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 //! # Considerations for unsafe code
127 //! **Warning: This section is not normative and is subject to change, possibly
128 //! being relaxed in the future! It is a simplified summary of the rules
129 //! currently implemented in the compiler.**
131 //! The aliasing rules for `Box<T>` are the same as for `&mut T`. `Box<T>`
132 //! asserts uniqueness over its content. Using raw pointers derived from a box
133 //! after that box has been mutated through, moved or borrowed as `&mut T`
134 //! is not allowed. For more guidance on working with box from unsafe code, see
135 //! [rust-lang/unsafe-code-guidelines#326][ucg#326].
138 //! [ucg#198]: https://github.com/rust-lang/unsafe-code-guidelines/issues/198
139 //! [ucg#326]: https://github.com/rust-lang/unsafe-code-guidelines/issues/326
140 //! [dereferencing]: core::ops::Deref
141 //! [`Box::<T>::from_raw(value)`]: Box::from_raw
142 //! [`Global`]: crate::alloc::Global
143 //! [`Layout`]: crate::alloc::Layout
144 //! [`Layout::for_value(&*value)`]: crate::alloc::Layout::for_value
145 //! [valid]: ptr#safety
147 #![stable(feature = "rust1", since = "1.0.0")]
150 use core::async_iter::AsyncIterator;
152 use core::cmp::Ordering;
153 use core::convert::{From, TryFrom};
155 use core::future::Future;
156 use core::hash::{Hash, Hasher};
157 #[cfg(not(no_global_oom_handling))]
158 use core::iter::FromIterator;
159 use core::iter::{FusedIterator, Iterator};
160 use core::marker::{Destruct, Unpin, Unsize};
163 CoerceUnsized, Deref, DerefMut, DispatchFromDyn, Generator, GeneratorState, Receiver,
166 use core::ptr::{self, Unique};
167 use core::task::{Context, Poll};
169 #[cfg(not(no_global_oom_handling))]
170 use crate::alloc::{handle_alloc_error, WriteCloneIntoRaw};
171 use crate::alloc::{AllocError, Allocator, Global, Layout};
172 #[cfg(not(no_global_oom_handling))]
173 use crate::borrow::Cow;
174 use crate::raw_vec::RawVec;
175 #[cfg(not(no_global_oom_handling))]
176 use crate::str::from_boxed_utf8_unchecked;
177 #[cfg(not(no_global_oom_handling))]
180 #[unstable(feature = "thin_box", issue = "92791")]
181 pub use thin::ThinBox;
185 /// A pointer type for heap allocation.
187 /// See the [module-level documentation](../../std/boxed/index.html) for more.
188 #[lang = "owned_box"]
190 #[stable(feature = "rust1", since = "1.0.0")]
191 // The declaration of the `Box` struct must be kept in sync with the
192 // `alloc::alloc::box_free` function or ICEs will happen. See the comment
193 // on `box_free` for more details.
196 #[unstable(feature = "allocator_api", issue = "32838")] A: Allocator = Global,
200 /// Allocates memory on the heap and then places `x` into it.
202 /// This doesn't actually allocate if `T` is zero-sized.
207 /// let five = Box::new(5);
209 #[cfg(all(not(no_global_oom_handling), not(bootstrap)))]
211 #[stable(feature = "rust1", since = "1.0.0")]
213 pub fn new(x: T) -> Self {
218 /// Allocates memory on the heap and then places `x` into it.
220 /// This doesn't actually allocate if `T` is zero-sized.
225 /// let five = Box::new(5);
227 #[cfg(all(not(no_global_oom_handling), bootstrap))]
229 #[stable(feature = "rust1", since = "1.0.0")]
231 pub fn new(x: T) -> Self {
235 /// Constructs a new box with uninitialized contents.
240 /// #![feature(new_uninit)]
242 /// let mut five = Box::<u32>::new_uninit();
244 /// let five = unsafe {
245 /// // Deferred initialization:
246 /// five.as_mut_ptr().write(5);
248 /// five.assume_init()
251 /// assert_eq!(*five, 5)
253 #[cfg(not(no_global_oom_handling))]
254 #[unstable(feature = "new_uninit", issue = "63291")]
257 pub fn new_uninit() -> Box<mem::MaybeUninit<T>> {
258 Self::new_uninit_in(Global)
261 /// Constructs a new `Box` with uninitialized contents, with the memory
262 /// being filled with `0` bytes.
264 /// See [`MaybeUninit::zeroed`][zeroed] for examples of correct and incorrect usage
270 /// #![feature(new_uninit)]
272 /// let zero = Box::<u32>::new_zeroed();
273 /// let zero = unsafe { zero.assume_init() };
275 /// assert_eq!(*zero, 0)
278 /// [zeroed]: mem::MaybeUninit::zeroed
279 #[cfg(not(no_global_oom_handling))]
281 #[unstable(feature = "new_uninit", issue = "63291")]
283 pub fn new_zeroed() -> Box<mem::MaybeUninit<T>> {
284 Self::new_zeroed_in(Global)
287 /// Constructs a new `Pin<Box<T>>`. If `T` does not implement `Unpin`, then
288 /// `x` will be pinned in memory and unable to be moved.
289 #[cfg(not(no_global_oom_handling))]
290 #[stable(feature = "pin", since = "1.33.0")]
293 pub fn pin(x: T) -> Pin<Box<T>> {
294 (#[cfg_attr(not(bootstrap), rustc_box)]
299 /// Allocates memory on the heap then places `x` into it,
300 /// returning an error if the allocation fails
302 /// This doesn't actually allocate if `T` is zero-sized.
307 /// #![feature(allocator_api)]
309 /// let five = Box::try_new(5)?;
310 /// # Ok::<(), std::alloc::AllocError>(())
312 #[unstable(feature = "allocator_api", issue = "32838")]
314 pub fn try_new(x: T) -> Result<Self, AllocError> {
315 Self::try_new_in(x, Global)
318 /// Constructs a new box with uninitialized contents on the heap,
319 /// returning an error if the allocation fails
324 /// #![feature(allocator_api, new_uninit)]
326 /// let mut five = Box::<u32>::try_new_uninit()?;
328 /// let five = unsafe {
329 /// // Deferred initialization:
330 /// five.as_mut_ptr().write(5);
332 /// five.assume_init()
335 /// assert_eq!(*five, 5);
336 /// # Ok::<(), std::alloc::AllocError>(())
338 #[unstable(feature = "allocator_api", issue = "32838")]
339 // #[unstable(feature = "new_uninit", issue = "63291")]
341 pub fn try_new_uninit() -> Result<Box<mem::MaybeUninit<T>>, AllocError> {
342 Box::try_new_uninit_in(Global)
345 /// Constructs a new `Box` with uninitialized contents, with the memory
346 /// being filled with `0` bytes on the heap
348 /// See [`MaybeUninit::zeroed`][zeroed] for examples of correct and incorrect usage
354 /// #![feature(allocator_api, new_uninit)]
356 /// let zero = Box::<u32>::try_new_zeroed()?;
357 /// let zero = unsafe { zero.assume_init() };
359 /// assert_eq!(*zero, 0);
360 /// # Ok::<(), std::alloc::AllocError>(())
363 /// [zeroed]: mem::MaybeUninit::zeroed
364 #[unstable(feature = "allocator_api", issue = "32838")]
365 // #[unstable(feature = "new_uninit", issue = "63291")]
367 pub fn try_new_zeroed() -> Result<Box<mem::MaybeUninit<T>>, AllocError> {
368 Box::try_new_zeroed_in(Global)
372 impl<T, A: Allocator> Box<T, A> {
373 /// Allocates memory in the given allocator then places `x` into it.
375 /// This doesn't actually allocate if `T` is zero-sized.
380 /// #![feature(allocator_api)]
382 /// use std::alloc::System;
384 /// let five = Box::new_in(5, System);
386 #[cfg(not(no_global_oom_handling))]
387 #[unstable(feature = "allocator_api", issue = "32838")]
388 #[rustc_const_unstable(feature = "const_box", issue = "92521")]
391 pub const fn new_in(x: T, alloc: A) -> Self
393 A: ~const Allocator + ~const Destruct,
395 let mut boxed = Self::new_uninit_in(alloc);
397 boxed.as_mut_ptr().write(x);
402 /// Allocates memory in the given allocator then places `x` into it,
403 /// returning an error if the allocation fails
405 /// This doesn't actually allocate if `T` is zero-sized.
