1 // Copyright 2012-2014 The Rust Project Developers. See the COPYRIGHT
2 // file at the top-level directory of this distribution and at
3 // http://rust-lang.org/COPYRIGHT.
5 // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
6 // http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
7 // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
8 // option. This file may not be copied, modified, or distributed
9 // except according to those terms.
11 //! Shareable mutable containers.
13 //! Rust memory safety is based on this rule: Given an object `T`, it is only possible to
14 //! have one of the following:
16 //! - Having several immutable references (`&T`) to the object (also known as **aliasing**).
17 //! - Having one mutable reference (`&mut T`) to the object (also known as **mutability**).
19 //! This is enforced by the Rust compiler. However, there are situations where this rule is not
20 //! flexible enough. Sometimes it is required to have multiple references to an object and yet
23 //! Shareable mutable containers exist to permit mutability in a controlled manner, even in the
24 //! presence of aliasing. Both `Cell<T>` and `RefCell<T>` allows to do this in a single threaded
25 //! way. However, neither `Cell<T>` nor `RefCell<T>` are thread safe (they do not implement
26 //! `Sync`). If you need to do aliasing and mutation between multiple threads it is possible to
27 //! use [`Mutex`](../../std/sync/struct.Mutex.html),
28 //! [`RwLock`](../../std/sync/struct.RwLock.html) or
29 //! [`atomic`](../../core/sync/atomic/index.html) types.
31 //! Values of the `Cell<T>` and `RefCell<T>` types may be mutated through shared references (i.e.
32 //! the common `&T` type), whereas most Rust types can only be mutated through unique (`&mut T`)
33 //! references. We say that `Cell<T>` and `RefCell<T>` provide 'interior mutability', in contrast
34 //! with typical Rust types that exhibit 'inherited mutability'.
36 //! Cell types come in two flavors: `Cell<T>` and `RefCell<T>`. `Cell<T>` implements interior
37 //! mutability by moving values in and out of the `Cell<T>`. To use references instead of values,
38 //! one must use the `RefCell<T>` type, acquiring a write lock before mutating. `Cell<T>` provides
39 //! methods to retrieve and change the current interior value:
41 //! - For types that implement `Copy`, the `get` method retrieves the current interior value.
42 //! - For types that implement `Default`, the `take` method replaces the current interior value
43 //! with `Default::default()` and returns the replaced value.
44 //! - For all types, the `replace` method replaces the current interior value and returns the
45 //! replaced value and the `into_inner` method consumes the `Cell<T>` and returns the interior
46 //! value. Additionally, the `set` method replaces the interior value, dropping the replaced
49 //! `RefCell<T>` uses Rust's lifetimes to implement 'dynamic borrowing', a process whereby one can
50 //! claim temporary, exclusive, mutable access to the inner value. Borrows for `RefCell<T>`s are
51 //! tracked 'at runtime', unlike Rust's native reference types which are entirely tracked
52 //! statically, at compile time. Because `RefCell<T>` borrows are dynamic it is possible to attempt
53 //! to borrow a value that is already mutably borrowed; when this happens it results in thread
56 //! # When to choose interior mutability
58 //! The more common inherited mutability, where one must have unique access to mutate a value, is
59 //! one of the key language elements that enables Rust to reason strongly about pointer aliasing,
60 //! statically preventing crash bugs. Because of that, inherited mutability is preferred, and
61 //! interior mutability is something of a last resort. Since cell types enable mutation where it
62 //! would otherwise be disallowed though, there are occasions when interior mutability might be
63 //! appropriate, or even *must* be used, e.g.
65 //! * Introducing mutability 'inside' of something immutable
66 //! * Implementation details of logically-immutable methods.
67 //! * Mutating implementations of `Clone`.
69 //! ## Introducing mutability 'inside' of something immutable
71 //! Many shared smart pointer types, including `Rc<T>` and `Arc<T>`, provide containers that can be
72 //! cloned and shared between multiple parties. Because the contained values may be
73 //! multiply-aliased, they can only be borrowed with `&`, not `&mut`. Without cells it would be
74 //! impossible to mutate data inside of these smart pointers at all.
76 //! It's very common then to put a `RefCell<T>` inside shared pointer types to reintroduce
80 //! use std::collections::HashMap;
81 //! use std::cell::RefCell;
85 //! let shared_map: Rc<RefCell<_>> = Rc::new(RefCell::new(HashMap::new()));
86 //! shared_map.borrow_mut().insert("africa", 92388);
87 //! shared_map.borrow_mut().insert("kyoto", 11837);
88 //! shared_map.borrow_mut().insert("piccadilly", 11826);
89 //! shared_map.borrow_mut().insert("marbles", 38);
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 //! // Create a new scope to contain the lifetime of the
116 //! // dynamic borrow
118 //! // Take a reference to the inside of cache cell
119 //! let mut cache = self.span_tree_cache.borrow_mut();
120 //! if cache.is_some() {
121 //! return cache.as_ref().unwrap().clone();
124 //! let span_tree = self.calc_span_tree();
125 //! *cache = Some(span_tree);
128 //! // Recursive call to return the just-cached value.
129 //! // Note that if we had not let the previous borrow
130 //! // of the cache fall out of scope then the subsequent
131 //! // recursive borrow would cause a dynamic thread panic.
132 //! // This is the major hazard of using `RefCell`.
