1 // Copyright 2012-2013 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` and `RefCell` types may be mutated through
14 //! shared references (i.e. the common `&T` type), whereas most Rust
15 //! types can only be mutated through unique (`&mut T`) references. We
16 //! say that `Cell` and `RefCell` provide *interior mutability*, in
17 //! contrast with typical Rust types that exhibit *inherited
20 //! Cell types come in two flavors: `Cell` and `RefCell`. `Cell`
21 //! provides `get` and `set` methods that change the
22 //! interior value with a single method call. `Cell` though is only
23 //! compatible with types that implement `Copy`. For other types,
24 //! one must use the `RefCell` type, acquiring a write lock before
27 //! `RefCell` uses Rust's lifetimes to implement *dynamic borrowing*,
28 //! a process whereby one can claim temporary, exclusive, mutable
29 //! access to the inner value. Borrows for `RefCell`s are tracked *at
30 //! runtime*, unlike Rust's native reference types which are entirely
31 //! tracked statically, at compile time. Because `RefCell` borrows are
32 //! dynamic it is possible to attempt to borrow a value that is
33 //! already mutably borrowed; when this happens it results in task
36 //! # When to choose interior mutability
38 //! The more common inherited mutability, where one must have unique
39 //! access to mutate a value, is one of the key language elements that
40 //! enables Rust to reason strongly about pointer aliasing, statically
41 //! preventing crash bugs. Because of that, inherited mutability is
42 //! preferred, and interior mutability is something of a last
43 //! resort. Since cell types enable mutation where it would otherwise
44 //! be disallowed though, there are occasions when interior
45 //! mutability might be appropriate, or even *must* be used, e.g.
47 //! * Introducing inherited mutability roots to shared types.
48 //! * Implementation details of logically-immutable methods.
49 //! * Mutating implementations of `clone`.
51 //! ## Introducing inherited mutability roots to shared types
53 //! Shared smart pointer types, including `Rc` and `Arc`, provide
54 //! containers that can be cloned and shared between multiple parties.
55 //! Because the contained values may be multiply-aliased, they can
56 //! only be borrowed as shared references, not mutable references.
57 //! Without cells it would be impossible to mutate data inside of
58 //! shared boxes at all!
60 //! It's very common then to put a `RefCell` inside shared pointer
61 //! types to reintroduce mutability:
64 //! use std::collections::HashMap;
65 //! use std::cell::RefCell;
69 //! let shared_map: Rc<RefCell<_>> = Rc::new(RefCell::new(HashMap::new()));
70 //! shared_map.borrow_mut().insert("africa", 92388i);
71 //! shared_map.borrow_mut().insert("kyoto", 11837i);
72 //! shared_map.borrow_mut().insert("piccadilly", 11826i);
73 //! shared_map.borrow_mut().insert("marbles", 38i);
77 //! ## Implementation details of logically-immutable methods
79 //! Occasionally it may be desirable not to expose in an API that
80 //! there is mutation happening "under the hood". This may be because
81 //! logically the operation is immutable, but e.g. caching forces the
82 //! implementation to perform mutation; or because you must employ
83 //! mutation to implement a trait method that was originally defined
87 //! use std::cell::RefCell;
90 //! edges: Vec<(uint, uint)>,
91 //! span_tree_cache: RefCell<Option<Vec<(uint, uint)>>>
95 //! fn minimum_spanning_tree(&self) -> Vec<(uint, uint)> {
96 //! // Create a new scope to contain the lifetime of the
99 //! // Take a reference to the inside of cache cell
100 //! let mut cache = self.span_tree_cache.borrow_mut();
101 //! if cache.is_some() {
102 //! return cache.as_ref().unwrap().clone();
105 //! let span_tree = self.calc_span_tree();
106 //! *cache = Some(span_tree);
109 //! // Recursive call to return the just-cached value.
110 //! // Note that if we had not let the previous borrow
111 //! // of the cache fall out of scope then the subsequent
112 //! // recursive borrow would cause a dynamic task panic.
113 //! // This is the major hazard of using `RefCell`.
