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;
140 //! use std::marker::PhantomData;
142 //! struct Rc<T: ?Sized> {
143 //! ptr: NonNull<RcBox<T>>,
144 //! phantom: PhantomData<RcBox<T>>,
147 //! struct RcBox<T: ?Sized> {
148 //! strong: Cell<usize>,
149 //! refcount: Cell<usize>,
153 //! impl<T: ?Sized> Clone for Rc<T> {
154 //! fn clone(&self) -> Rc<T> {
155 //! self.inc_strong();
158 //! phantom: PhantomData,
163 //! trait RcBoxPtr<T: ?Sized> {
165 //! fn inner(&self) -> &RcBox<T>;
167 //! fn strong(&self) -> usize {
168 //! self.inner().strong.get()
171 //! fn inc_strong(&self) {
174 //! .set(self.strong()
176 //! .unwrap_or_else(|| unsafe { abort() }));
180 //! impl<T: ?Sized> RcBoxPtr<T> for Rc<T> {
181 //! fn inner(&self) -> &RcBox<T> {
183 //! self.ptr.as_ref()
190 // ignore-tidy-undocumented-unsafe
192 #![stable(feature = "rust1", since = "1.0.0")]
194 use crate::cmp::Ordering;
195 use crate::fmt::{self, Debug, Display};
196 use crate::marker::Unsize;
198 use crate::ops::{Deref, DerefMut, CoerceUnsized};
201 /// A mutable memory location.
205 /// In this example, you can see that `Cell<T>` enables mutation inside an
206 /// immutable struct. In other words, it enables "interior mutability".
209 /// use std::cell::Cell;
211 /// struct SomeStruct {
212 /// regular_field: u8,
213 /// special_field: Cell<u8>,
216 /// let my_struct = SomeStruct {
217 /// regular_field: 0,
218 /// special_field: Cell::new(1),
221 /// let new_value = 100;
223 /// // ERROR: `my_struct` is immutable
224 /// // my_struct.regular_field = new_value;
226 /// // WORKS: although `my_struct` is immutable, `special_field` is a `Cell`,
227 /// // which can always be mutated
228 /// my_struct.special_field.set(new_value);
229 /// assert_eq!(my_struct.special_field.get(), new_value);
232 /// See the [module-level documentation](index.html) for more.
233 #[stable(feature = "rust1", since = "1.0.0")]
235 pub struct Cell<T: ?Sized> {
236 value: UnsafeCell<T>,
239 #[stable(feature = "rust1", since = "1.0.0")]
240 unsafe impl<T: ?Sized> Send for Cell<T> where T: Send {}
242 #[stable(feature = "rust1", since = "1.0.0")]
243 impl<T: ?Sized> !Sync for Cell<T> {}
245 #[stable(feature = "rust1", since = "1.0.0")]
246 impl<T:Copy> Clone for Cell<T> {
248 fn clone(&self) -> Cell<T> {
249 Cell::new(self.get())
253 #[stable(feature = "rust1", since = "1.0.0")]
254 impl<T: Default> Default for Cell<T> {
255 /// Creates a `Cell<T>`, with the `Default` value for T.
257 fn default() -> Cell<T> {
258 Cell::new(Default::default())
262 #[stable(feature = "rust1", since = "1.0.0")]
263 impl<T: PartialEq + Copy> PartialEq for Cell<T> {
265 fn eq(&self, other: &Cell<T>) -> bool {
266 self.get() == other.get()
270 #[stable(feature = "cell_eq", since = "1.2.0")]
271 impl<T: Eq + Copy> Eq for Cell<T> {}
273 #[stable(feature = "cell_ord", since = "1.10.0")]
274 impl<T: PartialOrd + Copy> PartialOrd for Cell<T> {
276 fn partial_cmp(&self, other: &Cell<T>) -> Option<Ordering> {
277 self.get().partial_cmp(&other.get())
281 fn lt(&self, other: &Cell<T>) -> bool {
282 self.get() < other.get()
286 fn le(&self, other: &Cell<T>) -> bool {
287 self.get() <= other.get()
291 fn gt(&self, other: &Cell<T>) -> bool {
292 self.get() > other.get()
296 fn ge(&self, other: &Cell<T>) -> bool {
297 self.get() >= other.get()
301 #[stable(feature = "cell_ord", since = "1.10.0")]
302 impl<T: Ord + Copy> Ord for Cell<T> {
304 fn cmp(&self, other: &Cell<T>) -> Ordering {
305 self.get().cmp(&other.get())
309 #[stable(feature = "cell_from", since = "1.12.0")]
310 impl<T> From<T> for Cell<T> {
311 fn from(t: T) -> Cell<T> {
317 /// Creates a new `Cell` containing the given value.
322 /// use std::cell::Cell;
324 /// let c = Cell::new(5);
326 #[stable(feature = "rust1", since = "1.0.0")]
328 pub const fn new(value: T) -> Cell<T> {
330 value: UnsafeCell::new(value),
334 /// Sets the contained value.
339 /// use std::cell::Cell;
341 /// let c = Cell::new(5);
346 #[stable(feature = "rust1", since = "1.0.0")]
347 pub fn set(&self, val: T) {
348 let old = self.replace(val);
352 /// Swaps the values of two Cells.