133 //! self.minimum_spanning_tree()
135 //! # fn calc_span_tree(&self) -> Vec<(i32, i32)> { vec![] }
139 //! ## Mutating implementations of `Clone`
141 //! This is simply a special - but common - case of the previous: hiding mutability for operations
142 //! that appear to be immutable. The `clone` method is expected to not change the source value, and
143 //! is declared to take `&self`, not `&mut self`. Therefore any mutation that happens in the
144 //! `clone` method must use cell types. For example, `Rc<T>` maintains its reference counts within a
148 //! #![feature(core_intrinsics)]
149 //! use std::cell::Cell;
150 //! use std::ptr::NonNull;
151 //! use std::intrinsics::abort;
153 //! struct Rc<T: ?Sized> {
154 //! ptr: NonNull<RcBox<T>>
157 //! struct RcBox<T: ?Sized> {
158 //! strong: Cell<usize>,
159 //! refcount: Cell<usize>,
163 //! impl<T: ?Sized> Clone for Rc<T> {
164 //! fn clone(&self) -> Rc<T> {
165 //! self.inc_strong();
166 //! Rc { ptr: self.ptr }
170 //! trait RcBoxPtr<T: ?Sized> {
172 //! fn inner(&self) -> &RcBox<T>;
174 //! fn strong(&self) -> usize {
175 //! self.inner().strong.get()
178 //! fn inc_strong(&self) {
181 //! .set(self.strong()
183 //! .unwrap_or_else(|| unsafe { abort() }));
187 //! impl<T: ?Sized> RcBoxPtr<T> for Rc<T> {
188 //! fn inner(&self) -> &RcBox<T> {
190 //! self.ptr.as_ref()
197 #![stable(feature = "rust1", since = "1.0.0")]
200 use fmt::{self, Debug, Display};
203 use ops::{Deref, DerefMut, CoerceUnsized};
206 /// A mutable memory location.
210 /// In this example, you can see that `Cell<T>` enables mutation inside an
211 /// immutable struct. In other words, it enables "interior mutability".
214 /// use std::cell::Cell;
216 /// struct SomeStruct {
217 /// regular_field: u8,
218 /// special_field: Cell<u8>,
221 /// let my_struct = SomeStruct {
222 /// regular_field: 0,
223 /// special_field: Cell::new(1),
226 /// let new_value = 100;
228 /// // ERROR: `my_struct` is immutable
229 /// // my_struct.regular_field = new_value;
231 /// // WORKS: although `my_struct` is immutable, `special_field` is a `Cell`,
232 /// // which can always be mutated
233 /// my_struct.special_field.set(new_value);
234 /// assert_eq!(my_struct.special_field.get(), new_value);
237 /// See the [module-level documentation](index.html) for more.
238 #[stable(feature = "rust1", since = "1.0.0")]
240 pub struct Cell<T: ?Sized> {
241 value: UnsafeCell<T>,
244 impl<T:Copy> Cell<T> {
245 /// Returns a copy of the contained value.
250 /// use std::cell::Cell;
252 /// let c = Cell::new(5);
254 /// let five = c.get();
257 #[stable(feature = "rust1", since = "1.0.0")]
258 pub fn get(&self) -> T {
259 unsafe{ *self.value.get() }
262 /// Updates the contained value using a function and returns the new value.
267 /// #![feature(cell_update)]
269 /// use std::cell::Cell;
271 /// let c = Cell::new(5);
272 /// let new = c.update(|x| x + 1);
274 /// assert_eq!(new, 6);
275 /// assert_eq!(c.get(), 6);
278 #[unstable(feature = "cell_update", issue = "50186")]
279 pub fn update<F>(&self, f: F) -> T
283 let old = self.get();
290 #[stable(feature = "rust1", since = "1.0.0")]
291 unsafe impl<T: ?Sized> Send for Cell<T> where T: Send {}
293 #[stable(feature = "rust1", since = "1.0.0")]
294 impl<T: ?Sized> !Sync for Cell<T> {}
296 #[stable(feature = "rust1", since = "1.0.0")]
297 impl<T:Copy> Clone for Cell<T> {
299 fn clone(&self) -> Cell<T> {
300 Cell::new(self.get())
304 #[stable(feature = "rust1", since = "1.0.0")]
305 impl<T:Default> Default for Cell<T> {
306 /// Creates a `Cell<T>`, with the `Default` value for T.
308 fn default() -> Cell<T> {
309 Cell::new(Default::default())
313 #[stable(feature = "rust1", since = "1.0.0")]
314 impl<T:PartialEq + Copy> PartialEq for Cell<T> {
316 fn eq(&self, other: &Cell<T>) -> bool {
317 self.get() == other.get()
321 #[stable(feature = "cell_eq", since = "1.2.0")]
322 impl<T:Eq + Copy> Eq for Cell<T> {}
324 #[stable(feature = "cell_ord", since = "1.10.0")]
325 impl<T:PartialOrd + Copy> PartialOrd for Cell<T> {
327 fn partial_cmp(&self, other: &Cell<T>) -> Option<Ordering> {
328 self.get().partial_cmp(&other.get())
332 fn lt(&self, other: &Cell<T>) -> bool {
333 self.get() < other.get()
337 fn le(&self, other: &Cell<T>) -> bool {
338 self.get() <= other.get()
342 fn gt(&self, other: &Cell<T>) -> bool {
343 self.get() > other.get()
347 fn ge(&self, other: &Cell<T>) -> bool {
348 self.get() >= other.get()
352 #[stable(feature = "cell_ord", since = "1.10.0")]
353 impl<T:Ord + Copy> Ord for Cell<T> {
355 fn cmp(&self, other: &Cell<T>) -> Ordering {
356 self.get().cmp(&other.get())
360 #[stable(feature = "cell_from", since = "1.12.0")]
361 impl<T> From<T> for Cell<T> {
362 fn from(t: T) -> Cell<T> {
368 /// Creates a new `Cell` containing the given value.
373 /// use std::cell::Cell;
375 /// let c = Cell::new(5);
377 #[stable(feature = "rust1", since = "1.0.0")]
379 pub const fn new(value: T) -> Cell<T> {
381 value: UnsafeCell::new(value),
385 /// Sets the contained value.
