src/liballoc/boxed.rs

   1 // Copyright 2012-2015 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 //! A pointer type for heap allocation.
  12 //!
  13 //! `Box<T>`, casually referred to as a 'box', provides the simplest form of heap allocation in
  14 //! Rust. Boxes provide ownership for this allocation, and drop their contents when they go out of
  15 //! scope.
  16 //!
  17 //! # Examples
  18 //!
  19 //! Creating a box:
  20 //!
  21 //! ```
  22 //! let x = Box::new(5);
  23 //! ```
  24 //!
  25 //! Creating a recursive data structure:
  26 //!
  27 //! ```
  28 //! #[derive(Debug)]
  29 //! enum List<T> {
  30 //!     Cons(T, Box<List<T>>),
  31 //!     Nil,
  32 //! }
  33 //!
  34 //! fn main() {
  35 //!     let list: List<i32> = List::Cons(1, Box::new(List::Cons(2, Box::new(List::Nil))));
  36 //!     println!("{:?}", list);
  37 //! }
  38 //! ```
  39 //!
  40 //! This will print `Cons(1, Cons(2, Nil))`.
  41 //!
  42 //! Recursive structures must be boxed, because if the definition of `Cons` looked like this:
  43 //!
  44 //! ```rust,ignore
  45 //! Cons(T, List<T>),
  46 //! ```
  47 //!
  48 //! It wouldn't work. This is because the size of a `List` depends on how many elements are in the
  49 //! list, and so we don't know how much memory to allocate for a `Cons`. By introducing a `Box`,
  50 //! which has a defined size, we know how big `Cons` needs to be.
  51
  52 #![stable(feature = "rust1", since = "1.0.0")]
  53
  54 use core::prelude::*;
  55
  56 use core::any::Any;
  57 use core::cmp::Ordering;
  58 use core::fmt;
  59 use core::hash::{self, Hash};
  60 use core::mem;
  61 use core::ops::{Deref, DerefMut};
  62 use core::ptr::{Unique};
  63 use core::raw::{TraitObject};
  64
  65 #[cfg(not(stage0))]
  66 use core::marker::Unsize;
  67 #[cfg(not(stage0))]
  68 use core::ops::CoerceUnsized;
  69
  70 /// A value that represents the heap. This is the default place that the `box`
  71 /// keyword allocates into when no place is supplied.
  72 ///
  73 /// The following two examples are equivalent:
  74 ///
  75 /// ```
  76 /// # #![feature(alloc)]
  77 /// #![feature(box_syntax)]
  78 /// use std::boxed::HEAP;
  79 ///
  80 /// fn main() {
  81 ///     let foo = box(HEAP) 5;
  82 ///     let foo = box 5;
  83 /// }
  84 /// ```
  85 #[lang = "exchange_heap"]
  86 #[unstable(feature = "alloc",
  87            reason = "may be renamed; uncertain about custom allocator design")]
  88 pub static HEAP: () = ();
  89
  90 /// A pointer type for heap allocation.
  91 ///
  92 /// See the [module-level documentation](../../std/boxed/index.html) for more.
  93 #[lang = "owned_box"]
  94 #[stable(feature = "rust1", since = "1.0.0")]
  95 #[fundamental]
  96 pub struct Box<T>(Unique<T>);
  97
  98 impl<T> Box<T> {
  99     /// Allocates memory on the heap and then moves `x` into it.
 100     ///
 101     /// # Examples
 102     ///
 103     /// ```
 104     /// let x = Box::new(5);
 105     /// ```
 106     #[stable(feature = "rust1", since = "1.0.0")]
 107     #[inline(always)]
 108     pub fn new(x: T) -> Box<T> {
 109         box x
 110     }
 111 }
 112
 113 impl<T : ?Sized> Box<T> {
 114     /// Constructs a box from the raw pointer.
 115     ///
 116     /// After this function call, pointer is owned by resulting box.
