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, Unsize};
151 use ops::{Deref, DerefMut, Drop, FnOnce, CoerceUnsized};
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`.
222 /// #![feature(as_unsafe_cell)]
224 /// use std::cell::Cell;
226 /// let c = Cell::new(5);
228 /// let uc = c.as_unsafe_cell();
231 #[unstable(feature = "as_unsafe_cell", issue = "27708")]
232 pub fn as_unsafe_cell(&self) -> &UnsafeCell<T> {
236 /// Returns a mutable reference to the underlying data.
238 /// This call borrows `Cell` mutably (at compile-time) which guarantees
239 /// that we possess the only reference.
241 #[unstable(feature = "cell_get_mut", issue = "33444")]
242 pub fn get_mut(&mut self) -> &mut T {
244 &mut *self.value.get()
249 #[stable(feature = "rust1", since = "1.0.0")]
250 unsafe impl<T> Send for Cell<T> where T: Send {}
252 #[stable(feature = "rust1", since = "1.0.0")]
253 impl<T> !Sync for Cell<T> {}
255 #[stable(feature = "rust1", since = "1.0.0")]
256 impl<T:Copy> Clone for Cell<T> {
258 fn clone(&self) -> Cell<T> {
259 Cell::new(self.get())
263 #[stable(feature = "rust1", since = "1.0.0")]
264 impl<T:Default + Copy> Default for Cell<T> {
266 fn default() -> Cell<T> {
267 Cell::new(Default::default())
271 #[stable(feature = "rust1", since = "1.0.0")]
272 impl<T:PartialEq + Copy> PartialEq for Cell<T> {
274 fn eq(&self, other: &Cell<T>) -> bool {
275 self.get() == other.get()
279 #[stable(feature = "cell_eq", since = "1.2.0")]
280 impl<T:Eq + Copy> Eq for Cell<T> {}
282 /// A mutable memory location with dynamically checked borrow rules
284 /// See the [module-level documentation](index.html) for more.
285 #[stable(feature = "rust1", since = "1.0.0")]
286 pub struct RefCell<T: ?Sized> {
287 borrow: Cell<BorrowFlag>,
288 value: UnsafeCell<T>,
291 /// An enumeration of values returned from the `state` method on a `RefCell<T>`.
292 #[derive(Copy, Clone, PartialEq, Eq, Debug)]
293 #[unstable(feature = "borrow_state", issue = "27733")]
294 pub enum BorrowState {
295 /// The cell is currently being read, there is at least one active `borrow`.
297 /// The cell is currently being written to, there is an active `borrow_mut`.
299 /// There are no outstanding borrows on this cell.
303 // Values [1, MAX-1] represent the number of `Ref` active
304 // (will not outgrow its range since `usize` is the size of the address space)
305 type BorrowFlag = usize;
306 const UNUSED: BorrowFlag = 0;
307 const WRITING: BorrowFlag = !0;
310 /// Creates a new `RefCell` containing `value`.
315 /// use std::cell::RefCell;
317 /// let c = RefCell::new(5);
319 #[stable(feature = "rust1", since = "1.0.0")]
321 pub const fn new(value: T) -> RefCell<T> {
323 value: UnsafeCell::new(value),
324 borrow: Cell::new(UNUSED),
328 /// Consumes the `RefCell`, returning the wrapped value.
333 /// use std::cell::RefCell;
335 /// let c = RefCell::new(5);
337 /// let five = c.into_inner();
339 #[stable(feature = "rust1", since = "1.0.0")]
341 pub fn into_inner(self) -> T {
342 // Since this function takes `self` (the `RefCell`) by value, the
343 // compiler statically verifies that it is not currently borrowed.
344 // Therefore the following assertion is just a `debug_assert!`.
345 debug_assert!(self.borrow.get() == UNUSED);
346 unsafe { self.value.into_inner() }
350 impl<T: ?Sized> RefCell<T> {
351 /// Query the current state of this `RefCell`
353 /// The returned value can be dispatched on to determine if a call to
354 /// `borrow` or `borrow_mut` would succeed.
355 #[unstable(feature = "borrow_state", issue = "27733")]
357 pub fn borrow_state(&self) -> BorrowState {
358 match self.borrow.get() {
359 WRITING => BorrowState::Writing,
360 UNUSED => BorrowState::Unused,
361 _ => BorrowState::Reading,
365 /// Immutably borrows the wrapped value.
367 /// The borrow lasts until the returned `Ref` exits scope. Multiple
368 /// immutable borrows can be taken out at the same time.
372 /// Panics if the value is currently mutably borrowed.