410 /// #![feature(allocator_api)]
412 /// use std::alloc::System;
414 /// let five = Box::try_new_in(5, System)?;
415 /// # Ok::<(), std::alloc::AllocError>(())
417 #[unstable(feature = "allocator_api", issue = "32838")]
418 #[rustc_const_unstable(feature = "const_box", issue = "92521")]
420 pub const fn try_new_in(x: T, alloc: A) -> Result<Self, AllocError>
423 A: ~const Allocator + ~const Destruct,
425 let mut boxed = Self::try_new_uninit_in(alloc)?;
427 boxed.as_mut_ptr().write(x);
428 Ok(boxed.assume_init())
432 /// Constructs a new box with uninitialized contents in the provided allocator.
437 /// #![feature(allocator_api, new_uninit)]
439 /// use std::alloc::System;
441 /// let mut five = Box::<u32, _>::new_uninit_in(System);
443 /// let five = unsafe {
444 /// // Deferred initialization:
445 /// five.as_mut_ptr().write(5);
447 /// five.assume_init()
450 /// assert_eq!(*five, 5)
452 #[unstable(feature = "allocator_api", issue = "32838")]
453 #[rustc_const_unstable(feature = "const_box", issue = "92521")]
454 #[cfg(not(no_global_oom_handling))]
456 // #[unstable(feature = "new_uninit", issue = "63291")]
457 pub const fn new_uninit_in(alloc: A) -> Box<mem::MaybeUninit<T>, A>
459 A: ~const Allocator + ~const Destruct,
461 let layout = Layout::new::<mem::MaybeUninit<T>>();
462 // NOTE: Prefer match over unwrap_or_else since closure sometimes not inlineable.
463 // That would make code size bigger.
464 match Box::try_new_uninit_in(alloc) {
466 Err(_) => handle_alloc_error(layout),
470 /// Constructs a new box with uninitialized contents in the provided allocator,
471 /// returning an error if the allocation fails
476 /// #![feature(allocator_api, new_uninit)]
478 /// use std::alloc::System;
480 /// let mut five = Box::<u32, _>::try_new_uninit_in(System)?;
482 /// let five = unsafe {
483 /// // Deferred initialization:
484 /// five.as_mut_ptr().write(5);
486 /// five.assume_init()
489 /// assert_eq!(*five, 5);
490 /// # Ok::<(), std::alloc::AllocError>(())
492 #[unstable(feature = "allocator_api", issue = "32838")]
493 // #[unstable(feature = "new_uninit", issue = "63291")]
494 #[rustc_const_unstable(feature = "const_box", issue = "92521")]
495 pub const fn try_new_uninit_in(alloc: A) -> Result<Box<mem::MaybeUninit<T>, A>, AllocError>
497 A: ~const Allocator + ~const Destruct,
499 let layout = Layout::new::<mem::MaybeUninit<T>>();
500 let ptr = alloc.allocate(layout)?.cast();
501 unsafe { Ok(Box::from_raw_in(ptr.as_ptr(), alloc)) }
504 /// Constructs a new `Box` with uninitialized contents, with the memory
505 /// being filled with `0` bytes in the provided allocator.
507 /// See [`MaybeUninit::zeroed`][zeroed] for examples of correct and incorrect usage
513 /// #![feature(allocator_api, new_uninit)]
515 /// use std::alloc::System;
517 /// let zero = Box::<u32, _>::new_zeroed_in(System);
518 /// let zero = unsafe { zero.assume_init() };
520 /// assert_eq!(*zero, 0)
523 /// [zeroed]: mem::MaybeUninit::zeroed
524 #[unstable(feature = "allocator_api", issue = "32838")]
525 #[rustc_const_unstable(feature = "const_box", issue = "92521")]
526 #[cfg(not(no_global_oom_handling))]
527 // #[unstable(feature = "new_uninit", issue = "63291")]
529 pub const fn new_zeroed_in(alloc: A) -> Box<mem::MaybeUninit<T>, A>
531 A: ~const Allocator + ~const Destruct,
533 let layout = Layout::new::<mem::MaybeUninit<T>>();
534 // NOTE: Prefer match over unwrap_or_else since closure sometimes not inlineable.
535 // That would make code size bigger.
536 match Box::try_new_zeroed_in(alloc) {
538 Err(_) => handle_alloc_error(layout),
542 /// Constructs a new `Box` with uninitialized contents, with the memory
543 /// being filled with `0` bytes in the provided allocator,
544 /// returning an error if the allocation fails,
546 /// See [`MaybeUninit::zeroed`][zeroed] for examples of correct and incorrect usage
552 /// #![feature(allocator_api, new_uninit)]
554 /// use std::alloc::System;
556 /// let zero = Box::<u32, _>::try_new_zeroed_in(System)?;
557 /// let zero = unsafe { zero.assume_init() };
559 /// assert_eq!(*zero, 0);
560 /// # Ok::<(), std::alloc::AllocError>(())
563 /// [zeroed]: mem::MaybeUninit::zeroed
564 #[unstable(feature = "allocator_api", issue = "32838")]
565 // #[unstable(feature = "new_uninit", issue = "63291")]
566 #[rustc_const_unstable(feature = "const_box", issue = "92521")]
567 pub const fn try_new_zeroed_in(alloc: A) -> Result<Box<mem::MaybeUninit<T>, A>, AllocError>
569 A: ~const Allocator + ~const Destruct,
571 let layout = Layout::new::<mem::MaybeUninit<T>>();
572 let ptr = alloc.allocate_zeroed(layout)?.cast();
573 unsafe { Ok(Box::from_raw_in(ptr.as_ptr(), alloc)) }
576 /// Constructs a new `Pin<Box<T, A>>`. If `T` does not implement `Unpin`, then
577 /// `x` will be pinned in memory and unable to be moved.
578 #[cfg(not(no_global_oom_handling))]
579 #[unstable(feature = "allocator_api", issue = "32838")]
580 #[rustc_const_unstable(feature = "const_box", issue = "92521")]
583 pub const fn pin_in(x: T, alloc: A) -> Pin<Self>
585 A: 'static + ~const Allocator + ~const Destruct,
587 Self::into_pin(Self::new_in(x, alloc))
590 /// Converts a `Box<T>` into a `Box<[T]>`
592 /// This conversion does not allocate on the heap and happens in place.
593 #[unstable(feature = "box_into_boxed_slice", issue = "71582")]
594 #[rustc_const_unstable(feature = "const_box", issue = "92521")]
595 pub const fn into_boxed_slice(boxed: Self) -> Box<[T], A> {
596 let (raw, alloc) = Box::into_raw_with_allocator(boxed);
597 unsafe { Box::from_raw_in(raw as *mut [T; 1], alloc) }
600 /// Consumes the `Box`, returning the wrapped value.
605 /// #![feature(box_into_inner)]
607 /// let c = Box::new(5);
609 /// assert_eq!(Box::into_inner(c), 5);
611 #[unstable(feature = "box_into_inner", issue = "80437")]
612 #[rustc_const_unstable(feature = "const_box", issue = "92521")]
614 pub const fn into_inner(boxed: Self) -> T
616 Self: ~const Destruct,
623 /// Constructs a new boxed slice with uninitialized contents.
628 /// #![feature(new_uninit)]
630 /// let mut values = Box::<[u32]>::new_uninit_slice(3);
632 /// let values = unsafe {
633 /// // Deferred initialization:
634 /// values[0].as_mut_ptr().write(1);
635 /// values[1].as_mut_ptr().write(2);
636 /// values[2].as_mut_ptr().write(3);
638 /// values.assume_init()
641 /// assert_eq!(*values, [1, 2, 3])
643 #[cfg(not(no_global_oom_handling))]
644 #[unstable(feature = "new_uninit", issue = "63291")]
646 pub fn new_uninit_slice(len: usize) -> Box<[mem::MaybeUninit<T>]> {
647 unsafe { RawVec::with_capacity(len).into_box(len) }
650 /// Constructs a new boxed slice with uninitialized contents, with the memory
651 /// being filled with `0` bytes.
