RefCell<T> and the Interior Mutability Pattern
Interior mutability is a design pattern in Rust that allows you to mutate
data even when there are immutable references to that data; normally, this
action is disallowed by the borrowing rules. To mutate data, the pattern uses
unsafe code inside a data structure to bend Rust’s usual rules that govern
mutation and borrowing. We haven’t yet covered unsafe code; we will in Chapter
19. We can use types that use the interior mutability pattern when we can
ensure that the borrowing rules will be followed at runtime, even though the
compiler can’t guarantee that. The unsafe code involved is then wrapped in a
safe API, and the outer type is still immutable.
Let’s explore this concept by looking at the RefCell<T> type that follows the
interior mutability pattern.
Enforcing Borrowing Rules at Runtime with RefCell<T>
Unlike Rc<T>, the RefCell<T> type represents single ownership over the data
it holds. So, what makes RefCell<T> different from a type like Box<T>?
Recall the borrowing rules you learned in Chapter 4:
- At any given time, you can have either (but not both of) one mutable reference or any number of immutable references.
- References must always be valid.
With references and Box<T>, the borrowing rules’ invariants are enforced at
compile time. With RefCell<T>, these invariants are enforced at runtime.
With references, if you break these rules, you’ll get a compiler error. With
RefCell<T>, if you break these rules, your program will panic and exit.
The advantages of checking the borrowing rules at compile time are that errors will be caught sooner in the development process, and there is no impact on runtime performance because all the analysis is completed beforehand. For those reasons, checking the borrowing rules at compile time is the best choice in the majority of cases, which is why this is Rust’s default.
The advantage of checking the borrowing rules at runtime instead is that certain memory-safe scenarios are then allowed, whereas they are disallowed by the compile-time checks. Static analysis, like the Rust compiler, is inherently conservative. Some properties of code are impossible to detect by analyzing the code: the most famous example is the Halting Problem, which is beyond the scope of this book but is an interesting topic to research.
Because some analysis is impossible, if the Rust compiler can’t be sure the
code complies with the ownership rules, it might reject a correct program; in
this way, it’s conservative. If Rust accepted an incorrect program, users
wouldn’t be able to trust in the guarantees Rust makes. However, if Rust
rejects a correct program, the programmer will be inconvenienced, but nothing
catastrophic can occur. The RefCell<T> type is useful when you’re sure your
code follows the borrowing rules but the compiler is unable to understand and
guarantee that.
Similar to Rc<T>, RefCell<T> is only for use in single-threaded scenarios
and will give you a compile-time error if you try using it in a multithreaded
context. We’ll talk about how to get the functionality of RefCell<T> in a
multithreaded program in Chapter 16.
Here is a recap of the reasons to choose Box<T>, Rc<T>, or RefCell<T>:
Rc<T>enables multiple owners of the same data;Box<T>andRefCell<T>have single owners.Box<T>allows immutable or mutable borrows checked at compile time;Rc<T>allows only immutable borrows checked at compile time;RefCell<T>allows immutable or mutable borrows checked at runtime.- Because
RefCell<T>allows mutable borrows checked at runtime, you can mutate the value inside theRefCell<T>even when theRefCell<T>is immutable.
Mutating the value inside an immutable value is the interior mutability pattern. Let’s look at a situation in which interior mutability is useful and examine how it’s possible.
Interior Mutability: A Mutable Borrow to an Immutable Value
A consequence of the borrowing rules is that when you have an immutable value, you can’t borrow it mutably. For example, this code won’t compile:
fn main() {
let x = 5;
let y = &mut x;
}
If you tried to compile this code, you’d get the following error:
$ cargo run
Compiling borrowing v0.1.0 (file:///projects/borrowing)
error[E0596]: cannot borrow `x` as mutable, as it is not declared as mutable
--> src/main.rs:3:13
|
2 | let x = 5;
| - help: consider changing this to be mutable: `mut x`
3 | let y = &mut x;
| ^^^^^^ cannot borrow as mutable
For more information about this error, try `rustc --explain E0596`.
error: could not compile `borrowing` due to previous error
However, there are situations in which it would be useful for a value to mutate
itself in its methods but appear immutable to other code. Code outside the
value’s methods would not be able to mutate the value. Using RefCell<T> is
one way to get the ability to have interior mutability. But RefCell<T>
doesn’t get around the borrowing rules completely: the borrow checker in the
compiler allows this interior mutability, and the borrowing rules are checked
at runtime instead. If you violate the rules, you’ll get a panic! instead of
a compiler error.
