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 //! Values of the `Cell<T>` and `RefCell<T>` types may be mutated through shared references (i.e.
14 //! the common `&T` type), whereas most Rust types can only be mutated through unique (`&mut T`)
15 //! references. We say that `Cell<T>` and `RefCell<T>` provide 'interior mutability', in contrast
16 //! with typical Rust types that exhibit 'inherited mutability'.
18 //! Cell types come in two flavors: `Cell<T>` and `RefCell<T>`. `Cell<T>` implements interior
19 //! mutability by moving values in and out of the `Cell<T>`. To use references instead of values,
20 //! one must use the `RefCell<T>` type, acquiring a write lock before mutating. `Cell<T>` provides
21 //! methods to retrieve and change the current interior value:
23 //! - For types that implement `Copy`, the `get` method retrieves the current interior value.
24 //! - For types that implement `Default`, the `take` method replaces the current interior value
25 //! with `Default::default()` and returns the replaced value.
26 //! - For all types, the `replace` method replaces the current interior value and returns the
27 //! replaced value and the `into_inner` method consumes the `Cell<T>` and returns the interior
28 //! value. Additionally, the `set` method replaces the interior value, dropping the replaced
31 //! `RefCell<T>` uses Rust's lifetimes to implement 'dynamic borrowing', a process whereby one can
32 //! claim temporary, exclusive, mutable access to the inner value. Borrows for `RefCell<T>`s are
33 //! tracked 'at runtime', unlike Rust's native reference types which are entirely tracked
34 //! statically, at compile time. Because `RefCell<T>` borrows are dynamic it is possible to attempt
35 //! to borrow a value that is already mutably borrowed; when this happens it results in thread
38 //! # When to choose interior mutability
40 //! The more common inherited mutability, where one must have unique access to mutate a value, is
41 //! one of the key language elements that enables Rust to reason strongly about pointer aliasing,
42 //! statically preventing crash bugs. Because of that, inherited mutability is preferred, and
43 //! interior mutability is something of a last resort. Since cell types enable mutation where it
44 //! would otherwise be disallowed though, there are occasions when interior mutability might be
45 //! appropriate, or even *must* be used, e.g.
47 //! * Introducing mutability 'inside' of something immutable
48 //! * Implementation details of logically-immutable methods.
49 //! * Mutating implementations of `Clone`.
51 //! ## Introducing mutability 'inside' of something immutable
53 //! Many shared smart pointer types, including `Rc<T>` and `Arc<T>`, provide containers that can be
54 //! cloned and shared between multiple parties. Because the contained values may be
55 //! multiply-aliased, they can only be borrowed with `&`, not `&mut`. Without cells it would be
56 //! impossible to mutate data inside of these smart pointers at all.
58 //! It's very common then to put a `RefCell<T>` inside shared pointer types to reintroduce
62 //! use std::collections::HashMap;
63 //! use std::cell::RefCell;
67 //! let shared_map: Rc<RefCell<_>> = Rc::new(RefCell::new(HashMap::new()));
68 //! shared_map.borrow_mut().insert("africa", 92388);
69 //! shared_map.borrow_mut().insert("kyoto", 11837);
70 //! shared_map.borrow_mut().insert("piccadilly", 11826);
71 //! shared_map.borrow_mut().insert("marbles", 38);
75 //! Note that this example uses `Rc<T>` and not `Arc<T>`. `RefCell<T>`s are for single-threaded
76 //! scenarios. Consider using `RwLock<T>` or `Mutex<T>` if you need shared mutability in a
77 //! multi-threaded situation.
79 //! ## Implementation details of logically-immutable methods
81 //! Occasionally it may be desirable not to expose in an API that there is mutation happening
82 //! "under the hood". This may be because logically the operation is immutable, but e.g. caching
83 //! forces the implementation to perform mutation; or because you must employ mutation to implement
84 //! a trait method that was originally defined to take `&self`.