653 /// See [`MaybeUninit::zeroed`][zeroed] for examples of correct and incorrect usage
659 /// #![feature(new_uninit)]
661 /// let values = Box::<[u32]>::new_zeroed_slice(3);
662 /// let values = unsafe { values.assume_init() };
664 /// assert_eq!(*values, [0, 0, 0])
667 /// [zeroed]: mem::MaybeUninit::zeroed
668 #[cfg(not(no_global_oom_handling))]
669 #[unstable(feature = "new_uninit", issue = "63291")]
671 pub fn new_zeroed_slice(len: usize) -> Box<[mem::MaybeUninit<T>]> {
672 unsafe { RawVec::with_capacity_zeroed(len).into_box(len) }
675 /// Constructs a new boxed slice with uninitialized contents. Returns an error if
676 /// the allocation fails
681 /// #![feature(allocator_api, new_uninit)]
683 /// let mut values = Box::<[u32]>::try_new_uninit_slice(3)?;
684 /// let values = unsafe {
685 /// // Deferred initialization:
686 /// values[0].as_mut_ptr().write(1);
687 /// values[1].as_mut_ptr().write(2);
688 /// values[2].as_mut_ptr().write(3);
689 /// values.assume_init()
692 /// assert_eq!(*values, [1, 2, 3]);
693 /// # Ok::<(), std::alloc::AllocError>(())
695 #[unstable(feature = "allocator_api", issue = "32838")]
697 pub fn try_new_uninit_slice(len: usize) -> Result<Box<[mem::MaybeUninit<T>]>, AllocError> {
699 let layout = match Layout::array::<mem::MaybeUninit<T>>(len) {
701 Err(_) => return Err(AllocError),
703 let ptr = Global.allocate(layout)?;
704 Ok(RawVec::from_raw_parts_in(ptr.as_mut_ptr() as *mut _, len, Global).into_box(len))
708 /// Constructs a new boxed slice with uninitialized contents, with the memory
709 /// being filled with `0` bytes. Returns an error if the allocation fails
711 /// See [`MaybeUninit::zeroed`][zeroed] for examples of correct and incorrect usage
717 /// #![feature(allocator_api, new_uninit)]
719 /// let values = Box::<[u32]>::try_new_zeroed_slice(3)?;
720 /// let values = unsafe { values.assume_init() };
722 /// assert_eq!(*values, [0, 0, 0]);
723 /// # Ok::<(), std::alloc::AllocError>(())
726 /// [zeroed]: mem::MaybeUninit::zeroed
727 #[unstable(feature = "allocator_api", issue = "32838")]
729 pub fn try_new_zeroed_slice(len: usize) -> Result<Box<[mem::MaybeUninit<T>]>, AllocError> {
731 let layout = match Layout::array::<mem::MaybeUninit<T>>(len) {
733 Err(_) => return Err(AllocError),
735 let ptr = Global.allocate_zeroed(layout)?;
736 Ok(RawVec::from_raw_parts_in(ptr.as_mut_ptr() as *mut _, len, Global).into_box(len))
741 impl<T, A: Allocator> Box<[T], A> {
742 /// Constructs a new boxed slice with uninitialized contents in the provided allocator.
747 /// #![feature(allocator_api, new_uninit)]
749 /// use std::alloc::System;
751 /// let mut values = Box::<[u32], _>::new_uninit_slice_in(3, System);
753 /// let values = unsafe {
754 /// // Deferred initialization:
755 /// values[0].as_mut_ptr().write(1);
756 /// values[1].as_mut_ptr().write(2);
757 /// values[2].as_mut_ptr().write(3);
759 /// values.assume_init()
762 /// assert_eq!(*values, [1, 2, 3])
764 #[cfg(not(no_global_oom_handling))]
765 #[unstable(feature = "allocator_api", issue = "32838")]
766 // #[unstable(feature = "new_uninit", issue = "63291")]
768 pub fn new_uninit_slice_in(len: usize, alloc: A) -> Box<[mem::MaybeUninit<T>], A> {
769 unsafe { RawVec::with_capacity_in(len, alloc).into_box(len) }
772 /// Constructs a new boxed slice with uninitialized contents in the provided allocator,
773 /// with the memory being filled with `0` bytes.
775 /// See [`MaybeUninit::zeroed`][zeroed] for examples of correct and incorrect usage
781 /// #![feature(allocator_api, new_uninit)]
783 /// use std::alloc::System;
785 /// let values = Box::<[u32], _>::new_zeroed_slice_in(3, System);
786 /// let values = unsafe { values.assume_init() };
788 /// assert_eq!(*values, [0, 0, 0])
791 /// [zeroed]: mem::MaybeUninit::zeroed
792 #[cfg(not(no_global_oom_handling))]
793 #[unstable(feature = "allocator_api", issue = "32838")]
794 // #[unstable(feature = "new_uninit", issue = "63291")]
796 pub fn new_zeroed_slice_in(len: usize, alloc: A) -> Box<[mem::MaybeUninit<T>], A> {
797 unsafe { RawVec::with_capacity_zeroed_in(len, alloc).into_box(len) }
801 impl<T, A: Allocator> Box<mem::MaybeUninit<T>, A> {
802 /// Converts to `Box<T, A>`.
806 /// As with [`MaybeUninit::assume_init`],
807 /// it is up to the caller to guarantee that the value
808 /// really is in an initialized state.
809 /// Calling this when the content is not yet fully initialized
810 /// causes immediate undefined behavior.
812 /// [`MaybeUninit::assume_init`]: mem::MaybeUninit::assume_init
817 /// #![feature(new_uninit)]
819 /// let mut five = Box::<u32>::new_uninit();
821 /// let five: Box<u32> = unsafe {
822 /// // Deferred initialization:
823 /// five.as_mut_ptr().write(5);
825 /// five.assume_init()
828 /// assert_eq!(*five, 5)
830 #[unstable(feature = "new_uninit", issue = "63291")]
831 #[rustc_const_unstable(feature = "const_box", issue = "92521")]
833 pub const unsafe fn assume_init(self) -> Box<T, A> {
834 let (raw, alloc) = Box::into_raw_with_allocator(self);
835 unsafe { Box::from_raw_in(raw as *mut T, alloc) }
838 /// Writes the value and converts to `Box<T, A>`.
840 /// This method converts the box similarly to [`Box::assume_init`] but
841 /// writes `value` into it before conversion thus guaranteeing safety.
842 /// In some scenarios use of this method may improve performance because
843 /// the compiler may be able to optimize copying from stack.
848 /// #![feature(new_uninit)]
850 /// let big_box = Box::<[usize; 1024]>::new_uninit();
852 /// let mut array = [0; 1024];
853 /// for (i, place) in array.iter_mut().enumerate() {
857 /// // The optimizer may be able to elide this copy, so previous code writes
858 /// // to heap directly.
859 /// let big_box = Box::write(big_box, array);
861 /// for (i, x) in big_box.iter().enumerate() {
862 /// assert_eq!(*x, i);
865 #[unstable(feature = "new_uninit", issue = "63291")]
866 #[rustc_const_unstable(feature = "const_box", issue = "92521")]
868 pub const fn write(mut boxed: Self, value: T) -> Box<T, A> {
870 (*boxed).write(value);
876 impl<T, A: Allocator> Box<[mem::MaybeUninit<T>], A> {
877 /// Converts to `Box<[T], A>`.
881 /// As with [`MaybeUninit::assume_init`],
882 /// it is up to the caller to guarantee that the values
883 /// really are in an initialized state.
884 /// Calling this when the content is not yet fully initialized
885 /// causes immediate undefined behavior.
