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 //! #![feature(core_intrinsics)]
137 //! use std::cell::Cell;
138 //! use std::ptr::NonNull;
139 //! use std::intrinsics::abort;
141 //! struct Rc<T: ?Sized> {
142 //! ptr: NonNull<RcBox<T>>
145 //! struct RcBox<T: ?Sized> {
146 //! strong: Cell<usize>,
147 //! refcount: Cell<usize>,
151 //! impl<T: ?Sized> Clone for Rc<T> {
152 //! fn clone(&self) -> Rc<T> {
153 //! self.inc_strong();
154 //! Rc { ptr: self.ptr }
158 //! trait RcBoxPtr<T: ?Sized> {
160 //! fn inner(&self) -> &RcBox<T>;
162 //! fn strong(&self) -> usize {
163 //! self.inner().strong.get()
166 //! fn inc_strong(&self) {
169 //! .set(self.strong()
171 //! .unwrap_or_else(|| unsafe { abort() }));
175 //! impl<T: ?Sized> RcBoxPtr<T> for Rc<T> {
176 //! fn inner(&self) -> &RcBox<T> {
178 //! self.ptr.as_ref()
185 #![stable(feature = "rust1", since = "1.0.0")]
187 use crate::cmp::Ordering;
188 use crate::fmt::{self, Debug, Display};
189 use crate::marker::Unsize;
191 use crate::ops::{Deref, DerefMut, CoerceUnsized};
194 /// A mutable memory location.
198 /// In this example, you can see that `Cell<T>` enables mutation inside an
199 /// immutable struct. In other words, it enables "interior mutability".
202 /// use std::cell::Cell;
204 /// struct SomeStruct {
205 /// regular_field: u8,
206 /// special_field: Cell<u8>,
209 /// let my_struct = SomeStruct {
210 /// regular_field: 0,
211 /// special_field: Cell::new(1),
214 /// let new_value = 100;
216 /// // ERROR: `my_struct` is immutable
217 /// // my_struct.regular_field = new_value;
219 /// // WORKS: although `my_struct` is immutable, `special_field` is a `Cell`,
220 /// // which can always be mutated
221 /// my_struct.special_field.set(new_value);
222 /// assert_eq!(my_struct.special_field.get(), new_value);
225 /// See the [module-level documentation](index.html) for more.
226 #[stable(feature = "rust1", since = "1.0.0")]
228 pub struct Cell<T: ?Sized> {
229 value: UnsafeCell<T>,
232 impl<T:Copy> Cell<T> {
233 /// Returns a copy of the contained value.
238 /// use std::cell::Cell;
240 /// let c = Cell::new(5);
242 /// let five = c.get();
245 #[stable(feature = "rust1", since = "1.0.0")]
246 pub fn get(&self) -> T {
247 unsafe{ *self.value.get() }
250 /// Updates the contained value using a function and returns the new value.
255 /// #![feature(cell_update)]
257 /// use std::cell::Cell;
259 /// let c = Cell::new(5);
260 /// let new = c.update(|x| x + 1);
262 /// assert_eq!(new, 6);
263 /// assert_eq!(c.get(), 6);
266 #[unstable(feature = "cell_update", issue = "50186")]
267 pub fn update<F>(&self, f: F) -> T
271 let old = self.get();
278 #[stable(feature = "rust1", since = "1.0.0")]
279 unsafe impl<T: ?Sized> Send for Cell<T> where T: Send {}
281 #[stable(feature = "rust1", since = "1.0.0")]
282 impl<T: ?Sized> !Sync for Cell<T> {}
284 #[stable(feature = "rust1", since = "1.0.0")]
285 impl<T:Copy> Clone for Cell<T> {
287 fn clone(&self) -> Cell<T> {
288 Cell::new(self.get())
292 #[stable(feature = "rust1", since = "1.0.0")]
293 impl<T:Default> Default for Cell<T> {
294 /// Creates a `Cell<T>`, with the `Default` value for T.
296 fn default() -> Cell<T> {
297 Cell::new(Default::default())
301 #[stable(feature = "rust1", since = "1.0.0")]
302 impl<T:PartialEq + Copy> PartialEq for Cell<T> {
304 fn eq(&self, other: &Cell<T>) -> bool {
305 self.get() == other.get()
309 #[stable(feature = "cell_eq", since = "1.2.0")]
310 impl<T:Eq + Copy> Eq for Cell<T> {}
312 #[stable(feature = "cell_ord", since = "1.10.0")]
313 impl<T:PartialOrd + Copy> PartialOrd for Cell<T> {
315 fn partial_cmp(&self, other: &Cell<T>) -> Option<Ordering> {
316 self.get().partial_cmp(&other.get())
320 fn lt(&self, other: &Cell<T>) -> bool {
321 self.get() < other.get()
325 fn le(&self, other: &Cell<T>) -> bool {
326 self.get() <= other.get()
330 fn gt(&self, other: &Cell<T>) -> bool {
331 self.get() > other.get()
335 fn ge(&self, other: &Cell<T>) -> bool {
336 self.get() >= other.get()
340 #[stable(feature = "cell_ord", since = "1.10.0")]
341 impl<T:Ord + Copy> Ord for Cell<T> {
343 fn cmp(&self, other: &Cell<T>) -> Ordering {
344 self.get().cmp(&other.get())
348 #[stable(feature = "cell_from", since = "1.12.0")]
349 impl<T> From<T> for Cell<T> {
350 fn from(t: T) -> Cell<T> {
356 /// Creates a new `Cell` containing the given value.
