Module std::cell

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. Both Cell<T> and RefCell<T> allow doing this in a single-threaded way. However, neither Cell<T> nor RefCell<T> are thread safe (they do not implement Sync). If you need to do aliasing and mutation between multiple threads it is possible to use Mutex<T>, RwLock<T> or atomic types.

Values of the Cell<T> and RefCell<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 that Cell<T> and RefCell<T> provide ‘interior mutability’, in contrast with typical Rust types that exhibit ‘inherited mutability’.

Cell types come in two flavors: Cell<T> and RefCell<T>. Cell<T> implements interior mutability by moving values in and out of the Cell<T>. To use references instead of values, one must use the RefCell<T> type, acquiring a write lock before mutating. Cell<T> provides methods to retrieve and change the current interior value:

• For types that implement Copy, the get method retrieves the current interior value.
• For types that implement Default, the take method replaces the current interior value with Default::default() and returns the replaced value.
• For all types, the replace method replaces the current interior value and returns the replaced value and the into_inner method consumes the Cell<T> and returns the interior value. Additionally, the set method replaces the interior value, dropping the replaced value.

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. Because RefCell<T> borrows are dynamic it is possible to attempt to borrow a value that is already mutably borrowed; when this happens it results in thread panic.

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);
}

Note 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::RefCell;

struct Graph {
    edges: Vec<(i32, i32)>,
    span_tree_cache: RefCell<Option<Vec<(i32, i32)>>>
}

impl Graph {
    fn minimum_spanning_tree(&self) -> Vec<(i32, i32)> {
        self.span_tree_cache.borrow_mut()
            .get_or_insert_with(|| self.calc_span_tree())
            .clone()
    }

    fn calc_span_tree(&self) -> Vec<(i32, i32)> {
        // Expensive computation goes here
        vec![]
    }
}

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()
       }
   }
}

Structs