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 /// Here you can see how using `Cell<T>` allows to use mutable field inside
211 /// immutable struct (which is also called '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, because my_struct is immutable
229 /// // my_struct.regular_field = new_value;
231 /// // WORKS, although `my_struct` is immutable, field `special_field` is mutable because it is Cell
232 /// my_struct.special_field.set(new_value);
233 /// assert_eq!(my_struct.special_field.get(), new_value);
236 /// See the [module-level documentation](index.html) for more.
237 #[stable(feature = "rust1", since = "1.0.0")]
239 pub struct Cell<T: ?Sized> {
240 value: UnsafeCell<T>,
243 impl<T:Copy> Cell<T> {
244 /// Returns a copy of the contained value.
249 /// use std::cell::Cell;
251 /// let c = Cell::new(5);
253 /// let five = c.get();
256 #[stable(feature = "rust1", since = "1.0.0")]
257 pub fn get(&self) -> T {
258 unsafe{ *self.value.get() }
261 /// Updates the contained value using a function and returns the new value.
266 /// #![feature(cell_update)]
268 /// use std::cell::Cell;
270 /// let c = Cell::new(5);
271 /// let new = c.update(|x| x + 1);
273 /// assert_eq!(new, 6);
274 /// assert_eq!(c.get(), 6);
277 #[unstable(feature = "cell_update", issue = "50186")]
278 pub fn update<F>(&self, f: F) -> T
282 let old = self.get();
289 #[stable(feature = "rust1", since = "1.0.0")]
290 unsafe impl<T: ?Sized> Send for Cell<T> where T: Send {}
292 #[stable(feature = "rust1", since = "1.0.0")]
293 impl<T: ?Sized> !Sync for Cell<T> {}
295 #[stable(feature = "rust1", since = "1.0.0")]
296 impl<T:Copy> Clone for Cell<T> {
298 fn clone(&self) -> Cell<T> {
299 Cell::new(self.get())
303 #[stable(feature = "rust1", since = "1.0.0")]
304 impl<T:Default> Default for Cell<T> {
305 /// Creates a `Cell<T>`, with the `Default` value for T.
307 fn default() -> Cell<T> {
308 Cell::new(Default::default())
312 #[stable(feature = "rust1", since = "1.0.0")]
313 impl<T:PartialEq + Copy> PartialEq for Cell<T> {
315 fn eq(&self, other: &Cell<T>) -> bool {
316 self.get() == other.get()
320 #[stable(feature = "cell_eq", since = "1.2.0")]
321 impl<T:Eq + Copy> Eq for Cell<T> {}
323 #[stable(feature = "cell_ord", since = "1.10.0")]
324 impl<T:PartialOrd + Copy> PartialOrd for Cell<T> {
326 fn partial_cmp(&self, other: &Cell<T>) -> Option<Ordering> {
327 self.get().partial_cmp(&other.get())
331 fn lt(&self, other: &Cell<T>) -> bool {
332 self.get() < other.get()
336 fn le(&self, other: &Cell<T>) -> bool {
337 self.get() <= other.get()
341 fn gt(&self, other: &Cell<T>) -> bool {
342 self.get() > other.get()
346 fn ge(&self, other: &Cell<T>) -> bool {
347 self.get() >= other.get()
351 #[stable(feature = "cell_ord", since = "1.10.0")]
352 impl<T:Ord + Copy> Ord for Cell<T> {
354 fn cmp(&self, other: &Cell<T>) -> Ordering {
355 self.get().cmp(&other.get())
359 #[stable(feature = "cell_from", since = "1.12.0")]
360 impl<T> From<T> for Cell<T> {
361 fn from(t: T) -> Cell<T> {
367 /// Creates a new `Cell` containing the given value.
372 /// use std::cell::Cell;
374 /// let c = Cell::new(5);
376 #[stable(feature = "rust1", since = "1.0.0")]
378 pub const fn new(value: T) -> Cell<T> {
380 value: UnsafeCell::new(value),
384 /// Sets the contained value.
389 /// use std::cell::Cell;
391 /// let c = Cell::new(5);
396 #[stable(feature = "rust1", since = "1.0.0")]
397 pub fn set(&self, val: T) {
398 let old = self.replace(val);
402 /// Swaps the values of two Cells.
403 /// Difference with `std::mem::swap` is that this function doesn't require `&mut` reference.
408 /// use std::cell::Cell;
410 /// let c1 = Cell::new(5i32);
411 /// let c2 = Cell::new(10i32);
413 /// assert_eq!(10, c1.get());
414 /// assert_eq!(5, c2.get());
417 #[stable(feature = "move_cell", since = "1.17.0")]
418 pub fn swap(&self, other: &Self) {
419 if ptr::eq(self, other) {
423 ptr::swap(self.value.get(), other.value.get());
427 /// Replaces the contained value, and returns it.
