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.
290 /// Constructing and pinning of the `Box` can also be done in two steps: `Box::pin(x)`
291 /// does the same as <code>[Box::into_pin]\([Box::new]\(x))</code>. Consider using
292 /// [`into_pin`](Box::into_pin) if you already have a `Box<T>`, or if you want to
293 /// construct a (pinned) `Box` in a different way than with [`Box::new`].
294 #[cfg(not(no_global_oom_handling))]
295 #[stable(feature = "pin", since = "1.33.0")]
298 pub fn pin(x: T) -> Pin<Box<T>> {
299 (#[cfg_attr(not(bootstrap), rustc_box)]
304 /// Allocates memory on the heap then places `x` into it,
305 /// returning an error if the allocation fails
307 /// This doesn't actually allocate if `T` is zero-sized.
312 /// #![feature(allocator_api)]
314 /// let five = Box::try_new(5)?;
315 /// # Ok::<(), std::alloc::AllocError>(())
317 #[unstable(feature = "allocator_api", issue = "32838")]
319 pub fn try_new(x: T) -> Result<Self, AllocError> {
320 Self::try_new_in(x, Global)
323 /// Constructs a new box with uninitialized contents on the heap,
324 /// returning an error if the allocation fails
329 /// #![feature(allocator_api, new_uninit)]
331 /// let mut five = Box::<u32>::try_new_uninit()?;
333 /// let five = unsafe {
334 /// // Deferred initialization:
335 /// five.as_mut_ptr().write(5);
337 /// five.assume_init()
340 /// assert_eq!(*five, 5);
341 /// # Ok::<(), std::alloc::AllocError>(())
343 #[unstable(feature = "allocator_api", issue = "32838")]
344 // #[unstable(feature = "new_uninit", issue = "63291")]
346 pub fn try_new_uninit() -> Result<Box<mem::MaybeUninit<T>>, AllocError> {
347 Box::try_new_uninit_in(Global)
350 /// Constructs a new `Box` with uninitialized contents, with the memory
351 /// being filled with `0` bytes on the heap
353 /// See [`MaybeUninit::zeroed`][zeroed] for examples of correct and incorrect usage
359 /// #![feature(allocator_api, new_uninit)]
361 /// let zero = Box::<u32>::try_new_zeroed()?;
362 /// let zero = unsafe { zero.assume_init() };
364 /// assert_eq!(*zero, 0);
365 /// # Ok::<(), std::alloc::AllocError>(())
368 /// [zeroed]: mem::MaybeUninit::zeroed
369 #[unstable(feature = "allocator_api", issue = "32838")]
370 // #[unstable(feature = "new_uninit", issue = "63291")]
372 pub fn try_new_zeroed() -> Result<Box<mem::MaybeUninit<T>>, AllocError> {
373 Box::try_new_zeroed_in(Global)
377 impl<T, A: Allocator> Box<T, A> {
378 /// Allocates memory in the given allocator then places `x` into it.
380 /// This doesn't actually allocate if `T` is zero-sized.
385 /// #![feature(allocator_api)]
387 /// use std::alloc::System;
389 /// let five = Box::new_in(5, System);
391 #[cfg(not(no_global_oom_handling))]
392 #[unstable(feature = "allocator_api", issue = "32838")]
393 #[rustc_const_unstable(feature = "const_box", issue = "92521")]
396 pub const fn new_in(x: T, alloc: A) -> Self
398 A: ~const Allocator + ~const Destruct,
400 let mut boxed = Self::new_uninit_in(alloc);
402 boxed.as_mut_ptr().write(x);
407 /// Allocates memory in the given allocator then places `x` into it,
408 /// returning an error if the allocation fails
410 /// This doesn't actually allocate if `T` is zero-sized.
415 /// #![feature(allocator_api)]
417 /// use std::alloc::System;
419 /// let five = Box::try_new_in(5, System)?;
420 /// # Ok::<(), std::alloc::AllocError>(())
422 #[unstable(feature = "allocator_api", issue = "32838")]
423 #[rustc_const_unstable(feature = "const_box", issue = "92521")]
425 pub const fn try_new_in(x: T, alloc: A) -> Result<Self, AllocError>
428 A: ~const Allocator + ~const Destruct,
430 let mut boxed = Self::try_new_uninit_in(alloc)?;
432 boxed.as_mut_ptr().write(x);
433 Ok(boxed.assume_init())
437 /// Constructs a new box with uninitialized contents in the provided allocator.
442 /// #![feature(allocator_api, new_uninit)]
444 /// use std::alloc::System;
446 /// let mut five = Box::<u32, _>::new_uninit_in(System);
448 /// let five = unsafe {
449 /// // Deferred initialization:
450 /// five.as_mut_ptr().write(5);
452 /// five.assume_init()
455 /// assert_eq!(*five, 5)
457 #[unstable(feature = "allocator_api", issue = "32838")]
458 #[rustc_const_unstable(feature = "const_box", issue = "92521")]
459 #[cfg(not(no_global_oom_handling))]
461 // #[unstable(feature = "new_uninit", issue = "63291")]
462 pub const fn new_uninit_in(alloc: A) -> Box<mem::MaybeUninit<T>, A>
464 A: ~const Allocator + ~const Destruct,
466 let layout = Layout::new::<mem::MaybeUninit<T>>();
467 // NOTE: Prefer match over unwrap_or_else since closure sometimes not inlineable.
468 // That would make code size bigger.
469 match Box::try_new_uninit_in(alloc) {
471 Err(_) => handle_alloc_error(layout),
475 /// Constructs a new box with uninitialized contents in the provided allocator,
476 /// returning an error if the allocation fails
481 /// #![feature(allocator_api, new_uninit)]
483 /// use std::alloc::System;
485 /// let mut five = Box::<u32, _>::try_new_uninit_in(System)?;
487 /// let five = unsafe {
488 /// // Deferred initialization:
489 /// five.as_mut_ptr().write(5);
491 /// five.assume_init()
494 /// assert_eq!(*five, 5);
495 /// # Ok::<(), std::alloc::AllocError>(())
497 #[unstable(feature = "allocator_api", issue = "32838")]
498 // #[unstable(feature = "new_uninit", issue = "63291")]
499 #[rustc_const_unstable(feature = "const_box", issue = "92521")]
500 pub const fn try_new_uninit_in(alloc: A) -> Result<Box<mem::MaybeUninit<T>, A>, AllocError>
502 A: ~const Allocator + ~const Destruct,
504 let layout = Layout::new::<mem::MaybeUninit<T>>();
505 let ptr = alloc.allocate(layout)?.cast();
506 unsafe { Ok(Box::from_raw_in(ptr.as_ptr(), alloc)) }
509 /// Constructs a new `Box` with uninitialized contents, with the memory
510 /// being filled with `0` bytes in the provided allocator.
512 /// See [`MaybeUninit::zeroed`][zeroed] for examples of correct and incorrect usage
518 /// #![feature(allocator_api, new_uninit)]
520 /// use std::alloc::System;
522 /// let zero = Box::<u32, _>::new_zeroed_in(System);
523 /// let zero = unsafe { zero.assume_init() };
525 /// assert_eq!(*zero, 0)
528 /// [zeroed]: mem::MaybeUninit::zeroed
529 #[unstable(feature = "allocator_api", issue = "32838")]
530 #[rustc_const_unstable(feature = "const_box", issue = "92521")]
531 #[cfg(not(no_global_oom_handling))]
532 // #[unstable(feature = "new_uninit", issue = "63291")]
534 pub const fn new_zeroed_in(alloc: A) -> Box<mem::MaybeUninit<T>, A>
536 A: ~const Allocator + ~const Destruct,
538 let layout = Layout::new::<mem::MaybeUninit<T>>();
539 // NOTE: Prefer match over unwrap_or_else since closure sometimes not inlineable.
