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")]
191 pub fn new(x: T) -> Self {
195 /// Constructs a new box with uninitialized contents.
200 /// #![feature(new_uninit)]
202 /// let mut five = Box::<u32>::new_uninit();
204 /// let five = unsafe {
205 /// // Deferred initialization:
206 /// five.as_mut_ptr().write(5);
208 /// five.assume_init()
211 /// assert_eq!(*five, 5)
213 #[cfg(not(no_global_oom_handling))]
214 #[unstable(feature = "new_uninit", issue = "63291")]
217 pub fn new_uninit() -> Box<mem::MaybeUninit<T>> {
218 Self::new_uninit_in(Global)
221 /// Constructs a new `Box` with uninitialized contents, with the memory
222 /// being filled with `0` bytes.
224 /// See [`MaybeUninit::zeroed`][zeroed] for examples of correct and incorrect usage
230 /// #![feature(new_uninit)]
232 /// let zero = Box::<u32>::new_zeroed();
233 /// let zero = unsafe { zero.assume_init() };
235 /// assert_eq!(*zero, 0)
238 /// [zeroed]: mem::MaybeUninit::zeroed
239 #[cfg(not(no_global_oom_handling))]
241 #[unstable(feature = "new_uninit", issue = "63291")]
243 pub fn new_zeroed() -> Box<mem::MaybeUninit<T>> {
244 Self::new_zeroed_in(Global)
247 /// Constructs a new `Pin<Box<T>>`. If `T` does not implement `Unpin`, then
248 /// `x` will be pinned in memory and unable to be moved.
249 #[cfg(not(no_global_oom_handling))]
250 #[stable(feature = "pin", since = "1.33.0")]
253 pub fn pin(x: T) -> Pin<Box<T>> {
257 /// Allocates memory on the heap then places `x` into it,
258 /// returning an error if the allocation fails
260 /// This doesn't actually allocate if `T` is zero-sized.
265 /// #![feature(allocator_api)]
267 /// let five = Box::try_new(5)?;
268 /// # Ok::<(), std::alloc::AllocError>(())
270 #[unstable(feature = "allocator_api", issue = "32838")]
272 pub fn try_new(x: T) -> Result<Self, AllocError> {
273 Self::try_new_in(x, Global)
276 /// Constructs a new box with uninitialized contents on the heap,
277 /// returning an error if the allocation fails
282 /// #![feature(allocator_api, new_uninit)]
284 /// let mut five = Box::<u32>::try_new_uninit()?;
286 /// let five = unsafe {
287 /// // Deferred initialization:
288 /// five.as_mut_ptr().write(5);
290 /// five.assume_init()
293 /// assert_eq!(*five, 5);
294 /// # Ok::<(), std::alloc::AllocError>(())
296 #[unstable(feature = "allocator_api", issue = "32838")]
297 // #[unstable(feature = "new_uninit", issue = "63291")]
299 pub fn try_new_uninit() -> Result<Box<mem::MaybeUninit<T>>, AllocError> {
300 Box::try_new_uninit_in(Global)
303 /// Constructs a new `Box` with uninitialized contents, with the memory
304 /// being filled with `0` bytes on the heap
306 /// See [`MaybeUninit::zeroed`][zeroed] for examples of correct and incorrect usage
312 /// #![feature(allocator_api, new_uninit)]
314 /// let zero = Box::<u32>::try_new_zeroed()?;
315 /// let zero = unsafe { zero.assume_init() };
317 /// assert_eq!(*zero, 0);
318 /// # Ok::<(), std::alloc::AllocError>(())
321 /// [zeroed]: mem::MaybeUninit::zeroed
322 #[unstable(feature = "allocator_api", issue = "32838")]
323 // #[unstable(feature = "new_uninit", issue = "63291")]
325 pub fn try_new_zeroed() -> Result<Box<mem::MaybeUninit<T>>, AllocError> {
326 Box::try_new_zeroed_in(Global)
330 impl<T, A: Allocator> Box<T, A> {
331 /// Allocates memory in the given allocator then places `x` into it.
333 /// This doesn't actually allocate if `T` is zero-sized.
338 /// #![feature(allocator_api)]
340 /// use std::alloc::System;
342 /// let five = Box::new_in(5, System);
344 #[cfg(not(no_global_oom_handling))]
345 #[unstable(feature = "allocator_api", issue = "32838")]
348 pub fn new_in(x: T, alloc: A) -> Self {
349 let mut boxed = Self::new_uninit_in(alloc);
351 boxed.as_mut_ptr().write(x);
356 /// Allocates memory in the given allocator then places `x` into it,
357 /// returning an error if the allocation fails
359 /// This doesn't actually allocate if `T` is zero-sized.
364 /// #![feature(allocator_api)]
366 /// use std::alloc::System;
368 /// let five = Box::try_new_in(5, System)?;
369 /// # Ok::<(), std::alloc::AllocError>(())
371 #[unstable(feature = "allocator_api", issue = "32838")]
373 pub fn try_new_in(x: T, alloc: A) -> Result<Self, AllocError> {
374 let mut boxed = Self::try_new_uninit_in(alloc)?;
376 boxed.as_mut_ptr().write(x);
377 Ok(boxed.assume_init())
381 /// Constructs a new box with uninitialized contents in the provided allocator.
386 /// #![feature(allocator_api, new_uninit)]
388 /// use std::alloc::System;
390 /// let mut five = Box::<u32, _>::new_uninit_in(System);
392 /// let five = unsafe {
393 /// // Deferred initialization:
394 /// five.as_mut_ptr().write(5);
396 /// five.assume_init()
399 /// assert_eq!(*five, 5)
401 #[unstable(feature = "allocator_api", issue = "32838")]
402 #[cfg(not(no_global_oom_handling))]
404 // #[unstable(feature = "new_uninit", issue = "63291")]
405 pub fn new_uninit_in(alloc: A) -> Box<mem::MaybeUninit<T>, A> {
406 let layout = Layout::new::<mem::MaybeUninit<T>>();
407 // NOTE: Prefer match over unwrap_or_else since closure sometimes not inlineable.
408 // That would make code size bigger.
409 match Box::try_new_uninit_in(alloc) {
411 Err(_) => handle_alloc_error(layout),
415 /// Constructs a new box with uninitialized contents in the provided allocator,
416 /// returning an error if the allocation fails
421 /// #![feature(allocator_api, new_uninit)]
423 /// use std::alloc::System;
425 /// let mut five = Box::<u32, _>::try_new_uninit_in(System)?;
427 /// let five = unsafe {
428 /// // Deferred initialization:
429 /// five.as_mut_ptr().write(5);
431 /// five.assume_init()
434 /// assert_eq!(*five, 5);
435 /// # Ok::<(), std::alloc::AllocError>(())
437 #[unstable(feature = "allocator_api", issue = "32838")]
438 // #[unstable(feature = "new_uninit", issue = "63291")]
439 pub fn try_new_uninit_in(alloc: A) -> Result<Box<mem::MaybeUninit<T>, A>, AllocError> {
440 let layout = Layout::new::<mem::MaybeUninit<T>>();
441 let ptr = alloc.allocate(layout)?.cast();
442 unsafe { Ok(Box::from_raw_in(ptr.as_ptr(), alloc)) }
445 /// Constructs a new `Box` with uninitialized contents, with the memory
446 /// being filled with `0` bytes in the provided allocator.
