1 //! A pointer type for heap allocation.
3 //! [`Box<T>`], casually referred to as a 'box', provides the simplest form of
4 //! heap allocation in Rust. Boxes provide ownership for this allocation, and
5 //! drop their contents when they go out of scope. Boxes also ensure that they
6 //! never allocate more than `isize::MAX` bytes.
10 //! Move a value from the stack to the heap by creating a [`Box`]:
14 //! let boxed: Box<u8> = Box::new(val);
17 //! Move a value from a [`Box`] back to the stack by [dereferencing]:
20 //! let boxed: Box<u8> = Box::new(5);
21 //! let val: u8 = *boxed;
24 //! Creating a recursive data structure:
29 //! Cons(T, Box<List<T>>),
33 //! let list: List<i32> = List::Cons(1, Box::new(List::Cons(2, Box::new(List::Nil))));
34 //! println!("{:?}", list);
37 //! This will print `Cons(1, Cons(2, Nil))`.
39 //! Recursive structures must be boxed, because if the definition of `Cons`
42 //! ```compile_fail,E0072
48 //! It wouldn't work. This is because the size of a `List` depends on how many
49 //! elements are in the list, and so we don't know how much memory to allocate
50 //! for a `Cons`. By introducing a [`Box<T>`], which has a defined size, we know how
51 //! big `Cons` needs to be.
55 //! For non-zero-sized values, a [`Box`] will use the [`Global`] allocator for
56 //! its allocation. It is valid to convert both ways between a [`Box`] and a
57 //! raw pointer allocated with the [`Global`] allocator, given that the
58 //! [`Layout`] used with the allocator is correct for the type. More precisely,
59 //! a `value: *mut T` that has been allocated with the [`Global`] allocator
60 //! with `Layout::for_value(&*value)` may be converted into a box using
61 //! [`Box::<T>::from_raw(value)`]. Conversely, the memory backing a `value: *mut
62 //! T` obtained from [`Box::<T>::into_raw`] may be deallocated using the
63 //! [`Global`] allocator with [`Layout::for_value(&*value)`].
65 //! For zero-sized values, the `Box` pointer still has to be [valid] for reads
66 //! and writes and sufficiently aligned. In particular, casting any aligned
67 //! non-zero integer literal to a raw pointer produces a valid pointer, but a
68 //! pointer pointing into previously allocated memory that since got freed is
69 //! not valid. The recommended way to build a Box to a ZST if `Box::new` cannot
70 //! be used is to use [`ptr::NonNull::dangling`].
72 //! So long as `T: Sized`, a `Box<T>` is guaranteed to be represented
73 //! as a single pointer and is also ABI-compatible with C pointers
74 //! (i.e. the C type `T*`). This means that if you have extern "C"
75 //! Rust functions that will be called from C, you can define those
76 //! Rust functions using `Box<T>` types, and use `T*` as corresponding
77 //! type on the C side. As an example, consider this C header which
78 //! declares functions that create and destroy some kind of `Foo`
84 //! /* Returns ownership to the caller */
85 //! struct Foo* foo_new(void);
87 //! /* Takes ownership from the caller; no-op when invoked with null */
88 //! void foo_delete(struct Foo*);
91 //! These two functions might be implemented in Rust as follows. Here, the
92 //! `struct Foo*` type from C is translated to `Box<Foo>`, which captures
93 //! the ownership constraints. Note also that the nullable argument to
94 //! `foo_delete` is represented in Rust as `Option<Box<Foo>>`, since `Box<Foo>`
102 //! pub extern "C" fn foo_new() -> Box<Foo> {
107 //! pub extern "C" fn foo_delete(_: Option<Box<Foo>>) {}
110 //! Even though `Box<T>` has the same representation and C ABI as a C pointer,
111 //! this does not mean that you can convert an arbitrary `T*` into a `Box<T>`
112 //! and expect things to work. `Box<T>` values will always be fully aligned,
113 //! non-null pointers. Moreover, the destructor for `Box<T>` will attempt to
114 //! free the value with the global allocator. In general, the best practice
115 //! is to only use `Box<T>` for pointers that originated from the global
118 //! **Important.** At least at present, you should avoid using
119 //! `Box<T>` types for functions that are defined in C but invoked
120 //! from Rust. In those cases, you should directly mirror the C types
121 //! as closely as possible. Using types like `Box<T>` where the C
122 //! definition is just using `T*` can lead to undefined behavior, as
123 //! described in [rust-lang/unsafe-code-guidelines#198][ucg#198].
125 //! [ucg#198]: https://github.com/rust-lang/unsafe-code-guidelines/issues/198
126 //! [dereferencing]: core::ops::Deref
127 //! [`Box::<T>::from_raw(value)`]: Box::from_raw
128 //! [`Global`]: crate::alloc::Global
129 //! [`Layout`]: crate::alloc::Layout
130 //! [`Layout::for_value(&*value)`]: crate::alloc::Layout::for_value
131 //! [valid]: ptr#safety
133 #![stable(feature = "rust1", since = "1.0.0")]
137 use core::cmp::Ordering;
138 use core::convert::{From, TryFrom};
140 use core::future::Future;
141 use core::hash::{Hash, Hasher};
142 #[cfg(not(no_global_oom_handling))]
143 use core::iter::FromIterator;
144 use core::iter::{FusedIterator, Iterator};
145 use core::marker::{Unpin, Unsize};
148 CoerceUnsized, Deref, DerefMut, DispatchFromDyn, Generator, GeneratorState, Receiver,
151 use core::ptr::{self, Unique};
152 use core::stream::Stream;
153 use core::task::{Context, Poll};
155 #[cfg(not(no_global_oom_handling))]
156 use crate::alloc::{handle_alloc_error, WriteCloneIntoRaw};
157 use crate::alloc::{AllocError, Allocator, Global, Layout};
158 #[cfg(not(no_global_oom_handling))]
159 use crate::borrow::Cow;
160 use crate::raw_vec::RawVec;
161 #[cfg(not(no_global_oom_handling))]
162 use crate::str::from_boxed_utf8_unchecked;
163 #[cfg(not(no_global_oom_handling))]
166 /// A pointer type for heap allocation.
168 /// See the [module-level documentation](../../std/boxed/index.html) for more.
169 #[lang = "owned_box"]
171 #[stable(feature = "rust1", since = "1.0.0")]
174 #[unstable(feature = "allocator_api", issue = "32838")] A: Allocator = Global,
178 /// Allocates memory on the heap and then places `x` into it.
180 /// This doesn't actually allocate if `T` is zero-sized.
185 /// let five = Box::new(5);
187 #[cfg(not(no_global_oom_handling))]
189 #[stable(feature = "rust1", since = "1.0.0")]
190 pub fn new(x: T) -> Self {
194 /// Constructs a new box with uninitialized contents.
199 /// #![feature(new_uninit)]
201 /// let mut five = Box::<u32>::new_uninit();
203 /// let five = unsafe {
204 /// // Deferred initialization:
205 /// five.as_mut_ptr().write(5);
207 /// five.assume_init()
210 /// assert_eq!(*five, 5)
212 #[cfg(not(no_global_oom_handling))]
213 #[unstable(feature = "new_uninit", issue = "63291")]
215 pub fn new_uninit() -> Box<mem::MaybeUninit<T>> {
216 Self::new_uninit_in(Global)
219 /// Constructs a new `Box` with uninitialized contents, with the memory
220 /// being filled with `0` bytes.