540 // That would make code size bigger.
541 match Box::try_new_zeroed_in(alloc) {
543 Err(_) => handle_alloc_error(layout),
547 /// Constructs a new `Box` with uninitialized contents, with the memory
548 /// being filled with `0` bytes in the provided allocator,
549 /// returning an error if the allocation fails,
551 /// See [`MaybeUninit::zeroed`][zeroed] for examples of correct and incorrect usage
557 /// #![feature(allocator_api, new_uninit)]
559 /// use std::alloc::System;
561 /// let zero = Box::<u32, _>::try_new_zeroed_in(System)?;
562 /// let zero = unsafe { zero.assume_init() };
564 /// assert_eq!(*zero, 0);
565 /// # Ok::<(), std::alloc::AllocError>(())
568 /// [zeroed]: mem::MaybeUninit::zeroed
569 #[unstable(feature = "allocator_api", issue = "32838")]
570 // #[unstable(feature = "new_uninit", issue = "63291")]
571 #[rustc_const_unstable(feature = "const_box", issue = "92521")]
572 pub const fn try_new_zeroed_in(alloc: A) -> Result<Box<mem::MaybeUninit<T>, A>, AllocError>
574 A: ~const Allocator + ~const Destruct,
576 let layout = Layout::new::<mem::MaybeUninit<T>>();
577 let ptr = alloc.allocate_zeroed(layout)?.cast();
578 unsafe { Ok(Box::from_raw_in(ptr.as_ptr(), alloc)) }
581 /// Constructs a new `Pin<Box<T, A>>`. If `T` does not implement [`Unpin`], then
582 /// `x` will be pinned in memory and unable to be moved.
584 /// Constructing and pinning of the `Box` can also be done in two steps: `Box::pin_in(x, alloc)`
585 /// does the same as <code>[Box::into_pin]\([Box::new_in]\(x, alloc))</code>. Consider using
586 /// [`into_pin`](Box::into_pin) if you already have a `Box<T, A>`, or if you want to
587 /// construct a (pinned) `Box` in a different way than with [`Box::new_in`].
588 #[cfg(not(no_global_oom_handling))]
589 #[unstable(feature = "allocator_api", issue = "32838")]
590 #[rustc_const_unstable(feature = "const_box", issue = "92521")]
593 pub const fn pin_in(x: T, alloc: A) -> Pin<Self>
595 A: 'static + ~const Allocator + ~const Destruct,
597 Self::into_pin(Self::new_in(x, alloc))
600 /// Converts a `Box<T>` into a `Box<[T]>`
602 /// This conversion does not allocate on the heap and happens in place.
603 #[unstable(feature = "box_into_boxed_slice", issue = "71582")]
604 #[rustc_const_unstable(feature = "const_box", issue = "92521")]
605 pub const fn into_boxed_slice(boxed: Self) -> Box<[T], A> {
606 let (raw, alloc) = Box::into_raw_with_allocator(boxed);
607 unsafe { Box::from_raw_in(raw as *mut [T; 1], alloc) }
610 /// Consumes the `Box`, returning the wrapped value.
615 /// #![feature(box_into_inner)]
617 /// let c = Box::new(5);
619 /// assert_eq!(Box::into_inner(c), 5);
621 #[unstable(feature = "box_into_inner", issue = "80437")]
622 #[rustc_const_unstable(feature = "const_box", issue = "92521")]
624 pub const fn into_inner(boxed: Self) -> T
626 Self: ~const Destruct,
633 /// Constructs a new boxed slice with uninitialized contents.
638 /// #![feature(new_uninit)]
640 /// let mut values = Box::<[u32]>::new_uninit_slice(3);
642 /// let values = unsafe {
643 /// // Deferred initialization:
644 /// values[0].as_mut_ptr().write(1);
645 /// values[1].as_mut_ptr().write(2);
646 /// values[2].as_mut_ptr().write(3);
648 /// values.assume_init()
651 /// assert_eq!(*values, [1, 2, 3])
653 #[cfg(not(no_global_oom_handling))]
654 #[unstable(feature = "new_uninit", issue = "63291")]
656 pub fn new_uninit_slice(len: usize) -> Box<[mem::MaybeUninit<T>]> {
657 unsafe { RawVec::with_capacity(len).into_box(len) }
660 /// Constructs a new boxed slice with uninitialized contents, with the memory
661 /// being filled with `0` bytes.
663 /// See [`MaybeUninit::zeroed`][zeroed] for examples of correct and incorrect usage
669 /// #![feature(new_uninit)]
671 /// let values = Box::<[u32]>::new_zeroed_slice(3);
672 /// let values = unsafe { values.assume_init() };
674 /// assert_eq!(*values, [0, 0, 0])
677 /// [zeroed]: mem::MaybeUninit::zeroed
678 #[cfg(not(no_global_oom_handling))]
679 #[unstable(feature = "new_uninit", issue = "63291")]
681 pub fn new_zeroed_slice(len: usize) -> Box<[mem::MaybeUninit<T>]> {
682 unsafe { RawVec::with_capacity_zeroed(len).into_box(len) }
685 /// Constructs a new boxed slice with uninitialized contents. Returns an error if
686 /// the allocation fails
691 /// #![feature(allocator_api, new_uninit)]
693 /// let mut values = Box::<[u32]>::try_new_uninit_slice(3)?;
694 /// let values = unsafe {
695 /// // Deferred initialization:
696 /// values[0].as_mut_ptr().write(1);
697 /// values[1].as_mut_ptr().write(2);
698 /// values[2].as_mut_ptr().write(3);
699 /// values.assume_init()
702 /// assert_eq!(*values, [1, 2, 3]);
703 /// # Ok::<(), std::alloc::AllocError>(())
705 #[unstable(feature = "allocator_api", issue = "32838")]
707 pub fn try_new_uninit_slice(len: usize) -> Result<Box<[mem::MaybeUninit<T>]>, AllocError> {
709 let layout = match Layout::array::<mem::MaybeUninit<T>>(len) {
711 Err(_) => return Err(AllocError),
713 let ptr = Global.allocate(layout)?;
714 Ok(RawVec::from_raw_parts_in(ptr.as_mut_ptr() as *mut _, len, Global).into_box(len))
718 /// Constructs a new boxed slice with uninitialized contents, with the memory
719 /// being filled with `0` bytes. Returns an error if the allocation fails
721 /// See [`MaybeUninit::zeroed`][zeroed] for examples of correct and incorrect usage
727 /// #![feature(allocator_api, new_uninit)]
729 /// let values = Box::<[u32]>::try_new_zeroed_slice(3)?;
730 /// let values = unsafe { values.assume_init() };
732 /// assert_eq!(*values, [0, 0, 0]);
733 /// # Ok::<(), std::alloc::AllocError>(())
736 /// [zeroed]: mem::MaybeUninit::zeroed
737 #[unstable(feature = "allocator_api", issue = "32838")]
739 pub fn try_new_zeroed_slice(len: usize) -> Result<Box<[mem::MaybeUninit<T>]>, AllocError> {
741 let layout = match Layout::array::<mem::MaybeUninit<T>>(len) {
743 Err(_) => return Err(AllocError),
745 let ptr = Global.allocate_zeroed(layout)?;
746 Ok(RawVec::from_raw_parts_in(ptr.as_mut_ptr() as *mut _, len, Global).into_box(len))
751 impl<T, A: Allocator> Box<[T], A> {
752 /// Constructs a new boxed slice with uninitialized contents in the provided allocator.