887 /// [`MaybeUninit::assume_init`]: mem::MaybeUninit::assume_init
892 /// #![feature(new_uninit)]
894 /// let mut values = Box::<[u32]>::new_uninit_slice(3);
896 /// let values = unsafe {
897 /// // Deferred initialization:
898 /// values[0].as_mut_ptr().write(1);
899 /// values[1].as_mut_ptr().write(2);
900 /// values[2].as_mut_ptr().write(3);
902 /// values.assume_init()
905 /// assert_eq!(*values, [1, 2, 3])
907 #[unstable(feature = "new_uninit", issue = "63291")]
909 pub unsafe fn assume_init(self) -> Box<[T], A> {
910 let (raw, alloc) = Box::into_raw_with_allocator(self);
911 unsafe { Box::from_raw_in(raw as *mut [T], alloc) }
915 impl<T: ?Sized> Box<T> {
916 /// Constructs a box from a raw pointer.
918 /// After calling this function, the raw pointer is owned by the
919 /// resulting `Box`. Specifically, the `Box` destructor will call
920 /// the destructor of `T` and free the allocated memory. For this
921 /// to be safe, the memory must have been allocated in accordance
922 /// with the [memory layout] used by `Box` .
926 /// This function is unsafe because improper use may lead to
927 /// memory problems. For example, a double-free may occur if the
928 /// function is called twice on the same raw pointer.
930 /// The safety conditions are described in the [memory layout] section.
934 /// Recreate a `Box` which was previously converted to a raw pointer
935 /// using [`Box::into_raw`]:
937 /// let x = Box::new(5);
938 /// let ptr = Box::into_raw(x);
939 /// let x = unsafe { Box::from_raw(ptr) };
941 /// Manually create a `Box` from scratch by using the global allocator:
943 /// use std::alloc::{alloc, Layout};
946 /// let ptr = alloc(Layout::new::<i32>()) as *mut i32;
947 /// // In general .write is required to avoid attempting to destruct
948 /// // the (uninitialized) previous contents of `ptr`, though for this
949 /// // simple example `*ptr = 5` would have worked as well.
951 /// let x = Box::from_raw(ptr);
955 /// [memory layout]: self#memory-layout
956 /// [`Layout`]: crate::Layout
957 #[stable(feature = "box_raw", since = "1.4.0")]
959 pub unsafe fn from_raw(raw: *mut T) -> Self {
960 unsafe { Self::from_raw_in(raw, Global) }
964 impl<T: ?Sized, A: Allocator> Box<T, A> {
965 /// Constructs a box from a raw pointer in the given allocator.
967 /// After calling this function, the raw pointer is owned by the
968 /// resulting `Box`. Specifically, the `Box` destructor will call
969 /// the destructor of `T` and free the allocated memory. For this
970 /// to be safe, the memory must have been allocated in accordance
971 /// with the [memory layout] used by `Box` .
975 /// This function is unsafe because improper use may lead to
976 /// memory problems. For example, a double-free may occur if the
977 /// function is called twice on the same raw pointer.
982 /// Recreate a `Box` which was previously converted to a raw pointer
983 /// using [`Box::into_raw_with_allocator`]:
985 /// #![feature(allocator_api)]
987 /// use std::alloc::System;
989 /// let x = Box::new_in(5, System);
990 /// let (ptr, alloc) = Box::into_raw_with_allocator(x);
991 /// let x = unsafe { Box::from_raw_in(ptr, alloc) };
993 /// Manually create a `Box` from scratch by using the system allocator:
995 /// #![feature(allocator_api, slice_ptr_get)]
997 /// use std::alloc::{Allocator, Layout, System};
1000 /// let ptr = System.allocate(Layout::new::<i32>())?.as_mut_ptr() as *mut i32;
1001 /// // In general .write is required to avoid attempting to destruct
1002 /// // the (uninitialized) previous contents of `ptr`, though for this
1003 /// // simple example `*ptr = 5` would have worked as well.
1005 /// let x = Box::from_raw_in(ptr, System);
1007 /// # Ok::<(), std::alloc::AllocError>(())
1010 /// [memory layout]: self#memory-layout
1011 /// [`Layout`]: crate::Layout
1012 #[unstable(feature = "allocator_api", issue = "32838")]
1013 #[rustc_const_unstable(feature = "const_box", issue = "92521")]
1015 pub const unsafe fn from_raw_in(raw: *mut T, alloc: A) -> Self {
1016 Box(unsafe { Unique::new_unchecked(raw) }, alloc)
1019 /// Consumes the `Box`, returning a wrapped raw pointer.
1021 /// The pointer will be properly aligned and non-null.
1023 /// After calling this function, the caller is responsible for the
1024 /// memory previously managed by the `Box`. In particular, the
1025 /// caller should properly destroy `T` and release the memory, taking
1026 /// into account the [memory layout] used by `Box`. The easiest way to
1027 /// do this is to convert the raw pointer back into a `Box` with the
1028 /// [`Box::from_raw`] function, allowing the `Box` destructor to perform
1031 /// Note: this is an associated function, which means that you have
1032 /// to call it as `Box::into_raw(b)` instead of `b.into_raw()`. This
1033 /// is so that there is no conflict with a method on the inner type.
1036 /// Converting the raw pointer back into a `Box` with [`Box::from_raw`]
1037 /// for automatic cleanup:
1039 /// let x = Box::new(String::from("Hello"));
1040 /// let ptr = Box::into_raw(x);
1041 /// let x = unsafe { Box::from_raw(ptr) };
1043 /// Manual cleanup by explicitly running the destructor and deallocating
1046 /// use std::alloc::{dealloc, Layout};
1049 /// let x = Box::new(String::from("Hello"));
1050 /// let p = Box::into_raw(x);
1052 /// ptr::drop_in_place(p);
1053 /// dealloc(p as *mut u8, Layout::new::<String>());
1057 /// [memory layout]: self#memory-layout
1058 #[stable(feature = "box_raw", since = "1.4.0")]
1060 pub fn into_raw(b: Self) -> *mut T {
1061 Self::into_raw_with_allocator(b).0
1064 /// Consumes the `Box`, returning a wrapped raw pointer and the allocator.
1066 /// The pointer will be properly aligned and non-null.
1068 /// After calling this function, the caller is responsible for the
1069 /// memory previously managed by the `Box`. In particular, the
1070 /// caller should properly destroy `T` and release the memory, taking
1071 /// into account the [memory layout] used by `Box`. The easiest way to
1072 /// do this is to convert the raw pointer back into a `Box` with the
1073 /// [`Box::from_raw_in`] function, allowing the `Box` destructor to perform
1076 /// Note: this is an associated function, which means that you have
1077 /// to call it as `Box::into_raw_with_allocator(b)` instead of `b.into_raw_with_allocator()`. This
1078 /// is so that there is no conflict with a method on the inner type.
1081 /// Converting the raw pointer back into a `Box` with [`Box::from_raw_in`]
1082 /// for automatic cleanup:
1084 /// #![feature(allocator_api)]
1086 /// use std::alloc::System;
1088 /// let x = Box::new_in(String::from("Hello"), System);
1089 /// let (ptr, alloc) = Box::into_raw_with_allocator(x);
1090 /// let x = unsafe { Box::from_raw_in(ptr, alloc) };
1092 /// Manual cleanup by explicitly running the destructor and deallocating
1095 /// #![feature(allocator_api)]
1097 /// use std::alloc::{Allocator, Layout, System};
1098 /// use std::ptr::{self, NonNull};
1100 /// let x = Box::new_in(String::from("Hello"), System);
1101 /// let (ptr, alloc) = Box::into_raw_with_allocator(x);
1103 /// ptr::drop_in_place(ptr);
1104 /// let non_null = NonNull::new_unchecked(ptr);
1105 /// alloc.deallocate(non_null.cast(), Layout::new::<String>());
1109 /// [memory layout]: self#memory-layout
1110 #[unstable(feature = "allocator_api", issue = "32838")]
1111 #[rustc_const_unstable(feature = "const_box", issue = "92521")]
1113 pub const fn into_raw_with_allocator(b: Self) -> (*mut T, A) {
1114 let (leaked, alloc) = Box::into_unique(b);
1115 (leaked.as_ptr(), alloc)
1119 feature = "ptr_internals",
1121 reason = "use `Box::leak(b).into()` or `Unique::from(Box::leak(b))` instead"
1123 #[rustc_const_unstable(feature = "const_box", issue = "92521")]
1126 pub const fn into_unique(b: Self) -> (Unique<T>, A) {
1127 // Box is recognized as a "unique pointer" by Stacked Borrows, but internally it is a
1128 // raw pointer for the type system. Turning it directly into a raw pointer would not be
1129 // recognized as "releasing" the unique pointer to permit aliased raw accesses,
1130 // so all raw pointer methods have to go through `Box::leak`. Turning *that* to a raw pointer
1131 // behaves correctly.