222 /// See [`MaybeUninit::zeroed`][zeroed] for examples of correct and incorrect usage
228 /// #![feature(new_uninit)]
230 /// let zero = Box::<u32>::new_zeroed();
231 /// let zero = unsafe { zero.assume_init() };
233 /// assert_eq!(*zero, 0)
236 /// [zeroed]: mem::MaybeUninit::zeroed
237 #[cfg(not(no_global_oom_handling))]
239 #[unstable(feature = "new_uninit", issue = "63291")]
240 pub fn new_zeroed() -> Box<mem::MaybeUninit<T>> {
241 Self::new_zeroed_in(Global)
244 /// Constructs a new `Pin<Box<T>>`. If `T` does not implement `Unpin`, then
245 /// `x` will be pinned in memory and unable to be moved.
246 #[cfg(not(no_global_oom_handling))]
247 #[stable(feature = "pin", since = "1.33.0")]
249 pub fn pin(x: T) -> Pin<Box<T>> {
253 /// Allocates memory on the heap then places `x` into it,
254 /// returning an error if the allocation fails
256 /// This doesn't actually allocate if `T` is zero-sized.
261 /// #![feature(allocator_api)]
263 /// let five = Box::try_new(5)?;
264 /// # Ok::<(), std::alloc::AllocError>(())
266 #[unstable(feature = "allocator_api", issue = "32838")]
268 pub fn try_new(x: T) -> Result<Self, AllocError> {
269 Self::try_new_in(x, Global)
272 /// Constructs a new box with uninitialized contents on the heap,
273 /// returning an error if the allocation fails
278 /// #![feature(allocator_api, new_uninit)]
280 /// let mut five = Box::<u32>::try_new_uninit()?;
282 /// let five = unsafe {
283 /// // Deferred initialization:
284 /// five.as_mut_ptr().write(5);
286 /// five.assume_init()
289 /// assert_eq!(*five, 5);
290 /// # Ok::<(), std::alloc::AllocError>(())
292 #[unstable(feature = "allocator_api", issue = "32838")]
293 // #[unstable(feature = "new_uninit", issue = "63291")]
295 pub fn try_new_uninit() -> Result<Box<mem::MaybeUninit<T>>, AllocError> {
296 Box::try_new_uninit_in(Global)
299 /// Constructs a new `Box` with uninitialized contents, with the memory
300 /// being filled with `0` bytes on the heap
302 /// See [`MaybeUninit::zeroed`][zeroed] for examples of correct and incorrect usage
308 /// #![feature(allocator_api, new_uninit)]
310 /// let zero = Box::<u32>::try_new_zeroed()?;
311 /// let zero = unsafe { zero.assume_init() };
313 /// assert_eq!(*zero, 0);
314 /// # Ok::<(), std::alloc::AllocError>(())
317 /// [zeroed]: mem::MaybeUninit::zeroed
318 #[unstable(feature = "allocator_api", issue = "32838")]
319 // #[unstable(feature = "new_uninit", issue = "63291")]
321 pub fn try_new_zeroed() -> Result<Box<mem::MaybeUninit<T>>, AllocError> {
322 Box::try_new_zeroed_in(Global)
326 impl<T, A: Allocator> Box<T, A> {
327 /// Allocates memory in the given allocator then places `x` into it.
329 /// This doesn't actually allocate if `T` is zero-sized.
334 /// #![feature(allocator_api)]
336 /// use std::alloc::System;
338 /// let five = Box::new_in(5, System);
340 #[cfg(not(no_global_oom_handling))]
341 #[unstable(feature = "allocator_api", issue = "32838")]
343 pub fn new_in(x: T, alloc: A) -> Self {
344 let mut boxed = Self::new_uninit_in(alloc);
346 boxed.as_mut_ptr().write(x);
351 /// Allocates memory in the given allocator then places `x` into it,
352 /// returning an error if the allocation fails
354 /// This doesn't actually allocate if `T` is zero-sized.
359 /// #![feature(allocator_api)]
361 /// use std::alloc::System;
363 /// let five = Box::try_new_in(5, System)?;
364 /// # Ok::<(), std::alloc::AllocError>(())
366 #[unstable(feature = "allocator_api", issue = "32838")]
368 pub fn try_new_in(x: T, alloc: A) -> Result<Self, AllocError> {
369 let mut boxed = Self::try_new_uninit_in(alloc)?;
371 boxed.as_mut_ptr().write(x);
372 Ok(boxed.assume_init())
376 /// Constructs a new box with uninitialized contents in the provided allocator.
381 /// #![feature(allocator_api, new_uninit)]
383 /// use std::alloc::System;
385 /// let mut five = Box::<u32, _>::new_uninit_in(System);
387 /// let five = unsafe {
388 /// // Deferred initialization:
389 /// five.as_mut_ptr().write(5);
391 /// five.assume_init()
394 /// assert_eq!(*five, 5)
396 #[unstable(feature = "allocator_api", issue = "32838")]
397 #[cfg(not(no_global_oom_handling))]
398 // #[unstable(feature = "new_uninit", issue = "63291")]
399 pub fn new_uninit_in(alloc: A) -> Box<mem::MaybeUninit<T>, A> {
400 let layout = Layout::new::<mem::MaybeUninit<T>>();
401 // NOTE: Prefer match over unwrap_or_else since closure sometimes not inlineable.
402 // That would make code size bigger.
403 match Box::try_new_uninit_in(alloc) {
405 Err(_) => handle_alloc_error(layout),
409 /// Constructs a new box with uninitialized contents in the provided allocator,
410 /// returning an error if the allocation fails
415 /// #![feature(allocator_api, new_uninit)]
417 /// use std::alloc::System;
419 /// let mut five = Box::<u32, _>::try_new_uninit_in(System)?;
421 /// let five = unsafe {
422 /// // Deferred initialization:
423 /// five.as_mut_ptr().write(5);
425 /// five.assume_init()
428 /// assert_eq!(*five, 5);
429 /// # Ok::<(), std::alloc::AllocError>(())
431 #[unstable(feature = "allocator_api", issue = "32838")]
432 // #[unstable(feature = "new_uninit", issue = "63291")]
433 pub fn try_new_uninit_in(alloc: A) -> Result<Box<mem::MaybeUninit<T>, A>, AllocError> {
434 let layout = Layout::new::<mem::MaybeUninit<T>>();
435 let ptr = alloc.allocate(layout)?.cast();
436 unsafe { Ok(Box::from_raw_in(ptr.as_ptr(), alloc)) }
439 /// Constructs a new `Box` with uninitialized contents, with the memory
440 /// being filled with `0` bytes in the provided allocator.
442 /// See [`MaybeUninit::zeroed`][zeroed] for examples of correct and incorrect usage
448 /// #![feature(allocator_api, new_uninit)]
450 /// use std::alloc::System;
452 /// let zero = Box::<u32, _>::new_zeroed_in(System);
453 /// let zero = unsafe { zero.assume_init() };
455 /// assert_eq!(*zero, 0)
458 /// [zeroed]: mem::MaybeUninit::zeroed
459 #[unstable(feature = "allocator_api", issue = "32838")]
460 #[cfg(not(no_global_oom_handling))]
461 // #[unstable(feature = "new_uninit", issue = "63291")]
462 pub fn new_zeroed_in(alloc: A) -> Box<mem::MaybeUninit<T>, A> {
463 let layout = Layout::new::<mem::MaybeUninit<T>>();
464 // NOTE: Prefer match over unwrap_or_else since closure sometimes not inlineable.
465 // That would make code size bigger.
