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>` provides `get` and `set`
19 //! methods that change the interior value with a single method call. `Cell<T>` though is only
20 //! compatible with types that implement `Copy`. For other types, one must use the `RefCell<T>`
21 //! type, acquiring a write lock before mutating.
23 //! `RefCell<T>` uses Rust's lifetimes to implement 'dynamic borrowing', a process whereby one can
24 //! claim temporary, exclusive, mutable access to the inner value. Borrows for `RefCell<T>`s are
25 //! tracked 'at runtime', unlike Rust's native reference types which are entirely tracked
26 //! statically, at compile time. Because `RefCell<T>` borrows are dynamic it is possible to attempt
27 //! to borrow a value that is already mutably borrowed; when this happens it results in thread
30 //! # When to choose interior mutability
32 //! The more common inherited mutability, where one must have unique access to mutate a value, is
33 //! one of the key language elements that enables Rust to reason strongly about pointer aliasing,
34 //! statically preventing crash bugs. Because of that, inherited mutability is preferred, and
35 //! interior mutability is something of a last resort. Since cell types enable mutation where it
36 //! would otherwise be disallowed though, there are occasions when interior mutability might be
37 //! appropriate, or even *must* be used, e.g.
39 //! * Introducing mutability 'inside' of something immutable
40 //! * Implementation details of logically-immutable methods.
41 //! * Mutating implementations of `Clone`.
43 //! ## Introducing mutability 'inside' of something immutable
45 //! Many shared smart pointer types, including `Rc<T>` and `Arc<T>`, provide containers that can be
46 //! cloned and shared between multiple parties. Because the contained values may be
47 //! multiply-aliased, they can only be borrowed with `&`, not `&mut`. Without cells it would be
48 //! impossible to mutate data inside of these smart pointers at all.
50 //! It's very common then to put a `RefCell<T>` inside shared pointer types to reintroduce
54 //! use std::collections::HashMap;
55 //! use std::cell::RefCell;
59 //! let shared_map: Rc<RefCell<_>> = Rc::new(RefCell::new(HashMap::new()));
60 //! shared_map.borrow_mut().insert("africa", 92388);
61 //! shared_map.borrow_mut().insert("kyoto", 11837);
62 //! shared_map.borrow_mut().insert("piccadilly", 11826);
63 //! shared_map.borrow_mut().insert("marbles", 38);
67 //! Note that this example uses `Rc<T>` and not `Arc<T>`. `RefCell<T>`s are for single-threaded
68 //! scenarios. Consider using `RwLock<T>` or `Mutex<T>` if you need shared mutability in a
69 //! multi-threaded situation.
71 //! ## Implementation details of logically-immutable methods
73 //! Occasionally it may be desirable not to expose in an API that there is mutation happening
74 //! "under the hood". This may be because logically the operation is immutable, but e.g. caching
75 //! forces the implementation to perform mutation; or because you must employ mutation to implement
76 //! a trait method that was originally defined to take `&self`.
79 //! # #![allow(dead_code)]
80 //! use std::cell::RefCell;
83 //! edges: Vec<(i32, i32)>,
84 //! span_tree_cache: RefCell<Option<Vec<(i32, i32)>>>
88 //! fn minimum_spanning_tree(&self) -> Vec<(i32, i32)> {
89 //! // Create a new scope to contain the lifetime of the
92 //! // Take a reference to the inside of cache cell
93 //! let mut cache = self.span_tree_cache.borrow_mut();
94 //! if cache.is_some() {
95 //! return cache.as_ref().unwrap().clone();
98 //! let span_tree = self.calc_span_tree();
99 //! *cache = Some(span_tree);
102 //! // Recursive call to return the just-cached value.
103 //! // Note that if we had not let the previous borrow
104 //! // of the cache fall out of scope then the subsequent
105 //! // recursive borrow would cause a dynamic thread panic.
106 //! // This is the major hazard of using `RefCell`.
