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
  14 //! heap allocation in Rust. Boxes provide ownership for this allocation, and
  15 //! drop their contents when they go out of 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`
  43 //! looked like this:
  44 //!
  45 //! ```rust,ignore
  46 //! Cons(T, List<T>),
  47 //! ```
  48 //!
  49 //! It wouldn't work. This is because the size of a `List` depends on how many
  50 //! elements are in the list, and so we don't know how much memory to allocate
  51 //! for a `Cons`. By introducing a `Box`, which has a defined size, we know how
  52 //! big `Cons` needs to be.
  53
  54 #![stable(feature = "rust1", since = "1.0.0")]
  55
  56 use core::prelude::*;
  57
  58 use heap;
  59 use raw_vec::RawVec;
  60
  61 use core::any::Any;
  62 use core::cmp::Ordering;
  63 use core::fmt;
  64 use core::hash::{self, Hash};
  65 use core::marker::{self, Unsize};
  66 use core::mem;
  67 use core::ops::{CoerceUnsized, Deref, DerefMut};
  68 use core::ops::{Placer, Boxed, Place, InPlace, BoxPlace};
  69 use core::ptr::{self, Unique};
  70 use core::raw::{TraitObject};
  71
  72 /// A value that represents the heap. This is the default place that the `box`
  73 /// keyword allocates into when no place is supplied.
  74 ///
  75 /// The following two examples are equivalent:
  76 ///
  77 /// ```
  78 /// #![feature(box_heap)]
  79 ///
  80 /// #![feature(box_syntax, placement_in_syntax)]
  81 /// use std::boxed::HEAP;
  82 ///
  83 /// fn main() {
  84 ///     let foo = box(HEAP) 5;
  85 ///     let foo = box 5;
  86 /// }
  87 /// ```
  88 #[lang = "exchange_heap"]
  89 #[unstable(feature = "box_heap",
  90            reason = "may be renamed; uncertain about custom allocator design")]
  91 #[allow(deprecated)]
  92 pub const HEAP: ExchangeHeapSingleton =
  93     ExchangeHeapSingleton { _force_singleton: () };
  94
  95 /// This the singleton type used solely for `boxed::HEAP`.
  96 #[unstable(feature = "box_heap",
  97            reason = "may be renamed; uncertain about custom allocator design")]
  98 #[derive(Copy, Clone)]
  99 pub struct ExchangeHeapSingleton { _force_singleton: () }
 100
 101 /// A pointer type for heap allocation.
 102 ///
 103 /// See the [module-level documentation](../../std/boxed/index.html) for more.
 104 #[lang = "owned_box"]
 105 #[stable(feature = "rust1", since = "1.0.0")]
 106 #[fundamental]
 107 pub struct Box<T: ?Sized>(Unique<T>);
 108
 109 /// `IntermediateBox` represents uninitialized backing storage for `Box`.
 110 ///
 111 /// FIXME (pnkfelix): Ideally we would just reuse `Box<T>` instead of
 112 /// introducing a separate `IntermediateBox<T>`; but then you hit
 113 /// issues when you e.g. attempt to destructure an instance of `Box`,
 114 /// since it is a lang item and so it gets special handling by the
 115 /// compiler.  Easier just to make this parallel type for now.
 116 ///
 117 /// FIXME (pnkfelix): Currently the `box` protocol only supports
 118 /// creating instances of sized types. This IntermediateBox is
 119 /// designed to be forward-compatible with a future protocol that
 120 /// supports creating instances of unsized types; that is why the type
 121 /// parameter has the `?Sized` generalization marker, and is also why
 122 /// this carries an explicit size. However, it probably does not need
 123 /// to carry the explicit alignment; that is just a work-around for
 124 /// the fact that the `align_of` intrinsic currently requires the
 125 /// input type to be Sized (which I do not think is strictly
 126 /// necessary).
 127 #[unstable(feature = "placement_in", reason = "placement box design is still being worked out.")]
