1 //! Shareable mutable containers.
3 //! Rust memory safety is based on this rule: Given an object `T`, it is only possible to
4 //! have one of the following:
6 //! - Having several immutable references (`&T`) to the object (also known as **aliasing**).
7 //! - Having one mutable reference (`&mut T`) to the object (also known as **mutability**).
9 //! This is enforced by the Rust compiler. However, there are situations where this rule is not
10 //! flexible enough. Sometimes it is required to have multiple references to an object and yet
13 //! Shareable mutable containers exist to permit mutability in a controlled manner, even in the
14 //! presence of aliasing. Both `Cell<T>` and `RefCell<T>` allow doing this in a single-threaded
15 //! way. However, neither `Cell<T>` nor `RefCell<T>` are thread safe (they do not implement
16 //! `Sync`). If you need to do aliasing and mutation between multiple threads it is possible to
17 //! use [`Mutex`](../../std/sync/struct.Mutex.html),
18 //! [`RwLock`](../../std/sync/struct.RwLock.html) or
19 //! [`atomic`](../../core/sync/atomic/index.html) types.
21 //! Values of the `Cell<T>` and `RefCell<T>` types may be mutated through shared references (i.e.
22 //! the common `&T` type), whereas most Rust types can only be mutated through unique (`&mut T`)
23 //! references. We say that `Cell<T>` and `RefCell<T>` provide 'interior mutability', in contrast
24 //! with typical Rust types that exhibit 'inherited mutability'.
26 //! Cell types come in two flavors: `Cell<T>` and `RefCell<T>`. `Cell<T>` implements interior
27 //! mutability by moving values in and out of the `Cell<T>`. To use references instead of values,
28 //! one must use the `RefCell<T>` type, acquiring a write lock before mutating. `Cell<T>` provides
29 //! methods to retrieve and change the current interior value:
31 //! - For types that implement `Copy`, the `get` method retrieves the current interior value.
32 //! - For types that implement `Default`, the `take` method replaces the current interior value
33 //! with `Default::default()` and returns the replaced value.
34 //! - For all types, the `replace` method replaces the current interior value and returns the
35 //! replaced value and the `into_inner` method consumes the `Cell<T>` and returns the interior
36 //! value. Additionally, the `set` method replaces the interior value, dropping the replaced
39 //! `RefCell<T>` uses Rust's lifetimes to implement 'dynamic borrowing', a process whereby one can
40 //! claim temporary, exclusive, mutable access to the inner value. Borrows for `RefCell<T>`s are
41 //! tracked 'at runtime', unlike Rust's native reference types which are entirely tracked
42 //! statically, at compile time. Because `RefCell<T>` borrows are dynamic it is possible to attempt
43 //! to borrow a value that is already mutably borrowed; when this happens it results in thread
46 //! # When to choose interior mutability
48 //! The more common inherited mutability, where one must have unique access to mutate a value, is
49 //! one of the key language elements that enables Rust to reason strongly about pointer aliasing,
50 //! statically preventing crash bugs. Because of that, inherited mutability is preferred, and
51 //! interior mutability is something of a last resort. Since cell types enable mutation where it
52 //! would otherwise be disallowed though, there are occasions when interior mutability might be
53 //! appropriate, or even *must* be used, e.g.
55 //! * Introducing mutability 'inside' of something immutable
56 //! * Implementation details of logically-immutable methods.
57 //! * Mutating implementations of `Clone`.
59 //! ## Introducing mutability 'inside' of something immutable
61 //! Many shared smart pointer types, including `Rc<T>` and `Arc<T>`, provide containers that can be
62 //! cloned and shared between multiple parties. Because the contained values may be
63 //! multiply-aliased, they can only be borrowed with `&`, not `&mut`. Without cells it would be
64 //! impossible to mutate data inside of these smart pointers at all.
66 //! It's very common then to put a `RefCell<T>` inside shared pointer types to reintroduce
70 //! use std::cell::{RefCell, RefMut};
71 //! use std::collections::HashMap;
75 //! let shared_map: Rc<RefCell<_>> = Rc::new(RefCell::new(HashMap::new()));
76 //! // Create a new block to limit the scope of the dynamic borrow
78 //! let mut map: RefMut<_> = shared_map.borrow_mut();
79 //! map.insert("africa", 92388);
80 //! map.insert("kyoto", 11837);
81 //! map.insert("piccadilly", 11826);
82 //! map.insert("marbles", 38);
85 //! // Note that if we had not let the previous borrow of the cache fall out
86 //! // of scope then the subsequent borrow would cause a dynamic thread panic.
87 //! // This is the major hazard of using `RefCell`.
88 //! let total: i32 = shared_map.borrow().values().sum();
89 //! println!("{}", total);
93 //! Note that this example uses `Rc<T>` and not `Arc<T>`. `RefCell<T>`s are for single-threaded
94 //! scenarios. Consider using `RwLock<T>` or `Mutex<T>` if you need shared mutability in a
95 //! multi-threaded situation.
97 //! ## Implementation details of logically-immutable methods
99 //! Occasionally it may be desirable not to expose in an API that there is mutation happening
100 //! "under the hood". This may be because logically the operation is immutable, but e.g., caching
101 //! forces the implementation to perform mutation; or because you must employ mutation to implement
102 //! a trait method that was originally defined to take `&self`.
105 //! # #![allow(dead_code)]
106 //! use std::cell::RefCell;
109 //! edges: Vec<(i32, i32)>,
110 //! span_tree_cache: RefCell<Option<Vec<(i32, i32)>>>
114 //! fn minimum_spanning_tree(&self) -> Vec<(i32, i32)> {
115 //! self.span_tree_cache.borrow_mut()
116 //! .get_or_insert_with(|| self.calc_span_tree())
120 //! fn calc_span_tree(&self) -> Vec<(i32, i32)> {
121 //! // Expensive computation goes here
127 //! ## Mutating implementations of `Clone`
129 //! This is simply a special - but common - case of the previous: hiding mutability for operations
130 //! that appear to be immutable. The `clone` method is expected to not change the source value, and
131 //! is declared to take `&self`, not `&mut self`. Therefore, any mutation that happens in the
132 //! `clone` method must use cell types. For example, `Rc<T>` maintains its reference counts within a
136 //! use std::cell::Cell;
137 //! use std::ptr::NonNull;
138 //! use std::process::abort;
139 //! use std::marker::PhantomData;
141 //! struct Rc<T: ?Sized> {
142 //! ptr: NonNull<RcBox<T>>,
143 //! phantom: PhantomData<RcBox<T>>,
146 //! struct RcBox<T: ?Sized> {
147 //! strong: Cell<usize>,
148 //! refcount: Cell<usize>,
152 //! impl<T: ?Sized> Clone for Rc<T> {
153 //! fn clone(&self) -> Rc<T> {
154 //! self.inc_strong();
157 //! phantom: PhantomData,
162 //! trait RcBoxPtr<T: ?Sized> {
164 //! fn inner(&self) -> &RcBox<T>;
166 //! fn strong(&self) -> usize {
167 //! self.inner().strong.get()
170 //! fn inc_strong(&self) {
173 //! .set(self.strong()
175 //! .unwrap_or_else(|| abort() ));
179 //! impl<T: ?Sized> RcBoxPtr<T> for Rc<T> {
180 //! fn inner(&self) -> &RcBox<T> {
182 //! self.ptr.as_ref()
189 #![stable(feature = "rust1", since = "1.0.0")]
191 use crate::cmp::Ordering;
192 use crate::fmt::{self, Debug, Display};
193 use crate::marker::Unsize;
195 use crate::ops::{CoerceUnsized, Deref, DerefMut};
198 /// A mutable memory location.