107 //! self.minimum_spanning_tree()
109 //! # fn calc_span_tree(&self) -> Vec<(i32, i32)> { vec![] }
113 //! ## Mutating implementations of `Clone`
115 //! This is simply a special - but common - case of the previous: hiding mutability for operations
116 //! that appear to be immutable. The `clone` method is expected to not change the source value, and
117 //! is declared to take `&self`, not `&mut self`. Therefore any mutation that happens in the
118 //! `clone` method must use cell types. For example, `Rc<T>` maintains its reference counts within a
122 //! use std::cell::Cell;
125 //! ptr: *mut RcBox<T>
128 //! struct RcBox<T> {
129 //! # #[allow(dead_code)]
131 //! refcount: Cell<usize>
134 //! impl<T> Clone for Rc<T> {
135 //! fn clone(&self) -> Rc<T> {
137 //! (*self.ptr).refcount.set((*self.ptr).refcount.get() + 1);
138 //! Rc { ptr: self.ptr }
145 #![stable(feature = "rust1", since = "1.0.0")]
148 use cmp::{PartialEq, Eq, PartialOrd, Ord, Ordering};
149 use default::Default;
150 use fmt::{self, Debug, Display};
151 use marker::{Copy, PhantomData, Send, Sync, Sized, Unsize};
152 use ops::{Deref, DerefMut, Drop, FnOnce, CoerceUnsized};
154 use option::Option::{None, Some};
156 use result::Result::{Ok, Err};
158 /// A mutable memory location that admits only `Copy` data.
160 /// See the [module-level documentation](index.html) for more.
161 #[stable(feature = "rust1", since = "1.0.0")]
163 value: UnsafeCell<T>,
166 impl<T:Copy> Cell<T> {
167 /// Creates a new `Cell` containing the given value.
172 /// use std::cell::Cell;
174 /// let c = Cell::new(5);
176 #[stable(feature = "rust1", since = "1.0.0")]
178 pub const fn new(value: T) -> Cell<T> {
180 value: UnsafeCell::new(value),
184 /// Returns a copy of the contained value.
189 /// use std::cell::Cell;
191 /// let c = Cell::new(5);
193 /// let five = c.get();
196 #[stable(feature = "rust1", since = "1.0.0")]
197 pub fn get(&self) -> T {
198 unsafe{ *self.value.get() }
201 /// Sets the contained value.
206 /// use std::cell::Cell;
208 /// let c = Cell::new(5);
213 #[stable(feature = "rust1", since = "1.0.0")]
214 pub fn set(&self, value: T) {
216 *self.value.get() = value;
220 /// Returns a reference to the underlying `UnsafeCell`.
225 /// #![feature(as_unsafe_cell)]
227 /// use std::cell::Cell;
229 /// let c = Cell::new(5);
231 /// let uc = c.as_unsafe_cell();
234 #[unstable(feature = "as_unsafe_cell", issue = "27708")]
235 pub fn as_unsafe_cell(&self) -> &UnsafeCell<T> {
239 /// Returns a mutable reference to the underlying data.
241 /// This call borrows `Cell` mutably (at compile-time) which guarantees
242 /// that we possess the only reference.
247 /// use std::cell::Cell;
249 /// let mut c = Cell::new(5);
250 /// *c.get_mut() += 1;
252 /// assert_eq!(c.get(), 6);
255 #[stable(feature = "cell_get_mut", since = "1.11.0")]
256 pub fn get_mut(&mut self) -> &mut T {
258 &mut *self.value.get()
263 #[stable(feature = "rust1", since = "1.0.0")]
264 unsafe impl<T> Send for Cell<T> where T: Send {}
266 #[stable(feature = "rust1", since = "1.0.0")]
267 impl<T> !Sync for Cell<T> {}
269 #[stable(feature = "rust1", since = "1.0.0")]
270 impl<T:Copy> Clone for Cell<T> {
272 fn clone(&self) -> Cell<T> {
273 Cell::new(self.get())
277 #[stable(feature = "rust1", since = "1.0.0")]
278 impl<T:Default + Copy> Default for Cell<T> {
280 fn default() -> Cell<T> {
281 Cell::new(Default::default())
285 #[stable(feature = "rust1", since = "1.0.0")]
286 impl<T:PartialEq + Copy> PartialEq for Cell<T> {
288 fn eq(&self, other: &Cell<T>) -> bool {
289 self.get() == other.get()
293 #[stable(feature = "cell_eq", since = "1.2.0")]
294 impl<T:Eq + Copy> Eq for Cell<T> {}
296 #[stable(feature = "cell_ord", since = "1.10.0")]
297 impl<T:PartialOrd + Copy> PartialOrd for Cell<T> {
299 fn partial_cmp(&self, other: &Cell<T>) -> Option<Ordering> {
300 self.get().partial_cmp(&other.get())
304 fn lt(&self, other: &Cell<T>) -> bool {
305 self.get() < other.get()
309 fn le(&self, other: &Cell<T>) -> bool {
310 self.get() <= other.get()
314 fn gt(&self, other: &Cell<T>) -> bool {
315 self.get() > other.get()
319 fn ge(&self, other: &Cell<T>) -> bool {
320 self.get() >= other.get()
324 #[stable(feature = "cell_ord", since = "1.10.0")]
325 impl<T:Ord + Copy> Ord for Cell<T> {
327 fn cmp(&self, other: &Cell<T>) -> Ordering {
328 self.get().cmp(&other.get())
332 /// A mutable memory location with dynamically checked borrow rules
334 /// See the [module-level documentation](index.html) for more.