377 /// use std::cell::RefCell;
379 /// let c = RefCell::new(5);
381 /// let borrowed_five = c.borrow();
382 /// let borrowed_five2 = c.borrow();
385 /// An example of panic:
388 /// use std::cell::RefCell;
391 /// let result = thread::spawn(move || {
392 /// let c = RefCell::new(5);
393 /// let m = c.borrow_mut();
395 /// let b = c.borrow(); // this causes a panic
398 /// assert!(result.is_err());
400 #[stable(feature = "rust1", since = "1.0.0")]
402 pub fn borrow(&self) -> Ref<T> {
403 match BorrowRef::new(&self.borrow) {
405 value: unsafe { &*self.value.get() },
408 None => panic!("RefCell<T> already mutably borrowed"),
412 /// Mutably borrows the wrapped value.
414 /// The borrow lasts until the returned `RefMut` exits scope. The value
415 /// cannot be borrowed while this borrow is active.
419 /// Panics if the value is currently borrowed.
424 /// use std::cell::RefCell;
426 /// let c = RefCell::new(5);
428 /// *c.borrow_mut() = 7;
430 /// assert_eq!(*c.borrow(), 7);
433 /// An example of panic:
436 /// use std::cell::RefCell;
439 /// let result = thread::spawn(move || {
440 /// let c = RefCell::new(5);
441 /// let m = c.borrow();
443 /// let b = c.borrow_mut(); // this causes a panic
446 /// assert!(result.is_err());
448 #[stable(feature = "rust1", since = "1.0.0")]
450 pub fn borrow_mut(&self) -> RefMut<T> {
451 match BorrowRefMut::new(&self.borrow) {
453 value: unsafe { &mut *self.value.get() },
456 None => panic!("RefCell<T> already borrowed"),
460 /// Returns a reference to the underlying `UnsafeCell`.
462 /// This can be used to circumvent `RefCell`'s safety checks.
464 /// This function is `unsafe` because `UnsafeCell`'s field is public.
466 #[unstable(feature = "as_unsafe_cell", issue = "27708")]
467 pub unsafe fn as_unsafe_cell(&self) -> &UnsafeCell<T> {
471 /// Returns a mutable reference to the underlying data.
473 /// This call borrows `RefCell` mutably (at compile-time) so there is no
474 /// need for dynamic checks.
476 #[unstable(feature = "cell_get_mut", issue="33444")]
477 pub fn get_mut(&mut self) -> &mut T {
479 &mut *self.value.get()
484 #[stable(feature = "rust1", since = "1.0.0")]
485 unsafe impl<T: ?Sized> Send for RefCell<T> where T: Send {}
487 #[stable(feature = "rust1", since = "1.0.0")]
488 impl<T: ?Sized> !Sync for RefCell<T> {}
490 #[stable(feature = "rust1", since = "1.0.0")]
491 impl<T: Clone> Clone for RefCell<T> {
493 fn clone(&self) -> RefCell<T> {
494 RefCell::new(self.borrow().clone())
498 #[stable(feature = "rust1", since = "1.0.0")]
499 impl<T:Default> Default for RefCell<T> {
501 fn default() -> RefCell<T> {
502 RefCell::new(Default::default())
506 #[stable(feature = "rust1", since = "1.0.0")]
507 impl<T: ?Sized + PartialEq> PartialEq for RefCell<T> {
509 fn eq(&self, other: &RefCell<T>) -> bool {
510 *self.borrow() == *other.borrow()
514 #[stable(feature = "cell_eq", since = "1.2.0")]
515 impl<T: ?Sized + Eq> Eq for RefCell<T> {}
517 struct BorrowRef<'b> {
518 borrow: &'b Cell<BorrowFlag>,
521 impl<'b> BorrowRef<'b> {
523 fn new(borrow: &'b Cell<BorrowFlag>) -> Option<BorrowRef<'b>> {
528 Some(BorrowRef { borrow: borrow })
534 impl<'b> Drop for BorrowRef<'b> {
537 let borrow = self.borrow.get();
538 debug_assert!(borrow != WRITING && borrow != UNUSED);
539 self.borrow.set(borrow - 1);
543 impl<'b> Clone for BorrowRef<'b> {
545 fn clone(&self) -> BorrowRef<'b> {
546 // Since this Ref exists, we know the borrow flag
547 // is not set to WRITING.
548 let borrow = self.borrow.get();
549 debug_assert!(borrow != WRITING && borrow != UNUSED);
550 self.borrow.set(borrow + 1);
551 BorrowRef { borrow: self.borrow }
555 /// Wraps a borrowed reference to a value in a `RefCell` box.
556 /// A wrapper type for an immutably borrowed value from a `RefCell<T>`.
558 /// See the [module-level documentation](index.html) for more.