448 /// See [`MaybeUninit::zeroed`][zeroed] for examples of correct and incorrect usage
454 /// #![feature(allocator_api, new_uninit)]
456 /// use std::alloc::System;
458 /// let zero = Box::<u32, _>::new_zeroed_in(System);
459 /// let zero = unsafe { zero.assume_init() };
461 /// assert_eq!(*zero, 0)
464 /// [zeroed]: mem::MaybeUninit::zeroed
465 #[unstable(feature = "allocator_api", issue = "32838")]
466 #[cfg(not(no_global_oom_handling))]
467 // #[unstable(feature = "new_uninit", issue = "63291")]
469 pub fn new_zeroed_in(alloc: A) -> Box<mem::MaybeUninit<T>, A> {
470 let layout = Layout::new::<mem::MaybeUninit<T>>();
471 // NOTE: Prefer match over unwrap_or_else since closure sometimes not inlineable.
472 // That would make code size bigger.
473 match Box::try_new_zeroed_in(alloc) {
475 Err(_) => handle_alloc_error(layout),
479 /// Constructs a new `Box` with uninitialized contents, with the memory
480 /// being filled with `0` bytes in the provided allocator,
481 /// returning an error if the allocation fails,
483 /// See [`MaybeUninit::zeroed`][zeroed] for examples of correct and incorrect usage
489 /// #![feature(allocator_api, new_uninit)]
491 /// use std::alloc::System;
493 /// let zero = Box::<u32, _>::try_new_zeroed_in(System)?;
494 /// let zero = unsafe { zero.assume_init() };
496 /// assert_eq!(*zero, 0);
497 /// # Ok::<(), std::alloc::AllocError>(())
500 /// [zeroed]: mem::MaybeUninit::zeroed
501 #[unstable(feature = "allocator_api", issue = "32838")]
502 // #[unstable(feature = "new_uninit", issue = "63291")]
503 pub fn try_new_zeroed_in(alloc: A) -> Result<Box<mem::MaybeUninit<T>, A>, AllocError> {
504 let layout = Layout::new::<mem::MaybeUninit<T>>();
505 let ptr = alloc.allocate_zeroed(layout)?.cast();
506 unsafe { Ok(Box::from_raw_in(ptr.as_ptr(), alloc)) }
509 /// Constructs a new `Pin<Box<T, A>>`. If `T` does not implement `Unpin`, then
510 /// `x` will be pinned in memory and unable to be moved.
511 #[cfg(not(no_global_oom_handling))]
512 #[unstable(feature = "allocator_api", issue = "32838")]
515 pub fn pin_in(x: T, alloc: A) -> Pin<Self>
519 Self::new_in(x, alloc).into()
522 /// Converts a `Box<T>` into a `Box<[T]>`
524 /// This conversion does not allocate on the heap and happens in place.
525 #[unstable(feature = "box_into_boxed_slice", issue = "71582")]
526 pub fn into_boxed_slice(boxed: Self) -> Box<[T], A> {
527 let (raw, alloc) = Box::into_raw_with_allocator(boxed);
528 unsafe { Box::from_raw_in(raw as *mut [T; 1], alloc) }
531 /// Consumes the `Box`, returning the wrapped value.
536 /// #![feature(box_into_inner)]
538 /// let c = Box::new(5);
540 /// assert_eq!(Box::into_inner(c), 5);
542 #[unstable(feature = "box_into_inner", issue = "80437")]
544 pub fn into_inner(boxed: Self) -> T {
550 /// Constructs a new boxed slice with uninitialized contents.
555 /// #![feature(new_uninit)]
557 /// let mut values = Box::<[u32]>::new_uninit_slice(3);
559 /// let values = unsafe {
560 /// // Deferred initialization:
561 /// values[0].as_mut_ptr().write(1);
562 /// values[1].as_mut_ptr().write(2);
563 /// values[2].as_mut_ptr().write(3);
565 /// values.assume_init()
568 /// assert_eq!(*values, [1, 2, 3])
570 #[cfg(not(no_global_oom_handling))]
571 #[unstable(feature = "new_uninit", issue = "63291")]
573 pub fn new_uninit_slice(len: usize) -> Box<[mem::MaybeUninit<T>]> {
574 unsafe { RawVec::with_capacity(len).into_box(len) }
577 /// Constructs a new boxed slice with uninitialized contents, with the memory
578 /// being filled with `0` bytes.
580 /// See [`MaybeUninit::zeroed`][zeroed] for examples of correct and incorrect usage
586 /// #![feature(new_uninit)]
588 /// let values = Box::<[u32]>::new_zeroed_slice(3);
589 /// let values = unsafe { values.assume_init() };
591 /// assert_eq!(*values, [0, 0, 0])
594 /// [zeroed]: mem::MaybeUninit::zeroed
595 #[cfg(not(no_global_oom_handling))]
596 #[unstable(feature = "new_uninit", issue = "63291")]
598 pub fn new_zeroed_slice(len: usize) -> Box<[mem::MaybeUninit<T>]> {
599 unsafe { RawVec::with_capacity_zeroed(len).into_box(len) }
602 /// Constructs a new boxed slice with uninitialized contents. Returns an error if
603 /// the allocation fails
608 /// #![feature(allocator_api, new_uninit)]
610 /// let mut values = Box::<[u32]>::try_new_uninit_slice(3)?;
611 /// let values = unsafe {
612 /// // Deferred initialization:
613 /// values[0].as_mut_ptr().write(1);
614 /// values[1].as_mut_ptr().write(2);
615 /// values[2].as_mut_ptr().write(3);
616 /// values.assume_init()
619 /// assert_eq!(*values, [1, 2, 3]);
620 /// # Ok::<(), std::alloc::AllocError>(())
622 #[unstable(feature = "allocator_api", issue = "32838")]
624 pub fn try_new_uninit_slice(len: usize) -> Result<Box<[mem::MaybeUninit<T>]>, AllocError> {
626 let layout = match Layout::array::<mem::MaybeUninit<T>>(len) {
628 Err(_) => return Err(AllocError),
630 let ptr = Global.allocate(layout)?;
631 Ok(RawVec::from_raw_parts_in(ptr.as_mut_ptr() as *mut _, len, Global).into_box(len))
635 /// Constructs a new boxed slice with uninitialized contents, with the memory
636 /// being filled with `0` bytes. Returns an error if the allocation fails
638 /// See [`MaybeUninit::zeroed`][zeroed] for examples of correct and incorrect usage
644 /// #![feature(allocator_api, new_uninit)]
646 /// let values = Box::<[u32]>::try_new_zeroed_slice(3)?;
647 /// let values = unsafe { values.assume_init() };
649 /// assert_eq!(*values, [0, 0, 0]);
650 /// # Ok::<(), std::alloc::AllocError>(())
653 /// [zeroed]: mem::MaybeUninit::zeroed
654 #[unstable(feature = "allocator_api", issue = "32838")]
656 pub fn try_new_zeroed_slice(len: usize) -> Result<Box<[mem::MaybeUninit<T>]>, AllocError> {
658 let layout = match Layout::array::<mem::MaybeUninit<T>>(len) {
660 Err(_) => return Err(AllocError),
662 let ptr = Global.allocate_zeroed(layout)?;
663 Ok(RawVec::from_raw_parts_in(ptr.as_mut_ptr() as *mut _, len, Global).into_box(len))
668 impl<T, A: Allocator> Box<[T], A> {
669 /// Constructs a new boxed slice with uninitialized contents in the provided allocator.