432 /// use std::cell::Cell;
434 /// let cell = Cell::new(5);
435 /// assert_eq!(cell.get(), 5);
436 /// assert_eq!(cell.replace(10), 5);
437 /// assert_eq!(cell.get(), 10);
439 #[stable(feature = "move_cell", since = "1.17.0")]
440 pub fn replace(&self, val: T) -> T {
441 mem::replace(unsafe { &mut *self.value.get() }, val)
444 /// Unwraps the value.
449 /// use std::cell::Cell;
451 /// let c = Cell::new(5);
452 /// let five = c.into_inner();
454 /// assert_eq!(five, 5);
456 #[stable(feature = "move_cell", since = "1.17.0")]
457 pub fn into_inner(self) -> T {
458 self.value.into_inner()
462 impl<T: ?Sized> Cell<T> {
463 /// Returns a raw pointer to the underlying data in this cell.
468 /// use std::cell::Cell;
470 /// let c = Cell::new(5);
472 /// let ptr = c.as_ptr();
475 #[stable(feature = "cell_as_ptr", since = "1.12.0")]
476 pub fn as_ptr(&self) -> *mut T {
480 /// Returns a mutable reference to the underlying data.
482 /// This call borrows `Cell` mutably (at compile-time) which guarantees
483 /// that we possess the only reference.
488 /// use std::cell::Cell;
490 /// let mut c = Cell::new(5);
491 /// *c.get_mut() += 1;
493 /// assert_eq!(c.get(), 6);
496 #[stable(feature = "cell_get_mut", since = "1.11.0")]
497 pub fn get_mut(&mut self) -> &mut T {
499 &mut *self.value.get()
503 /// Returns a `&Cell<T>` from a `&mut T`
508 /// #![feature(as_cell)]
509 /// use std::cell::Cell;
511 /// let slice: &mut [i32] = &mut [1, 2, 3];
512 /// let cell_slice: &Cell<[i32]> = Cell::from_mut(slice);
513 /// let slice_cell: &[Cell<i32>] = cell_slice.as_slice_of_cells();
515 /// assert_eq!(slice_cell.len(), 3);
518 #[unstable(feature = "as_cell", issue="43038")]
519 pub fn from_mut(t: &mut T) -> &Cell<T> {
521 &*(t as *mut T as *const Cell<T>)
526 impl<T: Default> Cell<T> {
527 /// Takes the value of the cell, leaving `Default::default()` in its place.
532 /// use std::cell::Cell;
534 /// let c = Cell::new(5);
535 /// let five = c.take();
537 /// assert_eq!(five, 5);
538 /// assert_eq!(c.into_inner(), 0);
540 #[stable(feature = "move_cell", since = "1.17.0")]
541 pub fn take(&self) -> T {
542 self.replace(Default::default())
546 #[unstable(feature = "coerce_unsized", issue = "27732")]
547 impl<T: CoerceUnsized<U>, U> CoerceUnsized<Cell<U>> for Cell<T> {}
550 /// Returns a `&[Cell<T>]` from a `&Cell<[T]>`
555 /// #![feature(as_cell)]
556 /// use std::cell::Cell;
558 /// let slice: &mut [i32] = &mut [1, 2, 3];
559 /// let cell_slice: &Cell<[i32]> = Cell::from_mut(slice);
560 /// let slice_cell: &[Cell<i32>] = cell_slice.as_slice_of_cells();
562 /// assert_eq!(slice_cell.len(), 3);
564 #[unstable(feature = "as_cell", issue="43038")]
565 pub fn as_slice_of_cells(&self) -> &[Cell<T>] {
567 &*(self as *const Cell<[T]> as *const [Cell<T>])
572 /// A mutable memory location with dynamically checked borrow rules
574 /// See the [module-level documentation](index.html) for more.
575 #[stable(feature = "rust1", since = "1.0.0")]
576 pub struct RefCell<T: ?Sized> {
577 borrow: Cell<BorrowFlag>,
578 value: UnsafeCell<T>,
581 /// An error returned by [`RefCell::try_borrow`](struct.RefCell.html#method.try_borrow).
582 #[stable(feature = "try_borrow", since = "1.13.0")]
583 pub struct BorrowError {
587 #[stable(feature = "try_borrow", since = "1.13.0")]
588 impl Debug for BorrowError {
589 fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
590 f.debug_struct("BorrowError").finish()
594 #[stable(feature = "try_borrow", since = "1.13.0")]
595 impl Display for BorrowError {
596 fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
597 Display::fmt("already mutably borrowed", f)
601 /// An error returned by [`RefCell::try_borrow_mut`](struct.RefCell.html#method.try_borrow_mut).