390 /// use std::cell::Cell;
392 /// let c = Cell::new(5);
397 #[stable(feature = "rust1", since = "1.0.0")]
398 pub fn set(&self, val: T) {
399 let old = self.replace(val);
403 /// Swaps the values of two Cells.
404 /// Difference with `std::mem::swap` is that this function doesn't require `&mut` reference.
409 /// use std::cell::Cell;
411 /// let c1 = Cell::new(5i32);
412 /// let c2 = Cell::new(10i32);
414 /// assert_eq!(10, c1.get());
415 /// assert_eq!(5, c2.get());
418 #[stable(feature = "move_cell", since = "1.17.0")]
419 pub fn swap(&self, other: &Self) {
420 if ptr::eq(self, other) {
424 ptr::swap(self.value.get(), other.value.get());
428 /// Replaces the contained value, and returns it.
433 /// use std::cell::Cell;
435 /// let cell = Cell::new(5);
436 /// assert_eq!(cell.get(), 5);
437 /// assert_eq!(cell.replace(10), 5);
438 /// assert_eq!(cell.get(), 10);
440 #[stable(feature = "move_cell", since = "1.17.0")]
441 pub fn replace(&self, val: T) -> T {
442 mem::replace(unsafe { &mut *self.value.get() }, val)
445 /// Unwraps the value.
450 /// use std::cell::Cell;
452 /// let c = Cell::new(5);
453 /// let five = c.into_inner();
455 /// assert_eq!(five, 5);
457 #[stable(feature = "move_cell", since = "1.17.0")]
458 pub fn into_inner(self) -> T {
459 self.value.into_inner()
463 impl<T: ?Sized> Cell<T> {
464 /// Returns a raw pointer to the underlying data in this cell.
469 /// use std::cell::Cell;
471 /// let c = Cell::new(5);
473 /// let ptr = c.as_ptr();
476 #[stable(feature = "cell_as_ptr", since = "1.12.0")]
477 pub const fn as_ptr(&self) -> *mut T {
481 /// Returns a mutable reference to the underlying data.
483 /// This call borrows `Cell` mutably (at compile-time) which guarantees
484 /// that we possess the only reference.
489 /// use std::cell::Cell;
491 /// let mut c = Cell::new(5);
492 /// *c.get_mut() += 1;
494 /// assert_eq!(c.get(), 6);
497 #[stable(feature = "cell_get_mut", since = "1.11.0")]
498 pub fn get_mut(&mut self) -> &mut T {
500 &mut *self.value.get()
504 /// Returns a `&Cell<T>` from a `&mut T`
509 /// #![feature(as_cell)]
510 /// use std::cell::Cell;
512 /// let slice: &mut [i32] = &mut [1, 2, 3];
513 /// let cell_slice: &Cell<[i32]> = Cell::from_mut(slice);
514 /// let slice_cell: &[Cell<i32>] = cell_slice.as_slice_of_cells();
516 /// assert_eq!(slice_cell.len(), 3);
519 #[unstable(feature = "as_cell", issue="43038")]
520 pub fn from_mut(t: &mut T) -> &Cell<T> {
522 &*(t as *mut T as *const Cell<T>)
527 impl<T: Default> Cell<T> {
528 /// Takes the value of the cell, leaving `Default::default()` in its place.
533 /// use std::cell::Cell;
535 /// let c = Cell::new(5);
536 /// let five = c.take();
538 /// assert_eq!(five, 5);
539 /// assert_eq!(c.into_inner(), 0);
541 #[stable(feature = "move_cell", since = "1.17.0")]
542 pub fn take(&self) -> T {
543 self.replace(Default::default())
547 #[unstable(feature = "coerce_unsized", issue = "27732")]
548 impl<T: CoerceUnsized<U>, U> CoerceUnsized<Cell<U>> for Cell<T> {}
551 /// Returns a `&[Cell<T>]` from a `&Cell<[T]>`
556 /// #![feature(as_cell)]
557 /// use std::cell::Cell;
559 /// let slice: &mut [i32] = &mut [1, 2, 3];
560 /// let cell_slice: &Cell<[i32]> = Cell::from_mut(slice);
561 /// let slice_cell: &[Cell<i32>] = cell_slice.as_slice_of_cells();
563 /// assert_eq!(slice_cell.len(), 3);
565 #[unstable(feature = "as_cell", issue="43038")]
566 pub fn as_slice_of_cells(&self) -> &[Cell<T>] {
568 &*(self as *const Cell<[T]> as *const [Cell<T>])
573 /// A mutable memory location with dynamically checked borrow rules
575 /// See the [module-level documentation](index.html) for more.
576 #[stable(feature = "rust1", since = "1.0.0")]
577 pub struct RefCell<T: ?Sized> {
578 borrow: Cell<BorrowFlag>,
579 value: UnsafeCell<T>,
582 /// An error returned by [`RefCell::try_borrow`](struct.RefCell.html#method.try_borrow).
583 #[stable(feature = "try_borrow", since = "1.13.0")]
584 pub struct BorrowError {
588 #[stable(feature = "try_borrow", since = "1.13.0")]
589 impl Debug for BorrowError {
590 fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
591 f.debug_struct("BorrowError").finish()
595 #[stable(feature = "try_borrow", since = "1.13.0")]
596 impl Display for BorrowError {
597 fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
598 Display::fmt("already mutably borrowed", f)
602 /// An error returned by [`RefCell::try_borrow_mut`](struct.RefCell.html#method.try_borrow_mut).
603 #[stable(feature = "try_borrow", since = "1.13.0")]
604 pub struct BorrowMutError {
608 #[stable(feature = "try_borrow", since = "1.13.0")]
609 impl Debug for BorrowMutError {
610 fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
611 f.debug_struct("BorrowMutError").finish()
615 #[stable(feature = "try_borrow", since = "1.13.0")]
616 impl Display for BorrowMutError {
617 fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
618 Display::fmt("already borrowed", f)
622 // Positive values represent the number of `Ref` active. Negative values
623 // represent the number of `RefMut` active. Multiple `RefMut`s can only be
624 // active at a time if they refer to distinct, nonoverlapping components of a
625 // `RefCell` (e.g., different ranges of a slice).