202 /// In this example, you can see that `Cell<T>` enables mutation inside an
203 /// immutable struct. In other words, it enables "interior mutability".
206 /// use std::cell::Cell;
208 /// struct SomeStruct {
209 /// regular_field: u8,
210 /// special_field: Cell<u8>,
213 /// let my_struct = SomeStruct {
214 /// regular_field: 0,
215 /// special_field: Cell::new(1),
218 /// let new_value = 100;
220 /// // ERROR: `my_struct` is immutable
221 /// // my_struct.regular_field = new_value;
223 /// // WORKS: although `my_struct` is immutable, `special_field` is a `Cell`,
224 /// // which can always be mutated
225 /// my_struct.special_field.set(new_value);
226 /// assert_eq!(my_struct.special_field.get(), new_value);
229 /// See the [module-level documentation](self) for more.
230 #[stable(feature = "rust1", since = "1.0.0")]
232 pub struct Cell<T: ?Sized> {
233 value: UnsafeCell<T>,
236 #[stable(feature = "rust1", since = "1.0.0")]
237 unsafe impl<T: ?Sized> Send for Cell<T> where T: Send {}
239 #[stable(feature = "rust1", since = "1.0.0")]
240 impl<T: ?Sized> !Sync for Cell<T> {}
242 #[stable(feature = "rust1", since = "1.0.0")]
243 impl<T: Copy> Clone for Cell<T> {
245 fn clone(&self) -> Cell<T> {
246 Cell::new(self.get())
250 #[stable(feature = "rust1", since = "1.0.0")]
251 impl<T: Default> Default for Cell<T> {
252 /// Creates a `Cell<T>`, with the `Default` value for T.
254 fn default() -> Cell<T> {
255 Cell::new(Default::default())
259 #[stable(feature = "rust1", since = "1.0.0")]
260 impl<T: PartialEq + Copy> PartialEq for Cell<T> {
262 fn eq(&self, other: &Cell<T>) -> bool {
263 self.get() == other.get()
267 #[stable(feature = "cell_eq", since = "1.2.0")]
268 impl<T: Eq + Copy> Eq for Cell<T> {}
270 #[stable(feature = "cell_ord", since = "1.10.0")]
271 impl<T: PartialOrd + Copy> PartialOrd for Cell<T> {
273 fn partial_cmp(&self, other: &Cell<T>) -> Option<Ordering> {
274 self.get().partial_cmp(&other.get())
278 fn lt(&self, other: &Cell<T>) -> bool {
279 self.get() < other.get()
283 fn le(&self, other: &Cell<T>) -> bool {
284 self.get() <= other.get()
288 fn gt(&self, other: &Cell<T>) -> bool {
289 self.get() > other.get()
293 fn ge(&self, other: &Cell<T>) -> bool {
294 self.get() >= other.get()
298 #[stable(feature = "cell_ord", since = "1.10.0")]
299 impl<T: Ord + Copy> Ord for Cell<T> {
301 fn cmp(&self, other: &Cell<T>) -> Ordering {
302 self.get().cmp(&other.get())
306 #[stable(feature = "cell_from", since = "1.12.0")]
307 impl<T> From<T> for Cell<T> {
308 fn from(t: T) -> Cell<T> {
314 /// Creates a new `Cell` containing the given value.
319 /// use std::cell::Cell;
321 /// let c = Cell::new(5);
323 #[stable(feature = "rust1", since = "1.0.0")]
324 #[rustc_const_stable(feature = "const_cell_new", since = "1.32.0")]
326 pub const fn new(value: T) -> Cell<T> {
327 Cell { value: UnsafeCell::new(value) }
330 /// Sets the contained value.
335 /// use std::cell::Cell;
337 /// let c = Cell::new(5);
342 #[stable(feature = "rust1", since = "1.0.0")]
343 pub fn set(&self, val: T) {
344 let old = self.replace(val);
348 /// Swaps the values of two Cells.
349 /// Difference with `std::mem::swap` is that this function doesn't require `&mut` reference.
354 /// use std::cell::Cell;
356 /// let c1 = Cell::new(5i32);
357 /// let c2 = Cell::new(10i32);
359 /// assert_eq!(10, c1.get());
360 /// assert_eq!(5, c2.get());
363 #[stable(feature = "move_cell", since = "1.17.0")]
364 pub fn swap(&self, other: &Self) {
365 if ptr::eq(self, other) {
368 // SAFETY: This can be risky if called from separate threads, but `Cell`
369 // is `!Sync` so this won't happen. This also won't invalidate any
370 // pointers since `Cell` makes sure nothing else will be pointing into
371 // either of these `Cell`s.
373 ptr::swap(self.value.get(), other.value.get());
377 /// Replaces the contained value, and returns it.
382 /// use std::cell::Cell;
384 /// let cell = Cell::new(5);
385 /// assert_eq!(cell.get(), 5);
386 /// assert_eq!(cell.replace(10), 5);
387 /// assert_eq!(cell.get(), 10);
389 #[stable(feature = "move_cell", since = "1.17.0")]
390 pub fn replace(&self, val: T) -> T {
391 // SAFETY: This can cause data races if called from a separate thread,
392 // but `Cell` is `!Sync` so this won't happen.
393 mem::replace(unsafe { &mut *self.value.get() }, val)
396 /// Unwraps the value.
401 /// use std::cell::Cell;
403 /// let c = Cell::new(5);
404 /// let five = c.into_inner();
406 /// assert_eq!(five, 5);
408 #[stable(feature = "move_cell", since = "1.17.0")]
409 #[rustc_const_unstable(feature = "const_cell_into_inner", issue = "78729")]
410 pub const fn into_inner(self) -> T {
411 self.value.into_inner()
415 impl<T: Copy> Cell<T> {
416 /// Returns a copy of the contained value.
421 /// use std::cell::Cell;
423 /// let c = Cell::new(5);
425 /// let five = c.get();
428 #[stable(feature = "rust1", since = "1.0.0")]
429 pub fn get(&self) -> T {
430 // SAFETY: This can cause data races if called from a separate thread,
431 // but `Cell` is `!Sync` so this won't happen.
432 unsafe { *self.value.get() }
435 /// Updates the contained value using a function and returns the new value.
440 /// #![feature(cell_update)]
442 /// use std::cell::Cell;
444 /// let c = Cell::new(5);
445 /// let new = c.update(|x| x + 1);
447 /// assert_eq!(new, 6);
448 /// assert_eq!(c.get(), 6);
451 #[unstable(feature = "cell_update", issue = "50186")]
452 pub fn update<F>(&self, f: F) -> T
456 let old = self.get();
463 impl<T: ?Sized> Cell<T> {
464 /// Returns a raw pointer to the underlying data in this cell.
469 /// use std::cell::Cell;
471 /// let c = Cell::new(5);
473 /// let ptr = c.as_ptr();
476 #[stable(feature = "cell_as_ptr", since = "1.12.0")]
477 #[rustc_const_stable(feature = "const_cell_as_ptr", since = "1.32.0")]
478 pub const fn as_ptr(&self) -> *mut T {
482 /// Returns a mutable reference to the underlying data.
484 /// This call borrows `Cell` mutably (at compile-time) which guarantees
485 /// that we possess the only reference.
490 /// use std::cell::Cell;
492 /// let mut c = Cell::new(5);
493 /// *c.get_mut() += 1;
495 /// assert_eq!(c.get(), 6);
498 #[stable(feature = "cell_get_mut", since = "1.11.0")]
499 pub fn get_mut(&mut self) -> &mut T {
503 /// Returns a `&Cell<T>` from a `&mut T`
508 /// use std::cell::Cell;
510 /// let slice: &mut [i32] = &mut [1, 2, 3];
511 /// let cell_slice: &Cell<[i32]> = Cell::from_mut(slice);
512 /// let slice_cell: &[Cell<i32>] = cell_slice.as_slice_of_cells();
514 /// assert_eq!(slice_cell.len(), 3);
517 #[stable(feature = "as_cell", since = "1.37.0")]
518 pub fn from_mut(t: &mut T) -> &Cell<T> {
519 // SAFETY: `&mut` ensures unique access.