 117     /// In particular, it means that `Box` destructor calls destructor
 118     /// of `T` and releases memory. Since the way `Box` allocates and
 119     /// releases memory is unspecified, the only valid pointer to pass
 120     /// to this function is the one taken from another `Box` with
 121     /// `boxed::into_raw` function.
 122     ///
 123     /// Function is unsafe, because improper use of this function may
 124     /// lead to memory problems like double-free, for example if the
 125     /// function is called twice on the same raw pointer.
 126     #[unstable(feature = "alloc",
 127                reason = "may be renamed or moved out of Box scope")]
 128     #[inline]
 129     pub unsafe fn from_raw(raw: *mut T) -> Self {
 130         mem::transmute(raw)
 131     }
 132 }
 133
 134 /// Consumes the `Box`, returning the wrapped raw pointer.
 135 ///
 136 /// After call to this function, caller is responsible for the memory
 137 /// previously managed by `Box`, in particular caller should properly
 138 /// destroy `T` and release memory. The proper way to do it is to
 139 /// convert pointer back to `Box` with `Box::from_raw` function, because
 140 /// `Box` does not specify, how memory is allocated.
 141 ///
 142 /// Function is unsafe, because result of this function is no longer
 143 /// automatically managed that may lead to memory or other resource
 144 /// leak.
 145 ///
 146 /// # Examples
 147 /// ```
 148 /// # #![feature(alloc)]
 149 /// use std::boxed;
 150 ///
 151 /// let seventeen = Box::new(17u32);
 152 /// let raw = unsafe { boxed::into_raw(seventeen) };
 153 /// let boxed_again = unsafe { Box::from_raw(raw) };
 154 /// ```
 155 #[unstable(feature = "alloc",
 156            reason = "may be renamed")]
 157 #[inline]
 158 pub unsafe fn into_raw<T : ?Sized>(b: Box<T>) -> *mut T {
 159     mem::transmute(b)
 160 }
 161
 162 #[stable(feature = "rust1", since = "1.0.0")]
 163 impl<T: Default> Default for Box<T> {
 164     #[stable(feature = "rust1", since = "1.0.0")]
 165     fn default() -> Box<T> { box Default::default() }
 166 }
 167
 168 #[stable(feature = "rust1", since = "1.0.0")]
 169 impl<T> Default for Box<[T]> {
 170     #[stable(feature = "rust1", since = "1.0.0")]
 171     fn default() -> Box<[T]> { Box::<[T; 0]>::new([]) }
 172 }
 173
 174 #[stable(feature = "rust1", since = "1.0.0")]
 175 impl<T: Clone> Clone for Box<T> {
 176     /// Returns a new box with a `clone()` of this box's contents.
 177     ///
 178     /// # Examples
 179     ///
 180     /// ```
 181     /// let x = Box::new(5);
 182     /// let y = x.clone();
 183     /// ```
 184     #[inline]
 185     fn clone(&self) -> Box<T> { box {(**self).clone()} }
 186
 187     /// Copies `source`'s contents into `self` without creating a new allocation.