 128 pub struct IntermediateBox<T: ?Sized>{
 129     ptr: *mut u8,
 130     size: usize,
 131     align: usize,
 132     marker: marker::PhantomData<*mut T>,
 133 }
 134
 135 impl<T> Place<T> for IntermediateBox<T> {
 136     fn pointer(&mut self) -> *mut T {
 137         unsafe { ::core::mem::transmute(self.ptr) }
 138     }
 139 }
 140
 141 unsafe fn finalize<T>(b: IntermediateBox<T>) -> Box<T> {
 142     let p = b.ptr as *mut T;
 143     mem::forget(b);
 144     mem::transmute(p)
 145 }
 146
 147 fn make_place<T>() -> IntermediateBox<T> {
 148     let size = mem::size_of::<T>();
 149     let align = mem::align_of::<T>();
 150
 151     let p = if size == 0 {
 152         heap::EMPTY as *mut u8
 153     } else {
 154         let p = unsafe {
 155             heap::allocate(size, align)
 156         };
 157         if p.is_null() {
 158             panic!("Box make_place allocation failure.");
 159         }
 160         p
 161     };
 162
 163     IntermediateBox { ptr: p, size: size, align: align, marker: marker::PhantomData }
 164 }
 165
 166 impl<T> BoxPlace<T> for IntermediateBox<T> {
 167     fn make_place() -> IntermediateBox<T> { make_place() }
 168 }
 169
 170 impl<T> InPlace<T> for IntermediateBox<T> {
 171     type Owner = Box<T>;
 172     unsafe fn finalize(self) -> Box<T> { finalize(self) }
 173 }
 174
 175 impl<T> Boxed for Box<T> {
 176     type Data = T;
 177     type Place = IntermediateBox<T>;
 178     unsafe fn finalize(b: IntermediateBox<T>) -> Box<T> { finalize(b) }
 179 }
 180
 181 impl<T> Placer<T> for ExchangeHeapSingleton {
 182     type Place = IntermediateBox<T>;
 183
 184     fn make_place(self) -> IntermediateBox<T> {
 185         make_place()
 186     }
 187 }
 188
 189 impl<T: ?Sized> Drop for IntermediateBox<T> {
 190     fn drop(&mut self) {
 191         if self.size > 0 {
 192             unsafe {
 193                 heap::deallocate(self.ptr, self.size, self.align)
 194             }
 195         }
 196     }
 197 }
 198
 199 impl<T> Box<T> {
 200     /// Allocates memory on the heap and then moves `x` into it.
 201     ///
 202     /// # Examples
 203     ///
 204     /// ```
 205     /// let x = Box::new(5);
 206     /// ```
 207     #[stable(feature = "rust1", since = "1.0.0")]
 208     #[inline(always)]
 209     pub fn new(x: T) -> Box<T> {
 210         box x
 211     }
 212 }
 213
 214 impl<T : ?Sized> Box<T> {
 215     /// Constructs a box from the raw pointer.
 216     ///
 217     /// After this function call, pointer is owned by resulting box.
 218     /// In particular, it means that `Box` destructor calls destructor
 219     /// of `T` and releases memory. Since the way `Box` allocates and
 220     /// releases memory is unspecified, the only valid pointer to pass
 221     /// to this function is the one taken from another `Box` with
 222     /// `Box::into_raw` function.
 223     ///
 224     /// Function is unsafe, because improper use of this function may
 225     /// lead to memory problems like double-free, for example if the
 226     /// function is called twice on the same raw pointer.
 227     #[unstable(feature = "box_raw",
 228                reason = "may be renamed or moved out of Box scope")]
 229     #[inline]
 230     // NB: may want to be called from_ptr, see comments on CStr::from_ptr
 231     pub unsafe fn from_raw(raw: *mut T) -> Self {
 232         mem::transmute(raw)
 233     }
 234
 235     /// Consumes the `Box`, returning the wrapped raw pointer.
 236     ///
 237     /// After call to this function, caller is responsible for the memory
 238     /// previously managed by `Box`, in particular caller should properly
 239     /// destroy `T` and release memory. The proper way to do it is to
 240     /// convert pointer back to `Box` with `Box::from_raw` function, because
 241     /// `Box` does not specify, how memory is allocated.
 242     ///
 243     /// # Examples
 244     /// ```
 245     /// #![feature(box_raw)]
 246     ///
 247     /// let seventeen = Box::new(17u32);
 248     /// let raw = Box::into_raw(seventeen);
 249     /// let boxed_again = unsafe { Box::from_raw(raw) };
 250     /// ```
 251     #[unstable(feature = "box_raw", reason = "may be renamed")]
 252     #[inline]
 253     // NB: may want to be called into_ptr, see comments on CStr::from_ptr
 254     pub fn into_raw(b: Box<T>) -> *mut T {
 255         unsafe { mem::transmute(b) }
 256     }
 257 }
 258
 259 /// Consumes the `Box`, returning the wrapped raw pointer.