627 // `Ref` and `RefMut` are both two words in size, and so there will likely never
628 // be enough `Ref`s or `RefMut`s in existence to overflow half of the `usize`
629 // range. Thus, a `BorrowFlag` will probably never overflow or underflow.
630 // However, this is not a guarantee, as a pathological program could repeatedly
631 // create and then mem::forget `Ref`s or `RefMut`s. Thus, all code must
632 // explicitly check for overflow and underflow in order to avoid unsafety, or at
633 // least behave correctly in the event that overflow or underflow happens (e.g.,
634 // see BorrowRef::new).
635 type BorrowFlag = isize;
636 const UNUSED: BorrowFlag = 0;
639 fn is_writing(x: BorrowFlag) -> bool {
644 fn is_reading(x: BorrowFlag) -> bool {
649 /// Creates a new `RefCell` containing `value`.
654 /// use std::cell::RefCell;
656 /// let c = RefCell::new(5);
658 #[stable(feature = "rust1", since = "1.0.0")]
660 pub const fn new(value: T) -> RefCell<T> {
662 value: UnsafeCell::new(value),
663 borrow: Cell::new(UNUSED),
667 /// Consumes the `RefCell`, returning the wrapped value.
672 /// use std::cell::RefCell;
674 /// let c = RefCell::new(5);
676 /// let five = c.into_inner();
678 #[stable(feature = "rust1", since = "1.0.0")]
680 pub fn into_inner(self) -> T {
681 // Since this function takes `self` (the `RefCell`) by value, the
682 // compiler statically verifies that it is not currently borrowed.
683 // Therefore the following assertion is just a `debug_assert!`.
684 debug_assert!(self.borrow.get() == UNUSED);
685 self.value.into_inner()
688 /// Replaces the wrapped value with a new one, returning the old value,
689 /// without deinitializing either one.
691 /// This function corresponds to [`std::mem::replace`](../mem/fn.replace.html).
695 /// Panics if the value is currently borrowed.
700 /// use std::cell::RefCell;
701 /// let cell = RefCell::new(5);
702 /// let old_value = cell.replace(6);
703 /// assert_eq!(old_value, 5);
704 /// assert_eq!(cell, RefCell::new(6));
707 #[stable(feature = "refcell_replace", since="1.24.0")]
708 pub fn replace(&self, t: T) -> T {
709 mem::replace(&mut *self.borrow_mut(), t)
712 /// Replaces the wrapped value with a new one computed from `f`, returning
713 /// the old value, without deinitializing either one.
715 /// This function corresponds to [`std::mem::replace`](../mem/fn.replace.html).
719 /// Panics if the value is currently borrowed.
724 /// #![feature(refcell_replace_swap)]
725 /// use std::cell::RefCell;
726 /// let cell = RefCell::new(5);
727 /// let old_value = cell.replace_with(|&mut old| old + 1);
728 /// assert_eq!(old_value, 5);
729 /// assert_eq!(cell, RefCell::new(6));
732 #[unstable(feature = "refcell_replace_swap", issue="43570")]
733 pub fn replace_with<F: FnOnce(&mut T) -> T>(&self, f: F) -> T {
734 let mut_borrow = &mut *self.borrow_mut();
735 let replacement = f(mut_borrow);
736 mem::replace(mut_borrow, replacement)
739 /// Swaps the wrapped value of `self` with the wrapped value of `other`,
740 /// without deinitializing either one.
742 /// This function corresponds to [`std::mem::swap`](../mem/fn.swap.html).
746 /// Panics if the value in either `RefCell` is currently borrowed.
751 /// use std::cell::RefCell;
752 /// let c = RefCell::new(5);
753 /// let d = RefCell::new(6);
755 /// assert_eq!(c, RefCell::new(6));
756 /// assert_eq!(d, RefCell::new(5));
759 #[stable(feature = "refcell_swap", since="1.24.0")]
760 pub fn swap(&self, other: &Self) {
761 mem::swap(&mut *self.borrow_mut(), &mut *other.borrow_mut())
765 impl<T: ?Sized> RefCell<T> {
766 /// Immutably borrows the wrapped value.
768 /// The borrow lasts until the returned `Ref` exits scope. Multiple
769 /// immutable borrows can be taken out at the same time.
773 /// Panics if the value is currently mutably borrowed. For a non-panicking variant, use
774 /// [`try_borrow`](#method.try_borrow).
779 /// use std::cell::RefCell;
781 /// let c = RefCell::new(5);
783 /// let borrowed_five = c.borrow();
784 /// let borrowed_five2 = c.borrow();
787 /// An example of panic:
790 /// use std::cell::RefCell;
793 /// let result = thread::spawn(move || {
794 /// let c = RefCell::new(5);
795 /// let m = c.borrow_mut();
797 /// let b = c.borrow(); // this causes a panic
800 /// assert!(result.is_err());
802 #[stable(feature = "rust1", since = "1.0.0")]
804 pub fn borrow(&self) -> Ref<T> {
805 self.try_borrow().expect("already mutably borrowed")
808 /// Immutably borrows the wrapped value, returning an error if the value is currently mutably
811 /// The borrow lasts until the returned `Ref` exits scope. Multiple immutable borrows can be
812 /// taken out at the same time.
814 /// This is the non-panicking variant of [`borrow`](#method.borrow).
