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>` allows to do 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::collections::HashMap;
71 //! use std::cell::RefCell;
75 //! let shared_map: Rc<RefCell<_>> = Rc::new(RefCell::new(HashMap::new()));
76 //! shared_map.borrow_mut().insert("africa", 92388);
77 //! shared_map.borrow_mut().insert("kyoto", 11837);
78 //! shared_map.borrow_mut().insert("piccadilly", 11826);
79 //! shared_map.borrow_mut().insert("marbles", 38);
83 //! Note that this example uses `Rc<T>` and not `Arc<T>`. `RefCell<T>`s are for single-threaded
84 //! scenarios. Consider using `RwLock<T>` or `Mutex<T>` if you need shared mutability in a
85 //! multi-threaded situation.
87 //! ## Implementation details of logically-immutable methods
89 //! Occasionally it may be desirable not to expose in an API that there is mutation happening
90 //! "under the hood". This may be because logically the operation is immutable, but e.g., caching
91 //! forces the implementation to perform mutation; or because you must employ mutation to implement
92 //! a trait method that was originally defined to take `&self`.
95 //! # #![allow(dead_code)]
96 //! use std::cell::RefCell;
99 //! edges: Vec<(i32, i32)>,
100 //! span_tree_cache: RefCell<Option<Vec<(i32, i32)>>>
104 //! fn minimum_spanning_tree(&self) -> Vec<(i32, i32)> {
105 //! // Create a new scope to contain the lifetime of the
106 //! // dynamic borrow
108 //! // Take a reference to the inside of cache cell
109 //! let mut cache = self.span_tree_cache.borrow_mut();
110 //! if cache.is_some() {
111 //! return cache.as_ref().unwrap().clone();
114 //! let span_tree = self.calc_span_tree();
115 //! *cache = Some(span_tree);
118 //! // Recursive call to return the just-cached value.
119 //! // Note that if we had not let the previous borrow
120 //! // of the cache fall out of scope then the subsequent
121 //! // recursive borrow would cause a dynamic thread panic.
122 //! // This is the major hazard of using `RefCell`.
123 //! self.minimum_spanning_tree()
125 //! # fn calc_span_tree(&self) -> Vec<(i32, i32)> { vec![] }
129 //! ## Mutating implementations of `Clone`
131 //! This is simply a special - but common - case of the previous: hiding mutability for operations
132 //! that appear to be immutable. The `clone` method is expected to not change the source value, and
133 //! is declared to take `&self`, not `&mut self`. Therefore, any mutation that happens in the
134 //! `clone` method must use cell types. For example, `Rc<T>` maintains its reference counts within a
138 //! #![feature(core_intrinsics)]
139 //! use std::cell::Cell;
140 //! use std::ptr::NonNull;
141 //! use std::intrinsics::abort;
143 //! struct Rc<T: ?Sized> {
144 //! ptr: NonNull<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();
156 //! Rc { ptr: self.ptr }
160 //! trait RcBoxPtr<T: ?Sized> {
162 //! fn inner(&self) -> &RcBox<T>;
164 //! fn strong(&self) -> usize {
165 //! self.inner().strong.get()
168 //! fn inc_strong(&self) {
171 //! .set(self.strong()
173 //! .unwrap_or_else(|| unsafe { abort() }));
177 //! impl<T: ?Sized> RcBoxPtr<T> for Rc<T> {
178 //! fn inner(&self) -> &RcBox<T> {
180 //! self.ptr.as_ref()
187 #![stable(feature = "rust1", since = "1.0.0")]
190 use fmt::{self, Debug, Display};
193 use ops::{Deref, DerefMut, CoerceUnsized};
196 /// A mutable memory location.
200 /// In this example, you can see that `Cell<T>` enables mutation inside an
201 /// immutable struct. In other words, it enables "interior mutability".
204 /// use std::cell::Cell;
206 /// struct SomeStruct {
207 /// regular_field: u8,
208 /// special_field: Cell<u8>,
211 /// let my_struct = SomeStruct {
212 /// regular_field: 0,
213 /// special_field: Cell::new(1),
216 /// let new_value = 100;
218 /// // ERROR: `my_struct` is immutable
219 /// // my_struct.regular_field = new_value;
221 /// // WORKS: although `my_struct` is immutable, `special_field` is a `Cell`,
222 /// // which can always be mutated
223 /// my_struct.special_field.set(new_value);
224 /// assert_eq!(my_struct.special_field.get(), new_value);
227 /// See the [module-level documentation](index.html) for more.
228 #[stable(feature = "rust1", since = "1.0.0")]
230 pub struct Cell<T: ?Sized> {
231 value: UnsafeCell<T>,
234 impl<T:Copy> Cell<T> {
235 /// Returns a copy of the contained value.
240 /// use std::cell::Cell;
242 /// let c = Cell::new(5);
244 /// let five = c.get();
247 #[stable(feature = "rust1", since = "1.0.0")]
248 pub fn get(&self) -> T {
249 unsafe{ *self.value.get() }
252 /// Updates the contained value using a function and returns the new value.
