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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 heap;
57 use raw_vec::RawVec;
58
59 use core::any::Any;
60 use core::borrow;
61 use core::cmp::Ordering;
62 use core::fmt;
63 use core::hash::{self, Hash};
64 use core::marker::{self, Unsize};
65 use core::mem;
66 use core::ops::{CoerceUnsized, Deref, DerefMut};
67 use core::ops::{Placer, Boxed, Place, InPlace, BoxPlace};
68 use core::ptr::{self, Unique};
69 use core::raw::TraitObject;
70 use core::convert::From;
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<i32> = in HEAP { 5 };
85 /// let foo = box 5;
86 /// }
87 /// ```
88 #[unstable(feature = "box_heap",
89 reason = "may be renamed; uncertain about custom allocator design",
90 issue = "27779")]
91 pub const HEAP: ExchangeHeapSingleton = ExchangeHeapSingleton { _force_singleton: () };
92
93 /// This the singleton type used solely for `boxed::HEAP`.
94 #[unstable(feature = "box_heap",
95 reason = "may be renamed; uncertain about custom allocator design",
96 issue = "27779")]
97 #[derive(Copy, Clone)]
98 pub struct ExchangeHeapSingleton {
99 _force_singleton: (),
100 }
101
102 /// A pointer type for heap allocation.
103 ///
104 /// See the [module-level documentation](../../std/boxed/index.html) for more.
105 #[lang = "owned_box"]
106 #[stable(feature = "rust1", since = "1.0.0")]
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",
128 reason = "placement box design is still being worked out.",
129 issue = "27779")]
130 pub struct IntermediateBox<T: ?Sized> {
131 ptr: *mut u8,
132 size: usize,
133 align: usize,
134 marker: marker::PhantomData<*mut T>,
135 }
136
137 #[unstable(feature = "placement_in",
138 reason = "placement box design is still being worked out.",
139 issue = "27779")]
140 impl<T> Place<T> for IntermediateBox<T> {
141 fn pointer(&mut self) -> *mut T {
142 self.ptr as *mut T
143 }
144 }
145
146 unsafe fn finalize<T>(b: IntermediateBox<T>) -> Box<T> {
147 let p = b.ptr as *mut T;
148 mem::forget(b);
149 mem::transmute(p)
150 }
151
152 fn make_place<T>() -> IntermediateBox<T> {
153 let size = mem::size_of::<T>();
154 let align = mem::align_of::<T>();
155
156 let p = if size == 0 {
157 heap::EMPTY as *mut u8
158 } else {
159 let p = unsafe { heap::allocate(size, align) };
160 if p.is_null() {
161 panic!("Box make_place allocation failure.");
162 }
163 p
164 };
165
166 IntermediateBox {
167 ptr: p,
168 size: size,
169 align: align,
170 marker: marker::PhantomData,
171 }
172 }
173
174 #[unstable(feature = "placement_in",
175 reason = "placement box design is still being worked out.",
176 issue = "27779")]
177 impl<T> BoxPlace<T> for IntermediateBox<T> {
178 fn make_place() -> IntermediateBox<T> {
179 make_place()
180 }
181 }
182
183 #[unstable(feature = "placement_in",
184 reason = "placement box design is still being worked out.",
185 issue = "27779")]
186 impl<T> InPlace<T> for IntermediateBox<T> {
187 type Owner = Box<T>;
188 unsafe fn finalize(self) -> Box<T> {
189 finalize(self)
190 }
191 }
192
193 #[unstable(feature = "placement_new_protocol", issue = "27779")]
194 impl<T> Boxed for Box<T> {
195 type Data = T;
196 type Place = IntermediateBox<T>;
197 unsafe fn finalize(b: IntermediateBox<T>) -> Box<T> {
198 finalize(b)
199 }
200 }
201
202 #[unstable(feature = "placement_in",
203 reason = "placement box design is still being worked out.",
204 issue = "27779")]
205 impl<T> Placer<T> for ExchangeHeapSingleton {
206 type Place = IntermediateBox<T>;
207
208 fn make_place(self) -> IntermediateBox<T> {
209 make_place()
210 }
211 }
212
213 #[unstable(feature = "placement_in",
214 reason = "placement box design is still being worked out.",
215 issue = "27779")]
216 impl<T: ?Sized> Drop for IntermediateBox<T> {
217 fn drop(&mut self) {
218 if self.size > 0 {
219 unsafe { heap::deallocate(self.ptr, self.size, self.align) }
220 }
221 }
222 }
223
224 impl<T> Box<T> {
225 /// Allocates memory on the heap and then places `x` into it.