353 /// Difference with `std::mem::swap` is that this function doesn't require `&mut` reference.
358 /// use std::cell::Cell;
360 /// let c1 = Cell::new(5i32);
361 /// let c2 = Cell::new(10i32);
363 /// assert_eq!(10, c1.get());
364 /// assert_eq!(5, c2.get());
367 #[stable(feature = "move_cell", since = "1.17.0")]
368 pub fn swap(&self, other: &Self) {
369 if ptr::eq(self, other) {
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 mem::replace(unsafe { &mut *self.value.get() }, val)
394 /// Unwraps the value.
399 /// use std::cell::Cell;
401 /// let c = Cell::new(5);
402 /// let five = c.into_inner();
404 /// assert_eq!(five, 5);
406 #[stable(feature = "move_cell", since = "1.17.0")]
407 pub fn into_inner(self) -> T {
408 self.value.into_inner()
412 impl<T:Copy> Cell<T> {
413 /// Returns a copy of the contained value.
418 /// use std::cell::Cell;
420 /// let c = Cell::new(5);
422 /// let five = c.get();
425 #[stable(feature = "rust1", since = "1.0.0")]
426 pub fn get(&self) -> T {
427 unsafe{ *self.value.get() }
430 /// Updates the contained value using a function and returns the new value.
435 /// #![feature(cell_update)]
437 /// use std::cell::Cell;
439 /// let c = Cell::new(5);
440 /// let new = c.update(|x| x + 1);
442 /// assert_eq!(new, 6);
443 /// assert_eq!(c.get(), 6);
446 #[unstable(feature = "cell_update", issue = "50186")]
447 pub fn update<F>(&self, f: F) -> T
451 let old = self.get();
458 impl<T: ?Sized> Cell<T> {
459 /// Returns a raw pointer to the underlying data in this cell.
464 /// use std::cell::Cell;
466 /// let c = Cell::new(5);
468 /// let ptr = c.as_ptr();
471 #[stable(feature = "cell_as_ptr", since = "1.12.0")]
472 pub const fn as_ptr(&self) -> *mut T {
476 /// Returns a mutable reference to the underlying data.
478 /// This call borrows `Cell` mutably (at compile-time) which guarantees
479 /// that we possess the only reference.
484 /// use std::cell::Cell;
486 /// let mut c = Cell::new(5);
487 /// *c.get_mut() += 1;
489 /// assert_eq!(c.get(), 6);
492 #[stable(feature = "cell_get_mut", since = "1.11.0")]
493 pub fn get_mut(&mut self) -> &mut T {
495 &mut *self.value.get()
499 /// Returns a `&Cell<T>` from a `&mut T`
504 /// use std::cell::Cell;
506 /// let slice: &mut [i32] = &mut [1, 2, 3];
507 /// let cell_slice: &Cell<[i32]> = Cell::from_mut(slice);
508 /// let slice_cell: &[Cell<i32>] = cell_slice.as_slice_of_cells();
510 /// assert_eq!(slice_cell.len(), 3);
513 #[stable(feature = "as_cell", since = "1.37.0")]
514 pub fn from_mut(t: &mut T) -> &Cell<T> {
516 &*(t as *mut T as *const Cell<T>)
521 impl<T: Default> Cell<T> {
522 /// Takes the value of the cell, leaving `Default::default()` in its place.
527 /// use std::cell::Cell;
529 /// let c = Cell::new(5);
530 /// let five = c.take();
532 /// assert_eq!(five, 5);
533 /// assert_eq!(c.into_inner(), 0);
535 #[stable(feature = "move_cell", since = "1.17.0")]
536 pub fn take(&self) -> T {
537 self.replace(Default::default())
541 #[unstable(feature = "coerce_unsized", issue = "27732")]
542 impl<T: CoerceUnsized<U>, U> CoerceUnsized<Cell<U>> for Cell<T> {}
545 /// Returns a `&[Cell<T>]` from a `&Cell<[T]>`
550 /// use std::cell::Cell;
552 /// let slice: &mut [i32] = &mut [1, 2, 3];
553 /// let cell_slice: &Cell<[i32]> = Cell::from_mut(slice);
554 /// let slice_cell: &[Cell<i32>] = cell_slice.as_slice_of_cells();
556 /// assert_eq!(slice_cell.len(), 3);
558 #[stable(feature = "as_cell", since = "1.37.0")]
559 pub fn as_slice_of_cells(&self) -> &[Cell<T>] {
561 &*(self as *const Cell<[T]> as *const [Cell<T>])
566 /// A mutable memory location with dynamically checked borrow rules
568 /// See the [module-level documentation](index.html) for more.
569 #[stable(feature = "rust1", since = "1.0.0")]
570 pub struct RefCell<T: ?Sized> {
571 borrow: Cell<BorrowFlag>,
572 value: UnsafeCell<T>,
575 /// An error returned by [`RefCell::try_borrow`](struct.RefCell.html#method.try_borrow).
576 #[stable(feature = "try_borrow", since = "1.13.0")]
577 pub struct BorrowError {
581 #[stable(feature = "try_borrow", since = "1.13.0")]
582 impl Debug for BorrowError {
583 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
584 f.debug_struct("BorrowError").finish()
588 #[stable(feature = "try_borrow", since = "1.13.0")]
589 impl Display for BorrowError {
590 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
591 Display::fmt("already mutably borrowed", f)
595 /// An error returned by [`RefCell::try_borrow_mut`](struct.RefCell.html#method.try_borrow_mut).