335 #[stable(feature = "rust1", since = "1.0.0")]
336 pub struct RefCell<T: ?Sized> {
337 borrow: Cell<BorrowFlag>,
338 value: UnsafeCell<T>,
341 /// An enumeration of values returned from the `state` method on a `RefCell<T>`.
342 #[derive(Copy, Clone, PartialEq, Eq, Debug)]
343 #[unstable(feature = "borrow_state", issue = "27733")]
344 pub enum BorrowState {
345 /// The cell is currently being read, there is at least one active `borrow`.
347 /// The cell is currently being written to, there is an active `borrow_mut`.
349 /// There are no outstanding borrows on this cell.
353 /// An error returned by [`RefCell::try_borrow`](struct.RefCell.html#method.try_borrow).
354 #[unstable(feature = "try_borrow", issue = "35070")]
355 pub struct BorrowError<'a, T: 'a + ?Sized> {
356 marker: PhantomData<&'a RefCell<T>>,
359 #[unstable(feature = "try_borrow", issue = "35070")]
360 impl<'a, T: ?Sized> Debug for BorrowError<'a, T> {
361 fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
362 f.debug_struct("BorrowError").finish()
366 #[unstable(feature = "try_borrow", issue = "35070")]
367 impl<'a, T: ?Sized> Display for BorrowError<'a, T> {
368 fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
369 Display::fmt("already mutably borrowed", f)
373 /// An error returned by [`RefCell::try_borrow_mut`](struct.RefCell.html#method.try_borrow_mut).
374 #[unstable(feature = "try_borrow", issue = "35070")]
375 pub struct BorrowMutError<'a, T: 'a + ?Sized> {
376 marker: PhantomData<&'a RefCell<T>>,
379 #[unstable(feature = "try_borrow", issue = "35070")]
380 impl<'a, T: ?Sized> Debug for BorrowMutError<'a, T> {
381 fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
382 f.debug_struct("BorrowMutError").finish()
386 #[unstable(feature = "try_borrow", issue = "35070")]
387 impl<'a, T: ?Sized> Display for BorrowMutError<'a, T> {
388 fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
389 Display::fmt("already borrowed", f)
393 // Values [1, MAX-1] represent the number of `Ref` active
394 // (will not outgrow its range since `usize` is the size of the address space)
395 type BorrowFlag = usize;
396 const UNUSED: BorrowFlag = 0;
397 const WRITING: BorrowFlag = !0;
400 /// Creates a new `RefCell` containing `value`.
405 /// use std::cell::RefCell;
407 /// let c = RefCell::new(5);
409 #[stable(feature = "rust1", since = "1.0.0")]
411 pub const fn new(value: T) -> RefCell<T> {
413 value: UnsafeCell::new(value),
414 borrow: Cell::new(UNUSED),
418 /// Consumes the `RefCell`, returning the wrapped value.
423 /// use std::cell::RefCell;
425 /// let c = RefCell::new(5);
427 /// let five = c.into_inner();
429 #[stable(feature = "rust1", since = "1.0.0")]
431 pub fn into_inner(self) -> T {
432 // Since this function takes `self` (the `RefCell`) by value, the
433 // compiler statically verifies that it is not currently borrowed.
434 // Therefore the following assertion is just a `debug_assert!`.
435 debug_assert!(self.borrow.get() == UNUSED);
436 unsafe { self.value.into_inner() }
440 impl<T: ?Sized> RefCell<T> {
441 /// Query the current state of this `RefCell`
443 /// The returned value can be dispatched on to determine if a call to
444 /// `borrow` or `borrow_mut` would succeed.