361 /// use std::cell::Cell;
363 /// let c = Cell::new(5);
365 #[stable(feature = "rust1", since = "1.0.0")]
367 pub const fn new(value: T) -> Cell<T> {
369 value: UnsafeCell::new(value),
373 /// Sets the contained value.
378 /// use std::cell::Cell;
380 /// let c = Cell::new(5);
385 #[stable(feature = "rust1", since = "1.0.0")]
386 pub fn set(&self, val: T) {
387 let old = self.replace(val);
391 /// Swaps the values of two Cells.
392 /// Difference with `std::mem::swap` is that this function doesn't require `&mut` reference.
397 /// use std::cell::Cell;
399 /// let c1 = Cell::new(5i32);
400 /// let c2 = Cell::new(10i32);
402 /// assert_eq!(10, c1.get());
403 /// assert_eq!(5, c2.get());
406 #[stable(feature = "move_cell", since = "1.17.0")]
407 pub fn swap(&self, other: &Self) {
408 if ptr::eq(self, other) {
412 ptr::swap(self.value.get(), other.value.get());
416 /// Replaces the contained value, and returns it.
421 /// use std::cell::Cell;
423 /// let cell = Cell::new(5);
424 /// assert_eq!(cell.get(), 5);
425 /// assert_eq!(cell.replace(10), 5);
426 /// assert_eq!(cell.get(), 10);
428 #[stable(feature = "move_cell", since = "1.17.0")]
429 pub fn replace(&self, val: T) -> T {
430 mem::replace(unsafe { &mut *self.value.get() }, val)
433 /// Unwraps the value.
438 /// use std::cell::Cell;
440 /// let c = Cell::new(5);
441 /// let five = c.into_inner();
443 /// assert_eq!(five, 5);
445 #[stable(feature = "move_cell", since = "1.17.0")]
446 pub fn into_inner(self) -> T {
447 self.value.into_inner()
451 impl<T: ?Sized> Cell<T> {
452 /// Returns a raw pointer to the underlying data in this cell.
457 /// use std::cell::Cell;
459 /// let c = Cell::new(5);
461 /// let ptr = c.as_ptr();
464 #[stable(feature = "cell_as_ptr", since = "1.12.0")]
465 pub const fn as_ptr(&self) -> *mut T {
469 /// Returns a mutable reference to the underlying data.
471 /// This call borrows `Cell` mutably (at compile-time) which guarantees
472 /// that we possess the only reference.
477 /// use std::cell::Cell;
479 /// let mut c = Cell::new(5);
480 /// *c.get_mut() += 1;
482 /// assert_eq!(c.get(), 6);
485 #[stable(feature = "cell_get_mut", since = "1.11.0")]
486 pub fn get_mut(&mut self) -> &mut T {
488 &mut *self.value.get()
492 /// Returns a `&Cell<T>` from a `&mut T`
497 /// #![feature(as_cell)]
498 /// use std::cell::Cell;
500 /// let slice: &mut [i32] = &mut [1, 2, 3];
501 /// let cell_slice: &Cell<[i32]> = Cell::from_mut(slice);
502 /// let slice_cell: &[Cell<i32>] = cell_slice.as_slice_of_cells();
504 /// assert_eq!(slice_cell.len(), 3);
507 #[unstable(feature = "as_cell", issue="43038")]
508 pub fn from_mut(t: &mut T) -> &Cell<T> {
510 &*(t as *mut T as *const Cell<T>)
515 impl<T: Default> Cell<T> {
516 /// Takes the value of the cell, leaving `Default::default()` in its place.
521 /// use std::cell::Cell;
523 /// let c = Cell::new(5);
524 /// let five = c.take();
526 /// assert_eq!(five, 5);
527 /// assert_eq!(c.into_inner(), 0);
529 #[stable(feature = "move_cell", since = "1.17.0")]
530 pub fn take(&self) -> T {
531 self.replace(Default::default())
535 #[unstable(feature = "coerce_unsized", issue = "27732")]
536 impl<T: CoerceUnsized<U>, U> CoerceUnsized<Cell<U>> for Cell<T> {}
539 /// Returns a `&[Cell<T>]` from a `&Cell<[T]>`
544 /// #![feature(as_cell)]
545 /// use std::cell::Cell;
547 /// let slice: &mut [i32] = &mut [1, 2, 3];
548 /// let cell_slice: &Cell<[i32]> = Cell::from_mut(slice);
549 /// let slice_cell: &[Cell<i32>] = cell_slice.as_slice_of_cells();
551 /// assert_eq!(slice_cell.len(), 3);
553 #[unstable(feature = "as_cell", issue="43038")]
554 pub fn as_slice_of_cells(&self) -> &[Cell<T>] {
556 &*(self as *const Cell<[T]> as *const [Cell<T>])
561 /// A mutable memory location with dynamically checked borrow rules
563 /// See the [module-level documentation](index.html) for more.