 188     ///
 189     /// # Examples
 190     ///
 191     /// ```
 192     /// # #![feature(alloc, core)]
 193     /// let x = Box::new(5);
 194     /// let mut y = Box::new(10);
 195     ///
 196     /// y.clone_from(&x);
 197     ///
 198     /// assert_eq!(*y, 5);
 199     /// ```
 200     #[inline]
 201     fn clone_from(&mut self, source: &Box<T>) {
 202         (**self).clone_from(&(**source));
 203     }
 204 }
 205
 206 #[stable(feature = "rust1", since = "1.0.0")]
 207 impl<T: ?Sized + PartialEq> PartialEq for Box<T> {
 208     #[inline]
 209     fn eq(&self, other: &Box<T>) -> bool { PartialEq::eq(&**self, &**other) }
 210     #[inline]
 211     fn ne(&self, other: &Box<T>) -> bool { PartialEq::ne(&**self, &**other) }
 212 }
 213 #[stable(feature = "rust1", since = "1.0.0")]
 214 impl<T: ?Sized + PartialOrd> PartialOrd for Box<T> {
 215     #[inline]
 216     fn partial_cmp(&self, other: &Box<T>) -> Option<Ordering> {
 217         PartialOrd::partial_cmp(&**self, &**other)
 218     }
 219     #[inline]
 220     fn lt(&self, other: &Box<T>) -> bool { PartialOrd::lt(&**self, &**other) }
 221     #[inline]
 222     fn le(&self, other: &Box<T>) -> bool { PartialOrd::le(&**self, &**other) }
 223     #[inline]
 224     fn ge(&self, other: &Box<T>) -> bool { PartialOrd::ge(&**self, &**other) }
 225     #[inline]
 226     fn gt(&self, other: &Box<T>) -> bool { PartialOrd::gt(&**self, &**other) }
 227 }
 228 #[stable(feature = "rust1", since = "1.0.0")]
 229 impl<T: ?Sized + Ord> Ord for Box<T> {
 230     #[inline]
 231     fn cmp(&self, other: &Box<T>) -> Ordering {
 232         Ord::cmp(&**self, &**other)
 233     }
 234 }
 235 #[stable(feature = "rust1", since = "1.0.0")]
 236 impl<T: ?Sized + Eq> Eq for Box<T> {}
 237
 238 #[stable(feature = "rust1", since = "1.0.0")]
 239 impl<T: ?Sized + Hash> Hash for Box<T> {
 240     fn hash<H: hash::Hasher>(&self, state: &mut H) {
 241         (**self).hash(state);
 242     }
 243 }
 244
 245 impl Box<Any> {
 246     #[inline]
 247     #[stable(feature = "rust1", since = "1.0.0")]
 248     /// Attempt to downcast the box to a concrete type.
 249     pub fn downcast<T: Any>(self) -> Result<Box<T>, Box<Any>> {
 250         if self.is::<T>() {
 251             unsafe {
 252                 // Get the raw representation of the trait object
 253                 let raw = into_raw(self);
 254                 let to: TraitObject =
 255                     mem::transmute::<*mut Any, TraitObject>(raw);
 256
 257                 // Extract the data pointer
 258                 Ok(Box::from_raw(to.data as *mut T))
 259             }
 260         } else {
 261             Err(self)
 262         }
 263     }
 264 }
 265
 266 impl Box<Any + Send> {
 267     #[inline]
 268     #[stable(feature = "rust1", since = "1.0.0")]
 269     /// Attempt to downcast the box to a concrete type.
 270     pub fn downcast<T: Any>(self) -> Result<Box<T>, Box<Any + Send>> {
 271         <Box<Any>>::downcast(self).map_err(|s| unsafe {
 272             // reapply the Send marker
 273             mem::transmute::<Box<Any>, Box<Any + Send>>(s)
 274         })
 275     }
 276 }
 277
 278 #[stable(feature = "rust1", since = "1.0.0")]
 279 impl<T: fmt::Display + ?Sized> fmt::Display for Box<T> {
 280     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
 281         fmt::Display::fmt(&**self, f)
 282     }
 283 }
 284
 285 #[stable(feature = "rust1", since = "1.0.0")]
 286 impl<T: fmt::Debug + ?Sized> fmt::Debug for Box<T> {
 287     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
 288         fmt::Debug::fmt(&**self, f)
 289     }
 290 }
 291
 292 #[stable(feature = "rust1", since = "1.0.0")]
 293 impl<T> fmt::Pointer for Box<T> {