602 #[stable(feature = "try_borrow", since = "1.13.0")]
603 pub struct BorrowMutError {
607 #[stable(feature = "try_borrow", since = "1.13.0")]
608 impl Debug for BorrowMutError {
609 fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
610 f.debug_struct("BorrowMutError").finish()
614 #[stable(feature = "try_borrow", since = "1.13.0")]
615 impl Display for BorrowMutError {
616 fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
617 Display::fmt("already borrowed", f)
621 // Positive values represent the number of `Ref` active. Negative values
622 // represent the number of `RefMut` active. Multiple `RefMut`s can only be
623 // active at a time if they refer to distinct, nonoverlapping components of a
624 // `RefCell` (e.g., different ranges of a slice).
626 // `Ref` and `RefMut` are both two words in size, and so there will likely never
627 // be enough `Ref`s or `RefMut`s in existence to overflow half of the `usize`
628 // range. Thus, a `BorrowFlag` will probably never overflow or underflow.
629 // However, this is not a guarantee, as a pathological program could repeatedly
630 // create and then mem::forget `Ref`s or `RefMut`s. Thus, all code must
631 // explicitly check for overflow and underflow in order to avoid unsafety, or at
632 // least behave correctly in the event that overflow or underflow happens (e.g.,
633 // see BorrowRef::new).
634 type BorrowFlag = isize;
635 const UNUSED: BorrowFlag = 0;
638 fn is_writing(x: BorrowFlag) -> bool {
643 fn is_reading(x: BorrowFlag) -> bool {
648 /// Creates a new `RefCell` containing `value`.
653 /// use std::cell::RefCell;
655 /// let c = RefCell::new(5);
657 #[stable(feature = "rust1", since = "1.0.0")]
659 pub const fn new(value: T) -> RefCell<T> {
661 value: UnsafeCell::new(value),
662 borrow: Cell::new(UNUSED),
666 /// Consumes the `RefCell`, returning the wrapped value.
671 /// use std::cell::RefCell;
673 /// let c = RefCell::new(5);
675 /// let five = c.into_inner();
677 #[stable(feature = "rust1", since = "1.0.0")]
679 pub fn into_inner(self) -> T {
680 // Since this function takes `self` (the `RefCell`) by value, the
681 // compiler statically verifies that it is not currently borrowed.
682 // Therefore the following assertion is just a `debug_assert!`.
683 debug_assert!(self.borrow.get() == UNUSED);
684 self.value.into_inner()
687 /// Replaces the wrapped value with a new one, returning the old value,
688 /// without deinitializing either one.
690 /// This function corresponds to [`std::mem::replace`](../mem/fn.replace.html).
694 /// Panics if the value is currently borrowed.
699 /// use std::cell::RefCell;
700 /// let cell = RefCell::new(5);
701 /// let old_value = cell.replace(6);
702 /// assert_eq!(old_value, 5);
703 /// assert_eq!(cell, RefCell::new(6));
706 #[stable(feature = "refcell_replace", since="1.24.0")]
707 pub fn replace(&self, t: T) -> T {
708 mem::replace(&mut *self.borrow_mut(), t)
711 /// Replaces the wrapped value with a new one computed from `f`, returning
712 /// the old value, without deinitializing either one.
714 /// This function corresponds to [`std::mem::replace`](../mem/fn.replace.html).
718 /// Panics if the value is currently borrowed.
723 /// #![feature(refcell_replace_swap)]
724 /// use std::cell::RefCell;
725 /// let cell = RefCell::new(5);
726 /// let old_value = cell.replace_with(|&mut old| old + 1);
727 /// assert_eq!(old_value, 5);
728 /// assert_eq!(cell, RefCell::new(6));
731 #[unstable(feature = "refcell_replace_swap", issue="43570")]
732 pub fn replace_with<F: FnOnce(&mut T) -> T>(&self, f: F) -> T {
733 let mut_borrow = &mut *self.borrow_mut();
734 let replacement = f(mut_borrow);
735 mem::replace(mut_borrow, replacement)
738 /// Swaps the wrapped value of `self` with the wrapped value of `other`,
739 /// without deinitializing either one.
741 /// This function corresponds to [`std::mem::swap`](../mem/fn.swap.html).
745 /// Panics if the value in either `RefCell` is currently borrowed.
750 /// use std::cell::RefCell;
751 /// let c = RefCell::new(5);
752 /// let d = RefCell::new(6);
754 /// assert_eq!(c, RefCell::new(6));
755 /// assert_eq!(d, RefCell::new(5));
758 #[stable(feature = "refcell_swap", since="1.24.0")]
759 pub fn swap(&self, other: &Self) {
760 mem::swap(&mut *self.borrow_mut(), &mut *other.borrow_mut())
764 impl<T: ?Sized> RefCell<T> {
765 /// Immutably borrows the wrapped value.
767 /// The borrow lasts until the returned `Ref` exits scope. Multiple
768 /// immutable borrows can be taken out at the same time.
772 /// Panics if the value is currently mutably borrowed. For a non-panicking variant, use
773 /// [`try_borrow`](#method.try_borrow).