87 //! # #![allow(dead_code)]
88 //! use std::cell::RefCell;
91 //! edges: Vec<(i32, i32)>,
92 //! span_tree_cache: RefCell<Option<Vec<(i32, i32)>>>
96 //! fn minimum_spanning_tree(&self) -> Vec<(i32, i32)> {
97 //! // Create a new scope to contain the lifetime of the
100 //! // Take a reference to the inside of cache cell
101 //! let mut cache = self.span_tree_cache.borrow_mut();
102 //! if cache.is_some() {
103 //! return cache.as_ref().unwrap().clone();
106 //! let span_tree = self.calc_span_tree();
107 //! *cache = Some(span_tree);
110 //! // Recursive call to return the just-cached value.
111 //! // Note that if we had not let the previous borrow
112 //! // of the cache fall out of scope then the subsequent
113 //! // recursive borrow would cause a dynamic thread panic.
114 //! // This is the major hazard of using `RefCell`.
115 //! self.minimum_spanning_tree()
117 //! # fn calc_span_tree(&self) -> Vec<(i32, i32)> { vec![] }
121 //! ## Mutating implementations of `Clone`
123 //! This is simply a special - but common - case of the previous: hiding mutability for operations
124 //! that appear to be immutable. The `clone` method is expected to not change the source value, and
125 //! is declared to take `&self`, not `&mut self`. Therefore any mutation that happens in the
126 //! `clone` method must use cell types. For example, `Rc<T>` maintains its reference counts within a
130 //! #![feature(core_intrinsics)]
131 //! #![feature(shared)]
132 //! use std::cell::Cell;
133 //! use std::ptr::Shared;
134 //! use std::intrinsics::abort;
135 //! use std::intrinsics::assume;
137 //! struct Rc<T: ?Sized> {
138 //! ptr: Shared<RcBox<T>>
141 //! struct RcBox<T: ?Sized> {
142 //! strong: Cell<usize>,
143 //! refcount: Cell<usize>,
147 //! impl<T: ?Sized> Clone for Rc<T> {
148 //! fn clone(&self) -> Rc<T> {
149 //! self.inc_strong();
150 //! Rc { ptr: self.ptr }
154 //! trait RcBoxPtr<T: ?Sized> {
156 //! fn inner(&self) -> &RcBox<T>;
158 //! fn strong(&self) -> usize {
159 //! self.inner().strong.get()
162 //! fn inc_strong(&self) {
165 //! .set(self.strong()
167 //! .unwrap_or_else(|| unsafe { abort() }));
171 //! impl<T: ?Sized> RcBoxPtr<T> for Rc<T> {
172 //! fn inner(&self) -> &RcBox<T> {
174 //! assume(!(*(&self.ptr as *const _ as *const *const ())).is_null());
182 #![stable(feature = "rust1", since = "1.0.0")]
185 use fmt::{self, Debug, Display};
188 use ops::{Deref, DerefMut, CoerceUnsized};
191 /// A mutable memory location.
193 /// See the [module-level documentation](index.html) for more.
194 #[stable(feature = "rust1", since = "1.0.0")]
196 value: UnsafeCell<T>,
199 impl<T:Copy> Cell<T> {
200 /// Returns a copy of the contained value.
205 /// use std::cell::Cell;
207 /// let c = Cell::new(5);
209 /// let five = c.get();
212 #[stable(feature = "rust1", since = "1.0.0")]
213 pub fn get(&self) -> T {
214 unsafe{ *self.value.get() }
218 #[stable(feature = "rust1", since = "1.0.0")]
219 unsafe impl<T> Send for Cell<T> where T: Send {}
221 #[stable(feature = "rust1", since = "1.0.0")]
222 impl<T> !Sync for Cell<T> {}
224 #[stable(feature = "rust1", since = "1.0.0")]
225 impl<T:Copy> Clone for Cell<T> {
227 fn clone(&self) -> Cell<T> {
228 Cell::new(self.get())
232 #[stable(feature = "rust1", since = "1.0.0")]
233 impl<T:Default> Default for Cell<T> {
234 /// Creates a `Cell<T>`, with the `Default` value for T.