449 /// #![feature(borrow_state)]
451 /// use std::cell::{BorrowState, RefCell};
453 /// let c = RefCell::new(5);
455 /// match c.borrow_state() {
456 /// BorrowState::Writing => println!("Cannot be borrowed"),
457 /// BorrowState::Reading => println!("Cannot be borrowed mutably"),
458 /// BorrowState::Unused => println!("Can be borrowed (mutably as well)"),
461 #[unstable(feature = "borrow_state", issue = "27733")]
463 pub fn borrow_state(&self) -> BorrowState {
464 match self.borrow.get() {
465 WRITING => BorrowState::Writing,
466 UNUSED => BorrowState::Unused,
467 _ => BorrowState::Reading,
471 /// Immutably borrows the wrapped value.
473 /// The borrow lasts until the returned `Ref` exits scope. Multiple
474 /// immutable borrows can be taken out at the same time.
478 /// Panics if the value is currently mutably borrowed. For a non-panicking variant, use
479 /// [`try_borrow`](#method.try_borrow).
484 /// use std::cell::RefCell;
486 /// let c = RefCell::new(5);
488 /// let borrowed_five = c.borrow();
489 /// let borrowed_five2 = c.borrow();
492 /// An example of panic:
495 /// use std::cell::RefCell;
498 /// let result = thread::spawn(move || {
499 /// let c = RefCell::new(5);
500 /// let m = c.borrow_mut();
502 /// let b = c.borrow(); // this causes a panic
505 /// assert!(result.is_err());
507 #[stable(feature = "rust1", since = "1.0.0")]
509 pub fn borrow(&self) -> Ref<T> {
510 self.try_borrow().expect("already mutably borrowed")
513 /// Immutably borrows the wrapped value, returning an error if the value is currently mutably
516 /// The borrow lasts until the returned `Ref` exits scope. Multiple immutable borrows can be
517 /// taken out at the same time.
519 /// This is the non-panicking variant of [`borrow`](#method.borrow).
524 /// #![feature(try_borrow)]
526 /// use std::cell::RefCell;
528 /// let c = RefCell::new(5);
531 /// let m = c.borrow_mut();
532 /// assert!(c.try_borrow().is_err());
536 /// let m = c.borrow();
537 /// assert!(c.try_borrow().is_ok());
540 #[unstable(feature = "try_borrow", issue = "35070")]
542 pub fn try_borrow(&self) -> Result<Ref<T>, BorrowError<T>> {
543 match BorrowRef::new(&self.borrow) {
545 value: unsafe { &*self.value.get() },
548 None => Err(BorrowError { marker: PhantomData }),
552 /// Mutably borrows the wrapped value.
554 /// The borrow lasts until the returned `RefMut` exits scope. The value
555 /// cannot be borrowed while this borrow is active.
559 /// Panics if the value is currently borrowed. For a non-panicking variant, use
560 /// [`try_borrow_mut`](#method.try_borrow_mut).
565 /// use std::cell::RefCell;
567 /// let c = RefCell::new(5);
569 /// *c.borrow_mut() = 7;
571 /// assert_eq!(*c.borrow(), 7);
574 /// An example of panic:
577 /// use std::cell::RefCell;
580 /// let result = thread::spawn(move || {
581 /// let c = RefCell::new(5);
582 /// let m = c.borrow();
584 /// let b = c.borrow_mut(); // this causes a panic
587 /// assert!(result.is_err());
589 #[stable(feature = "rust1", since = "1.0.0")]
591 pub fn borrow_mut(&self) -> RefMut<T> {
592 self.try_borrow_mut().expect("already borrowed")
595 /// Mutably borrows the wrapped value, returning an error if the value is currently borrowed.
597 /// The borrow lasts until the returned `RefMut` exits scope. The value cannot be borrowed
598 /// while this borrow is active.
600 /// This is the non-panicking variant of [`borrow_mut`](#method.borrow_mut).
605 /// #![feature(try_borrow)]
607 /// use std::cell::RefCell;
609 /// let c = RefCell::new(5);
612 /// let m = c.borrow();
613 /// assert!(c.try_borrow_mut().is_err());
616 /// assert!(c.try_borrow_mut().is_ok());
618 #[unstable(feature = "try_borrow", issue = "35070")]
620 pub fn try_borrow_mut(&self) -> Result<RefMut<T>, BorrowMutError<T>> {
621 match BorrowRefMut::new(&self.borrow) {
622 Some(b) => Ok(RefMut {
623 value: unsafe { &mut *self.value.get() },
626 None => Err(BorrowMutError { marker: PhantomData }),
630 /// Returns a reference to the underlying `UnsafeCell`.