564 #[stable(feature = "rust1", since = "1.0.0")]
565 pub struct RefCell<T: ?Sized> {
566 borrow: Cell<BorrowFlag>,
567 value: UnsafeCell<T>,
570 /// An error returned by [`RefCell::try_borrow`](struct.RefCell.html#method.try_borrow).
571 #[stable(feature = "try_borrow", since = "1.13.0")]
572 pub struct BorrowError {
576 #[stable(feature = "try_borrow", since = "1.13.0")]
577 impl Debug for BorrowError {
578 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
579 f.debug_struct("BorrowError").finish()
583 #[stable(feature = "try_borrow", since = "1.13.0")]
584 impl Display for BorrowError {
585 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
586 Display::fmt("already mutably borrowed", f)
590 /// An error returned by [`RefCell::try_borrow_mut`](struct.RefCell.html#method.try_borrow_mut).
591 #[stable(feature = "try_borrow", since = "1.13.0")]
592 pub struct BorrowMutError {
596 #[stable(feature = "try_borrow", since = "1.13.0")]
597 impl Debug for BorrowMutError {
598 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
599 f.debug_struct("BorrowMutError").finish()
603 #[stable(feature = "try_borrow", since = "1.13.0")]
604 impl Display for BorrowMutError {
605 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
606 Display::fmt("already borrowed", f)
610 // Positive values represent the number of `Ref` active. Negative values
611 // represent the number of `RefMut` active. Multiple `RefMut`s can only be
612 // active at a time if they refer to distinct, nonoverlapping components of a
613 // `RefCell` (e.g., different ranges of a slice).
615 // `Ref` and `RefMut` are both two words in size, and so there will likely never
616 // be enough `Ref`s or `RefMut`s in existence to overflow half of the `usize`
617 // range. Thus, a `BorrowFlag` will probably never overflow or underflow.
618 // However, this is not a guarantee, as a pathological program could repeatedly
619 // create and then mem::forget `Ref`s or `RefMut`s. Thus, all code must
620 // explicitly check for overflow and underflow in order to avoid unsafety, or at
621 // least behave correctly in the event that overflow or underflow happens (e.g.,
622 // see BorrowRef::new).
623 type BorrowFlag = isize;
624 const UNUSED: BorrowFlag = 0;
627 fn is_writing(x: BorrowFlag) -> bool {
632 fn is_reading(x: BorrowFlag) -> bool {
637 /// Creates a new `RefCell` containing `value`.
642 /// use std::cell::RefCell;
644 /// let c = RefCell::new(5);
646 #[stable(feature = "rust1", since = "1.0.0")]
648 pub const fn new(value: T) -> RefCell<T> {
650 value: UnsafeCell::new(value),
651 borrow: Cell::new(UNUSED),
655 /// Consumes the `RefCell`, returning the wrapped value.
660 /// use std::cell::RefCell;
662 /// let c = RefCell::new(5);
664 /// let five = c.into_inner();
666 #[stable(feature = "rust1", since = "1.0.0")]
668 pub fn into_inner(self) -> T {
669 // Since this function takes `self` (the `RefCell`) by value, the
670 // compiler statically verifies that it is not currently borrowed.
671 // Therefore the following assertion is just a `debug_assert!`.
672 debug_assert!(self.borrow.get() == UNUSED);
673 self.value.into_inner()
676 /// Replaces the wrapped value with a new one, returning the old value,
677 /// without deinitializing either one.
679 /// This function corresponds to [`std::mem::replace`](../mem/fn.replace.html).
683 /// Panics if the value is currently borrowed.
688 /// use std::cell::RefCell;
689 /// let cell = RefCell::new(5);
690 /// let old_value = cell.replace(6);
691 /// assert_eq!(old_value, 5);
692 /// assert_eq!(cell, RefCell::new(6));
695 #[stable(feature = "refcell_replace", since="1.24.0")]
696 pub fn replace(&self, t: T) -> T {
697 mem::replace(&mut *self.borrow_mut(), t)
700 /// Replaces the wrapped value with a new one computed from `f`, returning
701 /// the old value, without deinitializing either one.
705 /// Panics if the value is currently borrowed.
710 /// use std::cell::RefCell;
711 /// let cell = RefCell::new(5);
712 /// let old_value = cell.replace_with(|&mut old| old + 1);
713 /// assert_eq!(old_value, 5);
714 /// assert_eq!(cell, RefCell::new(6));
717 #[stable(feature = "refcell_replace_swap", since="1.35.0")]
718 pub fn replace_with<F: FnOnce(&mut T) -> T>(&self, f: F) -> T {
719 let mut_borrow = &mut *self.borrow_mut();
720 let replacement = f(mut_borrow);
721 mem::replace(mut_borrow, replacement)
724 /// Swaps the wrapped value of `self` with the wrapped value of `other`,
725 /// without deinitializing either one.
727 /// This function corresponds to [`std::mem::swap`](../mem/fn.swap.html).
731 /// Panics if the value in either `RefCell` is currently borrowed.
