1 // Copyright 2012-2014 The Rust Project Developers. See the COPYRIGHT
2 // file at the top-level directory of this distribution and at
3 // http://rust-lang.org/COPYRIGHT.
5 // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
6 // http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
7 // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
8 // option. This file may not be copied, modified, or distributed
9 // except according to those terms.
11 //! Shareable mutable containers.
13 //! Rust memory safety is based on this rule: Given an object `T`, it is only possible to
14 //! have one of the following:
16 //! - Having several immutable references (`&T`) to the object (also known as **aliasing**).
17 //! - Having one mutable reference (`&mut T`) to the object (also known as **mutability**).
19 //! This is enforced by the Rust compiler. However, there are situations where this rule is not
20 //! flexible enough. Sometimes it is required to have multiple references to an object and yet
23 //! Shareable mutable containers exist to permit mutability in a controlled manner, even in the
24 //! presence of aliasing. Both `Cell<T>` and `RefCell<T>` allows to do this in a single threaded
25 //! way. However, neither `Cell<T>` nor `RefCell<T>` are thread safe (they do not implement
26 //! `Sync`). If you need to do aliasing and mutation between multiple threads it is possible to
27 //! use [`Mutex`](../../std/sync/struct.Mutex.html),
28 //! [`RwLock`](../../std/sync/struct.RwLock.html) or
29 //! [`atomic`](../../core/sync/atomic/index.html) types.
31 //! Values of the `Cell<T>` and `RefCell<T>` types may be mutated through shared references (i.e.
32 //! the common `&T` type), whereas most Rust types can only be mutated through unique (`&mut T`)
33 //! references. We say that `Cell<T>` and `RefCell<T>` provide 'interior mutability', in contrast
34 //! with typical Rust types that exhibit 'inherited mutability'.
36 //! Cell types come in two flavors: `Cell<T>` and `RefCell<T>`. `Cell<T>` implements interior
37 //! mutability by moving values in and out of the `Cell<T>`. To use references instead of values,
38 //! one must use the `RefCell<T>` type, acquiring a write lock before mutating. `Cell<T>` provides
39 //! methods to retrieve and change the current interior value:
41 //! - For types that implement `Copy`, the `get` method retrieves the current interior value.
42 //! - For types that implement `Default`, the `take` method replaces the current interior value
43 //! with `Default::default()` and returns the replaced value.
44 //! - For all types, the `replace` method replaces the current interior value and returns the
45 //! replaced value and the `into_inner` method consumes the `Cell<T>` and returns the interior
46 //! value. Additionally, the `set` method replaces the interior value, dropping the replaced
49 //! `RefCell<T>` uses Rust's lifetimes to implement 'dynamic borrowing', a process whereby one can
50 //! claim temporary, exclusive, mutable access to the inner value. Borrows for `RefCell<T>`s are
51 //! tracked 'at runtime', unlike Rust's native reference types which are entirely tracked
52 //! statically, at compile time. Because `RefCell<T>` borrows are dynamic it is possible to attempt
53 //! to borrow a value that is already mutably borrowed; when this happens it results in thread
56 //! # When to choose interior mutability
58 //! The more common inherited mutability, where one must have unique access to mutate a value, is
59 //! one of the key language elements that enables Rust to reason strongly about pointer aliasing,
60 //! statically preventing crash bugs. Because of that, inherited mutability is preferred, and
61 //! interior mutability is something of a last resort. Since cell types enable mutation where it
62 //! would otherwise be disallowed though, there are occasions when interior mutability might be
63 //! appropriate, or even *must* be used, e.g.
65 //! * Introducing mutability 'inside' of something immutable
66 //! * Implementation details of logically-immutable methods.
67 //! * Mutating implementations of `Clone`.
69 //! ## Introducing mutability 'inside' of something immutable
71 //! Many shared smart pointer types, including `Rc<T>` and `Arc<T>`, provide containers that can be
72 //! cloned and shared between multiple parties. Because the contained values may be
73 //! multiply-aliased, they can only be borrowed with `&`, not `&mut`. Without cells it would be
74 //! impossible to mutate data inside of these smart pointers at all.
76 //! It's very common then to put a `RefCell<T>` inside shared pointer types to reintroduce
80 //! use std::collections::HashMap;
81 //! use std::cell::RefCell;
85 //! let shared_map: Rc<RefCell<_>> = Rc::new(RefCell::new(HashMap::new()));
86 //! shared_map.borrow_mut().insert("africa", 92388);
87 //! shared_map.borrow_mut().insert("kyoto", 11837);
88 //! shared_map.borrow_mut().insert("piccadilly", 11826);
89 //! shared_map.borrow_mut().insert("marbles", 38);
93 //! Note that this example uses `Rc<T>` and not `Arc<T>`. `RefCell<T>`s are for single-threaded
94 //! scenarios. Consider using `RwLock<T>` or `Mutex<T>` if you need shared mutability in a
95 //! multi-threaded situation.
97 //! ## Implementation details of logically-immutable methods
99 //! Occasionally it may be desirable not to expose in an API that there is mutation happening
100 //! "under the hood". This may be because logically the operation is immutable, but e.g. caching
101 //! forces the implementation to perform mutation; or because you must employ mutation to implement
102 //! a trait method that was originally defined to take `&self`.
105 //! # #![allow(dead_code)]
106 //! use std::cell::RefCell;
109 //! edges: Vec<(i32, i32)>,
110 //! span_tree_cache: RefCell<Option<Vec<(i32, i32)>>>
114 //! fn minimum_spanning_tree(&self) -> Vec<(i32, i32)> {
115 //! // Create a new scope to contain the lifetime of the
116 //! // dynamic borrow
118 //! // Take a reference to the inside of cache cell
119 //! let mut cache = self.span_tree_cache.borrow_mut();
120 //! if cache.is_some() {
121 //! return cache.as_ref().unwrap().clone();
124 //! let span_tree = self.calc_span_tree();
125 //! *cache = Some(span_tree);
128 //! // Recursive call to return the just-cached value.