520 unsafe { &*(t as *mut T as *const Cell<T>) }
524 impl<T: Default> Cell<T> {
525 /// Takes the value of the cell, leaving `Default::default()` in its place.
530 /// use std::cell::Cell;
532 /// let c = Cell::new(5);
533 /// let five = c.take();
535 /// assert_eq!(five, 5);
536 /// assert_eq!(c.into_inner(), 0);
538 #[stable(feature = "move_cell", since = "1.17.0")]
539 pub fn take(&self) -> T {
540 self.replace(Default::default())
544 #[unstable(feature = "coerce_unsized", issue = "27732")]
545 impl<T: CoerceUnsized<U>, U> CoerceUnsized<Cell<U>> for Cell<T> {}
548 /// Returns a `&[Cell<T>]` from a `&Cell<[T]>`
553 /// use std::cell::Cell;
555 /// let slice: &mut [i32] = &mut [1, 2, 3];
556 /// let cell_slice: &Cell<[i32]> = Cell::from_mut(slice);
557 /// let slice_cell: &[Cell<i32>] = cell_slice.as_slice_of_cells();
559 /// assert_eq!(slice_cell.len(), 3);
561 #[stable(feature = "as_cell", since = "1.37.0")]
562 pub fn as_slice_of_cells(&self) -> &[Cell<T>] {
563 // SAFETY: `Cell<T>` has the same memory layout as `T`.
564 unsafe { &*(self as *const Cell<[T]> as *const [Cell<T>]) }
568 /// A mutable memory location with dynamically checked borrow rules
570 /// See the [module-level documentation](self) for more.
571 #[stable(feature = "rust1", since = "1.0.0")]
572 pub struct RefCell<T: ?Sized> {
573 borrow: Cell<BorrowFlag>,
574 value: UnsafeCell<T>,
577 /// An error returned by [`RefCell::try_borrow`].
578 #[stable(feature = "try_borrow", since = "1.13.0")]
579 pub struct BorrowError {
583 #[stable(feature = "try_borrow", since = "1.13.0")]
584 impl Debug for BorrowError {
585 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
586 f.debug_struct("BorrowError").finish()
590 #[stable(feature = "try_borrow", since = "1.13.0")]
591 impl Display for BorrowError {
592 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
593 Display::fmt("already mutably borrowed", f)
597 /// An error returned by [`RefCell::try_borrow_mut`].
598 #[stable(feature = "try_borrow", since = "1.13.0")]
599 pub struct BorrowMutError {
603 #[stable(feature = "try_borrow", since = "1.13.0")]
604 impl Debug for BorrowMutError {
605 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
606 f.debug_struct("BorrowMutError").finish()
610 #[stable(feature = "try_borrow", since = "1.13.0")]
611 impl Display for BorrowMutError {
612 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
613 Display::fmt("already borrowed", f)
617 // Positive values represent the number of `Ref` active. Negative values
618 // represent the number of `RefMut` active. Multiple `RefMut`s can only be
619 // active at a time if they refer to distinct, nonoverlapping components of a
620 // `RefCell` (e.g., different ranges of a slice).
622 // `Ref` and `RefMut` are both two words in size, and so there will likely never
623 // be enough `Ref`s or `RefMut`s in existence to overflow half of the `usize`
624 // range. Thus, a `BorrowFlag` will probably never overflow or underflow.
625 // However, this is not a guarantee, as a pathological program could repeatedly
626 // create and then mem::forget `Ref`s or `RefMut`s. Thus, all code must
627 // explicitly check for overflow and underflow in order to avoid unsafety, or at
628 // least behave correctly in the event that overflow or underflow happens (e.g.,
629 // see BorrowRef::new).
630 type BorrowFlag = isize;
631 const UNUSED: BorrowFlag = 0;
634 fn is_writing(x: BorrowFlag) -> bool {
639 fn is_reading(x: BorrowFlag) -> bool {
644 /// Creates a new `RefCell` containing `value`.
649 /// use std::cell::RefCell;
651 /// let c = RefCell::new(5);
653 #[stable(feature = "rust1", since = "1.0.0")]
654 #[rustc_const_stable(feature = "const_refcell_new", since = "1.32.0")]
656 pub const fn new(value: T) -> RefCell<T> {
657 RefCell { value: UnsafeCell::new(value), borrow: Cell::new(UNUSED) }
660 /// Consumes the `RefCell`, returning the wrapped value.
665 /// use std::cell::RefCell;
667 /// let c = RefCell::new(5);
669 /// let five = c.into_inner();
671 #[stable(feature = "rust1", since = "1.0.0")]
672 #[rustc_const_unstable(feature = "const_cell_into_inner", issue = "78729")]
674 pub const fn into_inner(self) -> T {
675 // Since this function takes `self` (the `RefCell`) by value, the
676 // compiler statically verifies that it is not currently borrowed.
677 self.value.into_inner()
680 /// Replaces the wrapped value with a new one, returning the old value,
681 /// without deinitializing either one.
683 /// This function corresponds to [`std::mem::replace`](../mem/fn.replace.html).
687 /// Panics if the value is currently borrowed.
692 /// use std::cell::RefCell;
693 /// let cell = RefCell::new(5);
694 /// let old_value = cell.replace(6);
695 /// assert_eq!(old_value, 5);
696 /// assert_eq!(cell, RefCell::new(6));
699 #[stable(feature = "refcell_replace", since = "1.24.0")]
701 pub fn replace(&self, t: T) -> T {
702 mem::replace(&mut *self.borrow_mut(), t)
705 /// Replaces the wrapped value with a new one computed from `f`, returning
706 /// the old value, without deinitializing either one.
710 /// Panics if the value is currently borrowed.
715 /// use std::cell::RefCell;
716 /// let cell = RefCell::new(5);
717 /// let old_value = cell.replace_with(|&mut old| old + 1);
718 /// assert_eq!(old_value, 5);
719 /// assert_eq!(cell, RefCell::new(6));
722 #[stable(feature = "refcell_replace_swap", since = "1.35.0")]
724 pub fn replace_with<F: FnOnce(&mut T) -> T>(&self, f: F) -> T {
725 let mut_borrow = &mut *self.borrow_mut();
726 let replacement = f(mut_borrow);
727 mem::replace(mut_borrow, replacement)
730 /// Swaps the wrapped value of `self` with the wrapped value of `other`,
731 /// without deinitializing either one.
733 /// This function corresponds to [`std::mem::swap`](../mem/fn.swap.html).
737 /// Panics if the value in either `RefCell` is currently borrowed.
742 /// use std::cell::RefCell;
743 /// let c = RefCell::new(5);
744 /// let d = RefCell::new(6);
746 /// assert_eq!(c, RefCell::new(6));
747 /// assert_eq!(d, RefCell::new(5));
750 #[stable(feature = "refcell_swap", since = "1.24.0")]
751 pub fn swap(&self, other: &Self) {
752 mem::swap(&mut *self.borrow_mut(), &mut *other.borrow_mut())
756 impl<T: ?Sized> RefCell<T> {
757 /// Immutably borrows the wrapped value.
759 /// The borrow lasts until the returned `Ref` exits scope. Multiple
760 /// immutable borrows can be taken out at the same time.
