src/libcore/cell.rs

   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.
   4 //
   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.
  10
  11 //! Shareable mutable containers.
  12 //!
  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
  18 //! mutability*.
  19 //!
  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
  25 //! mutating.
  26 //!
  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
  34 //! panic.
  35 //!
  36 //! # When to choose interior mutability
  37 //!
  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.
  46 //!
  47 //! * Introducing inherited mutability roots to shared types.
  48 //! * Implementation details of logically-immutable methods.
  49 //! * Mutating implementations of `clone`.
  50 //!
  51 //! ## Introducing inherited mutability roots to shared types
  52 //!
  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!
  59 //!
  60 //! It's very common then to put a `RefCell` inside shared pointer
  61 //! types to reintroduce mutability:
  62 //!
  63 //! ```
  64 //! use std::collections::HashMap;
  65 //! use std::cell::RefCell;
  66 //! use std::rc::Rc;
  67 //!
  68 //! fn main() {
  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);
  74 //! }
  75 //! ```
  76 //!
  77 //! ## Implementation details of logically-immutable methods
  78 //!
  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
  84 //! to take `&self`.
  85 //!
  86 //! ```
  87 //! use std::cell::RefCell;
  88 //!
  89 //! struct Graph {
  90 //!     edges: Vec<(uint, uint)>,
  91 //!     span_tree_cache: RefCell<Option<Vec<(uint, uint)>>>
  92 //! }
  93 //!
  94 //! impl Graph {
  95 //!     fn minimum_spanning_tree(&self) -> Vec<(uint, uint)> {
  96 //!         // Create a new scope to contain the lifetime of the
  97 //!         // dynamic borrow
  98 //!         {
  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();
 103 //!             }
 104 //!
 105 //!             let span_tree = self.calc_span_tree();
 106 //!             *cache = Some(span_tree);
 107 //!         }
 108 //!
 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()
 115 //!     }
 116 //! #   fn calc_span_tree(&self) -> Vec<(uint, uint)> { vec![] }
 117 //! }
 118 //! # fn main() { }
 119 //! ```
 120 //!
 121 //! ## Mutating implementations of `clone`
 122 //!
 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
 129 //! `Cell`.
 130 //!
 131 //! ```
 132 //! use std::cell::Cell;
 133 //!
 134 //! struct Rc<T> {
 135 //!     ptr: *mut RcBox<T>
 136 //! }
 137 //!
 138 //! struct RcBox<T> {
 139 //!     value: T,
 140 //!     refcount: Cell<uint>
 141 //! }
 142 //!
 143 //! impl<T> Clone for Rc<T> {
 144 //!     fn clone(&self) -> Rc<T> {
 145 //!         unsafe {
 146 //!             (*self.ptr).refcount.set((*self.ptr).refcount.get() + 1);
 147 //!             Rc { ptr: self.ptr }
 148 //!         }
 149 //!     }
 150 //! }
 151 //! ```
 152 //!
 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
 157
 158 #![stable]
 159
 160 use clone::Clone;
 161 use cmp::PartialEq;
 162 use default::Default;
 163 use marker::{Copy, Send};
 164 use ops::{Deref, DerefMut, Drop};
 165 use option::Option;
 166 use option::Option::{None, Some};
 167
 168 /// A mutable memory location that admits only `Copy` data.
 169 #[stable]
 170 pub struct Cell<T> {
 171     value: UnsafeCell<T>,
 172 }
 173
 174 impl<T:Copy> Cell<T> {
 175     /// Creates a new `Cell` containing the given value.
 176     #[stable]
 177     pub fn new(value: T) -> Cell<T> {
 178         Cell {
 179             value: UnsafeCell::new(value),
 180         }
 181     }
 182
 183     /// Returns a copy of the contained value.
 184     #[inline]
 185     #[stable]
 186     pub fn get(&self) -> T {
 187         unsafe{ *self.value.get() }
 188     }
 189
 190     /// Sets the contained value.
 191     #[inline]
 192     #[stable]
 193     pub fn set(&self, value: T) {
 194         unsafe {
 195             *self.value.get() = value;
 196         }
 197     }
 198
 199     /// Get a reference to the underlying `UnsafeCell`.
 200     ///
 201     /// This can be used to circumvent `Cell`'s safety checks.
 202     ///
 203     /// This function is `unsafe` because `UnsafeCell`'s field is public.