236 fn default() -> Cell<T> {
237 Cell::new(Default::default())
241 #[stable(feature = "rust1", since = "1.0.0")]
242 impl<T:PartialEq + Copy> PartialEq for Cell<T> {
244 fn eq(&self, other: &Cell<T>) -> bool {
245 self.get() == other.get()
249 #[stable(feature = "cell_eq", since = "1.2.0")]
250 impl<T:Eq + Copy> Eq for Cell<T> {}
252 #[stable(feature = "cell_ord", since = "1.10.0")]
253 impl<T:PartialOrd + Copy> PartialOrd for Cell<T> {
255 fn partial_cmp(&self, other: &Cell<T>) -> Option<Ordering> {
256 self.get().partial_cmp(&other.get())
260 fn lt(&self, other: &Cell<T>) -> bool {
261 self.get() < other.get()
265 fn le(&self, other: &Cell<T>) -> bool {
266 self.get() <= other.get()
270 fn gt(&self, other: &Cell<T>) -> bool {
271 self.get() > other.get()
275 fn ge(&self, other: &Cell<T>) -> bool {
276 self.get() >= other.get()
280 #[stable(feature = "cell_ord", since = "1.10.0")]
281 impl<T:Ord + Copy> Ord for Cell<T> {
283 fn cmp(&self, other: &Cell<T>) -> Ordering {
284 self.get().cmp(&other.get())
288 #[stable(feature = "cell_from", since = "1.12.0")]
289 impl<T> From<T> for Cell<T> {
290 fn from(t: T) -> Cell<T> {
296 /// Creates a new `Cell` containing the given value.
301 /// use std::cell::Cell;
303 /// let c = Cell::new(5);
305 #[stable(feature = "rust1", since = "1.0.0")]
307 pub const fn new(value: T) -> Cell<T> {
309 value: UnsafeCell::new(value),
313 /// Returns a raw pointer to the underlying data in this cell.
318 /// use std::cell::Cell;
320 /// let c = Cell::new(5);
322 /// let ptr = c.as_ptr();
325 #[stable(feature = "cell_as_ptr", since = "1.12.0")]
326 pub fn as_ptr(&self) -> *mut T {
330 /// Returns a mutable reference to the underlying data.
332 /// This call borrows `Cell` mutably (at compile-time) which guarantees
333 /// that we possess the only reference.
338 /// use std::cell::Cell;
340 /// let mut c = Cell::new(5);
341 /// *c.get_mut() += 1;
343 /// assert_eq!(c.get(), 6);
346 #[stable(feature = "cell_get_mut", since = "1.11.0")]
347 pub fn get_mut(&mut self) -> &mut T {
349 &mut *self.value.get()
353 /// Sets the contained value.
358 /// use std::cell::Cell;
360 /// let c = Cell::new(5);
365 #[stable(feature = "rust1", since = "1.0.0")]
366 pub fn set(&self, val: T) {
367 let old = self.replace(val);
371 /// Swaps the values of two Cells.
372 /// Difference with `std::mem::swap` is that this function doesn't require `&mut` reference.
377 /// use std::cell::Cell;
379 /// let c1 = Cell::new(5i32);
380 /// let c2 = Cell::new(10i32);
382 /// assert_eq!(10, c1.get());
383 /// assert_eq!(5, c2.get());
386 #[stable(feature = "move_cell", since = "1.17.0")]
387 pub fn swap(&self, other: &Self) {
388 if ptr::eq(self, other) {
392 ptr::swap(self.value.get(), other.value.get());
396 /// Replaces the contained value.
401 /// use std::cell::Cell;
403 /// let c = Cell::new(5);
404 /// let old = c.replace(10);
406 /// assert_eq!(5, old);
408 #[stable(feature = "move_cell", since = "1.17.0")]
409 pub fn replace(&self, val: T) -> T {
410 mem::replace(unsafe { &mut *self.value.get() }, val)
413 /// Unwraps the value.
418 /// use std::cell::Cell;
420 /// let c = Cell::new(5);
421 /// let five = c.into_inner();
423 /// assert_eq!(five, 5);
425 #[stable(feature = "move_cell", since = "1.17.0")]
426 pub fn into_inner(self) -> T {
427 unsafe { self.value.into_inner() }
431 impl<T: Default> Cell<T> {
432 /// Takes the value of the cell, leaving `Default::default()` in its place.