596 #[stable(feature = "try_borrow", since = "1.13.0")]
597 pub struct BorrowMutError {
601 #[stable(feature = "try_borrow", since = "1.13.0")]
602 impl Debug for BorrowMutError {
603 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
604 f.debug_struct("BorrowMutError").finish()
608 #[stable(feature = "try_borrow", since = "1.13.0")]
609 impl Display for BorrowMutError {
610 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
611 Display::fmt("already borrowed", f)
615 // Positive values represent the number of `Ref` active. Negative values
616 // represent the number of `RefMut` active. Multiple `RefMut`s can only be
617 // active at a time if they refer to distinct, nonoverlapping components of a
618 // `RefCell` (e.g., different ranges of a slice).
620 // `Ref` and `RefMut` are both two words in size, and so there will likely never
621 // be enough `Ref`s or `RefMut`s in existence to overflow half of the `usize`
622 // range. Thus, a `BorrowFlag` will probably never overflow or underflow.
623 // However, this is not a guarantee, as a pathological program could repeatedly
624 // create and then mem::forget `Ref`s or `RefMut`s. Thus, all code must
625 // explicitly check for overflow and underflow in order to avoid unsafety, or at
626 // least behave correctly in the event that overflow or underflow happens (e.g.,
627 // see BorrowRef::new).
628 type BorrowFlag = isize;
629 const UNUSED: BorrowFlag = 0;
632 fn is_writing(x: BorrowFlag) -> bool {
637 fn is_reading(x: BorrowFlag) -> bool {
642 /// Creates a new `RefCell` containing `value`.
647 /// use std::cell::RefCell;
649 /// let c = RefCell::new(5);
651 #[stable(feature = "rust1", since = "1.0.0")]
653 pub const fn new(value: T) -> RefCell<T> {
655 value: UnsafeCell::new(value),
656 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")]
673 pub fn into_inner(self) -> T {
674 // Since this function takes `self` (the `RefCell`) by value, the
675 // compiler statically verifies that it is not currently borrowed.
676 // Therefore the following assertion is just a `debug_assert!`.
677 debug_assert!(self.borrow.get() == UNUSED);
678 self.value.into_inner()
681 /// Replaces the wrapped value with a new one, returning the old value,
682 /// without deinitializing either one.
684 /// This function corresponds to [`std::mem::replace`](../mem/fn.replace.html).
688 /// Panics if the value is currently borrowed.
693 /// use std::cell::RefCell;
694 /// let cell = RefCell::new(5);
695 /// let old_value = cell.replace(6);
696 /// assert_eq!(old_value, 5);
697 /// assert_eq!(cell, RefCell::new(6));
700 #[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")]
723 pub fn replace_with<F: FnOnce(&mut T) -> T>(&self, f: F) -> T {
724 let mut_borrow = &mut *self.borrow_mut();
725 let replacement = f(mut_borrow);
726 mem::replace(mut_borrow, replacement)
729 /// Swaps the wrapped value of `self` with the wrapped value of `other`,
730 /// without deinitializing either one.
732 /// This function corresponds to [`std::mem::swap`](../mem/fn.swap.html).
736 /// Panics if the value in either `RefCell` is currently borrowed.
741 /// use std::cell::RefCell;
742 /// let c = RefCell::new(5);
743 /// let d = RefCell::new(6);
745 /// assert_eq!(c, RefCell::new(6));
746 /// assert_eq!(d, RefCell::new(5));
749 #[stable(feature = "refcell_swap", since="1.24.0")]
750 pub fn swap(&self, other: &Self) {
751 mem::swap(&mut *self.borrow_mut(), &mut *other.borrow_mut())
755 impl<T: ?Sized> RefCell<T> {
756 /// Immutably borrows the wrapped value.
758 /// The borrow lasts until the returned `Ref` exits scope. Multiple
759 /// immutable borrows can be taken out at the same time.
763 /// Panics if the value is currently mutably borrowed. For a non-panicking variant, use
764 /// [`try_borrow`](#method.try_borrow).
769 /// use std::cell::RefCell;
771 /// let c = RefCell::new(5);
773 /// let borrowed_five = c.borrow();
774 /// let borrowed_five2 = c.borrow();
777 /// An example of panic:
780 /// use std::cell::RefCell;
783 /// let result = thread::spawn(move || {
784 /// let c = RefCell::new(5);
785 /// let m = c.borrow_mut();
787 /// let b = c.borrow(); // this causes a panic
790 /// assert!(result.is_err());
792 #[stable(feature = "rust1", since = "1.0.0")]
794 pub fn borrow(&self) -> Ref<'_, T> {
795 self.try_borrow().expect("already mutably borrowed")
798 /// Immutably borrows the wrapped value, returning an error if the value is currently mutably
801 /// The borrow lasts until the returned `Ref` exits scope. Multiple immutable borrows can be
802 /// taken out at the same time.
804 /// This is the non-panicking variant of [`borrow`](#method.borrow).