129 //! // Note that if we had not let the previous borrow
130 //! // of the cache fall out of scope then the subsequent
131 //! // recursive borrow would cause a dynamic thread panic.
132 //! // This is the major hazard of using `RefCell`.
133 //! self.minimum_spanning_tree()
135 //! # fn calc_span_tree(&self) -> Vec<(i32, i32)> { vec![] }
139 //! ## Mutating implementations of `Clone`
141 //! This is simply a special - but common - case of the previous: hiding mutability for operations
142 //! that appear to be immutable. The `clone` method is expected to not change the source value, and
143 //! is declared to take `&self`, not `&mut self`. Therefore any mutation that happens in the
144 //! `clone` method must use cell types. For example, `Rc<T>` maintains its reference counts within a
148 //! #![feature(core_intrinsics)]
149 //! use std::cell::Cell;
150 //! use std::ptr::NonNull;
151 //! use std::intrinsics::abort;
153 //! struct Rc<T: ?Sized> {
154 //! ptr: NonNull<RcBox<T>>
157 //! struct RcBox<T: ?Sized> {
158 //! strong: Cell<usize>,
159 //! refcount: Cell<usize>,
163 //! impl<T: ?Sized> Clone for Rc<T> {
164 //! fn clone(&self) -> Rc<T> {
165 //! self.inc_strong();
166 //! Rc { ptr: self.ptr }
170 //! trait RcBoxPtr<T: ?Sized> {
172 //! fn inner(&self) -> &RcBox<T>;
174 //! fn strong(&self) -> usize {
175 //! self.inner().strong.get()
178 //! fn inc_strong(&self) {
181 //! .set(self.strong()
183 //! .unwrap_or_else(|| unsafe { abort() }));
187 //! impl<T: ?Sized> RcBoxPtr<T> for Rc<T> {
188 //! fn inner(&self) -> &RcBox<T> {
190 //! self.ptr.as_ref()
197 #![stable(feature = "rust1", since = "1.0.0")]
200 use fmt
::{self, Debug, Display}
;
203 use ops
::{Deref, DerefMut, CoerceUnsized}
;
206 /// A mutable memory location.
210 /// Here you can see how using `Cell<T>` allows to use mutable field inside
211 /// immutable struct (which is also called 'interior mutability').
214 /// use std::cell::Cell;
216 /// struct SomeStruct {
217 /// regular_field: u8,
218 /// special_field: Cell<u8>,
221 /// let my_struct = SomeStruct {
222 /// regular_field: 0,
223 /// special_field: Cell::new(1),
226 /// let new_value = 100;
228 /// // ERROR, because my_struct is immutable
229 /// // my_struct.regular_field = new_value;
231 /// // WORKS, although `my_struct` is immutable, field `special_field` is mutable because it is Cell
232 /// my_struct.special_field.set(new_value);
233 /// assert_eq!(my_struct.special_field.get(), new_value);
236 /// See the [module-level documentation](index.html) for more.
237 #[stable(feature = "rust1", since = "1.0.0")]
239 value
: UnsafeCell
<T
>,
242 impl<T
:Copy
> Cell
<T
> {
243 /// Returns a copy of the contained value.
248 /// use std::cell::Cell;
250 /// let c = Cell::new(5);
252 /// let five = c.get();
255 #[stable(feature = "rust1", since = "1.0.0")]
256 pub fn get(&self) -> T
{
257 unsafe{ *self.value.get() }
260 /// Updates the contained value using a function and returns the new value.
265 /// #![feature(cell_update)]
267 /// use std::cell::Cell;
269 /// let c = Cell::new(5);
270 /// let new = c.update(|x| x + 1);
272 /// assert_eq!(new, 6);
273 /// assert_eq!(c.get(), 6);
276 #[unstable(feature = "cell_update", issue = "50186")]
277 pub fn update
<F
>(&self, f
: F
) -> T
281 let old
= self.get();
288 #[stable(feature = "rust1", since = "1.0.0")]
289 unsafe impl<T
> Send
for Cell
<T
> where T
: Send {}
291 #[stable(feature = "rust1", since = "1.0.0")]
292 impl<T
> !Sync
for Cell
<T
> {}
294 #[stable(feature = "rust1", since = "1.0.0")]
295 impl<T
:Copy
> Clone
for Cell
<T
> {
297 fn clone(&self) -> Cell
<T
> {
298 Cell
::new(self.get())
302 #[stable(feature = "rust1", since = "1.0.0")]
303 impl<T
:Default
> Default
for Cell
<T
> {
304 /// Creates a `Cell<T>`, with the `Default` value for T.
306 fn default() -> Cell
<T
> {
307 Cell
::new(Default
::default())
311 #[stable(feature = "rust1", since = "1.0.0")]
312 impl<T
:PartialEq
+ Copy
> PartialEq
for Cell
<T
> {
314 fn eq(&self, other
: &Cell
<T
>) -> bool
{
315 self.get() == other
.get()
319 #[stable(feature = "cell_eq", since = "1.2.0")]
320 impl<T
:Eq
+ Copy
> Eq
for Cell
<T
> {}
322 #[stable(feature = "cell_ord", since = "1.10.0")]
323 impl<T
:PartialOrd
+ Copy
> PartialOrd
for Cell
<T
> {
325 fn partial_cmp(&self, other
: &Cell
<T
>) -> Option
<Ordering
> {
326 self.get().partial_cmp(&other
.get())
330 fn lt(&self, other
: &Cell
<T
>) -> bool
{
331 self.get() < other
.get()
335 fn le(&self, other
: &Cell
<T
>) -> bool
{
336 self.get() <= other
.get()
340 fn gt(&self, other
: &Cell
<T
>) -> bool
{
341 self.get() > other
.get()
345 fn ge(&self, other
: &Cell
<T
>) -> bool
{
346 self.get() >= other
.get()
350 #[stable(feature = "cell_ord", since = "1.10.0")]
351 impl<T
:Ord
+ Copy
> Ord
for Cell
<T
> {
353 fn cmp(&self, other
: &Cell
<T
>) -> Ordering
{
354 self.get().cmp(&other
.get())
358 #[stable(feature = "cell_from", since = "1.12.0")]
359 impl<T
> From
<T
> for Cell
<T
> {
360 fn from(t
: T
) -> Cell
<T
> {
366 /// Creates a new `Cell` containing the given value.