764 /// Panics if the value is currently mutably borrowed. For a non-panicking variant, use
765 /// [`try_borrow`](#method.try_borrow).
770 /// use std::cell::RefCell;
772 /// let c = RefCell::new(5);
774 /// let borrowed_five = c.borrow();
775 /// let borrowed_five2 = c.borrow();
778 /// An example of panic:
781 /// use std::cell::RefCell;
783 /// let c = RefCell::new(5);
785 /// let m = c.borrow_mut();
786 /// let b = c.borrow(); // this causes a panic
788 #[stable(feature = "rust1", since = "1.0.0")]
791 pub fn borrow(&self) -> Ref<'_, T> {
792 self.try_borrow().expect("already mutably borrowed")
795 /// Immutably borrows the wrapped value, returning an error if the value is currently mutably
798 /// The borrow lasts until the returned `Ref` exits scope. Multiple immutable borrows can be
799 /// taken out at the same time.
801 /// This is the non-panicking variant of [`borrow`](#method.borrow).
806 /// use std::cell::RefCell;
808 /// let c = RefCell::new(5);
811 /// let m = c.borrow_mut();
812 /// assert!(c.try_borrow().is_err());
816 /// let m = c.borrow();
817 /// assert!(c.try_borrow().is_ok());
820 #[stable(feature = "try_borrow", since = "1.13.0")]
822 pub fn try_borrow(&self) -> Result<Ref<'_, T>, BorrowError> {
823 match BorrowRef::new(&self.borrow) {
824 // SAFETY: `BorrowRef` ensures that there is only immutable access
825 // to the value while borrowed.
826 Some(b) => Ok(Ref { value: unsafe { &*self.value.get() }, borrow: b }),
827 None => Err(BorrowError { _private: () }),
831 /// Mutably borrows the wrapped value.
833 /// The borrow lasts until the returned `RefMut` or all `RefMut`s derived
834 /// from it exit scope. The value cannot be borrowed while this borrow is
839 /// Panics if the value is currently borrowed. For a non-panicking variant, use
840 /// [`try_borrow_mut`](#method.try_borrow_mut).
845 /// use std::cell::RefCell;
847 /// let c = RefCell::new("hello".to_owned());
849 /// *c.borrow_mut() = "bonjour".to_owned();
851 /// assert_eq!(&*c.borrow(), "bonjour");
854 /// An example of panic:
857 /// use std::cell::RefCell;
859 /// let c = RefCell::new(5);
860 /// let m = c.borrow();
862 /// let b = c.borrow_mut(); // this causes a panic
864 #[stable(feature = "rust1", since = "1.0.0")]
867 pub fn borrow_mut(&self) -> RefMut<'_, T> {
868 self.try_borrow_mut().expect("already borrowed")
871 /// Mutably borrows the wrapped value, returning an error if the value is currently borrowed.
873 /// The borrow lasts until the returned `RefMut` or all `RefMut`s derived
874 /// from it exit scope. The value cannot be borrowed while this borrow is
877 /// This is the non-panicking variant of [`borrow_mut`](#method.borrow_mut).
882 /// use std::cell::RefCell;
884 /// let c = RefCell::new(5);
887 /// let m = c.borrow();
888 /// assert!(c.try_borrow_mut().is_err());
891 /// assert!(c.try_borrow_mut().is_ok());
893 #[stable(feature = "try_borrow", since = "1.13.0")]
895 pub fn try_borrow_mut(&self) -> Result<RefMut<'_, T>, BorrowMutError> {
896 match BorrowRefMut::new(&self.borrow) {
897 // SAFETY: `BorrowRef` guarantees unique access.
898 Some(b) => Ok(RefMut { value: unsafe { &mut *self.value.get() }, borrow: b }),
899 None => Err(BorrowMutError { _private: () }),
903 /// Returns a raw pointer to the underlying data in this cell.
908 /// use std::cell::RefCell;
910 /// let c = RefCell::new(5);
912 /// let ptr = c.as_ptr();
915 #[stable(feature = "cell_as_ptr", since = "1.12.0")]
916 pub fn as_ptr(&self) -> *mut T {
920 /// Returns a mutable reference to the underlying data.
922 /// This call borrows `RefCell` mutably (at compile-time) so there is no
923 /// need for dynamic checks.
925 /// However be cautious: this method expects `self` to be mutable, which is
926 /// generally not the case when using a `RefCell`. Take a look at the
927 /// [`borrow_mut`] method instead if `self` isn't mutable.
929 /// Also, please be aware that this method is only for special circumstances and is usually
930 /// not what you want. In case of doubt, use [`borrow_mut`] instead.
932 /// [`borrow_mut`]: RefCell::borrow_mut()
937 /// use std::cell::RefCell;
939 /// let mut c = RefCell::new(5);
940 /// *c.get_mut() += 1;
942 /// assert_eq!(c, RefCell::new(6));
945 #[stable(feature = "cell_get_mut", since = "1.11.0")]
946 pub fn get_mut(&mut self) -> &mut T {
950 /// Undo the effect of leaked guards on the borrow state of the `RefCell`.
952 /// This call is similar to [`get_mut`] but more specialized. It borrows `RefCell` mutably to
953 /// ensure no borrows exist and then resets the state tracking shared borrows. This is relevant
954 /// if some `Ref` or `RefMut` borrows have been leaked.
956 /// [`get_mut`]: RefCell::get_mut()
961 /// #![feature(cell_leak)]
962 /// use std::cell::RefCell;
964 /// let mut c = RefCell::new(0);
965 /// std::mem::forget(c.borrow_mut());
967 /// assert!(c.try_borrow().is_err());
969 /// assert!(c.try_borrow().is_ok());
971 #[unstable(feature = "cell_leak", issue = "69099")]
972 pub fn undo_leak(&mut self) -> &mut T {
973 *self.borrow.get_mut() = UNUSED;
977 /// Immutably borrows the wrapped value, returning an error if the value is
978 /// currently mutably borrowed.
982 /// Unlike `RefCell::borrow`, this method is unsafe because it does not
983 /// return a `Ref`, thus leaving the borrow flag untouched. Mutably
984 /// borrowing the `RefCell` while the reference returned by this method
985 /// is alive is undefined behaviour.
990 /// use std::cell::RefCell;
992 /// let c = RefCell::new(5);
995 /// let m = c.borrow_mut();
996 /// assert!(unsafe { c.try_borrow_unguarded() }.is_err());
1000 /// let m = c.borrow();
1001 /// assert!(unsafe { c.try_borrow_unguarded() }.is_ok());
1004 #[stable(feature = "borrow_state", since = "1.37.0")]
1006 pub unsafe fn try_borrow_unguarded(&self) -> Result<&T, BorrowError> {
1007 if !is_writing(self.borrow.get()) {
1008 // SAFETY: We check that nobody is actively writing now, but it is
1009 // the caller's responsibility to ensure that nobody writes until
1010 // the returned reference is no longer in use.
1011 // Also, `self.value.get()` refers to the value owned by `self`
1012 // and is thus guaranteed to be valid for the lifetime of `self`.
1013 Ok(unsafe { &*self.value.get() })
1015 Err(BorrowError { _private: () })
1020 impl<T: Default> RefCell<T> {
1021 /// Takes the wrapped value, leaving `Default::default()` in its place.
1025 /// Panics if the value is currently borrowed.