 294     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
 295         // It's not possible to extract the inner Uniq directly from the Box,
 296         // instead we cast it to a *const which aliases the Unique
 297         let ptr: *const T = &**self;
 298         fmt::Pointer::fmt(&ptr, f)
 299     }
 300 }
 301
 302 #[stable(feature = "rust1", since = "1.0.0")]
 303 impl<T: ?Sized> Deref for Box<T> {
 304     type Target = T;
 305
 306     fn deref(&self) -> &T { &**self }
 307 }
 308
 309 #[stable(feature = "rust1", since = "1.0.0")]
 310 impl<T: ?Sized> DerefMut for Box<T> {
 311     fn deref_mut(&mut self) -> &mut T { &mut **self }
 312 }
 313
 314 #[stable(feature = "rust1", since = "1.0.0")]
 315 impl<I: Iterator + ?Sized> Iterator for Box<I> {
 316     type Item = I::Item;
 317     fn next(&mut self) -> Option<I::Item> { (**self).next() }
 318     fn size_hint(&self) -> (usize, Option<usize>) { (**self).size_hint() }
 319 }
 320 #[stable(feature = "rust1", since = "1.0.0")]
 321 impl<I: DoubleEndedIterator + ?Sized> DoubleEndedIterator for Box<I> {
 322     fn next_back(&mut self) -> Option<I::Item> { (**self).next_back() }
 323 }
 324 #[stable(feature = "rust1", since = "1.0.0")]
 325 impl<I: ExactSizeIterator + ?Sized> ExactSizeIterator for Box<I> {}
 326
 327
 328 /// `FnBox` is a version of the `FnOnce` intended for use with boxed
 329 /// closure objects. The idea is that where one would normally store a
 330 /// `Box<FnOnce()>` in a data structure, you should use
 331 /// `Box<FnBox()>`. The two traits behave essentially the same, except
 332 /// that a `FnBox` closure can only be called if it is boxed. (Note
 333 /// that `FnBox` may be deprecated in the future if `Box<FnOnce()>`
 334 /// closures become directly usable.)
 335 ///
 336 /// ### Example
 337 ///
 338 /// Here is a snippet of code which creates a hashmap full of boxed
 339 /// once closures and then removes them one by one, calling each
 340 /// closure as it is removed. Note that the type of the closures
 341 /// stored in the map is `Box<FnBox() -> i32>` and not `Box<FnOnce()
 342 /// -> i32>`.
 343 ///
 344 /// ```
 345 /// #![feature(core)]
 346 ///
 347 /// use std::boxed::FnBox;
 348 /// use std::collections::HashMap;
 349 ///
 350 /// fn make_map() -> HashMap<i32, Box<FnBox() -> i32>> {
 351 ///     let mut map: HashMap<i32, Box<FnBox() -> i32>> = HashMap::new();
 352 ///     map.insert(1, Box::new(|| 22));
 353 ///     map.insert(2, Box::new(|| 44));
 354 ///     map
 355 /// }
 356 ///
 357 /// fn main() {
 358 ///     let mut map = make_map();
 359 ///     for i in &[1, 2] {
 360 ///         let f = map.remove(&i).unwrap();
 361 ///         assert_eq!(f(), i * 22);
 362 ///     }
 363 /// }
 364 /// ```
 365 #[rustc_paren_sugar]
 366 #[unstable(feature = "core", reason = "Newly introduced")]
 367 pub trait FnBox<A> {
 368     type Output;
 369
 370     fn call_box(self: Box<Self>, args: A) -> Self::Output;
 371 }
 372
 373 impl<A,F> FnBox<A> for F
 374     where F: FnOnce<A>
 375 {
 376     type Output = F::Output;
 377
 378     fn call_box(self: Box<F>, args: A) -> F::Output {
 379         self.call_once(args)
 380     }
 381 }
 382
 383 impl<'a,A,R> FnOnce<A> for Box<FnBox<A,Output=R>+'a> {
 384     type Output = R;
 385
 386     extern "rust-call" fn call_once(self, args: A) -> R {
 387         self.call_box(args)
 388     }
 389 }
 390
 391 impl<'a,A,R> FnOnce<A> for Box<FnBox<A,Output=R>+Send+'a> {
 392     type Output = R;
 393
 394     extern "rust-call" fn call_once(self, args: A) -> R {
 395         self.call_box(args)
 396     }
 397 }
 398
 399 #[cfg(not(stage0))]
 400 impl<T: ?Sized+Unsize<U>, U: ?Sized> CoerceUnsized<Box<U>> for Box<T> {}