674 /// #![feature(allocator_api, new_uninit)]
676 /// use std::alloc::System;
678 /// let mut values = Box::<[u32], _>::new_uninit_slice_in(3, System);
680 /// let values = unsafe {
681 /// // Deferred initialization:
682 /// values[0].as_mut_ptr().write(1);
683 /// values[1].as_mut_ptr().write(2);
684 /// values[2].as_mut_ptr().write(3);
686 /// values.assume_init()
689 /// assert_eq!(*values, [1, 2, 3])
691 #[cfg(not(no_global_oom_handling))]
692 #[unstable(feature = "allocator_api", issue = "32838")]
693 // #[unstable(feature = "new_uninit", issue = "63291")]
695 pub fn new_uninit_slice_in(len: usize, alloc: A) -> Box<[mem::MaybeUninit<T>], A> {
696 unsafe { RawVec::with_capacity_in(len, alloc).into_box(len) }
699 /// Constructs a new boxed slice with uninitialized contents in the provided allocator,
700 /// with the memory being filled with `0` bytes.
702 /// See [`MaybeUninit::zeroed`][zeroed] for examples of correct and incorrect usage
708 /// #![feature(allocator_api, new_uninit)]
710 /// use std::alloc::System;
712 /// let values = Box::<[u32], _>::new_zeroed_slice_in(3, System);
713 /// let values = unsafe { values.assume_init() };
715 /// assert_eq!(*values, [0, 0, 0])
718 /// [zeroed]: mem::MaybeUninit::zeroed
719 #[cfg(not(no_global_oom_handling))]
720 #[unstable(feature = "allocator_api", issue = "32838")]
721 // #[unstable(feature = "new_uninit", issue = "63291")]
723 pub fn new_zeroed_slice_in(len: usize, alloc: A) -> Box<[mem::MaybeUninit<T>], A> {
724 unsafe { RawVec::with_capacity_zeroed_in(len, alloc).into_box(len) }
728 impl<T, A: Allocator> Box<mem::MaybeUninit<T>, A> {
729 /// Converts to `Box<T, A>`.
733 /// As with [`MaybeUninit::assume_init`],
734 /// it is up to the caller to guarantee that the value
735 /// really is in an initialized state.
736 /// Calling this when the content is not yet fully initialized
737 /// causes immediate undefined behavior.
739 /// [`MaybeUninit::assume_init`]: mem::MaybeUninit::assume_init
744 /// #![feature(new_uninit)]
746 /// let mut five = Box::<u32>::new_uninit();
748 /// let five: Box<u32> = unsafe {
749 /// // Deferred initialization:
750 /// five.as_mut_ptr().write(5);
752 /// five.assume_init()
755 /// assert_eq!(*five, 5)
757 #[unstable(feature = "new_uninit", issue = "63291")]
759 pub unsafe fn assume_init(self) -> Box<T, A> {
760 let (raw, alloc) = Box::into_raw_with_allocator(self);
761 unsafe { Box::from_raw_in(raw as *mut T, alloc) }
765 impl<T, A: Allocator> Box<[mem::MaybeUninit<T>], A> {
766 /// Converts to `Box<[T], A>`.
770 /// As with [`MaybeUninit::assume_init`],
771 /// it is up to the caller to guarantee that the values
772 /// really are in an initialized state.
773 /// Calling this when the content is not yet fully initialized
774 /// causes immediate undefined behavior.
776 /// [`MaybeUninit::assume_init`]: mem::MaybeUninit::assume_init
781 /// #![feature(new_uninit)]
783 /// let mut values = Box::<[u32]>::new_uninit_slice(3);
785 /// let values = unsafe {
786 /// // Deferred initialization:
787 /// values[0].as_mut_ptr().write(1);
788 /// values[1].as_mut_ptr().write(2);
789 /// values[2].as_mut_ptr().write(3);
791 /// values.assume_init()
794 /// assert_eq!(*values, [1, 2, 3])
796 #[unstable(feature = "new_uninit", issue = "63291")]
798 pub unsafe fn assume_init(self) -> Box<[T], A> {
799 let (raw, alloc) = Box::into_raw_with_allocator(self);
800 unsafe { Box::from_raw_in(raw as *mut [T], alloc) }
804 impl<T: ?Sized> Box<T> {
805 /// Constructs a box from a raw pointer.
807 /// After calling this function, the raw pointer is owned by the
808 /// resulting `Box`. Specifically, the `Box` destructor will call
809 /// the destructor of `T` and free the allocated memory. For this
810 /// to be safe, the memory must have been allocated in accordance
811 /// with the [memory layout] used by `Box` .
815 /// This function is unsafe because improper use may lead to
816 /// memory problems. For example, a double-free may occur if the
817 /// function is called twice on the same raw pointer.
819 /// The safety conditions are described in the [memory layout] section.
823 /// Recreate a `Box` which was previously converted to a raw pointer
824 /// using [`Box::into_raw`]:
826 /// let x = Box::new(5);
827 /// let ptr = Box::into_raw(x);
828 /// let x = unsafe { Box::from_raw(ptr) };
830 /// Manually create a `Box` from scratch by using the global allocator:
832 /// use std::alloc::{alloc, Layout};
835 /// let ptr = alloc(Layout::new::<i32>()) as *mut i32;
836 /// // In general .write is required to avoid attempting to destruct
837 /// // the (uninitialized) previous contents of `ptr`, though for this
838 /// // simple example `*ptr = 5` would have worked as well.