466 match Box::try_new_zeroed_in(alloc) {
468 Err(_) => handle_alloc_error(layout),
472 /// Constructs a new `Box` with uninitialized contents, with the memory
473 /// being filled with `0` bytes in the provided allocator,
474 /// returning an error if the allocation fails,
476 /// See [`MaybeUninit::zeroed`][zeroed] for examples of correct and incorrect usage
482 /// #![feature(allocator_api, new_uninit)]
484 /// use std::alloc::System;
486 /// let zero = Box::<u32, _>::try_new_zeroed_in(System)?;
487 /// let zero = unsafe { zero.assume_init() };
489 /// assert_eq!(*zero, 0);
490 /// # Ok::<(), std::alloc::AllocError>(())
493 /// [zeroed]: mem::MaybeUninit::zeroed
494 #[unstable(feature = "allocator_api", issue = "32838")]
495 // #[unstable(feature = "new_uninit", issue = "63291")]
496 pub fn try_new_zeroed_in(alloc: A) -> Result<Box<mem::MaybeUninit<T>, A>, AllocError> {
497 let layout = Layout::new::<mem::MaybeUninit<T>>();
498 let ptr = alloc.allocate_zeroed(layout)?.cast();
499 unsafe { Ok(Box::from_raw_in(ptr.as_ptr(), alloc)) }
502 /// Constructs a new `Pin<Box<T, A>>`. If `T` does not implement `Unpin`, then
503 /// `x` will be pinned in memory and unable to be moved.
504 #[cfg(not(no_global_oom_handling))]
505 #[unstable(feature = "allocator_api", issue = "32838")]
507 pub fn pin_in(x: T, alloc: A) -> Pin<Self>
511 Self::new_in(x, alloc).into()
514 /// Converts a `Box<T>` into a `Box<[T]>`
516 /// This conversion does not allocate on the heap and happens in place.
517 #[unstable(feature = "box_into_boxed_slice", issue = "71582")]
518 pub fn into_boxed_slice(boxed: Self) -> Box<[T], A> {
519 let (raw, alloc) = Box::into_raw_with_allocator(boxed);
520 unsafe { Box::from_raw_in(raw as *mut [T; 1], alloc) }
523 /// Consumes the `Box`, returning the wrapped value.
528 /// #![feature(box_into_inner)]
530 /// let c = Box::new(5);
532 /// assert_eq!(Box::into_inner(c), 5);
534 #[unstable(feature = "box_into_inner", issue = "80437")]
536 pub fn into_inner(boxed: Self) -> T {
542 /// Constructs a new boxed slice with uninitialized contents.
547 /// #![feature(new_uninit)]
549 /// let mut values = Box::<[u32]>::new_uninit_slice(3);
551 /// let values = unsafe {
552 /// // Deferred initialization:
553 /// values[0].as_mut_ptr().write(1);
554 /// values[1].as_mut_ptr().write(2);
555 /// values[2].as_mut_ptr().write(3);
557 /// values.assume_init()
560 /// assert_eq!(*values, [1, 2, 3])
562 #[cfg(not(no_global_oom_handling))]
563 #[unstable(feature = "new_uninit", issue = "63291")]
564 pub fn new_uninit_slice(len: usize) -> Box<[mem::MaybeUninit<T>]> {
565 unsafe { RawVec::with_capacity(len).into_box(len) }
568 /// Constructs a new boxed slice with uninitialized contents, with the memory
569 /// being filled with `0` bytes.
571 /// See [`MaybeUninit::zeroed`][zeroed] for examples of correct and incorrect usage
577 /// #![feature(new_uninit)]
579 /// let values = Box::<[u32]>::new_zeroed_slice(3);
580 /// let values = unsafe { values.assume_init() };
582 /// assert_eq!(*values, [0, 0, 0])
585 /// [zeroed]: mem::MaybeUninit::zeroed
586 #[cfg(not(no_global_oom_handling))]
587 #[unstable(feature = "new_uninit", issue = "63291")]
588 pub fn new_zeroed_slice(len: usize) -> Box<[mem::MaybeUninit<T>]> {
589 unsafe { RawVec::with_capacity_zeroed(len).into_box(len) }
592 /// Constructs a new boxed slice with uninitialized contents. Returns an error if
593 /// the allocation fails
598 /// #![feature(allocator_api, new_uninit)]
600 /// let mut values = Box::<[u32]>::try_new_uninit_slice(3)?;
601 /// let values = unsafe {
602 /// // Deferred initialization:
603 /// values[0].as_mut_ptr().write(1);
604 /// values[1].as_mut_ptr().write(2);
605 /// values[2].as_mut_ptr().write(3);
606 /// values.assume_init()
609 /// assert_eq!(*values, [1, 2, 3]);
610 /// # Ok::<(), std::alloc::AllocError>(())
612 #[unstable(feature = "allocator_api", issue = "32838")]
614 pub fn try_new_uninit_slice(len: usize) -> Result<Box<[mem::MaybeUninit<T>]>, AllocError> {
616 let layout = match Layout::array::<mem::MaybeUninit<T>>(len) {
618 Err(_) => return Err(AllocError),
620 let ptr = Global.allocate(layout)?;
621 Ok(RawVec::from_raw_parts_in(ptr.as_mut_ptr() as *mut _, len, Global).into_box(len))
625 /// Constructs a new boxed slice with uninitialized contents, with the memory
626 /// being filled with `0` bytes. Returns an error if the allocation fails
628 /// See [`MaybeUninit::zeroed`][zeroed] for examples of correct and incorrect usage
634 /// #![feature(allocator_api, new_uninit)]
636 /// let values = Box::<[u32]>::try_new_zeroed_slice(3)?;
637 /// let values = unsafe { values.assume_init() };
639 /// assert_eq!(*values, [0, 0, 0]);
640 /// # Ok::<(), std::alloc::AllocError>(())
643 /// [zeroed]: mem::MaybeUninit::zeroed
644 #[unstable(feature = "allocator_api", issue = "32838")]
646 pub fn try_new_zeroed_slice(len: usize) -> Result<Box<[mem::MaybeUninit<T>]>, AllocError> {
648 let layout = match Layout::array::<mem::MaybeUninit<T>>(len) {
650 Err(_) => return Err(AllocError),
652 let ptr = Global.allocate_zeroed(layout)?;
653 Ok(RawVec::from_raw_parts_in(ptr.as_mut_ptr() as *mut _, len, Global).into_box(len))
658 impl<T, A: Allocator> Box<[T], A> {
659 /// Constructs a new boxed slice with uninitialized contents in the provided allocator.
664 /// #![feature(allocator_api, new_uninit)]
666 /// use std::alloc::System;
668 /// let mut values = Box::<[u32], _>::new_uninit_slice_in(3, System);
670 /// let values = unsafe {
671 /// // Deferred initialization:
672 /// values[0].as_mut_ptr().write(1);
673 /// values[1].as_mut_ptr().write(2);
674 /// values[2].as_mut_ptr().write(3);
676 /// values.assume_init()
679 /// assert_eq!(*values, [1, 2, 3])
681 #[cfg(not(no_global_oom_handling))]
682 #[unstable(feature = "allocator_api", issue = "32838")]
683 // #[unstable(feature = "new_uninit", issue = "63291")]
684 pub fn new_uninit_slice_in(len: usize, alloc: A) -> Box<[mem::MaybeUninit<T>], A> {
685 unsafe { RawVec::with_capacity_in(len, alloc).into_box(len) }
688 /// Constructs a new boxed slice with uninitialized contents in the provided allocator,
689 /// with the memory being filled with `0` bytes.