757 /// #![feature(allocator_api, new_uninit)]
759 /// use std::alloc::System;
761 /// let mut values = Box::<[u32], _>::new_uninit_slice_in(3, System);
763 /// let values = unsafe {
764 /// // Deferred initialization:
765 /// values[0].as_mut_ptr().write(1);
766 /// values[1].as_mut_ptr().write(2);
767 /// values[2].as_mut_ptr().write(3);
769 /// values.assume_init()
772 /// assert_eq!(*values, [1, 2, 3])
774 #[cfg(not(no_global_oom_handling))]
775 #[unstable(feature = "allocator_api", issue = "32838")]
776 // #[unstable(feature = "new_uninit", issue = "63291")]
778 pub fn new_uninit_slice_in(len: usize, alloc: A) -> Box<[mem::MaybeUninit<T>], A> {
779 unsafe { RawVec::with_capacity_in(len, alloc).into_box(len) }
782 /// Constructs a new boxed slice with uninitialized contents in the provided allocator,
783 /// with the memory being filled with `0` bytes.
785 /// See [`MaybeUninit::zeroed`][zeroed] for examples of correct and incorrect usage
791 /// #![feature(allocator_api, new_uninit)]
793 /// use std::alloc::System;
795 /// let values = Box::<[u32], _>::new_zeroed_slice_in(3, System);
796 /// let values = unsafe { values.assume_init() };
798 /// assert_eq!(*values, [0, 0, 0])
801 /// [zeroed]: mem::MaybeUninit::zeroed
802 #[cfg(not(no_global_oom_handling))]
803 #[unstable(feature = "allocator_api", issue = "32838")]
804 // #[unstable(feature = "new_uninit", issue = "63291")]
806 pub fn new_zeroed_slice_in(len: usize, alloc: A) -> Box<[mem::MaybeUninit<T>], A> {
807 unsafe { RawVec::with_capacity_zeroed_in(len, alloc).into_box(len) }
811 impl<T, A: Allocator> Box<mem::MaybeUninit<T>, A> {
812 /// Converts to `Box<T, A>`.
816 /// As with [`MaybeUninit::assume_init`],
817 /// it is up to the caller to guarantee that the value
818 /// really is in an initialized state.
819 /// Calling this when the content is not yet fully initialized
820 /// causes immediate undefined behavior.
822 /// [`MaybeUninit::assume_init`]: mem::MaybeUninit::assume_init
827 /// #![feature(new_uninit)]
829 /// let mut five = Box::<u32>::new_uninit();
831 /// let five: Box<u32> = unsafe {
832 /// // Deferred initialization:
833 /// five.as_mut_ptr().write(5);
835 /// five.assume_init()
838 /// assert_eq!(*five, 5)
840 #[unstable(feature = "new_uninit", issue = "63291")]
841 #[rustc_const_unstable(feature = "const_box", issue = "92521")]
843 pub const unsafe fn assume_init(self) -> Box<T, A> {
844 let (raw, alloc) = Box::into_raw_with_allocator(self);
845 unsafe { Box::from_raw_in(raw as *mut T, alloc) }
848 /// Writes the value and converts to `Box<T, A>`.
850 /// This method converts the box similarly to [`Box::assume_init`] but
851 /// writes `value` into it before conversion thus guaranteeing safety.
852 /// In some scenarios use of this method may improve performance because
853 /// the compiler may be able to optimize copying from stack.
858 /// #![feature(new_uninit)]
860 /// let big_box = Box::<[usize; 1024]>::new_uninit();
862 /// let mut array = [0; 1024];
863 /// for (i, place) in array.iter_mut().enumerate() {
867 /// // The optimizer may be able to elide this copy, so previous code writes
868 /// // to heap directly.
869 /// let big_box = Box::write(big_box, array);
871 /// for (i, x) in big_box.iter().enumerate() {
872 /// assert_eq!(*x, i);
875 #[unstable(feature = "new_uninit", issue = "63291")]
876 #[rustc_const_unstable(feature = "const_box", issue = "92521")]
878 pub const fn write(mut boxed: Self, value: T) -> Box<T, A> {
880 (*boxed).write(value);
886 impl<T, A: Allocator> Box<[mem::MaybeUninit<T>], A> {
887 /// Converts to `Box<[T], A>`.
891 /// As with [`MaybeUninit::assume_init`],
892 /// it is up to the caller to guarantee that the values
893 /// really are in an initialized state.
894 /// Calling this when the content is not yet fully initialized
895 /// causes immediate undefined behavior.
897 /// [`MaybeUninit::assume_init`]: mem::MaybeUninit::assume_init
902 /// #![feature(new_uninit)]
904 /// let mut values = Box::<[u32]>::new_uninit_slice(3);
906 /// let values = unsafe {
907 /// // Deferred initialization:
908 /// values[0].as_mut_ptr().write(1);
909 /// values[1].as_mut_ptr().write(2);
910 /// values[2].as_mut_ptr().write(3);
912 /// values.assume_init()
915 /// assert_eq!(*values, [1, 2, 3])
917 #[unstable(feature = "new_uninit", issue = "63291")]
919 pub unsafe fn assume_init(self) -> Box<[T], A> {
920 let (raw, alloc) = Box::into_raw_with_allocator(self);
921 unsafe { Box::from_raw_in(raw as *mut [T], alloc) }
925 impl<T: ?Sized> Box<T> {
926 /// Constructs a box from a raw pointer.
928 /// After calling this function, the raw pointer is owned by the
929 /// resulting `Box`. Specifically, the `Box` destructor will call
930 /// the destructor of `T` and free the allocated memory. For this
931 /// to be safe, the memory must have been allocated in accordance
932 /// with the [memory layout] used by `Box` .
936 /// This function is unsafe because improper use may lead to
937 /// memory problems. For example, a double-free may occur if the
938 /// function is called twice on the same raw pointer.
940 /// The safety conditions are described in the [memory layout] section.
944 /// Recreate a `Box` which was previously converted to a raw pointer
945 /// using [`Box::into_raw`]:
947 /// let x = Box::new(5);
948 /// let ptr = Box::into_raw(x);
949 /// let x = unsafe { Box::from_raw(ptr) };
951 /// Manually create a `Box` from scratch by using the global allocator:
953 /// use std::alloc::{alloc, Layout};
956 /// let ptr = alloc(Layout::new::<i32>()) as *mut i32;
957 /// // In general .write is required to avoid attempting to destruct
958 /// // the (uninitialized) previous contents of `ptr`, though for this
959 /// // simple example `*ptr = 5` would have worked as well.
961 /// let x = Box::from_raw(ptr);
965 /// [memory layout]: self#memory-layout
966 /// [`Layout`]: crate::Layout
967 #[stable(feature = "box_raw", since = "1.4.0")]
969 pub unsafe fn from_raw(raw: *mut T) -> Self {
970 unsafe { Self::from_raw_in(raw, Global) }
974 impl<T: ?Sized, A: Allocator> Box<T, A> {
975 /// Constructs a box from a raw pointer in the given allocator.
977 /// After calling this function, the raw pointer is owned by the
978 /// resulting `Box`. Specifically, the `Box` destructor will call
979 /// the destructor of `T` and free the allocated memory. For this
980 /// to be safe, the memory must have been allocated in accordance
981 /// with the [memory layout] used by `Box` .
985 /// This function is unsafe because improper use may lead to
986 /// memory problems. For example, a double-free may occur if the
987 /// function is called twice on the same raw pointer.