809 /// use std::cell::RefCell;
811 /// let c = RefCell::new(5);
814 /// let m = c.borrow_mut();
815 /// assert!(c.try_borrow().is_err());
819 /// let m = c.borrow();
820 /// assert!(c.try_borrow().is_ok());
823 #[stable(feature = "try_borrow", since = "1.13.0")]
825 pub fn try_borrow(&self) -> Result<Ref<'_, T>, BorrowError> {
826 match BorrowRef::new(&self.borrow) {
828 value: unsafe { &*self.value.get() },
831 None => Err(BorrowError { _private: () }),
835 /// Mutably borrows the wrapped value.
837 /// The borrow lasts until the returned `RefMut` or all `RefMut`s derived
838 /// from it exit scope. The value cannot be borrowed while this borrow is
843 /// Panics if the value is currently borrowed. For a non-panicking variant, use
844 /// [`try_borrow_mut`](#method.try_borrow_mut).
849 /// use std::cell::RefCell;
851 /// let c = RefCell::new(5);
853 /// *c.borrow_mut() = 7;
855 /// assert_eq!(*c.borrow(), 7);
858 /// An example of panic:
861 /// use std::cell::RefCell;
864 /// let result = thread::spawn(move || {
865 /// let c = RefCell::new(5);
866 /// let m = c.borrow();
868 /// let b = c.borrow_mut(); // this causes a panic
871 /// assert!(result.is_err());
873 #[stable(feature = "rust1", since = "1.0.0")]
875 pub fn borrow_mut(&self) -> RefMut<'_, T> {
876 self.try_borrow_mut().expect("already borrowed")
879 /// Mutably borrows the wrapped value, returning an error if the value is currently borrowed.
881 /// The borrow lasts until the returned `RefMut` or all `RefMut`s derived
882 /// from it exit scope. The value cannot be borrowed while this borrow is
885 /// This is the non-panicking variant of [`borrow_mut`](#method.borrow_mut).
890 /// use std::cell::RefCell;
892 /// let c = RefCell::new(5);
895 /// let m = c.borrow();
896 /// assert!(c.try_borrow_mut().is_err());
899 /// assert!(c.try_borrow_mut().is_ok());
901 #[stable(feature = "try_borrow", since = "1.13.0")]
903 pub fn try_borrow_mut(&self) -> Result<RefMut<'_, T>, BorrowMutError> {
904 match BorrowRefMut::new(&self.borrow) {
905 Some(b) => Ok(RefMut {
906 value: unsafe { &mut *self.value.get() },
909 None => Err(BorrowMutError { _private: () }),
913 /// Returns a raw pointer to the underlying data in this cell.
918 /// use std::cell::RefCell;
920 /// let c = RefCell::new(5);
922 /// let ptr = c.as_ptr();
925 #[stable(feature = "cell_as_ptr", since = "1.12.0")]
926 pub fn as_ptr(&self) -> *mut T {
930 /// Returns a mutable reference to the underlying data.
932 /// This call borrows `RefCell` mutably (at compile-time) so there is no
933 /// need for dynamic checks.
935 /// However be cautious: this method expects `self` to be mutable, which is
936 /// generally not the case when using a `RefCell`. Take a look at the
937 /// [`borrow_mut`] method instead if `self` isn't mutable.
939 /// Also, please be aware that this method is only for special circumstances and is usually
940 /// not what you want. In case of doubt, use [`borrow_mut`] instead.
942 /// [`borrow_mut`]: #method.borrow_mut
947 /// use std::cell::RefCell;
949 /// let mut c = RefCell::new(5);
950 /// *c.get_mut() += 1;
952 /// assert_eq!(c, RefCell::new(6));
955 #[stable(feature = "cell_get_mut", since = "1.11.0")]
956 pub fn get_mut(&mut self) -> &mut T {
958 &mut *self.value.get()
962 /// Immutably borrows the wrapped value, returning an error if the value is
963 /// currently mutably borrowed.
967 /// Unlike `RefCell::borrow`, this method is unsafe because it does not
968 /// return a `Ref`, thus leaving the borrow flag untouched. Mutably
969 /// borrowing the `RefCell` while the reference returned by this method
970 /// is alive is undefined behaviour.
975 /// use std::cell::RefCell;
977 /// let c = RefCell::new(5);
980 /// let m = c.borrow_mut();
981 /// assert!(unsafe { c.try_borrow_unguarded() }.is_err());
985 /// let m = c.borrow();
986 /// assert!(unsafe { c.try_borrow_unguarded() }.is_ok());
989 #[stable(feature = "borrow_state", since = "1.37.0")]
991 pub unsafe fn try_borrow_unguarded(&self) -> Result<&T, BorrowError> {
992 if !is_writing(self.borrow.get()) {
993 Ok(&*self.value.get())
995 Err(BorrowError { _private: () })
1000 #[stable(feature = "rust1", since = "1.0.0")]
1001 unsafe impl<T: ?Sized> Send for RefCell<T> where T: Send {}
1003 #[stable(feature = "rust1", since = "1.0.0")]
1004 impl<T: ?Sized> !Sync for RefCell<T> {}
1006 #[stable(feature = "rust1", since = "1.0.0")]
1007 impl<T: Clone> Clone for RefCell<T> {
1010 /// Panics if the value is currently mutably borrowed.