371 /// use std::cell::Cell;
373 /// let c = Cell::new(5);
375 #[stable(feature = "rust1", since = "1.0.0")]
377 pub const fn new(value
: T
) -> Cell
<T
> {
379 value
: UnsafeCell
::new(value
),
383 /// Returns a raw pointer to the underlying data in this cell.
388 /// use std::cell::Cell;
390 /// let c = Cell::new(5);
392 /// let ptr = c.as_ptr();
395 #[stable(feature = "cell_as_ptr", since = "1.12.0")]
396 pub fn as_ptr(&self) -> *mut T
{
400 /// Returns a mutable reference to the underlying data.
402 /// This call borrows `Cell` mutably (at compile-time) which guarantees
403 /// that we possess the only reference.
408 /// use std::cell::Cell;
410 /// let mut c = Cell::new(5);
411 /// *c.get_mut() += 1;
413 /// assert_eq!(c.get(), 6);
416 #[stable(feature = "cell_get_mut", since = "1.11.0")]
417 pub fn get_mut(&mut self) -> &mut T
{
419 &mut *self.value
.get()
423 /// Sets the contained value.
428 /// use std::cell::Cell;
430 /// let c = Cell::new(5);
435 #[stable(feature = "rust1", since = "1.0.0")]
436 pub fn set(&self, val
: T
) {
437 let old
= self.replace(val
);
441 /// Swaps the values of two Cells.
442 /// Difference with `std::mem::swap` is that this function doesn't require `&mut` reference.
447 /// use std::cell::Cell;
449 /// let c1 = Cell::new(5i32);
450 /// let c2 = Cell::new(10i32);
452 /// assert_eq!(10, c1.get());
453 /// assert_eq!(5, c2.get());
456 #[stable(feature = "move_cell", since = "1.17.0")]
457 pub fn swap(&self, other
: &Self) {
458 if ptr
::eq(self, other
) {
462 ptr
::swap(self.value
.get(), other
.value
.get());
466 /// Replaces the contained value, and returns it.
471 /// use std::cell::Cell;
473 /// let cell = Cell::new(5);
474 /// assert_eq!(cell.get(), 5);
475 /// assert_eq!(cell.replace(10), 5);
476 /// assert_eq!(cell.get(), 10);
478 #[stable(feature = "move_cell", since = "1.17.0")]
479 pub fn replace(&self, val
: T
) -> T
{
480 mem
::replace(unsafe { &mut *self.value.get() }
, val
)
483 /// Unwraps the value.
488 /// use std::cell::Cell;
490 /// let c = Cell::new(5);
491 /// let five = c.into_inner();
493 /// assert_eq!(five, 5);
495 #[stable(feature = "move_cell", since = "1.17.0")]
496 pub fn into_inner(self) -> T
{
497 self.value
.into_inner()
501 impl<T
: Default
> Cell
<T
> {
502 /// Takes the value of the cell, leaving `Default::default()` in its place.
507 /// use std::cell::Cell;
509 /// let c = Cell::new(5);
510 /// let five = c.take();
512 /// assert_eq!(five, 5);
513 /// assert_eq!(c.into_inner(), 0);
515 #[stable(feature = "move_cell", since = "1.17.0")]
516 pub fn take(&self) -> T
{
517 self.replace(Default
::default())
521 #[unstable(feature = "coerce_unsized", issue = "27732")]
522 impl<T
: CoerceUnsized
<U
>, U
> CoerceUnsized
<Cell
<U
>> for Cell
<T
> {}
524 /// A mutable memory location with dynamically checked borrow rules
526 /// See the [module-level documentation](index.html) for more.
527 #[stable(feature = "rust1", since = "1.0.0")]
528 pub struct RefCell
<T
: ?Sized
> {
529 borrow
: Cell
<BorrowFlag
>,
530 value
: UnsafeCell
<T
>,
533 /// An error returned by [`RefCell::try_borrow`](struct.RefCell.html#method.try_borrow).
534 #[stable(feature = "try_borrow", since = "1.13.0")]
535 pub struct BorrowError
{
539 #[stable(feature = "try_borrow", since = "1.13.0")]
540 impl Debug
for BorrowError
{
541 fn fmt(&self, f
: &mut fmt
::Formatter
) -> fmt
::Result
{
542 f
.debug_struct("BorrowError").finish()
546 #[stable(feature = "try_borrow", since = "1.13.0")]
547 impl Display
for BorrowError
{
548 fn fmt(&self, f
: &mut fmt
::Formatter
) -> fmt
::Result
{
549 Display
::fmt("already mutably borrowed", f
)
553 /// An error returned by [`RefCell::try_borrow_mut`](struct.RefCell.html#method.try_borrow_mut).
554 #[stable(feature = "try_borrow", since = "1.13.0")]
555 pub struct BorrowMutError
{
559 #[stable(feature = "try_borrow", since = "1.13.0")]
560 impl Debug
for BorrowMutError
{
561 fn fmt(&self, f
: &mut fmt
::Formatter
) -> fmt
::Result
{
562 f
.debug_struct("BorrowMutError").finish()
566 #[stable(feature = "try_borrow", since = "1.13.0")]
567 impl Display
for BorrowMutError
{
568 fn fmt(&self, f
: &mut fmt
::Formatter
) -> fmt
::Result
{
569 Display
::fmt("already borrowed", f
)
573 // Values [1, MIN_WRITING-1] represent the number of `Ref` active. Values in
574 // [MIN_WRITING, MAX-1] represent the number of `RefMut` active. Multiple
575 // `RefMut`s can only be active at a time if they refer to distinct,
576 // nonoverlapping components of a `RefCell` (e.g., different ranges of a slice).