992 /// Recreate a `Box` which was previously converted to a raw pointer
993 /// using [`Box::into_raw_with_allocator`]:
995 /// #![feature(allocator_api)]
997 /// use std::alloc::System;
999 /// let x = Box::new_in(5, System);
1000 /// let (ptr, alloc) = Box::into_raw_with_allocator(x);
1001 /// let x = unsafe { Box::from_raw_in(ptr, alloc) };
1003 /// Manually create a `Box` from scratch by using the system allocator:
1005 /// #![feature(allocator_api, slice_ptr_get)]
1007 /// use std::alloc::{Allocator, Layout, System};
1010 /// let ptr = System.allocate(Layout::new::<i32>())?.as_mut_ptr() as *mut i32;
1011 /// // In general .write is required to avoid attempting to destruct
1012 /// // the (uninitialized) previous contents of `ptr`, though for this
1013 /// // simple example `*ptr = 5` would have worked as well.
1015 /// let x = Box::from_raw_in(ptr, System);
1017 /// # Ok::<(), std::alloc::AllocError>(())
1020 /// [memory layout]: self#memory-layout
1021 /// [`Layout`]: crate::Layout
1022 #[unstable(feature = "allocator_api", issue = "32838")]
1023 #[rustc_const_unstable(feature = "const_box", issue = "92521")]
1025 pub const unsafe fn from_raw_in(raw: *mut T, alloc: A) -> Self {
1026 Box(unsafe { Unique::new_unchecked(raw) }, alloc)
1029 /// Consumes the `Box`, returning a wrapped raw pointer.
1031 /// The pointer will be properly aligned and non-null.
1033 /// After calling this function, the caller is responsible for the
1034 /// memory previously managed by the `Box`. In particular, the
1035 /// caller should properly destroy `T` and release the memory, taking
1036 /// into account the [memory layout] used by `Box`. The easiest way to
1037 /// do this is to convert the raw pointer back into a `Box` with the
1038 /// [`Box::from_raw`] function, allowing the `Box` destructor to perform
1041 /// Note: this is an associated function, which means that you have
1042 /// to call it as `Box::into_raw(b)` instead of `b.into_raw()`. This
1043 /// is so that there is no conflict with a method on the inner type.
1046 /// Converting the raw pointer back into a `Box` with [`Box::from_raw`]
1047 /// for automatic cleanup:
1049 /// let x = Box::new(String::from("Hello"));
1050 /// let ptr = Box::into_raw(x);
1051 /// let x = unsafe { Box::from_raw(ptr) };
1053 /// Manual cleanup by explicitly running the destructor and deallocating
1056 /// use std::alloc::{dealloc, Layout};
1059 /// let x = Box::new(String::from("Hello"));
1060 /// let p = Box::into_raw(x);
1062 /// ptr::drop_in_place(p);
1063 /// dealloc(p as *mut u8, Layout::new::<String>());
1067 /// [memory layout]: self#memory-layout
1068 #[stable(feature = "box_raw", since = "1.4.0")]
1070 pub fn into_raw(b: Self) -> *mut T {
1071 Self::into_raw_with_allocator(b).0
1074 /// Consumes the `Box`, returning a wrapped raw pointer and the allocator.
1076 /// The pointer will be properly aligned and non-null.
1078 /// After calling this function, the caller is responsible for the
1079 /// memory previously managed by the `Box`. In particular, the
1080 /// caller should properly destroy `T` and release the memory, taking
1081 /// into account the [memory layout] used by `Box`. The easiest way to
1082 /// do this is to convert the raw pointer back into a `Box` with the
1083 /// [`Box::from_raw_in`] function, allowing the `Box` destructor to perform
1086 /// Note: this is an associated function, which means that you have
1087 /// to call it as `Box::into_raw_with_allocator(b)` instead of `b.into_raw_with_allocator()`. This
1088 /// is so that there is no conflict with a method on the inner type.
1091 /// Converting the raw pointer back into a `Box` with [`Box::from_raw_in`]
1092 /// for automatic cleanup:
1094 /// #![feature(allocator_api)]
1096 /// use std::alloc::System;
1098 /// let x = Box::new_in(String::from("Hello"), System);
1099 /// let (ptr, alloc) = Box::into_raw_with_allocator(x);
1100 /// let x = unsafe { Box::from_raw_in(ptr, alloc) };
1102 /// Manual cleanup by explicitly running the destructor and deallocating
1105 /// #![feature(allocator_api)]
1107 /// use std::alloc::{Allocator, Layout, System};
1108 /// use std::ptr::{self, NonNull};
1110 /// let x = Box::new_in(String::from("Hello"), System);
1111 /// let (ptr, alloc) = Box::into_raw_with_allocator(x);
1113 /// ptr::drop_in_place(ptr);
1114 /// let non_null = NonNull::new_unchecked(ptr);
1115 /// alloc.deallocate(non_null.cast(), Layout::new::<String>());
1119 /// [memory layout]: self#memory-layout
1120 #[unstable(feature = "allocator_api", issue = "32838")]
1121 #[rustc_const_unstable(feature = "const_box", issue = "92521")]
1123 pub const fn into_raw_with_allocator(b: Self) -> (*mut T, A) {
1124 let (leaked, alloc) = Box::into_unique(b);
1125 (leaked.as_ptr(), alloc)
1129 feature = "ptr_internals",
1131 reason = "use `Box::leak(b).into()` or `Unique::from(Box::leak(b))` instead"
1133 #[rustc_const_unstable(feature = "const_box", issue = "92521")]
1136 pub const fn into_unique(b: Self) -> (Unique<T>, A) {
1137 // Box is recognized as a "unique pointer" by Stacked Borrows, but internally it is a
1138 // raw pointer for the type system. Turning it directly into a raw pointer would not be
1139 // recognized as "releasing" the unique pointer to permit aliased raw accesses,
1140 // so all raw pointer methods have to go through `Box::leak`. Turning *that* to a raw pointer
1141 // behaves correctly.
1142 let alloc = unsafe { ptr::read(&b.1) };
1143 (Unique::from(Box::leak(b)), alloc)
1146 /// Returns a reference to the underlying allocator.
1148 /// Note: this is an associated function, which means that you have
1149 /// to call it as `Box::allocator(&b)` instead of `b.allocator()`. This
1150 /// is so that there is no conflict with a method on the inner type.
1151 #[unstable(feature = "allocator_api", issue = "32838")]
1152 #[rustc_const_unstable(feature = "const_box", issue = "92521")]
1154 pub const fn allocator(b: &Self) -> &A {
1158 /// Consumes and leaks the `Box`, returning a mutable reference,
1159 /// `&'a mut T`. Note that the type `T` must outlive the chosen lifetime
1160 /// `'a`. If the type has only static references, or none at all, then this
1161 /// may be chosen to be `'static`.
1163 /// This function is mainly useful for data that lives for the remainder of
1164 /// the program's life. Dropping the returned reference will cause a memory
1165 /// leak. If this is not acceptable, the reference should first be wrapped
1166 /// with the [`Box::from_raw`] function producing a `Box`. This `Box` can
1167 /// then be dropped which will properly destroy `T` and release the
1168 /// allocated memory.
1170 /// Note: this is an associated function, which means that you have
1171 /// to call it as `Box::leak(b)` instead of `b.leak()`. This
1172 /// is so that there is no conflict with a method on the inner type.
1179 /// let x = Box::new(41);
1180 /// let static_ref: &'static mut usize = Box::leak(x);
1181 /// *static_ref += 1;
1182 /// assert_eq!(*static_ref, 42);
1188 /// let x = vec![1, 2, 3].into_boxed_slice();
1189 /// let static_ref = Box::leak(x);
1190 /// static_ref[0] = 4;
1191 /// assert_eq!(*static_ref, [4, 2, 3]);
1193 #[stable(feature = "box_leak", since = "1.26.0")]
1194 #[rustc_const_unstable(feature = "const_box", issue = "92521")]
1196 pub const fn leak<'a>(b: Self) -> &'a mut T
1200 unsafe { &mut *mem::ManuallyDrop::new(b).0.as_ptr() }
1203 /// Converts a `Box<T>` into a `Pin<Box<T>>`. If `T` does not implement [`Unpin`], then
1204 /// `*boxed` will be pinned in memory and unable to be moved.