257 /// #![feature(cell_update)]
259 /// use std::cell::Cell;
261 /// let c = Cell::new(5);
262 /// let new = c.update(|x| x + 1);
264 /// assert_eq!(new, 6);
265 /// assert_eq!(c.get(), 6);
268 #[unstable(feature = "cell_update", issue = "50186")]
269 pub fn update<F>(&self, f: F) -> T
273 let old = self.get();
280 #[stable(feature = "rust1", since = "1.0.0")]
281 unsafe impl<T: ?Sized> Send for Cell<T> where T: Send {}
283 #[stable(feature = "rust1", since = "1.0.0")]
284 impl<T: ?Sized> !Sync for Cell<T> {}
286 #[stable(feature = "rust1", since = "1.0.0")]
287 impl<T:Copy> Clone for Cell<T> {
289 fn clone(&self) -> Cell<T> {
290 Cell::new(self.get())
294 #[stable(feature = "rust1", since = "1.0.0")]
295 impl<T:Default> Default for Cell<T> {
296 /// Creates a `Cell<T>`, with the `Default` value for T.
298 fn default() -> Cell<T> {
299 Cell::new(Default::default())
303 #[stable(feature = "rust1", since = "1.0.0")]
304 impl<T:PartialEq + Copy> PartialEq for Cell<T> {
306 fn eq(&self, other: &Cell<T>) -> bool {
307 self.get() == other.get()
311 #[stable(feature = "cell_eq", since = "1.2.0")]
312 impl<T:Eq + Copy> Eq for Cell<T> {}
314 #[stable(feature = "cell_ord", since = "1.10.0")]
315 impl<T:PartialOrd + Copy> PartialOrd for Cell<T> {
317 fn partial_cmp(&self, other: &Cell<T>) -> Option<Ordering> {
318 self.get().partial_cmp(&other.get())
322 fn lt(&self, other: &Cell<T>) -> bool {
323 self.get() < other.get()
327 fn le(&self, other: &Cell<T>) -> bool {
328 self.get() <= other.get()
332 fn gt(&self, other: &Cell<T>) -> bool {
333 self.get() > other.get()
337 fn ge(&self, other: &Cell<T>) -> bool {
338 self.get() >= other.get()
342 #[stable(feature = "cell_ord", since = "1.10.0")]
343 impl<T:Ord + Copy> Ord for Cell<T> {
345 fn cmp(&self, other: &Cell<T>) -> Ordering {
346 self.get().cmp(&other.get())
350 #[stable(feature = "cell_from", since = "1.12.0")]
351 impl<T> From<T> for Cell<T> {
352 fn from(t: T) -> Cell<T> {
358 /// Creates a new `Cell` containing the given value.
363 /// use std::cell::Cell;
365 /// let c = Cell::new(5);
367 #[stable(feature = "rust1", since = "1.0.0")]
369 pub const fn new(value: T) -> Cell<T> {
371 value: UnsafeCell::new(value),
375 /// Sets the contained value.
380 /// use std::cell::Cell;
382 /// let c = Cell::new(5);
387 #[stable(feature = "rust1", since = "1.0.0")]
388 pub fn set(&self, val: T) {
389 let old = self.replace(val);
393 /// Swaps the values of two Cells.
394 /// Difference with `std::mem::swap` is that this function doesn't require `&mut` reference.
399 /// use std::cell::Cell;
401 /// let c1 = Cell::new(5i32);
402 /// let c2 = Cell::new(10i32);
404 /// assert_eq!(10, c1.get());
405 /// assert_eq!(5, c2.get());
408 #[stable(feature = "move_cell", since = "1.17.0")]
409 pub fn swap(&self, other: &Self) {
410 if ptr::eq(self, other) {
414 ptr::swap(self.value.get(), other.value.get());
418 /// Replaces the contained value, and returns it.
423 /// use std::cell::Cell;
425 /// let cell = Cell::new(5);
426 /// assert_eq!(cell.get(), 5);
427 /// assert_eq!(cell.replace(10), 5);
428 /// assert_eq!(cell.get(), 10);
430 #[stable(feature = "move_cell", since = "1.17.0")]
431 pub fn replace(&self, val: T) -> T {
432 mem::replace(unsafe { &mut *self.value.get() }, val)
435 /// Unwraps the value.
440 /// use std::cell::Cell;
442 /// let c = Cell::new(5);
443 /// let five = c.into_inner();
445 /// assert_eq!(five, 5);
447 #[stable(feature = "move_cell", since = "1.17.0")]
448 pub fn into_inner(self) -> T {
449 self.value.into_inner()
453 impl<T: ?Sized> Cell<T> {
454 /// Returns a raw pointer to the underlying data in this cell.
459 /// use std::cell::Cell;
461 /// let c = Cell::new(5);
463 /// let ptr = c.as_ptr();
466 #[stable(feature = "cell_as_ptr", since = "1.12.0")]
467 pub const fn as_ptr(&self) -> *mut T {
471 /// Returns a mutable reference to the underlying data.
473 /// This call borrows `Cell` mutably (at compile-time) which guarantees
474 /// that we possess the only reference.