840 /// let x = Box::from_raw(ptr);
844 /// [memory layout]: self#memory-layout
845 /// [`Layout`]: crate::Layout
846 #[stable(feature = "box_raw", since = "1.4.0")]
848 pub unsafe fn from_raw(raw: *mut T) -> Self {
849 unsafe { Self::from_raw_in(raw, Global) }
853 impl<T: ?Sized, A: Allocator> Box<T, A> {
854 /// Constructs a box from a raw pointer in the given allocator.
856 /// After calling this function, the raw pointer is owned by the
857 /// resulting `Box`. Specifically, the `Box` destructor will call
858 /// the destructor of `T` and free the allocated memory. For this
859 /// to be safe, the memory must have been allocated in accordance
860 /// with the [memory layout] used by `Box` .
864 /// This function is unsafe because improper use may lead to
865 /// memory problems. For example, a double-free may occur if the
866 /// function is called twice on the same raw pointer.
871 /// Recreate a `Box` which was previously converted to a raw pointer
872 /// using [`Box::into_raw_with_allocator`]:
874 /// #![feature(allocator_api)]
876 /// use std::alloc::System;
878 /// let x = Box::new_in(5, System);
879 /// let (ptr, alloc) = Box::into_raw_with_allocator(x);
880 /// let x = unsafe { Box::from_raw_in(ptr, alloc) };
882 /// Manually create a `Box` from scratch by using the system allocator:
884 /// #![feature(allocator_api, slice_ptr_get)]
886 /// use std::alloc::{Allocator, Layout, System};
889 /// let ptr = System.allocate(Layout::new::<i32>())?.as_mut_ptr() as *mut i32;
890 /// // In general .write is required to avoid attempting to destruct
891 /// // the (uninitialized) previous contents of `ptr`, though for this
892 /// // simple example `*ptr = 5` would have worked as well.
894 /// let x = Box::from_raw_in(ptr, System);
896 /// # Ok::<(), std::alloc::AllocError>(())
899 /// [memory layout]: self#memory-layout
900 /// [`Layout`]: crate::Layout
901 #[unstable(feature = "allocator_api", issue = "32838")]
903 pub unsafe fn from_raw_in(raw: *mut T, alloc: A) -> Self {
904 Box(unsafe { Unique::new_unchecked(raw) }, alloc)
907 /// Consumes the `Box`, returning a wrapped raw pointer.
909 /// The pointer will be properly aligned and non-null.
911 /// After calling this function, the caller is responsible for the
912 /// memory previously managed by the `Box`. In particular, the
913 /// caller should properly destroy `T` and release the memory, taking
914 /// into account the [memory layout] used by `Box`. The easiest way to
915 /// do this is to convert the raw pointer back into a `Box` with the
916 /// [`Box::from_raw`] function, allowing the `Box` destructor to perform
919 /// Note: this is an associated function, which means that you have
920 /// to call it as `Box::into_raw(b)` instead of `b.into_raw()`. This
921 /// is so that there is no conflict with a method on the inner type.
924 /// Converting the raw pointer back into a `Box` with [`Box::from_raw`]
925 /// for automatic cleanup:
927 /// let x = Box::new(String::from("Hello"));
928 /// let ptr = Box::into_raw(x);
929 /// let x = unsafe { Box::from_raw(ptr) };
931 /// Manual cleanup by explicitly running the destructor and deallocating
934 /// use std::alloc::{dealloc, Layout};
937 /// let x = Box::new(String::from("Hello"));
938 /// let p = Box::into_raw(x);
940 /// ptr::drop_in_place(p);
941 /// dealloc(p as *mut u8, Layout::new::<String>());
945 /// [memory layout]: self#memory-layout
946 #[stable(feature = "box_raw", since = "1.4.0")]
948 pub fn into_raw(b: Self) -> *mut T {
949 Self::into_raw_with_allocator(b).0
952 /// Consumes the `Box`, returning a wrapped raw pointer and the allocator.
954 /// The pointer will be properly aligned and non-null.
956 /// After calling this function, the caller is responsible for the
957 /// memory previously managed by the `Box`. In particular, the
958 /// caller should properly destroy `T` and release the memory, taking
959 /// into account the [memory layout] used by `Box`. The easiest way to
960 /// do this is to convert the raw pointer back into a `Box` with the
961 /// [`Box::from_raw_in`] function, allowing the `Box` destructor to perform
964 /// Note: this is an associated function, which means that you have
965 /// to call it as `Box::into_raw_with_allocator(b)` instead of `b.into_raw_with_allocator()`. This
966 /// is so that there is no conflict with a method on the inner type.
969 /// Converting the raw pointer back into a `Box` with [`Box::from_raw_in`]
970 /// for automatic cleanup:
972 /// #![feature(allocator_api)]
974 /// use std::alloc::System;
976 /// let x = Box::new_in(String::from("Hello"), System);
977 /// let (ptr, alloc) = Box::into_raw_with_allocator(x);
978 /// let x = unsafe { Box::from_raw_in(ptr, alloc) };
980 /// Manual cleanup by explicitly running the destructor and deallocating
983 /// #![feature(allocator_api)]
985 /// use std::alloc::{Allocator, Layout, System};
986 /// use std::ptr::{self, NonNull};
988 /// let x = Box::new_in(String::from("Hello"), System);
989 /// let (ptr, alloc) = Box::into_raw_with_allocator(x);
991 /// ptr::drop_in_place(ptr);
992 /// let non_null = NonNull::new_unchecked(ptr);
993 /// alloc.deallocate(non_null.cast(), Layout::new::<String>());
997 /// [memory layout]: self#memory-layout
998 #[unstable(feature = "allocator_api", issue = "32838")]
1000 pub fn into_raw_with_allocator(b: Self) -> (*mut T, A) {
1001 let (leaked, alloc) = Box::into_unique(b);
1002 (leaked.as_ptr(), alloc)
1006 feature = "ptr_internals",
1008 reason = "use `Box::leak(b).into()` or `Unique::from(Box::leak(b))` instead"
1012 pub fn into_unique(b: Self) -> (Unique<T>, A) {
1013 // Box is recognized as a "unique pointer" by Stacked Borrows, but internally it is a
1014 // raw pointer for the type system. Turning it directly into a raw pointer would not be
1015 // recognized as "releasing" the unique pointer to permit aliased raw accesses,
1016 // so all raw pointer methods have to go through `Box::leak`. Turning *that* to a raw pointer
1017 // behaves correctly.
1018 let alloc = unsafe { ptr::read(&b.1) };
1019 (Unique::from(Box::leak(b)), alloc)
1022 /// Returns a reference to the underlying allocator.
1024 /// Note: this is an associated function, which means that you have
1025 /// to call it as `Box::allocator(&b)` instead of `b.allocator()`. This
1026 /// is so that there is no conflict with a method on the inner type.