578 // `Ref` and `RefMut` are both two words in size, and so there will likely never
579 // be enough `Ref`s or `RefMut`s in existence to overflow half of the `usize`
580 // range. Thus, a `BorrowFlag` will probably never overflow. However, this is
581 // not a guarantee, as a pathological program could repeatedly create and then
582 // mem::forget `Ref`s or `RefMut`s. Thus, all code must explicitly check for
583 // overflow in order to avoid unsafety.
584 type BorrowFlag
= usize;
585 const UNUSED
: BorrowFlag
= 0;
586 const MIN_WRITING
: BorrowFlag
= (!0)/2 + 1; // 0b1000...
589 /// Creates a new `RefCell` containing `value`.
594 /// use std::cell::RefCell;
596 /// let c = RefCell::new(5);
598 #[stable(feature = "rust1", since = "1.0.0")]
600 pub const fn new(value
: T
) -> RefCell
<T
> {
602 value
: UnsafeCell
::new(value
),
603 borrow
: Cell
::new(UNUSED
),
607 /// Consumes the `RefCell`, returning the wrapped value.
612 /// use std::cell::RefCell;
614 /// let c = RefCell::new(5);
616 /// let five = c.into_inner();
618 #[stable(feature = "rust1", since = "1.0.0")]
620 pub fn into_inner(self) -> T
{
621 // Since this function takes `self` (the `RefCell`) by value, the
622 // compiler statically verifies that it is not currently borrowed.
623 // Therefore the following assertion is just a `debug_assert!`.
624 debug_assert
!(self.borrow
.get() == UNUSED
);
625 self.value
.into_inner()
628 /// Replaces the wrapped value with a new one, returning the old value,
629 /// without deinitializing either one.
631 /// This function corresponds to [`std::mem::replace`](../mem/fn.replace.html).
635 /// Panics if the value is currently borrowed.
640 /// use std::cell::RefCell;
641 /// let cell = RefCell::new(5);
642 /// let old_value = cell.replace(6);
643 /// assert_eq!(old_value, 5);
644 /// assert_eq!(cell, RefCell::new(6));
647 #[stable(feature = "refcell_replace", since="1.24.0")]
648 pub fn replace(&self, t
: T
) -> T
{
649 mem
::replace(&mut *self.borrow_mut(), t
)
652 /// Replaces the wrapped value with a new one computed from `f`, returning
653 /// the old value, without deinitializing either one.
655 /// This function corresponds to [`std::mem::replace`](../mem/fn.replace.html).
659 /// Panics if the value is currently borrowed.
664 /// #![feature(refcell_replace_swap)]
665 /// use std::cell::RefCell;
666 /// let cell = RefCell::new(5);
667 /// let old_value = cell.replace_with(|&mut old| old + 1);
668 /// assert_eq!(old_value, 5);
669 /// assert_eq!(cell, RefCell::new(6));
672 #[unstable(feature = "refcell_replace_swap", issue="43570")]
673 pub fn replace_with
<F
: FnOnce(&mut T
) -> T
>(&self, f
: F
) -> T
{
674 let mut_borrow
= &mut *self.borrow_mut();
675 let replacement
= f(mut_borrow
);
676 mem
::replace(mut_borrow
, replacement
)
679 /// Swaps the wrapped value of `self` with the wrapped value of `other`,
680 /// without deinitializing either one.
682 /// This function corresponds to [`std::mem::swap`](../mem/fn.swap.html).
686 /// Panics if the value in either `RefCell` is currently borrowed.
691 /// use std::cell::RefCell;
692 /// let c = RefCell::new(5);
693 /// let d = RefCell::new(6);
695 /// assert_eq!(c, RefCell::new(6));
696 /// assert_eq!(d, RefCell::new(5));
699 #[stable(feature = "refcell_swap", since="1.24.0")]
700 pub fn swap(&self, other
: &Self) {
701 mem
::swap(&mut *self.borrow_mut(), &mut *other
.borrow_mut())
705 impl<T
: ?Sized
> RefCell
<T
> {
706 /// Immutably borrows the wrapped value.
708 /// The borrow lasts until the returned `Ref` exits scope. Multiple
709 /// immutable borrows can be taken out at the same time.
713 /// Panics if the value is currently mutably borrowed. For a non-panicking variant, use
714 /// [`try_borrow`](#method.try_borrow).
719 /// use std::cell::RefCell;
721 /// let c = RefCell::new(5);
723 /// let borrowed_five = c.borrow();
724 /// let borrowed_five2 = c.borrow();
727 /// An example of panic:
730 /// use std::cell::RefCell;
733 /// let result = thread::spawn(move || {
734 /// let c = RefCell::new(5);
735 /// let m = c.borrow_mut();
737 /// let b = c.borrow(); // this causes a panic
740 /// assert!(result.is_err());
742 #[stable(feature = "rust1", since = "1.0.0")]
744 pub fn borrow(&self) -> Ref
<T
> {
745 self.try_borrow().expect("already mutably borrowed")
748 /// Immutably borrows the wrapped value, returning an error if the value is currently mutably
751 /// The borrow lasts until the returned `Ref` exits scope. Multiple immutable borrows can be
752 /// taken out at the same time.
754 /// This is the non-panicking variant of [`borrow`](#method.borrow).
759 /// use std::cell::RefCell;
761 /// let c = RefCell::new(5);
764 /// let m = c.borrow_mut();
765 /// assert!(c.try_borrow().is_err());
769 /// let m = c.borrow();
770 /// assert!(c.try_borrow().is_ok());
773 #[stable(feature = "try_borrow", since = "1.13.0")]
775 pub fn try_borrow(&self) -> Result
<Ref
<T
>, BorrowError
> {
776 match BorrowRef
::new(&self.borrow
) {
778 value
: unsafe { &*self.value.get() }
,
781 None
=> Err(BorrowError { _private: () }
),
785 /// Mutably borrows the wrapped value.
787 /// The borrow lasts until the returned `RefMut` or all `RefMut`s derived
788 /// from it exit scope. The value cannot be borrowed while this borrow is
793 /// Panics if the value is currently borrowed. For a non-panicking variant, use
794 /// [`try_borrow_mut`](#method.try_borrow_mut).