479 /// use std::cell::Cell;
481 /// let mut c = Cell::new(5);
482 /// *c.get_mut() += 1;
484 /// assert_eq!(c.get(), 6);
487 #[stable(feature = "cell_get_mut", since = "1.11.0")]
488 pub fn get_mut(&mut self) -> &mut T {
490 &mut *self.value.get()
494 /// Returns a `&Cell<T>` from a `&mut T`
499 /// #![feature(as_cell)]
500 /// use std::cell::Cell;
502 /// let slice: &mut [i32] = &mut [1, 2, 3];
503 /// let cell_slice: &Cell<[i32]> = Cell::from_mut(slice);
504 /// let slice_cell: &[Cell<i32>] = cell_slice.as_slice_of_cells();
506 /// assert_eq!(slice_cell.len(), 3);
509 #[unstable(feature = "as_cell", issue="43038")]
510 pub fn from_mut(t: &mut T) -> &Cell<T> {
512 &*(t as *mut T as *const Cell<T>)
517 impl<T: Default> Cell<T> {
518 /// Takes the value of the cell, leaving `Default::default()` in its place.
523 /// use std::cell::Cell;
525 /// let c = Cell::new(5);
526 /// let five = c.take();
528 /// assert_eq!(five, 5);
529 /// assert_eq!(c.into_inner(), 0);
531 #[stable(feature = "move_cell", since = "1.17.0")]
532 pub fn take(&self) -> T {
533 self.replace(Default::default())
537 #[unstable(feature = "coerce_unsized", issue = "27732")]
538 impl<T: CoerceUnsized<U>, U> CoerceUnsized<Cell<U>> for Cell<T> {}
541 /// Returns a `&[Cell<T>]` from a `&Cell<[T]>`
546 /// #![feature(as_cell)]
547 /// use std::cell::Cell;
549 /// let slice: &mut [i32] = &mut [1, 2, 3];
550 /// let cell_slice: &Cell<[i32]> = Cell::from_mut(slice);
551 /// let slice_cell: &[Cell<i32>] = cell_slice.as_slice_of_cells();
553 /// assert_eq!(slice_cell.len(), 3);
555 #[unstable(feature = "as_cell", issue="43038")]
556 pub fn as_slice_of_cells(&self) -> &[Cell<T>] {
558 &*(self as *const Cell<[T]> as *const [Cell<T>])
563 /// A mutable memory location with dynamically checked borrow rules
565 /// See the [module-level documentation](index.html) for more.
566 #[stable(feature = "rust1", since = "1.0.0")]
567 pub struct RefCell<T: ?Sized> {
568 borrow: Cell<BorrowFlag>,
569 value: UnsafeCell<T>,
572 /// An error returned by [`RefCell::try_borrow`](struct.RefCell.html#method.try_borrow).
573 #[stable(feature = "try_borrow", since = "1.13.0")]
574 pub struct BorrowError {
578 #[stable(feature = "try_borrow", since = "1.13.0")]
579 impl Debug for BorrowError {
580 fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
581 f.debug_struct("BorrowError").finish()
585 #[stable(feature = "try_borrow", since = "1.13.0")]
586 impl Display for BorrowError {
587 fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
588 Display::fmt("already mutably borrowed", f)
592 /// An error returned by [`RefCell::try_borrow_mut`](struct.RefCell.html#method.try_borrow_mut).
593 #[stable(feature = "try_borrow", since = "1.13.0")]
594 pub struct BorrowMutError {
598 #[stable(feature = "try_borrow", since = "1.13.0")]
599 impl Debug for BorrowMutError {
600 fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
601 f.debug_struct("BorrowMutError").finish()
605 #[stable(feature = "try_borrow", since = "1.13.0")]
606 impl Display for BorrowMutError {
607 fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
608 Display::fmt("already borrowed", f)
612 // Positive values represent the number of `Ref` active. Negative values
613 // represent the number of `RefMut` active. Multiple `RefMut`s can only be
614 // active at a time if they refer to distinct, nonoverlapping components of a
615 // `RefCell` (e.g., different ranges of a slice).
617 // `Ref` and `RefMut` are both two words in size, and so there will likely never
618 // be enough `Ref`s or `RefMut`s in existence to overflow half of the `usize`
619 // range. Thus, a `BorrowFlag` will probably never overflow or underflow.
620 // However, this is not a guarantee, as a pathological program could repeatedly
621 // create and then mem::forget `Ref`s or `RefMut`s. Thus, all code must
622 // explicitly check for overflow and underflow in order to avoid unsafety, or at
623 // least behave correctly in the event that overflow or underflow happens (e.g.,
624 // see BorrowRef::new).
625 type BorrowFlag = isize;
626 const UNUSED: BorrowFlag = 0;
629 fn is_writing(x: BorrowFlag) -> bool {
634 fn is_reading(x: BorrowFlag) -> bool {
639 /// Creates a new `RefCell` containing `value`.
644 /// use std::cell::RefCell;
646 /// let c = RefCell::new(5);
648 #[stable(feature = "rust1", since = "1.0.0")]
650 pub const fn new(value: T) -> RefCell<T> {
652 value: UnsafeCell::new(value),
653 borrow: Cell::new(UNUSED),
657 /// Consumes the `RefCell`, returning the wrapped value.
662 /// use std::cell::RefCell;
664 /// let c = RefCell::new(5);
666 /// let five = c.into_inner();
668 #[stable(feature = "rust1", since = "1.0.0")]
670 pub fn into_inner(self) -> T {
671 // Since this function takes `self` (the `RefCell`) by value, the
672 // compiler statically verifies that it is not currently borrowed.
