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};
149 use default::Default;
150 use marker::{Copy, Send, Sync, Sized};
151 use ops::{Deref, DerefMut, Drop, FnOnce};
153 use option::Option::{None, Some};
155 /// A mutable memory location that admits only `Copy` data.
157 /// See the [module-level documentation](index.html) for more.
158 #[stable(feature = "rust1", since = "1.0.0")]
160 value: UnsafeCell<T>,
163 impl<T:Copy> Cell<T> {
164 /// Creates a new `Cell` containing the given value.
169 /// use std::cell::Cell;
171 /// let c = Cell::new(5);
173 #[stable(feature = "rust1", since = "1.0.0")]
175 pub const fn new(value: T) -> Cell<T> {
177 value: UnsafeCell::new(value),
181 /// Returns a copy of the contained value.
186 /// use std::cell::Cell;
188 /// let c = Cell::new(5);
190 /// let five = c.get();
193 #[stable(feature = "rust1", since = "1.0.0")]
194 pub fn get(&self) -> T {
195 unsafe{ *self.value.get() }
198 /// Sets the contained value.
203 /// use std::cell::Cell;
205 /// let c = Cell::new(5);
210 #[stable(feature = "rust1", since = "1.0.0")]
211 pub fn set(&self, value: T) {
213 *self.value.get() = value;
217 /// Returns a reference to the underlying `UnsafeCell`.
221 /// This function is `unsafe` because `UnsafeCell`'s field is public.
226 /// #![feature(as_unsafe_cell)]
228 /// use std::cell::Cell;
230 /// let c = Cell::new(5);
232 /// let uc = unsafe { c.as_unsafe_cell() };
235 #[unstable(feature = "as_unsafe_cell", issue = "27708")]
236 pub unsafe fn as_unsafe_cell(&self) -> &UnsafeCell<T> {
241 #[stable(feature = "rust1", since = "1.0.0")]
242 unsafe impl<T> Send for Cell<T> where T: Send {}
244 #[stable(feature = "rust1", since = "1.0.0")]
245 impl<T:Copy> Clone for Cell<T> {
247 fn clone(&self) -> Cell<T> {
248 Cell::new(self.get())
252 #[stable(feature = "rust1", since = "1.0.0")]
253 impl<T:Default + Copy> Default for Cell<T> {
255 fn default() -> Cell<T> {
256 Cell::new(Default::default())
260 #[stable(feature = "rust1", since = "1.0.0")]
261 impl<T:PartialEq + Copy> PartialEq for Cell<T> {
263 fn eq(&self, other: &Cell<T>) -> bool {
264 self.get() == other.get()
268 #[stable(feature = "cell_eq", since = "1.2.0")]
269 impl<T:Eq + Copy> Eq for Cell<T> {}
271 /// A mutable memory location with dynamically checked borrow rules
273 /// See the [module-level documentation](index.html) for more.
274 #[stable(feature = "rust1", since = "1.0.0")]
275 pub struct RefCell<T: ?Sized> {
276 borrow: Cell<BorrowFlag>,
277 value: UnsafeCell<T>,
280 /// An enumeration of values returned from the `state` method on a `RefCell<T>`.
281 #[derive(Copy, Clone, PartialEq, Eq, Debug)]
282 #[unstable(feature = "borrow_state", issue = "27733")]
283 pub enum BorrowState {
284 /// The cell is currently being read, there is at least one active `borrow`.
286 /// The cell is currently being written to, there is an active `borrow_mut`.
288 /// There are no outstanding borrows on this cell.
292 // Values [1, MAX-1] represent the number of `Ref` active
293 // (will not outgrow its range since `usize` is the size of the address space)
294 type BorrowFlag = usize;
295 const UNUSED: BorrowFlag = 0;
296 const WRITING: BorrowFlag = !0;
299 /// Creates a new `RefCell` containing `value`.
304 /// use std::cell::RefCell;
306 /// let c = RefCell::new(5);
308 #[stable(feature = "rust1", since = "1.0.0")]
310 pub const fn new(value: T) -> RefCell<T> {
312 value: UnsafeCell::new(value),
313 borrow: Cell::new(UNUSED),
317 /// Consumes the `RefCell`, returning the wrapped value.
322 /// use std::cell::RefCell;
324 /// let c = RefCell::new(5);
326 /// let five = c.into_inner();
328 #[stable(feature = "rust1", since = "1.0.0")]
330 pub fn into_inner(self) -> T {
331 // Since this function takes `self` (the `RefCell`) by value, the
332 // compiler statically verifies that it is not currently borrowed.
333 // Therefore the following assertion is just a `debug_assert!`.