1206 /// This conversion does not allocate on the heap and happens in place.
1208 /// This is also available via [`From`].
1210 /// Constructing and pinning a `Box` with <code>Box::into_pin([Box::new]\(x))</code>
1211 /// can also be written more concisely using <code>[Box::pin]\(x)</code>.
1212 /// This `into_pin` method is useful if you already have a `Box<T>`, or you are
1213 /// constructing a (pinned) `Box` in a different way than with [`Box::new`].
1217 /// It's not recommended that crates add an impl like `From<Box<T>> for Pin<T>`,
1218 /// as it'll introduce an ambiguity when calling `Pin::from`.
1219 /// A demonstration of such a poor impl is shown below.
1222 /// # use std::pin::Pin;
1223 /// struct Foo; // A type defined in this crate.
1224 /// impl From<Box<()>> for Pin<Foo> {
1225 /// fn from(_: Box<()>) -> Pin<Foo> {
1230 /// let foo = Box::new(());
1231 /// let bar = Pin::from(foo);
1233 #[stable(feature = "box_into_pin", since = "1.63.0")]
1234 #[rustc_const_unstable(feature = "const_box", issue = "92521")]
1235 pub const fn into_pin(boxed: Self) -> Pin<Self>
1239 // It's not possible to move or replace the insides of a `Pin<Box<T>>`
1240 // when `T: !Unpin`, so it's safe to pin it directly without any
1241 // additional requirements.
1242 unsafe { Pin::new_unchecked(boxed) }
1246 #[stable(feature = "rust1", since = "1.0.0")]
1247 unsafe impl<#[may_dangle] T: ?Sized, A: Allocator> Drop for Box<T, A> {
1248 fn drop(&mut self) {
1249 // FIXME: Do nothing, drop is currently performed by compiler.
1253 #[cfg(not(no_global_oom_handling))]
1254 #[stable(feature = "rust1", since = "1.0.0")]
1255 impl<T: Default> Default for Box<T> {
1256 /// Creates a `Box<T>`, with the `Default` value for T.
1257 fn default() -> Self {
1258 #[cfg_attr(not(bootstrap), rustc_box)]
1259 Box::new(T::default())
1263 #[cfg(not(no_global_oom_handling))]
1264 #[stable(feature = "rust1", since = "1.0.0")]
1265 #[rustc_const_unstable(feature = "const_default_impls", issue = "87864")]
1266 impl<T> const Default for Box<[T]> {
1267 fn default() -> Self {
1268 let ptr: Unique<[T]> = Unique::<[T; 0]>::dangling();
1273 #[cfg(not(no_global_oom_handling))]
1274 #[stable(feature = "default_box_extra", since = "1.17.0")]
1275 #[rustc_const_unstable(feature = "const_default_impls", issue = "87864")]
1276 impl const Default for Box<str> {
1277 fn default() -> Self {
1278 // SAFETY: This is the same as `Unique::cast<U>` but with an unsized `U = str`.
1279 let ptr: Unique<str> = unsafe {
1280 let bytes: Unique<[u8]> = Unique::<[u8; 0]>::dangling();
1281 Unique::new_unchecked(bytes.as_ptr() as *mut str)
1287 #[cfg(not(no_global_oom_handling))]
1288 #[stable(feature = "rust1", since = "1.0.0")]
1289 impl<T: Clone, A: Allocator + Clone> Clone for Box<T, A> {
1290 /// Returns a new box with a `clone()` of this box's contents.
1295 /// let x = Box::new(5);
1296 /// let y = x.clone();
1298 /// // The value is the same
1299 /// assert_eq!(x, y);
1301 /// // But they are unique objects
1302 /// assert_ne!(&*x as *const i32, &*y as *const i32);
1305 fn clone(&self) -> Self {
1306 // Pre-allocate memory to allow writing the cloned value directly.
1307 let mut boxed = Self::new_uninit_in(self.1.clone());
1309 (**self).write_clone_into_raw(boxed.as_mut_ptr());
1314 /// Copies `source`'s contents into `self` without creating a new allocation.
1319 /// let x = Box::new(5);
1320 /// let mut y = Box::new(10);
1321 /// let yp: *const i32 = &*y;
1323 /// y.clone_from(&x);
1325 /// // The value is the same
1326 /// assert_eq!(x, y);
1328 /// // And no allocation occurred
1329 /// assert_eq!(yp, &*y);
1332 fn clone_from(&mut self, source: &Self) {
1333 (**self).clone_from(&(**source));
1337 #[cfg(not(no_global_oom_handling))]
1338 #[stable(feature = "box_slice_clone", since = "1.3.0")]
1339 impl Clone for Box<str> {
1340 fn clone(&self) -> Self {
1341 // this makes a copy of the data
1342 let buf: Box<[u8]> = self.as_bytes().into();
1343 unsafe { from_boxed_utf8_unchecked(buf) }
1347 #[stable(feature = "rust1", since = "1.0.0")]
1348 impl<T: ?Sized + PartialEq, A: Allocator> PartialEq for Box<T, A> {
1350 fn eq(&self, other: &Self) -> bool {
1351 PartialEq::eq(&**self, &**other)
1354 fn ne(&self, other: &Self) -> bool {
1355 PartialEq::ne(&**self, &**other)
1358 #[stable(feature = "rust1", since = "1.0.0")]
1359 impl<T: ?Sized + PartialOrd, A: Allocator> PartialOrd for Box<T, A> {
1361 fn partial_cmp(&self, other: &Self) -> Option<Ordering> {
1362 PartialOrd::partial_cmp(&**self, &**other)
1365 fn lt(&self, other: &Self) -> bool {
1366 PartialOrd::lt(&**self, &**other)
1369 fn le(&self, other: &Self) -> bool {
1370 PartialOrd::le(&**self, &**other)
1373 fn ge(&self, other: &Self) -> bool {
1374 PartialOrd::ge(&**self, &**other)
1377 fn gt(&self, other: &Self) -> bool {
1378 PartialOrd::gt(&**self, &**other)
1381 #[stable(feature = "rust1", since = "1.0.0")]
1382 impl<T: ?Sized + Ord, A: Allocator> Ord for Box<T, A> {
1384 fn cmp(&self, other: &Self) -> Ordering {
1385 Ord::cmp(&**self, &**other)
1388 #[stable(feature = "rust1", since = "1.0.0")]
1389 impl<T: ?Sized + Eq, A: Allocator> Eq for Box<T, A> {}
1391 #[stable(feature = "rust1", since = "1.0.0")]
1392 impl<T: ?Sized + Hash, A: Allocator> Hash for Box<T, A> {
1393 fn hash<H: Hasher>(&self, state: &mut H) {
1394 (**self).hash(state);
1398 #[stable(feature = "indirect_hasher_impl", since = "1.22.0")]
1399 impl<T: ?Sized + Hasher, A: Allocator> Hasher for Box<T, A> {
1400 fn finish(&self) -> u64 {
1403 fn write(&mut self, bytes: &[u8]) {
1404 (**self).write(bytes)
1406 fn write_u8(&mut self, i: u8) {
1407 (**self).write_u8(i)
1409 fn write_u16(&mut self, i: u16) {
1410 (**self).write_u16(i)
1412 fn write_u32(&mut self, i: u32) {
1413 (**self).write_u32(i)
1415 fn write_u64(&mut self, i: u64) {
1416 (**self).write_u64(i)
1418 fn write_u128(&mut self, i: u128) {
1419 (**self).write_u128(i)
1421 fn write_usize(&mut self, i: usize) {
1422 (**self).write_usize(i)
1424 fn write_i8(&mut self, i: i8) {
1425 (**self).write_i8(i)
1427 fn write_i16(&mut self, i: i16) {
1428 (**self).write_i16(i)
1430 fn write_i32(&mut self, i: i32) {
1431 (**self).write_i32(i)
1433 fn write_i64(&mut self, i: i64) {
1434 (**self).write_i64(i)
1436 fn write_i128(&mut self, i: i128) {
1437 (**self).write_i128(i)
1439 fn write_isize(&mut self, i: isize) {
1440 (**self).write_isize(i)
1442 fn write_length_prefix(&mut self, len: usize) {
1443 (**self).write_length_prefix(len)
1445 fn write_str(&mut self, s: &str) {
1446 (**self).write_str(s)
1450 #[cfg(not(no_global_oom_handling))]
1451 #[stable(feature = "from_for_ptrs", since = "1.6.0")]
1452 impl<T> From<T> for Box<T> {
1453 /// Converts a `T` into a `Box<T>`
1455 /// The conversion allocates on the heap and moves `t`
1456 /// from the stack into it.