673 // Therefore the following assertion is just a `debug_assert!`.
674 debug_assert!(self.borrow.get() == UNUSED);
675 self.value.into_inner()
678 /// Replaces the wrapped value with a new one, returning the old value,
679 /// without deinitializing either one.
681 /// This function corresponds to [`std::mem::replace`](../mem/fn.replace.html).
685 /// Panics if the value is currently borrowed.
690 /// use std::cell::RefCell;
691 /// let cell = RefCell::new(5);
692 /// let old_value = cell.replace(6);
693 /// assert_eq!(old_value, 5);
694 /// assert_eq!(cell, RefCell::new(6));
697 #[stable(feature = "refcell_replace", since="1.24.0")]
698 pub fn replace(&self, t: T) -> T {
699 mem::replace(&mut *self.borrow_mut(), t)
702 /// Replaces the wrapped value with a new one computed from `f`, returning
703 /// the old value, without deinitializing either one.
707 /// Panics if the value is currently borrowed.
712 /// use std::cell::RefCell;
713 /// let cell = RefCell::new(5);
714 /// let old_value = cell.replace_with(|&mut old| old + 1);
715 /// assert_eq!(old_value, 5);
716 /// assert_eq!(cell, RefCell::new(6));
719 #[stable(feature = "refcell_replace_swap", since="1.35.0")]
720 pub fn replace_with<F: FnOnce(&mut T) -> T>(&self, f: F) -> T {
721 let mut_borrow = &mut *self.borrow_mut();
722 let replacement = f(mut_borrow);
723 mem::replace(mut_borrow, replacement)
726 /// Swaps the wrapped value of `self` with the wrapped value of `other`,
727 /// without deinitializing either one.
729 /// This function corresponds to [`std::mem::swap`](../mem/fn.swap.html).
733 /// Panics if the value in either `RefCell` is currently borrowed.
738 /// use std::cell::RefCell;
739 /// let c = RefCell::new(5);
740 /// let d = RefCell::new(6);
742 /// assert_eq!(c, RefCell::new(6));
743 /// assert_eq!(d, RefCell::new(5));
746 #[stable(feature = "refcell_swap", since="1.24.0")]
747 pub fn swap(&self, other: &Self) {
748 mem::swap(&mut *self.borrow_mut(), &mut *other.borrow_mut())
752 impl<T: ?Sized> RefCell<T> {
753 /// Immutably borrows the wrapped value.
755 /// The borrow lasts until the returned `Ref` exits scope. Multiple
756 /// immutable borrows can be taken out at the same time.
760 /// Panics if the value is currently mutably borrowed. For a non-panicking variant, use
761 /// [`try_borrow`](#method.try_borrow).
766 /// use std::cell::RefCell;
768 /// let c = RefCell::new(5);
770 /// let borrowed_five = c.borrow();
771 /// let borrowed_five2 = c.borrow();
774 /// An example of panic:
777 /// use std::cell::RefCell;
780 /// let result = thread::spawn(move || {
781 /// let c = RefCell::new(5);
782 /// let m = c.borrow_mut();
784 /// let b = c.borrow(); // this causes a panic
787 /// assert!(result.is_err());
789 #[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) {
825 value: unsafe { &*self.value.get() },
828 None => Err(BorrowError { _private: () }),
832 /// Mutably borrows the wrapped value.
834 /// The borrow lasts until the returned `RefMut` or all `RefMut`s derived
835 /// from it exit scope. The value cannot be borrowed while this borrow is
840 /// Panics if the value is currently borrowed. For a non-panicking variant, use
841 /// [`try_borrow_mut`](#method.try_borrow_mut).
846 /// use std::cell::RefCell;
848 /// let c = RefCell::new(5);
850 /// *c.borrow_mut() = 7;
852 /// assert_eq!(*c.borrow(), 7);
855 /// An example of panic:
858 /// use std::cell::RefCell;
861 /// let result = thread::spawn(move || {
862 /// let c = RefCell::new(5);
863 /// let m = c.borrow();
865 /// let b = c.borrow_mut(); // this causes a panic
868 /// assert!(result.is_err());
870 #[stable(feature = "rust1", since = "1.0.0")]
872 pub fn borrow_mut(&self) -> RefMut<T> {
873 self.try_borrow_mut().expect("already borrowed")
876 /// Mutably borrows the wrapped value, returning an error if the value is currently borrowed.
878 /// The borrow lasts until the returned `RefMut` or all `RefMut`s derived
879 /// from it exit scope. The value cannot be borrowed while this borrow is
882 /// This is the non-panicking variant of [`borrow_mut`](#method.borrow_mut).
887 /// use std::cell::RefCell;
889 /// let c = RefCell::new(5);
892 /// let m = c.borrow();
893 /// assert!(c.try_borrow_mut().is_err());
896 /// assert!(c.try_borrow_mut().is_ok());
898 #[stable(feature = "try_borrow", since = "1.13.0")]
900 pub fn try_borrow_mut(&self) -> Result<RefMut<T>, BorrowMutError> {
901 match BorrowRefMut::new(&self.borrow) {
902 Some(b) => Ok(RefMut {
903 value: unsafe { &mut *self.value.get() },
906 None => Err(BorrowMutError { _private: () }),
910 /// Returns a raw pointer to the underlying data in this cell.