334 debug_assert!(self.borrow.get() == UNUSED);
335 unsafe { self.value.into_inner() }
339 impl<T: ?Sized> RefCell<T> {
340 /// Query the current state of this `RefCell`
342 /// The returned value can be dispatched on to determine if a call to
343 /// `borrow` or `borrow_mut` would succeed.
344 #[unstable(feature = "borrow_state", issue = "27733")]
346 pub fn borrow_state(&self) -> BorrowState {
347 match self.borrow.get() {
348 WRITING => BorrowState::Writing,
349 UNUSED => BorrowState::Unused,
350 _ => BorrowState::Reading,
354 /// Immutably borrows the wrapped value.
356 /// The borrow lasts until the returned `Ref` exits scope. Multiple
357 /// immutable borrows can be taken out at the same time.
361 /// Panics if the value is currently mutably borrowed.
366 /// use std::cell::RefCell;
368 /// let c = RefCell::new(5);
370 /// let borrowed_five = c.borrow();
371 /// let borrowed_five2 = c.borrow();
374 /// An example of panic:
377 /// use std::cell::RefCell;
380 /// let result = thread::spawn(move || {
381 /// let c = RefCell::new(5);
382 /// let m = c.borrow_mut();
384 /// let b = c.borrow(); // this causes a panic
387 /// assert!(result.is_err());
389 #[stable(feature = "rust1", since = "1.0.0")]
391 pub fn borrow(&self) -> Ref<T> {
392 match BorrowRef::new(&self.borrow) {
394 _value: unsafe { &*self.value.get() },
397 None => panic!("RefCell<T> already mutably borrowed"),
401 /// Mutably borrows the wrapped value.
403 /// The borrow lasts until the returned `RefMut` exits scope. The value
404 /// cannot be borrowed while this borrow is active.
408 /// Panics if the value is currently borrowed.
413 /// use std::cell::RefCell;
415 /// let c = RefCell::new(5);
417 /// *c.borrow_mut() = 7;
419 /// assert_eq!(*c.borrow(), 7);
422 /// An example of panic:
425 /// use std::cell::RefCell;
428 /// let result = thread::spawn(move || {
429 /// let c = RefCell::new(5);
430 /// let m = c.borrow();
432 /// let b = c.borrow_mut(); // this causes a panic
435 /// assert!(result.is_err());
437 #[stable(feature = "rust1", since = "1.0.0")]
439 pub fn borrow_mut(&self) -> RefMut<T> {
440 match BorrowRefMut::new(&self.borrow) {
442 _value: unsafe { &mut *self.value.get() },
445 None => panic!("RefCell<T> already borrowed"),
449 /// Returns a reference to the underlying `UnsafeCell`.
451 /// This can be used to circumvent `RefCell`'s safety checks.
453 /// This function is `unsafe` because `UnsafeCell`'s field is public.
455 #[unstable(feature = "as_unsafe_cell", issue = "27708")]
456 pub unsafe fn as_unsafe_cell(&self) -> &UnsafeCell<T> {
461 #[stable(feature = "rust1", since = "1.0.0")]
462 unsafe impl<T: ?Sized> Send for RefCell<T> where T: Send {}
464 #[stable(feature = "rust1", since = "1.0.0")]
465 impl<T: Clone> Clone for RefCell<T> {
467 fn clone(&self) -> RefCell<T> {
468 RefCell::new(self.borrow().clone())
472 #[stable(feature = "rust1", since = "1.0.0")]
473 impl<T:Default> Default for RefCell<T> {
475 fn default() -> RefCell<T> {
476 RefCell::new(Default::default())
480 #[stable(feature = "rust1", since = "1.0.0")]
481 impl<T: ?Sized + PartialEq> PartialEq for RefCell<T> {
483 fn eq(&self, other: &RefCell<T>) -> bool {
484 *self.borrow() == *other.borrow()
488 #[stable(feature = "cell_eq", since = "1.2.0")]
489 impl<T: ?Sized + Eq> Eq for RefCell<T> {}
491 struct BorrowRef<'b> {
492 _borrow: &'b Cell<BorrowFlag>,
495 impl<'b> BorrowRef<'b> {
497 fn new(borrow: &'b Cell<BorrowFlag>) -> Option<BorrowRef<'b>> {
502 Some(BorrowRef { _borrow: borrow })
508 impl<'b> Drop for BorrowRef<'b> {
511 let borrow = self._borrow.get();
512 debug_assert!(borrow != WRITING && borrow != UNUSED);
513 self._borrow.set(borrow - 1);
517 impl<'b> Clone for BorrowRef<'b> {
519 fn clone(&self) -> BorrowRef<'b> {
520 // Since this Ref exists, we know the borrow flag
521 // is not set to WRITING.
522 let borrow = self._borrow.get();
523 debug_assert!(borrow != WRITING && borrow != UNUSED);
524 self._borrow.set(borrow + 1);
525 BorrowRef { _borrow: self._borrow }
529 /// Wraps a borrowed reference to a value in a `RefCell` box.