691 /// See [`MaybeUninit::zeroed`][zeroed] for examples of correct and incorrect usage
697 /// #![feature(allocator_api, new_uninit)]
699 /// use std::alloc::System;
701 /// let values = Box::<[u32], _>::new_zeroed_slice_in(3, System);
702 /// let values = unsafe { values.assume_init() };
704 /// assert_eq!(*values, [0, 0, 0])
707 /// [zeroed]: mem::MaybeUninit::zeroed
708 #[cfg(not(no_global_oom_handling))]
709 #[unstable(feature = "allocator_api", issue = "32838")]
710 // #[unstable(feature = "new_uninit", issue = "63291")]
711 pub fn new_zeroed_slice_in(len: usize, alloc: A) -> Box<[mem::MaybeUninit<T>], A> {
712 unsafe { RawVec::with_capacity_zeroed_in(len, alloc).into_box(len) }
716 impl<T, A: Allocator> Box<mem::MaybeUninit<T>, A> {
717 /// Converts to `Box<T, A>`.
721 /// As with [`MaybeUninit::assume_init`],
722 /// it is up to the caller to guarantee that the value
723 /// really is in an initialized state.
724 /// Calling this when the content is not yet fully initialized
725 /// causes immediate undefined behavior.
727 /// [`MaybeUninit::assume_init`]: mem::MaybeUninit::assume_init
732 /// #![feature(new_uninit)]
734 /// let mut five = Box::<u32>::new_uninit();
736 /// let five: Box<u32> = unsafe {
737 /// // Deferred initialization:
738 /// five.as_mut_ptr().write(5);
740 /// five.assume_init()
743 /// assert_eq!(*five, 5)
745 #[unstable(feature = "new_uninit", issue = "63291")]
747 pub unsafe fn assume_init(self) -> Box<T, A> {
748 let (raw, alloc) = Box::into_raw_with_allocator(self);
749 unsafe { Box::from_raw_in(raw as *mut T, alloc) }
753 impl<T, A: Allocator> Box<[mem::MaybeUninit<T>], A> {
754 /// Converts to `Box<[T], A>`.
758 /// As with [`MaybeUninit::assume_init`],
759 /// it is up to the caller to guarantee that the values
760 /// really are in an initialized state.
761 /// Calling this when the content is not yet fully initialized
762 /// causes immediate undefined behavior.
764 /// [`MaybeUninit::assume_init`]: mem::MaybeUninit::assume_init
769 /// #![feature(new_uninit)]
771 /// let mut values = Box::<[u32]>::new_uninit_slice(3);
773 /// let values = unsafe {
774 /// // Deferred initialization:
775 /// values[0].as_mut_ptr().write(1);
776 /// values[1].as_mut_ptr().write(2);
777 /// values[2].as_mut_ptr().write(3);
779 /// values.assume_init()
782 /// assert_eq!(*values, [1, 2, 3])
784 #[unstable(feature = "new_uninit", issue = "63291")]
786 pub unsafe fn assume_init(self) -> Box<[T], A> {
787 let (raw, alloc) = Box::into_raw_with_allocator(self);
788 unsafe { Box::from_raw_in(raw as *mut [T], alloc) }
792 impl<T: ?Sized> Box<T> {
793 /// Constructs a box from a raw pointer.
795 /// After calling this function, the raw pointer is owned by the
796 /// resulting `Box`. Specifically, the `Box` destructor will call
797 /// the destructor of `T` and free the allocated memory. For this
798 /// to be safe, the memory must have been allocated in accordance
799 /// with the [memory layout] used by `Box` .
803 /// This function is unsafe because improper use may lead to
804 /// memory problems. For example, a double-free may occur if the
805 /// function is called twice on the same raw pointer.
807 /// The safety conditions are described in the [memory layout] section.
811 /// Recreate a `Box` which was previously converted to a raw pointer
812 /// using [`Box::into_raw`]:
814 /// let x = Box::new(5);
815 /// let ptr = Box::into_raw(x);
816 /// let x = unsafe { Box::from_raw(ptr) };
818 /// Manually create a `Box` from scratch by using the global allocator:
820 /// use std::alloc::{alloc, Layout};
823 /// let ptr = alloc(Layout::new::<i32>()) as *mut i32;
824 /// // In general .write is required to avoid attempting to destruct
825 /// // the (uninitialized) previous contents of `ptr`, though for this
826 /// // simple example `*ptr = 5` would have worked as well.
828 /// let x = Box::from_raw(ptr);
832 /// [memory layout]: self#memory-layout
833 /// [`Layout`]: crate::Layout
834 #[stable(feature = "box_raw", since = "1.4.0")]
836 pub unsafe fn from_raw(raw: *mut T) -> Self {
837 unsafe { Self::from_raw_in(raw, Global) }
841 impl<T: ?Sized, A: Allocator> Box<T, A> {
842 /// Constructs a box from a raw pointer in the given allocator.
844 /// After calling this function, the raw pointer is owned by the
845 /// resulting `Box`. Specifically, the `Box` destructor will call
846 /// the destructor of `T` and free the allocated memory. For this
847 /// to be safe, the memory must have been allocated in accordance
848 /// with the [memory layout] used by `Box` .
852 /// This function is unsafe because improper use may lead to
853 /// memory problems. For example, a double-free may occur if the
854 /// function is called twice on the same raw pointer.
859 /// Recreate a `Box` which was previously converted to a raw pointer
860 /// using [`Box::into_raw_with_allocator`]:
862 /// #![feature(allocator_api)]
864 /// use std::alloc::System;
866 /// let x = Box::new_in(5, System);
867 /// let (ptr, alloc) = Box::into_raw_with_allocator(x);
868 /// let x = unsafe { Box::from_raw_in(ptr, alloc) };
870 /// Manually create a `Box` from scratch by using the system allocator:
872 /// #![feature(allocator_api, slice_ptr_get)]
874 /// use std::alloc::{Allocator, Layout, System};
877 /// let ptr = System.allocate(Layout::new::<i32>())?.as_mut_ptr() as *mut i32;
878 /// // In general .write is required to avoid attempting to destruct
879 /// // the (uninitialized) previous contents of `ptr`, though for this
880 /// // simple example `*ptr = 5` would have worked as well.
882 /// let x = Box::from_raw_in(ptr, System);
884 /// # Ok::<(), std::alloc::AllocError>(())
887 /// [memory layout]: self#memory-layout
888 /// [`Layout`]: crate::Layout
889 #[unstable(feature = "allocator_api", issue = "32838")]
891 pub unsafe fn from_raw_in(raw: *mut T, alloc: A) -> Self {
892 Box(unsafe { Unique::new_unchecked(raw) }, alloc)
895 /// Consumes the `Box`, returning a wrapped raw pointer.
897 /// The pointer will be properly aligned and non-null.
899 /// After calling this function, the caller is responsible for the
900 /// memory previously managed by the `Box`. In particular, the
901 /// caller should properly destroy `T` and release the memory, taking
902 /// into account the [memory layout] used by `Box`. The easiest way to
903 /// do this is to convert the raw pointer back into a `Box` with the
904 /// [`Box::from_raw`] function, allowing the `Box` destructor to perform
907 /// Note: this is an associated function, which means that you have
908 /// to call it as `Box::into_raw(b)` instead of `b.into_raw()`. This
909 /// is so that there is no conflict with a method on the inner type.
912 /// Converting the raw pointer back into a `Box` with [`Box::from_raw`]
913 /// for automatic cleanup:
915 /// let x = Box::new(String::from("Hello"));
916 /// let ptr = Box::into_raw(x);
917 /// let x = unsafe { Box::from_raw(ptr) };
919 /// Manual cleanup by explicitly running the destructor and deallocating
922 /// use std::alloc::{dealloc, Layout};
925 /// let x = Box::new(String::from("Hello"));
926 /// let p = Box::into_raw(x);
928 /// ptr::drop_in_place(p);
929 /// dealloc(p as *mut u8, Layout::new::<String>());
933 /// [memory layout]: self#memory-layout
934 #[stable(feature = "box_raw", since = "1.4.0")]
936 pub fn into_raw(b: Self) -> *mut T {
937 Self::into_raw_with_allocator(b).0
940 /// Consumes the `Box`, returning a wrapped raw pointer and the allocator.