736 /// use std::cell::RefCell;
737 /// let c = RefCell::new(5);
738 /// let d = RefCell::new(6);
740 /// assert_eq!(c, RefCell::new(6));
741 /// assert_eq!(d, RefCell::new(5));
744 #[stable(feature = "refcell_swap", since="1.24.0")]
745 pub fn swap(&self, other: &Self) {
746 mem::swap(&mut *self.borrow_mut(), &mut *other.borrow_mut())
750 impl<T: ?Sized> RefCell<T> {
751 /// Immutably borrows the wrapped value.
753 /// The borrow lasts until the returned `Ref` exits scope. Multiple
754 /// immutable borrows can be taken out at the same time.
758 /// Panics if the value is currently mutably borrowed. For a non-panicking variant, use
759 /// [`try_borrow`](#method.try_borrow).
764 /// use std::cell::RefCell;
766 /// let c = RefCell::new(5);
768 /// let borrowed_five = c.borrow();
769 /// let borrowed_five2 = c.borrow();
772 /// An example of panic:
775 /// use std::cell::RefCell;
778 /// let result = thread::spawn(move || {
779 /// let c = RefCell::new(5);
780 /// let m = c.borrow_mut();
782 /// let b = c.borrow(); // this causes a panic
785 /// assert!(result.is_err());
787 #[stable(feature = "rust1", since = "1.0.0")]
789 pub fn borrow(&self) -> Ref<'_, T> {
790 self.try_borrow().expect("already mutably borrowed")
793 /// Immutably borrows the wrapped value, returning an error if the value is currently mutably
796 /// The borrow lasts until the returned `Ref` exits scope. Multiple immutable borrows can be
797 /// taken out at the same time.
799 /// This is the non-panicking variant of [`borrow`](#method.borrow).
804 /// use std::cell::RefCell;
806 /// let c = RefCell::new(5);
809 /// let m = c.borrow_mut();
810 /// assert!(c.try_borrow().is_err());
814 /// let m = c.borrow();
815 /// assert!(c.try_borrow().is_ok());
818 #[stable(feature = "try_borrow", since = "1.13.0")]
820 pub fn try_borrow(&self) -> Result<Ref<'_, T>, BorrowError> {
821 match BorrowRef::new(&self.borrow) {
823 value: unsafe { &*self.value.get() },
826 None => Err(BorrowError { _private: () }),
830 /// Mutably borrows the wrapped value.
832 /// The borrow lasts until the returned `RefMut` or all `RefMut`s derived
833 /// from it exit scope. The value cannot be borrowed while this borrow is
838 /// Panics if the value is currently borrowed. For a non-panicking variant, use
839 /// [`try_borrow_mut`](#method.try_borrow_mut).
844 /// use std::cell::RefCell;
846 /// let c = RefCell::new(5);
848 /// *c.borrow_mut() = 7;
850 /// assert_eq!(*c.borrow(), 7);
853 /// An example of panic:
856 /// use std::cell::RefCell;
859 /// let result = thread::spawn(move || {
860 /// let c = RefCell::new(5);
861 /// let m = c.borrow();
863 /// let b = c.borrow_mut(); // this causes a panic
866 /// assert!(result.is_err());
868 #[stable(feature = "rust1", since = "1.0.0")]
870 pub fn borrow_mut(&self) -> RefMut<'_, T> {
871 self.try_borrow_mut().expect("already borrowed")
874 /// Mutably borrows the wrapped value, returning an error if the value is currently borrowed.
876 /// The borrow lasts until the returned `RefMut` or all `RefMut`s derived
877 /// from it exit scope. The value cannot be borrowed while this borrow is
880 /// This is the non-panicking variant of [`borrow_mut`](#method.borrow_mut).
885 /// use std::cell::RefCell;
887 /// let c = RefCell::new(5);
890 /// let m = c.borrow();
891 /// assert!(c.try_borrow_mut().is_err());
894 /// assert!(c.try_borrow_mut().is_ok());
896 #[stable(feature = "try_borrow", since = "1.13.0")]
898 pub fn try_borrow_mut(&self) -> Result<RefMut<'_, T>, BorrowMutError> {
899 match BorrowRefMut::new(&self.borrow) {
900 Some(b) => Ok(RefMut {
901 value: unsafe { &mut *self.value.get() },
904 None => Err(BorrowMutError { _private: () }),
908 /// Returns a raw pointer to the underlying data in this cell.
913 /// use std::cell::RefCell;
915 /// let c = RefCell::new(5);
917 /// let ptr = c.as_ptr();
920 #[stable(feature = "cell_as_ptr", since = "1.12.0")]
921 pub fn as_ptr(&self) -> *mut T {
925 /// Returns a mutable reference to the underlying data.
927 /// This call borrows `RefCell` mutably (at compile-time) so there is no
928 /// need for dynamic checks.
930 /// However be cautious: this method expects `self` to be mutable, which is
931 /// generally not the case when using a `RefCell`. Take a look at the
932 /// [`borrow_mut`] method instead if `self` isn't mutable.
934 /// Also, please be aware that this method is only for special circumstances and is usually
935 /// not what you want. In case of doubt, use [`borrow_mut`] instead.
937 /// [`borrow_mut`]: #method.borrow_mut
942 /// use std::cell::RefCell;
944 /// let mut c = RefCell::new(5);
945 /// *c.get_mut() += 1;
947 /// assert_eq!(c, RefCell::new(6));
950 #[stable(feature = "cell_get_mut", since = "1.11.0")]
951 pub fn get_mut(&mut self) -> &mut T {
953 &mut *self.value.get()
957 /// Immutably borrows the wrapped value, returning an error if the value is
958 /// currently mutably borrowed.