1132 let alloc = unsafe { ptr::read(&b.1) };
1133 (Unique::from(Box::leak(b)), alloc)
1136 /// Returns a reference to the underlying allocator.
1138 /// Note: this is an associated function, which means that you have
1139 /// to call it as `Box::allocator(&b)` instead of `b.allocator()`. This
1140 /// is so that there is no conflict with a method on the inner type.
1141 #[unstable(feature = "allocator_api", issue = "32838")]
1142 #[rustc_const_unstable(feature = "const_box", issue = "92521")]
1144 pub const fn allocator(b: &Self) -> &A {
1148 /// Consumes and leaks the `Box`, returning a mutable reference,
1149 /// `&'a mut T`. Note that the type `T` must outlive the chosen lifetime
1150 /// `'a`. If the type has only static references, or none at all, then this
1151 /// may be chosen to be `'static`.
1153 /// This function is mainly useful for data that lives for the remainder of
1154 /// the program's life. Dropping the returned reference will cause a memory
1155 /// leak. If this is not acceptable, the reference should first be wrapped
1156 /// with the [`Box::from_raw`] function producing a `Box`. This `Box` can
1157 /// then be dropped which will properly destroy `T` and release the
1158 /// allocated memory.
1160 /// Note: this is an associated function, which means that you have
1161 /// to call it as `Box::leak(b)` instead of `b.leak()`. This
1162 /// is so that there is no conflict with a method on the inner type.
1169 /// let x = Box::new(41);
1170 /// let static_ref: &'static mut usize = Box::leak(x);
1171 /// *static_ref += 1;
1172 /// assert_eq!(*static_ref, 42);
1178 /// let x = vec![1, 2, 3].into_boxed_slice();
1179 /// let static_ref = Box::leak(x);
1180 /// static_ref[0] = 4;
1181 /// assert_eq!(*static_ref, [4, 2, 3]);
1183 #[stable(feature = "box_leak", since = "1.26.0")]
1184 #[rustc_const_unstable(feature = "const_box", issue = "92521")]
1186 pub const fn leak<'a>(b: Self) -> &'a mut T
1190 unsafe { &mut *mem::ManuallyDrop::new(b).0.as_ptr() }
1193 /// Converts a `Box<T>` into a `Pin<Box<T>>`
1195 /// This conversion does not allocate on the heap and happens in place.
1197 /// This is also available via [`From`].
1201 /// It's not recommended that crates add an impl like `From<Box<T>> for Pin<T>`,
1202 /// as it'll introduce an ambiguity when calling `Pin::from`.
1203 /// A demonstration of such a poor impl is shown below.
1206 /// # use std::pin::Pin;
1207 /// struct Foo; // A type defined in this crate.
1208 /// impl From<Box<()>> for Pin<Foo> {
1209 /// fn from(_: Box<()>) -> Pin<Foo> {
1214 /// let foo = Box::new(());
1215 /// let bar = Pin::from(foo);
1217 #[stable(feature = "box_into_pin", since = "1.63.0")]
1218 #[rustc_const_unstable(feature = "const_box", issue = "92521")]
1219 pub const fn into_pin(boxed: Self) -> Pin<Self>
1223 // It's not possible to move or replace the insides of a `Pin<Box<T>>`
1224 // when `T: !Unpin`, so it's safe to pin it directly without any
1225 // additional requirements.
1226 unsafe { Pin::new_unchecked(boxed) }
1230 #[stable(feature = "rust1", since = "1.0.0")]
1231 unsafe impl<#[may_dangle] T: ?Sized, A: Allocator> Drop for Box<T, A> {
1232 fn drop(&mut self) {
1233 // FIXME: Do nothing, drop is currently performed by compiler.
1237 #[cfg(not(no_global_oom_handling))]
1238 #[stable(feature = "rust1", since = "1.0.0")]
1239 impl<T: Default> Default for Box<T> {
1240 /// Creates a `Box<T>`, with the `Default` value for T.
1241 fn default() -> Self {
1242 #[cfg_attr(not(bootstrap), rustc_box)]
1243 Box::new(T::default())
1247 #[cfg(not(no_global_oom_handling))]
1248 #[stable(feature = "rust1", since = "1.0.0")]
1249 #[rustc_const_unstable(feature = "const_default_impls", issue = "87864")]
1250 impl<T> const Default for Box<[T]> {
1251 fn default() -> Self {
1252 let ptr: Unique<[T]> = Unique::<[T; 0]>::dangling();
1257 #[cfg(not(no_global_oom_handling))]
1258 #[stable(feature = "default_box_extra", since = "1.17.0")]
1259 #[rustc_const_unstable(feature = "const_default_impls", issue = "87864")]
1260 impl const Default for Box<str> {
1261 fn default() -> Self {
1262 // SAFETY: This is the same as `Unique::cast<U>` but with an unsized `U = str`.
1263 let ptr: Unique<str> = unsafe {
1264 let bytes: Unique<[u8]> = Unique::<[u8; 0]>::dangling();
1265 Unique::new_unchecked(bytes.as_ptr() as *mut str)
1271 #[cfg(not(no_global_oom_handling))]
1272 #[stable(feature = "rust1", since = "1.0.0")]
1273 impl<T: Clone, A: Allocator + Clone> Clone for Box<T, A> {
1274 /// Returns a new box with a `clone()` of this box's contents.
1279 /// let x = Box::new(5);
1280 /// let y = x.clone();
1282 /// // The value is the same
1283 /// assert_eq!(x, y);
1285 /// // But they are unique objects
1286 /// assert_ne!(&*x as *const i32, &*y as *const i32);
1289 fn clone(&self) -> Self {
1290 // Pre-allocate memory to allow writing the cloned value directly.
1291 let mut boxed = Self::new_uninit_in(self.1.clone());
1293 (**self).write_clone_into_raw(boxed.as_mut_ptr());
1298 /// Copies `source`'s contents into `self` without creating a new allocation.