1462 /// let boxed = Box::new(5);
1464 /// assert_eq!(Box::from(x), boxed);
1466 fn from(t: T) -> Self {
1471 #[stable(feature = "pin", since = "1.33.0")]
1472 #[rustc_const_unstable(feature = "const_box", issue = "92521")]
1473 impl<T: ?Sized, A: Allocator> const From<Box<T, A>> for Pin<Box<T, A>>
1477 /// Converts a `Box<T>` into a `Pin<Box<T>>`. If `T` does not implement [`Unpin`], then
1478 /// `*boxed` will be pinned in memory and unable to be moved.
1480 /// This conversion does not allocate on the heap and happens in place.
1482 /// This is also available via [`Box::into_pin`].
1484 /// Constructing and pinning a `Box` with <code><Pin<Box\<T>>>::from([Box::new]\(x))</code>
1485 /// can also be written more concisely using <code>[Box::pin]\(x)</code>.
1486 /// This `From` implementation is useful if you already have a `Box<T>`, or you are
1487 /// constructing a (pinned) `Box` in a different way than with [`Box::new`].
1488 fn from(boxed: Box<T, A>) -> Self {
1489 Box::into_pin(boxed)
1493 #[cfg(not(no_global_oom_handling))]
1494 #[stable(feature = "box_from_slice", since = "1.17.0")]
1495 impl<T: Copy> From<&[T]> for Box<[T]> {
1496 /// Converts a `&[T]` into a `Box<[T]>`
1498 /// This conversion allocates on the heap
1499 /// and performs a copy of `slice`.
1503 /// // create a &[u8] which will be used to create a Box<[u8]>
1504 /// let slice: &[u8] = &[104, 101, 108, 108, 111];
1505 /// let boxed_slice: Box<[u8]> = Box::from(slice);
1507 /// println!("{boxed_slice:?}");
1509 fn from(slice: &[T]) -> Box<[T]> {
1510 let len = slice.len();
1511 let buf = RawVec::with_capacity(len);
1513 ptr::copy_nonoverlapping(slice.as_ptr(), buf.ptr(), len);
1514 buf.into_box(slice.len()).assume_init()
1519 #[cfg(not(no_global_oom_handling))]
1520 #[stable(feature = "box_from_cow", since = "1.45.0")]
1521 impl<T: Copy> From<Cow<'_, [T]>> for Box<[T]> {
1522 /// Converts a `Cow<'_, [T]>` into a `Box<[T]>`
1524 /// When `cow` is the `Cow::Borrowed` variant, this
1525 /// conversion allocates on the heap and copies the
1526 /// underlying slice. Otherwise, it will try to reuse the owned
1527 /// `Vec`'s allocation.
1529 fn from(cow: Cow<'_, [T]>) -> Box<[T]> {
1531 Cow::Borrowed(slice) => Box::from(slice),
1532 Cow::Owned(slice) => Box::from(slice),
1537 #[cfg(not(no_global_oom_handling))]
1538 #[stable(feature = "box_from_slice", since = "1.17.0")]
1539 impl From<&str> for Box<str> {
1540 /// Converts a `&str` into a `Box<str>`
1542 /// This conversion allocates on the heap
1543 /// and performs a copy of `s`.
1548 /// let boxed: Box<str> = Box::from("hello");
1549 /// println!("{boxed}");
1552 fn from(s: &str) -> Box<str> {
1553 unsafe { from_boxed_utf8_unchecked(Box::from(s.as_bytes())) }
1557 #[cfg(not(no_global_oom_handling))]
1558 #[stable(feature = "box_from_cow", since = "1.45.0")]
1559 impl From<Cow<'_, str>> for Box<str> {
1560 /// Converts a `Cow<'_, str>` into a `Box<str>`
1562 /// When `cow` is the `Cow::Borrowed` variant, this
1563 /// conversion allocates on the heap and copies the
1564 /// underlying `str`. Otherwise, it will try to reuse the owned
1565 /// `String`'s allocation.
1570 /// use std::borrow::Cow;
1572 /// let unboxed = Cow::Borrowed("hello");
1573 /// let boxed: Box<str> = Box::from(unboxed);
1574 /// println!("{boxed}");
1578 /// # use std::borrow::Cow;
1579 /// let unboxed = Cow::Owned("hello".to_string());
1580 /// let boxed: Box<str> = Box::from(unboxed);
1581 /// println!("{boxed}");
1584 fn from(cow: Cow<'_, str>) -> Box<str> {
1586 Cow::Borrowed(s) => Box::from(s),
1587 Cow::Owned(s) => Box::from(s),
1592 #[stable(feature = "boxed_str_conv", since = "1.19.0")]
1593 impl<A: Allocator> From<Box<str, A>> for Box<[u8], A> {
1594 /// Converts a `Box<str>` into a `Box<[u8]>`
1596 /// This conversion does not allocate on the heap and happens in place.
1600 /// // create a Box<str> which will be used to create a Box<[u8]>
1601 /// let boxed: Box<str> = Box::from("hello");
1602 /// let boxed_str: Box<[u8]> = Box::from(boxed);
1604 /// // create a &[u8] which will be used to create a Box<[u8]>
1605 /// let slice: &[u8] = &[104, 101, 108, 108, 111];
1606 /// let boxed_slice = Box::from(slice);
1608 /// assert_eq!(boxed_slice, boxed_str);
1611 fn from(s: Box<str, A>) -> Self {
1612 let (raw, alloc) = Box::into_raw_with_allocator(s);
1613 unsafe { Box::from_raw_in(raw as *mut [u8], alloc) }
1617 #[cfg(not(no_global_oom_handling))]
1618 #[stable(feature = "box_from_array", since = "1.45.0")]
1619 impl<T, const N: usize> From<[T; N]> for Box<[T]> {
1620 /// Converts a `[T; N]` into a `Box<[T]>`
1622 /// This conversion moves the array to newly heap-allocated memory.
1627 /// let boxed: Box<[u8]> = Box::from([4, 2]);
1628 /// println!("{boxed:?}");
1630 fn from(array: [T; N]) -> Box<[T]> {
1631 #[cfg_attr(not(bootstrap), rustc_box)]
1636 #[stable(feature = "boxed_slice_try_from", since = "1.43.0")]
1637 impl<T, const N: usize> TryFrom<Box<[T]>> for Box<[T; N]> {
1638 type Error = Box<[T]>;
1640 /// Attempts to convert a `Box<[T]>` into a `Box<[T; N]>`.
1642 /// The conversion occurs in-place and does not require a
1643 /// new memory allocation.