942 /// The pointer will be properly aligned and non-null.
944 /// After calling this function, the caller is responsible for the
945 /// memory previously managed by the `Box`. In particular, the
946 /// caller should properly destroy `T` and release the memory, taking
947 /// into account the [memory layout] used by `Box`. The easiest way to
948 /// do this is to convert the raw pointer back into a `Box` with the
949 /// [`Box::from_raw_in`] function, allowing the `Box` destructor to perform
952 /// Note: this is an associated function, which means that you have
953 /// to call it as `Box::into_raw_with_allocator(b)` instead of `b.into_raw_with_allocator()`. This
954 /// is so that there is no conflict with a method on the inner type.
957 /// Converting the raw pointer back into a `Box` with [`Box::from_raw_in`]
958 /// for automatic cleanup:
960 /// #![feature(allocator_api)]
962 /// use std::alloc::System;
964 /// let x = Box::new_in(String::from("Hello"), System);
965 /// let (ptr, alloc) = Box::into_raw_with_allocator(x);
966 /// let x = unsafe { Box::from_raw_in(ptr, alloc) };
968 /// Manual cleanup by explicitly running the destructor and deallocating
971 /// #![feature(allocator_api)]
973 /// use std::alloc::{Allocator, Layout, System};
974 /// use std::ptr::{self, NonNull};
976 /// let x = Box::new_in(String::from("Hello"), System);
977 /// let (ptr, alloc) = Box::into_raw_with_allocator(x);
979 /// ptr::drop_in_place(ptr);
980 /// let non_null = NonNull::new_unchecked(ptr);
981 /// alloc.deallocate(non_null.cast(), Layout::new::<String>());
985 /// [memory layout]: self#memory-layout
986 #[unstable(feature = "allocator_api", issue = "32838")]
988 pub fn into_raw_with_allocator(b: Self) -> (*mut T, A) {
989 let (leaked, alloc) = Box::into_unique(b);
990 (leaked.as_ptr(), alloc)
994 feature = "ptr_internals",
996 reason = "use `Box::leak(b).into()` or `Unique::from(Box::leak(b))` instead"
1000 pub fn into_unique(b: Self) -> (Unique<T>, A) {
1001 // Box is recognized as a "unique pointer" by Stacked Borrows, but internally it is a
1002 // raw pointer for the type system. Turning it directly into a raw pointer would not be
1003 // recognized as "releasing" the unique pointer to permit aliased raw accesses,
1004 // so all raw pointer methods have to go through `Box::leak`. Turning *that* to a raw pointer
1005 // behaves correctly.
1006 let alloc = unsafe { ptr::read(&b.1) };
1007 (Unique::from(Box::leak(b)), alloc)
1010 /// Returns a reference to the underlying allocator.
1012 /// Note: this is an associated function, which means that you have
1013 /// to call it as `Box::allocator(&b)` instead of `b.allocator()`. This
1014 /// is so that there is no conflict with a method on the inner type.
1015 #[unstable(feature = "allocator_api", issue = "32838")]
1017 pub fn allocator(b: &Self) -> &A {
1021 /// Consumes and leaks the `Box`, returning a mutable reference,
1022 /// `&'a mut T`. Note that the type `T` must outlive the chosen lifetime
1023 /// `'a`. If the type has only static references, or none at all, then this
1024 /// may be chosen to be `'static`.
1026 /// This function is mainly useful for data that lives for the remainder of
1027 /// the program's life. Dropping the returned reference will cause a memory
1028 /// leak. If this is not acceptable, the reference should first be wrapped
1029 /// with the [`Box::from_raw`] function producing a `Box`. This `Box` can
1030 /// then be dropped which will properly destroy `T` and release the
1031 /// allocated memory.
1033 /// Note: this is an associated function, which means that you have
1034 /// to call it as `Box::leak(b)` instead of `b.leak()`. This
1035 /// is so that there is no conflict with a method on the inner type.
1042 /// let x = Box::new(41);
1043 /// let static_ref: &'static mut usize = Box::leak(x);
1044 /// *static_ref += 1;
1045 /// assert_eq!(*static_ref, 42);
1051 /// let x = vec![1, 2, 3].into_boxed_slice();
1052 /// let static_ref = Box::leak(x);
1053 /// static_ref[0] = 4;
1054 /// assert_eq!(*static_ref, [4, 2, 3]);
1056 #[stable(feature = "box_leak", since = "1.26.0")]
1058 pub fn leak<'a>(b: Self) -> &'a mut T
1062 unsafe { &mut *mem::ManuallyDrop::new(b).0.as_ptr() }
1065 /// Converts a `Box<T>` into a `Pin<Box<T>>`
1067 /// This conversion does not allocate on the heap and happens in place.
1069 /// This is also available via [`From`].
1070 #[unstable(feature = "box_into_pin", issue = "62370")]
1071 pub fn into_pin(boxed: Self) -> Pin<Self>
1075 // It's not possible to move or replace the insides of a `Pin<Box<T>>`
1076 // when `T: !Unpin`, so it's safe to pin it directly without any
1077 // additional requirements.
1078 unsafe { Pin::new_unchecked(boxed) }
1082 #[stable(feature = "rust1", since = "1.0.0")]
1083 unsafe impl<#[may_dangle] T: ?Sized, A: Allocator> Drop for Box<T, A> {
1084 fn drop(&mut self) {
1085 // FIXME: Do nothing, drop is currently performed by compiler.
1089 #[cfg(not(no_global_oom_handling))]
1090 #[stable(feature = "rust1", since = "1.0.0")]
1091 impl<T: Default> Default for Box<T> {
1092 /// Creates a `Box<T>`, with the `Default` value for T.
1093 fn default() -> Self {
1098 #[cfg(not(no_global_oom_handling))]
1099 #[stable(feature = "rust1", since = "1.0.0")]
1100 impl<T> Default for Box<[T]> {
1101 fn default() -> Self {
1102 Box::<[T; 0]>::new([])
1106 #[cfg(not(no_global_oom_handling))]
1107 #[stable(feature = "default_box_extra", since = "1.17.0")]
1108 impl Default for Box<str> {
1109 fn default() -> Self {
1110 unsafe { from_boxed_utf8_unchecked(Default::default()) }
1114 #[cfg(not(no_global_oom_handling))]
1115 #[stable(feature = "rust1", since = "1.0.0")]
1116 impl<T: Clone, A: Allocator + Clone> Clone for Box<T, A> {
1117 /// Returns a new box with a `clone()` of this box's contents.
1122 /// let x = Box::new(5);
1123 /// let y = x.clone();
1125 /// // The value is the same
1126 /// assert_eq!(x, y);
1128 /// // But they are unique objects
1129 /// assert_ne!(&*x as *const i32, &*y as *const i32);
1132 fn clone(&self) -> Self {
1133 // Pre-allocate memory to allow writing the cloned value directly.
1134 let mut boxed = Self::new_uninit_in(self.1.clone());
1136 (**self).write_clone_into_raw(boxed.as_mut_ptr());
1141 /// Copies `source`'s contents into `self` without creating a new allocation.