530 /// A wrapper type for an immutably borrowed value from a `RefCell<T>`.
532 /// See the [module-level documentation](index.html) for more.
533 #[stable(feature = "rust1", since = "1.0.0")]
534 pub struct Ref<'b, T: ?Sized + 'b> {
535 // FIXME #12808: strange name to try to avoid interfering with
536 // field accesses of the contained type via Deref
538 _borrow: BorrowRef<'b>,
541 #[stable(feature = "rust1", since = "1.0.0")]
542 impl<'b, T: ?Sized> Deref for Ref<'b, T> {
546 fn deref(&self) -> &T {
551 impl<'b, T: ?Sized> Ref<'b, T> {
554 /// The `RefCell` is already immutably borrowed, so this cannot fail.
556 /// This is an associated function that needs to be used as
557 /// `Ref::clone(...)`. A `Clone` implementation or a method would interfere
558 /// with the widespread use of `r.borrow().clone()` to clone the contents of
560 #[unstable(feature = "cell_extras",
561 reason = "likely to be moved to a method, pending language changes",
564 pub fn clone(orig: &Ref<'b, T>) -> Ref<'b, T> {
567 _borrow: orig._borrow.clone(),
571 /// Make a new `Ref` for a component of the borrowed data.
573 /// The `RefCell` is already immutably borrowed, so this cannot fail.
575 /// This is an associated function that needs to be used as `Ref::map(...)`.
576 /// A method would interfere with methods of the same name on the contents
577 /// of a `RefCell` used through `Deref`.
582 /// #![feature(cell_extras)]
584 /// use std::cell::{RefCell, Ref};
586 /// let c = RefCell::new((5, 'b'));
587 /// let b1: Ref<(u32, char)> = c.borrow();
588 /// let b2: Ref<u32> = Ref::map(b1, |t| &t.0);
589 /// assert_eq!(*b2, 5)
591 #[unstable(feature = "cell_extras", reason = "recently added",
594 pub fn map<U: ?Sized, F>(orig: Ref<'b, T>, f: F) -> Ref<'b, U>
595 where F: FnOnce(&T) -> &U
598 _value: f(orig._value),
599 _borrow: orig._borrow,
603 /// Make a new `Ref` for an optional component of the borrowed data, e.g. an
606 /// The `RefCell` is already immutably borrowed, so this cannot fail.
608 /// This is an associated function that needs to be used as
609 /// `Ref::filter_map(...)`. A method would interfere with methods of the
610 /// same name on the contents of a `RefCell` used through `Deref`.
615 /// # #![feature(cell_extras)]
616 /// use std::cell::{RefCell, Ref};
618 /// let c = RefCell::new(Ok(5));
619 /// let b1: Ref<Result<u32, ()>> = c.borrow();
620 /// let b2: Ref<u32> = Ref::filter_map(b1, |o| o.as_ref().ok()).unwrap();
621 /// assert_eq!(*b2, 5)
623 #[unstable(feature = "cell_extras", reason = "recently added",
626 pub fn filter_map<U: ?Sized, F>(orig: Ref<'b, T>, f: F) -> Option<Ref<'b, U>>
627 where F: FnOnce(&T) -> Option<&U>
629 f(orig._value).map(move |new| Ref {
631 _borrow: orig._borrow,
636 impl<'b, T: ?Sized> RefMut<'b, T> {
637 /// Make a new `RefMut` for a component of the borrowed data, e.g. an enum
640 /// The `RefCell` is already mutably borrowed, so this cannot fail.
642 /// This is an associated function that needs to be used as
643 /// `RefMut::map(...)`. A method would interfere with methods of the same
644 /// name on the contents of a `RefCell` used through `Deref`.
649 /// # #![feature(cell_extras)]
650 /// use std::cell::{RefCell, RefMut};
652 /// let c = RefCell::new((5, 'b'));
654 /// let b1: RefMut<(u32, char)> = c.borrow_mut();
655 /// let mut b2: RefMut<u32> = RefMut::map(b1, |t| &mut t.0);
656 /// assert_eq!(*b2, 5);
659 /// assert_eq!(*c.borrow(), (42, 'b'));
661 #[unstable(feature = "cell_extras", reason = "recently added",
664 pub fn map<U: ?Sized, F>(orig: RefMut<'b, T>, f: F) -> RefMut<'b, U>
665 where F: FnOnce(&mut T) -> &mut U
668 _value: f(orig._value),
669 _borrow: orig._borrow,
673 /// Make a new `RefMut` for an optional component of the borrowed data, e.g.
676 /// The `RefCell` is already mutably borrowed, so this cannot fail.