1647 /// Returns the old `Box<[T]>` in the `Err` variant if
1648 /// `boxed_slice.len()` does not equal `N`.
1649 fn try_from(boxed_slice: Box<[T]>) -> Result<Self, Self::Error> {
1650 if boxed_slice.len() == N {
1651 Ok(unsafe { Box::from_raw(Box::into_raw(boxed_slice) as *mut [T; N]) })
1658 impl<A: Allocator> Box<dyn Any, A> {
1659 /// Attempt to downcast the box to a concrete type.
1664 /// use std::any::Any;
1666 /// fn print_if_string(value: Box<dyn Any>) {
1667 /// if let Ok(string) = value.downcast::<String>() {
1668 /// println!("String ({}): {}", string.len(), string);
1672 /// let my_string = "Hello World".to_string();
1673 /// print_if_string(Box::new(my_string));
1674 /// print_if_string(Box::new(0i8));
1677 #[stable(feature = "rust1", since = "1.0.0")]
1678 pub fn downcast<T: Any>(self) -> Result<Box<T, A>, Self> {
1679 if self.is::<T>() { unsafe { Ok(self.downcast_unchecked::<T>()) } } else { Err(self) }
1682 /// Downcasts the box to a concrete type.
1684 /// For a safe alternative see [`downcast`].
1689 /// #![feature(downcast_unchecked)]
1691 /// use std::any::Any;
1693 /// let x: Box<dyn Any> = Box::new(1_usize);
1696 /// assert_eq!(*x.downcast_unchecked::<usize>(), 1);
1702 /// The contained value must be of type `T`. Calling this method
1703 /// with the incorrect type is *undefined behavior*.
1705 /// [`downcast`]: Self::downcast
1707 #[unstable(feature = "downcast_unchecked", issue = "90850")]
1708 pub unsafe fn downcast_unchecked<T: Any>(self) -> Box<T, A> {
1709 debug_assert!(self.is::<T>());
1711 let (raw, alloc): (*mut dyn Any, _) = Box::into_raw_with_allocator(self);
1712 Box::from_raw_in(raw as *mut T, alloc)
1717 impl<A: Allocator> Box<dyn Any + Send, A> {
1718 /// Attempt to downcast the box to a concrete type.
1723 /// use std::any::Any;
1725 /// fn print_if_string(value: Box<dyn Any + Send>) {
1726 /// if let Ok(string) = value.downcast::<String>() {
1727 /// println!("String ({}): {}", string.len(), string);
1731 /// let my_string = "Hello World".to_string();
1732 /// print_if_string(Box::new(my_string));
1733 /// print_if_string(Box::new(0i8));
1736 #[stable(feature = "rust1", since = "1.0.0")]
1737 pub fn downcast<T: Any>(self) -> Result<Box<T, A>, Self> {
1738 if self.is::<T>() { unsafe { Ok(self.downcast_unchecked::<T>()) } } else { Err(self) }
1741 /// Downcasts the box to a concrete type.
1743 /// For a safe alternative see [`downcast`].
1748 /// #![feature(downcast_unchecked)]
1750 /// use std::any::Any;
1752 /// let x: Box<dyn Any + Send> = Box::new(1_usize);
1755 /// assert_eq!(*x.downcast_unchecked::<usize>(), 1);
1761 /// The contained value must be of type `T`. Calling this method
1762 /// with the incorrect type is *undefined behavior*.
1764 /// [`downcast`]: Self::downcast
1766 #[unstable(feature = "downcast_unchecked", issue = "90850")]
1767 pub unsafe fn downcast_unchecked<T: Any>(self) -> Box<T, A> {
1768 debug_assert!(self.is::<T>());
1770 let (raw, alloc): (*mut (dyn Any + Send), _) = Box::into_raw_with_allocator(self);
1771 Box::from_raw_in(raw as *mut T, alloc)
1776 impl<A: Allocator> Box<dyn Any + Send + Sync, A> {
1777 /// Attempt to downcast the box to a concrete type.
1782 /// use std::any::Any;
1784 /// fn print_if_string(value: Box<dyn Any + Send + Sync>) {
1785 /// if let Ok(string) = value.downcast::<String>() {
1786 /// println!("String ({}): {}", string.len(), string);
1790 /// let my_string = "Hello World".to_string();
1791 /// print_if_string(Box::new(my_string));
1792 /// print_if_string(Box::new(0i8));
1795 #[stable(feature = "box_send_sync_any_downcast", since = "1.51.0")]
1796 pub fn downcast<T: Any>(self) -> Result<Box<T, A>, Self> {
1797 if self.is::<T>() { unsafe { Ok(self.downcast_unchecked::<T>()) } } else { Err(self) }
1800 /// Downcasts the box to a concrete type.
1802 /// For a safe alternative see [`downcast`].
1807 /// #![feature(downcast_unchecked)]
1809 /// use std::any::Any;
1811 /// let x: Box<dyn Any + Send + Sync> = Box::new(1_usize);
1814 /// assert_eq!(*x.downcast_unchecked::<usize>(), 1);
1820 /// The contained value must be of type `T`. Calling this method
1821 /// with the incorrect type is *undefined behavior*.
1823 /// [`downcast`]: Self::downcast
1825 #[unstable(feature = "downcast_unchecked", issue = "90850")]
1826 pub unsafe fn downcast_unchecked<T: Any>(self) -> Box<T, A> {
1827 debug_assert!(self.is::<T>());
1829 let (raw, alloc): (*mut (dyn Any + Send + Sync), _) =
1830 Box::into_raw_with_allocator(self);
1831 Box::from_raw_in(raw as *mut T, alloc)
1836 #[stable(feature = "rust1", since = "1.0.0")]
1837 impl<T: fmt::Display + ?Sized, A: Allocator> fmt::Display for Box<T, A> {
1838 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
1839 fmt::Display::fmt(&**self, f)
1843 #[stable(feature = "rust1", since = "1.0.0")]
1844 impl<T: fmt::Debug + ?Sized, A: Allocator> fmt::Debug for Box<T, A> {
1845 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
1846 fmt::Debug::fmt(&**self, f)
1850 #[stable(feature = "rust1", since = "1.0.0")]
1851 impl<T: ?Sized, A: Allocator> fmt::Pointer for Box<T, A> {
1852 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
1853 // It's not possible to extract the inner Uniq directly from the Box,
1854 // instead we cast it to a *const which aliases the Unique
1855 let ptr: *const T = &**self;
1856 fmt::Pointer::fmt(&ptr, f)
1860 #[stable(feature = "rust1", since = "1.0.0")]
1861 #[rustc_const_unstable(feature = "const_box", issue = "92521")]
1862 impl<T: ?Sized, A: Allocator> const Deref for Box<T, A> {
1865 fn deref(&self) -> &T {
1870 #[stable(feature = "rust1", since = "1.0.0")]
1871 #[rustc_const_unstable(feature = "const_box", issue = "92521")]
1872 impl<T: ?Sized, A: Allocator> const DerefMut for Box<T, A> {
1873 fn deref_mut(&mut self) -> &mut T {
1878 #[unstable(feature = "receiver_trait", issue = "none")]
1879 impl<T: ?Sized, A: Allocator> Receiver for Box<T, A> {}
1881 #[stable(feature = "rust1", since = "1.0.0")]
1882 impl<I: Iterator + ?Sized, A: Allocator> Iterator for Box<I, A> {
1883 type Item = I::Item;
1884 fn next(&mut self) -> Option<I::Item> {
1887 fn size_hint(&self) -> (usize, Option<usize>) {
1888 (**self).size_hint()
1890 fn nth(&mut self, n: usize) -> Option<I::Item> {
1893 fn last(self) -> Option<I::Item> {
1900 fn last(self) -> Option<Self::Item>;
1903 impl<I: Iterator + ?Sized, A: Allocator> BoxIter for Box<I, A> {
1904 type Item = I::Item;
1905 default fn last(self) -> Option<I::Item> {
1907 fn some<T>(_: Option<T>, x: T) -> Option<T> {
1911 self.fold(None, some)
1915 /// Specialization for sized `I`s that uses `I`s implementation of `last()`
1916 /// instead of the default.