1030 /// use std::cell::RefCell;
1032 /// let c = RefCell::new(5);
1033 /// let five = c.take();
1035 /// assert_eq!(five, 5);
1036 /// assert_eq!(c.into_inner(), 0);
1038 #[stable(feature = "refcell_take", since = "1.50.0")]
1039 pub fn take(&self) -> T {
1040 self.replace(Default::default())
1044 #[stable(feature = "rust1", since = "1.0.0")]
1045 unsafe impl<T: ?Sized> Send for RefCell<T> where T: Send {}
1047 #[stable(feature = "rust1", since = "1.0.0")]
1048 impl<T: ?Sized> !Sync for RefCell<T> {}
1050 #[stable(feature = "rust1", since = "1.0.0")]
1051 impl<T: Clone> Clone for RefCell<T> {
1054 /// Panics if the value is currently mutably borrowed.
1057 fn clone(&self) -> RefCell<T> {
1058 RefCell::new(self.borrow().clone())
1062 #[stable(feature = "rust1", since = "1.0.0")]
1063 impl<T: Default> Default for RefCell<T> {
1064 /// Creates a `RefCell<T>`, with the `Default` value for T.
1066 fn default() -> RefCell<T> {
1067 RefCell::new(Default::default())
1071 #[stable(feature = "rust1", since = "1.0.0")]
1072 impl<T: ?Sized + PartialEq> PartialEq for RefCell<T> {
1075 /// Panics if the value in either `RefCell` is currently borrowed.
1077 fn eq(&self, other: &RefCell<T>) -> bool {
1078 *self.borrow() == *other.borrow()
1082 #[stable(feature = "cell_eq", since = "1.2.0")]
1083 impl<T: ?Sized + Eq> Eq for RefCell<T> {}
1085 #[stable(feature = "cell_ord", since = "1.10.0")]
1086 impl<T: ?Sized + PartialOrd> PartialOrd for RefCell<T> {
1089 /// Panics if the value in either `RefCell` is currently borrowed.
1091 fn partial_cmp(&self, other: &RefCell<T>) -> Option<Ordering> {
1092 self.borrow().partial_cmp(&*other.borrow())
1097 /// Panics if the value in either `RefCell` is currently borrowed.
1099 fn lt(&self, other: &RefCell<T>) -> bool {
1100 *self.borrow() < *other.borrow()
1105 /// Panics if the value in either `RefCell` is currently borrowed.
1107 fn le(&self, other: &RefCell<T>) -> bool {
1108 *self.borrow() <= *other.borrow()
1113 /// Panics if the value in either `RefCell` is currently borrowed.
1115 fn gt(&self, other: &RefCell<T>) -> bool {
1116 *self.borrow() > *other.borrow()
1121 /// Panics if the value in either `RefCell` is currently borrowed.
1123 fn ge(&self, other: &RefCell<T>) -> bool {
1124 *self.borrow() >= *other.borrow()
1128 #[stable(feature = "cell_ord", since = "1.10.0")]
1129 impl<T: ?Sized + Ord> Ord for RefCell<T> {
1132 /// Panics if the value in either `RefCell` is currently borrowed.
1134 fn cmp(&self, other: &RefCell<T>) -> Ordering {
1135 self.borrow().cmp(&*other.borrow())
1139 #[stable(feature = "cell_from", since = "1.12.0")]
1140 impl<T> From<T> for RefCell<T> {
1141 fn from(t: T) -> RefCell<T> {
1146 #[unstable(feature = "coerce_unsized", issue = "27732")]
1147 impl<T: CoerceUnsized<U>, U> CoerceUnsized<RefCell<U>> for RefCell<T> {}
1149 struct BorrowRef<'b> {
1150 borrow: &'b Cell<BorrowFlag>,
1153 impl<'b> BorrowRef<'b> {
1155 fn new(borrow: &'b Cell<BorrowFlag>) -> Option<BorrowRef<'b>> {
1156 let b = borrow.get().wrapping_add(1);
1158 // Incrementing borrow can result in a non-reading value (<= 0) in these cases:
1159 // 1. It was < 0, i.e. there are writing borrows, so we can't allow a read borrow
1160 // due to Rust's reference aliasing rules
1161 // 2. It was isize::MAX (the max amount of reading borrows) and it overflowed
1162 // into isize::MIN (the max amount of writing borrows) so we can't allow
1163 // an additional read borrow because isize can't represent so many read borrows
1164 // (this can only happen if you mem::forget more than a small constant amount of
1165 // `Ref`s, which is not good practice)
1168 // Incrementing borrow can result in a reading value (> 0) in these cases:
1169 // 1. It was = 0, i.e. it wasn't borrowed, and we are taking the first read borrow
1170 // 2. It was > 0 and < isize::MAX, i.e. there were read borrows, and isize
1171 // is large enough to represent having one more read borrow
1173 Some(BorrowRef { borrow })
1178 impl Drop for BorrowRef<'_> {
1180 fn drop(&mut self) {
1181 let borrow = self.borrow.get();
1182 debug_assert!(is_reading(borrow));
1183 self.borrow.set(borrow - 1);
1187 impl Clone for BorrowRef<'_> {
1189 fn clone(&self) -> Self {
1190 // Since this Ref exists, we know the borrow flag
1191 // is a reading borrow.
1192 let borrow = self.borrow.get();
1193 debug_assert!(is_reading(borrow));
1194 // Prevent the borrow counter from overflowing into
1195 // a writing borrow.
1196 assert!(borrow != isize::MAX);
1197 self.borrow.set(borrow + 1);
1198 BorrowRef { borrow: self.borrow }
1202 /// Wraps a borrowed reference to a value in a `RefCell` box.
1203 /// A wrapper type for an immutably borrowed value from a `RefCell<T>`.
1205 /// See the [module-level documentation](self) for more.
1206 #[stable(feature = "rust1", since = "1.0.0")]
1207 pub struct Ref<'b, T: ?Sized + 'b> {
1209 borrow: BorrowRef<'b>,
1212 #[stable(feature = "rust1", since = "1.0.0")]
1213 impl<T: ?Sized> Deref for Ref<'_, T> {
1217 fn deref(&self) -> &T {
1222 impl<'b, T: ?Sized> Ref<'b, T> {
1225 /// The `RefCell` is already immutably borrowed, so this cannot fail.
1227 /// This is an associated function that needs to be used as
1228 /// `Ref::clone(...)`. A `Clone` implementation or a method would interfere
1229 /// with the widespread use of `r.borrow().clone()` to clone the contents of
1231 #[stable(feature = "cell_extras", since = "1.15.0")]
1233 pub fn clone(orig: &Ref<'b, T>) -> Ref<'b, T> {
1234 Ref { value: orig.value, borrow: orig.borrow.clone() }
1237 /// Makes a new `Ref` for a component of the borrowed data.
1239 /// The `RefCell` is already immutably borrowed, so this cannot fail.
1241 /// This is an associated function that needs to be used as `Ref::map(...)`.
1242 /// A method would interfere with methods of the same name on the contents
1243 /// of a `RefCell` used through `Deref`.
1248 /// use std::cell::{RefCell, Ref};
1250 /// let c = RefCell::new((5, 'b'));
1251 /// let b1: Ref<(u32, char)> = c.borrow();
1252 /// let b2: Ref<u32> = Ref::map(b1, |t| &t.0);
1253 /// assert_eq!(*b2, 5)
1255 #[stable(feature = "cell_map", since = "1.8.0")]
1257 pub fn map<U: ?Sized, F>(orig: Ref<'b, T>, f: F) -> Ref<'b, U>
1259 F: FnOnce(&T) -> &U,
1261 Ref { value: f(orig.value), borrow: orig.borrow }
1264 /// Makes a new `Ref` for an optional component of the borrowed data.
1266 /// The `RefCell` is already immutably borrowed, so this cannot fail.
1268 /// This is an associated function that needs to be used as
1269 /// `Ref::try_map(...)`. A method would interfere with methods of the same
1270 /// name on the contents of a `RefCell` used through `Deref`.