226 ///
227 /// # Examples
228 ///
229 /// ```
230 /// let five = Box::new(5);
231 /// ```
232 #[stable(feature = "rust1", since = "1.0.0")]
233 #[inline(always)]
234 pub fn new(x: T) -> Box<T> {
235 box x
236 }
237 }
238
239 impl<T: ?Sized> Box<T> {
240 /// Constructs a box from a raw pointer.
241 ///
242 /// After calling this function, the raw pointer is owned by the
243 /// resulting `Box`. Specifically, the `Box` destructor will call
244 /// the destructor of `T` and free the allocated memory. Since the
245 /// way `Box` allocates and releases memory is unspecified, the
246 /// only valid pointer to pass to this function is the one taken
247 /// from another `Box` via the `Box::into_raw` function.
248 ///
249 /// This function is unsafe because improper use may lead to
250 /// memory problems. For example, a double-free may occur if the
251 /// function is called twice on the same raw pointer.
252 #[stable(feature = "box_raw", since = "1.4.0")]
253 #[inline]
254 pub unsafe fn from_raw(raw: *mut T) -> Self {
255 mem::transmute(raw)
256 }
257
258 /// Consumes the `Box`, returning the wrapped raw pointer.
259 ///
260 /// After calling this function, the caller is responsible for the
261 /// memory previously managed by the `Box`. In particular, the
262 /// caller should properly destroy `T` and release the memory. The
263 /// proper way to do so is to convert the raw pointer back into a
264 /// `Box` with the `Box::from_raw` function.
265 ///
266 /// # Examples
267 ///
268 /// ```
269 /// let seventeen = Box::new(17);
270 /// let raw = Box::into_raw(seventeen);
271 /// let boxed_again = unsafe { Box::from_raw(raw) };
272 /// ```
273 #[stable(feature = "box_raw", since = "1.4.0")]
274 #[inline]
275 pub fn into_raw(b: Box<T>) -> *mut T {
276 unsafe { mem::transmute(b) }
277 }
278 }
279
280 #[stable(feature = "rust1", since = "1.0.0")]
281 impl<T: Default> Default for Box<T> {
282 fn default() -> Box<T> {
283 box Default::default()
284 }
285 }
286
287 #[stable(feature = "rust1", since = "1.0.0")]
288 impl<T> Default for Box<[T]> {
289 fn default() -> Box<[T]> {
290 Box::<[T; 0]>::new([])
291 }
292 }
293
294 #[stable(feature = "rust1", since = "1.0.0")]
295 impl<T: Clone> Clone for Box<T> {
296 /// Returns a new box with a `clone()` of this box's contents.
297 ///
298 /// # Examples
299 ///
300 /// ```
301 /// let x = Box::new(5);
302 /// let y = x.clone();
303 /// ```
304 #[rustfmt_skip]
305 #[inline]
306 fn clone(&self) -> Box<T> {
307 box { (**self).clone() }
308 }
309 /// Copies `source`'s contents into `self` without creating a new allocation.