1303 /// let x = Box::new(5);
1304 /// let mut y = Box::new(10);
1305 /// let yp: *const i32 = &*y;
1307 /// y.clone_from(&x);
1309 /// // The value is the same
1310 /// assert_eq!(x, y);
1312 /// // And no allocation occurred
1313 /// assert_eq!(yp, &*y);
1316 fn clone_from(&mut self, source: &Self) {
1317 (**self).clone_from(&(**source));
1321 #[cfg(not(no_global_oom_handling))]
1322 #[stable(feature = "box_slice_clone", since = "1.3.0")]
1323 impl Clone for Box<str> {
1324 fn clone(&self) -> Self {
1325 // this makes a copy of the data
1326 let buf: Box<[u8]> = self.as_bytes().into();
1327 unsafe { from_boxed_utf8_unchecked(buf) }
1331 #[stable(feature = "rust1", since = "1.0.0")]
1332 impl<T: ?Sized + PartialEq, A: Allocator> PartialEq for Box<T, A> {
1334 fn eq(&self, other: &Self) -> bool {
1335 PartialEq::eq(&**self, &**other)
1338 fn ne(&self, other: &Self) -> bool {
1339 PartialEq::ne(&**self, &**other)
1342 #[stable(feature = "rust1", since = "1.0.0")]
1343 impl<T: ?Sized + PartialOrd, A: Allocator> PartialOrd for Box<T, A> {
1345 fn partial_cmp(&self, other: &Self) -> Option<Ordering> {
1346 PartialOrd::partial_cmp(&**self, &**other)
1349 fn lt(&self, other: &Self) -> bool {
1350 PartialOrd::lt(&**self, &**other)
1353 fn le(&self, other: &Self) -> bool {
1354 PartialOrd::le(&**self, &**other)
1357 fn ge(&self, other: &Self) -> bool {
1358 PartialOrd::ge(&**self, &**other)
1361 fn gt(&self, other: &Self) -> bool {
1362 PartialOrd::gt(&**self, &**other)
1365 #[stable(feature = "rust1", since = "1.0.0")]
1366 impl<T: ?Sized + Ord, A: Allocator> Ord for Box<T, A> {
1368 fn cmp(&self, other: &Self) -> Ordering {
1369 Ord::cmp(&**self, &**other)
1372 #[stable(feature = "rust1", since = "1.0.0")]
1373 impl<T: ?Sized + Eq, A: Allocator> Eq for Box<T, A> {}
1375 #[stable(feature = "rust1", since = "1.0.0")]
1376 impl<T: ?Sized + Hash, A: Allocator> Hash for Box<T, A> {
1377 fn hash<H: Hasher>(&self, state: &mut H) {
1378 (**self).hash(state);
1382 #[stable(feature = "indirect_hasher_impl", since = "1.22.0")]
1383 impl<T: ?Sized + Hasher, A: Allocator> Hasher for Box<T, A> {
1384 fn finish(&self) -> u64 {
1387 fn write(&mut self, bytes: &[u8]) {
1388 (**self).write(bytes)
1390 fn write_u8(&mut self, i: u8) {
1391 (**self).write_u8(i)
1393 fn write_u16(&mut self, i: u16) {
1394 (**self).write_u16(i)
1396 fn write_u32(&mut self, i: u32) {
1397 (**self).write_u32(i)
1399 fn write_u64(&mut self, i: u64) {
1400 (**self).write_u64(i)
1402 fn write_u128(&mut self, i: u128) {
1403 (**self).write_u128(i)
1405 fn write_usize(&mut self, i: usize) {
1406 (**self).write_usize(i)
1408 fn write_i8(&mut self, i: i8) {
1409 (**self).write_i8(i)
1411 fn write_i16(&mut self, i: i16) {
1412 (**self).write_i16(i)
1414 fn write_i32(&mut self, i: i32) {
1415 (**self).write_i32(i)
1417 fn write_i64(&mut self, i: i64) {
1418 (**self).write_i64(i)
1420 fn write_i128(&mut self, i: i128) {
1421 (**self).write_i128(i)
1423 fn write_isize(&mut self, i: isize) {
1424 (**self).write_isize(i)
1426 fn write_length_prefix(&mut self, len: usize) {
1427 (**self).write_length_prefix(len)
1429 fn write_str(&mut self, s: &str) {
1430 (**self).write_str(s)
1434 #[cfg(not(no_global_oom_handling))]
1435 #[stable(feature = "from_for_ptrs", since = "1.6.0")]
1436 impl<T> From<T> for Box<T> {
1437 /// Converts a `T` into a `Box<T>`
1439 /// The conversion allocates on the heap and moves `t`
1440 /// from the stack into it.
1446 /// let boxed = Box::new(5);
1448 /// assert_eq!(Box::from(x), boxed);
1450 fn from(t: T) -> Self {
1455 #[stable(feature = "pin", since = "1.33.0")]
1456 #[rustc_const_unstable(feature = "const_box", issue = "92521")]
1457 impl<T: ?Sized, A: Allocator> const From<Box<T, A>> for Pin<Box<T, A>>
1461 /// Converts a `Box<T>` into a `Pin<Box<T>>`
1463 /// This conversion does not allocate on the heap and happens in place.
1464 fn from(boxed: Box<T, A>) -> Self {
1465 Box::into_pin(boxed)
1469 #[cfg(not(no_global_oom_handling))]
1470 #[stable(feature = "box_from_slice", since = "1.17.0")]
1471 impl<T: Copy> From<&[T]> for Box<[T]> {
1472 /// Converts a `&[T]` into a `Box<[T]>`
1474 /// This conversion allocates on the heap
1475 /// and performs a copy of `slice`.
1479 /// // create a &[u8] which will be used to create a Box<[u8]>
1480 /// let slice: &[u8] = &[104, 101, 108, 108, 111];
1481 /// let boxed_slice: Box<[u8]> = Box::from(slice);
1483 /// println!("{boxed_slice:?}");
1485 fn from(slice: &[T]) -> Box<[T]> {
1486 let len = slice.len();
1487 let buf = RawVec::with_capacity(len);
1489 ptr::copy_nonoverlapping(slice.as_ptr(), buf.ptr(), len);
1490 buf.into_box(slice.len()).assume_init()
1495 #[cfg(not(no_global_oom_handling))]
1496 #[stable(feature = "box_from_cow", since = "1.45.0")]
1497 impl<T: Copy> From<Cow<'_, [T]>> for Box<[T]> {
1498 /// Converts a `Cow<'_, [T]>` into a `Box<[T]>`
1500 /// When `cow` is the `Cow::Borrowed` variant, this
1501 /// conversion allocates on the heap and copies the
1502 /// underlying slice. Otherwise, it will try to reuse the owned
1503 /// `Vec`'s allocation.
1505 fn from(cow: Cow<'_, [T]>) -> Box<[T]> {
1507 Cow::Borrowed(slice) => Box::from(slice),
1508 Cow::Owned(slice) => Box::from(slice),
1513 #[cfg(not(no_global_oom_handling))]
1514 #[stable(feature = "box_from_slice", since = "1.17.0")]
1515 impl From<&str> for Box<str> {
1516 /// Converts a `&str` into a `Box<str>`
1518 /// This conversion allocates on the heap
1519 /// and performs a copy of `s`.
1524 /// let boxed: Box<str> = Box::from("hello");
1525 /// println!("{boxed}");
1528 fn from(s: &str) -> Box<str> {
1529 unsafe { from_boxed_utf8_unchecked(Box::from(s.as_bytes())) }
1533 #[cfg(not(no_global_oom_handling))]
1534 #[stable(feature = "box_from_cow", since = "1.45.0")]
1535 impl From<Cow<'_, str>> for Box<str> {
1536 /// Converts a `Cow<'_, str>` into a `Box<str>`
1538 /// When `cow` is the `Cow::Borrowed` variant, this
1539 /// conversion allocates on the heap and copies the
1540 /// underlying `str`. Otherwise, it will try to reuse the owned
1541 /// `String`'s allocation.
1546 /// use std::borrow::Cow;
1548 /// let unboxed = Cow::Borrowed("hello");
1549 /// let boxed: Box<str> = Box::from(unboxed);
1550 /// println!("{boxed}");
1554 /// # use std::borrow::Cow;
1555 /// let unboxed = Cow::Owned("hello".to_string());
1556 /// let boxed: Box<str> = Box::from(unboxed);
1557 /// println!("{boxed}");
1560 fn from(cow: Cow<'_, str>) -> Box<str> {
1562 Cow::Borrowed(s) => Box::from(s),
1563 Cow::Owned(s) => Box::from(s),
1568 #[stable(feature = "boxed_str_conv", since = "1.19.0")]
1569 impl<A: Allocator> From<Box<str, A>> for Box<[u8], A> {
1570 /// Converts a `Box<str>` into a `Box<[u8]>`
1572 /// This conversion does not allocate on the heap and happens in place.
1576 /// // create a Box<str> which will be used to create a Box<[u8]>
1577 /// let boxed: Box<str> = Box::from("hello");
1578 /// let boxed_str: Box<[u8]> = Box::from(boxed);
1580 /// // create a &[u8] which will be used to create a Box<[u8]>
1581 /// let slice: &[u8] = &[104, 101, 108, 108, 111];
1582 /// let boxed_slice = Box::from(slice);
1584 /// assert_eq!(boxed_slice, boxed_str);
1587 fn from(s: Box<str, A>) -> Self {
1588 let (raw, alloc) = Box::into_raw_with_allocator(s);
1589 unsafe { Box::from_raw_in(raw as *mut [u8], alloc) }
1593 #[cfg(not(no_global_oom_handling))]
1594 #[stable(feature = "box_from_array", since = "1.45.0")]
1595 impl<T, const N: usize> From<[T; N]> for Box<[T]> {
1596 /// Converts a `[T; N]` into a `Box<[T]>`
1598 /// This conversion moves the array to newly heap-allocated memory.