 260 ///
 261 /// After call to this function, caller is responsible for the memory
 262 /// previously managed by `Box`, in particular caller should properly
 263 /// destroy `T` and release memory. The proper way to do it is to
 264 /// convert pointer back to `Box` with `Box::from_raw` function, because
 265 /// `Box` does not specify, how memory is allocated.
 266 ///
 267 /// # Examples
 268 /// ```
 269 /// #![feature(box_raw)]
 270 ///
 271 /// use std::boxed;
 272 ///
 273 /// let seventeen = Box::new(17u32);
 274 /// let raw = boxed::into_raw(seventeen);
 275 /// let boxed_again = unsafe { Box::from_raw(raw) };
 276 /// ```
 277 #[unstable(feature = "box_raw", reason = "may be renamed")]
 278 #[deprecated(since = "1.2.0", reason = "renamed to Box::into_raw")]
 279 #[inline]
 280 pub fn into_raw<T : ?Sized>(b: Box<T>) -> *mut T {
 281     Box::into_raw(b)
 282 }
 283
 284 #[stable(feature = "rust1", since = "1.0.0")]
 285 impl<T: Default> Default for Box<T> {
 286     #[stable(feature = "rust1", since = "1.0.0")]
 287     fn default() -> Box<T> { box Default::default() }
 288 }
 289
 290 #[stable(feature = "rust1", since = "1.0.0")]
 291 impl<T> Default for Box<[T]> {
 292     #[stable(feature = "rust1", since = "1.0.0")]
 293     fn default() -> Box<[T]> { Box::<[T; 0]>::new([]) }
 294 }
 295
 296 #[stable(feature = "rust1", since = "1.0.0")]
 297 impl<T: Clone> Clone for Box<T> {
 298     /// Returns a new box with a `clone()` of this box's contents.
 299     ///
 300     /// # Examples
 301     ///
 302     /// ```
 303     /// let x = Box::new(5);
 304     /// let y = x.clone();
 305     /// ```
 306     #[inline]
 307     fn clone(&self) -> Box<T> { box {(**self).clone()} }
 308     /// Copies `source`'s contents into `self` without creating a new allocation.
 309     ///
 310     /// # Examples
 311     ///
 312     /// ```
 313     /// #![feature(box_raw)]
 314     ///
 315     /// let x = Box::new(5);
 316     /// let mut y = Box::new(10);
 317     ///
 318     /// y.clone_from(&x);
 319     ///
 320     /// assert_eq!(*y, 5);
 321     /// ```
 322     #[inline]
 323     fn clone_from(&mut self, source: &Box<T>) {
 324         (**self).clone_from(&(**source));
 325     }
 326 }
 327
 328
 329 #[stable(feature = "box_slice_clone", since = "1.3.0")]
 330 impl Clone for Box<str> {
 331     fn clone(&self) -> Self {
 332         let len = self.len();
 333         let buf = RawVec::with_capacity(len);
 334         unsafe {
 335             ptr::copy_nonoverlapping(self.as_ptr(), buf.ptr(), len);
 336             mem::transmute(buf.into_box()) // bytes to str ~magic
 337         }
 338     }
 339 }
 340
 341 #[stable(feature = "rust1", since = "1.0.0")]
 342 impl<T: ?Sized + PartialEq> PartialEq for Box<T> {
 343     #[inline]
 344     fn eq(&self, other: &Box<T>) -> bool { PartialEq::eq(&**self, &**other) }
 345     #[inline]
 346     fn ne(&self, other: &Box<T>) -> bool { PartialEq::ne(&**self, &**other) }
 347 }
 348 #[stable(feature = "rust1", since = "1.0.0")]
 349 impl<T: ?Sized + PartialOrd> PartialOrd for Box<T> {
 350     #[inline]
 351     fn partial_cmp(&self, other: &Box<T>) -> Option<Ordering> {
 352         PartialOrd::partial_cmp(&**self, &**other)
 353     }
 354     #[inline]
 355     fn lt(&self, other: &Box<T>) -> bool { PartialOrd::lt(&**self, &**other) }
 356     #[inline]
 357     fn le(&self, other: &Box<T>) -> bool { PartialOrd::le(&**self, &**other) }
 358     #[inline]
 359     fn ge(&self, other: &Box<T>) -> bool { PartialOrd::ge(&**self, &**other) }
 360     #[inline]
 361     fn gt(&self, other: &Box<T>) -> bool { PartialOrd::gt(&**self, &**other) }
 362 }
 363 #[stable(feature = "rust1", since = "1.0.0")]
 364 impl<T: ?Sized + Ord> Ord for Box<T> {
 365     #[inline]
 366     fn cmp(&self, other: &Box<T>) -> Ordering {
 367         Ord::cmp(&**self, &**other)
 368     }
 369 }
 370 #[stable(feature = "rust1", since = "1.0.0")]
 371 impl<T: ?Sized + Eq> Eq for Box<T> {}
 372
 373 #[stable(feature = "rust1", since = "1.0.0")]
 374 impl<T: ?Sized + Hash> Hash for Box<T> {
 375     fn hash<H: hash::Hasher>(&self, state: &mut H) {
 376         (**self).hash(state);
 377     }
 378 }
 379
 380 impl Box<Any> {
 381     #[inline]
 382     #[stable(feature = "rust1", since = "1.0.0")]
 383     /// Attempt to downcast the box to a concrete type.