799 /// use std::cell::RefCell;
801 /// let c = RefCell::new(5);
803 /// *c.borrow_mut() = 7;
805 /// assert_eq!(*c.borrow(), 7);
808 /// An example of panic:
811 /// use std::cell::RefCell;
814 /// let result = thread::spawn(move || {
815 /// let c = RefCell::new(5);
816 /// let m = c.borrow();
818 /// let b = c.borrow_mut(); // this causes a panic
821 /// assert!(result.is_err());
823 #[stable(feature = "rust1", since = "1.0.0")]
825 pub fn borrow_mut(&self) -> RefMut
<T
> {
826 self.try_borrow_mut().expect("already borrowed")
829 /// Mutably borrows the wrapped value, returning an error if the value is currently borrowed.
831 /// The borrow lasts until the returned `RefMut` or all `RefMut`s derived
832 /// from it exit scope. The value cannot be borrowed while this borrow is
835 /// This is the non-panicking variant of [`borrow_mut`](#method.borrow_mut).
840 /// use std::cell::RefCell;
842 /// let c = RefCell::new(5);
845 /// let m = c.borrow();
846 /// assert!(c.try_borrow_mut().is_err());
849 /// assert!(c.try_borrow_mut().is_ok());
851 #[stable(feature = "try_borrow", since = "1.13.0")]
853 pub fn try_borrow_mut(&self) -> Result
<RefMut
<T
>, BorrowMutError
> {
854 match BorrowRefMut
::new(&self.borrow
) {
855 Some(b
) => Ok(RefMut
{
856 value
: unsafe { &mut *self.value.get() }
,
859 None
=> Err(BorrowMutError { _private: () }
),
863 /// Returns a raw pointer to the underlying data in this cell.
868 /// use std::cell::RefCell;
870 /// let c = RefCell::new(5);
872 /// let ptr = c.as_ptr();
875 #[stable(feature = "cell_as_ptr", since = "1.12.0")]
876 pub fn as_ptr(&self) -> *mut T
{
880 /// Returns a mutable reference to the underlying data.
882 /// This call borrows `RefCell` mutably (at compile-time) so there is no
883 /// need for dynamic checks.
885 /// However be cautious: this method expects `self` to be mutable, which is
886 /// generally not the case when using a `RefCell`. Take a look at the
887 /// [`borrow_mut`] method instead if `self` isn't mutable.
889 /// Also, please be aware that this method is only for special circumstances and is usually
890 /// not what you want. In case of doubt, use [`borrow_mut`] instead.
892 /// [`borrow_mut`]: #method.borrow_mut
897 /// use std::cell::RefCell;
899 /// let mut c = RefCell::new(5);
900 /// *c.get_mut() += 1;
902 /// assert_eq!(c, RefCell::new(6));
905 #[stable(feature = "cell_get_mut", since = "1.11.0")]
906 pub fn get_mut(&mut self) -> &mut T
{
908 &mut *self.value
.get()
913 #[stable(feature = "rust1", since = "1.0.0")]
914 unsafe impl<T
: ?Sized
> Send
for RefCell
<T
> where T
: Send {}
916 #[stable(feature = "rust1", since = "1.0.0")]
917 impl<T
: ?Sized
> !Sync
for RefCell
<T
> {}
919 #[stable(feature = "rust1", since = "1.0.0")]
920 impl<T
: Clone
> Clone
for RefCell
<T
> {
923 /// Panics if the value is currently mutably borrowed.
925 fn clone(&self) -> RefCell
<T
> {
926 RefCell
::new(self.borrow().clone())
930 #[stable(feature = "rust1", since = "1.0.0")]
931 impl<T
:Default
> Default
for RefCell
<T
> {
932 /// Creates a `RefCell<T>`, with the `Default` value for T.
934 fn default() -> RefCell
<T
> {
935 RefCell
::new(Default
::default())
939 #[stable(feature = "rust1", since = "1.0.0")]
940 impl<T
: ?Sized
+ PartialEq
> PartialEq
for RefCell
<T
> {
943 /// Panics if the value in either `RefCell` is currently borrowed.
945 fn eq(&self, other
: &RefCell
<T
>) -> bool
{
946 *self.borrow() == *other
.borrow()
950 #[stable(feature = "cell_eq", since = "1.2.0")]
951 impl<T
: ?Sized
+ Eq
> Eq
for RefCell
<T
> {}
953 #[stable(feature = "cell_ord", since = "1.10.0")]
954 impl<T
: ?Sized
+ PartialOrd
> PartialOrd
for RefCell
<T
> {
957 /// Panics if the value in either `RefCell` is currently borrowed.
959 fn partial_cmp(&self, other
: &RefCell
<T
>) -> Option
<Ordering
> {
960 self.borrow().partial_cmp(&*other
.borrow())
965 /// Panics if the value in either `RefCell` is currently borrowed.
967 fn lt(&self, other
: &RefCell
<T
>) -> bool
{
968 *self.borrow() < *other
.borrow()
973 /// Panics if the value in either `RefCell` is currently borrowed.
975 fn le(&self, other
: &RefCell
<T
>) -> bool
{
976 *self.borrow() <= *other
.borrow()
981 /// Panics if the value in either `RefCell` is currently borrowed.
983 fn gt(&self, other
: &RefCell
<T
>) -> bool
{
984 *self.borrow() > *other
.borrow()
989 /// Panics if the value in either `RefCell` is currently borrowed.
991 fn ge(&self, other
: &RefCell
<T
>) -> bool
{
992 *self.borrow() >= *other
.borrow()
996 #[stable(feature = "cell_ord", since = "1.10.0")]
997 impl<T
: ?Sized
+ Ord
> Ord
for RefCell
<T
> {
1000 /// Panics if the value in either `RefCell` is currently borrowed.
