}
}
trait RcBoxPtr<T: ?Sized> {
fn inner(&self) -> &RcBox<T>;
fn strong(&self) -> usize {
self.inner().strong.get()
}
fn inc_strong(&self) {
self.inner()
.strong
.set(self.strong()
.checked_add(1)
.unwrap_or_else(|| abort() ));
}
}
impl<T: ?Sized> RcBoxPtr<T> for Rc<T> {
fn inner(&self) -> &RcBox<T> {
unsafe {
self.ptr.as_ref()
}
}
}
RunStructs§
- An error returned by
RefCell::try_borrow
. - An error returned by
RefCell::try_borrow_mut
. - A mutable memory location.
- A cell which can be written to only once.
- Wraps a borrowed reference to a value in a
RefCell
box. A wrapper type for an immutably borrowed value from aRefCell<T>
. - A mutable memory location with dynamically checked borrow rules
- A wrapper type for a mutably borrowed value from a
RefCell<T>
. - The core primitive for interior mutability in Rust.
- LazyCellExperimentalA value which is initialized on the first access.
- SyncUnsafeCellExperimental
UnsafeCell
, butSync
.