915 /// use std::cell::RefCell;
917 /// let c = RefCell::new(5);
919 /// let ptr = c.as_ptr();
922 #[stable(feature = "cell_as_ptr", since = "1.12.0")]
923 pub fn as_ptr(&self) -> *mut T {
927 /// Returns a mutable reference to the underlying data.
929 /// This call borrows `RefCell` mutably (at compile-time) so there is no
930 /// need for dynamic checks.
932 /// However be cautious: this method expects `self` to be mutable, which is
933 /// generally not the case when using a `RefCell`. Take a look at the
934 /// [`borrow_mut`] method instead if `self` isn't mutable.
936 /// Also, please be aware that this method is only for special circumstances and is usually
937 /// not what you want. In case of doubt, use [`borrow_mut`] instead.
939 /// [`borrow_mut`]: #method.borrow_mut
944 /// use std::cell::RefCell;
946 /// let mut c = RefCell::new(5);
947 /// *c.get_mut() += 1;
949 /// assert_eq!(c, RefCell::new(6));
952 #[stable(feature = "cell_get_mut", since = "1.11.0")]
953 pub fn get_mut(&mut self) -> &mut T {
955 &mut *self.value.get()
960 #[stable(feature = "rust1", since = "1.0.0")]
961 unsafe impl<T: ?Sized> Send for RefCell<T> where T: Send {}
963 #[stable(feature = "rust1", since = "1.0.0")]
964 impl<T: ?Sized> !Sync for RefCell<T> {}
966 #[stable(feature = "rust1", since = "1.0.0")]
967 impl<T: Clone> Clone for RefCell<T> {
970 /// Panics if the value is currently mutably borrowed.
972 fn clone(&self) -> RefCell<T> {
973 RefCell::new(self.borrow().clone())
977 #[stable(feature = "rust1", since = "1.0.0")]
978 impl<T:Default> Default for RefCell<T> {
979 /// Creates a `RefCell<T>`, with the `Default` value for T.
981 fn default() -> RefCell<T> {
982 RefCell::new(Default::default())
986 #[stable(feature = "rust1", since = "1.0.0")]
987 impl<T: ?Sized + PartialEq> PartialEq for RefCell<T> {
990 /// Panics if the value in either `RefCell` is currently borrowed.
992 fn eq(&self, other: &RefCell<T>) -> bool {
993 *self.borrow() == *other.borrow()
997 #[stable(feature = "cell_eq", since = "1.2.0")]
998 impl<T: ?Sized + Eq> Eq for RefCell<T> {}
1000 #[stable(feature = "cell_ord", since = "1.10.0")]
1001 impl<T: ?Sized + PartialOrd> PartialOrd for RefCell<T> {
1004 /// Panics if the value in either `RefCell` is currently borrowed.
1006 fn partial_cmp(&self, other: &RefCell<T>) -> Option<Ordering> {
1007 self.borrow().partial_cmp(&*other.borrow())
1012 /// Panics if the value in either `RefCell` is currently borrowed.
1014 fn lt(&self, other: &RefCell<T>) -> bool {
1015 *self.borrow() < *other.borrow()
1020 /// Panics if the value in either `RefCell` is currently borrowed.
1022 fn le(&self, other: &RefCell<T>) -> bool {
1023 *self.borrow() <= *other.borrow()
1028 /// Panics if the value in either `RefCell` is currently borrowed.
1030 fn gt(&self, other: &RefCell<T>) -> bool {
1031 *self.borrow() > *other.borrow()
1036 /// Panics if the value in either `RefCell` is currently borrowed.
1038 fn ge(&self, other: &RefCell<T>) -> bool {
1039 *self.borrow() >= *other.borrow()
1043 #[stable(feature = "cell_ord", since = "1.10.0")]
1044 impl<T: ?Sized + Ord> Ord for RefCell<T> {
1047 /// Panics if the value in either `RefCell` is currently borrowed.
1049 fn cmp(&self, other: &RefCell<T>) -> Ordering {
1050 self.borrow().cmp(&*other.borrow())
1054 #[stable(feature = "cell_from", since = "1.12.0")]
1055 impl<T> From<T> for RefCell<T> {
1056 fn from(t: T) -> RefCell<T> {
1061 #[unstable(feature = "coerce_unsized", issue = "27732")]
1062 impl<T: CoerceUnsized<U>, U> CoerceUnsized<RefCell<U>> for RefCell<T> {}
1064 struct BorrowRef<'b> {
1065 borrow: &'b Cell<BorrowFlag>,
1068 impl<'b> BorrowRef<'b> {
1070 fn new(borrow: &'b Cell<BorrowFlag>) -> Option<BorrowRef<'b>> {
1071 let b = borrow.get();
1072 if is_writing(b) || b == isize::max_value() {
1073 // If there's currently a writing borrow, or if incrementing the
1074 // refcount would overflow into a writing borrow.
1078 Some(BorrowRef { borrow })
1083 impl Drop for BorrowRef<'_> {
1085 fn drop(&mut self) {
1086 let borrow = self.borrow.get();
1087 debug_assert!(is_reading(borrow));
1088 self.borrow.set(borrow - 1);
1092 impl Clone for BorrowRef<'_> {
1094 fn clone(&self) -> Self {
1095 // Since this Ref exists, we know the borrow flag
1096 // is a reading borrow.