1146 /// let x = Box::new(5);
1147 /// let mut y = Box::new(10);
1148 /// let yp: *const i32 = &*y;
1150 /// y.clone_from(&x);
1152 /// // The value is the same
1153 /// assert_eq!(x, y);
1155 /// // And no allocation occurred
1156 /// assert_eq!(yp, &*y);
1159 fn clone_from(&mut self, source: &Self) {
1160 (**self).clone_from(&(**source));
1164 #[cfg(not(no_global_oom_handling))]
1165 #[stable(feature = "box_slice_clone", since = "1.3.0")]
1166 impl Clone for Box<str> {
1167 fn clone(&self) -> Self {
1168 // this makes a copy of the data
1169 let buf: Box<[u8]> = self.as_bytes().into();
1170 unsafe { from_boxed_utf8_unchecked(buf) }
1174 #[stable(feature = "rust1", since = "1.0.0")]
1175 impl<T: ?Sized + PartialEq, A: Allocator> PartialEq for Box<T, A> {
1177 fn eq(&self, other: &Self) -> bool {
1178 PartialEq::eq(&**self, &**other)
1181 fn ne(&self, other: &Self) -> bool {
1182 PartialEq::ne(&**self, &**other)
1185 #[stable(feature = "rust1", since = "1.0.0")]
1186 impl<T: ?Sized + PartialOrd, A: Allocator> PartialOrd for Box<T, A> {
1188 fn partial_cmp(&self, other: &Self) -> Option<Ordering> {
1189 PartialOrd::partial_cmp(&**self, &**other)
1192 fn lt(&self, other: &Self) -> bool {
1193 PartialOrd::lt(&**self, &**other)
1196 fn le(&self, other: &Self) -> bool {
1197 PartialOrd::le(&**self, &**other)
1200 fn ge(&self, other: &Self) -> bool {
1201 PartialOrd::ge(&**self, &**other)
1204 fn gt(&self, other: &Self) -> bool {
1205 PartialOrd::gt(&**self, &**other)
1208 #[stable(feature = "rust1", since = "1.0.0")]
1209 impl<T: ?Sized + Ord, A: Allocator> Ord for Box<T, A> {
1211 fn cmp(&self, other: &Self) -> Ordering {
1212 Ord::cmp(&**self, &**other)
1215 #[stable(feature = "rust1", since = "1.0.0")]
1216 impl<T: ?Sized + Eq, A: Allocator> Eq for Box<T, A> {}
1218 #[stable(feature = "rust1", since = "1.0.0")]
1219 impl<T: ?Sized + Hash, A: Allocator> Hash for Box<T, A> {
1220 fn hash<H: Hasher>(&self, state: &mut H) {
1221 (**self).hash(state);
1225 #[stable(feature = "indirect_hasher_impl", since = "1.22.0")]
1226 impl<T: ?Sized + Hasher, A: Allocator> Hasher for Box<T, A> {
1227 fn finish(&self) -> u64 {
1230 fn write(&mut self, bytes: &[u8]) {
1231 (**self).write(bytes)
1233 fn write_u8(&mut self, i: u8) {
1234 (**self).write_u8(i)
1236 fn write_u16(&mut self, i: u16) {
1237 (**self).write_u16(i)
1239 fn write_u32(&mut self, i: u32) {
1240 (**self).write_u32(i)
1242 fn write_u64(&mut self, i: u64) {
1243 (**self).write_u64(i)
1245 fn write_u128(&mut self, i: u128) {
1246 (**self).write_u128(i)
1248 fn write_usize(&mut self, i: usize) {
1249 (**self).write_usize(i)
1251 fn write_i8(&mut self, i: i8) {
1252 (**self).write_i8(i)
1254 fn write_i16(&mut self, i: i16) {
1255 (**self).write_i16(i)
1257 fn write_i32(&mut self, i: i32) {
1258 (**self).write_i32(i)
1260 fn write_i64(&mut self, i: i64) {
1261 (**self).write_i64(i)
1263 fn write_i128(&mut self, i: i128) {
1264 (**self).write_i128(i)
1266 fn write_isize(&mut self, i: isize) {
1267 (**self).write_isize(i)
1271 #[cfg(not(no_global_oom_handling))]
1272 #[stable(feature = "from_for_ptrs", since = "1.6.0")]
1273 impl<T> From<T> for Box<T> {
1274 /// Converts a `T` into a `Box<T>`
1276 /// The conversion allocates on the heap and moves `t`
1277 /// from the stack into it.
1283 /// let boxed = Box::new(5);
1285 /// assert_eq!(Box::from(x), boxed);
1287 fn from(t: T) -> Self {
1292 #[stable(feature = "pin", since = "1.33.0")]
1293 impl<T: ?Sized, A: Allocator> From<Box<T, A>> for Pin<Box<T, A>>
1297 /// Converts a `Box<T>` into a `Pin<Box<T>>`
1299 /// This conversion does not allocate on the heap and happens in place.
1300 fn from(boxed: Box<T, A>) -> Self {
1301 Box::into_pin(boxed)
1305 #[cfg(not(no_global_oom_handling))]
1306 #[stable(feature = "box_from_slice", since = "1.17.0")]
1307 impl<T: Copy> From<&[T]> for Box<[T]> {
1308 /// Converts a `&[T]` into a `Box<[T]>`
1310 /// This conversion allocates on the heap
1311 /// and performs a copy of `slice`.
1315 /// // create a &[u8] which will be used to create a Box<[u8]>
1316 /// let slice: &[u8] = &[104, 101, 108, 108, 111];
1317 /// let boxed_slice: Box<[u8]> = Box::from(slice);
1319 /// println!("{:?}", boxed_slice);
1321 fn from(slice: &[T]) -> Box<[T]> {
1322 let len = slice.len();
1323 let buf = RawVec::with_capacity(len);
1325 ptr::copy_nonoverlapping(slice.as_ptr(), buf.ptr(), len);
1326 buf.into_box(slice.len()).assume_init()
1331 #[cfg(not(no_global_oom_handling))]
1332 #[stable(feature = "box_from_cow", since = "1.45.0")]
1333 impl<T: Copy> From<Cow<'_, [T]>> for Box<[T]> {
1334 /// Converts a `Cow<'_, [T]>` into a `Box<[T]>`
1336 /// When `cow` is the `Cow::Borrowed` variant, this
1337 /// conversion allocates on the heap and copies the
1338 /// underlying slice. Otherwise, it will try to reuse the owned
1339 /// `Vec`'s allocation.
1341 fn from(cow: Cow<'_, [T]>) -> Box<[T]> {
1343 Cow::Borrowed(slice) => Box::from(slice),
1344 Cow::Owned(slice) => Box::from(slice),
1349 #[cfg(not(no_global_oom_handling))]
1350 #[stable(feature = "box_from_slice", since = "1.17.0")]
1351 impl From<&str> for Box<str> {
1352 /// Converts a `&str` into a `Box<str>`
1354 /// This conversion allocates on the heap
1355 /// and performs a copy of `s`.
1360 /// let boxed: Box<str> = Box::from("hello");
1361 /// println!("{}", boxed);
1364 fn from(s: &str) -> Box<str> {
1365 unsafe { from_boxed_utf8_unchecked(Box::from(s.as_bytes())) }
1369 #[cfg(not(no_global_oom_handling))]
1370 #[stable(feature = "box_from_cow", since = "1.45.0")]
1371 impl From<Cow<'_, str>> for Box<str> {
1372 /// Converts a `Cow<'_, str>` into a `Box<str>`
1374 /// When `cow` is the `Cow::Borrowed` variant, this
1375 /// conversion allocates on the heap and copies the
1376 /// underlying `str`. Otherwise, it will try to reuse the owned
1377 /// `String`'s allocation.