559 #[stable(feature = "rust1", since = "1.0.0")]
560 pub struct Ref<'b, T: ?Sized + 'b> {
562 borrow: BorrowRef<'b>,
565 #[stable(feature = "rust1", since = "1.0.0")]
566 impl<'b, T: ?Sized> Deref for Ref<'b, T> {
570 fn deref(&self) -> &T {
575 impl<'b, T: ?Sized> Ref<'b, T> {
578 /// The `RefCell` is already immutably borrowed, so this cannot fail.
580 /// This is an associated function that needs to be used as
581 /// `Ref::clone(...)`. A `Clone` implementation or a method would interfere
582 /// with the widespread use of `r.borrow().clone()` to clone the contents of
584 #[unstable(feature = "cell_extras",
585 reason = "likely to be moved to a method, pending language changes",
588 pub fn clone(orig: &Ref<'b, T>) -> Ref<'b, T> {
591 borrow: orig.borrow.clone(),
595 /// Make a new `Ref` for a component of the borrowed data.
597 /// The `RefCell` is already immutably borrowed, so this cannot fail.
599 /// This is an associated function that needs to be used as `Ref::map(...)`.
600 /// A method would interfere with methods of the same name on the contents
601 /// of a `RefCell` used through `Deref`.
606 /// use std::cell::{RefCell, Ref};
608 /// let c = RefCell::new((5, 'b'));
609 /// let b1: Ref<(u32, char)> = c.borrow();
610 /// let b2: Ref<u32> = Ref::map(b1, |t| &t.0);
611 /// assert_eq!(*b2, 5)
613 #[stable(feature = "cell_map", since = "1.8.0")]
615 pub fn map<U: ?Sized, F>(orig: Ref<'b, T>, f: F) -> Ref<'b, U>
616 where F: FnOnce(&T) -> &U
619 value: f(orig.value),
624 /// Make a new `Ref` for an optional component of the borrowed data, e.g. an
627 /// The `RefCell` is already immutably borrowed, so this cannot fail.
629 /// This is an associated function that needs to be used as
630 /// `Ref::filter_map(...)`. A method would interfere with methods of the
631 /// same name on the contents of a `RefCell` used through `Deref`.
636 /// # #![feature(cell_extras)]
637 /// use std::cell::{RefCell, Ref};
639 /// let c = RefCell::new(Ok(5));
640 /// let b1: Ref<Result<u32, ()>> = c.borrow();
641 /// let b2: Ref<u32> = Ref::filter_map(b1, |o| o.as_ref().ok()).unwrap();
642 /// assert_eq!(*b2, 5)
644 #[unstable(feature = "cell_extras", reason = "recently added",
646 #[rustc_deprecated(since = "1.8.0", reason = "can be built on `Ref::map`: \
647 https://crates.io/crates/ref_filter_map")]
649 pub fn filter_map<U: ?Sized, F>(orig: Ref<'b, T>, f: F) -> Option<Ref<'b, U>>
650 where F: FnOnce(&T) -> Option<&U>
652 f(orig.value).map(move |new| Ref {
659 #[unstable(feature = "coerce_unsized", issue = "27732")]
660 impl<'b, T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<Ref<'b, U>> for Ref<'b, T> {}
662 impl<'b, T: ?Sized> RefMut<'b, T> {
663 /// Make a new `RefMut` for a component of the borrowed data, e.g. an enum
666 /// The `RefCell` is already mutably borrowed, so this cannot fail.
668 /// This is an associated function that needs to be used as
669 /// `RefMut::map(...)`. A method would interfere with methods of the same
670 /// name on the contents of a `RefCell` used through `Deref`.
675 /// use std::cell::{RefCell, RefMut};
677 /// let c = RefCell::new((5, 'b'));
679 /// let b1: RefMut<(u32, char)> = c.borrow_mut();
680 /// let mut b2: RefMut<u32> = RefMut::map(b1, |t| &mut t.0);
681 /// assert_eq!(*b2, 5);
684 /// assert_eq!(*c.borrow(), (42, 'b'));
686 #[stable(feature = "cell_map", since = "1.8.0")]
688 pub fn map<U: ?Sized, F>(orig: RefMut<'b, T>, f: F) -> RefMut<'b, U>
689 where F: FnOnce(&mut T) -> &mut U
692 value: f(orig.value),
697 /// Make a new `RefMut` for an optional component of the borrowed data, e.g.
700 /// The `RefCell` is already mutably borrowed, so this cannot fail.
702 /// This is an associated function that needs to be used as
703 /// `RefMut::filter_map(...)`. A method would interfere with methods of the
704 /// same name on the contents of a `RefCell` used through `Deref`.