1027 #[unstable(feature = "allocator_api", issue = "32838")]
1029 pub fn allocator(b: &Self) -> &A {
1033 /// Consumes and leaks the `Box`, returning a mutable reference,
1034 /// `&'a mut T`. Note that the type `T` must outlive the chosen lifetime
1035 /// `'a`. If the type has only static references, or none at all, then this
1036 /// may be chosen to be `'static`.
1038 /// This function is mainly useful for data that lives for the remainder of
1039 /// the program's life. Dropping the returned reference will cause a memory
1040 /// leak. If this is not acceptable, the reference should first be wrapped
1041 /// with the [`Box::from_raw`] function producing a `Box`. This `Box` can
1042 /// then be dropped which will properly destroy `T` and release the
1043 /// allocated memory.
1045 /// Note: this is an associated function, which means that you have
1046 /// to call it as `Box::leak(b)` instead of `b.leak()`. This
1047 /// is so that there is no conflict with a method on the inner type.
1054 /// let x = Box::new(41);
1055 /// let static_ref: &'static mut usize = Box::leak(x);
1056 /// *static_ref += 1;
1057 /// assert_eq!(*static_ref, 42);
1063 /// let x = vec![1, 2, 3].into_boxed_slice();
1064 /// let static_ref = Box::leak(x);
1065 /// static_ref[0] = 4;
1066 /// assert_eq!(*static_ref, [4, 2, 3]);
1068 #[stable(feature = "box_leak", since = "1.26.0")]
1070 pub fn leak<'a>(b: Self) -> &'a mut T
1074 unsafe { &mut *mem::ManuallyDrop::new(b).0.as_ptr() }
1077 /// Converts a `Box<T>` into a `Pin<Box<T>>`
1079 /// This conversion does not allocate on the heap and happens in place.
1081 /// This is also available via [`From`].
1082 #[unstable(feature = "box_into_pin", issue = "62370")]
1083 pub fn into_pin(boxed: Self) -> Pin<Self>
1087 // It's not possible to move or replace the insides of a `Pin<Box<T>>`
1088 // when `T: !Unpin`, so it's safe to pin it directly without any
1089 // additional requirements.
1090 unsafe { Pin::new_unchecked(boxed) }
1094 #[stable(feature = "rust1", since = "1.0.0")]
1095 unsafe impl<#[may_dangle] T: ?Sized, A: Allocator> Drop for Box<T, A> {
1096 fn drop(&mut self) {
1097 // FIXME: Do nothing, drop is currently performed by compiler.
1101 #[cfg(not(no_global_oom_handling))]
1102 #[stable(feature = "rust1", since = "1.0.0")]
1103 impl<T: Default> Default for Box<T> {
1104 /// Creates a `Box<T>`, with the `Default` value for T.
1105 fn default() -> Self {
1110 #[cfg(not(no_global_oom_handling))]
1111 #[stable(feature = "rust1", since = "1.0.0")]
1112 impl<T> Default for Box<[T]> {
1113 fn default() -> Self {
1114 Box::<[T; 0]>::new([])
1118 #[cfg(not(no_global_oom_handling))]
1119 #[stable(feature = "default_box_extra", since = "1.17.0")]
1120 impl Default for Box<str> {
1121 fn default() -> Self {
1122 unsafe { from_boxed_utf8_unchecked(Default::default()) }
1126 #[cfg(not(no_global_oom_handling))]
1127 #[stable(feature = "rust1", since = "1.0.0")]
1128 impl<T: Clone, A: Allocator + Clone> Clone for Box<T, A> {
1129 /// Returns a new box with a `clone()` of this box's contents.
1134 /// let x = Box::new(5);
1135 /// let y = x.clone();
1137 /// // The value is the same
1138 /// assert_eq!(x, y);
1140 /// // But they are unique objects
1141 /// assert_ne!(&*x as *const i32, &*y as *const i32);
1144 fn clone(&self) -> Self {
1145 // Pre-allocate memory to allow writing the cloned value directly.
1146 let mut boxed = Self::new_uninit_in(self.1.clone());
1148 (**self).write_clone_into_raw(boxed.as_mut_ptr());
1153 /// Copies `source`'s contents into `self` without creating a new allocation.
1158 /// let x = Box::new(5);
1159 /// let mut y = Box::new(10);
1160 /// let yp: *const i32 = &*y;
1162 /// y.clone_from(&x);
1164 /// // The value is the same
1165 /// assert_eq!(x, y);
1167 /// // And no allocation occurred
1168 /// assert_eq!(yp, &*y);
1171 fn clone_from(&mut self, source: &Self) {
1172 (**self).clone_from(&(**source));
1176 #[cfg(not(no_global_oom_handling))]
1177 #[stable(feature = "box_slice_clone", since = "1.3.0")]
1178 impl Clone for Box<str> {
1179 fn clone(&self) -> Self {
1180 // this makes a copy of the data
1181 let buf: Box<[u8]> = self.as_bytes().into();
1182 unsafe { from_boxed_utf8_unchecked(buf) }
1186 #[stable(feature = "rust1", since = "1.0.0")]
1187 impl<T: ?Sized + PartialEq, A: Allocator> PartialEq for Box<T, A> {
1189 fn eq(&self, other: &Self) -> bool {
1190 PartialEq::eq(&**self, &**other)
1193 fn ne(&self, other: &Self) -> bool {
1194 PartialEq::ne(&**self, &**other)
1197 #[stable(feature = "rust1", since = "1.0.0")]
1198 impl<T: ?Sized + PartialOrd, A: Allocator> PartialOrd for Box<T, A> {
1200 fn partial_cmp(&self, other: &Self) -> Option<Ordering> {
1201 PartialOrd::partial_cmp(&**self, &**other)
1204 fn lt(&self, other: &Self) -> bool {
1205 PartialOrd::lt(&**self, &**other)
1208 fn le(&self, other: &Self) -> bool {
1209 PartialOrd::le(&**self, &**other)
1212 fn ge(&self, other: &Self) -> bool {
1213 PartialOrd::ge(&**self, &**other)
1216 fn gt(&self, other: &Self) -> bool {
1217 PartialOrd::gt(&**self, &**other)
1220 #[stable(feature = "rust1", since = "1.0.0")]
1221 impl<T: ?Sized + Ord, A: Allocator> Ord for Box<T, A> {
1223 fn cmp(&self, other: &Self) -> Ordering {
1224 Ord::cmp(&**self, &**other)
1227 #[stable(feature = "rust1", since = "1.0.0")]
1228 impl<T: ?Sized + Eq, A: Allocator> Eq for Box<T, A> {}
1230 #[stable(feature = "rust1", since = "1.0.0")]
1231 impl<T: ?Sized + Hash, A: Allocator> Hash for Box<T, A> {
1232 fn hash<H: Hasher>(&self, state: &mut H) {
1233 (**self).hash(state);
1237 #[stable(feature = "indirect_hasher_impl", since = "1.22.0")]
1238 impl<T: ?Sized + Hasher, A: Allocator> Hasher for Box<T, A> {
1239 fn finish(&self) -> u64 {
1242 fn write(&mut self, bytes: &[u8]) {
1243 (**self).write(bytes)
1245 fn write_u8(&mut self, i: u8) {
1246 (**self).write_u8(i)
1248 fn write_u16(&mut self, i: u16) {
1249 (**self).write_u16(i)
1251 fn write_u32(&mut self, i: u32) {
1252 (**self).write_u32(i)
1254 fn write_u64(&mut self, i: u64) {
1255 (**self).write_u64(i)
1257 fn write_u128(&mut self, i: u128) {
1258 (**self).write_u128(i)
1260 fn write_usize(&mut self, i: usize) {
1261 (**self).write_usize(i)
1263 fn write_i8(&mut self, i: i8) {
1264 (**self).write_i8(i)
1266 fn write_i16(&mut self, i: i16) {
1267 (**self).write_i16(i)
1269 fn write_i32(&mut self, i: i32) {
1270 (**self).write_i32(i)
1272 fn write_i64(&mut self, i: i64) {
1273 (**self).write_i64(i)
1275 fn write_i128(&mut self, i: i128) {
1276 (**self).write_i128(i)
1278 fn write_isize(&mut self, i: isize) {
1279 (**self).write_isize(i)
1283 #[cfg(not(no_global_oom_handling))]
1284 #[stable(feature = "from_for_ptrs", since = "1.6.0")]
1285 impl<T> From<T> for Box<T> {
1286 /// Converts a `T` into a `Box<T>`
1288 /// The conversion allocates on the heap and moves `t`
1289 /// from the stack into it.