Let’s work through a practical example where we can use RefCell<T> to mutate
an immutable value and see why that is useful.
A Use Case for Interior Mutability: Mock Objects
A test double is the general programming concept for a type used in place of another type during testing. Mock objects are specific types of test doubles that record what happens during a test so you can assert that the correct actions took place.
Rust doesn’t have objects in the same sense as other languages have objects, and Rust doesn’t have mock object functionality built into the standard library as some other languages do. However, you can definitely create a struct that will serve the same purposes as a mock object.
Here’s the scenario we’ll test: we’ll create a library that tracks a value against a maximum value and sends messages based on how close to the maximum value the current value is. This library could be used to keep track of a user’s quota for the number of API calls they’re allowed to make, for example.
Our library will only provide the functionality of tracking how close to the
maximum a value is and what the messages should be at what times. Applications
that use our library will be expected to provide the mechanism for sending the
messages: the application could put a message in the application, send an
email, send a text message, or something else. The library doesn’t need to know
that detail. All it needs is something that implements a trait we’ll provide
called Messenger. Listing 15-20 shows the library code:
Filename: src/lib.rs
pub trait Messenger {
fn send(&self, msg: &str);
}
pub struct LimitTracker<'a, T: Messenger> {
messenger: &'a T,
value: usize,
max: usize,
}
impl<'a, T> LimitTracker<'a, T>
where
T: Messenger,
{
pub fn new(messenger: &T, max: usize) -> LimitTracker<T> {
LimitTracker {
messenger,
value: 0,
max,
}
}
pub fn set_value(&mut self, value: usize) {
self.value = value;
let percentage_of_max = self.value as f64 / self.max as f64;
if percentage_of_max >= 1.0 {
self.messenger.send("Error: You are over your quota!");
} else if percentage_of_max >= 0.9 {
self.messenger
.send("Urgent warning: You've used up over 90% of your quota!");
} else if percentage_of_max >= 0.75 {
self.messenger
.send("Warning: You've used up over 75% of your quota!");
}
}
}
Listing 15-20: A library to keep track of how close a value is to a maximum value and warn when the value is at certain levels
One important part of this code is that the Messenger trait has one method
called send that takes an immutable reference to self and the text of the
message. This trait is the interface our mock object needs to implement so that
the mock can be used in the same way a real object is. The other important part
is that we want to test the behavior of the set_value method on the
LimitTracker. We can change what we pass in for the value parameter, but
set_value doesn’t return anything for us to make assertions on. We want to be
able to say that if we create a LimitTracker with something that implements
the Messenger trait and a particular value for max, when we pass different
numbers for value, the messenger is told to send the appropriate messages.
We need a mock object that, instead of sending an email or text message when we
call send, will only keep track of the messages it’s told to send. We can
create a new instance of the mock object, create a LimitTracker that uses the
mock object, call the set_value method on LimitTracker, and then check that
the mock object has the messages we expect. Listing 15-21 shows an attempt to
implement a mock object to do just that, but the borrow checker won’t allow it:
Filename: src/lib.rs
pub trait Messenger {
fn send(&self, msg: &str);
}
pub struct LimitTracker<'a, T: Messenger> {
messenger: &'a T,
value: usize,
max: usize,
}
impl<'a, T> LimitTracker<'a, T>
where
T: Messenger,
{
pub fn new(messenger: &T, max: usize) -> LimitTracker<T> {
LimitTracker {
messenger,
value: 0,
max,
}
}
pub fn set_value(&mut self, value: usize) {
self.value = value;
let percentage_of_max = self.value as f64 / self.max as f64;
if percentage_of_max >= 1.0 {
self.messenger.send("Error: You are over your quota!");
} else if percentage_of_max >= 0.9 {
self.messenger
.send("Urgent warning: You've used up over 90% of your quota!");
} else if percentage_of_max >= 0.75 {
self.messenger
.send("Warning: You've used up over 75% of your quota!");
}
}
}
#[cfg(test)]
mod tests {
use super::*;
struct MockMessenger {
sent_messages: Vec<String>,
}
impl MockMessenger {
fn new() -> MockMessenger {
MockMessenger {
sent_messages: vec![],
}
}
}
impl Messenger for MockMessenger {
fn send(&self, message: &str) {
self.sent_messages.push(String::from(message));
}
}
#[test]
fn it_sends_an_over_75_percent_warning_message() {
let mock_messenger = MockMessenger::new();
let mut limit_tracker = LimitTracker::new(&mock_messenger, 100);
limit_tracker.set_value(80);
assert_eq!(mock_messenger.sent_messages.len(), 1);
}
}
Listing 15-21: An attempt to implement a MockMessenger
that isn’t allowed by the borrow checker
This test code defines a MockMessenger struct that has a sent_messages
field with a Vec of String values to keep track of the messages it’s told
to send. We also define an associated function new to make it convenient to
create new MockMessenger values that start with an empty list of messages. We
then implement the Messenger trait for MockMessenger so we can give a
MockMessenger to a LimitTracker. In the definition of the send method, we
take the message passed in as a parameter and store it in the MockMessenger
list of sent_messages.