 204     #[inline]
 205     #[unstable]
 206     pub unsafe fn as_unsafe_cell<'a>(&'a self) -> &'a UnsafeCell<T> {
 207         &self.value
 208     }
 209 }
 210
 211 #[stable]
 212 unsafe impl<T> Send for Cell<T> where T: Send {}
 213
 214 #[stable]
 215 impl<T:Copy> Clone for Cell<T> {
 216     fn clone(&self) -> Cell<T> {
 217         Cell::new(self.get())
 218     }
 219 }
 220
 221 #[stable]
 222 impl<T:Default + Copy> Default for Cell<T> {
 223     #[stable]
 224     fn default() -> Cell<T> {
 225         Cell::new(Default::default())
 226     }
 227 }
 228
 229 #[stable]
 230 impl<T:PartialEq + Copy> PartialEq for Cell<T> {
 231     fn eq(&self, other: &Cell<T>) -> bool {
 232         self.get() == other.get()
 233     }
 234 }
 235
 236 /// A mutable memory location with dynamically checked borrow rules
 237 #[stable]
 238 pub struct RefCell<T> {
 239     value: UnsafeCell<T>,
 240     borrow: Cell<BorrowFlag>,
 241 }
 242
 243 // Values [1, MAX-1] represent the number of `Ref` active
 244 // (will not outgrow its range since `uint` is the size of the address space)
 245 type BorrowFlag = uint;
 246 const UNUSED: BorrowFlag = 0;
 247 const WRITING: BorrowFlag = -1;
 248
 249 impl<T> RefCell<T> {
 250     /// Create a new `RefCell` containing `value`
 251     #[stable]
 252     pub fn new(value: T) -> RefCell<T> {
 253         RefCell {
 254             value: UnsafeCell::new(value),
 255             borrow: Cell::new(UNUSED),
 256         }
 257     }
 258
 259     /// Consumes the `RefCell`, returning the wrapped value.
 260     #[stable]
 261     pub fn into_inner(self) -> T {
 262         // Since this function takes `self` (the `RefCell`) by value, the
 263         // compiler statically verifies that it is not currently borrowed.
 264         // Therefore the following assertion is just a `debug_assert!`.
 265         debug_assert!(self.borrow.get() == UNUSED);
 266         unsafe { self.value.into_inner() }
 267     }
 268
 269     /// Attempts to immutably borrow the wrapped value.
 270     ///
 271     /// The borrow lasts until the returned `Ref` exits scope. Multiple
 272     /// immutable borrows can be taken out at the same time.
 273     ///
 274     /// Returns `None` if the value is currently mutably borrowed.
 275     #[unstable = "may be renamed or removed"]
 276     pub fn try_borrow<'a>(&'a self) -> Option<Ref<'a, T>> {
 277         match BorrowRef::new(&self.borrow) {
 278             Some(b) => Some(Ref { _value: unsafe { &*self.value.get() }, _borrow: b }),
 279             None => None,
 280         }
 281     }
 282
 283     /// Immutably borrows the wrapped value.
 284     ///
 285     /// The borrow lasts until the returned `Ref` exits scope. Multiple
 286     /// immutable borrows can be taken out at the same time.
 287     ///
 288     /// # Panics
 289     ///
 290     /// Panics if the value is currently mutably borrowed.
 291     #[stable]
 292     pub fn borrow<'a>(&'a self) -> Ref<'a, T> {
 293         match self.try_borrow() {
 294             Some(ptr) => ptr,
 295             None => panic!("RefCell<T> already mutably borrowed")
 296         }
 297     }
 298
 299     /// Mutably borrows the wrapped value.
 300     ///
 301     /// The borrow lasts until the returned `RefMut` exits scope. The value
 302     /// cannot be borrowed while this borrow is active.
 303     ///
 304     /// Returns `None` if the value is currently borrowed.
 305     #[unstable = "may be renamed or removed"]
 306     pub fn try_borrow_mut<'a>(&'a self) -> Option<RefMut<'a, T>> {
 307         match BorrowRefMut::new(&self.borrow) {
 308             Some(b) => Some(RefMut { _value: unsafe { &mut *self.value.get() }, _borrow: b }),
 309             None => None,
 310         }
 311     }
 312
 313     /// Mutably borrows the wrapped value.
 314     ///
 315     /// The borrow lasts until the returned `RefMut` exits scope. The value
 316     /// cannot be borrowed while this borrow is active.