819 /// use std::cell::RefCell;
821 /// let c = RefCell::new(5);
824 /// let m = c.borrow_mut();
825 /// assert!(c.try_borrow().is_err());
829 /// let m = c.borrow();
830 /// assert!(c.try_borrow().is_ok());
833 #[stable(feature = "try_borrow", since = "1.13.0")]
835 pub fn try_borrow(&self) -> Result<Ref<T>, BorrowError> {
836 match BorrowRef::new(&self.borrow) {
838 value: unsafe { &*self.value.get() },
841 None => Err(BorrowError { _private: () }),
845 /// Mutably borrows the wrapped value.
847 /// The borrow lasts until the returned `RefMut` or all `RefMut`s derived
848 /// from it exit scope. The value cannot be borrowed while this borrow is
853 /// Panics if the value is currently borrowed. For a non-panicking variant, use
854 /// [`try_borrow_mut`](#method.try_borrow_mut).
859 /// use std::cell::RefCell;
861 /// let c = RefCell::new(5);
863 /// *c.borrow_mut() = 7;
865 /// assert_eq!(*c.borrow(), 7);
868 /// An example of panic:
871 /// use std::cell::RefCell;
874 /// let result = thread::spawn(move || {
875 /// let c = RefCell::new(5);
876 /// let m = c.borrow();
878 /// let b = c.borrow_mut(); // this causes a panic
881 /// assert!(result.is_err());
883 #[stable(feature = "rust1", since = "1.0.0")]
885 pub fn borrow_mut(&self) -> RefMut<T> {
886 self.try_borrow_mut().expect("already borrowed")
889 /// Mutably borrows the wrapped value, returning an error if the value is currently borrowed.
891 /// The borrow lasts until the returned `RefMut` or all `RefMut`s derived
892 /// from it exit scope. The value cannot be borrowed while this borrow is
895 /// This is the non-panicking variant of [`borrow_mut`](#method.borrow_mut).
900 /// use std::cell::RefCell;
902 /// let c = RefCell::new(5);
905 /// let m = c.borrow();
906 /// assert!(c.try_borrow_mut().is_err());
909 /// assert!(c.try_borrow_mut().is_ok());
911 #[stable(feature = "try_borrow", since = "1.13.0")]
913 pub fn try_borrow_mut(&self) -> Result<RefMut<T>, BorrowMutError> {
914 match BorrowRefMut::new(&self.borrow) {
915 Some(b) => Ok(RefMut {
916 value: unsafe { &mut *self.value.get() },
919 None => Err(BorrowMutError { _private: () }),
923 /// Returns a raw pointer to the underlying data in this cell.
928 /// use std::cell::RefCell;
930 /// let c = RefCell::new(5);
932 /// let ptr = c.as_ptr();
935 #[stable(feature = "cell_as_ptr", since = "1.12.0")]
936 pub fn as_ptr(&self) -> *mut T {
940 /// Returns a mutable reference to the underlying data.
942 /// This call borrows `RefCell` mutably (at compile-time) so there is no
943 /// need for dynamic checks.
945 /// However be cautious: this method expects `self` to be mutable, which is
946 /// generally not the case when using a `RefCell`. Take a look at the
947 /// [`borrow_mut`] method instead if `self` isn't mutable.
949 /// Also, please be aware that this method is only for special circumstances and is usually
950 /// not what you want. In case of doubt, use [`borrow_mut`] instead.
952 /// [`borrow_mut`]: #method.borrow_mut
957 /// use std::cell::RefCell;
959 /// let mut c = RefCell::new(5);
960 /// *c.get_mut() += 1;
962 /// assert_eq!(c, RefCell::new(6));
965 #[stable(feature = "cell_get_mut", since = "1.11.0")]
966 pub fn get_mut(&mut self) -> &mut T {
968 &mut *self.value.get()
973 #[stable(feature = "rust1", since = "1.0.0")]
974 unsafe impl<T: ?Sized> Send for RefCell<T> where T: Send {}
976 #[stable(feature = "rust1", since = "1.0.0")]
977 impl<T: ?Sized> !Sync for RefCell<T> {}
979 #[stable(feature = "rust1", since = "1.0.0")]
980 impl<T: Clone> Clone for RefCell<T> {
983 /// Panics if the value is currently mutably borrowed.
985 fn clone(&self) -> RefCell<T> {
986 RefCell::new(self.borrow().clone())
990 #[stable(feature = "rust1", since = "1.0.0")]
991 impl<T:Default> Default for RefCell<T> {
992 /// Creates a `RefCell<T>`, with the `Default` value for T.
994 fn default() -> RefCell<T> {
995 RefCell::new(Default::default())
999 #[stable(feature = "rust1", since = "1.0.0")]
1000 impl<T: ?Sized + PartialEq> PartialEq for RefCell<T> {
1003 /// Panics if the value in either `RefCell` is currently borrowed.
1005 fn eq(&self, other: &RefCell<T>) -> bool {
1006 *self.borrow() == *other.borrow()
1010 #[stable(feature = "cell_eq", since = "1.2.0")]
1011 impl<T: ?Sized + Eq> Eq for RefCell<T> {}
1013 #[stable(feature = "cell_ord", since = "1.10.0")]
1014 impl<T: ?Sized + PartialOrd> PartialOrd for RefCell<T> {
1017 /// Panics if the value in either `RefCell` is currently borrowed.
1019 fn partial_cmp(&self, other: &RefCell<T>) -> Option<Ordering> {
1020 self.borrow().partial_cmp(&*other.borrow())
1025 /// Panics if the value in either `RefCell` is currently borrowed.
1027 fn lt(&self, other: &RefCell<T>) -> bool {
1028 *self.borrow() < *other.borrow()
1033 /// Panics if the value in either `RefCell` is currently borrowed.
1035 fn le(&self, other: &RefCell<T>) -> bool {
1036 *self.borrow() <= *other.borrow()
1041 /// Panics if the value in either `RefCell` is currently borrowed.
1043 fn gt(&self, other: &RefCell<T>) -> bool {
1044 *self.borrow() > *other.borrow()
1049 /// Panics if the value in either `RefCell` is currently borrowed.