In the test, we’re testing what happens when the LimitTracker is told to set
value to something that is more than 75 percent of the max value. First, we
create a new MockMessenger, which will start with an empty list of messages.
Then we create a new LimitTracker and give it a reference to the new
MockMessenger and a max value of 100. We call the set_value method on the
LimitTracker with a value of 80, which is more than 75 percent of 100. Then
we assert that the list of messages that the MockMessenger is keeping track
of should now have one message in it.
However, there’s one problem with this test, as shown here:
$ cargo test
Compiling limit-tracker v0.1.0 (file:///projects/limit-tracker)
error[E0596]: cannot borrow `self.sent_messages` as mutable, as it is behind a `&` reference
--> src/lib.rs:58:13
|
2 | fn send(&self, msg: &str);
| ----- help: consider changing that to be a mutable reference: `&mut self`
...
58 | self.sent_messages.push(String::from(message));
| ^^^^^^^^^^^^^^^^^^ `self` is a `&` reference, so the data it refers to cannot be borrowed as mutable
For more information about this error, try `rustc --explain E0596`.
error: could not compile `limit-tracker` due to previous error
warning: build failed, waiting for other jobs to finish...
error: build failed
We can’t modify the MockMessenger to keep track of the messages, because the
send method takes an immutable reference to self. We also can’t take the
suggestion from the error text to use &mut self instead, because then the
signature of send wouldn’t match the signature in the Messenger trait
definition (feel free to try and see what error message you get).
This is a situation in which interior mutability can help! We’ll store the
sent_messages within a RefCell<T>, and then the send method will be
able to modify sent_messages to store the messages we’ve seen. Listing 15-22
shows what that looks like:
Filename: src/lib.rs
pub trait Messenger {
fn send(&self, msg: &str);
}
pub struct LimitTracker<'a, T: Messenger> {
messenger: &'a T,
value: usize,
max: usize,
}
impl<'a, T> LimitTracker<'a, T>
where
T: Messenger,
{
pub fn new(messenger: &T, max: usize) -> LimitTracker<T> {
LimitTracker {
messenger,
value: 0,
max,
}
}
pub fn set_value(&mut self, value: usize) {
self.value = value;
let percentage_of_max = self.value as f64 / self.max as f64;
if percentage_of_max >= 1.0 {
self.messenger.send("Error: You are over your quota!");
} else if percentage_of_max >= 0.9 {
self.messenger
.send("Urgent warning: You've used up over 90% of your quota!");
} else if percentage_of_max >= 0.75 {
self.messenger
.send("Warning: You've used up over 75% of your quota!");
}
}
}
#[cfg(test)]
mod tests {
use super::*;
use std::cell::RefCell;
struct MockMessenger {
sent_messages: RefCell<Vec<String>>,
}
impl MockMessenger {
fn new() -> MockMessenger {
MockMessenger {
sent_messages: RefCell::new(vec![]),
}
}
}
impl Messenger for MockMessenger {
fn send(&self, message: &str) {
self.sent_messages.borrow_mut().push(String::from(message));
}
}
#[test]
fn it_sends_an_over_75_percent_warning_message() {
// --snip--
let mock_messenger = MockMessenger::new();
let mut limit_tracker = LimitTracker::new(&mock_messenger, 100);
limit_tracker.set_value(80);
assert_eq!(mock_messenger.sent_messages.borrow().len(), 1);
}
}
Listing 15-22: Using RefCell<T> to mutate an inner
value while the outer value is considered immutable
The sent_messages field is now of type RefCell<Vec<String>> instead of
Vec<String>. In the new function, we create a new RefCell<Vec<String>>
instance around the empty vector.