310 ///
311 /// # Examples
312 ///
313 /// ```
314 /// let x = Box::new(5);
315 /// let mut y = Box::new(10);
316 ///
317 /// y.clone_from(&x);
318 ///
319 /// assert_eq!(*y, 5);
320 /// ```
321 #[inline]
322 fn clone_from(&mut self, source: &Box<T>) {
323 (**self).clone_from(&(**source));
324 }
325 }
326
327
328 #[stable(feature = "box_slice_clone", since = "1.3.0")]
329 impl Clone for Box<str> {
330 fn clone(&self) -> Self {
331 let len = self.len();
332 let buf = RawVec::with_capacity(len);
333 unsafe {
334 ptr::copy_nonoverlapping(self.as_ptr(), buf.ptr(), len);
335 mem::transmute(buf.into_box()) // bytes to str ~magic
336 }
337 }
338 }
339
340 #[stable(feature = "rust1", since = "1.0.0")]
341 impl<T: ?Sized + PartialEq> PartialEq for Box<T> {
342 #[inline]
343 fn eq(&self, other: &Box<T>) -> bool {
344 PartialEq::eq(&**self, &**other)
345 }
346 #[inline]
347 fn ne(&self, other: &Box<T>) -> bool {
348 PartialEq::ne(&**self, &**other)
349 }
350 }
351 #[stable(feature = "rust1", since = "1.0.0")]
352 impl<T: ?Sized + PartialOrd> PartialOrd for Box<T> {
353 #[inline]
354 fn partial_cmp(&self, other: &Box<T>) -> Option<Ordering> {
355 PartialOrd::partial_cmp(&**self, &**other)
356 }
357 #[inline]
358 fn lt(&self, other: &Box<T>) -> bool {
359 PartialOrd::lt(&**self, &**other)
360 }
361 #[inline]
362 fn le(&self, other: &Box<T>) -> bool {
363 PartialOrd::le(&**self, &**other)
364 }
365 #[inline]
366 fn ge(&self, other: &Box<T>) -> bool {
367 PartialOrd::ge(&**self, &**other)
368 }
369 #[inline]
370 fn gt(&self, other: &Box<T>) -> bool {
371 PartialOrd::gt(&**self, &**other)
372 }
373 }
374 #[stable(feature = "rust1", since = "1.0.0")]
375 impl<T: ?Sized + Ord> Ord for Box<T> {
376 #[inline]
377 fn cmp(&self, other: &Box<T>) -> Ordering {
378 Ord::cmp(&**self, &**other)
379 }
380 }
381 #[stable(feature = "rust1", since = "1.0.0")]
382 impl<T: ?Sized + Eq> Eq for Box<T> {}
383
384 #[stable(feature = "rust1", since = "1.0.0")]
385 impl<T: ?Sized + Hash> Hash for Box<T> {
386 fn hash<H: hash::Hasher>(&self, state: &mut H) {
387 (**self).hash(state);
388 }
389 }
390
391 #[stable(feature = "from_for_ptrs", since = "1.6.0")]
392 impl<T> From<T> for Box<T> {
393 fn from(t: T) -> Self {
394 Box::new(t)
395 }
396 }
397
398 impl Box<Any> {
399 #[inline]
400 #[stable(feature = "rust1", since = "1.0.0")]
401 /// Attempt to downcast the box to a concrete type.
402 pub fn downcast<T: Any>(self) -> Result<Box<T>, Box<Any>> {
403 if self.is::<T>() {
404 unsafe {
405 // Get the raw representation of the trait object
406 let raw = Box::into_raw(self);
407 let to: TraitObject = mem::transmute::<*mut Any, TraitObject>(raw);
408
409 // Extract the data pointer
410 Ok(Box::from_raw(to.data as *mut T))
411 }
412 } else {
413 Err(self)
414 }
415 }
416 }
417
418 impl Box<Any + Send> {
419 #[inline]
420 #[stable(feature = "rust1", since = "1.0.0")]
421 /// Attempt to downcast the box to a concrete type.