 384     pub fn downcast<T: Any>(self) -> Result<Box<T>, Box<Any>> {
 385         if self.is::<T>() {
 386             unsafe {
 387                 // Get the raw representation of the trait object
 388                 let raw = Box::into_raw(self);
 389                 let to: TraitObject =
 390                     mem::transmute::<*mut Any, TraitObject>(raw);
 391
 392                 // Extract the data pointer
 393                 Ok(Box::from_raw(to.data as *mut T))
 394             }
 395         } else {
 396             Err(self)
 397         }
 398     }
 399 }
 400
 401 impl Box<Any + Send> {
 402     #[inline]
 403     #[stable(feature = "rust1", since = "1.0.0")]
 404     /// Attempt to downcast the box to a concrete type.
 405     pub fn downcast<T: Any>(self) -> Result<Box<T>, Box<Any + Send>> {
 406         <Box<Any>>::downcast(self).map_err(|s| unsafe {
 407             // reapply the Send marker
 408             mem::transmute::<Box<Any>, Box<Any + Send>>(s)
 409         })
 410     }
 411 }
 412
 413 #[stable(feature = "rust1", since = "1.0.0")]
 414 impl<T: fmt::Display + ?Sized> fmt::Display for Box<T> {
 415     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
 416         fmt::Display::fmt(&**self, f)
 417     }
 418 }
 419
 420 #[stable(feature = "rust1", since = "1.0.0")]
 421 impl<T: fmt::Debug + ?Sized> fmt::Debug for Box<T> {
 422     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
 423         fmt::Debug::fmt(&**self, f)
 424     }
 425 }
 426
 427 #[stable(feature = "rust1", since = "1.0.0")]
 428 impl<T> fmt::Pointer for Box<T> {
 429     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
 430         // It's not possible to extract the inner Uniq directly from the Box,
 431         // instead we cast it to a *const which aliases the Unique
 432         let ptr: *const T = &**self;
 433         fmt::Pointer::fmt(&ptr, f)
 434     }
 435 }
 436
 437 #[stable(feature = "rust1", since = "1.0.0")]
 438 impl<T: ?Sized> Deref for Box<T> {
 439     type Target = T;
 440
 441     fn deref(&self) -> &T { &**self }
 442 }
 443
 444 #[stable(feature = "rust1", since = "1.0.0")]
 445 impl<T: ?Sized> DerefMut for Box<T> {
 446     fn deref_mut(&mut self) -> &mut T { &mut **self }
 447 }
 448
 449 #[stable(feature = "rust1", since = "1.0.0")]
 450 impl<I: Iterator + ?Sized> Iterator for Box<I> {
 451     type Item = I::Item;
 452     fn next(&mut self) -> Option<I::Item> { (**self).next() }
 453     fn size_hint(&self) -> (usize, Option<usize>) { (**self).size_hint() }
 454 }
 455 #[stable(feature = "rust1", since = "1.0.0")]
 456 impl<I: DoubleEndedIterator + ?Sized> DoubleEndedIterator for Box<I> {
 457     fn next_back(&mut self) -> Option<I::Item> { (**self).next_back() }
 458 }
 459 #[stable(feature = "rust1", since = "1.0.0")]
 460 impl<I: ExactSizeIterator + ?Sized> ExactSizeIterator for Box<I> {}
 461
 462
 463 /// `FnBox` is a version of the `FnOnce` intended for use with boxed
 464 /// closure objects. The idea is that where one would normally store a
 465 /// `Box<FnOnce()>` in a data structure, you should use
 466 /// `Box<FnBox()>`. The two traits behave essentially the same, except
 467 /// that a `FnBox` closure can only be called if it is boxed. (Note
 468 /// that `FnBox` may be deprecated in the future if `Box<FnOnce()>`
 469 /// closures become directly usable.)