437 /// use std::cell::Cell;
439 /// let c = Cell::new(5);
440 /// let five = c.take();
442 /// assert_eq!(five, 5);
443 /// assert_eq!(c.into_inner(), 0);
445 #[stable(feature = "move_cell", since = "1.17.0")]
446 pub fn take(&self) -> T {
447 self.replace(Default::default())
451 #[unstable(feature = "coerce_unsized", issue = "27732")]
452 impl<T: CoerceUnsized<U>, U> CoerceUnsized<Cell<U>> for Cell<T> {}
454 /// A mutable memory location with dynamically checked borrow rules
456 /// See the [module-level documentation](index.html) for more.
457 #[stable(feature = "rust1", since = "1.0.0")]
458 pub struct RefCell<T: ?Sized> {
459 borrow: Cell<BorrowFlag>,
460 value: UnsafeCell<T>,
463 /// An error returned by [`RefCell::try_borrow`](struct.RefCell.html#method.try_borrow).
464 #[stable(feature = "try_borrow", since = "1.13.0")]
465 pub struct BorrowError {
469 #[stable(feature = "try_borrow", since = "1.13.0")]
470 impl Debug for BorrowError {
471 fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
472 f.debug_struct("BorrowError").finish()
476 #[stable(feature = "try_borrow", since = "1.13.0")]
477 impl Display for BorrowError {
478 fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
479 Display::fmt("already mutably borrowed", f)
483 /// An error returned by [`RefCell::try_borrow_mut`](struct.RefCell.html#method.try_borrow_mut).
484 #[stable(feature = "try_borrow", since = "1.13.0")]
485 pub struct BorrowMutError {
489 #[stable(feature = "try_borrow", since = "1.13.0")]
490 impl Debug for BorrowMutError {
491 fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
492 f.debug_struct("BorrowMutError").finish()
496 #[stable(feature = "try_borrow", since = "1.13.0")]
497 impl Display for BorrowMutError {
498 fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
499 Display::fmt("already borrowed", f)
503 // Values [1, MAX-1] represent the number of `Ref` active
504 // (will not outgrow its range since `usize` is the size of the address space)
505 type BorrowFlag = usize;
506 const UNUSED: BorrowFlag = 0;
507 const WRITING: BorrowFlag = !0;
510 /// Creates a new `RefCell` containing `value`.
515 /// use std::cell::RefCell;
517 /// let c = RefCell::new(5);
519 #[stable(feature = "rust1", since = "1.0.0")]
521 pub const fn new(value: T) -> RefCell<T> {
523 value: UnsafeCell::new(value),
524 borrow: Cell::new(UNUSED),
528 /// Consumes the `RefCell`, returning the wrapped value.
533 /// use std::cell::RefCell;
535 /// let c = RefCell::new(5);
537 /// let five = c.into_inner();
539 #[stable(feature = "rust1", since = "1.0.0")]
541 pub fn into_inner(self) -> T {
542 // Since this function takes `self` (the `RefCell`) by value, the
543 // compiler statically verifies that it is not currently borrowed.
544 // Therefore the following assertion is just a `debug_assert!`.
545 debug_assert!(self.borrow.get() == UNUSED);
546 unsafe { self.value.into_inner() }
550 impl<T: ?Sized> RefCell<T> {
551 /// Immutably borrows the wrapped value.
553 /// The borrow lasts until the returned `Ref` exits scope. Multiple
554 /// immutable borrows can be taken out at the same time.
558 /// Panics if the value is currently mutably borrowed. For a non-panicking variant, use
559 /// [`try_borrow`](#method.try_borrow).
564 /// use std::cell::RefCell;
566 /// let c = RefCell::new(5);
568 /// let borrowed_five = c.borrow();
569 /// let borrowed_five2 = c.borrow();
572 /// An example of panic:
575 /// use std::cell::RefCell;
578 /// let result = thread::spawn(move || {
579 /// let c = RefCell::new(5);
580 /// let m = c.borrow_mut();
582 /// let b = c.borrow(); // this causes a panic
585 /// assert!(result.is_err());
587 #[stable(feature = "rust1", since = "1.0.0")]
589 pub fn borrow(&self) -> Ref<T> {
590 self.try_borrow().expect("already mutably borrowed")
593 /// Immutably borrows the wrapped value, returning an error if the value is currently mutably
596 /// The borrow lasts until the returned `Ref` exits scope. Multiple immutable borrows can be
597 /// taken out at the same time.
599 /// This is the non-panicking variant of [`borrow`](#method.borrow).