422 pub fn downcast<T: Any>(self) -> Result<Box<T>, Box<Any + Send>> {
423 <Box<Any>>::downcast(self).map_err(|s| unsafe {
424 // reapply the Send marker
425 mem::transmute::<Box<Any>, Box<Any + Send>>(s)
426 })
427 }
428 }
429
430 #[stable(feature = "rust1", since = "1.0.0")]
431 impl<T: fmt::Display + ?Sized> fmt::Display for Box<T> {
432 fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
433 fmt::Display::fmt(&**self, f)
434 }
435 }
436
437 #[stable(feature = "rust1", since = "1.0.0")]
438 impl<T: fmt::Debug + ?Sized> fmt::Debug for Box<T> {
439 fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
440 fmt::Debug::fmt(&**self, f)
441 }
442 }
443
444 #[stable(feature = "rust1", since = "1.0.0")]
445 impl<T: ?Sized> fmt::Pointer for Box<T> {
446 fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
447 // It's not possible to extract the inner Uniq directly from the Box,
448 // instead we cast it to a *const which aliases the Unique
449 let ptr: *const T = &**self;
450 fmt::Pointer::fmt(&ptr, f)
451 }
452 }
453
454 #[stable(feature = "rust1", since = "1.0.0")]
455 impl<T: ?Sized> Deref for Box<T> {
456 type Target = T;
457
458 fn deref(&self) -> &T {
459 &**self
460 }
461 }
462
463 #[stable(feature = "rust1", since = "1.0.0")]
464 impl<T: ?Sized> DerefMut for Box<T> {
465 fn deref_mut(&mut self) -> &mut T {
466 &mut **self
467 }
468 }
469
470 #[stable(feature = "rust1", since = "1.0.0")]
471 impl<I: Iterator + ?Sized> Iterator for Box<I> {
472 type Item = I::Item;
473 fn next(&mut self) -> Option<I::Item> {
474 (**self).next()
475 }
476 fn size_hint(&self) -> (usize, Option<usize>) {
477 (**self).size_hint()
478 }
479 }
480 #[stable(feature = "rust1", since = "1.0.0")]
481 impl<I: DoubleEndedIterator + ?Sized> DoubleEndedIterator for Box<I> {
482 fn next_back(&mut self) -> Option<I::Item> {
483 (**self).next_back()
484 }
485 }
486 #[stable(feature = "rust1", since = "1.0.0")]
487 impl<I: ExactSizeIterator + ?Sized> ExactSizeIterator for Box<I> {}
488
489
490 /// `FnBox` is a version of the `FnOnce` intended for use with boxed
491 /// closure objects. The idea is that where one would normally store a
492 /// `Box<FnOnce()>` in a data structure, you should use
493 /// `Box<FnBox()>`. The two traits behave essentially the same, except
494 /// that a `FnBox` closure can only be called if it is boxed. (Note
495 /// that `FnBox` may be deprecated in the future if `Box<FnOnce()>`
496 /// closures become directly usable.)
497 ///
498 /// ### Example
499 ///
500 /// Here is a snippet of code which creates a hashmap full of boxed
501 /// once closures and then removes them one by one, calling each
502 /// closure as it is removed. Note that the type of the closures
503 /// stored in the map is `Box<FnBox() -> i32>` and not `Box<FnOnce()
504 /// -> i32>`.