1002 fn cmp(&self, other
: &RefCell
<T
>) -> Ordering
{
1003 self.borrow().cmp(&*other
.borrow())
1007 #[stable(feature = "cell_from", since = "1.12.0")]
1008 impl<T
> From
<T
> for RefCell
<T
> {
1009 fn from(t
: T
) -> RefCell
<T
> {
1014 #[unstable(feature = "coerce_unsized", issue = "27732")]
1015 impl<T
: CoerceUnsized
<U
>, U
> CoerceUnsized
<RefCell
<U
>> for RefCell
<T
> {}
1017 struct BorrowRef
<'b
> {
1018 borrow
: &'b Cell
<BorrowFlag
>,
1021 impl<'b
> BorrowRef
<'b
> {
1023 fn new(borrow
: &'b Cell
<BorrowFlag
>) -> Option
<BorrowRef
<'b
>> {
1024 let b
= borrow
.get();
1025 if b
>= MIN_WRITING
{
1028 // Prevent the borrow counter from overflowing into
1029 // a writing borrow.
1030 assert
!(b
< MIN_WRITING
- 1);
1032 Some(BorrowRef { borrow }
)
1037 impl<'b
> Drop
for BorrowRef
<'b
> {
1039 fn drop(&mut self) {
1040 let borrow
= self.borrow
.get();
1041 debug_assert
!(borrow
< MIN_WRITING
&& borrow
!= UNUSED
);
1042 self.borrow
.set(borrow
- 1);
1046 impl<'b
> Clone
for BorrowRef
<'b
> {
1048 fn clone(&self) -> BorrowRef
<'b
> {
1049 // Since this Ref exists, we know the borrow flag
1050 // is not set to WRITING.
1051 let borrow
= self.borrow
.get();
1052 debug_assert
!(borrow
!= UNUSED
);
1053 // Prevent the borrow counter from overflowing into
1054 // a writing borrow.
1055 assert
!(borrow
< MIN_WRITING
- 1);
1056 self.borrow
.set(borrow
+ 1);
1057 BorrowRef { borrow: self.borrow }
1061 /// Wraps a borrowed reference to a value in a `RefCell` box.
1062 /// A wrapper type for an immutably borrowed value from a `RefCell<T>`.
1064 /// See the [module-level documentation](index.html) for more.
1065 #[stable(feature = "rust1", since = "1.0.0")]
1066 pub struct Ref
<'b
, T
: ?Sized
+ 'b
> {
1068 borrow
: BorrowRef
<'b
>,
1071 #[stable(feature = "rust1", since = "1.0.0")]
1072 impl<'b
, T
: ?Sized
> Deref
for Ref
<'b
, T
> {
1076 fn deref(&self) -> &T
{
1081 impl<'b
, T
: ?Sized
> Ref
<'b
, T
> {
1084 /// The `RefCell` is already immutably borrowed, so this cannot fail.
1086 /// This is an associated function that needs to be used as
1087 /// `Ref::clone(...)`. A `Clone` implementation or a method would interfere
1088 /// with the widespread use of `r.borrow().clone()` to clone the contents of
1090 #[stable(feature = "cell_extras", since = "1.15.0")]
1092 pub fn clone(orig
: &Ref
<'b
, T
>) -> Ref
<'b
, T
> {
1095 borrow
: orig
.borrow
.clone(),
1099 /// Make a new `Ref` for a component of the borrowed data.
1101 /// The `RefCell` is already immutably borrowed, so this cannot fail.
1103 /// This is an associated function that needs to be used as `Ref::map(...)`.
1104 /// A method would interfere with methods of the same name on the contents
1105 /// of a `RefCell` used through `Deref`.
1110 /// use std::cell::{RefCell, Ref};
1112 /// let c = RefCell::new((5, 'b'));
1113 /// let b1: Ref<(u32, char)> = c.borrow();
1114 /// let b2: Ref<u32> = Ref::map(b1, |t| &t.0);
1115 /// assert_eq!(*b2, 5)
1117 #[stable(feature = "cell_map", since = "1.8.0")]
1119 pub fn map
<U
: ?Sized
, F
>(orig
: Ref
<'b
, T
>, f
: F
) -> Ref
<'b
, U
>
1120 where F
: FnOnce(&T
) -> &U
1123 value
: f(orig
.value
),
1124 borrow
: orig
.borrow
,
1128 /// Split a `Ref` into multiple `Ref`s for different components of the
1131 /// The `RefCell` is already immutably borrowed, so this cannot fail.
1133 /// This is an associated function that needs to be used as
1134 /// `Ref::map_split(...)`. A method would interfere with methods of the same
1135 /// name on the contents of a `RefCell` used through `Deref`.
1140 /// #![feature(refcell_map_split)]
1141 /// use std::cell::{Ref, RefCell};
1143 /// let cell = RefCell::new([1, 2, 3, 4]);
1144 /// let borrow = cell.borrow();
1145 /// let (begin, end) = Ref::map_split(borrow, |slice| slice.split_at(2));
1146 /// assert_eq!(*begin, [1, 2]);
1147 /// assert_eq!(*end, [3, 4]);
1149 #[unstable(feature = "refcell_map_split", issue = "51476")]
1151 pub fn map_split
<U
: ?Sized
, V
: ?Sized
, F
>(orig
: Ref
<'b
, T
>, f
: F
) -> (Ref
<'b
, U
>, Ref
<'b
, V
>)
1152 where F
: FnOnce(&T
) -> (&U
, &V
)
1154 let (a
, b
) = f(orig
.value
);
1155 let borrow
= orig
.borrow
.clone();
1156 (Ref { value: a, borrow }
, Ref { value: b, borrow: orig.borrow }
)
1160 #[unstable(feature = "coerce_unsized", issue = "27732")]
1161 impl<'b
, T
: ?Sized
+ Unsize
<U
>, U
: ?Sized
> CoerceUnsized
<Ref
<'b
, U
>> for Ref
<'b
, T
> {}
1163 #[stable(feature = "std_guard_impls", since = "1.20.0")]
1164 impl<'a
, T
: ?Sized
+ fmt
::Display
> fmt
::Display
for Ref
<'a
, T
> {
1165 fn fmt(&self, f
: &mut fmt
::Formatter
) -> fmt
::Result
{
1170 impl<'b
, T
: ?Sized
> RefMut
<'b
, T
> {
1171 /// Make a new `RefMut` for a component of the borrowed data, e.g. an enum
1174 /// The `RefCell` is already mutably borrowed, so this cannot fail.