1917 #[stable(feature = "rust1", since = "1.0.0")]
1918 impl<I: Iterator, A: Allocator> BoxIter for Box<I, A> {
1919 fn last(self) -> Option<I::Item> {
1924 #[stable(feature = "rust1", since = "1.0.0")]
1925 impl<I: DoubleEndedIterator + ?Sized, A: Allocator> DoubleEndedIterator for Box<I, A> {
1926 fn next_back(&mut self) -> Option<I::Item> {
1927 (**self).next_back()
1929 fn nth_back(&mut self, n: usize) -> Option<I::Item> {
1930 (**self).nth_back(n)
1933 #[stable(feature = "rust1", since = "1.0.0")]
1934 impl<I: ExactSizeIterator + ?Sized, A: Allocator> ExactSizeIterator for Box<I, A> {
1935 fn len(&self) -> usize {
1938 fn is_empty(&self) -> bool {
1943 #[stable(feature = "fused", since = "1.26.0")]
1944 impl<I: FusedIterator + ?Sized, A: Allocator> FusedIterator for Box<I, A> {}
1946 #[stable(feature = "boxed_closure_impls", since = "1.35.0")]
1947 impl<Args, F: FnOnce<Args> + ?Sized, A: Allocator> FnOnce<Args> for Box<F, A> {
1948 type Output = <F as FnOnce<Args>>::Output;
1950 extern "rust-call" fn call_once(self, args: Args) -> Self::Output {
1951 <F as FnOnce<Args>>::call_once(*self, args)
1955 #[stable(feature = "boxed_closure_impls", since = "1.35.0")]
1956 impl<Args, F: FnMut<Args> + ?Sized, A: Allocator> FnMut<Args> for Box<F, A> {
1957 extern "rust-call" fn call_mut(&mut self, args: Args) -> Self::Output {
1958 <F as FnMut<Args>>::call_mut(self, args)
1962 #[stable(feature = "boxed_closure_impls", since = "1.35.0")]
1963 impl<Args, F: Fn<Args> + ?Sized, A: Allocator> Fn<Args> for Box<F, A> {
1964 extern "rust-call" fn call(&self, args: Args) -> Self::Output {
1965 <F as Fn<Args>>::call(self, args)
1969 #[unstable(feature = "coerce_unsized", issue = "27732")]
1970 impl<T: ?Sized + Unsize<U>, U: ?Sized, A: Allocator> CoerceUnsized<Box<U, A>> for Box<T, A> {}
1972 #[unstable(feature = "dispatch_from_dyn", issue = "none")]
1973 impl<T: ?Sized + Unsize<U>, U: ?Sized> DispatchFromDyn<Box<U>> for Box<T, Global> {}
1975 #[cfg(not(no_global_oom_handling))]
1976 #[stable(feature = "boxed_slice_from_iter", since = "1.32.0")]
1977 impl<I> FromIterator<I> for Box<[I]> {
1978 fn from_iter<T: IntoIterator<Item = I>>(iter: T) -> Self {
1979 iter.into_iter().collect::<Vec<_>>().into_boxed_slice()
1983 #[cfg(not(no_global_oom_handling))]
1984 #[stable(feature = "box_slice_clone", since = "1.3.0")]
1985 impl<T: Clone, A: Allocator + Clone> Clone for Box<[T], A> {
1986 fn clone(&self) -> Self {
1987 let alloc = Box::allocator(self).clone();
1988 self.to_vec_in(alloc).into_boxed_slice()
1991 fn clone_from(&mut self, other: &Self) {
1992 if self.len() == other.len() {
1993 self.clone_from_slice(&other);
1995 *self = other.clone();
2000 #[stable(feature = "box_borrow", since = "1.1.0")]
2001 impl<T: ?Sized, A: Allocator> borrow::Borrow<T> for Box<T, A> {
2002 fn borrow(&self) -> &T {
2007 #[stable(feature = "box_borrow", since = "1.1.0")]
2008 impl<T: ?Sized, A: Allocator> borrow::BorrowMut<T> for Box<T, A> {
2009 fn borrow_mut(&mut self) -> &mut T {
2014 #[stable(since = "1.5.0", feature = "smart_ptr_as_ref")]
2015 impl<T: ?Sized, A: Allocator> AsRef<T> for Box<T, A> {
2016 fn as_ref(&self) -> &T {
2021 #[stable(since = "1.5.0", feature = "smart_ptr_as_ref")]
2022 impl<T: ?Sized, A: Allocator> AsMut<T> for Box<T, A> {
2023 fn as_mut(&mut self) -> &mut T {
2030 * We could have chosen not to add this impl, and instead have written a
2031 * function of Pin<Box<T>> to Pin<T>. Such a function would not be sound,
2032 * because Box<T> implements Unpin even when T does not, as a result of
2035 * We chose this API instead of the alternative for a few reasons:
2036 * - Logically, it is helpful to understand pinning in regard to the
2037 * memory region being pointed to. For this reason none of the
2038 * standard library pointer types support projecting through a pin
2039 * (Box<T> is the only pointer type in std for which this would be
2041 * - It is in practice very useful to have Box<T> be unconditionally
2042 * Unpin because of trait objects, for which the structural auto
2043 * trait functionality does not apply (e.g., Box<dyn Foo> would
2044 * otherwise not be Unpin).
2046 * Another type with the same semantics as Box but only a conditional
2047 * implementation of `Unpin` (where `T: Unpin`) would be valid/safe, and
2048 * could have a method to project a Pin<T> from it.
2050 #[stable(feature = "pin", since = "1.33.0")]
2051 #[rustc_const_unstable(feature = "const_box", issue = "92521")]
2052 impl<T: ?Sized, A: Allocator> const Unpin for Box<T, A> where A: 'static {}
2054 #[unstable(feature = "generator_trait", issue = "43122")]
2055 impl<G: ?Sized + Generator<R> + Unpin, R, A: Allocator> Generator<R> for Box<G, A>
2059 type Yield = G::Yield;
2060 type Return = G::Return;
2062 fn resume(mut self: Pin<&mut Self>, arg: R) -> GeneratorState<Self::Yield, Self::Return> {
2063 G::resume(Pin::new(&mut *self), arg)
2067 #[unstable(feature = "generator_trait", issue = "43122")]
2068 impl<G: ?Sized + Generator<R>, R, A: Allocator> Generator<R> for Pin<Box<G, A>>
2072 type Yield = G::Yield;
2073 type Return = G::Return;
2075 fn resume(mut self: Pin<&mut Self>, arg: R) -> GeneratorState<Self::Yield, Self::Return> {
2076 G::resume((*self).as_mut(), arg)
2080 #[stable(feature = "futures_api", since = "1.36.0")]
2081 impl<F: ?Sized + Future + Unpin, A: Allocator> Future for Box<F, A>
2085 type Output = F::Output;
2087 fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
2088 F::poll(Pin::new(&mut *self), cx)
2092 #[unstable(feature = "async_iterator", issue = "79024")]
2093 impl<S: ?Sized + AsyncIterator + Unpin> AsyncIterator for Box<S> {
2094 type Item = S::Item;
2096 fn poll_next(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Option<Self::Item>> {
2097 Pin::new(&mut **self).poll_next(cx)
2100 fn size_hint(&self) -> (usize, Option<usize>) {
2101 (**self).size_hint()