604 /// use std::cell::RefCell;
606 /// let c = RefCell::new(5);
609 /// let m = c.borrow_mut();
610 /// assert!(c.try_borrow().is_err());
614 /// let m = c.borrow();
615 /// assert!(c.try_borrow().is_ok());
618 #[stable(feature = "try_borrow", since = "1.13.0")]
620 pub fn try_borrow(&self) -> Result<Ref<T>, BorrowError> {
621 match BorrowRef::new(&self.borrow) {
623 value: unsafe { &*self.value.get() },
626 None => Err(BorrowError { _private: () }),
630 /// Mutably borrows the wrapped value.
632 /// The borrow lasts until the returned `RefMut` exits scope. The value
633 /// cannot be borrowed while this borrow is active.
637 /// Panics if the value is currently borrowed. For a non-panicking variant, use
638 /// [`try_borrow_mut`](#method.try_borrow_mut).
643 /// use std::cell::RefCell;
645 /// let c = RefCell::new(5);
647 /// *c.borrow_mut() = 7;
649 /// assert_eq!(*c.borrow(), 7);
652 /// An example of panic:
655 /// use std::cell::RefCell;
658 /// let result = thread::spawn(move || {
659 /// let c = RefCell::new(5);
660 /// let m = c.borrow();
662 /// let b = c.borrow_mut(); // this causes a panic
665 /// assert!(result.is_err());
667 #[stable(feature = "rust1", since = "1.0.0")]
669 pub fn borrow_mut(&self) -> RefMut<T> {
670 self.try_borrow_mut().expect("already borrowed")
673 /// Mutably borrows the wrapped value, returning an error if the value is currently borrowed.
675 /// The borrow lasts until the returned `RefMut` exits scope. The value cannot be borrowed
676 /// while this borrow is active.
678 /// This is the non-panicking variant of [`borrow_mut`](#method.borrow_mut).
683 /// use std::cell::RefCell;
685 /// let c = RefCell::new(5);
688 /// let m = c.borrow();
689 /// assert!(c.try_borrow_mut().is_err());
692 /// assert!(c.try_borrow_mut().is_ok());
694 #[stable(feature = "try_borrow", since = "1.13.0")]
696 pub fn try_borrow_mut(&self) -> Result<RefMut<T>, BorrowMutError> {
697 match BorrowRefMut::new(&self.borrow) {
698 Some(b) => Ok(RefMut {
699 value: unsafe { &mut *self.value.get() },
702 None => Err(BorrowMutError { _private: () }),
706 /// Returns a raw pointer to the underlying data in this cell.
711 /// use std::cell::RefCell;
713 /// let c = RefCell::new(5);
715 /// let ptr = c.as_ptr();
718 #[stable(feature = "cell_as_ptr", since = "1.12.0")]
719 pub fn as_ptr(&self) -> *mut T {
723 /// Returns a mutable reference to the underlying data.
725 /// This call borrows `RefCell` mutably (at compile-time) so there is no
726 /// need for dynamic checks.
728 /// However be cautious: this method expects `self` to be mutable, which is
729 /// generally not the case when using a `RefCell`. Take a look at the
730 /// [`borrow_mut`] method instead if `self` isn't mutable.
732 /// Also, please be aware that this method is only for special circumstances and is usually
733 /// not you want. In case of doubt, use [`borrow_mut`] instead.
735 /// [`borrow_mut`]: #method.borrow_mut
740 /// use std::cell::RefCell;
742 /// let mut c = RefCell::new(5);
743 /// *c.get_mut() += 1;
745 /// assert_eq!(c, RefCell::new(6));
748 #[stable(feature = "cell_get_mut", since = "1.11.0")]
749 pub fn get_mut(&mut self) -> &mut T {
751 &mut *self.value.get()
756 #[stable(feature = "rust1", since = "1.0.0")]
757 unsafe impl<T: ?Sized> Send for RefCell<T> where T: Send {}
759 #[stable(feature = "rust1", since = "1.0.0")]
760 impl<T: ?Sized> !Sync for RefCell<T> {}
762 #[stable(feature = "rust1", since = "1.0.0")]
763 impl<T: Clone> Clone for RefCell<T> {
765 fn clone(&self) -> RefCell<T> {
766 RefCell::new(self.borrow().clone())
770 #[stable(feature = "rust1", since = "1.0.0")]
771 impl<T:Default> Default for RefCell<T> {
772 /// Creates a `RefCell<T>`, with the `Default` value for T.