1051 fn ge(&self, other: &RefCell<T>) -> bool {
1052 *self.borrow() >= *other.borrow()
1056 #[stable(feature = "cell_ord", since = "1.10.0")]
1057 impl<T: ?Sized + Ord> Ord for RefCell<T> {
1060 /// Panics if the value in either `RefCell` is currently borrowed.
1062 fn cmp(&self, other: &RefCell<T>) -> Ordering {
1063 self.borrow().cmp(&*other.borrow())
1067 #[stable(feature = "cell_from", since = "1.12.0")]
1068 impl<T> From<T> for RefCell<T> {
1069 fn from(t: T) -> RefCell<T> {
1074 #[unstable(feature = "coerce_unsized", issue = "27732")]
1075 impl<T: CoerceUnsized<U>, U> CoerceUnsized<RefCell<U>> for RefCell<T> {}
1077 struct BorrowRef<'b> {
1078 borrow: &'b Cell<BorrowFlag>,
1081 impl<'b> BorrowRef<'b> {
1083 fn new(borrow: &'b Cell<BorrowFlag>) -> Option<BorrowRef<'b>> {
1084 let b = borrow.get();
1085 if is_writing(b) || b == isize::max_value() {
1086 // If there's currently a writing borrow, or if incrementing the
1087 // refcount would overflow into a writing borrow.
1091 Some(BorrowRef { borrow })
1096 impl Drop for BorrowRef<'_> {
1098 fn drop(&mut self) {
1099 let borrow = self.borrow.get();
1100 debug_assert!(is_reading(borrow));
1101 self.borrow.set(borrow - 1);
1105 impl Clone for BorrowRef<'_> {
1107 fn clone(&self) -> Self {
1108 // Since this Ref exists, we know the borrow flag
1109 // is a reading borrow.
1110 let borrow = self.borrow.get();
1111 debug_assert!(is_reading(borrow));
1112 // Prevent the borrow counter from overflowing into
1113 // a writing borrow.
1114 assert!(borrow != isize::max_value());
1115 self.borrow.set(borrow + 1);
1116 BorrowRef { borrow: self.borrow }
1120 /// Wraps a borrowed reference to a value in a `RefCell` box.
1121 /// A wrapper type for an immutably borrowed value from a `RefCell<T>`.
1123 /// See the [module-level documentation](index.html) for more.
1124 #[stable(feature = "rust1", since = "1.0.0")]
1125 pub struct Ref<'b, T: ?Sized + 'b> {
1127 borrow: BorrowRef<'b>,
1130 #[stable(feature = "rust1", since = "1.0.0")]
1131 impl<T: ?Sized> Deref for Ref<'_, T> {
1135 fn deref(&self) -> &T {
1140 impl<'b, T: ?Sized> Ref<'b, T> {
1143 /// The `RefCell` is already immutably borrowed, so this cannot fail.
1145 /// This is an associated function that needs to be used as
1146 /// `Ref::clone(...)`. A `Clone` implementation or a method would interfere
1147 /// with the widespread use of `r.borrow().clone()` to clone the contents of
1149 #[stable(feature = "cell_extras", since = "1.15.0")]
1151 pub fn clone(orig: &Ref<'b, T>) -> Ref<'b, T> {
1154 borrow: orig.borrow.clone(),
1158 /// Make a new `Ref` for a component of the borrowed data.
1160 /// The `RefCell` is already immutably borrowed, so this cannot fail.
1162 /// This is an associated function that needs to be used as `Ref::map(...)`.
1163 /// A method would interfere with methods of the same name on the contents
1164 /// of a `RefCell` used through `Deref`.
1169 /// use std::cell::{RefCell, Ref};
1171 /// let c = RefCell::new((5, 'b'));
1172 /// let b1: Ref<(u32, char)> = c.borrow();
1173 /// let b2: Ref<u32> = Ref::map(b1, |t| &t.0);
1174 /// assert_eq!(*b2, 5)
1176 #[stable(feature = "cell_map", since = "1.8.0")]
1178 pub fn map<U: ?Sized, F>(orig: Ref<'b, T>, f: F) -> Ref<'b, U>
1179 where F: FnOnce(&T) -> &U
1182 value: f(orig.value),
1183 borrow: orig.borrow,
1187 /// Split a `Ref` into multiple `Ref`s for different components of the
1190 /// The `RefCell` is already immutably borrowed, so this cannot fail.
1192 /// This is an associated function that needs to be used as
1193 /// `Ref::map_split(...)`. A method would interfere with methods of the same
1194 /// name on the contents of a `RefCell` used through `Deref`.
1199 /// #![feature(refcell_map_split)]
1200 /// use std::cell::{Ref, RefCell};
1202 /// let cell = RefCell::new([1, 2, 3, 4]);
1203 /// let borrow = cell.borrow();
1204 /// let (begin, end) = Ref::map_split(borrow, |slice| slice.split_at(2));
1205 /// assert_eq!(*begin, [1, 2]);
1206 /// assert_eq!(*end, [3, 4]);
1208 #[unstable(feature = "refcell_map_split", issue = "51476")]
1210 pub fn map_split<U: ?Sized, V: ?Sized, F>(orig: Ref<'b, T>, f: F) -> (Ref<'b, U>, Ref<'b, V>)
1211 where F: FnOnce(&T) -> (&U, &V)
1213 let (a, b) = f(orig.value);
1214 let borrow = orig.borrow.clone();
1215 (Ref { value: a, borrow }, Ref { value: b, borrow: orig.borrow })
1219 #[unstable(feature = "coerce_unsized", issue = "27732")]
1220 impl<'b, T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<Ref<'b, U>> for Ref<'b, T> {}
1222 #[stable(feature = "std_guard_impls", since = "1.20.0")]
1223 impl<T: ?Sized + fmt::Display> fmt::Display for Ref<'_, T> {
1224 fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
1229 impl<'b, T: ?Sized> RefMut<'b, T> {
1230 /// Make a new `RefMut` for a component of the borrowed data, e.g., an enum
1233 /// The `RefCell` is already mutably borrowed, so this cannot fail.