778 /// use std::cell::RefCell;
780 /// let c = RefCell::new(5);
782 /// let borrowed_five = c.borrow();
783 /// let borrowed_five2 = c.borrow();
786 /// An example of panic:
789 /// use std::cell::RefCell;
792 /// let result = thread::spawn(move || {
793 /// let c = RefCell::new(5);
794 /// let m = c.borrow_mut();
796 /// let b = c.borrow(); // this causes a panic
799 /// assert!(result.is_err());
801 #[stable(feature = "rust1", since = "1.0.0")]
803 pub fn borrow(&self) -> Ref<T> {
804 self.try_borrow().expect("already mutably borrowed")
807 /// Immutably borrows the wrapped value, returning an error if the value is currently mutably
810 /// The borrow lasts until the returned `Ref` exits scope. Multiple immutable borrows can be
811 /// taken out at the same time.
813 /// This is the non-panicking variant of [`borrow`](#method.borrow).
818 /// use std::cell::RefCell;
820 /// let c = RefCell::new(5);
823 /// let m = c.borrow_mut();
824 /// assert!(c.try_borrow().is_err());
828 /// let m = c.borrow();
829 /// assert!(c.try_borrow().is_ok());
832 #[stable(feature = "try_borrow", since = "1.13.0")]
834 pub fn try_borrow(&self) -> Result<Ref<T>, BorrowError> {
835 match BorrowRef::new(&self.borrow) {
837 value: unsafe { &*self.value.get() },
840 None => Err(BorrowError { _private: () }),
844 /// Mutably borrows the wrapped value.
846 /// The borrow lasts until the returned `RefMut` or all `RefMut`s derived
847 /// from it exit scope. The value cannot be borrowed while this borrow is
852 /// Panics if the value is currently borrowed. For a non-panicking variant, use
853 /// [`try_borrow_mut`](#method.try_borrow_mut).
858 /// use std::cell::RefCell;
860 /// let c = RefCell::new(5);
862 /// *c.borrow_mut() = 7;
864 /// assert_eq!(*c.borrow(), 7);
867 /// An example of panic:
870 /// use std::cell::RefCell;
873 /// let result = thread::spawn(move || {
874 /// let c = RefCell::new(5);
875 /// let m = c.borrow();
877 /// let b = c.borrow_mut(); // this causes a panic
880 /// assert!(result.is_err());
882 #[stable(feature = "rust1", since = "1.0.0")]
884 pub fn borrow_mut(&self) -> RefMut<T> {
885 self.try_borrow_mut().expect("already borrowed")
888 /// Mutably borrows the wrapped value, returning an error if the value is currently borrowed.
890 /// The borrow lasts until the returned `RefMut` or all `RefMut`s derived
891 /// from it exit scope. The value cannot be borrowed while this borrow is
894 /// This is the non-panicking variant of [`borrow_mut`](#method.borrow_mut).
899 /// use std::cell::RefCell;
901 /// let c = RefCell::new(5);
904 /// let m = c.borrow();
905 /// assert!(c.try_borrow_mut().is_err());
908 /// assert!(c.try_borrow_mut().is_ok());
910 #[stable(feature = "try_borrow", since = "1.13.0")]
912 pub fn try_borrow_mut(&self) -> Result<RefMut<T>, BorrowMutError> {
913 match BorrowRefMut::new(&self.borrow) {
914 Some(b) => Ok(RefMut {
915 value: unsafe { &mut *self.value.get() },
918 None => Err(BorrowMutError { _private: () }),
922 /// Returns a raw pointer to the underlying data in this cell.
927 /// use std::cell::RefCell;
929 /// let c = RefCell::new(5);
931 /// let ptr = c.as_ptr();
934 #[stable(feature = "cell_as_ptr", since = "1.12.0")]
935 pub fn as_ptr(&self) -> *mut T {
939 /// Returns a mutable reference to the underlying data.
941 /// This call borrows `RefCell` mutably (at compile-time) so there is no
942 /// need for dynamic checks.
944 /// However be cautious: this method expects `self` to be mutable, which is
945 /// generally not the case when using a `RefCell`. Take a look at the
946 /// [`borrow_mut`] method instead if `self` isn't mutable.
948 /// Also, please be aware that this method is only for special circumstances and is usually
949 /// not what you want. In case of doubt, use [`borrow_mut`] instead.
951 /// [`borrow_mut`]: #method.borrow_mut
956 /// use std::cell::RefCell;
958 /// let mut c = RefCell::new(5);
959 /// *c.get_mut() += 1;
961 /// assert_eq!(c, RefCell::new(6));
964 #[stable(feature = "cell_get_mut", since = "1.11.0")]
965 pub fn get_mut(&mut self) -> &mut T {
967 &mut *self.value.get()
972 #[stable(feature = "rust1", since = "1.0.0")]
973 unsafe impl<T: ?Sized> Send for RefCell<T> where T: Send {}
975 #[stable(feature = "rust1", since = "1.0.0")]
976 impl<T: ?Sized> !Sync for RefCell<T> {}
978 #[stable(feature = "rust1", since = "1.0.0")]
979 impl<T: Clone> Clone for RefCell<T> {
982 /// Panics if the value is currently mutably borrowed.