1176 /// This is an associated function that needs to be used as
1177 /// `RefMut::map(...)`. A method would interfere with methods of the same
1178 /// name on the contents of a `RefCell` used through `Deref`.
1183 /// use std::cell::{RefCell, RefMut};
1185 /// let c = RefCell::new((5, 'b'));
1187 /// let b1: RefMut<(u32, char)> = c.borrow_mut();
1188 /// let mut b2: RefMut<u32> = RefMut::map(b1, |t| &mut t.0);
1189 /// assert_eq!(*b2, 5);
1192 /// assert_eq!(*c.borrow(), (42, 'b'));
1194 #[stable(feature = "cell_map", since = "1.8.0")]
1196 pub fn map
<U
: ?Sized
, F
>(orig
: RefMut
<'b
, T
>, f
: F
) -> RefMut
<'b
, U
>
1197 where F
: FnOnce(&mut T
) -> &mut U
1199 // FIXME(nll-rfc#40): fix borrow-check
1200 let RefMut { value, borrow }
= orig
;
1207 /// Split a `RefMut` into multiple `RefMut`s for different components of the
1210 /// The underlying `RefCell` will remain mutably borrowed until both
1211 /// returned `RefMut`s go out of scope.
1213 /// The `RefCell` is already mutably borrowed, so this cannot fail.
1215 /// This is an associated function that needs to be used as
1216 /// `RefMut::map_split(...)`. A method would interfere with methods of the
1217 /// same name on the contents of a `RefCell` used through `Deref`.
1222 /// #![feature(refcell_map_split)]
1223 /// use std::cell::{RefCell, RefMut};
1225 /// let cell = RefCell::new([1, 2, 3, 4]);
1226 /// let borrow = cell.borrow_mut();
1227 /// let (mut begin, mut end) = RefMut::map_split(borrow, |slice| slice.split_at_mut(2));
1228 /// assert_eq!(*begin, [1, 2]);
1229 /// assert_eq!(*end, [3, 4]);
1230 /// begin.copy_from_slice(&[4, 3]);
1231 /// end.copy_from_slice(&[2, 1]);
1233 #[unstable(feature = "refcell_map_split", issue = "51476")]
1235 pub fn map_split
<U
: ?Sized
, V
: ?Sized
, F
>(
1236 orig
: RefMut
<'b
, T
>, f
: F
1237 ) -> (RefMut
<'b
, U
>, RefMut
<'b
, V
>)
1238 where F
: FnOnce(&mut T
) -> (&mut U
, &mut V
)
1240 let (a
, b
) = f(orig
.value
);
1241 let borrow
= orig
.borrow
.clone();
1242 (RefMut { value: a, borrow }
, RefMut { value: b, borrow: orig.borrow }
)
1246 struct BorrowRefMut
<'b
> {
1247 borrow
: &'b Cell
<BorrowFlag
>,
1250 impl<'b
> Drop
for BorrowRefMut
<'b
> {
1252 fn drop(&mut self) {
1253 let borrow
= self.borrow
.get();
1254 debug_assert
!(borrow
>= MIN_WRITING
);
1255 self.borrow
.set(if borrow
== MIN_WRITING
{
1263 impl<'b
> BorrowRefMut
<'b
> {
1265 fn new(borrow
: &'b Cell
<BorrowFlag
>) -> Option
<BorrowRefMut
<'b
>> {
1266 // NOTE: Unlike BorrowRefMut::clone, new is called to create the initial
1267 // mutable reference, and so there must currently be no existing
1268 // references. Thus, while clone increments the mutable refcount, here
1269 // we simply go directly from UNUSED to MIN_WRITING.
1270 match borrow
.get() {
1272 borrow
.set(MIN_WRITING
);
1273 Some(BorrowRefMut { borrow: borrow }
)
1279 // Clone a `BorrowRefMut`.
1281 // This is only valid if each `BorrowRefMut` is used to track a mutable
1282 // reference to a distinct, nonoverlapping range of the original object.
1283 // This isn't in a Clone impl so that code doesn't call this implicitly.
1285 fn clone(&self) -> BorrowRefMut
<'b
> {
1286 let borrow
= self.borrow
.get();
1287 debug_assert
!(borrow
>= MIN_WRITING
);
1288 // Prevent the borrow counter from overflowing.
1289 assert
!(borrow
!= !0);
1290 self.borrow
.set(borrow
+ 1);
1291 BorrowRefMut { borrow: self.borrow }
1295 /// A wrapper type for a mutably borrowed value from a `RefCell<T>`.
1297 /// See the [module-level documentation](index.html) for more.
1298 #[stable(feature = "rust1", since = "1.0.0")]
1299 pub struct RefMut
<'b
, T
: ?Sized
+ 'b
> {
1301 borrow
: BorrowRefMut
<'b
>,
1304 #[stable(feature = "rust1", since = "1.0.0")]
1305 impl<'b
, T
: ?Sized
> Deref
for RefMut
<'b
, T
> {
1309 fn deref(&self) -> &T
{
1314 #[stable(feature = "rust1", since = "1.0.0")]
1315 impl<'b
, T
: ?Sized
> DerefMut
for RefMut
<'b
, T
> {
1317 fn deref_mut(&mut self) -> &mut T
{
1322 #[unstable(feature = "coerce_unsized", issue = "27732")]
1323 impl<'b
, T
: ?Sized
+ Unsize
<U
>, U
: ?Sized
> CoerceUnsized
<RefMut
<'b
, U
>> for RefMut
<'b
, T
> {}
1325 #[stable(feature = "std_guard_impls", since = "1.20.0")]
1326 impl<'a
, T
: ?Sized
+ fmt
::Display
> fmt
::Display
for RefMut
<'a
, T
> {
1327 fn fmt(&self, f
: &mut fmt
::Formatter
) -> fmt
::Result
{
1332 /// The core primitive for interior mutability in Rust.