1295 /// let boxed = Box::new(5);
1297 /// assert_eq!(Box::from(x), boxed);
1299 fn from(t: T) -> Self {
1304 #[stable(feature = "pin", since = "1.33.0")]
1305 impl<T: ?Sized, A: Allocator> From<Box<T, A>> for Pin<Box<T, A>>
1309 /// Converts a `Box<T>` into a `Pin<Box<T>>`
1311 /// This conversion does not allocate on the heap and happens in place.
1312 fn from(boxed: Box<T, A>) -> Self {
1313 Box::into_pin(boxed)
1317 #[cfg(not(no_global_oom_handling))]
1318 #[stable(feature = "box_from_slice", since = "1.17.0")]
1319 impl<T: Copy> From<&[T]> for Box<[T]> {
1320 /// Converts a `&[T]` into a `Box<[T]>`
1322 /// This conversion allocates on the heap
1323 /// and performs a copy of `slice`.
1327 /// // create a &[u8] which will be used to create a Box<[u8]>
1328 /// let slice: &[u8] = &[104, 101, 108, 108, 111];
1329 /// let boxed_slice: Box<[u8]> = Box::from(slice);
1331 /// println!("{:?}", boxed_slice);
1333 fn from(slice: &[T]) -> Box<[T]> {
1334 let len = slice.len();
1335 let buf = RawVec::with_capacity(len);
1337 ptr::copy_nonoverlapping(slice.as_ptr(), buf.ptr(), len);
1338 buf.into_box(slice.len()).assume_init()
1343 #[cfg(not(no_global_oom_handling))]
1344 #[stable(feature = "box_from_cow", since = "1.45.0")]
1345 impl<T: Copy> From<Cow<'_, [T]>> for Box<[T]> {
1346 /// Converts a `Cow<'_, [T]>` into a `Box<[T]>`
1348 /// When `cow` is the `Cow::Borrowed` variant, this
1349 /// conversion allocates on the heap and copies the
1350 /// underlying slice. Otherwise, it will try to reuse the owned
1351 /// `Vec`'s allocation.
1353 fn from(cow: Cow<'_, [T]>) -> Box<[T]> {
1355 Cow::Borrowed(slice) => Box::from(slice),
1356 Cow::Owned(slice) => Box::from(slice),
1361 #[cfg(not(no_global_oom_handling))]
1362 #[stable(feature = "box_from_slice", since = "1.17.0")]
1363 impl From<&str> for Box<str> {
1364 /// Converts a `&str` into a `Box<str>`
1366 /// This conversion allocates on the heap
1367 /// and performs a copy of `s`.
1372 /// let boxed: Box<str> = Box::from("hello");
1373 /// println!("{}", boxed);
1376 fn from(s: &str) -> Box<str> {
1377 unsafe { from_boxed_utf8_unchecked(Box::from(s.as_bytes())) }
1381 #[cfg(not(no_global_oom_handling))]
1382 #[stable(feature = "box_from_cow", since = "1.45.0")]
1383 impl From<Cow<'_, str>> for Box<str> {
1384 /// Converts a `Cow<'_, str>` into a `Box<str>`
1386 /// When `cow` is the `Cow::Borrowed` variant, this
1387 /// conversion allocates on the heap and copies the
1388 /// underlying `str`. Otherwise, it will try to reuse the owned
1389 /// `String`'s allocation.
1394 /// use std::borrow::Cow;
1396 /// let unboxed = Cow::Borrowed("hello");
1397 /// let boxed: Box<str> = Box::from(unboxed);
1398 /// println!("{}", boxed);
1402 /// # use std::borrow::Cow;
1403 /// let unboxed = Cow::Owned("hello".to_string());
1404 /// let boxed: Box<str> = Box::from(unboxed);
1405 /// println!("{}", boxed);
1408 fn from(cow: Cow<'_, str>) -> Box<str> {
1410 Cow::Borrowed(s) => Box::from(s),
1411 Cow::Owned(s) => Box::from(s),
1416 #[stable(feature = "boxed_str_conv", since = "1.19.0")]
1417 impl<A: Allocator> From<Box<str, A>> for Box<[u8], A> {
1418 /// Converts a `Box<str>` into a `Box<[u8]>`
1420 /// This conversion does not allocate on the heap and happens in place.
1424 /// // create a Box<str> which will be used to create a Box<[u8]>
1425 /// let boxed: Box<str> = Box::from("hello");
1426 /// let boxed_str: Box<[u8]> = Box::from(boxed);
1428 /// // create a &[u8] which will be used to create a Box<[u8]>
1429 /// let slice: &[u8] = &[104, 101, 108, 108, 111];
1430 /// let boxed_slice = Box::from(slice);
1432 /// assert_eq!(boxed_slice, boxed_str);
1435 fn from(s: Box<str, A>) -> Self {
1436 let (raw, alloc) = Box::into_raw_with_allocator(s);
1437 unsafe { Box::from_raw_in(raw as *mut [u8], alloc) }
1441 #[cfg(not(no_global_oom_handling))]
1442 #[stable(feature = "box_from_array", since = "1.45.0")]
1443 impl<T, const N: usize> From<[T; N]> for Box<[T]> {
1444 /// Converts a `[T; N]` into a `Box<[T]>`
1446 /// This conversion moves the array to newly heap-allocated memory.