1275 /// #![feature(cell_try_map)]
1277 /// use std::cell::{RefCell, Ref};
1279 /// let c = RefCell::new(vec![1, 2, 3]);
1280 /// let b1: Ref<Vec<u32>> = c.borrow();
1281 /// let b2: Option<Ref<u32>> = Ref::try_map(b1, |v| v.get(1));
1282 /// assert_eq!(b2.as_deref(), Some(&2))
1284 #[unstable(feature = "cell_try_map", reason = "recently added", issue = "none")]
1286 pub fn try_map<U: ?Sized, F>(orig: Ref<'b, T>, f: F) -> Option<Ref<'b, U>>
1288 F: FnOnce(&T) -> Option<&U>,
1290 let value = f(orig.value)?;
1291 Some(Ref { value, borrow: orig.borrow })
1294 /// Splits a `Ref` into multiple `Ref`s for different components of the
1297 /// The `RefCell` is already immutably borrowed, so this cannot fail.
1299 /// This is an associated function that needs to be used as
1300 /// `Ref::map_split(...)`. A method would interfere with methods of the same
1301 /// name on the contents of a `RefCell` used through `Deref`.
1306 /// use std::cell::{Ref, RefCell};
1308 /// let cell = RefCell::new([1, 2, 3, 4]);
1309 /// let borrow = cell.borrow();
1310 /// let (begin, end) = Ref::map_split(borrow, |slice| slice.split_at(2));
1311 /// assert_eq!(*begin, [1, 2]);
1312 /// assert_eq!(*end, [3, 4]);
1314 #[stable(feature = "refcell_map_split", since = "1.35.0")]
1316 pub fn map_split<U: ?Sized, V: ?Sized, F>(orig: Ref<'b, T>, f: F) -> (Ref<'b, U>, Ref<'b, V>)
1318 F: FnOnce(&T) -> (&U, &V),
1320 let (a, b) = f(orig.value);
1321 let borrow = orig.borrow.clone();
1322 (Ref { value: a, borrow }, Ref { value: b, borrow: orig.borrow })
1325 /// Convert into a reference to the underlying data.
1327 /// The underlying `RefCell` can never be mutably borrowed from again and will always appear
1328 /// already immutably borrowed. It is not a good idea to leak more than a constant number of
1329 /// references. The `RefCell` can be immutably borrowed again if only a smaller number of leaks
1330 /// have occurred in total.
1332 /// This is an associated function that needs to be used as
1333 /// `Ref::leak(...)`. A method would interfere with methods of the
1334 /// same name on the contents of a `RefCell` used through `Deref`.
1339 /// #![feature(cell_leak)]
1340 /// use std::cell::{RefCell, Ref};
1341 /// let cell = RefCell::new(0);
1343 /// let value = Ref::leak(cell.borrow());
1344 /// assert_eq!(*value, 0);
1346 /// assert!(cell.try_borrow().is_ok());
1347 /// assert!(cell.try_borrow_mut().is_err());
1349 #[unstable(feature = "cell_leak", issue = "69099")]
1350 pub fn leak(orig: Ref<'b, T>) -> &'b T {
1351 // By forgetting this Ref we ensure that the borrow counter in the RefCell can't go back to
1352 // UNUSED within the lifetime `'b`. Resetting the reference tracking state would require a
1353 // unique reference to the borrowed RefCell. No further mutable references can be created
1354 // from the original cell.
1355 mem::forget(orig.borrow);
1360 #[unstable(feature = "coerce_unsized", issue = "27732")]
1361 impl<'b, T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<Ref<'b, U>> for Ref<'b, T> {}
1363 #[stable(feature = "std_guard_impls", since = "1.20.0")]
1364 impl<T: ?Sized + fmt::Display> fmt::Display for Ref<'_, T> {
1365 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
1370 impl<'b, T: ?Sized> RefMut<'b, T> {
1371 /// Makes a new `RefMut` for a component of the borrowed data, e.g., an enum
1374 /// The `RefCell` is already mutably borrowed, so this cannot fail.
1376 /// This is an associated function that needs to be used as
1377 /// `RefMut::map(...)`. A method would interfere with methods of the same
1378 /// name on the contents of a `RefCell` used through `Deref`.
1383 /// use std::cell::{RefCell, RefMut};
1385 /// let c = RefCell::new((5, 'b'));
1387 /// let b1: RefMut<(u32, char)> = c.borrow_mut();
1388 /// let mut b2: RefMut<u32> = RefMut::map(b1, |t| &mut t.0);
1389 /// assert_eq!(*b2, 5);
1392 /// assert_eq!(*c.borrow(), (42, 'b'));
1394 #[stable(feature = "cell_map", since = "1.8.0")]
1396 pub fn map<U: ?Sized, F>(orig: RefMut<'b, T>, f: F) -> RefMut<'b, U>
1398 F: FnOnce(&mut T) -> &mut U,
1400 // FIXME(nll-rfc#40): fix borrow-check
1401 let RefMut { value, borrow } = orig;
1402 RefMut { value: f(value), borrow }
1405 /// Makes a new `RefMut` for an optional component of the borrowed data.
1407 /// The `RefCell` is already mutably borrowed, so this cannot fail.
1409 /// This is an associated function that needs to be used as
1410 /// `RefMut::try_map(...)`. A method would interfere with methods of the
1411 /// same name on the contents of a `RefCell` used through `Deref`.
1416 /// #![feature(cell_try_map)]
1418 /// use std::cell::{RefCell, RefMut};
1420 /// let c = RefCell::new(vec![1, 2, 3]);
1423 /// let b1: RefMut<Vec<u32>> = c.borrow_mut();
1424 /// let mut b2: Option<RefMut<u32>> = RefMut::try_map(b1, |v| v.get_mut(1));
1426 /// if let Some(mut b2) = b2 {
1431 /// assert_eq!(*c.borrow(), vec![1, 4, 3]);
1433 #[unstable(feature = "cell_try_map", reason = "recently added", issue = "none")]
1435 pub fn try_map<U: ?Sized, F>(orig: RefMut<'b, T>, f: F) -> Option<RefMut<'b, U>>
1437 F: FnOnce(&mut T) -> Option<&mut U>,
1439 // FIXME(nll-rfc#40): fix borrow-check
1440 let RefMut { value, borrow } = orig;
1441 let value = f(value)?;
1442 Some(RefMut { value, borrow })
1445 /// Splits a `RefMut` into multiple `RefMut`s for different components of the
1448 /// The underlying `RefCell` will remain mutably borrowed until both
1449 /// returned `RefMut`s go out of scope.
1451 /// The `RefCell` is already mutably borrowed, so this cannot fail.
1453 /// This is an associated function that needs to be used as
1454 /// `RefMut::map_split(...)`. A method would interfere with methods of the
1455 /// same name on the contents of a `RefCell` used through `Deref`.
1460 /// use std::cell::{RefCell, RefMut};
1462 /// let cell = RefCell::new([1, 2, 3, 4]);
1463 /// let borrow = cell.borrow_mut();
1464 /// let (mut begin, mut end) = RefMut::map_split(borrow, |slice| slice.split_at_mut(2));
1465 /// assert_eq!(*begin, [1, 2]);
1466 /// assert_eq!(*end, [3, 4]);
1467 /// begin.copy_from_slice(&[4, 3]);
1468 /// end.copy_from_slice(&[2, 1]);
1470 #[stable(feature = "refcell_map_split", since = "1.35.0")]
1472 pub fn map_split<U: ?Sized, V: ?Sized, F>(
1473 orig: RefMut<'b, T>,
1475 ) -> (RefMut<'b, U>, RefMut<'b, V>)
1477 F: FnOnce(&mut T) -> (&mut U, &mut V),
1479 let (a, b) = f(orig.value);
1480 let borrow = orig.borrow.clone();
1481 (RefMut { value: a, borrow }, RefMut { value: b, borrow: orig.borrow })
1484 /// Convert into a mutable reference to the underlying data.