1603 /// let boxed: Box<[u8]> = Box::from([4, 2]);
1604 /// println!("{boxed:?}");
1606 fn from(array: [T; N]) -> Box<[T]> {
1607 #[cfg_attr(not(bootstrap), rustc_box)]
1612 #[stable(feature = "boxed_slice_try_from", since = "1.43.0")]
1613 impl<T, const N: usize> TryFrom<Box<[T]>> for Box<[T; N]> {
1614 type Error = Box<[T]>;
1616 /// Attempts to convert a `Box<[T]>` into a `Box<[T; N]>`.
1618 /// The conversion occurs in-place and does not require a
1619 /// new memory allocation.
1623 /// Returns the old `Box<[T]>` in the `Err` variant if
1624 /// `boxed_slice.len()` does not equal `N`.
1625 fn try_from(boxed_slice: Box<[T]>) -> Result<Self, Self::Error> {
1626 if boxed_slice.len() == N {
1627 Ok(unsafe { Box::from_raw(Box::into_raw(boxed_slice) as *mut [T; N]) })
1634 impl<A: Allocator> Box<dyn Any, A> {
1635 /// Attempt to downcast the box to a concrete type.
1640 /// use std::any::Any;
1642 /// fn print_if_string(value: Box<dyn Any>) {
1643 /// if let Ok(string) = value.downcast::<String>() {
1644 /// println!("String ({}): {}", string.len(), string);
1648 /// let my_string = "Hello World".to_string();
1649 /// print_if_string(Box::new(my_string));
1650 /// print_if_string(Box::new(0i8));
1653 #[stable(feature = "rust1", since = "1.0.0")]
1654 pub fn downcast<T: Any>(self) -> Result<Box<T, A>, Self> {
1655 if self.is::<T>() { unsafe { Ok(self.downcast_unchecked::<T>()) } } else { Err(self) }
1658 /// Downcasts the box to a concrete type.
1660 /// For a safe alternative see [`downcast`].
1665 /// #![feature(downcast_unchecked)]
1667 /// use std::any::Any;
1669 /// let x: Box<dyn Any> = Box::new(1_usize);
1672 /// assert_eq!(*x.downcast_unchecked::<usize>(), 1);
1678 /// The contained value must be of type `T`. Calling this method
1679 /// with the incorrect type is *undefined behavior*.
1681 /// [`downcast`]: Self::downcast
1683 #[unstable(feature = "downcast_unchecked", issue = "90850")]
1684 pub unsafe fn downcast_unchecked<T: Any>(self) -> Box<T, A> {
1685 debug_assert!(self.is::<T>());
1687 let (raw, alloc): (*mut dyn Any, _) = Box::into_raw_with_allocator(self);
1688 Box::from_raw_in(raw as *mut T, alloc)
1693 impl<A: Allocator> Box<dyn Any + Send, A> {
1694 /// Attempt to downcast the box to a concrete type.
1699 /// use std::any::Any;
1701 /// fn print_if_string(value: Box<dyn Any + Send>) {
1702 /// if let Ok(string) = value.downcast::<String>() {
1703 /// println!("String ({}): {}", string.len(), string);
1707 /// let my_string = "Hello World".to_string();
1708 /// print_if_string(Box::new(my_string));
1709 /// print_if_string(Box::new(0i8));
1712 #[stable(feature = "rust1", since = "1.0.0")]
1713 pub fn downcast<T: Any>(self) -> Result<Box<T, A>, Self> {
1714 if self.is::<T>() { unsafe { Ok(self.downcast_unchecked::<T>()) } } else { Err(self) }
1717 /// Downcasts the box to a concrete type.
1719 /// For a safe alternative see [`downcast`].
1724 /// #![feature(downcast_unchecked)]
1726 /// use std::any::Any;
1728 /// let x: Box<dyn Any + Send> = Box::new(1_usize);
1731 /// assert_eq!(*x.downcast_unchecked::<usize>(), 1);
1737 /// The contained value must be of type `T`. Calling this method
1738 /// with the incorrect type is *undefined behavior*.
1740 /// [`downcast`]: Self::downcast
1742 #[unstable(feature = "downcast_unchecked", issue = "90850")]
1743 pub unsafe fn downcast_unchecked<T: Any>(self) -> Box<T, A> {
1744 debug_assert!(self.is::<T>());
1746 let (raw, alloc): (*mut (dyn Any + Send), _) = Box::into_raw_with_allocator(self);
1747 Box::from_raw_in(raw as *mut T, alloc)
1752 impl<A: Allocator> Box<dyn Any + Send + Sync, A> {
1753 /// Attempt to downcast the box to a concrete type.
1758 /// use std::any::Any;
1760 /// fn print_if_string(value: Box<dyn Any + Send + Sync>) {
1761 /// if let Ok(string) = value.downcast::<String>() {
1762 /// println!("String ({}): {}", string.len(), string);
1766 /// let my_string = "Hello World".to_string();
1767 /// print_if_string(Box::new(my_string));
1768 /// print_if_string(Box::new(0i8));
1771 #[stable(feature = "box_send_sync_any_downcast", since = "1.51.0")]
1772 pub fn downcast<T: Any>(self) -> Result<Box<T, A>, Self> {
1773 if self.is::<T>() { unsafe { Ok(self.downcast_unchecked::<T>()) } } else { Err(self) }
1776 /// Downcasts the box to a concrete type.
1778 /// For a safe alternative see [`downcast`].
1783 /// #![feature(downcast_unchecked)]
1785 /// use std::any::Any;
1787 /// let x: Box<dyn Any + Send + Sync> = Box::new(1_usize);
1790 /// assert_eq!(*x.downcast_unchecked::<usize>(), 1);
1796 /// The contained value must be of type `T`. Calling this method
1797 /// with the incorrect type is *undefined behavior*.
1799 /// [`downcast`]: Self::downcast
1801 #[unstable(feature = "downcast_unchecked", issue = "90850")]
1802 pub unsafe fn downcast_unchecked<T: Any>(self) -> Box<T, A> {
1803 debug_assert!(self.is::<T>());
1805 let (raw, alloc): (*mut (dyn Any + Send + Sync), _) =
1806 Box::into_raw_with_allocator(self);
1807 Box::from_raw_in(raw as *mut T, alloc)
1812 #[stable(feature = "rust1", since = "1.0.0")]
1813 impl<T: fmt::Display + ?Sized, A: Allocator> fmt::Display for Box<T, A> {
1814 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
1815 fmt::Display::fmt(&**self, f)
1819 #[stable(feature = "rust1", since = "1.0.0")]
1820 impl<T: fmt::Debug + ?Sized, A: Allocator> fmt::Debug for Box<T, A> {
1821 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
1822 fmt::Debug::fmt(&**self, f)
1826 #[stable(feature = "rust1", since = "1.0.0")]
1827 impl<T: ?Sized, A: Allocator> fmt::Pointer for Box<T, A> {
1828 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
1829 // It's not possible to extract the inner Uniq directly from the Box,
1830 // instead we cast it to a *const which aliases the Unique
1831 let ptr: *const T = &**self;
1832 fmt::Pointer::fmt(&ptr, f)
1836 #[stable(feature = "rust1", since = "1.0.0")]
1837 #[rustc_const_unstable(feature = "const_box", issue = "92521")]
1838 impl<T: ?Sized, A: Allocator> const Deref for Box<T, A> {
1841 fn deref(&self) -> &T {
1846 #[stable(feature = "rust1", since = "1.0.0")]
1847 #[rustc_const_unstable(feature = "const_box", issue = "92521")]
1848 impl<T: ?Sized, A: Allocator> const DerefMut for Box<T, A> {
1849 fn deref_mut(&mut self) -> &mut T {
1854 #[unstable(feature = "receiver_trait", issue = "none")]
1855 impl<T: ?Sized, A: Allocator> Receiver for Box<T, A> {}
1857 #[stable(feature = "rust1", since = "1.0.0")]
1858 impl<I: Iterator + ?Sized, A: Allocator> Iterator for Box<I, A> {
1859 type Item = I::Item;
1860 fn next(&mut self) -> Option<I::Item> {
1863 fn size_hint(&self) -> (usize, Option<usize>) {
1864 (**self).size_hint()
1866 fn nth(&mut self, n: usize) -> Option<I::Item> {
1869 fn last(self) -> Option<I::Item> {
1876 fn last(self) -> Option<Self::Item>;
1879 impl<I: Iterator + ?Sized, A: Allocator> BoxIter for Box<I, A> {
1880 type Item = I::Item;
1881 default fn last(self) -> Option<I::Item> {
1883 fn some<T>(_: Option<T>, x: T) -> Option<T> {
1887 self.fold(None, some)
1891 /// Specialization for sized `I`s that uses `I`s implementation of `last()`
1892 /// instead of the default.