632 /// This can be used to circumvent `RefCell`'s safety checks.
634 /// This function is `unsafe` because `UnsafeCell`'s field is public.
639 /// #![feature(as_unsafe_cell)]
641 /// use std::cell::RefCell;
643 /// let c = RefCell::new(5);
644 /// let c = unsafe { c.as_unsafe_cell() };
647 #[unstable(feature = "as_unsafe_cell", issue = "27708")]
648 pub unsafe fn as_unsafe_cell(&self) -> &UnsafeCell<T> {
652 /// Returns a mutable reference to the underlying data.
654 /// This call borrows `RefCell` mutably (at compile-time) so there is no
655 /// need for dynamic checks.
660 /// use std::cell::RefCell;
662 /// let mut c = RefCell::new(5);
663 /// *c.get_mut() += 1;
665 /// assert_eq!(c, RefCell::new(6));
668 #[stable(feature = "cell_get_mut", since = "1.11.0")]
669 pub fn get_mut(&mut self) -> &mut T {
671 &mut *self.value.get()
676 #[stable(feature = "rust1", since = "1.0.0")]
677 unsafe impl<T: ?Sized> Send for RefCell<T> where T: Send {}
679 #[stable(feature = "rust1", since = "1.0.0")]
680 impl<T: ?Sized> !Sync for RefCell<T> {}
682 #[stable(feature = "rust1", since = "1.0.0")]
683 impl<T: Clone> Clone for RefCell<T> {
685 fn clone(&self) -> RefCell<T> {
686 RefCell::new(self.borrow().clone())
690 #[stable(feature = "rust1", since = "1.0.0")]
691 impl<T:Default> Default for RefCell<T> {
693 fn default() -> RefCell<T> {
694 RefCell::new(Default::default())
698 #[stable(feature = "rust1", since = "1.0.0")]
699 impl<T: ?Sized + PartialEq> PartialEq for RefCell<T> {
701 fn eq(&self, other: &RefCell<T>) -> bool {
702 *self.borrow() == *other.borrow()
706 #[stable(feature = "cell_eq", since = "1.2.0")]
707 impl<T: ?Sized + Eq> Eq for RefCell<T> {}
709 #[stable(feature = "cell_ord", since = "1.10.0")]
710 impl<T: ?Sized + PartialOrd> PartialOrd for RefCell<T> {
712 fn partial_cmp(&self, other: &RefCell<T>) -> Option<Ordering> {
713 self.borrow().partial_cmp(&*other.borrow())
717 fn lt(&self, other: &RefCell<T>) -> bool {
718 *self.borrow() < *other.borrow()
722 fn le(&self, other: &RefCell<T>) -> bool {
723 *self.borrow() <= *other.borrow()
727 fn gt(&self, other: &RefCell<T>) -> bool {
728 *self.borrow() > *other.borrow()
732 fn ge(&self, other: &RefCell<T>) -> bool {
733 *self.borrow() >= *other.borrow()
737 #[stable(feature = "cell_ord", since = "1.10.0")]
738 impl<T: ?Sized + Ord> Ord for RefCell<T> {
740 fn cmp(&self, other: &RefCell<T>) -> Ordering {
741 self.borrow().cmp(&*other.borrow())
745 struct BorrowRef<'b> {
746 borrow: &'b Cell<BorrowFlag>,
749 impl<'b> BorrowRef<'b> {
751 fn new(borrow: &'b Cell<BorrowFlag>) -> Option<BorrowRef<'b>> {
756 Some(BorrowRef { borrow: borrow })
762 impl<'b> Drop for BorrowRef<'b> {
765 let borrow = self.borrow.get();
766 debug_assert!(borrow != WRITING && borrow != UNUSED);
767 self.borrow.set(borrow - 1);
771 impl<'b> Clone for BorrowRef<'b> {
773 fn clone(&self) -> BorrowRef<'b> {
774 // Since this Ref exists, we know the borrow flag
775 // is not set to WRITING.
776 let borrow = self.borrow.get();
777 debug_assert!(borrow != UNUSED);
778 // Prevent the borrow counter from overflowing.