1012 fn clone(&self) -> RefCell<T> {
1013 RefCell::new(self.borrow().clone())
1017 #[stable(feature = "rust1", since = "1.0.0")]
1018 impl<T: Default> Default for RefCell<T> {
1019 /// Creates a `RefCell<T>`, with the `Default` value for T.
1021 fn default() -> RefCell<T> {
1022 RefCell::new(Default::default())
1026 #[stable(feature = "rust1", since = "1.0.0")]
1027 impl<T: ?Sized + PartialEq> PartialEq for RefCell<T> {
1030 /// Panics if the value in either `RefCell` is currently borrowed.
1032 fn eq(&self, other: &RefCell<T>) -> bool {
1033 *self.borrow() == *other.borrow()
1037 #[stable(feature = "cell_eq", since = "1.2.0")]
1038 impl<T: ?Sized + Eq> Eq for RefCell<T> {}
1040 #[stable(feature = "cell_ord", since = "1.10.0")]
1041 impl<T: ?Sized + PartialOrd> PartialOrd for RefCell<T> {
1044 /// Panics if the value in either `RefCell` is currently borrowed.
1046 fn partial_cmp(&self, other: &RefCell<T>) -> Option<Ordering> {
1047 self.borrow().partial_cmp(&*other.borrow())
1052 /// Panics if the value in either `RefCell` is currently borrowed.
1054 fn lt(&self, other: &RefCell<T>) -> bool {
1055 *self.borrow() < *other.borrow()
1060 /// Panics if the value in either `RefCell` is currently borrowed.
1062 fn le(&self, other: &RefCell<T>) -> bool {
1063 *self.borrow() <= *other.borrow()
1068 /// Panics if the value in either `RefCell` is currently borrowed.
1070 fn gt(&self, other: &RefCell<T>) -> bool {
1071 *self.borrow() > *other.borrow()
1076 /// Panics if the value in either `RefCell` is currently borrowed.
1078 fn ge(&self, other: &RefCell<T>) -> bool {
1079 *self.borrow() >= *other.borrow()
1083 #[stable(feature = "cell_ord", since = "1.10.0")]
1084 impl<T: ?Sized + Ord> Ord for RefCell<T> {
1087 /// Panics if the value in either `RefCell` is currently borrowed.
1089 fn cmp(&self, other: &RefCell<T>) -> Ordering {
1090 self.borrow().cmp(&*other.borrow())
1094 #[stable(feature = "cell_from", since = "1.12.0")]
1095 impl<T> From<T> for RefCell<T> {
1096 fn from(t: T) -> RefCell<T> {
1101 #[unstable(feature = "coerce_unsized", issue = "27732")]
1102 impl<T: CoerceUnsized<U>, U> CoerceUnsized<RefCell<U>> for RefCell<T> {}
1104 struct BorrowRef<'b> {
1105 borrow: &'b Cell<BorrowFlag>,
1108 impl<'b> BorrowRef<'b> {
1110 fn new(borrow: &'b Cell<BorrowFlag>) -> Option<BorrowRef<'b>> {
1111 let b = borrow.get().wrapping_add(1);
1113 // Incrementing borrow can result in a non-reading value (<= 0) in these cases:
1114 // 1. It was < 0, i.e. there are writing borrows, so we can't allow a read borrow
1115 // due to Rust's reference aliasing rules
1116 // 2. It was isize::max_value() (the max amount of reading borrows) and it overflowed
1117 // into isize::min_value() (the max amount of writing borrows) so we can't allow
1118 // an additional read borrow because isize can't represent so many read borrows
1119 // (this can only happen if you mem::forget more than a small constant amount of
1120 // `Ref`s, which is not good practice)
1123 // Incrementing borrow can result in a reading value (> 0) in these cases:
1124 // 1. It was = 0, i.e. it wasn't borrowed, and we are taking the first read borrow
1125 // 2. It was > 0 and < isize::max_value(), i.e. there were read borrows, and isize
1126 // is large enough to represent having one more read borrow
1128 Some(BorrowRef { borrow })
1133 impl Drop for BorrowRef<'_> {
1135 fn drop(&mut self) {
1136 let borrow = self.borrow.get();
1137 debug_assert!(is_reading(borrow));
1138 self.borrow.set(borrow - 1);
1142 impl Clone for BorrowRef<'_> {
1144 fn clone(&self) -> Self {
1145 // Since this Ref exists, we know the borrow flag
1146 // is a reading borrow.
1147 let borrow = self.borrow.get();
1148 debug_assert!(is_reading(borrow));
1149 // Prevent the borrow counter from overflowing into
1150 // a writing borrow.
1151 assert!(borrow != isize::max_value());
1152 self.borrow.set(borrow + 1);
1153 BorrowRef { borrow: self.borrow }
1157 /// Wraps a borrowed reference to a value in a `RefCell` box.
1158 /// A wrapper type for an immutably borrowed value from a `RefCell<T>`.
1160 /// See the [module-level documentation](index.html) for more.
1161 #[stable(feature = "rust1", since = "1.0.0")]
1162 pub struct Ref<'b, T: ?Sized + 'b> {
1164 borrow: BorrowRef<'b>,
1167 #[stable(feature = "rust1", since = "1.0.0")]
1168 impl<T: ?Sized> Deref for Ref<'_, T> {
1172 fn deref(&self) -> &T {
1177 impl<'b, T: ?Sized> Ref<'b, T> {
1180 /// The `RefCell` is already immutably borrowed, so this cannot fail.