774 fn default() -> RefCell<T> {
775 RefCell::new(Default::default())
779 #[stable(feature = "rust1", since = "1.0.0")]
780 impl<T: ?Sized + PartialEq> PartialEq for RefCell<T> {
782 fn eq(&self, other: &RefCell<T>) -> bool {
783 *self.borrow() == *other.borrow()
787 #[stable(feature = "cell_eq", since = "1.2.0")]
788 impl<T: ?Sized + Eq> Eq for RefCell<T> {}
790 #[stable(feature = "cell_ord", since = "1.10.0")]
791 impl<T: ?Sized + PartialOrd> PartialOrd for RefCell<T> {
793 fn partial_cmp(&self, other: &RefCell<T>) -> Option<Ordering> {
794 self.borrow().partial_cmp(&*other.borrow())
798 fn lt(&self, other: &RefCell<T>) -> bool {
799 *self.borrow() < *other.borrow()
803 fn le(&self, other: &RefCell<T>) -> bool {
804 *self.borrow() <= *other.borrow()
808 fn gt(&self, other: &RefCell<T>) -> bool {
809 *self.borrow() > *other.borrow()
813 fn ge(&self, other: &RefCell<T>) -> bool {
814 *self.borrow() >= *other.borrow()
818 #[stable(feature = "cell_ord", since = "1.10.0")]
819 impl<T: ?Sized + Ord> Ord for RefCell<T> {
821 fn cmp(&self, other: &RefCell<T>) -> Ordering {
822 self.borrow().cmp(&*other.borrow())
826 #[stable(feature = "cell_from", since = "1.12.0")]
827 impl<T> From<T> for RefCell<T> {
828 fn from(t: T) -> RefCell<T> {
833 #[unstable(feature = "coerce_unsized", issue = "27732")]
834 impl<T: CoerceUnsized<U>, U> CoerceUnsized<RefCell<U>> for RefCell<T> {}
836 struct BorrowRef<'b> {
837 borrow: &'b Cell<BorrowFlag>,
840 impl<'b> BorrowRef<'b> {
842 fn new(borrow: &'b Cell<BorrowFlag>) -> Option<BorrowRef<'b>> {
847 Some(BorrowRef { borrow: borrow })
853 impl<'b> Drop for BorrowRef<'b> {
856 let borrow = self.borrow.get();
857 debug_assert!(borrow != WRITING && borrow != UNUSED);
858 self.borrow.set(borrow - 1);
862 impl<'b> Clone for BorrowRef<'b> {
864 fn clone(&self) -> BorrowRef<'b> {
865 // Since this Ref exists, we know the borrow flag
866 // is not set to WRITING.
867 let borrow = self.borrow.get();
868 debug_assert!(borrow != UNUSED);
869 // Prevent the borrow counter from overflowing.
870 assert!(borrow != WRITING);
871 self.borrow.set(borrow + 1);
872 BorrowRef { borrow: self.borrow }
876 /// Wraps a borrowed reference to a value in a `RefCell` box.
877 /// A wrapper type for an immutably borrowed value from a `RefCell<T>`.
879 /// See the [module-level documentation](index.html) for more.
880 #[stable(feature = "rust1", since = "1.0.0")]
881 pub struct Ref<'b, T: ?Sized + 'b> {
883 borrow: BorrowRef<'b>,
886 #[stable(feature = "rust1", since = "1.0.0")]
887 impl<'b, T: ?Sized> Deref for Ref<'b, T> {
891 fn deref(&self) -> &T {
896 impl<'b, T: ?Sized> Ref<'b, T> {
899 /// The `RefCell` is already immutably borrowed, so this cannot fail.
901 /// This is an associated function that needs to be used as
902 /// `Ref::clone(...)`. A `Clone` implementation or a method would interfere
903 /// with the widespread use of `r.borrow().clone()` to clone the contents of
905 #[stable(feature = "cell_extras", since = "1.15.0")]
907 pub fn clone(orig: &Ref<'b, T>) -> Ref<'b, T> {
910 borrow: orig.borrow.clone(),
914 /// Make a new `Ref` for a component of the borrowed data.