1235 /// This is an associated function that needs to be used as
1236 /// `RefMut::map(...)`. A method would interfere with methods of the same
1237 /// name on the contents of a `RefCell` used through `Deref`.
1242 /// use std::cell::{RefCell, RefMut};
1244 /// let c = RefCell::new((5, 'b'));
1246 /// let b1: RefMut<(u32, char)> = c.borrow_mut();
1247 /// let mut b2: RefMut<u32> = RefMut::map(b1, |t| &mut t.0);
1248 /// assert_eq!(*b2, 5);
1251 /// assert_eq!(*c.borrow(), (42, 'b'));
1253 #[stable(feature = "cell_map", since = "1.8.0")]
1255 pub fn map<U: ?Sized, F>(orig: RefMut<'b, T>, f: F) -> RefMut<'b, U>
1256 where F: FnOnce(&mut T) -> &mut U
1258 // FIXME(nll-rfc#40): fix borrow-check
1259 let RefMut { value, borrow } = orig;
1266 /// Split a `RefMut` into multiple `RefMut`s for different components of the
1269 /// The underlying `RefCell` will remain mutably borrowed until both
1270 /// returned `RefMut`s go out of scope.
1272 /// The `RefCell` is already mutably borrowed, so this cannot fail.
1274 /// This is an associated function that needs to be used as
1275 /// `RefMut::map_split(...)`. A method would interfere with methods of the
1276 /// same name on the contents of a `RefCell` used through `Deref`.
1281 /// #![feature(refcell_map_split)]
1282 /// use std::cell::{RefCell, RefMut};
1284 /// let cell = RefCell::new([1, 2, 3, 4]);
1285 /// let borrow = cell.borrow_mut();
1286 /// let (mut begin, mut end) = RefMut::map_split(borrow, |slice| slice.split_at_mut(2));
1287 /// assert_eq!(*begin, [1, 2]);
1288 /// assert_eq!(*end, [3, 4]);
1289 /// begin.copy_from_slice(&[4, 3]);
1290 /// end.copy_from_slice(&[2, 1]);
1292 #[unstable(feature = "refcell_map_split", issue = "51476")]
1294 pub fn map_split<U: ?Sized, V: ?Sized, F>(
1295 orig: RefMut<'b, T>, f: F
1296 ) -> (RefMut<'b, U>, RefMut<'b, V>)
1297 where F: FnOnce(&mut T) -> (&mut U, &mut V)
1299 let (a, b) = f(orig.value);
1300 let borrow = orig.borrow.clone();
1301 (RefMut { value: a, borrow }, RefMut { value: b, borrow: orig.borrow })
1305 struct BorrowRefMut<'b> {
1306 borrow: &'b Cell<BorrowFlag>,
1309 impl Drop for BorrowRefMut<'_> {
1311 fn drop(&mut self) {
1312 let borrow = self.borrow.get();
1313 debug_assert!(is_writing(borrow));
1314 self.borrow.set(borrow + 1);
1318 impl<'b> BorrowRefMut<'b> {
1320 fn new(borrow: &'b Cell<BorrowFlag>) -> Option<BorrowRefMut<'b>> {
1321 // NOTE: Unlike BorrowRefMut::clone, new is called to create the initial
1322 // mutable reference, and so there must currently be no existing
1323 // references. Thus, while clone increments the mutable refcount, here
1324 // we explicitly only allow going from UNUSED to UNUSED - 1.
1325 match borrow.get() {
1327 borrow.set(UNUSED - 1);
1328 Some(BorrowRefMut { borrow })
1334 // Clone a `BorrowRefMut`.
1336 // This is only valid if each `BorrowRefMut` is used to track a mutable
1337 // reference to a distinct, nonoverlapping range of the original object.
1338 // This isn't in a Clone impl so that code doesn't call this implicitly.
1340 fn clone(&self) -> BorrowRefMut<'b> {
1341 let borrow = self.borrow.get();
1342 debug_assert!(is_writing(borrow));
1343 // Prevent the borrow counter from underflowing.
1344 assert!(borrow != isize::min_value());
1345 self.borrow.set(borrow - 1);
1346 BorrowRefMut { borrow: self.borrow }
1350 /// A wrapper type for a mutably borrowed value from a `RefCell<T>`.
1352 /// See the [module-level documentation](index.html) for more.
1353 #[stable(feature = "rust1", since = "1.0.0")]
1354 pub struct RefMut<'b, T: ?Sized + 'b> {
1356 borrow: BorrowRefMut<'b>,
1359 #[stable(feature = "rust1", since = "1.0.0")]
1360 impl<T: ?Sized> Deref for RefMut<'_, T> {
1364 fn deref(&self) -> &T {
1369 #[stable(feature = "rust1", since = "1.0.0")]
1370 impl<T: ?Sized> DerefMut for RefMut<'_, T> {
1372 fn deref_mut(&mut self) -> &mut T {
1377 #[unstable(feature = "coerce_unsized", issue = "27732")]
1378 impl<'b, T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<RefMut<'b, U>> for RefMut<'b, T> {}
1380 #[stable(feature = "std_guard_impls", since = "1.20.0")]
1381 impl<T: ?Sized + fmt::Display> fmt::Display for RefMut<'_, T> {
1382 fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
1387 /// The core primitive for interior mutability in Rust.
1389 /// `UnsafeCell<T>` is a type that wraps some `T` and indicates unsafe interior operations on the
1390 /// wrapped type. Types with an `UnsafeCell<T>` field are considered to have an 'unsafe interior'.