779 assert!(borrow != WRITING);
780 self.borrow.set(borrow + 1);
781 BorrowRef { borrow: self.borrow }
785 /// Wraps a borrowed reference to a value in a `RefCell` box.
786 /// A wrapper type for an immutably borrowed value from a `RefCell<T>`.
788 /// See the [module-level documentation](index.html) for more.
789 #[stable(feature = "rust1", since = "1.0.0")]
790 pub struct Ref<'b, T: ?Sized + 'b> {
792 borrow: BorrowRef<'b>,
795 #[stable(feature = "rust1", since = "1.0.0")]
796 impl<'b, T: ?Sized> Deref for Ref<'b, T> {
800 fn deref(&self) -> &T {
805 impl<'b, T: ?Sized> Ref<'b, T> {
808 /// The `RefCell` is already immutably borrowed, so this cannot fail.
810 /// This is an associated function that needs to be used as
811 /// `Ref::clone(...)`. A `Clone` implementation or a method would interfere
812 /// with the widespread use of `r.borrow().clone()` to clone the contents of
814 #[unstable(feature = "cell_extras",
815 reason = "likely to be moved to a method, pending language changes",
818 pub fn clone(orig: &Ref<'b, T>) -> Ref<'b, T> {
821 borrow: orig.borrow.clone(),
825 /// Make a new `Ref` for a component of the borrowed data.
827 /// The `RefCell` is already immutably borrowed, so this cannot fail.
829 /// This is an associated function that needs to be used as `Ref::map(...)`.
830 /// A method would interfere with methods of the same name on the contents
831 /// of a `RefCell` used through `Deref`.
836 /// use std::cell::{RefCell, Ref};
838 /// let c = RefCell::new((5, 'b'));
839 /// let b1: Ref<(u32, char)> = c.borrow();
840 /// let b2: Ref<u32> = Ref::map(b1, |t| &t.0);
841 /// assert_eq!(*b2, 5)
843 #[stable(feature = "cell_map", since = "1.8.0")]
845 pub fn map<U: ?Sized, F>(orig: Ref<'b, T>, f: F) -> Ref<'b, U>
846 where F: FnOnce(&T) -> &U
849 value: f(orig.value),
855 #[unstable(feature = "coerce_unsized", issue = "27732")]
856 impl<'b, T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<Ref<'b, U>> for Ref<'b, T> {}
858 impl<'b, T: ?Sized> RefMut<'b, T> {
859 /// Make a new `RefMut` for a component of the borrowed data, e.g. an enum
862 /// The `RefCell` is already mutably borrowed, so this cannot fail.
864 /// This is an associated function that needs to be used as
865 /// `RefMut::map(...)`. A method would interfere with methods of the same
866 /// name on the contents of a `RefCell` used through `Deref`.
871 /// use std::cell::{RefCell, RefMut};
873 /// let c = RefCell::new((5, 'b'));
875 /// let b1: RefMut<(u32, char)> = c.borrow_mut();
876 /// let mut b2: RefMut<u32> = RefMut::map(b1, |t| &mut t.0);
877 /// assert_eq!(*b2, 5);
880 /// assert_eq!(*c.borrow(), (42, 'b'));
882 #[stable(feature = "cell_map", since = "1.8.0")]
884 pub fn map<U: ?Sized, F>(orig: RefMut<'b, T>, f: F) -> RefMut<'b, U>
885 where F: FnOnce(&mut T) -> &mut U
888 value: f(orig.value),
894 struct BorrowRefMut<'b> {
895 borrow: &'b Cell<BorrowFlag>,
898 impl<'b> Drop for BorrowRefMut<'b> {
901 let borrow = self.borrow.get();
902 debug_assert!(borrow == WRITING);
903 self.borrow.set(UNUSED);
907 impl<'b> BorrowRefMut<'b> {
909 fn new(borrow: &'b Cell<BorrowFlag>) -> Option<BorrowRefMut<'b>> {
913 Some(BorrowRefMut { borrow: borrow })
920 /// A wrapper type for a mutably borrowed value from a `RefCell<T>`.
922 /// See the [module-level documentation](index.html) for more.