1893 #[stable(feature = "rust1", since = "1.0.0")]
1894 impl<I: Iterator, A: Allocator> BoxIter for Box<I, A> {
1895 fn last(self) -> Option<I::Item> {
1900 #[stable(feature = "rust1", since = "1.0.0")]
1901 impl<I: DoubleEndedIterator + ?Sized, A: Allocator> DoubleEndedIterator for Box<I, A> {
1902 fn next_back(&mut self) -> Option<I::Item> {
1903 (**self).next_back()
1905 fn nth_back(&mut self, n: usize) -> Option<I::Item> {
1906 (**self).nth_back(n)
1909 #[stable(feature = "rust1", since = "1.0.0")]
1910 impl<I: ExactSizeIterator + ?Sized, A: Allocator> ExactSizeIterator for Box<I, A> {
1911 fn len(&self) -> usize {
1914 fn is_empty(&self) -> bool {
1919 #[stable(feature = "fused", since = "1.26.0")]
1920 impl<I: FusedIterator + ?Sized, A: Allocator> FusedIterator for Box<I, A> {}
1922 #[stable(feature = "boxed_closure_impls", since = "1.35.0")]
1923 impl<Args, F: FnOnce<Args> + ?Sized, A: Allocator> FnOnce<Args> for Box<F, A> {
1924 type Output = <F as FnOnce<Args>>::Output;
1926 extern "rust-call" fn call_once(self, args: Args) -> Self::Output {
1927 <F as FnOnce<Args>>::call_once(*self, args)
1931 #[stable(feature = "boxed_closure_impls", since = "1.35.0")]
1932 impl<Args, F: FnMut<Args> + ?Sized, A: Allocator> FnMut<Args> for Box<F, A> {
1933 extern "rust-call" fn call_mut(&mut self, args: Args) -> Self::Output {
1934 <F as FnMut<Args>>::call_mut(self, args)
1938 #[stable(feature = "boxed_closure_impls", since = "1.35.0")]
1939 impl<Args, F: Fn<Args> + ?Sized, A: Allocator> Fn<Args> for Box<F, A> {
1940 extern "rust-call" fn call(&self, args: Args) -> Self::Output {
1941 <F as Fn<Args>>::call(self, args)
1945 #[unstable(feature = "coerce_unsized", issue = "27732")]
1946 impl<T: ?Sized + Unsize<U>, U: ?Sized, A: Allocator> CoerceUnsized<Box<U, A>> for Box<T, A> {}
1948 #[unstable(feature = "dispatch_from_dyn", issue = "none")]
1949 impl<T: ?Sized + Unsize<U>, U: ?Sized> DispatchFromDyn<Box<U>> for Box<T, Global> {}
1951 #[cfg(not(no_global_oom_handling))]
1952 #[stable(feature = "boxed_slice_from_iter", since = "1.32.0")]
1953 impl<I> FromIterator<I> for Box<[I]> {
1954 fn from_iter<T: IntoIterator<Item = I>>(iter: T) -> Self {
1955 iter.into_iter().collect::<Vec<_>>().into_boxed_slice()
1959 #[cfg(not(no_global_oom_handling))]
1960 #[stable(feature = "box_slice_clone", since = "1.3.0")]
1961 impl<T: Clone, A: Allocator + Clone> Clone for Box<[T], A> {
1962 fn clone(&self) -> Self {
1963 let alloc = Box::allocator(self).clone();
1964 self.to_vec_in(alloc).into_boxed_slice()
1967 fn clone_from(&mut self, other: &Self) {
1968 if self.len() == other.len() {
1969 self.clone_from_slice(&other);
1971 *self = other.clone();
1976 #[stable(feature = "box_borrow", since = "1.1.0")]
1977 impl<T: ?Sized, A: Allocator> borrow::Borrow<T> for Box<T, A> {
1978 fn borrow(&self) -> &T {
1983 #[stable(feature = "box_borrow", since = "1.1.0")]
1984 impl<T: ?Sized, A: Allocator> borrow::BorrowMut<T> for Box<T, A> {
1985 fn borrow_mut(&mut self) -> &mut T {
1990 #[stable(since = "1.5.0", feature = "smart_ptr_as_ref")]
1991 impl<T: ?Sized, A: Allocator> AsRef<T> for Box<T, A> {
1992 fn as_ref(&self) -> &T {
1997 #[stable(since = "1.5.0", feature = "smart_ptr_as_ref")]
1998 impl<T: ?Sized, A: Allocator> AsMut<T> for Box<T, A> {
1999 fn as_mut(&mut self) -> &mut T {
2006 * We could have chosen not to add this impl, and instead have written a
2007 * function of Pin<Box<T>> to Pin<T>. Such a function would not be sound,
2008 * because Box<T> implements Unpin even when T does not, as a result of
2011 * We chose this API instead of the alternative for a few reasons:
2012 * - Logically, it is helpful to understand pinning in regard to the
2013 * memory region being pointed to. For this reason none of the
2014 * standard library pointer types support projecting through a pin
2015 * (Box<T> is the only pointer type in std for which this would be
2017 * - It is in practice very useful to have Box<T> be unconditionally
2018 * Unpin because of trait objects, for which the structural auto
2019 * trait functionality does not apply (e.g., Box<dyn Foo> would
2020 * otherwise not be Unpin).
2022 * Another type with the same semantics as Box but only a conditional
2023 * implementation of `Unpin` (where `T: Unpin`) would be valid/safe, and
2024 * could have a method to project a Pin<T> from it.
2026 #[stable(feature = "pin", since = "1.33.0")]
2027 #[rustc_const_unstable(feature = "const_box", issue = "92521")]
2028 impl<T: ?Sized, A: Allocator> const Unpin for Box<T, A> where A: 'static {}
2030 #[unstable(feature = "generator_trait", issue = "43122")]
2031 impl<G: ?Sized + Generator<R> + Unpin, R, A: Allocator> Generator<R> for Box<G, A>
2035 type Yield = G::Yield;
2036 type Return = G::Return;
2038 fn resume(mut self: Pin<&mut Self>, arg: R) -> GeneratorState<Self::Yield, Self::Return> {
2039 G::resume(Pin::new(&mut *self), arg)
2043 #[unstable(feature = "generator_trait", issue = "43122")]
2044 impl<G: ?Sized + Generator<R>, R, A: Allocator> Generator<R> for Pin<Box<G, A>>
2048 type Yield = G::Yield;
2049 type Return = G::Return;
2051 fn resume(mut self: Pin<&mut Self>, arg: R) -> GeneratorState<Self::Yield, Self::Return> {
2052 G::resume((*self).as_mut(), arg)
2056 #[stable(feature = "futures_api", since = "1.36.0")]
2057 impl<F: ?Sized + Future + Unpin, A: Allocator> Future for Box<F, A>
2061 type Output = F::Output;
2063 fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
2064 F::poll(Pin::new(&mut *self), cx)
2068 #[unstable(feature = "async_iterator", issue = "79024")]
2069 impl<S: ?Sized + AsyncIterator + Unpin> AsyncIterator for Box<S> {
2070 type Item = S::Item;
2072 fn poll_next(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Option<Self::Item>> {
2073 Pin::new(&mut **self).poll_next(cx)
2076 fn size_hint(&self) -> (usize, Option<usize>) {
2077 (**self).size_hint()