1451 /// let boxed: Box<[u8]> = Box::from([4, 2]);
1452 /// println!("{:?}", boxed);
1454 fn from(array: [T; N]) -> Box<[T]> {
1459 #[stable(feature = "boxed_slice_try_from", since = "1.43.0")]
1460 impl<T, const N: usize> TryFrom<Box<[T]>> for Box<[T; N]> {
1461 type Error = Box<[T]>;
1463 /// Attempts to convert a `Box<[T]>` into a `Box<[T; N]>`.
1465 /// The conversion occurs in-place and does not require a
1466 /// new memory allocation.
1470 /// Returns the old `Box<[T]>` in the `Err` variant if
1471 /// `boxed_slice.len()` does not equal `N`.
1472 fn try_from(boxed_slice: Box<[T]>) -> Result<Self, Self::Error> {
1473 if boxed_slice.len() == N {
1474 Ok(unsafe { Box::from_raw(Box::into_raw(boxed_slice) as *mut [T; N]) })
1481 impl<A: Allocator> Box<dyn Any, A> {
1483 #[stable(feature = "rust1", since = "1.0.0")]
1484 /// Attempt to downcast the box to a concrete type.
1489 /// use std::any::Any;
1491 /// fn print_if_string(value: Box<dyn Any>) {
1492 /// if let Ok(string) = value.downcast::<String>() {
1493 /// println!("String ({}): {}", string.len(), string);
1497 /// let my_string = "Hello World".to_string();
1498 /// print_if_string(Box::new(my_string));
1499 /// print_if_string(Box::new(0i8));
1501 pub fn downcast<T: Any>(self) -> Result<Box<T, A>, Self> {
1504 let (raw, alloc): (*mut dyn Any, _) = Box::into_raw_with_allocator(self);
1505 Ok(Box::from_raw_in(raw as *mut T, alloc))
1513 impl<A: Allocator> Box<dyn Any + Send, A> {
1515 #[stable(feature = "rust1", since = "1.0.0")]
1516 /// Attempt to downcast the box to a concrete type.
1521 /// use std::any::Any;
1523 /// fn print_if_string(value: Box<dyn Any + Send>) {
1524 /// if let Ok(string) = value.downcast::<String>() {
1525 /// println!("String ({}): {}", string.len(), string);
1529 /// let my_string = "Hello World".to_string();
1530 /// print_if_string(Box::new(my_string));
1531 /// print_if_string(Box::new(0i8));
1533 pub fn downcast<T: Any>(self) -> Result<Box<T, A>, Self> {
1536 let (raw, alloc): (*mut (dyn Any + Send), _) = Box::into_raw_with_allocator(self);
1537 Ok(Box::from_raw_in(raw as *mut T, alloc))
1545 impl<A: Allocator> Box<dyn Any + Send + Sync, A> {
1547 #[stable(feature = "box_send_sync_any_downcast", since = "1.51.0")]
1548 /// Attempt to downcast the box to a concrete type.
1553 /// use std::any::Any;
1555 /// fn print_if_string(value: Box<dyn Any + Send + Sync>) {
1556 /// if let Ok(string) = value.downcast::<String>() {
1557 /// println!("String ({}): {}", string.len(), string);
1561 /// let my_string = "Hello World".to_string();
1562 /// print_if_string(Box::new(my_string));
1563 /// print_if_string(Box::new(0i8));
1565 pub fn downcast<T: Any>(self) -> Result<Box<T, A>, Self> {
1568 let (raw, alloc): (*mut (dyn Any + Send + Sync), _) =
1569 Box::into_raw_with_allocator(self);
1570 Ok(Box::from_raw_in(raw as *mut T, alloc))
1578 #[stable(feature = "rust1", since = "1.0.0")]
1579 impl<T: fmt::Display + ?Sized, A: Allocator> fmt::Display for Box<T, A> {
1580 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
1581 fmt::Display::fmt(&**self, f)
1585 #[stable(feature = "rust1", since = "1.0.0")]
1586 impl<T: fmt::Debug + ?Sized, A: Allocator> fmt::Debug for Box<T, A> {
1587 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
1588 fmt::Debug::fmt(&**self, f)
1592 #[stable(feature = "rust1", since = "1.0.0")]
1593 impl<T: ?Sized, A: Allocator> fmt::Pointer for Box<T, A> {
1594 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
1595 // It's not possible to extract the inner Uniq directly from the Box,
1596 // instead we cast it to a *const which aliases the Unique
1597 let ptr: *const T = &**self;
1598 fmt::Pointer::fmt(&ptr, f)
1602 #[stable(feature = "rust1", since = "1.0.0")]
1603 impl<T: ?Sized, A: Allocator> Deref for Box<T, A> {
1606 fn deref(&self) -> &T {
1611 #[stable(feature = "rust1", since = "1.0.0")]
1612 impl<T: ?Sized, A: Allocator> DerefMut for Box<T, A> {
1613 fn deref_mut(&mut self) -> &mut T {
1618 #[unstable(feature = "receiver_trait", issue = "none")]
1619 impl<T: ?Sized, A: Allocator> Receiver for Box<T, A> {}
1621 #[stable(feature = "rust1", since = "1.0.0")]
1622 impl<I: Iterator + ?Sized, A: Allocator> Iterator for Box<I, A> {
1623 type Item = I::Item;
1624 fn next(&mut self) -> Option<I::Item> {
1627 fn size_hint(&self) -> (usize, Option<usize>) {
1628 (**self).size_hint()
1630 fn nth(&mut self, n: usize) -> Option<I::Item> {
1633 fn last(self) -> Option<I::Item> {
1640 fn last(self) -> Option<Self::Item>;
1643 impl<I: Iterator + ?Sized, A: Allocator> BoxIter for Box<I, A> {
1644 type Item = I::Item;
1645 default fn last(self) -> Option<I::Item> {
1647 fn some<T>(_: Option<T>, x: T) -> Option<T> {
1651 self.fold(None, some)
1655 /// Specialization for sized `I`s that uses `I`s implementation of `last()`
1656 /// instead of the default.