1486 /// The underlying `RefCell` can not be borrowed from again and will always appear already
1487 /// mutably borrowed, making the returned reference the only to the interior.
1489 /// This is an associated function that needs to be used as
1490 /// `RefMut::leak(...)`. A method would interfere with methods of the
1491 /// same name on the contents of a `RefCell` used through `Deref`.
1496 /// #![feature(cell_leak)]
1497 /// use std::cell::{RefCell, RefMut};
1498 /// let cell = RefCell::new(0);
1500 /// let value = RefMut::leak(cell.borrow_mut());
1501 /// assert_eq!(*value, 0);
1504 /// assert!(cell.try_borrow_mut().is_err());
1506 #[unstable(feature = "cell_leak", issue = "69099")]
1507 pub fn leak(orig: RefMut<'b, T>) -> &'b mut T {
1508 // By forgetting this BorrowRefMut we ensure that the borrow counter in the RefCell can't
1509 // go back to UNUSED within the lifetime `'b`. Resetting the reference tracking state would
1510 // require a unique reference to the borrowed RefCell. No further references can be created
1511 // from the original cell within that lifetime, making the current borrow the only
1512 // reference for the remaining lifetime.
1513 mem::forget(orig.borrow);
1518 struct BorrowRefMut<'b> {
1519 borrow: &'b Cell<BorrowFlag>,
1522 impl Drop for BorrowRefMut<'_> {
1524 fn drop(&mut self) {
1525 let borrow = self.borrow.get();
1526 debug_assert!(is_writing(borrow));
1527 self.borrow.set(borrow + 1);
1531 impl<'b> BorrowRefMut<'b> {
1533 fn new(borrow: &'b Cell<BorrowFlag>) -> Option<BorrowRefMut<'b>> {
1534 // NOTE: Unlike BorrowRefMut::clone, new is called to create the initial
1535 // mutable reference, and so there must currently be no existing
1536 // references. Thus, while clone increments the mutable refcount, here
1537 // we explicitly only allow going from UNUSED to UNUSED - 1.
1538 match borrow.get() {
1540 borrow.set(UNUSED - 1);
1541 Some(BorrowRefMut { borrow })
1547 // Clones a `BorrowRefMut`.
1549 // This is only valid if each `BorrowRefMut` is used to track a mutable
1550 // reference to a distinct, nonoverlapping range of the original object.
1551 // This isn't in a Clone impl so that code doesn't call this implicitly.
1553 fn clone(&self) -> BorrowRefMut<'b> {
1554 let borrow = self.borrow.get();
1555 debug_assert!(is_writing(borrow));
1556 // Prevent the borrow counter from underflowing.
1557 assert!(borrow != isize::MIN);
1558 self.borrow.set(borrow - 1);
1559 BorrowRefMut { borrow: self.borrow }
1563 /// A wrapper type for a mutably borrowed value from a `RefCell<T>`.
1565 /// See the [module-level documentation](self) for more.
1566 #[stable(feature = "rust1", since = "1.0.0")]
1567 pub struct RefMut<'b, T: ?Sized + 'b> {
1569 borrow: BorrowRefMut<'b>,
1572 #[stable(feature = "rust1", since = "1.0.0")]
1573 impl<T: ?Sized> Deref for RefMut<'_, T> {
1577 fn deref(&self) -> &T {
1582 #[stable(feature = "rust1", since = "1.0.0")]
1583 impl<T: ?Sized> DerefMut for RefMut<'_, T> {
1585 fn deref_mut(&mut self) -> &mut T {
1590 #[unstable(feature = "coerce_unsized", issue = "27732")]
1591 impl<'b, T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<RefMut<'b, U>> for RefMut<'b, T> {}
1593 #[stable(feature = "std_guard_impls", since = "1.20.0")]
1594 impl<T: ?Sized + fmt::Display> fmt::Display for RefMut<'_, T> {
1595 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
1600 /// The core primitive for interior mutability in Rust.
1602 /// `UnsafeCell<T>` is a type that wraps some `T` and indicates unsafe interior operations on the
1603 /// wrapped type. Types with an `UnsafeCell<T>` field are considered to have an 'unsafe interior'.
1604 /// The `UnsafeCell<T>` type is the only legal way to obtain aliasable data that is considered
1605 /// mutable. In general, transmuting an `&T` type into an `&mut T` is considered undefined behavior.
1607 /// If you have a reference `&SomeStruct`, then normally in Rust all fields of `SomeStruct` are
1608 /// immutable. The compiler makes optimizations based on the knowledge that `&T` is not mutably
1609 /// aliased or mutated, and that `&mut T` is unique. `UnsafeCell<T>` is the only core language
1610 /// feature to work around the restriction that `&T` may not be mutated. All other types that
1611 /// allow internal mutability, such as `Cell<T>` and `RefCell<T>`, use `UnsafeCell` to wrap their
1612 /// internal data. There is *no* legal way to obtain aliasing `&mut`, not even with `UnsafeCell<T>`.
1614 /// The `UnsafeCell` API itself is technically very simple: [`.get()`] gives you a raw pointer
1615 /// `*mut T` to its contents. It is up to _you_ as the abstraction designer to use that raw pointer
1618 /// [`.get()`]: `UnsafeCell::get`
1620 /// The precise Rust aliasing rules are somewhat in flux, but the main points are not contentious:
1622 /// - If you create a safe reference with lifetime `'a` (either a `&T` or `&mut T`
1623 /// reference) that is accessible by safe code (for example, because you returned it),
1624 /// then you must not access the data in any way that contradicts that reference for the
1625 /// remainder of `'a`. For example, this means that if you take the `*mut T` from an
1626 /// `UnsafeCell<T>` and cast it to an `&T`, then the data in `T` must remain immutable
1627 /// (modulo any `UnsafeCell` data found within `T`, of course) until that reference's
1628 /// lifetime expires. Similarly, if you create a `&mut T` reference that is released to
1629 /// safe code, then you must not access the data within the `UnsafeCell` until that
1630 /// reference expires.
1632 /// - At all times, you must avoid data races. If multiple threads have access to
1633 /// the same `UnsafeCell`, then any writes must have a proper happens-before relation to all other
1634 /// accesses (or use atomics).
1636 /// To assist with proper design, the following scenarios are explicitly declared legal
1637 /// for single-threaded code:
1639 /// 1. A `&T` reference can be released to safe code and there it can co-exist with other `&T`
1640 /// references, but not with a `&mut T`
1642 /// 2. A `&mut T` reference may be released to safe code provided neither other `&mut T` nor `&T`
1643 /// co-exist with it. A `&mut T` must always be unique.
1645 /// Note that whilst mutating the contents of an `&UnsafeCell<T>` (even while other
1646 /// `&UnsafeCell<T>` references alias the cell) is
1647 /// ok (provided you enforce the above invariants some other way), it is still undefined behavior
1648 /// to have multiple `&mut UnsafeCell<T>` aliases. That is, `UnsafeCell` is a wrapper
1649 /// designed to have a special interaction with _shared_ accesses (_i.e._, through an
1650 /// `&UnsafeCell<_>` reference); there is no magic whatsoever when dealing with _exclusive_
1651 /// accesses (_e.g._, through an `&mut UnsafeCell<_>`): neither the cell nor the wrapped value
1652 /// may be aliased for the duration of that `&mut` borrow.