962 /// Unlike `RefCell::borrow`, this method is unsafe because it does not
963 /// return a `Ref`, thus leaving the borrow flag untouched. Mutably
964 /// borrowing the `RefCell` while the reference returned by this method
965 /// is alive is undefined behaviour.
970 /// use std::cell::RefCell;
972 /// let c = RefCell::new(5);
975 /// let m = c.borrow_mut();
976 /// assert!(unsafe { c.try_borrow_unguarded() }.is_err());
980 /// let m = c.borrow();
981 /// assert!(unsafe { c.try_borrow_unguarded() }.is_ok());
984 #[stable(feature = "borrow_state", since = "1.37.0")]
986 pub unsafe fn try_borrow_unguarded(&self) -> Result<&T, BorrowError> {
987 if !is_writing(self.borrow.get()) {
988 Ok(&*self.value.get())
990 Err(BorrowError { _private: () })
995 #[stable(feature = "rust1", since = "1.0.0")]
996 unsafe impl<T: ?Sized> Send for RefCell<T> where T: Send {}
998 #[stable(feature = "rust1", since = "1.0.0")]
999 impl<T: ?Sized> !Sync for RefCell<T> {}
1001 #[stable(feature = "rust1", since = "1.0.0")]
1002 impl<T: Clone> Clone for RefCell<T> {
1005 /// Panics if the value is currently mutably borrowed.
1007 fn clone(&self) -> RefCell<T> {
1008 RefCell::new(self.borrow().clone())
1012 #[stable(feature = "rust1", since = "1.0.0")]
1013 impl<T:Default> Default for RefCell<T> {
1014 /// Creates a `RefCell<T>`, with the `Default` value for T.
1016 fn default() -> RefCell<T> {
1017 RefCell::new(Default::default())
1021 #[stable(feature = "rust1", since = "1.0.0")]
1022 impl<T: ?Sized + PartialEq> PartialEq for RefCell<T> {
1025 /// Panics if the value in either `RefCell` is currently borrowed.
1027 fn eq(&self, other: &RefCell<T>) -> bool {
1028 *self.borrow() == *other.borrow()
1032 #[stable(feature = "cell_eq", since = "1.2.0")]
1033 impl<T: ?Sized + Eq> Eq for RefCell<T> {}
1035 #[stable(feature = "cell_ord", since = "1.10.0")]
1036 impl<T: ?Sized + PartialOrd> PartialOrd for RefCell<T> {
1039 /// Panics if the value in either `RefCell` is currently borrowed.
1041 fn partial_cmp(&self, other: &RefCell<T>) -> Option<Ordering> {
1042 self.borrow().partial_cmp(&*other.borrow())
1047 /// Panics if the value in either `RefCell` is currently borrowed.
1049 fn lt(&self, other: &RefCell<T>) -> bool {
1050 *self.borrow() < *other.borrow()
1055 /// Panics if the value in either `RefCell` is currently borrowed.
1057 fn le(&self, other: &RefCell<T>) -> bool {
1058 *self.borrow() <= *other.borrow()
1063 /// Panics if the value in either `RefCell` is currently borrowed.
1065 fn gt(&self, other: &RefCell<T>) -> bool {
1066 *self.borrow() > *other.borrow()
1071 /// Panics if the value in either `RefCell` is currently borrowed.
1073 fn ge(&self, other: &RefCell<T>) -> bool {
1074 *self.borrow() >= *other.borrow()
1078 #[stable(feature = "cell_ord", since = "1.10.0")]
1079 impl<T: ?Sized + Ord> Ord for RefCell<T> {
1082 /// Panics if the value in either `RefCell` is currently borrowed.
1084 fn cmp(&self, other: &RefCell<T>) -> Ordering {
1085 self.borrow().cmp(&*other.borrow())
1089 #[stable(feature = "cell_from", since = "1.12.0")]
1090 impl<T> From<T> for RefCell<T> {
1091 fn from(t: T) -> RefCell<T> {
1096 #[unstable(feature = "coerce_unsized", issue = "27732")]
1097 impl<T: CoerceUnsized<U>, U> CoerceUnsized<RefCell<U>> for RefCell<T> {}
1099 struct BorrowRef<'b> {
1100 borrow: &'b Cell<BorrowFlag>,
1103 impl<'b> BorrowRef<'b> {
1105 fn new(borrow: &'b Cell<BorrowFlag>) -> Option<BorrowRef<'b>> {
1106 let b = borrow.get();
1107 if is_writing(b) || b == isize::max_value() {
1108 // If there's currently a writing borrow, or if incrementing the
1109 // refcount would overflow into a writing borrow.
1113 Some(BorrowRef { borrow })
1118 impl Drop for BorrowRef<'_> {
1120 fn drop(&mut self) {
1121 let borrow = self.borrow.get();
1122 debug_assert!(is_reading(borrow));
1123 self.borrow.set(borrow - 1);
1127 impl Clone for BorrowRef<'_> {
1129 fn clone(&self) -> Self {
1130 // Since this Ref exists, we know the borrow flag
1131 // is a reading borrow.