923 #[stable(feature = "rust1", since = "1.0.0")]
924 pub struct RefMut<'b, T: ?Sized + 'b> {
926 borrow: BorrowRefMut<'b>,
929 #[stable(feature = "rust1", since = "1.0.0")]
930 impl<'b, T: ?Sized> Deref for RefMut<'b, T> {
934 fn deref(&self) -> &T {
939 #[stable(feature = "rust1", since = "1.0.0")]
940 impl<'b, T: ?Sized> DerefMut for RefMut<'b, T> {
942 fn deref_mut(&mut self) -> &mut T {
947 #[unstable(feature = "coerce_unsized", issue = "27732")]
948 impl<'b, T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<RefMut<'b, U>> for RefMut<'b, T> {}
950 /// The core primitive for interior mutability in Rust.
952 /// `UnsafeCell<T>` is a type that wraps some `T` and indicates unsafe interior operations on the
953 /// wrapped type. Types with an `UnsafeCell<T>` field are considered to have an 'unsafe interior'.
954 /// The `UnsafeCell<T>` type is the only legal way to obtain aliasable data that is considered
955 /// mutable. In general, transmuting an `&T` type into an `&mut T` is considered undefined behavior.
957 /// The compiler makes optimizations based on the knowledge that `&T` is not mutably aliased or
958 /// mutated, and that `&mut T` is unique. When building abstractions like `Cell`, `RefCell`,
959 /// `Mutex`, etc, you need to turn these optimizations off. `UnsafeCell` is the only legal way
960 /// to do this. When `UnsafeCell<T>` is immutably aliased, it is still safe to obtain a mutable
961 /// reference to its interior and/or to mutate it. However, it is up to the abstraction designer
962 /// to ensure that no two mutable references obtained this way are active at the same time, and
963 /// that there are no active mutable references or mutations when an immutable reference is obtained
964 /// from the cell. This is often done via runtime checks.
966 /// Note that while mutating or mutably aliasing the contents of an `& UnsafeCell<T>` is
967 /// okay (provided you enforce the invariants some other way); it is still undefined behavior
968 /// to have multiple `&mut UnsafeCell<T>` aliases.
971 /// Types like `Cell<T>` and `RefCell<T>` use this type to wrap their internal data.
976 /// use std::cell::UnsafeCell;
977 /// use std::marker::Sync;
979 /// # #[allow(dead_code)]
980 /// struct NotThreadSafe<T> {
981 /// value: UnsafeCell<T>,
984 /// unsafe impl<T> Sync for NotThreadSafe<T> {}
986 #[lang = "unsafe_cell"]
987 #[stable(feature = "rust1", since = "1.0.0")]
988 pub struct UnsafeCell<T: ?Sized> {
992 #[stable(feature = "rust1", since = "1.0.0")]
993 impl<T: ?Sized> !Sync for UnsafeCell<T> {}
995 impl<T> UnsafeCell<T> {
996 /// Constructs a new instance of `UnsafeCell` which will wrap the specified
999 /// All access to the inner value through methods is `unsafe`.
1004 /// use std::cell::UnsafeCell;
1006 /// let uc = UnsafeCell::new(5);
1008 #[stable(feature = "rust1", since = "1.0.0")]
1010 pub const fn new(value: T) -> UnsafeCell<T> {
1011 UnsafeCell { value: value }
1014 /// Unwraps the value.
1018 /// This function is unsafe because this thread or another thread may currently be
1019 /// inspecting the inner value.
1024 /// use std::cell::UnsafeCell;
1026 /// let uc = UnsafeCell::new(5);
1028 /// let five = unsafe { uc.into_inner() };
1031 #[stable(feature = "rust1", since = "1.0.0")]
1032 pub unsafe fn into_inner(self) -> T {
1037 impl<T: ?Sized> UnsafeCell<T> {
1038 /// Gets a mutable pointer to the wrapped value.
1040 /// This can be cast to a pointer of any kind.
1041 /// Ensure that the access is unique when casting to
1042 /// `&mut T`, and ensure that there are no mutations or mutable
1043 /// aliases going on when casting to `&T`
1048 /// use std::cell::UnsafeCell;
1050 /// let uc = UnsafeCell::new(5);
1052 /// let five = uc.get();
1055 #[stable(feature = "rust1", since = "1.0.0")]
1056 pub fn get(&self) -> *mut T {
1057 &self.value as *const T as *mut T
1061 #[stable(feature = "unsafe_cell_default", since = "1.9.0")]
1062 impl<T: Default> Default for UnsafeCell<T> {
1063 fn default() -> UnsafeCell<T> {
1064 UnsafeCell::new(Default::default())