1334 /// `UnsafeCell<T>` is a type that wraps some `T` and indicates unsafe interior operations on the
1335 /// wrapped type. Types with an `UnsafeCell<T>` field are considered to have an 'unsafe interior'.
1336 /// The `UnsafeCell<T>` type is the only legal way to obtain aliasable data that is considered
1337 /// mutable. In general, transmuting an `&T` type into an `&mut T` is considered undefined behavior.
1339 /// If you have a reference `&SomeStruct`, then normally in Rust all fields of `SomeStruct` are
1340 /// immutable. The compiler makes optimizations based on the knowledge that `&T` is not mutably
1341 /// aliased or mutated, and that `&mut T` is unique. `UnsafeCell<T>` is the only core language
1342 /// feature to work around this restriction. All other types that allow internal mutability, such as
1343 /// `Cell<T>` and `RefCell<T>`, use `UnsafeCell` to wrap their internal data.
1345 /// The `UnsafeCell` API itself is technically very simple: it gives you a raw pointer `*mut T` to
1346 /// its contents. It is up to _you_ as the abstraction designer to use that raw pointer correctly.
1348 /// The precise Rust aliasing rules are somewhat in flux, but the main points are not contentious:
1350 /// - If you create a safe reference with lifetime `'a` (either a `&T` or `&mut T`
1351 /// reference) that is accessible by safe code (for example, because you returned it),
1352 /// then you must not access the data in any way that contradicts that reference for the
1353 /// remainder of `'a`. For example, this means that if you take the `*mut T` from an
1354 /// `UnsafeCell<T>` and cast it to an `&T`, then the data in `T` must remain immutable
1355 /// (modulo any `UnsafeCell` data found within `T`, of course) until that reference's
1356 /// lifetime expires. Similarly, if you create a `&mut T` reference that is released to
1357 /// safe code, then you must not access the data within the `UnsafeCell` until that
1358 /// reference expires.
1360 /// - At all times, you must avoid data races. If multiple threads have access to
1361 /// the same `UnsafeCell`, then any writes must have a proper happens-before relation to all other
1362 /// accesses (or use atomics).
1364 /// To assist with proper design, the following scenarios are explicitly declared legal
1365 /// for single-threaded code:
1367 /// 1. A `&T` reference can be released to safe code and there it can co-exist with other `&T`
1368 /// references, but not with a `&mut T`
1370 /// 2. A `&mut T` reference may be released to safe code provided neither other `&mut T` nor `&T`
1371 /// co-exist with it. A `&mut T` must always be unique.
1373 /// Note that while mutating or mutably aliasing the contents of an `&UnsafeCell<T>` is
1374 /// okay (provided you enforce the invariants some other way), it is still undefined behavior
1375 /// to have multiple `&mut UnsafeCell<T>` aliases.
1380 /// use std::cell::UnsafeCell;
1381 /// use std::marker::Sync;
1383 /// # #[allow(dead_code)]
1384 /// struct NotThreadSafe<T> {
1385 /// value: UnsafeCell<T>,
1388 /// unsafe impl<T> Sync for NotThreadSafe<T> {}
1390 #[lang = "unsafe_cell"]
1391 #[stable(feature = "rust1", since = "1.0.0")]
1392 pub struct UnsafeCell
<T
: ?Sized
> {
1396 #[stable(feature = "rust1", since = "1.0.0")]
1397 impl<T
: ?Sized
> !Sync
for UnsafeCell
<T
> {}
1399 impl<T
> UnsafeCell
<T
> {
1400 /// Constructs a new instance of `UnsafeCell` which will wrap the specified
1403 /// All access to the inner value through methods is `unsafe`.
1408 /// use std::cell::UnsafeCell;
1410 /// let uc = UnsafeCell::new(5);
1412 #[stable(feature = "rust1", since = "1.0.0")]
1414 pub const fn new(value
: T
) -> UnsafeCell
<T
> {
1415 UnsafeCell { value: value }
1418 /// Unwraps the value.
1423 /// use std::cell::UnsafeCell;
1425 /// let uc = UnsafeCell::new(5);
1427 /// let five = uc.into_inner();
1430 #[stable(feature = "rust1", since = "1.0.0")]
1431 pub fn into_inner(self) -> T
{
1436 impl<T
: ?Sized
> UnsafeCell
<T
> {
1437 /// Gets a mutable pointer to the wrapped value.
1439 /// This can be cast to a pointer of any kind.
1440 /// Ensure that the access is unique (no active references, mutable or not)
1441 /// when casting to `&mut T`, and ensure that there are no mutations
1442 /// or mutable aliases going on when casting to `&T`
1447 /// use std::cell::UnsafeCell;
1449 /// let uc = UnsafeCell::new(5);
1451 /// let five = uc.get();
1454 #[stable(feature = "rust1", since = "1.0.0")]
1455 pub fn get(&self) -> *mut T
{
1456 &self.value
as *const T
as *mut T
1460 #[stable(feature = "unsafe_cell_default", since = "1.10.0")]
1461 impl<T
: Default
> Default
for UnsafeCell
<T
> {
1462 /// Creates an `UnsafeCell`, with the `Default` value for T.
1463 fn default() -> UnsafeCell
<T
> {
1464 UnsafeCell
::new(Default
::default())
1468 #[stable(feature = "cell_from", since = "1.12.0")]
1469 impl<T
> From
<T
> for UnsafeCell
<T
> {
1470 fn from(t
: T
) -> UnsafeCell
<T
> {
1475 #[unstable(feature = "coerce_unsized", issue = "27732")]
1476 impl<T
: CoerceUnsized
<U
>, U
> CoerceUnsized
<UnsafeCell
<U
>> for UnsafeCell
<T
> {}
1479 fn assert_coerce_unsized(a
: UnsafeCell
<&i32>, b
: Cell
<&i32>, c
: RefCell
<&i32>) {
1480 let _
: UnsafeCell
<&Send
> = a
;
1481 let _
: Cell
<&Send
> = b
;
1482 let _
: RefCell
<&Send
> = c
;