1132 let borrow = self.borrow.get();
1133 debug_assert!(is_reading(borrow));
1134 // Prevent the borrow counter from overflowing into
1135 // a writing borrow.
1136 assert!(borrow != isize::max_value());
1137 self.borrow.set(borrow + 1);
1138 BorrowRef { borrow: self.borrow }
1142 /// Wraps a borrowed reference to a value in a `RefCell` box.
1143 /// A wrapper type for an immutably borrowed value from a `RefCell<T>`.
1145 /// See the [module-level documentation](index.html) for more.
1146 #[stable(feature = "rust1", since = "1.0.0")]
1147 pub struct Ref<'b, T: ?Sized + 'b> {
1149 borrow: BorrowRef<'b>,
1152 #[stable(feature = "rust1", since = "1.0.0")]
1153 impl<T: ?Sized> Deref for Ref<'_, T> {
1157 fn deref(&self) -> &T {
1162 impl<'b, T: ?Sized> Ref<'b, T> {
1165 /// The `RefCell` is already immutably borrowed, so this cannot fail.
1167 /// This is an associated function that needs to be used as
1168 /// `Ref::clone(...)`. A `Clone` implementation or a method would interfere
1169 /// with the widespread use of `r.borrow().clone()` to clone the contents of
1171 #[stable(feature = "cell_extras", since = "1.15.0")]
1173 pub fn clone(orig: &Ref<'b, T>) -> Ref<'b, T> {
1176 borrow: orig.borrow.clone(),
1180 /// Makes a new `Ref` for a component of the borrowed data.
1182 /// The `RefCell` is already immutably borrowed, so this cannot fail.
1184 /// This is an associated function that needs to be used as `Ref::map(...)`.
1185 /// A method would interfere with methods of the same name on the contents
1186 /// of a `RefCell` used through `Deref`.
1191 /// use std::cell::{RefCell, Ref};
1193 /// let c = RefCell::new((5, 'b'));
1194 /// let b1: Ref<(u32, char)> = c.borrow();
1195 /// let b2: Ref<u32> = Ref::map(b1, |t| &t.0);
1196 /// assert_eq!(*b2, 5)
1198 #[stable(feature = "cell_map", since = "1.8.0")]
1200 pub fn map<U: ?Sized, F>(orig: Ref<'b, T>, f: F) -> Ref<'b, U>
1201 where F: FnOnce(&T) -> &U
1204 value: f(orig.value),
1205 borrow: orig.borrow,
1209 /// Splits a `Ref` into multiple `Ref`s for different components of the
1212 /// The `RefCell` is already immutably borrowed, so this cannot fail.
1214 /// This is an associated function that needs to be used as
1215 /// `Ref::map_split(...)`. A method would interfere with methods of the same
1216 /// name on the contents of a `RefCell` used through `Deref`.
1221 /// use std::cell::{Ref, RefCell};
1223 /// let cell = RefCell::new([1, 2, 3, 4]);
1224 /// let borrow = cell.borrow();
1225 /// let (begin, end) = Ref::map_split(borrow, |slice| slice.split_at(2));
1226 /// assert_eq!(*begin, [1, 2]);
1227 /// assert_eq!(*end, [3, 4]);
1229 #[stable(feature = "refcell_map_split", since = "1.35.0")]
1231 pub fn map_split<U: ?Sized, V: ?Sized, F>(orig: Ref<'b, T>, f: F) -> (Ref<'b, U>, Ref<'b, V>)
1232 where F: FnOnce(&T) -> (&U, &V)
1234 let (a, b) = f(orig.value);
1235 let borrow = orig.borrow.clone();
1236 (Ref { value: a, borrow }, Ref { value: b, borrow: orig.borrow })
1240 #[unstable(feature = "coerce_unsized", issue = "27732")]
1241 impl<'b, T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<Ref<'b, U>> for Ref<'b, T> {}
1243 #[stable(feature = "std_guard_impls", since = "1.20.0")]
1244 impl<T: ?Sized + fmt::Display> fmt::Display for Ref<'_, T> {
1245 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
1250 impl<'b, T: ?Sized> RefMut<'b, T> {
1251 /// Makes a new `RefMut` for a component of the borrowed data, e.g., an enum
1254 /// The `RefCell` is already mutably borrowed, so this cannot fail.
1256 /// This is an associated function that needs to be used as
1257 /// `RefMut::map(...)`. A method would interfere with methods of the same
1258 /// name on the contents of a `RefCell` used through `Deref`.
1263 /// use std::cell::{RefCell, RefMut};
1265 /// let c = RefCell::new((5, 'b'));
1267 /// let b1: RefMut<(u32, char)> = c.borrow_mut();
1268 /// let mut b2: RefMut<u32> = RefMut::map(b1, |t| &mut t.0);
1269 /// assert_eq!(*b2, 5);
1272 /// assert_eq!(*c.borrow(), (42, 'b'));
1274 #[stable(feature = "cell_map", since = "1.8.0")]
1276 pub fn map<U: ?Sized, F>(orig: RefMut<'b, T>, f: F) -> RefMut<'b, U>
1277 where F: FnOnce(&mut T) -> &mut U
1279 // FIXME(nll-rfc#40): fix borrow-check
1280 let RefMut { value, borrow } = orig;
1287 /// Splits a `RefMut` into multiple `RefMut`s for different components of the
1290 /// The underlying `RefCell` will remain mutably borrowed until both
1291 /// returned `RefMut`s go out of scope.