505 ///
506 /// ```
507 /// #![feature(fnbox)]
508 ///
509 /// use std::boxed::FnBox;
510 /// use std::collections::HashMap;
511 ///
512 /// fn make_map() -> HashMap<i32, Box<FnBox() -> i32>> {
513 /// let mut map: HashMap<i32, Box<FnBox() -> i32>> = HashMap::new();
514 /// map.insert(1, Box::new(|| 22));
515 /// map.insert(2, Box::new(|| 44));
516 /// map
517 /// }
518 ///
519 /// fn main() {
520 /// let mut map = make_map();
521 /// for i in &[1, 2] {
522 /// let f = map.remove(&i).unwrap();
523 /// assert_eq!(f(), i * 22);
524 /// }
525 /// }
526 /// ```
527 #[rustc_paren_sugar]
528 #[unstable(feature = "fnbox", reason = "Newly introduced", issue = "28796")]
529 pub trait FnBox<A> {
530 type Output;
531
532 fn call_box(self: Box<Self>, args: A) -> Self::Output;
533 }
534
535 #[unstable(feature = "fnbox", reason = "Newly introduced", issue = "28796")]
536 impl<A, F> FnBox<A> for F where F: FnOnce<A>
537 {
538 type Output = F::Output;
539
540 fn call_box(self: Box<F>, args: A) -> F::Output {
541 self.call_once(args)
542 }
543 }
544
545 #[unstable(feature = "fnbox", reason = "Newly introduced", issue = "28796")]
546 impl<'a, A, R> FnOnce<A> for Box<FnBox<A, Output = R> + 'a> {
547 type Output = R;
548
549 extern "rust-call" fn call_once(self, args: A) -> R {
550 self.call_box(args)
551 }
552 }
553
554 #[unstable(feature = "fnbox", reason = "Newly introduced", issue = "28796")]
555 impl<'a, A, R> FnOnce<A> for Box<FnBox<A, Output = R> + Send + 'a> {
556 type Output = R;
557
558 extern "rust-call" fn call_once(self, args: A) -> R {
559 self.call_box(args)
560 }
561 }
562
563 #[unstable(feature = "coerce_unsized", issue = "27732")]
564 impl<T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<Box<U>> for Box<T> {}
565
566 #[stable(feature = "box_slice_clone", since = "1.3.0")]
567 impl<T: Clone> Clone for Box<[T]> {
568 fn clone(&self) -> Self {
569 let mut new = BoxBuilder {
570 data: RawVec::with_capacity(self.len()),
571 len: 0,
572 };
573
574 let mut target = new.data.ptr();
575
576 for item in self.iter() {
577 unsafe {
578 ptr::write(target, item.clone());
579 target = target.offset(1);
580 };
581
582 new.len += 1;
583 }
584
585 return unsafe { new.into_box() };
586
587 // Helper type for responding to panics correctly.
588 struct BoxBuilder<T> {
589 data: RawVec<T>,
590 len: usize,
591 }
592
593 impl<T> BoxBuilder<T> {
594 unsafe fn into_box(self) -> Box<[T]> {
595 let raw = ptr::read(&self.data);
596 mem::forget(self);
597 raw.into_box()
598 }
599 }
600
601 impl<T> Drop for BoxBuilder<T> {
602 fn drop(&mut self) {
603 let mut data = self.data.ptr();
604 let max = unsafe { data.offset(self.len as isize) };
605
606 while data != max {
607 unsafe {
608 ptr::read(data);
609 data = data.offset(1);
610 }
611 }
612 }
613 }
614 }
615 }
616
617 #[stable(feature = "rust1", since = "1.0.0")]
618 impl<T: ?Sized> borrow::Borrow<T> for Box<T> {
619 fn borrow(&self) -> &T {
620 &**self
621 }
622 }
623
624 #[stable(feature = "rust1", since = "1.0.0")]
625 impl<T: ?Sized> borrow::BorrowMut<T> for Box<T> {
626 fn borrow_mut(&mut self) -> &mut T {
627 &mut **self
628 }
629 }
630
631 #[stable(since = "1.5.0", feature = "smart_ptr_as_ref")]
632 impl<T: ?Sized> AsRef<T> for Box<T> {
633 fn as_ref(&self) -> &T {
634 &**self
635 }
636 }
637
638 #[stable(since = "1.5.0", feature = "smart_ptr_as_ref")]
639 impl<T: ?Sized> AsMut<T> for Box<T> {
640 fn as_mut(&mut self) -> &mut T {
641 &mut **self
642 }
643 }