984 fn clone(&self) -> RefCell<T> {
985 RefCell::new(self.borrow().clone())
989 #[stable(feature = "rust1", since = "1.0.0")]
990 impl<T:Default> Default for RefCell<T> {
991 /// Creates a `RefCell<T>`, with the `Default` value for T.
993 fn default() -> RefCell<T> {
994 RefCell::new(Default::default())
998 #[stable(feature = "rust1", since = "1.0.0")]
999 impl<T: ?Sized + PartialEq> PartialEq for RefCell<T> {
1002 /// Panics if the value in either `RefCell` is currently borrowed.
1004 fn eq(&self, other: &RefCell<T>) -> bool {
1005 *self.borrow() == *other.borrow()
1009 #[stable(feature = "cell_eq", since = "1.2.0")]
1010 impl<T: ?Sized + Eq> Eq for RefCell<T> {}
1012 #[stable(feature = "cell_ord", since = "1.10.0")]
1013 impl<T: ?Sized + PartialOrd> PartialOrd for RefCell<T> {
1016 /// Panics if the value in either `RefCell` is currently borrowed.
1018 fn partial_cmp(&self, other: &RefCell<T>) -> Option<Ordering> {
1019 self.borrow().partial_cmp(&*other.borrow())
1024 /// Panics if the value in either `RefCell` is currently borrowed.
1026 fn lt(&self, other: &RefCell<T>) -> bool {
1027 *self.borrow() < *other.borrow()
1032 /// Panics if the value in either `RefCell` is currently borrowed.
1034 fn le(&self, other: &RefCell<T>) -> bool {
1035 *self.borrow() <= *other.borrow()
1040 /// Panics if the value in either `RefCell` is currently borrowed.
1042 fn gt(&self, other: &RefCell<T>) -> bool {
1043 *self.borrow() > *other.borrow()
1048 /// Panics if the value in either `RefCell` is currently borrowed.
1050 fn ge(&self, other: &RefCell<T>) -> bool {
1051 *self.borrow() >= *other.borrow()
1055 #[stable(feature = "cell_ord", since = "1.10.0")]
1056 impl<T: ?Sized + Ord> Ord for RefCell<T> {
1059 /// Panics if the value in either `RefCell` is currently borrowed.
1061 fn cmp(&self, other: &RefCell<T>) -> Ordering {
1062 self.borrow().cmp(&*other.borrow())
1066 #[stable(feature = "cell_from", since = "1.12.0")]
1067 impl<T> From<T> for RefCell<T> {
1068 fn from(t: T) -> RefCell<T> {
1073 #[unstable(feature = "coerce_unsized", issue = "27732")]
1074 impl<T: CoerceUnsized<U>, U> CoerceUnsized<RefCell<U>> for RefCell<T> {}
1076 struct BorrowRef<'b> {
1077 borrow: &'b Cell<BorrowFlag>,
1080 impl<'b> BorrowRef<'b> {
1082 fn new(borrow: &'b Cell<BorrowFlag>) -> Option<BorrowRef<'b>> {
1083 let b = borrow.get();
1084 if is_writing(b) || b == isize::max_value() {
1085 // If there's currently a writing borrow, or if incrementing the
1086 // refcount would overflow into a writing borrow.
1090 Some(BorrowRef { borrow })
1095 impl<'b> Drop for BorrowRef<'b> {
1097 fn drop(&mut self) {
1098 let borrow = self.borrow.get();
1099 debug_assert!(is_reading(borrow));
1100 self.borrow.set(borrow - 1);
1104 impl<'b> Clone for BorrowRef<'b> {
1106 fn clone(&self) -> BorrowRef<'b> {
1107 // Since this Ref exists, we know the borrow flag
1108 // is a reading borrow.
1109 let borrow = self.borrow.get();
1110 debug_assert!(is_reading(borrow));
1111 // Prevent the borrow counter from overflowing into
1112 // a writing borrow.
1113 assert!(borrow != isize::max_value());
1114 self.borrow.set(borrow + 1);
1115 BorrowRef { borrow: self.borrow }
1119 /// Wraps a borrowed reference to a value in a `RefCell` box.
1120 /// A wrapper type for an immutably borrowed value from a `RefCell<T>`.
1122 /// See the [module-level documentation](index.html) for more.
1123 #[stable(feature = "rust1", since = "1.0.0")]
1124 pub struct Ref<'b, T: ?Sized + 'b> {
1126 borrow: BorrowRef<'b>,
1129 #[stable(feature = "rust1", since = "1.0.0")]
1130 impl<'b, T: ?Sized> Deref for Ref<'b, T> {
1134 fn deref(&self) -> &T {
1139 impl<'b, T: ?Sized> Ref<'b, T> {
1142 /// The `RefCell` is already immutably borrowed, so this cannot fail.