1657 #[stable(feature = "rust1", since = "1.0.0")]
1658 impl<I: Iterator, A: Allocator> BoxIter for Box<I, A> {
1659 fn last(self) -> Option<I::Item> {
1664 #[stable(feature = "rust1", since = "1.0.0")]
1665 impl<I: DoubleEndedIterator + ?Sized, A: Allocator> DoubleEndedIterator for Box<I, A> {
1666 fn next_back(&mut self) -> Option<I::Item> {
1667 (**self).next_back()
1669 fn nth_back(&mut self, n: usize) -> Option<I::Item> {
1670 (**self).nth_back(n)
1673 #[stable(feature = "rust1", since = "1.0.0")]
1674 impl<I: ExactSizeIterator + ?Sized, A: Allocator> ExactSizeIterator for Box<I, A> {
1675 fn len(&self) -> usize {
1678 fn is_empty(&self) -> bool {
1683 #[stable(feature = "fused", since = "1.26.0")]
1684 impl<I: FusedIterator + ?Sized, A: Allocator> FusedIterator for Box<I, A> {}
1686 #[stable(feature = "boxed_closure_impls", since = "1.35.0")]
1687 impl<Args, F: FnOnce<Args> + ?Sized, A: Allocator> FnOnce<Args> for Box<F, A> {
1688 type Output = <F as FnOnce<Args>>::Output;
1690 extern "rust-call" fn call_once(self, args: Args) -> Self::Output {
1691 <F as FnOnce<Args>>::call_once(*self, args)
1695 #[stable(feature = "boxed_closure_impls", since = "1.35.0")]
1696 impl<Args, F: FnMut<Args> + ?Sized, A: Allocator> FnMut<Args> for Box<F, A> {
1697 extern "rust-call" fn call_mut(&mut self, args: Args) -> Self::Output {
1698 <F as FnMut<Args>>::call_mut(self, args)
1702 #[stable(feature = "boxed_closure_impls", since = "1.35.0")]
1703 impl<Args, F: Fn<Args> + ?Sized, A: Allocator> Fn<Args> for Box<F, A> {
1704 extern "rust-call" fn call(&self, args: Args) -> Self::Output {
1705 <F as Fn<Args>>::call(self, args)
1709 #[unstable(feature = "coerce_unsized", issue = "27732")]
1710 impl<T: ?Sized + Unsize<U>, U: ?Sized, A: Allocator> CoerceUnsized<Box<U, A>> for Box<T, A> {}
1712 #[unstable(feature = "dispatch_from_dyn", issue = "none")]
1713 impl<T: ?Sized + Unsize<U>, U: ?Sized> DispatchFromDyn<Box<U>> for Box<T, Global> {}
1715 #[cfg(not(no_global_oom_handling))]
1716 #[stable(feature = "boxed_slice_from_iter", since = "1.32.0")]
1717 impl<I> FromIterator<I> for Box<[I]> {
1718 fn from_iter<T: IntoIterator<Item = I>>(iter: T) -> Self {
1719 iter.into_iter().collect::<Vec<_>>().into_boxed_slice()
1723 #[cfg(not(no_global_oom_handling))]
1724 #[stable(feature = "box_slice_clone", since = "1.3.0")]
1725 impl<T: Clone, A: Allocator + Clone> Clone for Box<[T], A> {
1726 fn clone(&self) -> Self {
1727 let alloc = Box::allocator(self).clone();
1728 self.to_vec_in(alloc).into_boxed_slice()
1731 fn clone_from(&mut self, other: &Self) {
1732 if self.len() == other.len() {
1733 self.clone_from_slice(&other);
1735 *self = other.clone();
1740 #[stable(feature = "box_borrow", since = "1.1.0")]
1741 impl<T: ?Sized, A: Allocator> borrow::Borrow<T> for Box<T, A> {
1742 fn borrow(&self) -> &T {
1747 #[stable(feature = "box_borrow", since = "1.1.0")]
1748 impl<T: ?Sized, A: Allocator> borrow::BorrowMut<T> for Box<T, A> {
1749 fn borrow_mut(&mut self) -> &mut T {
1754 #[stable(since = "1.5.0", feature = "smart_ptr_as_ref")]
1755 impl<T: ?Sized, A: Allocator> AsRef<T> for Box<T, A> {
1756 fn as_ref(&self) -> &T {
1761 #[stable(since = "1.5.0", feature = "smart_ptr_as_ref")]
1762 impl<T: ?Sized, A: Allocator> AsMut<T> for Box<T, A> {
1763 fn as_mut(&mut self) -> &mut T {
1770 * We could have chosen not to add this impl, and instead have written a
1771 * function of Pin<Box<T>> to Pin<T>. Such a function would not be sound,
1772 * because Box<T> implements Unpin even when T does not, as a result of
1775 * We chose this API instead of the alternative for a few reasons:
1776 * - Logically, it is helpful to understand pinning in regard to the
1777 * memory region being pointed to. For this reason none of the
1778 * standard library pointer types support projecting through a pin
1779 * (Box<T> is the only pointer type in std for which this would be
1781 * - It is in practice very useful to have Box<T> be unconditionally
1782 * Unpin because of trait objects, for which the structural auto
1783 * trait functionality does not apply (e.g., Box<dyn Foo> would
1784 * otherwise not be Unpin).
1786 * Another type with the same semantics as Box but only a conditional
1787 * implementation of `Unpin` (where `T: Unpin`) would be valid/safe, and
1788 * could have a method to project a Pin<T> from it.
1790 #[stable(feature = "pin", since = "1.33.0")]
1791 impl<T: ?Sized, A: Allocator> Unpin for Box<T, A> where A: 'static {}
1793 #[unstable(feature = "generator_trait", issue = "43122")]
1794 impl<G: ?Sized + Generator<R> + Unpin, R, A: Allocator> Generator<R> for Box<G, A>
1798 type Yield = G::Yield;
1799 type Return = G::Return;
1801 fn resume(mut self: Pin<&mut Self>, arg: R) -> GeneratorState<Self::Yield, Self::Return> {
1802 G::resume(Pin::new(&mut *self), arg)
1806 #[unstable(feature = "generator_trait", issue = "43122")]
1807 impl<G: ?Sized + Generator<R>, R, A: Allocator> Generator<R> for Pin<Box<G, A>>
1811 type Yield = G::Yield;
1812 type Return = G::Return;
1814 fn resume(mut self: Pin<&mut Self>, arg: R) -> GeneratorState<Self::Yield, Self::Return> {
1815 G::resume((*self).as_mut(), arg)
1819 #[stable(feature = "futures_api", since = "1.36.0")]
1820 impl<F: ?Sized + Future + Unpin, A: Allocator> Future for Box<F, A>
1824 type Output = F::Output;
1826 fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
1827 F::poll(Pin::new(&mut *self), cx)
1831 #[unstable(feature = "async_stream", issue = "79024")]
1832 impl<S: ?Sized + Stream + Unpin> Stream for Box<S> {
1833 type Item = S::Item;
1835 fn poll_next(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Option<Self::Item>> {
1836 Pin::new(&mut **self).poll_next(cx)
1839 fn size_hint(&self) -> (usize, Option<usize>) {
1840 (**self).size_hint()