For the implementation of the send method, the first parameter is still an
immutable borrow of self, which matches the trait definition. We call
borrow_mut on the RefCell<Vec<String>> in self.sent_messages to get a
mutable reference to the value inside the RefCell<Vec<String>>, which is
the vector. Then we can call push on the mutable reference to the vector to
keep track of the messages sent during the test.
The last change we have to make is in the assertion: to see how many items are
in the inner vector, we call borrow on the RefCell<Vec<String>> to get an
immutable reference to the vector.
Now that you’ve seen how to use RefCell<T>, let’s dig into how it works!
Keeping Track of Borrows at Runtime with RefCell<T>
When creating immutable and mutable references, we use the & and &mut
syntax, respectively. With RefCell<T>, we use the borrow and borrow_mut
methods, which are part of the safe API that belongs to RefCell<T>. The
borrow method returns the smart pointer type Ref<T>, and borrow_mut
returns the smart pointer type RefMut<T>. Both types implement Deref, so we
can treat them like regular references.
The RefCell<T> keeps track of how many Ref<T> and RefMut<T> smart
pointers are currently active. Every time we call borrow, the RefCell<T>
increases its count of how many immutable borrows are active. When a Ref<T>
value goes out of scope, the count of immutable borrows goes down by one. Just
like the compile-time borrowing rules, RefCell<T> lets us have many immutable
borrows or one mutable borrow at any point in time.
If we try to violate these rules, rather than getting a compiler error as we
would with references, the implementation of RefCell<T> will panic at
runtime. Listing 15-23 shows a modification of the implementation of send in
Listing 15-22. We’re deliberately trying to create two mutable borrows active
for the same scope to illustrate that RefCell<T> prevents us from doing this
at runtime.
Filename: src/lib.rs
pub trait Messenger {
fn send(&self, msg: &str);
}
pub struct LimitTracker<'a, T: Messenger> {
messenger: &'a T,
value: usize,
max: usize,
}
impl<'a, T> LimitTracker<'a, T>
where
T: Messenger,
{
pub fn new(messenger: &T, max: usize) -> LimitTracker<T> {
LimitTracker {
messenger,
value: 0,
max,
}
}
pub fn set_value(&mut self, value: usize) {
self.value = value;
let percentage_of_max = self.value as f64 / self.max as f64;
if percentage_of_max >= 1.0 {
self.messenger.send("Error: You are over your quota!");
} else if percentage_of_max >= 0.9 {
self.messenger
.send("Urgent warning: You've used up over 90% of your quota!");
} else if percentage_of_max >= 0.75 {
self.messenger
.send("Warning: You've used up over 75% of your quota!");
}
}
}
#[cfg(test)]
mod tests {
use super::*;
use std::cell::RefCell;
struct MockMessenger {
sent_messages: RefCell<Vec<String>>,
}
impl MockMessenger {
fn new() -> MockMessenger {
MockMessenger {
sent_messages: RefCell::new(vec![]),
}
}
}
impl Messenger for MockMessenger {
fn send(&self, message: &str) {
let mut one_borrow = self.sent_messages.borrow_mut();
let mut two_borrow = self.sent_messages.borrow_mut();
one_borrow.push(String::from(message));
two_borrow.push(String::from(message));
}
}
#[test]
fn it_sends_an_over_75_percent_warning_message() {
let mock_messenger = MockMessenger::new();
let mut limit_tracker = LimitTracker::new(&mock_messenger, 100);
limit_tracker.set_value(80);
assert_eq!(mock_messenger.sent_messages.borrow().len(), 1);
}
}
Listing 15-23: Creating two mutable references in the
same scope to see that RefCell<T> will panic
We create a variable one_borrow for the RefMut<T> smart pointer returned
from borrow_mut. Then we create another mutable borrow in the same way in the
variable two_borrow. This makes two mutable references in the same scope,
which isn’t allowed. When we run the tests for our library, the code in Listing
15-23 will compile without any errors, but the test will fail:
$ cargo test
Compiling limit-tracker v0.1.0 (file:///projects/limit-tracker)
Finished test [unoptimized + debuginfo] target(s) in 0.91s
Running unittests (target/debug/deps/limit_tracker-e599811fa246dbde)
running 1 test
test tests::it_sends_an_over_75_percent_warning_message ... FAILED
failures:
---- tests::it_sends_an_over_75_percent_warning_message stdout ----
thread 'main' panicked at 'already borrowed: BorrowMutError', src/lib.rs:60:53
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
failures:
tests::it_sends_an_over_75_percent_warning_message
test result: FAILED. 0 passed; 1 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.00s
error: test failed, to rerun pass '--lib'
Notice that the code panicked with the message already borrowed: BorrowMutError. This is how RefCell<T> handles violations of the borrowing
rules at runtime.