1144 /// This is an associated function that needs to be used as
1145 /// `Ref::clone(...)`. A `Clone` implementation or a method would interfere
1146 /// with the widespread use of `r.borrow().clone()` to clone the contents of
1148 #[stable(feature = "cell_extras", since = "1.15.0")]
1150 pub fn clone(orig: &Ref<'b, T>) -> Ref<'b, T> {
1153 borrow: orig.borrow.clone(),
1157 /// Make a new `Ref` for a component of the borrowed data.
1159 /// The `RefCell` is already immutably borrowed, so this cannot fail.
1161 /// This is an associated function that needs to be used as `Ref::map(...)`.
1162 /// A method would interfere with methods of the same name on the contents
1163 /// of a `RefCell` used through `Deref`.
1168 /// use std::cell::{RefCell, Ref};
1170 /// let c = RefCell::new((5, 'b'));
1171 /// let b1: Ref<(u32, char)> = c.borrow();
1172 /// let b2: Ref<u32> = Ref::map(b1, |t| &t.0);
1173 /// assert_eq!(*b2, 5)
1175 #[stable(feature = "cell_map", since = "1.8.0")]
1177 pub fn map<U: ?Sized, F>(orig: Ref<'b, T>, f: F) -> Ref<'b, U>
1178 where F: FnOnce(&T) -> &U
1181 value: f(orig.value),
1182 borrow: orig.borrow,
1186 /// Split a `Ref` into multiple `Ref`s for different components of the
1189 /// The `RefCell` is already immutably borrowed, so this cannot fail.
1191 /// This is an associated function that needs to be used as
1192 /// `Ref::map_split(...)`. A method would interfere with methods of the same
1193 /// name on the contents of a `RefCell` used through `Deref`.
1198 /// #![feature(refcell_map_split)]
1199 /// use std::cell::{Ref, RefCell};
1201 /// let cell = RefCell::new([1, 2, 3, 4]);
1202 /// let borrow = cell.borrow();
1203 /// let (begin, end) = Ref::map_split(borrow, |slice| slice.split_at(2));
1204 /// assert_eq!(*begin, [1, 2]);
1205 /// assert_eq!(*end, [3, 4]);
1207 #[unstable(feature = "refcell_map_split", issue = "51476")]
1209 pub fn map_split<U: ?Sized, V: ?Sized, F>(orig: Ref<'b, T>, f: F) -> (Ref<'b, U>, Ref<'b, V>)
1210 where F: FnOnce(&T) -> (&U, &V)
1212 let (a, b) = f(orig.value);
1213 let borrow = orig.borrow.clone();
1214 (Ref { value: a, borrow }, Ref { value: b, borrow: orig.borrow })
1218 #[unstable(feature = "coerce_unsized", issue = "27732")]
1219 impl<'b, T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<Ref<'b, U>> for Ref<'b, T> {}
1221 #[stable(feature = "std_guard_impls", since = "1.20.0")]
1222 impl<'a, T: ?Sized + fmt::Display> fmt::Display for Ref<'a, T> {
1223 fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
1228 impl<'b, T: ?Sized> RefMut<'b, T> {
1229 /// Make a new `RefMut` for a component of the borrowed data, e.g. an enum
1232 /// The `RefCell` is already mutably borrowed, so this cannot fail.
1234 /// This is an associated function that needs to be used as
1235 /// `RefMut::map(...)`. A method would interfere with methods of the same
1236 /// name on the contents of a `RefCell` used through `Deref`.
1241 /// use std::cell::{RefCell, RefMut};
1243 /// let c = RefCell::new((5, 'b'));
1245 /// let b1: RefMut<(u32, char)> = c.borrow_mut();
1246 /// let mut b2: RefMut<u32> = RefMut::map(b1, |t| &mut t.0);
1247 /// assert_eq!(*b2, 5);
1250 /// assert_eq!(*c.borrow(), (42, 'b'));
1252 #[stable(feature = "cell_map", since = "1.8.0")]
1254 pub fn map<U: ?Sized, F>(orig: RefMut<'b, T>, f: F) -> RefMut<'b, U>
1255 where F: FnOnce(&mut T) -> &mut U
1257 // FIXME(nll-rfc#40): fix borrow-check
1258 let RefMut { value, borrow } = orig;
1265 /// Split a `RefMut` into multiple `RefMut`s for different components of the
1268 /// The underlying `RefCell` will remain mutably borrowed until both
1269 /// returned `RefMut`s go out of scope.
1271 /// The `RefCell` is already mutably borrowed, so this cannot fail.
1273 /// This is an associated function that needs to be used as
1274 /// `RefMut::map_split(...)`. A method would interfere with methods of the
1275 /// same name on the contents of a `RefCell` used through `Deref`.