1182 /// This is an associated function that needs to be used as
1183 /// `Ref::clone(...)`. A `Clone` implementation or a method would interfere
1184 /// with the widespread use of `r.borrow().clone()` to clone the contents of
1186 #[stable(feature = "cell_extras", since = "1.15.0")]
1188 pub fn clone(orig: &Ref<'b, T>) -> Ref<'b, T> {
1191 borrow: orig.borrow.clone(),
1195 /// Makes a new `Ref` for a component of the borrowed data.
1197 /// The `RefCell` is already immutably borrowed, so this cannot fail.
1199 /// This is an associated function that needs to be used as `Ref::map(...)`.
1200 /// A method would interfere with methods of the same name on the contents
1201 /// of a `RefCell` used through `Deref`.
1206 /// use std::cell::{RefCell, Ref};
1208 /// let c = RefCell::new((5, 'b'));
1209 /// let b1: Ref<(u32, char)> = c.borrow();
1210 /// let b2: Ref<u32> = Ref::map(b1, |t| &t.0);
1211 /// assert_eq!(*b2, 5)
1213 #[stable(feature = "cell_map", since = "1.8.0")]
1215 pub fn map<U: ?Sized, F>(orig: Ref<'b, T>, f: F) -> Ref<'b, U>
1216 where F: FnOnce(&T) -> &U
1219 value: f(orig.value),
1220 borrow: orig.borrow,
1224 /// Splits a `Ref` into multiple `Ref`s for different components of the
1227 /// The `RefCell` is already immutably borrowed, so this cannot fail.
1229 /// This is an associated function that needs to be used as
1230 /// `Ref::map_split(...)`. A method would interfere with methods of the same
1231 /// name on the contents of a `RefCell` used through `Deref`.
1236 /// use std::cell::{Ref, RefCell};
1238 /// let cell = RefCell::new([1, 2, 3, 4]);
1239 /// let borrow = cell.borrow();
1240 /// let (begin, end) = Ref::map_split(borrow, |slice| slice.split_at(2));
1241 /// assert_eq!(*begin, [1, 2]);
1242 /// assert_eq!(*end, [3, 4]);
1244 #[stable(feature = "refcell_map_split", since = "1.35.0")]
1246 pub fn map_split<U: ?Sized, V: ?Sized, F>(orig: Ref<'b, T>, f: F) -> (Ref<'b, U>, Ref<'b, V>)
1247 where F: FnOnce(&T) -> (&U, &V)
1249 let (a, b) = f(orig.value);
1250 let borrow = orig.borrow.clone();
1251 (Ref { value: a, borrow }, Ref { value: b, borrow: orig.borrow })
1255 #[unstable(feature = "coerce_unsized", issue = "27732")]
1256 impl<'b, T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<Ref<'b, U>> for Ref<'b, T> {}
1258 #[stable(feature = "std_guard_impls", since = "1.20.0")]
1259 impl<T: ?Sized + fmt::Display> fmt::Display for Ref<'_, T> {
1260 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
1265 impl<'b, T: ?Sized> RefMut<'b, T> {
1266 /// Makes a new `RefMut` for a component of the borrowed data, e.g., an enum
1269 /// The `RefCell` is already mutably borrowed, so this cannot fail.
1271 /// This is an associated function that needs to be used as
1272 /// `RefMut::map(...)`. A method would interfere with methods of the same
1273 /// name on the contents of a `RefCell` used through `Deref`.
1278 /// use std::cell::{RefCell, RefMut};
1280 /// let c = RefCell::new((5, 'b'));
1282 /// let b1: RefMut<(u32, char)> = c.borrow_mut();
1283 /// let mut b2: RefMut<u32> = RefMut::map(b1, |t| &mut t.0);
1284 /// assert_eq!(*b2, 5);
1287 /// assert_eq!(*c.borrow(), (42, 'b'));
1289 #[stable(feature = "cell_map", since = "1.8.0")]
1291 pub fn map<U: ?Sized, F>(orig: RefMut<'b, T>, f: F) -> RefMut<'b, U>
1292 where F: FnOnce(&mut T) -> &mut U
1294 // FIXME(nll-rfc#40): fix borrow-check
1295 let RefMut { value, borrow } = orig;
1302 /// Splits a `RefMut` into multiple `RefMut`s for different components of the
1305 /// The underlying `RefCell` will remain mutably borrowed until both
1306 /// returned `RefMut`s go out of scope.
1308 /// The `RefCell` is already mutably borrowed, so this cannot fail.
1310 /// This is an associated function that needs to be used as
1311 /// `RefMut::map_split(...)`. A method would interfere with methods of the
1312 /// same name on the contents of a `RefCell` used through `Deref`.