1382 /// use std::borrow::Cow;
1384 /// let unboxed = Cow::Borrowed("hello");
1385 /// let boxed: Box<str> = Box::from(unboxed);
1386 /// println!("{}", boxed);
1390 /// # use std::borrow::Cow;
1391 /// let unboxed = Cow::Owned("hello".to_string());
1392 /// let boxed: Box<str> = Box::from(unboxed);
1393 /// println!("{}", boxed);
1396 fn from(cow: Cow<'_, str>) -> Box<str> {
1398 Cow::Borrowed(s) => Box::from(s),
1399 Cow::Owned(s) => Box::from(s),
1404 #[stable(feature = "boxed_str_conv", since = "1.19.0")]
1405 impl<A: Allocator> From<Box<str, A>> for Box<[u8], A> {
1406 /// Converts a `Box<str>` into a `Box<[u8]>`
1408 /// This conversion does not allocate on the heap and happens in place.
1412 /// // create a Box<str> which will be used to create a Box<[u8]>
1413 /// let boxed: Box<str> = Box::from("hello");
1414 /// let boxed_str: Box<[u8]> = Box::from(boxed);
1416 /// // create a &[u8] which will be used to create a Box<[u8]>
1417 /// let slice: &[u8] = &[104, 101, 108, 108, 111];
1418 /// let boxed_slice = Box::from(slice);
1420 /// assert_eq!(boxed_slice, boxed_str);
1423 fn from(s: Box<str, A>) -> Self {
1424 let (raw, alloc) = Box::into_raw_with_allocator(s);
1425 unsafe { Box::from_raw_in(raw as *mut [u8], alloc) }
1429 #[cfg(not(no_global_oom_handling))]
1430 #[stable(feature = "box_from_array", since = "1.45.0")]
1431 impl<T, const N: usize> From<[T; N]> for Box<[T]> {
1432 /// Converts a `[T; N]` into a `Box<[T]>`
1434 /// This conversion moves the array to newly heap-allocated memory.
1439 /// let boxed: Box<[u8]> = Box::from([4, 2]);
1440 /// println!("{:?}", boxed);
1442 fn from(array: [T; N]) -> Box<[T]> {
1447 #[stable(feature = "boxed_slice_try_from", since = "1.43.0")]
1448 impl<T, const N: usize> TryFrom<Box<[T]>> for Box<[T; N]> {
1449 type Error = Box<[T]>;
1451 /// Attempts to convert a `Box<[T]>` into a `Box<[T; N]>`.
1453 /// The conversion occurs in-place and does not require a
1454 /// new memory allocation.
1458 /// Returns the old `Box<[T]>` in the `Err` variant if
1459 /// `boxed_slice.len()` does not equal `N`.
1460 fn try_from(boxed_slice: Box<[T]>) -> Result<Self, Self::Error> {
1461 if boxed_slice.len() == N {
1462 Ok(unsafe { Box::from_raw(Box::into_raw(boxed_slice) as *mut [T; N]) })
1469 impl<A: Allocator> Box<dyn Any, A> {
1471 #[stable(feature = "rust1", since = "1.0.0")]
1472 /// Attempt to downcast the box to a concrete type.
1477 /// use std::any::Any;
1479 /// fn print_if_string(value: Box<dyn Any>) {
1480 /// if let Ok(string) = value.downcast::<String>() {
1481 /// println!("String ({}): {}", string.len(), string);
1485 /// let my_string = "Hello World".to_string();
1486 /// print_if_string(Box::new(my_string));
1487 /// print_if_string(Box::new(0i8));
1489 pub fn downcast<T: Any>(self) -> Result<Box<T, A>, Self> {
1492 let (raw, alloc): (*mut dyn Any, _) = Box::into_raw_with_allocator(self);
1493 Ok(Box::from_raw_in(raw as *mut T, alloc))
1501 impl<A: Allocator> Box<dyn Any + Send, A> {
1503 #[stable(feature = "rust1", since = "1.0.0")]
1504 /// Attempt to downcast the box to a concrete type.
1509 /// use std::any::Any;
1511 /// fn print_if_string(value: Box<dyn Any + Send>) {
1512 /// if let Ok(string) = value.downcast::<String>() {
1513 /// println!("String ({}): {}", string.len(), string);
1517 /// let my_string = "Hello World".to_string();
1518 /// print_if_string(Box::new(my_string));
1519 /// print_if_string(Box::new(0i8));
1521 pub fn downcast<T: Any>(self) -> Result<Box<T, A>, Self> {
1524 let (raw, alloc): (*mut (dyn Any + Send), _) = Box::into_raw_with_allocator(self);
1525 Ok(Box::from_raw_in(raw as *mut T, alloc))
1533 impl<A: Allocator> Box<dyn Any + Send + Sync, A> {
1535 #[stable(feature = "box_send_sync_any_downcast", since = "1.51.0")]
1536 /// Attempt to downcast the box to a concrete type.
1541 /// use std::any::Any;
1543 /// fn print_if_string(value: Box<dyn Any + Send + Sync>) {
1544 /// if let Ok(string) = value.downcast::<String>() {
1545 /// println!("String ({}): {}", string.len(), string);
1549 /// let my_string = "Hello World".to_string();
1550 /// print_if_string(Box::new(my_string));
1551 /// print_if_string(Box::new(0i8));
1553 pub fn downcast<T: Any>(self) -> Result<Box<T, A>, Self> {
1556 let (raw, alloc): (*mut (dyn Any + Send + Sync), _) =
1557 Box::into_raw_with_allocator(self);
1558 Ok(Box::from_raw_in(raw as *mut T, alloc))
1566 #[stable(feature = "rust1", since = "1.0.0")]
1567 impl<T: fmt::Display + ?Sized, A: Allocator> fmt::Display for Box<T, A> {
1568 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
1569 fmt::Display::fmt(&**self, f)
1573 #[stable(feature = "rust1", since = "1.0.0")]
1574 impl<T: fmt::Debug + ?Sized, A: Allocator> fmt::Debug for Box<T, A> {
1575 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
1576 fmt::Debug::fmt(&**self, f)
1580 #[stable(feature = "rust1", since = "1.0.0")]
1581 impl<T: ?Sized, A: Allocator> fmt::Pointer for Box<T, A> {
1582 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
1583 // It's not possible to extract the inner Uniq directly from the Box,
1584 // instead we cast it to a *const which aliases the Unique
1585 let ptr: *const T = &**self;
1586 fmt::Pointer::fmt(&ptr, f)
1590 #[stable(feature = "rust1", since = "1.0.0")]
1591 impl<T: ?Sized, A: Allocator> Deref for Box<T, A> {
1594 fn deref(&self) -> &T {
1599 #[stable(feature = "rust1", since = "1.0.0")]
1600 impl<T: ?Sized, A: Allocator> DerefMut for Box<T, A> {
1601 fn deref_mut(&mut self) -> &mut T {
1606 #[unstable(feature = "receiver_trait", issue = "none")]
1607 impl<T: ?Sized, A: Allocator> Receiver for Box<T, A> {}
1609 #[stable(feature = "rust1", since = "1.0.0")]
1610 impl<I: Iterator + ?Sized, A: Allocator> Iterator for Box<I, A> {
1611 type Item = I::Item;
1612 fn next(&mut self) -> Option<I::Item> {
1615 fn size_hint(&self) -> (usize, Option<usize>) {
1616 (**self).size_hint()
1618 fn nth(&mut self, n: usize) -> Option<I::Item> {
1621 fn last(self) -> Option<I::Item> {
1628 fn last(self) -> Option<Self::Item>;
1631 impl<I: Iterator + ?Sized, A: Allocator> BoxIter for Box<I, A> {
1632 type Item = I::Item;
1633 default fn last(self) -> Option<I::Item> {
1635 fn some<T>(_: Option<T>, x: T) -> Option<T> {
1639 self.fold(None, some)
1643 /// Specialization for sized `I`s that uses `I`s implementation of `last()`
1644 /// instead of the default.