1097 let borrow = self.borrow.get();
1098 debug_assert!(is_reading(borrow));
1099 // Prevent the borrow counter from overflowing into
1100 // a writing borrow.
1101 assert!(borrow != isize::max_value());
1102 self.borrow.set(borrow + 1);
1103 BorrowRef { borrow: self.borrow }
1107 /// Wraps a borrowed reference to a value in a `RefCell` box.
1108 /// A wrapper type for an immutably borrowed value from a `RefCell<T>`.
1110 /// See the [module-level documentation](index.html) for more.
1111 #[stable(feature = "rust1", since = "1.0.0")]
1112 pub struct Ref<'b, T: ?Sized + 'b> {
1114 borrow: BorrowRef<'b>,
1117 #[stable(feature = "rust1", since = "1.0.0")]
1118 impl<T: ?Sized> Deref for Ref<'_, T> {
1122 fn deref(&self) -> &T {
1127 impl<'b, T: ?Sized> Ref<'b, T> {
1130 /// The `RefCell` is already immutably borrowed, so this cannot fail.
1132 /// This is an associated function that needs to be used as
1133 /// `Ref::clone(...)`. A `Clone` implementation or a method would interfere
1134 /// with the widespread use of `r.borrow().clone()` to clone the contents of
1136 #[stable(feature = "cell_extras", since = "1.15.0")]
1138 pub fn clone(orig: &Ref<'b, T>) -> Ref<'b, T> {
1141 borrow: orig.borrow.clone(),
1145 /// Makes a new `Ref` for a component of the borrowed data.
1147 /// The `RefCell` is already immutably borrowed, so this cannot fail.
1149 /// This is an associated function that needs to be used as `Ref::map(...)`.
1150 /// A method would interfere with methods of the same name on the contents
1151 /// of a `RefCell` used through `Deref`.
1156 /// use std::cell::{RefCell, Ref};
1158 /// let c = RefCell::new((5, 'b'));
1159 /// let b1: Ref<(u32, char)> = c.borrow();
1160 /// let b2: Ref<u32> = Ref::map(b1, |t| &t.0);
1161 /// assert_eq!(*b2, 5)
1163 #[stable(feature = "cell_map", since = "1.8.0")]
1165 pub fn map<U: ?Sized, F>(orig: Ref<'b, T>, f: F) -> Ref<'b, U>
1166 where F: FnOnce(&T) -> &U
1169 value: f(orig.value),
1170 borrow: orig.borrow,
1174 /// Splits a `Ref` into multiple `Ref`s for different components of the
1177 /// The `RefCell` is already immutably borrowed, so this cannot fail.
1179 /// This is an associated function that needs to be used as
1180 /// `Ref::map_split(...)`. A method would interfere with methods of the same
1181 /// name on the contents of a `RefCell` used through `Deref`.
1186 /// use std::cell::{Ref, RefCell};
1188 /// let cell = RefCell::new([1, 2, 3, 4]);
1189 /// let borrow = cell.borrow();
1190 /// let (begin, end) = Ref::map_split(borrow, |slice| slice.split_at(2));
1191 /// assert_eq!(*begin, [1, 2]);
1192 /// assert_eq!(*end, [3, 4]);
1194 #[stable(feature = "refcell_map_split", since = "1.35.0")]
1196 pub fn map_split<U: ?Sized, V: ?Sized, F>(orig: Ref<'b, T>, f: F) -> (Ref<'b, U>, Ref<'b, V>)
1197 where F: FnOnce(&T) -> (&U, &V)
1199 let (a, b) = f(orig.value);
1200 let borrow = orig.borrow.clone();
1201 (Ref { value: a, borrow }, Ref { value: b, borrow: orig.borrow })
1205 #[unstable(feature = "coerce_unsized", issue = "27732")]
1206 impl<'b, T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<Ref<'b, U>> for Ref<'b, T> {}
1208 #[stable(feature = "std_guard_impls", since = "1.20.0")]
1209 impl<T: ?Sized + fmt::Display> fmt::Display for Ref<'_, T> {
1210 fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
1215 impl<'b, T: ?Sized> RefMut<'b, T> {
1216 /// Makes a new `RefMut` for a component of the borrowed data, e.g., an enum
1219 /// The `RefCell` is already mutably borrowed, so this cannot fail.
1221 /// This is an associated function that needs to be used as
1222 /// `RefMut::map(...)`. A method would interfere with methods of the same
1223 /// name on the contents of a `RefCell` used through `Deref`.
1228 /// use std::cell::{RefCell, RefMut};
1230 /// let c = RefCell::new((5, 'b'));
1232 /// let b1: RefMut<(u32, char)> = c.borrow_mut();
1233 /// let mut b2: RefMut<u32> = RefMut::map(b1, |t| &mut t.0);
1234 /// assert_eq!(*b2, 5);
1237 /// assert_eq!(*c.borrow(), (42, 'b'));
1239 #[stable(feature = "cell_map", since = "1.8.0")]
1241 pub fn map<U: ?Sized, F>(orig: RefMut<'b, T>, f: F) -> RefMut<'b, U>
1242 where F: FnOnce(&mut T) -> &mut U
1244 // FIXME(nll-rfc#40): fix borrow-check
1245 let RefMut { value, borrow } = orig;
1252 /// Splits a `RefMut` into multiple `RefMut`s for different components of the
1255 /// The underlying `RefCell` will remain mutably borrowed until both
1256 /// returned `RefMut`s go out of scope.