 317     ///
 318     /// # Panics
 319     ///
 320     /// Panics if the value is currently borrowed.
 321     #[stable]
 322     pub fn borrow_mut<'a>(&'a self) -> RefMut<'a, T> {
 323         match self.try_borrow_mut() {
 324             Some(ptr) => ptr,
 325             None => panic!("RefCell<T> already borrowed")
 326         }
 327     }
 328
 329     /// Get a reference to the underlying `UnsafeCell`.
 330     ///
 331     /// This can be used to circumvent `RefCell`'s safety checks.
 332     ///
 333     /// This function is `unsafe` because `UnsafeCell`'s field is public.
 334     #[inline]
 335     #[unstable]
 336     pub unsafe fn as_unsafe_cell<'a>(&'a self) -> &'a UnsafeCell<T> {
 337         &self.value
 338     }
 339 }
 340
 341 #[stable]
 342 unsafe impl<T> Send for RefCell<T> where T: Send {}
 343
 344 #[stable]
 345 impl<T: Clone> Clone for RefCell<T> {
 346     fn clone(&self) -> RefCell<T> {
 347         RefCell::new(self.borrow().clone())
 348     }
 349 }
 350
 351 #[stable]
 352 impl<T:Default> Default for RefCell<T> {
 353     #[stable]
 354     fn default() -> RefCell<T> {
 355         RefCell::new(Default::default())
 356     }
 357 }
 358
 359 #[stable]
 360 impl<T: PartialEq> PartialEq for RefCell<T> {
 361     fn eq(&self, other: &RefCell<T>) -> bool {
 362         *self.borrow() == *other.borrow()
 363     }
 364 }
 365
 366 struct BorrowRef<'b> {
 367     _borrow: &'b Cell<BorrowFlag>,
 368 }
 369
 370 impl<'b> BorrowRef<'b> {
 371     fn new(borrow: &'b Cell<BorrowFlag>) -> Option<BorrowRef<'b>> {
 372         match borrow.get() {
 373             WRITING => None,
 374             b => {
 375                 borrow.set(b + 1);
 376                 Some(BorrowRef { _borrow: borrow })
 377             },
 378         }
 379     }
 380 }
 381
 382 #[unsafe_destructor]
 383 impl<'b> Drop for BorrowRef<'b> {
 384     fn drop(&mut self) {
 385         let borrow = self._borrow.get();
 386         debug_assert!(borrow != WRITING && borrow != UNUSED);
 387         self._borrow.set(borrow - 1);
 388     }
 389 }
 390
 391 impl<'b> Clone for BorrowRef<'b> {
 392     fn clone(&self) -> BorrowRef<'b> {
 393         // Since this Ref exists, we know the borrow flag
 394         // is not set to WRITING.
 395         let borrow = self._borrow.get();
 396         debug_assert!(borrow != WRITING && borrow != UNUSED);
 397         self._borrow.set(borrow + 1);
 398         BorrowRef { _borrow: self._borrow }
 399     }
 400 }
 401
 402 /// Wraps a borrowed reference to a value in a `RefCell` box.
 403 #[stable]
 404 pub struct Ref<'b, T:'b> {
 405     // FIXME #12808: strange name to try to avoid interfering with
 406     // field accesses of the contained type via Deref
 407     _value: &'b T,
 408     _borrow: BorrowRef<'b>,
 409 }
 410
 411 #[stable]
 412 impl<'b, T> Deref for Ref<'b, T> {
 413     type Target = T;
 414
 415     #[inline]
 416     fn deref<'a>(&'a self) -> &'a T {
 417         self._value
 418     }
 419 }
 420
 421 /// Copy a `Ref`.
 422 ///
 423 /// The `RefCell` is already immutably borrowed, so this cannot fail.
 424 ///
 425 /// A `Clone` implementation would interfere with the widespread
 426 /// use of `r.borrow().clone()` to clone the contents of a `RefCell`.