1645 #[stable(feature = "rust1", since = "1.0.0")]
1646 impl<I: Iterator, A: Allocator> BoxIter for Box<I, A> {
1647 fn last(self) -> Option<I::Item> {
1652 #[stable(feature = "rust1", since = "1.0.0")]
1653 impl<I: DoubleEndedIterator + ?Sized, A: Allocator> DoubleEndedIterator for Box<I, A> {
1654 fn next_back(&mut self) -> Option<I::Item> {
1655 (**self).next_back()
1657 fn nth_back(&mut self, n: usize) -> Option<I::Item> {
1658 (**self).nth_back(n)
1661 #[stable(feature = "rust1", since = "1.0.0")]
1662 impl<I: ExactSizeIterator + ?Sized, A: Allocator> ExactSizeIterator for Box<I, A> {
1663 fn len(&self) -> usize {
1666 fn is_empty(&self) -> bool {
1671 #[stable(feature = "fused", since = "1.26.0")]
1672 impl<I: FusedIterator + ?Sized, A: Allocator> FusedIterator for Box<I, A> {}
1674 #[stable(feature = "boxed_closure_impls", since = "1.35.0")]
1675 impl<Args, F: FnOnce<Args> + ?Sized, A: Allocator> FnOnce<Args> for Box<F, A> {
1676 type Output = <F as FnOnce<Args>>::Output;
1678 extern "rust-call" fn call_once(self, args: Args) -> Self::Output {
1679 <F as FnOnce<Args>>::call_once(*self, args)
1683 #[stable(feature = "boxed_closure_impls", since = "1.35.0")]
1684 impl<Args, F: FnMut<Args> + ?Sized, A: Allocator> FnMut<Args> for Box<F, A> {
1685 extern "rust-call" fn call_mut(&mut self, args: Args) -> Self::Output {
1686 <F as FnMut<Args>>::call_mut(self, args)
1690 #[stable(feature = "boxed_closure_impls", since = "1.35.0")]
1691 impl<Args, F: Fn<Args> + ?Sized, A: Allocator> Fn<Args> for Box<F, A> {
1692 extern "rust-call" fn call(&self, args: Args) -> Self::Output {
1693 <F as Fn<Args>>::call(self, args)
1697 #[unstable(feature = "coerce_unsized", issue = "27732")]
1698 impl<T: ?Sized + Unsize<U>, U: ?Sized, A: Allocator> CoerceUnsized<Box<U, A>> for Box<T, A> {}
1700 #[unstable(feature = "dispatch_from_dyn", issue = "none")]
1701 impl<T: ?Sized + Unsize<U>, U: ?Sized> DispatchFromDyn<Box<U>> for Box<T, Global> {}
1703 #[cfg(not(no_global_oom_handling))]
1704 #[stable(feature = "boxed_slice_from_iter", since = "1.32.0")]
1705 impl<I> FromIterator<I> for Box<[I]> {
1706 fn from_iter<T: IntoIterator<Item = I>>(iter: T) -> Self {
1707 iter.into_iter().collect::<Vec<_>>().into_boxed_slice()
1711 #[cfg(not(no_global_oom_handling))]
1712 #[stable(feature = "box_slice_clone", since = "1.3.0")]
1713 impl<T: Clone, A: Allocator + Clone> Clone for Box<[T], A> {
1714 fn clone(&self) -> Self {
1715 let alloc = Box::allocator(self).clone();
1716 self.to_vec_in(alloc).into_boxed_slice()
1719 fn clone_from(&mut self, other: &Self) {
1720 if self.len() == other.len() {
1721 self.clone_from_slice(&other);
1723 *self = other.clone();
1728 #[stable(feature = "box_borrow", since = "1.1.0")]
1729 impl<T: ?Sized, A: Allocator> borrow::Borrow<T> for Box<T, A> {
1730 fn borrow(&self) -> &T {
1735 #[stable(feature = "box_borrow", since = "1.1.0")]
1736 impl<T: ?Sized, A: Allocator> borrow::BorrowMut<T> for Box<T, A> {
1737 fn borrow_mut(&mut self) -> &mut T {
1742 #[stable(since = "1.5.0", feature = "smart_ptr_as_ref")]
1743 impl<T: ?Sized, A: Allocator> AsRef<T> for Box<T, A> {
1744 fn as_ref(&self) -> &T {
1749 #[stable(since = "1.5.0", feature = "smart_ptr_as_ref")]
1750 impl<T: ?Sized, A: Allocator> AsMut<T> for Box<T, A> {
1751 fn as_mut(&mut self) -> &mut T {
1758 * We could have chosen not to add this impl, and instead have written a
1759 * function of Pin<Box<T>> to Pin<T>. Such a function would not be sound,
1760 * because Box<T> implements Unpin even when T does not, as a result of
1763 * We chose this API instead of the alternative for a few reasons:
1764 * - Logically, it is helpful to understand pinning in regard to the
1765 * memory region being pointed to. For this reason none of the
1766 * standard library pointer types support projecting through a pin
1767 * (Box<T> is the only pointer type in std for which this would be
1769 * - It is in practice very useful to have Box<T> be unconditionally
1770 * Unpin because of trait objects, for which the structural auto
1771 * trait functionality does not apply (e.g., Box<dyn Foo> would
1772 * otherwise not be Unpin).
1774 * Another type with the same semantics as Box but only a conditional
1775 * implementation of `Unpin` (where `T: Unpin`) would be valid/safe, and
1776 * could have a method to project a Pin<T> from it.
1778 #[stable(feature = "pin", since = "1.33.0")]
1779 impl<T: ?Sized, A: Allocator> Unpin for Box<T, A> where A: 'static {}
1781 #[unstable(feature = "generator_trait", issue = "43122")]
1782 impl<G: ?Sized + Generator<R> + Unpin, R, A: Allocator> Generator<R> for Box<G, A>
1786 type Yield = G::Yield;
1787 type Return = G::Return;
1789 fn resume(mut self: Pin<&mut Self>, arg: R) -> GeneratorState<Self::Yield, Self::Return> {
1790 G::resume(Pin::new(&mut *self), arg)
1794 #[unstable(feature = "generator_trait", issue = "43122")]
1795 impl<G: ?Sized + Generator<R>, R, A: Allocator> Generator<R> for Pin<Box<G, A>>
1799 type Yield = G::Yield;
1800 type Return = G::Return;
1802 fn resume(mut self: Pin<&mut Self>, arg: R) -> GeneratorState<Self::Yield, Self::Return> {
1803 G::resume((*self).as_mut(), arg)
1807 #[stable(feature = "futures_api", since = "1.36.0")]
1808 impl<F: ?Sized + Future + Unpin, A: Allocator> Future for Box<F, A>
1812 type Output = F::Output;
1814 fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
1815 F::poll(Pin::new(&mut *self), cx)
1819 #[unstable(feature = "async_stream", issue = "79024")]
1820 impl<S: ?Sized + Stream + Unpin> Stream for Box<S> {
1821 type Item = S::Item;
1823 fn poll_next(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Option<Self::Item>> {
1824 Pin::new(&mut **self).poll_next(cx)
1827 fn size_hint(&self) -> (usize, Option<usize>) {
1828 (**self).size_hint()