1280 /// #![feature(refcell_map_split)]
1281 /// use std::cell::{RefCell, RefMut};
1283 /// let cell = RefCell::new([1, 2, 3, 4]);
1284 /// let borrow = cell.borrow_mut();
1285 /// let (mut begin, mut end) = RefMut::map_split(borrow, |slice| slice.split_at_mut(2));
1286 /// assert_eq!(*begin, [1, 2]);
1287 /// assert_eq!(*end, [3, 4]);
1288 /// begin.copy_from_slice(&[4, 3]);
1289 /// end.copy_from_slice(&[2, 1]);
1291 #[unstable(feature = "refcell_map_split", issue = "51476")]
1293 pub fn map_split<U: ?Sized, V: ?Sized, F>(
1294 orig: RefMut<'b, T>, f: F
1295 ) -> (RefMut<'b, U>, RefMut<'b, V>)
1296 where F: FnOnce(&mut T) -> (&mut U, &mut V)
1298 let (a, b) = f(orig.value);
1299 let borrow = orig.borrow.clone();
1300 (RefMut { value: a, borrow }, RefMut { value: b, borrow: orig.borrow })
1304 struct BorrowRefMut<'b> {
1305 borrow: &'b Cell<BorrowFlag>,
1308 impl<'b> Drop for BorrowRefMut<'b> {
1310 fn drop(&mut self) {
1311 let borrow = self.borrow.get();
1312 debug_assert!(is_writing(borrow));
1313 self.borrow.set(borrow + 1);
1317 impl<'b> BorrowRefMut<'b> {
1319 fn new(borrow: &'b Cell<BorrowFlag>) -> Option<BorrowRefMut<'b>> {
1320 // NOTE: Unlike BorrowRefMut::clone, new is called to create the initial
1321 // mutable reference, and so there must currently be no existing
1322 // references. Thus, while clone increments the mutable refcount, here
1323 // we explicitly only allow going from UNUSED to UNUSED - 1.
1324 match borrow.get() {
1326 borrow.set(UNUSED - 1);
1327 Some(BorrowRefMut { borrow: borrow })
1333 // Clone a `BorrowRefMut`.
1335 // This is only valid if each `BorrowRefMut` is used to track a mutable
1336 // reference to a distinct, nonoverlapping range of the original object.
1337 // This isn't in a Clone impl so that code doesn't call this implicitly.
1339 fn clone(&self) -> BorrowRefMut<'b> {
1340 let borrow = self.borrow.get();
1341 debug_assert!(is_writing(borrow));
1342 // Prevent the borrow counter from underflowing.
1343 assert!(borrow != isize::min_value());
1344 self.borrow.set(borrow - 1);
1345 BorrowRefMut { borrow: self.borrow }
1349 /// A wrapper type for a mutably borrowed value from a `RefCell<T>`.
1351 /// See the [module-level documentation](index.html) for more.
1352 #[stable(feature = "rust1", since = "1.0.0")]
1353 pub struct RefMut<'b, T: ?Sized + 'b> {
1355 borrow: BorrowRefMut<'b>,
1358 #[stable(feature = "rust1", since = "1.0.0")]
1359 impl<'b, T: ?Sized> Deref for RefMut<'b, T> {
1363 fn deref(&self) -> &T {
1368 #[stable(feature = "rust1", since = "1.0.0")]
1369 impl<'b, T: ?Sized> DerefMut for RefMut<'b, T> {
1371 fn deref_mut(&mut self) -> &mut T {
1376 #[unstable(feature = "coerce_unsized", issue = "27732")]
1377 impl<'b, T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<RefMut<'b, U>> for RefMut<'b, T> {}
1379 #[stable(feature = "std_guard_impls", since = "1.20.0")]
1380 impl<'a, T: ?Sized + fmt::Display> fmt::Display for RefMut<'a, T> {
1381 fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
1386 /// The core primitive for interior mutability in Rust.
1388 /// `UnsafeCell<T>` is a type that wraps some `T` and indicates unsafe interior operations on the
1389 /// wrapped type. Types with an `UnsafeCell<T>` field are considered to have an 'unsafe interior'.
1390 /// The `UnsafeCell<T>` type is the only legal way to obtain aliasable data that is considered
1391 /// mutable. In general, transmuting an `&T` type into an `&mut T` is considered undefined behavior.
1393 /// If you have a reference `&SomeStruct`, then normally in Rust all fields of `SomeStruct` are
1394 /// immutable. The compiler makes optimizations based on the knowledge that `&T` is not mutably
1395 /// aliased or mutated, and that `&mut T` is unique. `UnsafeCell<T>` is the only core language
1396 /// feature to work around this restriction. All other types that allow internal mutability, such as
1397 /// `Cell<T>` and `RefCell<T>`, use `UnsafeCell` to wrap their internal data.
1399 /// The `UnsafeCell` API itself is technically very simple: it gives you a raw pointer `*mut T` to
1400 /// its contents. It is up to _you_ as the abstraction designer to use that raw pointer correctly.