Catching borrowing errors at runtime rather than compile time means that you
would find a mistake in your code later in the development process and possibly
not until your code was deployed to production. Also, your code would incur a
small runtime performance penalty as a result of keeping track of the borrows
at runtime rather than compile time. However, using RefCell<T> makes it
possible to write a mock object that can modify itself to keep track of the
messages it has seen while you’re using it in a context where only immutable
values are allowed. You can use RefCell<T> despite its trade-offs to get more
functionality than regular references provide.
Having Multiple Owners of Mutable Data by Combining Rc<T> and RefCell<T>
A common way to use RefCell<T> is in combination with Rc<T>. Recall that
Rc<T> lets you have multiple owners of some data, but it only gives immutable
access to that data. If you have an Rc<T> that holds a RefCell<T>, you can
get a value that can have multiple owners and that you can mutate!
For example, recall the cons list example in Listing 15-18 where we used
Rc<T> to allow multiple lists to share ownership of another list. Because
Rc<T> holds only immutable values, we can’t change any of the values in the
list once we’ve created them. Let’s add in RefCell<T> to gain the ability to
change the values in the lists. Listing 15-24 shows that by using a
RefCell<T> in the Cons definition, we can modify the value stored in all
the lists:
Filename: src/main.rs
#[derive(Debug)] enum List { Cons(Rc<RefCell<i32>>, Rc<List>), Nil, } use crate::List::{Cons, Nil}; use std::cell::RefCell; use std::rc::Rc; fn main() { let value = Rc::new(RefCell::new(5)); let a = Rc::new(Cons(Rc::clone(&value), Rc::new(Nil))); let b = Cons(Rc::new(RefCell::new(3)), Rc::clone(&a)); let c = Cons(Rc::new(RefCell::new(4)), Rc::clone(&a)); *value.borrow_mut() += 10; println!("a after = {:?}", a); println!("b after = {:?}", b); println!("c after = {:?}", c); }
Listing 15-24: Using Rc<RefCell<i32>> to create a
List that we can mutate
We create a value that is an instance of Rc<RefCell<i32>> and store it in a
variable named value so we can access it directly later. Then we create a
List in a with a Cons variant that holds value. We need to clone
value so both a and value have ownership of the inner 5 value rather
than transferring ownership from value to a or having a borrow from
value.
We wrap the list a in an Rc<T> so when we create lists b and c, they
can both refer to a, which is what we did in Listing 15-18.
After we’ve created the lists in a, b, and c, we add 10 to the value in
value. We do this by calling borrow_mut on value, which uses the
automatic dereferencing feature we discussed in Chapter 5 (see the section
“Where’s the -> Operator?”) to
dereference the Rc<T> to the inner RefCell<T> value. The borrow_mut
method returns a RefMut<T> smart pointer, and we use the dereference operator
on it and change the inner value.
When we print a, b, and c, we can see that they all have the modified
value of 15 rather than 5:
$ cargo run
Compiling cons-list v0.1.0 (file:///projects/cons-list)
Finished dev [unoptimized + debuginfo] target(s) in 0.63s
Running `target/debug/cons-list`
a after = Cons(RefCell { value: 15 }, Nil)
b after = Cons(RefCell { value: 3 }, Cons(RefCell { value: 15 }, Nil))
c after = Cons(RefCell { value: 4 }, Cons(RefCell { value: 15 }, Nil))
This technique is pretty neat! By using RefCell<T>, we have an outwardly
immutable List value. But we can use the methods on RefCell<T> that provide
access to its interior mutability so we can modify our data when we need to.
The runtime checks of the borrowing rules protect us from data races, and it’s
sometimes worth trading a bit of speed for this flexibility in our data
structures.
The standard library has other types that provide interior mutability, such as
Cell<T>, which is similar except that instead of giving references to the
inner value, the value is copied in and out of the Cell<T>. There’s also
Mutex<T>, which offers interior mutability that’s safe to use across threads;
we’ll discuss its use in Chapter 16. Check out the standard library docs for
more details on the differences between these types.