678 /// This is an associated function that needs to be used as
679 /// `RefMut::filter_map(...)`. A method would interfere with methods of the
680 /// same name on the contents of a `RefCell` used through `Deref`.
685 /// # #![feature(cell_extras)]
686 /// use std::cell::{RefCell, RefMut};
688 /// let c = RefCell::new(Ok(5));
690 /// let b1: RefMut<Result<u32, ()>> = c.borrow_mut();
691 /// let mut b2: RefMut<u32> = RefMut::filter_map(b1, |o| {
694 /// assert_eq!(*b2, 5);
697 /// assert_eq!(*c.borrow(), Ok(42));
699 #[unstable(feature = "cell_extras", reason = "recently added",
702 pub fn filter_map<U: ?Sized, F>(orig: RefMut<'b, T>, f: F) -> Option<RefMut<'b, U>>
703 where F: FnOnce(&mut T) -> Option<&mut U>
705 let RefMut { _value, _borrow } = orig;
706 f(_value).map(move |new| RefMut {
713 struct BorrowRefMut<'b> {
714 _borrow: &'b Cell<BorrowFlag>,
717 impl<'b> Drop for BorrowRefMut<'b> {
720 let borrow = self._borrow.get();
721 debug_assert!(borrow == WRITING);
722 self._borrow.set(UNUSED);
726 impl<'b> BorrowRefMut<'b> {
728 fn new(borrow: &'b Cell<BorrowFlag>) -> Option<BorrowRefMut<'b>> {
732 Some(BorrowRefMut { _borrow: borrow })
739 /// A wrapper type for a mutably borrowed value from a `RefCell<T>`.
741 /// See the [module-level documentation](index.html) for more.
742 #[stable(feature = "rust1", since = "1.0.0")]
743 pub struct RefMut<'b, T: ?Sized + 'b> {
744 // FIXME #12808: strange name to try to avoid interfering with
745 // field accesses of the contained type via Deref
747 _borrow: BorrowRefMut<'b>,
750 #[stable(feature = "rust1", since = "1.0.0")]
751 impl<'b, T: ?Sized> Deref for RefMut<'b, T> {
755 fn deref(&self) -> &T {
760 #[stable(feature = "rust1", since = "1.0.0")]
761 impl<'b, T: ?Sized> DerefMut for RefMut<'b, T> {
763 fn deref_mut(&mut self) -> &mut T {
768 /// The core primitive for interior mutability in Rust.
770 /// `UnsafeCell<T>` is a type that wraps some `T` and indicates unsafe interior operations on the
771 /// wrapped type. Types with an `UnsafeCell<T>` field are considered to have an 'unsafe interior'.
772 /// The `UnsafeCell<T>` type is the only legal way to obtain aliasable data that is considered
773 /// mutable. In general, transmuting an `&T` type into an `&mut T` is considered undefined behavior.
775 /// Types like `Cell<T>` and `RefCell<T>` use this type to wrap their internal data.
780 /// use std::cell::UnsafeCell;
781 /// use std::marker::Sync;
783 /// # #[allow(dead_code)]
784 /// struct NotThreadSafe<T> {
785 /// value: UnsafeCell<T>,
788 /// unsafe impl<T> Sync for NotThreadSafe<T> {}
790 #[lang = "unsafe_cell"]
791 #[stable(feature = "rust1", since = "1.0.0")]
792 pub struct UnsafeCell<T: ?Sized> {
796 #[stable(feature = "rust1", since = "1.0.0")]
797 impl<T: ?Sized> !Sync for UnsafeCell<T> {}
799 impl<T> UnsafeCell<T> {
800 /// Constructs a new instance of `UnsafeCell` which will wrap the specified
803 /// All access to the inner value through methods is `unsafe`.
808 /// use std::cell::UnsafeCell;
810 /// let uc = UnsafeCell::new(5);
812 #[stable(feature = "rust1", since = "1.0.0")]
814 pub const fn new(value: T) -> UnsafeCell<T> {
815 UnsafeCell { value: value }
818 /// Unwraps the value.
822 /// This function is unsafe because this thread or another thread may currently be
823 /// inspecting the inner value.
828 /// use std::cell::UnsafeCell;
830 /// let uc = UnsafeCell::new(5);
832 /// let five = unsafe { uc.into_inner() };
835 #[stable(feature = "rust1", since = "1.0.0")]
836 pub unsafe fn into_inner(self) -> T {
841 impl<T: ?Sized> UnsafeCell<T> {
842 /// Gets a mutable pointer to the wrapped value.
847 /// use std::cell::UnsafeCell;
849 /// let uc = UnsafeCell::new(5);
851 /// let five = uc.get();
854 #[stable(feature = "rust1", since = "1.0.0")]
855 pub fn get(&self) -> *mut T {
856 &self.value as *const T as *mut T