 427 #[unstable = "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> {
 429     Ref {
 430         _value: orig._value,
 431         _borrow: orig._borrow.clone(),
 432     }
 433 }
 434
 435 struct BorrowRefMut<'b> {
 436     _borrow: &'b Cell<BorrowFlag>,
 437 }
 438
 439 #[unsafe_destructor]
 440 impl<'b> Drop for BorrowRefMut<'b> {
 441     fn drop(&mut self) {
 442         let borrow = self._borrow.get();
 443         debug_assert!(borrow == WRITING);
 444         self._borrow.set(UNUSED);
 445     }
 446 }
 447
 448 impl<'b> BorrowRefMut<'b> {
 449     fn new(borrow: &'b Cell<BorrowFlag>) -> Option<BorrowRefMut<'b>> {
 450         match borrow.get() {
 451             UNUSED => {
 452                 borrow.set(WRITING);
 453                 Some(BorrowRefMut { _borrow: borrow })
 454             },
 455             _ => None,
 456         }
 457     }
 458 }
 459
 460 /// Wraps a mutable borrowed reference to a value in a `RefCell` box.
 461 #[stable]
 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
 465     _value: &'b mut T,
 466     _borrow: BorrowRefMut<'b>,
 467 }
 468
 469 #[stable]
 470 impl<'b, T> Deref for RefMut<'b, T> {
 471     type Target = T;
 472
 473     #[inline]
 474     fn deref<'a>(&'a self) -> &'a T {
 475         self._value
 476     }
 477 }
 478
 479 #[stable]
 480 impl<'b, T> DerefMut for RefMut<'b, T> {
 481     #[inline]
 482     fn deref_mut<'a>(&'a mut self) -> &'a mut T {
 483         self._value
 484     }
 485 }
 486
 487 /// The core primitive for interior mutability in Rust.
 488 ///
 489 /// `UnsafeCell` type that wraps a type T and indicates unsafe interior
 490 /// operations on the wrapped type. Types with an `UnsafeCell<T>` field are
 491 /// considered to have an *unsafe interior*. The `UnsafeCell` type is the only
 492 /// legal way to obtain aliasable data that is considered mutable. In general,
 493 /// transmuting an &T type into an &mut T is considered undefined behavior.
 494 ///
 495 /// Although it is possible to put an `UnsafeCell<T>` into static item, it is
 496 /// not permitted to take the address of the static item if the item is not
 497 /// declared as mutable. This rule exists because immutable static items are
 498 /// stored in read-only memory, and thus any attempt to mutate their interior
 499 /// can cause segfaults. Immutable static items containing `UnsafeCell<T>`
 500 /// instances are still useful as read-only initializers, however, so we do not
 501 /// forbid them altogether.
 502 ///
 503 /// Types like `Cell` and `RefCell` use this type to wrap their internal data.
 504 ///
 505 /// `UnsafeCell` doesn't opt-out from any kind, instead, types with an
 506 /// `UnsafeCell` interior are expected to opt-out from kinds themselves.
 507 ///
 508 /// # Example:
 509 ///
 510 /// ```rust
 511 /// use std::cell::UnsafeCell;
 512 /// use std::marker;
 513 ///
 514 /// struct NotThreadSafe<T> {
 515 ///     value: UnsafeCell<T>,
 516 ///     marker: marker::NoSync
 517 /// }
 518 /// ```
 519 ///
 520 /// **NOTE:** `UnsafeCell<T>` fields are public to allow static initializers. It
 521 /// is not recommended to access its fields directly, `get` should be used
 522 /// instead.
 523 #[lang="unsafe"]
 524 #[stable]
 525 pub struct UnsafeCell<T> {
 526     /// Wrapped value
 527     ///
 528     /// This field should not be accessed directly, it is made public for static
 529     /// initializers.
 530     #[unstable]
 531     pub value: T,
 532 }
 533
 534 impl<T> UnsafeCell<T> {
 535     /// Construct a new instance of `UnsafeCell` which will wrap the specified
 536     /// value.
 537     ///
 538     /// All access to the inner value through methods is `unsafe`, and it is
 539     /// highly discouraged to access the fields directly.
 540     #[stable]
 541     pub fn new(value: T) -> UnsafeCell<T> {
 542         UnsafeCell { value: value }
 543     }
 544
 545     /// Gets a mutable pointer to the wrapped value.
 546     #[inline]
 547     #[stable]
 548     pub fn get(&self) -> *mut T { &self.value as *const T as *mut T }
 549
 550     /// Unwraps the value
 551     ///
 552     /// This function is unsafe because there is no guarantee that this or other
 553     /// tasks are currently inspecting the inner value.
 554     #[inline]
 555     #[stable]
 556     pub unsafe fn into_inner(self) -> T { self.value }
 557 }