916 /// The `RefCell` is already immutably borrowed, so this cannot fail.
918 /// This is an associated function that needs to be used as `Ref::map(...)`.
919 /// A method would interfere with methods of the same name on the contents
920 /// of a `RefCell` used through `Deref`.
925 /// use std::cell::{RefCell, Ref};
927 /// let c = RefCell::new((5, 'b'));
928 /// let b1: Ref<(u32, char)> = c.borrow();
929 /// let b2: Ref<u32> = Ref::map(b1, |t| &t.0);
930 /// assert_eq!(*b2, 5)
932 #[stable(feature = "cell_map", since = "1.8.0")]
934 pub fn map<U: ?Sized, F>(orig: Ref<'b, T>, f: F) -> Ref<'b, U>
935 where F: FnOnce(&T) -> &U
938 value: f(orig.value),
944 #[unstable(feature = "coerce_unsized", issue = "27732")]
945 impl<'b, T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<Ref<'b, U>> for Ref<'b, T> {}
947 impl<'b, T: ?Sized> RefMut<'b, T> {
948 /// Make a new `RefMut` for a component of the borrowed data, e.g. an enum
951 /// The `RefCell` is already mutably borrowed, so this cannot fail.
953 /// This is an associated function that needs to be used as
954 /// `RefMut::map(...)`. A method would interfere with methods of the same
955 /// name on the contents of a `RefCell` used through `Deref`.
960 /// use std::cell::{RefCell, RefMut};
962 /// let c = RefCell::new((5, 'b'));
964 /// let b1: RefMut<(u32, char)> = c.borrow_mut();
965 /// let mut b2: RefMut<u32> = RefMut::map(b1, |t| &mut t.0);
966 /// assert_eq!(*b2, 5);
969 /// assert_eq!(*c.borrow(), (42, 'b'));
971 #[stable(feature = "cell_map", since = "1.8.0")]
973 pub fn map<U: ?Sized, F>(orig: RefMut<'b, T>, f: F) -> RefMut<'b, U>
974 where F: FnOnce(&mut T) -> &mut U
977 value: f(orig.value),
983 struct BorrowRefMut<'b> {
984 borrow: &'b Cell<BorrowFlag>,
987 impl<'b> Drop for BorrowRefMut<'b> {
990 let borrow = self.borrow.get();
991 debug_assert!(borrow == WRITING);
992 self.borrow.set(UNUSED);
996 impl<'b> BorrowRefMut<'b> {
998 fn new(borrow: &'b Cell<BorrowFlag>) -> Option<BorrowRefMut<'b>> {
1001 borrow.set(WRITING);
1002 Some(BorrowRefMut { borrow: borrow })
1009 /// A wrapper type for a mutably borrowed value from a `RefCell<T>`.
1011 /// See the [module-level documentation](index.html) for more.
1012 #[stable(feature = "rust1", since = "1.0.0")]
1013 pub struct RefMut<'b, T: ?Sized + 'b> {
1015 borrow: BorrowRefMut<'b>,
1018 #[stable(feature = "rust1", since = "1.0.0")]
1019 impl<'b, T: ?Sized> Deref for RefMut<'b, T> {
1023 fn deref(&self) -> &T {
1028 #[stable(feature = "rust1", since = "1.0.0")]
1029 impl<'b, T: ?Sized> DerefMut for RefMut<'b, T> {
1031 fn deref_mut(&mut self) -> &mut T {
1036 #[unstable(feature = "coerce_unsized", issue = "27732")]
1037 impl<'b, T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<RefMut<'b, U>> for RefMut<'b, T> {}
1039 /// The core primitive for interior mutability in Rust.
1041 /// `UnsafeCell<T>` is a type that wraps some `T` and indicates unsafe interior operations on the
1042 /// wrapped type. Types with an `UnsafeCell<T>` field are considered to have an 'unsafe interior'.
1043 /// The `UnsafeCell<T>` type is the only legal way to obtain aliasable data that is considered
1044 /// mutable. In general, transmuting an `&T` type into an `&mut T` is considered undefined behavior.