709 /// # #![feature(cell_extras)]
710 /// use std::cell::{RefCell, RefMut};
712 /// let c = RefCell::new(Ok(5));
714 /// let b1: RefMut<Result<u32, ()>> = c.borrow_mut();
715 /// let mut b2: RefMut<u32> = RefMut::filter_map(b1, |o| {
718 /// assert_eq!(*b2, 5);
721 /// assert_eq!(*c.borrow(), Ok(42));
723 #[unstable(feature = "cell_extras", reason = "recently added",
725 #[rustc_deprecated(since = "1.8.0", reason = "can be built on `RefMut::map`: \
726 https://crates.io/crates/ref_filter_map")]
728 pub fn filter_map<U: ?Sized, F>(orig: RefMut<'b, T>, f: F) -> Option<RefMut<'b, U>>
729 where F: FnOnce(&mut T) -> Option<&mut U>
731 let RefMut { value, borrow } = orig;
732 f(value).map(move |new| RefMut {
739 struct BorrowRefMut<'b> {
740 borrow: &'b Cell<BorrowFlag>,
743 impl<'b> Drop for BorrowRefMut<'b> {
746 let borrow = self.borrow.get();
747 debug_assert!(borrow == WRITING);
748 self.borrow.set(UNUSED);
752 impl<'b> BorrowRefMut<'b> {
754 fn new(borrow: &'b Cell<BorrowFlag>) -> Option<BorrowRefMut<'b>> {
758 Some(BorrowRefMut { borrow: borrow })
765 /// A wrapper type for a mutably borrowed value from a `RefCell<T>`.
767 /// See the [module-level documentation](index.html) for more.
768 #[stable(feature = "rust1", since = "1.0.0")]
769 pub struct RefMut<'b, T: ?Sized + 'b> {
771 borrow: BorrowRefMut<'b>,
774 #[stable(feature = "rust1", since = "1.0.0")]
775 impl<'b, T: ?Sized> Deref for RefMut<'b, T> {
779 fn deref(&self) -> &T {
784 #[stable(feature = "rust1", since = "1.0.0")]
785 impl<'b, T: ?Sized> DerefMut for RefMut<'b, T> {
787 fn deref_mut(&mut self) -> &mut T {
792 #[unstable(feature = "coerce_unsized", issue = "27732")]
793 impl<'b, T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<RefMut<'b, U>> for RefMut<'b, T> {}
795 /// The core primitive for interior mutability in Rust.
797 /// `UnsafeCell<T>` is a type that wraps some `T` and indicates unsafe interior operations on the
798 /// wrapped type. Types with an `UnsafeCell<T>` field are considered to have an 'unsafe interior'.
799 /// The `UnsafeCell<T>` type is the only legal way to obtain aliasable data that is considered
800 /// mutable. In general, transmuting an `&T` type into an `&mut T` is considered undefined behavior.
802 /// Types like `Cell<T>` and `RefCell<T>` use this type to wrap their internal data.
807 /// use std::cell::UnsafeCell;
808 /// use std::marker::Sync;
810 /// # #[allow(dead_code)]
811 /// struct NotThreadSafe<T> {
812 /// value: UnsafeCell<T>,
815 /// unsafe impl<T> Sync for NotThreadSafe<T> {}
817 #[lang = "unsafe_cell"]
818 #[stable(feature = "rust1", since = "1.0.0")]
819 pub struct UnsafeCell<T: ?Sized> {
823 #[stable(feature = "rust1", since = "1.0.0")]
824 impl<T: ?Sized> !Sync for UnsafeCell<T> {}
826 impl<T> UnsafeCell<T> {
827 /// Constructs a new instance of `UnsafeCell` which will wrap the specified
830 /// All access to the inner value through methods is `unsafe`.
835 /// use std::cell::UnsafeCell;
837 /// let uc = UnsafeCell::new(5);
839 #[stable(feature = "rust1", since = "1.0.0")]
841 pub const fn new(value: T) -> UnsafeCell<T> {
842 UnsafeCell { value: value }
845 /// Unwraps the value.
849 /// This function is unsafe because this thread or another thread may currently be
850 /// inspecting the inner value.
855 /// use std::cell::UnsafeCell;
857 /// let uc = UnsafeCell::new(5);
859 /// let five = unsafe { uc.into_inner() };
862 #[stable(feature = "rust1", since = "1.0.0")]
863 pub unsafe fn into_inner(self) -> T {
868 impl<T: ?Sized> UnsafeCell<T> {
869 /// Gets a mutable pointer to the wrapped value.
874 /// use std::cell::UnsafeCell;
876 /// let uc = UnsafeCell::new(5);
878 /// let five = uc.get();
881 #[stable(feature = "rust1", since = "1.0.0")]
882 pub fn get(&self) -> *mut T {
883 &self.value as *const T as *mut T
887 #[stable(feature = "unsafe_cell_default", since = "1.9.0")]
888 impl<T: Default> Default for UnsafeCell<T> {
889 fn default() -> UnsafeCell<T> {
890 UnsafeCell::new(Default::default())