1293 /// The `RefCell` is already mutably borrowed, so this cannot fail.
1295 /// This is an associated function that needs to be used as
1296 /// `RefMut::map_split(...)`. A method would interfere with methods of the
1297 /// same name on the contents of a `RefCell` used through `Deref`.
1302 /// use std::cell::{RefCell, RefMut};
1304 /// let cell = RefCell::new([1, 2, 3, 4]);
1305 /// let borrow = cell.borrow_mut();
1306 /// let (mut begin, mut end) = RefMut::map_split(borrow, |slice| slice.split_at_mut(2));
1307 /// assert_eq!(*begin, [1, 2]);
1308 /// assert_eq!(*end, [3, 4]);
1309 /// begin.copy_from_slice(&[4, 3]);
1310 /// end.copy_from_slice(&[2, 1]);
1312 #[stable(feature = "refcell_map_split", since = "1.35.0")]
1314 pub fn map_split<U: ?Sized, V: ?Sized, F>(
1315 orig: RefMut<'b, T>, f: F
1316 ) -> (RefMut<'b, U>, RefMut<'b, V>)
1317 where F: FnOnce(&mut T) -> (&mut U, &mut V)
1319 let (a, b) = f(orig.value);
1320 let borrow = orig.borrow.clone();
1321 (RefMut { value: a, borrow }, RefMut { value: b, borrow: orig.borrow })
1325 struct BorrowRefMut<'b> {
1326 borrow: &'b Cell<BorrowFlag>,
1329 impl Drop for BorrowRefMut<'_> {
1331 fn drop(&mut self) {
1332 let borrow = self.borrow.get();
1333 debug_assert!(is_writing(borrow));
1334 self.borrow.set(borrow + 1);
1338 impl<'b> BorrowRefMut<'b> {
1340 fn new(borrow: &'b Cell<BorrowFlag>) -> Option<BorrowRefMut<'b>> {
1341 // NOTE: Unlike BorrowRefMut::clone, new is called to create the initial
1342 // mutable reference, and so there must currently be no existing
1343 // references. Thus, while clone increments the mutable refcount, here
1344 // we explicitly only allow going from UNUSED to UNUSED - 1.
1345 match borrow.get() {
1347 borrow.set(UNUSED - 1);
1348 Some(BorrowRefMut { borrow })
1354 // Clone a `BorrowRefMut`.
1356 // This is only valid if each `BorrowRefMut` is used to track a mutable
1357 // reference to a distinct, nonoverlapping range of the original object.
1358 // This isn't in a Clone impl so that code doesn't call this implicitly.
1360 fn clone(&self) -> BorrowRefMut<'b> {
1361 let borrow = self.borrow.get();
1362 debug_assert!(is_writing(borrow));
1363 // Prevent the borrow counter from underflowing.
1364 assert!(borrow != isize::min_value());
1365 self.borrow.set(borrow - 1);
1366 BorrowRefMut { borrow: self.borrow }
1370 /// A wrapper type for a mutably borrowed value from a `RefCell<T>`.
1372 /// See the [module-level documentation](index.html) for more.
1373 #[stable(feature = "rust1", since = "1.0.0")]
1374 pub struct RefMut<'b, T: ?Sized + 'b> {
1376 borrow: BorrowRefMut<'b>,
1379 #[stable(feature = "rust1", since = "1.0.0")]
1380 impl<T: ?Sized> Deref for RefMut<'_, T> {
1384 fn deref(&self) -> &T {
1389 #[stable(feature = "rust1", since = "1.0.0")]
1390 impl<T: ?Sized> DerefMut for RefMut<'_, T> {
1392 fn deref_mut(&mut self) -> &mut T {
1397 #[unstable(feature = "coerce_unsized", issue = "27732")]
1398 impl<'b, T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<RefMut<'b, U>> for RefMut<'b, T> {}
1400 #[stable(feature = "std_guard_impls", since = "1.20.0")]
1401 impl<T: ?Sized + fmt::Display> fmt::Display for RefMut<'_, T> {
1402 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
1407 /// The core primitive for interior mutability in Rust.
1409 /// `UnsafeCell<T>` is a type that wraps some `T` and indicates unsafe interior operations on the
1410 /// wrapped type. Types with an `UnsafeCell<T>` field are considered to have an 'unsafe interior'.
1411 /// The `UnsafeCell<T>` type is the only legal way to obtain aliasable data that is considered
1412 /// mutable. In general, transmuting an `&T` type into an `&mut T` is considered undefined behavior.
1414 /// If you have a reference `&SomeStruct`, then normally in Rust all fields of `SomeStruct` are
1415 /// immutable. The compiler makes optimizations based on the knowledge that `&T` is not mutably
1416 /// aliased or mutated, and that `&mut T` is unique. `UnsafeCell<T>` is the only core language
1417 /// feature to work around this restriction. All other types that allow internal mutability, such as
1418 /// `Cell<T>` and `RefCell<T>`, use `UnsafeCell` to wrap their internal data.