114 //! self.minimum_spanning_tree()
116 //! # fn calc_span_tree(&self) -> Vec<(uint, uint)> { vec![] }
121 //! ## Mutating implementations of `clone`
123 //! This is simply a special - but common - case of the previous:
124 //! hiding mutability for operations that appear to be immutable.
125 //! The `clone` method is expected to not change the source value, and
126 //! is declared to take `&self`, not `&mut self`. Therefore any
127 //! mutation that happens in the `clone` method must use cell
128 //! types. For example, `Rc` maintains its reference counts within a
132 //! use std::cell::Cell;
135 //! ptr: *mut RcBox<T>
138 //! struct RcBox<T> {
140 //! refcount: Cell<uint>
143 //! impl<T> Clone for Rc<T> {
144 //! fn clone(&self) -> Rc<T> {
146 //! (*self.ptr).refcount.set((*self.ptr).refcount.get() + 1);
147 //! Rc { ptr: self.ptr }
153 // FIXME: Explain difference between Cell and RefCell
154 // FIXME: Downsides to interior mutability
155 // FIXME: Can't be shared between threads. Dynamic borrows
156 // FIXME: Relationship to Atomic types and RWLock
160 use default::Default;
161 use kinds::{Copy, Send};
162 use ops::{Deref, DerefMut, Drop};
164 use option::Option::{None, Some};
166 /// A mutable memory location that admits only `Copy` data.
169 value: UnsafeCell<T>,
172 impl<T:Copy> Cell<T> {
173 /// Creates a new `Cell` containing the given value.
175 pub fn new(value: T) -> Cell<T> {
177 value: UnsafeCell::new(value),
181 /// Returns a copy of the contained value.
184 pub fn get(&self) -> T {
185 unsafe{ *self.value.get() }
188 /// Sets the contained value.
191 pub fn set(&self, value: T) {
193 *self.value.get() = value;
197 /// Get a reference to the underlying `UnsafeCell`.
199 /// This can be used to circumvent `Cell`'s safety checks.
201 /// This function is `unsafe` because `UnsafeCell`'s field is public.
204 pub unsafe fn as_unsafe_cell<'a>(&'a self) -> &'a UnsafeCell<T> {
210 unsafe impl<T> Send for Cell<T> where T: Send {}
213 impl<T:Copy> Clone for Cell<T> {
214 fn clone(&self) -> Cell<T> {
215 Cell::new(self.get())
220 impl<T:Default + Copy> Default for Cell<T> {
222 fn default() -> Cell<T> {
223 Cell::new(Default::default())
227 #[unstable = "waiting for `PartialEq` trait to become stable"]
228 impl<T:PartialEq + Copy> PartialEq for Cell<T> {
229 fn eq(&self, other: &Cell<T>) -> bool {
230 self.get() == other.get()
234 /// A mutable memory location with dynamically checked borrow rules
236 pub struct RefCell<T> {
237 value: UnsafeCell<T>,
238 borrow: Cell<BorrowFlag>,
241 // Values [1, MAX-1] represent the number of `Ref` active
242 // (will not outgrow its range since `uint` is the size of the address space)
243 type BorrowFlag = uint;
244 const UNUSED: BorrowFlag = 0;
245 const WRITING: BorrowFlag = -1;
248 /// Create a new `RefCell` containing `value`
250 pub fn new(value: T) -> RefCell<T> {
252 value: UnsafeCell::new(value),
253 borrow: Cell::new(UNUSED),
257 /// Consumes the `RefCell`, returning the wrapped value.
259 pub fn into_inner(self) -> T {
260 // Since this function takes `self` (the `RefCell`) by value, the
261 // compiler statically verifies that it is not currently borrowed.
262 // Therefore the following assertion is just a `debug_assert!`.
263 debug_assert!(self.borrow.get() == UNUSED);
264 unsafe { self.value.into_inner() }
267 /// Deprecated, use into_inner() instead
268 #[deprecated = "renamed to into_inner()"]
269 pub fn unwrap(self) -> T { self.into_inner() }
271 /// Attempts to immutably borrow the wrapped value.
273 /// The borrow lasts until the returned `Ref` exits scope. Multiple
274 /// immutable borrows can be taken out at the same time.