1258 /// The `RefCell` is already mutably borrowed, so this cannot fail.
1260 /// This is an associated function that needs to be used as
1261 /// `RefMut::map_split(...)`. A method would interfere with methods of the
1262 /// same name on the contents of a `RefCell` used through `Deref`.
1267 /// use std::cell::{RefCell, RefMut};
1269 /// let cell = RefCell::new([1, 2, 3, 4]);
1270 /// let borrow = cell.borrow_mut();
1271 /// let (mut begin, mut end) = RefMut::map_split(borrow, |slice| slice.split_at_mut(2));
1272 /// assert_eq!(*begin, [1, 2]);
1273 /// assert_eq!(*end, [3, 4]);
1274 /// begin.copy_from_slice(&[4, 3]);
1275 /// end.copy_from_slice(&[2, 1]);
1277 #[stable(feature = "refcell_map_split", since = "1.35.0")]
1279 pub fn map_split<U: ?Sized, V: ?Sized, F>(
1280 orig: RefMut<'b, T>, f: F
1281 ) -> (RefMut<'b, U>, RefMut<'b, V>)
1282 where F: FnOnce(&mut T) -> (&mut U, &mut V)
1284 let (a, b) = f(orig.value);
1285 let borrow = orig.borrow.clone();
1286 (RefMut { value: a, borrow }, RefMut { value: b, borrow: orig.borrow })
1290 struct BorrowRefMut<'b> {
1291 borrow: &'b Cell<BorrowFlag>,
1294 impl Drop for BorrowRefMut<'_> {
1296 fn drop(&mut self) {
1297 let borrow = self.borrow.get();
1298 debug_assert!(is_writing(borrow));
1299 self.borrow.set(borrow + 1);
1303 impl<'b> BorrowRefMut<'b> {
1305 fn new(borrow: &'b Cell<BorrowFlag>) -> Option<BorrowRefMut<'b>> {
1306 // NOTE: Unlike BorrowRefMut::clone, new is called to create the initial
1307 // mutable reference, and so there must currently be no existing
1308 // references. Thus, while clone increments the mutable refcount, here
1309 // we explicitly only allow going from UNUSED to UNUSED - 1.
1310 match borrow.get() {
1312 borrow.set(UNUSED - 1);
1313 Some(BorrowRefMut { borrow })
1319 // Clone a `BorrowRefMut`.
1321 // This is only valid if each `BorrowRefMut` is used to track a mutable
1322 // reference to a distinct, nonoverlapping range of the original object.
1323 // This isn't in a Clone impl so that code doesn't call this implicitly.
1325 fn clone(&self) -> BorrowRefMut<'b> {
1326 let borrow = self.borrow.get();
1327 debug_assert!(is_writing(borrow));
1328 // Prevent the borrow counter from underflowing.
1329 assert!(borrow != isize::min_value());
1330 self.borrow.set(borrow - 1);
1331 BorrowRefMut { borrow: self.borrow }
1335 /// A wrapper type for a mutably borrowed value from a `RefCell<T>`.
1337 /// See the [module-level documentation](index.html) for more.
1338 #[stable(feature = "rust1", since = "1.0.0")]
1339 pub struct RefMut<'b, T: ?Sized + 'b> {
1341 borrow: BorrowRefMut<'b>,
1344 #[stable(feature = "rust1", since = "1.0.0")]
1345 impl<T: ?Sized> Deref for RefMut<'_, T> {
1349 fn deref(&self) -> &T {
1354 #[stable(feature = "rust1", since = "1.0.0")]
1355 impl<T: ?Sized> DerefMut for RefMut<'_, T> {
1357 fn deref_mut(&mut self) -> &mut T {
1362 #[unstable(feature = "coerce_unsized", issue = "27732")]
1363 impl<'b, T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<RefMut<'b, U>> for RefMut<'b, T> {}
1365 #[stable(feature = "std_guard_impls", since = "1.20.0")]
1366 impl<T: ?Sized + fmt::Display> fmt::Display for RefMut<'_, T> {
1367 fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
1372 /// The core primitive for interior mutability in Rust.
1374 /// `UnsafeCell<T>` is a type that wraps some `T` and indicates unsafe interior operations on the
1375 /// wrapped type. Types with an `UnsafeCell<T>` field are considered to have an 'unsafe interior'.
1376 /// The `UnsafeCell<T>` type is the only legal way to obtain aliasable data that is considered
1377 /// mutable. In general, transmuting an `&T` type into an `&mut T` is considered undefined behavior.
1379 /// If you have a reference `&SomeStruct`, then normally in Rust all fields of `SomeStruct` are
1380 /// immutable. The compiler makes optimizations based on the knowledge that `&T` is not mutably
1381 /// aliased or mutated, and that `&mut T` is unique. `UnsafeCell<T>` is the only core language
1382 /// feature to work around this restriction. All other types that allow internal mutability, such as
1383 /// `Cell<T>` and `RefCell<T>`, use `UnsafeCell` to wrap their internal data.