1391 /// The `UnsafeCell<T>` type is the only legal way to obtain aliasable data that is considered
1392 /// mutable. In general, transmuting an `&T` type into an `&mut T` is considered undefined behavior.
1394 /// If you have a reference `&SomeStruct`, then normally in Rust all fields of `SomeStruct` are
1395 /// immutable. The compiler makes optimizations based on the knowledge that `&T` is not mutably
1396 /// aliased or mutated, and that `&mut T` is unique. `UnsafeCell<T>` is the only core language
1397 /// feature to work around this restriction. All other types that allow internal mutability, such as
1398 /// `Cell<T>` and `RefCell<T>`, use `UnsafeCell` to wrap their internal data.
1400 /// The `UnsafeCell` API itself is technically very simple: it gives you a raw pointer `*mut T` to
1401 /// its contents. It is up to _you_ as the abstraction designer to use that raw pointer correctly.
1403 /// The precise Rust aliasing rules are somewhat in flux, but the main points are not contentious:
1405 /// - If you create a safe reference with lifetime `'a` (either a `&T` or `&mut T`
1406 /// reference) that is accessible by safe code (for example, because you returned it),
1407 /// then you must not access the data in any way that contradicts that reference for the
1408 /// remainder of `'a`. For example, this means that if you take the `*mut T` from an
1409 /// `UnsafeCell<T>` and cast it to an `&T`, then the data in `T` must remain immutable
1410 /// (modulo any `UnsafeCell` data found within `T`, of course) until that reference's
1411 /// lifetime expires. Similarly, if you create a `&mut T` reference that is released to
1412 /// safe code, then you must not access the data within the `UnsafeCell` until that
1413 /// reference expires.
1415 /// - At all times, you must avoid data races. If multiple threads have access to
1416 /// the same `UnsafeCell`, then any writes must have a proper happens-before relation to all other
1417 /// accesses (or use atomics).
1419 /// To assist with proper design, the following scenarios are explicitly declared legal
1420 /// for single-threaded code:
1422 /// 1. A `&T` reference can be released to safe code and there it can co-exist with other `&T`
1423 /// references, but not with a `&mut T`
1425 /// 2. A `&mut T` reference may be released to safe code provided neither other `&mut T` nor `&T`
1426 /// co-exist with it. A `&mut T` must always be unique.
1428 /// Note that while mutating or mutably aliasing the contents of an `&UnsafeCell<T>` is
1429 /// okay (provided you enforce the invariants some other way), it is still undefined behavior
1430 /// to have multiple `&mut UnsafeCell<T>` aliases.
1435 /// use std::cell::UnsafeCell;
1436 /// use std::marker::Sync;
1438 /// # #[allow(dead_code)]
1439 /// struct NotThreadSafe<T> {
1440 /// value: UnsafeCell<T>,
1443 /// unsafe impl<T> Sync for NotThreadSafe<T> {}
1445 #[lang = "unsafe_cell"]
1446 #[stable(feature = "rust1", since = "1.0.0")]
1447 #[repr(transparent)]
1448 pub struct UnsafeCell<T: ?Sized> {
1452 #[stable(feature = "rust1", since = "1.0.0")]
1453 impl<T: ?Sized> !Sync for UnsafeCell<T> {}
1455 impl<T> UnsafeCell<T> {
1456 /// Constructs a new instance of `UnsafeCell` which will wrap the specified
1459 /// All access to the inner value through methods is `unsafe`.
1464 /// use std::cell::UnsafeCell;
1466 /// let uc = UnsafeCell::new(5);
1468 #[stable(feature = "rust1", since = "1.0.0")]
1470 pub const fn new(value: T) -> UnsafeCell<T> {
1471 UnsafeCell { value }
1474 /// Unwraps the value.
1479 /// use std::cell::UnsafeCell;
1481 /// let uc = UnsafeCell::new(5);
1483 /// let five = uc.into_inner();
1486 #[stable(feature = "rust1", since = "1.0.0")]
1487 pub fn into_inner(self) -> T {
1492 impl<T: ?Sized> UnsafeCell<T> {
1493 /// Gets a mutable pointer to the wrapped value.
1495 /// This can be cast to a pointer of any kind.
1496 /// Ensure that the access is unique (no active references, mutable or not)
1497 /// when casting to `&mut T`, and ensure that there are no mutations
1498 /// or mutable aliases going on when casting to `&T`
1503 /// use std::cell::UnsafeCell;
1505 /// let uc = UnsafeCell::new(5);
1507 /// let five = uc.get();
1510 #[stable(feature = "rust1", since = "1.0.0")]
1511 pub const fn get(&self) -> *mut T {
1512 // We can just cast the pointer from `UnsafeCell<T>` to `T` because of
1513 // #[repr(transparent)]
1514 self as *const UnsafeCell<T> as *const T as *mut T
1518 #[stable(feature = "unsafe_cell_default", since = "1.10.0")]
1519 impl<T: Default> Default for UnsafeCell<T> {
1520 /// Creates an `UnsafeCell`, with the `Default` value for T.
1521 fn default() -> UnsafeCell<T> {
1522 UnsafeCell::new(Default::default())
1526 #[stable(feature = "cell_from", since = "1.12.0")]
1527 impl<T> From<T> for UnsafeCell<T> {
1528 fn from(t: T) -> UnsafeCell<T> {
1533 #[unstable(feature = "coerce_unsized", issue = "27732")]
1534 impl<T: CoerceUnsized<U>, U> CoerceUnsized<UnsafeCell<U>> for UnsafeCell<T> {}
1537 fn assert_coerce_unsized(a: UnsafeCell<&i32>, b: Cell<&i32>, c: RefCell<&i32>) {
1538 let _: UnsafeCell<&dyn Send> = a;
1539 let _: Cell<&dyn Send> = b;
1540 let _: RefCell<&dyn Send> = c;