276 /// Returns `None` if the value is currently mutably borrowed.
277 #[unstable = "may be renamed or removed"]
278 pub fn try_borrow<'a>(&'a self) -> Option<Ref<'a, T>> {
279 match BorrowRef::new(&self.borrow) {
280 Some(b) => Some(Ref { _value: unsafe { &*self.value.get() }, _borrow: b }),
285 /// Immutably borrows the wrapped value.
287 /// The borrow lasts until the returned `Ref` exits scope. Multiple
288 /// immutable borrows can be taken out at the same time.
292 /// Panics if the value is currently mutably borrowed.
294 pub fn borrow<'a>(&'a self) -> Ref<'a, T> {
295 match self.try_borrow() {
297 None => panic!("RefCell<T> already mutably borrowed")
301 /// Mutably borrows the wrapped value.
303 /// The borrow lasts until the returned `RefMut` exits scope. The value
304 /// cannot be borrowed while this borrow is active.
306 /// Returns `None` if the value is currently borrowed.
307 #[unstable = "may be renamed or removed"]
308 pub fn try_borrow_mut<'a>(&'a self) -> Option<RefMut<'a, T>> {
309 match BorrowRefMut::new(&self.borrow) {
310 Some(b) => Some(RefMut { _value: unsafe { &mut *self.value.get() }, _borrow: b }),
315 /// Mutably borrows the wrapped value.
317 /// The borrow lasts until the returned `RefMut` exits scope. The value
318 /// cannot be borrowed while this borrow is active.
322 /// Panics if the value is currently borrowed.
324 pub fn borrow_mut<'a>(&'a self) -> RefMut<'a, T> {
325 match self.try_borrow_mut() {
327 None => panic!("RefCell<T> already borrowed")
331 /// Get a reference to the underlying `UnsafeCell`.
333 /// This can be used to circumvent `RefCell`'s safety checks.
335 /// This function is `unsafe` because `UnsafeCell`'s field is public.
338 pub unsafe fn as_unsafe_cell<'a>(&'a self) -> &'a UnsafeCell<T> {
344 unsafe impl<T> Send for RefCell<T> where T: Send {}
347 impl<T: Clone> Clone for RefCell<T> {
348 fn clone(&self) -> RefCell<T> {
349 RefCell::new(self.borrow().clone())
354 impl<T:Default> Default for RefCell<T> {
356 fn default() -> RefCell<T> {
357 RefCell::new(Default::default())
361 #[unstable = "waiting for `PartialEq` to become stable"]
362 impl<T: PartialEq> PartialEq for RefCell<T> {
363 fn eq(&self, other: &RefCell<T>) -> bool {
364 *self.borrow() == *other.borrow()
368 struct BorrowRef<'b> {
369 _borrow: &'b Cell<BorrowFlag>,
372 impl<'b> BorrowRef<'b> {
373 fn new(borrow: &'b Cell<BorrowFlag>) -> Option<BorrowRef<'b>> {
378 Some(BorrowRef { _borrow: borrow })
385 impl<'b> Drop for BorrowRef<'b> {
387 let borrow = self._borrow.get();
388 debug_assert!(borrow != WRITING && borrow != UNUSED);
389 self._borrow.set(borrow - 1);
393 impl<'b> Clone for BorrowRef<'b> {
394 fn clone(&self) -> BorrowRef<'b> {
395 // Since this Ref exists, we know the borrow flag
396 // is not set to WRITING.
397 let borrow = self._borrow.get();
398 debug_assert!(borrow != WRITING && borrow != UNUSED);
399 self._borrow.set(borrow + 1);
400 BorrowRef { _borrow: self._borrow }
404 /// Wraps a borrowed reference to a value in a `RefCell` box.
406 pub struct Ref<'b, T:'b> {
407 // FIXME #12808: strange name to try to avoid interfering with
408 // field accesses of the contained type via Deref
410 _borrow: BorrowRef<'b>,
413 #[unstable = "waiting for `Deref` to become stable"]
414 impl<'b, T> Deref<T> for Ref<'b, T> {
416 fn deref<'a>(&'a self) -> &'a T {
423 /// The `RefCell` is already immutably borrowed, so this cannot fail.