 470 ///
 471 /// ### Example
 472 ///
 473 /// Here is a snippet of code which creates a hashmap full of boxed
 474 /// once closures and then removes them one by one, calling each
 475 /// closure as it is removed. Note that the type of the closures
 476 /// stored in the map is `Box<FnBox() -> i32>` and not `Box<FnOnce()
 477 /// -> i32>`.
 478 ///
 479 /// ```
 480 /// #![feature(fnbox)]
 481 ///
 482 /// use std::boxed::FnBox;
 483 /// use std::collections::HashMap;
 484 ///
 485 /// fn make_map() -> HashMap<i32, Box<FnBox() -> i32>> {
 486 ///     let mut map: HashMap<i32, Box<FnBox() -> i32>> = HashMap::new();
 487 ///     map.insert(1, Box::new(|| 22));
 488 ///     map.insert(2, Box::new(|| 44));
 489 ///     map
 490 /// }
 491 ///
 492 /// fn main() {
 493 ///     let mut map = make_map();
 494 ///     for i in &[1, 2] {
 495 ///         let f = map.remove(&i).unwrap();
 496 ///         assert_eq!(f(), i * 22);
 497 ///     }
 498 /// }
 499 /// ```
 500 #[rustc_paren_sugar]
 501 #[unstable(feature = "fnbox", reason = "Newly introduced")]
 502 pub trait FnBox<A> {
 503     type Output;
 504
 505     fn call_box(self: Box<Self>, args: A) -> Self::Output;
 506 }
 507
 508 impl<A,F> FnBox<A> for F
 509     where F: FnOnce<A>
 510 {
 511     type Output = F::Output;
 512
 513     fn call_box(self: Box<F>, args: A) -> F::Output {
 514         self.call_once(args)
 515     }
 516 }
 517
 518 impl<'a,A,R> FnOnce<A> for Box<FnBox<A,Output=R>+'a> {
 519     type Output = R;
 520
 521     extern "rust-call" fn call_once(self, args: A) -> R {
 522         self.call_box(args)
 523     }
 524 }
 525
 526 impl<'a,A,R> FnOnce<A> for Box<FnBox<A,Output=R>+Send+'a> {
 527     type Output = R;
 528
 529     extern "rust-call" fn call_once(self, args: A) -> R {
 530         self.call_box(args)
 531     }
 532 }
 533
 534 impl<T: ?Sized+Unsize<U>, U: ?Sized> CoerceUnsized<Box<U>> for Box<T> {}
 535
 536 #[stable(feature = "box_slice_clone", since = "1.3.0")]
 537 impl<T: Clone> Clone for Box<[T]> {
 538     fn clone(&self) -> Self {
 539         let mut new = BoxBuilder {
 540             data: RawVec::with_capacity(self.len()),
 541             len: 0
 542         };
 543
 544         let mut target = new.data.ptr();
 545
 546         for item in self.iter() {
 547             unsafe {
 548                 ptr::write(target, item.clone());
 549                 target = target.offset(1);
 550             };
 551
 552             new.len += 1;
 553         }
 554
 555         return unsafe { new.into_box() };
 556
 557         // Helper type for responding to panics correctly.
 558         struct BoxBuilder<T> {
 559             data: RawVec<T>,
 560             len: usize,
 561         }
 562
 563         impl<T> BoxBuilder<T> {
 564             unsafe fn into_box(self) -> Box<[T]> {
 565                 let raw = ptr::read(&self.data);
 566                 mem::forget(self);
 567                 raw.into_box()
 568             }
 569         }
 570
 571         impl<T> Drop for BoxBuilder<T> {
 572             fn drop(&mut self) {
 573                 let mut data = self.data.ptr();
 574                 let max = unsafe { data.offset(self.len as isize) };
 575
 576                 while data != max {
 577                     unsafe {
 578                         ptr::read(data);
 579                         data = data.offset(1);
 580                     }
 581                 }
 582             }
 583         }
 584     }
 585 }
 586