1385 /// The `UnsafeCell` API itself is technically very simple: it gives you a raw pointer `*mut T` to
1386 /// its contents. It is up to _you_ as the abstraction designer to use that raw pointer correctly.
1388 /// The precise Rust aliasing rules are somewhat in flux, but the main points are not contentious:
1390 /// - If you create a safe reference with lifetime `'a` (either a `&T` or `&mut T`
1391 /// reference) that is accessible by safe code (for example, because you returned it),
1392 /// then you must not access the data in any way that contradicts that reference for the
1393 /// remainder of `'a`. For example, this means that if you take the `*mut T` from an
1394 /// `UnsafeCell<T>` and cast it to an `&T`, then the data in `T` must remain immutable
1395 /// (modulo any `UnsafeCell` data found within `T`, of course) until that reference's
1396 /// lifetime expires. Similarly, if you create a `&mut T` reference that is released to
1397 /// safe code, then you must not access the data within the `UnsafeCell` until that
1398 /// reference expires.
1400 /// - At all times, you must avoid data races. If multiple threads have access to
1401 /// the same `UnsafeCell`, then any writes must have a proper happens-before relation to all other
1402 /// accesses (or use atomics).
1404 /// To assist with proper design, the following scenarios are explicitly declared legal
1405 /// for single-threaded code:
1407 /// 1. A `&T` reference can be released to safe code and there it can co-exist with other `&T`
1408 /// references, but not with a `&mut T`
1410 /// 2. A `&mut T` reference may be released to safe code provided neither other `&mut T` nor `&T`
1411 /// co-exist with it. A `&mut T` must always be unique.
1413 /// Note that while mutating or mutably aliasing the contents of an `&UnsafeCell<T>` is
1414 /// ok (provided you enforce the invariants some other way), it is still undefined behavior
1415 /// to have multiple `&mut UnsafeCell<T>` aliases.
1420 /// use std::cell::UnsafeCell;
1422 /// # #[allow(dead_code)]
1423 /// struct NotThreadSafe<T> {
1424 /// value: UnsafeCell<T>,
1427 /// unsafe impl<T> Sync for NotThreadSafe<T> {}
1429 #[lang = "unsafe_cell"]
1430 #[stable(feature = "rust1", since = "1.0.0")]
1431 #[repr(transparent)]
1432 pub struct UnsafeCell<T: ?Sized> {
1436 #[stable(feature = "rust1", since = "1.0.0")]
1437 impl<T: ?Sized> !Sync for UnsafeCell<T> {}
1439 impl<T> UnsafeCell<T> {
1440 /// Constructs a new instance of `UnsafeCell` which will wrap the specified
1443 /// All access to the inner value through methods is `unsafe`.
1448 /// use std::cell::UnsafeCell;
1450 /// let uc = UnsafeCell::new(5);
1452 #[stable(feature = "rust1", since = "1.0.0")]
1454 pub const fn new(value: T) -> UnsafeCell<T> {
1455 UnsafeCell { value }
1458 /// Unwraps the value.
1463 /// use std::cell::UnsafeCell;
1465 /// let uc = UnsafeCell::new(5);
1467 /// let five = uc.into_inner();
1470 #[stable(feature = "rust1", since = "1.0.0")]
1471 pub fn into_inner(self) -> T {
1476 impl<T: ?Sized> UnsafeCell<T> {
1477 /// Gets a mutable pointer to the wrapped value.
1479 /// This can be cast to a pointer of any kind.
1480 /// Ensure that the access is unique (no active references, mutable or not)
1481 /// when casting to `&mut T`, and ensure that there are no mutations
1482 /// or mutable aliases going on when casting to `&T`
1487 /// use std::cell::UnsafeCell;
1489 /// let uc = UnsafeCell::new(5);
1491 /// let five = uc.get();
1494 #[stable(feature = "rust1", since = "1.0.0")]
1495 pub const fn get(&self) -> *mut T {
1496 // We can just cast the pointer from `UnsafeCell<T>` to `T` because of
1497 // #[repr(transparent)]
1498 self as *const UnsafeCell<T> as *const T as *mut T
1502 #[stable(feature = "unsafe_cell_default", since = "1.10.0")]
1503 impl<T: Default> Default for UnsafeCell<T> {
1504 /// Creates an `UnsafeCell`, with the `Default` value for T.
1505 fn default() -> UnsafeCell<T> {
1506 UnsafeCell::new(Default::default())
1510 #[stable(feature = "cell_from", since = "1.12.0")]
1511 impl<T> From<T> for UnsafeCell<T> {
1512 fn from(t: T) -> UnsafeCell<T> {
1517 #[unstable(feature = "coerce_unsized", issue = "27732")]
1518 impl<T: CoerceUnsized<U>, U> CoerceUnsized<UnsafeCell<U>> for UnsafeCell<T> {}
1521 fn assert_coerce_unsized(a: UnsafeCell<&i32>, b: Cell<&i32>, c: RefCell<&i32>) {
1522 let _: UnsafeCell<&dyn Send> = a;
1523 let _: Cell<&dyn Send> = b;
1524 let _: RefCell<&dyn Send> = c;