425 /// A `Clone` implementation would interfere with the widespread
426 /// use of `r.borrow().clone()` to clone the contents of a `RefCell`.
427 #[experimental = "likely to be moved to a method, pending language changes"]
428 pub fn clone_ref<'b, T:Clone>(orig: &Ref<'b, T>) -> Ref<'b, T> {
431 _borrow: orig._borrow.clone(),
435 struct BorrowRefMut<'b> {
436 _borrow: &'b Cell<BorrowFlag>,
440 impl<'b> Drop for BorrowRefMut<'b> {
442 let borrow = self._borrow.get();
443 debug_assert!(borrow == WRITING);
444 self._borrow.set(UNUSED);
448 impl<'b> BorrowRefMut<'b> {
449 fn new(borrow: &'b Cell<BorrowFlag>) -> Option<BorrowRefMut<'b>> {
453 Some(BorrowRefMut { _borrow: borrow })
460 /// Wraps a mutable borrowed reference to a value in a `RefCell` box.
462 pub struct RefMut<'b, T:'b> {
463 // FIXME #12808: strange name to try to avoid interfering with
464 // field accesses of the contained type via Deref
466 _borrow: BorrowRefMut<'b>,
469 #[unstable = "waiting for `Deref` to become stable"]
470 impl<'b, T> Deref<T> for RefMut<'b, T> {
472 fn deref<'a>(&'a self) -> &'a T {
477 #[unstable = "waiting for `DerefMut` to become stable"]
478 impl<'b, T> DerefMut<T> for RefMut<'b, T> {
480 fn deref_mut<'a>(&'a mut self) -> &'a mut T {
485 /// The core primitive for interior mutability in Rust.
487 /// `UnsafeCell` type that wraps a type T and indicates unsafe interior
488 /// operations on the wrapped type. Types with an `UnsafeCell<T>` field are
489 /// considered to have an *unsafe interior*. The `UnsafeCell` type is the only
490 /// legal way to obtain aliasable data that is considered mutable. In general,
491 /// transmuting an &T type into an &mut T is considered undefined behavior.
493 /// Although it is possible to put an `UnsafeCell<T>` into static item, it is
494 /// not permitted to take the address of the static item if the item is not
495 /// declared as mutable. This rule exists because immutable static items are
496 /// stored in read-only memory, and thus any attempt to mutate their interior
497 /// can cause segfaults. Immutable static items containing `UnsafeCell<T>`
498 /// instances are still useful as read-only initializers, however, so we do not
499 /// forbid them altogether.
501 /// Types like `Cell` and `RefCell` use this type to wrap their internal data.
503 /// `UnsafeCell` doesn't opt-out from any kind, instead, types with an
504 /// `UnsafeCell` interior are expected to opt-out from kinds themselves.
509 /// use std::cell::UnsafeCell;
510 /// use std::kinds::marker;
512 /// struct NotThreadSafe<T> {
513 /// value: UnsafeCell<T>,
514 /// marker: marker::NoSync
518 /// **NOTE:** `UnsafeCell<T>` fields are public to allow static initializers. It
519 /// is not recommended to access its fields directly, `get` should be used
523 pub struct UnsafeCell<T> {
526 /// This field should not be accessed directly, it is made public for static
532 impl<T> UnsafeCell<T> {
533 /// Construct a new instance of `UnsafeCell` which will wrap the specified
536 /// All access to the inner value through methods is `unsafe`, and it is
537 /// highly discouraged to access the fields directly.
539 pub fn new(value: T) -> UnsafeCell<T> {
540 UnsafeCell { value: value }
543 /// Gets a mutable pointer to the wrapped value.
546 pub fn get(&self) -> *mut T { &self.value as *const T as *mut T }
548 /// Unwraps the value
550 /// This function is unsafe because there is no guarantee that this or other
551 /// tasks are currently inspecting the inner value.
554 pub unsafe fn into_inner(self) -> T { self.value }
556 /// Deprecated, use into_inner() instead
557 #[deprecated = "renamed to into_inner()"]
558 pub unsafe fn unwrap(self) -> T { self.into_inner() }