1420 /// The `UnsafeCell` API itself is technically very simple: it gives you a raw pointer `*mut T` to
1421 /// its contents. It is up to _you_ as the abstraction designer to use that raw pointer correctly.
1423 /// The precise Rust aliasing rules are somewhat in flux, but the main points are not contentious:
1425 /// - If you create a safe reference with lifetime `'a` (either a `&T` or `&mut T`
1426 /// reference) that is accessible by safe code (for example, because you returned it),
1427 /// then you must not access the data in any way that contradicts that reference for the
1428 /// remainder of `'a`. For example, this means that if you take the `*mut T` from an
1429 /// `UnsafeCell<T>` and cast it to an `&T`, then the data in `T` must remain immutable
1430 /// (modulo any `UnsafeCell` data found within `T`, of course) until that reference's
1431 /// lifetime expires. Similarly, if you create a `&mut T` reference that is released to
1432 /// safe code, then you must not access the data within the `UnsafeCell` until that
1433 /// reference expires.
1435 /// - At all times, you must avoid data races. If multiple threads have access to
1436 /// the same `UnsafeCell`, then any writes must have a proper happens-before relation to all other
1437 /// accesses (or use atomics).
1439 /// To assist with proper design, the following scenarios are explicitly declared legal
1440 /// for single-threaded code:
1442 /// 1. A `&T` reference can be released to safe code and there it can co-exist with other `&T`
1443 /// references, but not with a `&mut T`
1445 /// 2. A `&mut T` reference may be released to safe code provided neither other `&mut T` nor `&T`
1446 /// co-exist with it. A `&mut T` must always be unique.
1448 /// Note that while mutating or mutably aliasing the contents of an `&UnsafeCell<T>` is
1449 /// ok (provided you enforce the invariants some other way), it is still undefined behavior
1450 /// to have multiple `&mut UnsafeCell<T>` aliases.
1455 /// use std::cell::UnsafeCell;
1457 /// # #[allow(dead_code)]
1458 /// struct NotThreadSafe<T> {
1459 /// value: UnsafeCell<T>,
1462 /// unsafe impl<T> Sync for NotThreadSafe<T> {}
1464 #[lang = "unsafe_cell"]
1465 #[stable(feature = "rust1", since = "1.0.0")]
1466 #[repr(transparent)]
1467 pub struct UnsafeCell<T: ?Sized> {
1471 #[stable(feature = "rust1", since = "1.0.0")]
1472 impl<T: ?Sized> !Sync for UnsafeCell<T> {}
1474 impl<T> UnsafeCell<T> {
1475 /// Constructs a new instance of `UnsafeCell` which will wrap the specified
1478 /// All access to the inner value through methods is `unsafe`.
1483 /// use std::cell::UnsafeCell;
1485 /// let uc = UnsafeCell::new(5);
1487 #[stable(feature = "rust1", since = "1.0.0")]
1489 pub const fn new(value: T) -> UnsafeCell<T> {
1490 UnsafeCell { value }
1493 /// Unwraps the value.
1498 /// use std::cell::UnsafeCell;
1500 /// let uc = UnsafeCell::new(5);
1502 /// let five = uc.into_inner();
1505 #[stable(feature = "rust1", since = "1.0.0")]
1506 pub fn into_inner(self) -> T {
1511 impl<T: ?Sized> UnsafeCell<T> {
1512 /// Gets a mutable pointer to the wrapped value.
1514 /// This can be cast to a pointer of any kind.
1515 /// Ensure that the access is unique (no active references, mutable or not)
1516 /// when casting to `&mut T`, and ensure that there are no mutations
1517 /// or mutable aliases going on when casting to `&T`
1522 /// use std::cell::UnsafeCell;
1524 /// let uc = UnsafeCell::new(5);
1526 /// let five = uc.get();
1529 #[stable(feature = "rust1", since = "1.0.0")]
1530 pub const fn get(&self) -> *mut T {
1531 // We can just cast the pointer from `UnsafeCell<T>` to `T` because of
1532 // #[repr(transparent)]
1533 self as *const UnsafeCell<T> as *const T as *mut T
1537 #[stable(feature = "unsafe_cell_default", since = "1.10.0")]
1538 impl<T: Default> Default for UnsafeCell<T> {
1539 /// Creates an `UnsafeCell`, with the `Default` value for T.
1540 fn default() -> UnsafeCell<T> {
1541 UnsafeCell::new(Default::default())
1545 #[stable(feature = "cell_from", since = "1.12.0")]
1546 impl<T> From<T> for UnsafeCell<T> {
1547 fn from(t: T) -> UnsafeCell<T> {
1552 #[unstable(feature = "coerce_unsized", issue = "27732")]
1553 impl<T: CoerceUnsized<U>, U> CoerceUnsized<UnsafeCell<U>> for UnsafeCell<T> {}
1556 fn assert_coerce_unsized(a: UnsafeCell<&i32>, b: Cell<&i32>, c: RefCell<&i32>) {
1557 let _: UnsafeCell<&dyn Send> = a;
1558 let _: Cell<&dyn Send> = b;
1559 let _: RefCell<&dyn Send> = c;