1317 /// use std::cell::{RefCell, RefMut};
1319 /// let cell = RefCell::new([1, 2, 3, 4]);
1320 /// let borrow = cell.borrow_mut();
1321 /// let (mut begin, mut end) = RefMut::map_split(borrow, |slice| slice.split_at_mut(2));
1322 /// assert_eq!(*begin, [1, 2]);
1323 /// assert_eq!(*end, [3, 4]);
1324 /// begin.copy_from_slice(&[4, 3]);
1325 /// end.copy_from_slice(&[2, 1]);
1327 #[stable(feature = "refcell_map_split", since = "1.35.0")]
1329 pub fn map_split<U: ?Sized, V: ?Sized, F>(
1330 orig: RefMut<'b, T>, f: F
1331 ) -> (RefMut<'b, U>, RefMut<'b, V>)
1332 where F: FnOnce(&mut T) -> (&mut U, &mut V)
1334 let (a, b) = f(orig.value);
1335 let borrow = orig.borrow.clone();
1336 (RefMut { value: a, borrow }, RefMut { value: b, borrow: orig.borrow })
1340 struct BorrowRefMut<'b> {
1341 borrow: &'b Cell<BorrowFlag>,
1344 impl Drop for BorrowRefMut<'_> {
1346 fn drop(&mut self) {
1347 let borrow = self.borrow.get();
1348 debug_assert!(is_writing(borrow));
1349 self.borrow.set(borrow + 1);
1353 impl<'b> BorrowRefMut<'b> {
1355 fn new(borrow: &'b Cell<BorrowFlag>) -> Option<BorrowRefMut<'b>> {
1356 // NOTE: Unlike BorrowRefMut::clone, new is called to create the initial
1357 // mutable reference, and so there must currently be no existing
1358 // references. Thus, while clone increments the mutable refcount, here
1359 // we explicitly only allow going from UNUSED to UNUSED - 1.
1360 match borrow.get() {
1362 borrow.set(UNUSED - 1);
1363 Some(BorrowRefMut { borrow })
1369 // Clones a `BorrowRefMut`.
1371 // This is only valid if each `BorrowRefMut` is used to track a mutable
1372 // reference to a distinct, nonoverlapping range of the original object.
1373 // This isn't in a Clone impl so that code doesn't call this implicitly.
1375 fn clone(&self) -> BorrowRefMut<'b> {
1376 let borrow = self.borrow.get();
1377 debug_assert!(is_writing(borrow));
1378 // Prevent the borrow counter from underflowing.
1379 assert!(borrow != isize::min_value());
1380 self.borrow.set(borrow - 1);
1381 BorrowRefMut { borrow: self.borrow }
1385 /// A wrapper type for a mutably borrowed value from a `RefCell<T>`.
1387 /// See the [module-level documentation](index.html) for more.
1388 #[stable(feature = "rust1", since = "1.0.0")]
1389 pub struct RefMut<'b, T: ?Sized + 'b> {
1391 borrow: BorrowRefMut<'b>,
1394 #[stable(feature = "rust1", since = "1.0.0")]
1395 impl<T: ?Sized> Deref for RefMut<'_, T> {
1399 fn deref(&self) -> &T {
1404 #[stable(feature = "rust1", since = "1.0.0")]
1405 impl<T: ?Sized> DerefMut for RefMut<'_, T> {
1407 fn deref_mut(&mut self) -> &mut T {
1412 #[unstable(feature = "coerce_unsized", issue = "27732")]
1413 impl<'b, T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<RefMut<'b, U>> for RefMut<'b, T> {}
1415 #[stable(feature = "std_guard_impls", since = "1.20.0")]
1416 impl<T: ?Sized + fmt::Display> fmt::Display for RefMut<'_, T> {
1417 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
1422 /// The core primitive for interior mutability in Rust.
1424 /// `UnsafeCell<T>` is a type that wraps some `T` and indicates unsafe interior operations on the
1425 /// wrapped type. Types with an `UnsafeCell<T>` field are considered to have an 'unsafe interior'.
1426 /// The `UnsafeCell<T>` type is the only legal way to obtain aliasable data that is considered
1427 /// mutable. In general, transmuting an `&T` type into an `&mut T` is considered undefined behavior.
1429 /// If you have a reference `&SomeStruct`, then normally in Rust all fields of `SomeStruct` are
1430 /// immutable. The compiler makes optimizations based on the knowledge that `&T` is not mutably
1431 /// aliased or mutated, and that `&mut T` is unique. `UnsafeCell<T>` is the only core language
1432 /// feature to work around the restriction that `&T` may not be mutated. All other types that
1433 /// allow internal mutability, such as `Cell<T>` and `RefCell<T>`, use `UnsafeCell` to wrap their
1434 /// internal data. There is *no* legal way to obtain aliasing `&mut`, not even with `UnsafeCell<T>`.
1436 /// The `UnsafeCell` API itself is technically very simple: it gives you a raw pointer `*mut T` to
1437 /// its contents. It is up to _you_ as the abstraction designer to use that raw pointer correctly.
1439 /// The precise Rust aliasing rules are somewhat in flux, but the main points are not contentious:
1441 /// - If you create a safe reference with lifetime `'a` (either a `&T` or `&mut T`
1442 /// reference) that is accessible by safe code (for example, because you returned it),
1443 /// then you must not access the data in any way that contradicts that reference for the
1444 /// remainder of `'a`. For example, this means that if you take the `*mut T` from an
1445 /// `UnsafeCell<T>` and cast it to an `&T`, then the data in `T` must remain immutable
1446 /// (modulo any `UnsafeCell` data found within `T`, of course) until that reference's
1447 /// lifetime expires. Similarly, if you create a `&mut T` reference that is released to
1448 /// safe code, then you must not access the data within the `UnsafeCell` until that
1449 /// reference expires.