1653 /// This is showcased by the [`.get_mut()`] accessor, which is a _safe_ getter that yields
1656 /// [`.get_mut()`]: `UnsafeCell::get_mut`
1660 /// Here is an example showcasing how to soundly mutate the contents of an `UnsafeCell<_>` despite
1661 /// there being multiple references aliasing the cell:
1664 /// use std::cell::UnsafeCell;
1666 /// let x: UnsafeCell<i32> = 42.into();
1667 /// // Get multiple / concurrent / shared references to the same `x`.
1668 /// let (p1, p2): (&UnsafeCell<i32>, &UnsafeCell<i32>) = (&x, &x);
1671 /// // SAFETY: within this scope there are no other references to `x`'s contents,
1672 /// // so ours is effectively unique.
1673 /// let p1_exclusive: &mut i32 = &mut *p1.get(); // -- borrow --+
1674 /// *p1_exclusive += 27; // |
1675 /// } // <---------- cannot go beyond this point -------------------+
1678 /// // SAFETY: within this scope nobody expects to have exclusive access to `x`'s contents,
1679 /// // so we can have multiple shared accesses concurrently.
1680 /// let p2_shared: &i32 = &*p2.get();
1681 /// assert_eq!(*p2_shared, 42 + 27);
1682 /// let p1_shared: &i32 = &*p1.get();
1683 /// assert_eq!(*p1_shared, *p2_shared);
1687 /// The following example showcases the fact that exclusive access to an `UnsafeCell<T>`
1688 /// implies exclusive access to its `T`:
1691 /// #![forbid(unsafe_code)] // with exclusive accesses,
1692 /// // `UnsafeCell` is a transparent no-op wrapper,
1693 /// // so no need for `unsafe` here.
1694 /// use std::cell::UnsafeCell;
1696 /// let mut x: UnsafeCell<i32> = 42.into();
1698 /// // Get a compile-time-checked unique reference to `x`.
1699 /// let p_unique: &mut UnsafeCell<i32> = &mut x;
1700 /// // With an exclusive reference, we can mutate the contents for free.
1701 /// *p_unique.get_mut() = 0;
1702 /// // Or, equivalently:
1703 /// x = UnsafeCell::new(0);
1705 /// // When we own the value, we can extract the contents for free.
1706 /// let contents: i32 = x.into_inner();
1707 /// assert_eq!(contents, 0);
1709 #[lang = "unsafe_cell"]
1710 #[stable(feature = "rust1", since = "1.0.0")]
1711 #[repr(transparent)]
1712 #[repr(no_niche)] // rust-lang/rust#68303.
1713 pub struct UnsafeCell<T: ?Sized> {
1717 #[stable(feature = "rust1", since = "1.0.0")]
1718 impl<T: ?Sized> !Sync for UnsafeCell<T> {}
1720 impl<T> UnsafeCell<T> {
1721 /// Constructs a new instance of `UnsafeCell` which will wrap the specified
1724 /// All access to the inner value through methods is `unsafe`.
1729 /// use std::cell::UnsafeCell;
1731 /// let uc = UnsafeCell::new(5);
1733 #[stable(feature = "rust1", since = "1.0.0")]
1734 #[rustc_const_stable(feature = "const_unsafe_cell_new", since = "1.32.0")]
1736 pub const fn new(value: T) -> UnsafeCell<T> {
1737 UnsafeCell { value }
1740 /// Unwraps the value.
1745 /// use std::cell::UnsafeCell;
1747 /// let uc = UnsafeCell::new(5);
1749 /// let five = uc.into_inner();
1752 #[stable(feature = "rust1", since = "1.0.0")]
1753 #[rustc_const_unstable(feature = "const_cell_into_inner", issue = "78729")]
1754 pub const fn into_inner(self) -> T {
1759 impl<T: ?Sized> UnsafeCell<T> {
1760 /// Gets a mutable pointer to the wrapped value.
1762 /// This can be cast to a pointer of any kind.
1763 /// Ensure that the access is unique (no active references, mutable or not)
1764 /// when casting to `&mut T`, and ensure that there are no mutations
1765 /// or mutable aliases going on when casting to `&T`
1770 /// use std::cell::UnsafeCell;
1772 /// let uc = UnsafeCell::new(5);
1774 /// let five = uc.get();
1777 #[stable(feature = "rust1", since = "1.0.0")]
1778 #[rustc_const_stable(feature = "const_unsafecell_get", since = "1.32.0")]
1779 pub const fn get(&self) -> *mut T {
1780 // We can just cast the pointer from `UnsafeCell<T>` to `T` because of
1781 // #[repr(transparent)]. This exploits libstd's special status, there is
1782 // no guarantee for user code that this will work in future versions of the compiler!
1783 self as *const UnsafeCell<T> as *const T as *mut T
1786 /// Returns a mutable reference to the underlying data.
1788 /// This call borrows the `UnsafeCell` mutably (at compile-time) which
1789 /// guarantees that we possess the only reference.
1794 /// use std::cell::UnsafeCell;
1796 /// let mut c = UnsafeCell::new(5);
1797 /// *c.get_mut() += 1;
1799 /// assert_eq!(*c.get_mut(), 6);
1802 #[stable(feature = "unsafe_cell_get_mut", since = "1.50.0")]
1803 pub fn get_mut(&mut self) -> &mut T {
1807 /// Gets a mutable pointer to the wrapped value.
1808 /// The difference to [`get`] is that this function accepts a raw pointer,
1809 /// which is useful to avoid the creation of temporary references.
1811 /// The result can be cast to a pointer of any kind.
1812 /// Ensure that the access is unique (no active references, mutable or not)
1813 /// when casting to `&mut T`, and ensure that there are no mutations
1814 /// or mutable aliases going on when casting to `&T`.
1816 /// [`get`]: UnsafeCell::get()
1820 /// Gradual initialization of an `UnsafeCell` requires `raw_get`, as
1821 /// calling `get` would require creating a reference to uninitialized data:
1824 /// #![feature(unsafe_cell_raw_get)]
1825 /// use std::cell::UnsafeCell;
1826 /// use std::mem::MaybeUninit;
1828 /// let m = MaybeUninit::<UnsafeCell<i32>>::uninit();
1829 /// unsafe { UnsafeCell::raw_get(m.as_ptr()).write(5); }
1830 /// let uc = unsafe { m.assume_init() };
1832 /// assert_eq!(uc.into_inner(), 5);
1835 #[unstable(feature = "unsafe_cell_raw_get", issue = "66358")]
1836 pub const fn raw_get(this: *const Self) -> *mut T {
1837 // We can just cast the pointer from `UnsafeCell<T>` to `T` because of
1838 // #[repr(transparent)]. This exploits libstd's special status, there is
1839 // no guarantee for user code that this will work in future versions of the compiler!
1840 this as *const T as *mut T
1844 #[stable(feature = "unsafe_cell_default", since = "1.10.0")]
1845 impl<T: Default> Default for UnsafeCell<T> {
1846 /// Creates an `UnsafeCell`, with the `Default` value for T.
1847 fn default() -> UnsafeCell<T> {
1848 UnsafeCell::new(Default::default())
1852 #[stable(feature = "cell_from", since = "1.12.0")]
1853 impl<T> From<T> for UnsafeCell<T> {
1854 fn from(t: T) -> UnsafeCell<T> {
1859 #[unstable(feature = "coerce_unsized", issue = "27732")]
1860 impl<T: CoerceUnsized<U>, U> CoerceUnsized<UnsafeCell<U>> for UnsafeCell<T> {}
1863 fn assert_coerce_unsized(a: UnsafeCell<&i32>, b: Cell<&i32>, c: RefCell<&i32>) {
1864 let _: UnsafeCell<&dyn Send> = a;
1865 let _: Cell<&dyn Send> = b;
1866 let _: RefCell<&dyn Send> = c;