# 1.0.0[−][src]Module core::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>` allows to do 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`, `RwLock` 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::collections::HashMap;
    use std::cell::RefCell;
    use std::rc::Rc;

    fn main() {
        let shared_map: Rc<RefCell<_>> = Rc::new(RefCell::new(HashMap::new()));
        shared_map.borrow_mut().insert("africa", 92388);
        shared_map.borrow_mut().insert("kyoto", 11837);
        shared_map.borrow_mut().insert("piccadilly", 11826);
        shared_map.borrow_mut().insert("marbles", 38);
    }Run

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)> {
            // Create a new scope to contain the lifetime of the
            // dynamic borrow
            {
                // Take a reference to the inside of cache cell
                let mut cache = self.span_tree_cache.borrow_mut();
                if cache.is_some() {
                    return cache.as_ref().unwrap().clone();
                }

                let span_tree = self.calc_span_tree();
                *cache = Some(span_tree);
            }

            // Recursive call to return the just-cached value.
            // Note that if we had not let the previous borrow
            // of the cache fall out of scope then the subsequent
            // recursive borrow would cause a dynamic thread panic.
            // This is the major hazard of using `RefCell`.
            self.minimum_spanning_tree()
        }
    }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>`.

    #![feature(core_intrinsics)]
    use std::cell::Cell;
    use std::ptr::NonNull;
    use std::intrinsics::abort;

    struct Rc<T: ?Sized> {
        ptr: NonNull<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 }
        }
    }

    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(|| unsafe { abort() }));
        }
    }

    impl<T: ?Sized> RcBoxPtr<T> for Rc<T> {
       fn inner(&self) -> &RcBox<T> {
           unsafe {
               self.ptr.as_ref()
           }
       }
    }Run

## Structs

BorrowError | An error returned by `RefCell::try_borrow`.
BorrowMutError | An error returned by `RefCell::try_borrow_mut`.
Cell | A mutable memory location.
Ref | Wraps a borrowed reference to a value in a `RefCell` box. A wrapper type for an immutably borrowed value from a `RefCell<T>`.
RefCell | A mutable memory location with dynamically checked borrow rules
RefMut | A wrapper type for a mutably borrowed value from a `RefCell<T>`.
UnsafeCell | The core primitive for interior mutability in Rust.

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