1046 /// The compiler makes optimizations based on the knowledge that `&T` is not mutably aliased or
1047 /// mutated, and that `&mut T` is unique. When building abstractions like `Cell`, `RefCell`,
1048 /// `Mutex`, etc, you need to turn these optimizations off. `UnsafeCell` is the only legal way
1049 /// to do this. When `UnsafeCell<T>` is immutably aliased, it is still safe to obtain a mutable
1050 /// reference to its interior and/or to mutate it. However, it is up to the abstraction designer
1051 /// to ensure that no two mutable references obtained this way are active at the same time, and
1052 /// that there are no active mutable references or mutations when an immutable reference is obtained
1053 /// from the cell. This is often done via runtime checks.
1055 /// Note that while mutating or mutably aliasing the contents of an `& UnsafeCell<T>` is
1056 /// okay (provided you enforce the invariants some other way); it is still undefined behavior
1057 /// to have multiple `&mut UnsafeCell<T>` aliases.
1060 /// Types like `Cell<T>` and `RefCell<T>` use this type to wrap their internal data.
1065 /// use std::cell::UnsafeCell;
1066 /// use std::marker::Sync;
1068 /// # #[allow(dead_code)]
1069 /// struct NotThreadSafe<T> {
1070 /// value: UnsafeCell<T>,
1073 /// unsafe impl<T> Sync for NotThreadSafe<T> {}
1075 #[lang = "unsafe_cell"]
1076 #[stable(feature = "rust1", since = "1.0.0")]
1077 pub struct UnsafeCell<T: ?Sized> {
1081 #[stable(feature = "rust1", since = "1.0.0")]
1082 impl<T: ?Sized> !Sync for UnsafeCell<T> {}
1084 impl<T> UnsafeCell<T> {
1085 /// Constructs a new instance of `UnsafeCell` which will wrap the specified
1088 /// All access to the inner value through methods is `unsafe`.
1093 /// use std::cell::UnsafeCell;
1095 /// let uc = UnsafeCell::new(5);
1097 #[stable(feature = "rust1", since = "1.0.0")]
1099 pub const fn new(value: T) -> UnsafeCell<T> {
1100 UnsafeCell { value: value }
1103 /// Unwraps the value.
1107 /// This function is unsafe because this thread or another thread may currently be
1108 /// inspecting the inner value.
1113 /// use std::cell::UnsafeCell;
1115 /// let uc = UnsafeCell::new(5);
1117 /// let five = unsafe { uc.into_inner() };
1120 #[stable(feature = "rust1", since = "1.0.0")]
1121 pub unsafe fn into_inner(self) -> T {
1126 impl<T: ?Sized> UnsafeCell<T> {
1127 /// Gets a mutable pointer to the wrapped value.
1129 /// This can be cast to a pointer of any kind.
1130 /// Ensure that the access is unique when casting to
1131 /// `&mut T`, and ensure that there are no mutations or mutable
1132 /// aliases going on when casting to `&T`
1137 /// use std::cell::UnsafeCell;
1139 /// let uc = UnsafeCell::new(5);
1141 /// let five = uc.get();
1144 #[stable(feature = "rust1", since = "1.0.0")]
1145 pub fn get(&self) -> *mut T {
1146 &self.value as *const T as *mut T
1150 #[stable(feature = "unsafe_cell_default", since = "1.9.0")]
1151 impl<T: Default> Default for UnsafeCell<T> {
1152 /// Creates an `UnsafeCell`, with the `Default` value for T.
1153 fn default() -> UnsafeCell<T> {
1154 UnsafeCell::new(Default::default())
1158 #[stable(feature = "cell_from", since = "1.12.0")]
1159 impl<T> From<T> for UnsafeCell<T> {
1160 fn from(t: T) -> UnsafeCell<T> {
1165 #[unstable(feature = "coerce_unsized", issue = "27732")]
1166 impl<T: CoerceUnsized<U>, U> CoerceUnsized<UnsafeCell<U>> for UnsafeCell<T> {}
1169 fn assert_coerce_unsized(a: UnsafeCell<&i32>, b: Cell<&i32>, c: RefCell<&i32>) {
1170 let _: UnsafeCell<&Send> = a;
1171 let _: Cell<&Send> = b;
1172 let _: RefCell<&Send> = c;