1451 /// - At all times, you must avoid data races. If multiple threads have access to
1452 /// the same `UnsafeCell`, then any writes must have a proper happens-before relation to all other
1453 /// accesses (or use atomics).
1455 /// To assist with proper design, the following scenarios are explicitly declared legal
1456 /// for single-threaded code:
1458 /// 1. A `&T` reference can be released to safe code and there it can co-exist with other `&T`
1459 /// references, but not with a `&mut T`
1461 /// 2. A `&mut T` reference may be released to safe code provided neither other `&mut T` nor `&T`
1462 /// co-exist with it. A `&mut T` must always be unique.
1464 /// Note that while mutating or mutably aliasing the contents of an `&UnsafeCell<T>` is
1465 /// ok (provided you enforce the invariants some other way), it is still undefined behavior
1466 /// to have multiple `&mut UnsafeCell<T>` aliases.
1471 /// use std::cell::UnsafeCell;
1473 /// # #[allow(dead_code)]
1474 /// struct NotThreadSafe<T> {
1475 /// value: UnsafeCell<T>,
1478 /// unsafe impl<T> Sync for NotThreadSafe<T> {}
1480 #[lang = "unsafe_cell"]
1481 #[stable(feature = "rust1", since = "1.0.0")]
1482 #[repr(transparent)]
1483 pub struct UnsafeCell<T: ?Sized> {
1487 #[stable(feature = "rust1", since = "1.0.0")]
1488 impl<T: ?Sized> !Sync for UnsafeCell<T> {}
1490 impl<T> UnsafeCell<T> {
1491 /// Constructs a new instance of `UnsafeCell` which will wrap the specified
1494 /// All access to the inner value through methods is `unsafe`.
1499 /// use std::cell::UnsafeCell;
1501 /// let uc = UnsafeCell::new(5);
1503 #[stable(feature = "rust1", since = "1.0.0")]
1505 pub const fn new(value: T) -> UnsafeCell<T> {
1506 UnsafeCell { value }
1509 /// Unwraps the value.
1514 /// use std::cell::UnsafeCell;
1516 /// let uc = UnsafeCell::new(5);
1518 /// let five = uc.into_inner();
1521 #[stable(feature = "rust1", since = "1.0.0")]
1522 pub fn into_inner(self) -> T {
1527 impl<T: ?Sized> UnsafeCell<T> {
1528 /// Gets a mutable pointer to the wrapped value.
1530 /// This can be cast to a pointer of any kind.
1531 /// Ensure that the access is unique (no active references, mutable or not)
1532 /// when casting to `&mut T`, and ensure that there are no mutations
1533 /// or mutable aliases going on when casting to `&T`
1538 /// use std::cell::UnsafeCell;
1540 /// let uc = UnsafeCell::new(5);
1542 /// let five = uc.get();
1545 #[stable(feature = "rust1", since = "1.0.0")]
1546 pub const fn get(&self) -> *mut T {
1547 // We can just cast the pointer from `UnsafeCell<T>` to `T` because of
1548 // #[repr(transparent)]
1549 self as *const UnsafeCell<T> as *const T as *mut T
1552 /// Gets a mutable pointer to the wrapped value.
1554 /// This can be cast to a pointer of any kind.
1555 /// Ensure that the access is unique (no active references, mutable or not)
1556 /// when casting to `&mut T`, and ensure that there are no mutations
1557 /// or mutable aliases going on when casting to `&T`.
1561 /// Gradual initialization of an `UnsafeCell`:
1564 /// #![feature(unsafe_cell_raw_get)]
1565 /// use std::cell::UnsafeCell;
1566 /// use std::mem::MaybeUninit;
1568 /// let m = MaybeUninit::<UnsafeCell<i32>>::uninit();
1569 /// unsafe { m.as_ptr().raw_get().write(5); }
1570 /// let uc = unsafe { m.assume_init() };
1572 /// assert_eq!(uc.into_inner(), 5);
1575 #[unstable(feature = "unsafe_cell_raw_get", issue = "0")]
1576 pub const fn raw_get(self: *const Self) -> *mut T {
1577 // We can just cast the pointer from `UnsafeCell<T>` to `T` because of
1578 // #[repr(transparent)]
1579 self as *const T as *mut T
1583 #[stable(feature = "unsafe_cell_default", since = "1.10.0")]
1584 impl<T: Default> Default for UnsafeCell<T> {
1585 /// Creates an `UnsafeCell`, with the `Default` value for T.
1586 fn default() -> UnsafeCell<T> {
1587 UnsafeCell::new(Default::default())
1591 #[stable(feature = "cell_from", since = "1.12.0")]
1592 impl<T> From<T> for UnsafeCell<T> {
1593 fn from(t: T) -> UnsafeCell<T> {
1598 #[unstable(feature = "coerce_unsized", issue = "27732")]
1599 impl<T: CoerceUnsized<U>, U> CoerceUnsized<UnsafeCell<U>> for UnsafeCell<T> {}
1602 fn assert_coerce_unsized(a: UnsafeCell<&i32>, b: Cell<&i32>, c: RefCell<&i32>) {
1603 let _: UnsafeCell<&dyn Send> = a;
1604 let _: Cell<&dyn Send> = b;
1605 let _: RefCell<&dyn Send> = c;