1402 /// The precise Rust aliasing rules are somewhat in flux, but the main points are not contentious:
1404 /// - If you create a safe reference with lifetime `'a` (either a `&T` or `&mut T`
1405 /// reference) that is accessible by safe code (for example, because you returned it),
1406 /// then you must not access the data in any way that contradicts that reference for the
1407 /// remainder of `'a`. For example, this means that if you take the `*mut T` from an
1408 /// `UnsafeCell<T>` and cast it to an `&T`, then the data in `T` must remain immutable
1409 /// (modulo any `UnsafeCell` data found within `T`, of course) until that reference's
1410 /// lifetime expires. Similarly, if you create a `&mut T` reference that is released to
1411 /// safe code, then you must not access the data within the `UnsafeCell` until that
1412 /// reference expires.
1414 /// - At all times, you must avoid data races. If multiple threads have access to
1415 /// the same `UnsafeCell`, then any writes must have a proper happens-before relation to all other
1416 /// accesses (or use atomics).
1418 /// To assist with proper design, the following scenarios are explicitly declared legal
1419 /// for single-threaded code:
1421 /// 1. A `&T` reference can be released to safe code and there it can co-exist with other `&T`
1422 /// references, but not with a `&mut T`
1424 /// 2. A `&mut T` reference may be released to safe code provided neither other `&mut T` nor `&T`
1425 /// co-exist with it. A `&mut T` must always be unique.
1427 /// Note that while mutating or mutably aliasing the contents of an `&UnsafeCell<T>` is
1428 /// okay (provided you enforce the invariants some other way), it is still undefined behavior
1429 /// to have multiple `&mut UnsafeCell<T>` aliases.
1434 /// use std::cell::UnsafeCell;
1435 /// use std::marker::Sync;
1437 /// # #[allow(dead_code)]
1438 /// struct NotThreadSafe<T> {
1439 /// value: UnsafeCell<T>,
1442 /// unsafe impl<T> Sync for NotThreadSafe<T> {}
1444 #[lang = "unsafe_cell"]
1445 #[stable(feature = "rust1", since = "1.0.0")]
1446 #[repr(transparent)]
1447 pub struct UnsafeCell<T: ?Sized> {
1451 #[stable(feature = "rust1", since = "1.0.0")]
1452 impl<T: ?Sized> !Sync for UnsafeCell<T> {}
1454 impl<T> UnsafeCell<T> {
1455 /// Constructs a new instance of `UnsafeCell` which will wrap the specified
1458 /// All access to the inner value through methods is `unsafe`.
1463 /// use std::cell::UnsafeCell;
1465 /// let uc = UnsafeCell::new(5);
1467 #[stable(feature = "rust1", since = "1.0.0")]
1469 pub const fn new(value: T) -> UnsafeCell<T> {
1470 UnsafeCell { value: value }
1473 /// Unwraps the value.
1478 /// use std::cell::UnsafeCell;
1480 /// let uc = UnsafeCell::new(5);
1482 /// let five = uc.into_inner();
1485 #[stable(feature = "rust1", since = "1.0.0")]
1486 pub fn into_inner(self) -> T {
1491 impl<T: ?Sized> UnsafeCell<T> {
1492 /// Gets a mutable pointer to the wrapped value.
1494 /// This can be cast to a pointer of any kind.
1495 /// Ensure that the access is unique (no active references, mutable or not)
1496 /// when casting to `&mut T`, and ensure that there are no mutations
1497 /// or mutable aliases going on when casting to `&T`
1502 /// use std::cell::UnsafeCell;
1504 /// let uc = UnsafeCell::new(5);
1506 /// let five = uc.get();
1509 #[stable(feature = "rust1", since = "1.0.0")]
1510 pub fn get(&self) -> *mut T {
1511 &self.value as *const T as *mut T
1515 #[stable(feature = "unsafe_cell_default", since = "1.10.0")]
1516 impl<T: Default> Default for UnsafeCell<T> {
1517 /// Creates an `UnsafeCell`, with the `Default` value for T.
1518 fn default() -> UnsafeCell<T> {
1519 UnsafeCell::new(Default::default())
1523 #[stable(feature = "cell_from", since = "1.12.0")]
1524 impl<T> From<T> for UnsafeCell<T> {
1525 fn from(t: T) -> UnsafeCell<T> {
1530 #[unstable(feature = "coerce_unsized", issue = "27732")]
1531 impl<T: CoerceUnsized<U>, U> CoerceUnsized<UnsafeCell<U>> for UnsafeCell<T> {}
1534 fn assert_coerce_unsized(a: UnsafeCell<&i32>, b: Cell<&i32>, c: RefCell<&i32>) {
1535 let _: UnsafeCell<&dyn Send> = a;
1536 let _: Cell<&dyn Send> = b;
1537 let _: RefCell<&dyn Send> = c;