1 //! The `Box<T>` 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)))]
211 #[stable(feature = "rust1", since = "1.0.0")]
213 pub fn new(x: T) -> Self {
218 /// Constructs a new box with uninitialized contents.
223 /// #![feature(new_uninit)]
225 /// let mut five = Box::<u32>::new_uninit();
227 /// let five = unsafe {
228 /// // Deferred initialization:
229 /// five.as_mut_ptr().write(5);
231 /// five.assume_init()
234 /// assert_eq!(*five, 5)
236 #[cfg(not(no_global_oom_handling))]
237 #[unstable(feature = "new_uninit", issue = "63291")]
240 pub fn new_uninit() -> Box<mem::MaybeUninit<T>> {
241 Self::new_uninit_in(Global)
244 /// Constructs a new `Box` with uninitialized contents, with the memory
245 /// being filled with `0` bytes.
247 /// See [`MaybeUninit::zeroed`][zeroed] for examples of correct and incorrect usage
253 /// #![feature(new_uninit)]
255 /// let zero = Box::<u32>::new_zeroed();
256 /// let zero = unsafe { zero.assume_init() };
258 /// assert_eq!(*zero, 0)
261 /// [zeroed]: mem::MaybeUninit::zeroed
262 #[cfg(not(no_global_oom_handling))]
264 #[unstable(feature = "new_uninit", issue = "63291")]
266 pub fn new_zeroed() -> Box<mem::MaybeUninit<T>> {
267 Self::new_zeroed_in(Global)
270 /// Constructs a new `Pin<Box<T>>`. If `T` does not implement [`Unpin`], then
271 /// `x` will be pinned in memory and unable to be moved.
273 /// Constructing and pinning of the `Box` can also be done in two steps: `Box::pin(x)`
274 /// does the same as <code>[Box::into_pin]\([Box::new]\(x))</code>. Consider using
275 /// [`into_pin`](Box::into_pin) if you already have a `Box<T>`, or if you want to
276 /// construct a (pinned) `Box` in a different way than with [`Box::new`].
277 #[cfg(not(no_global_oom_handling))]
278 #[stable(feature = "pin", since = "1.33.0")]
281 pub fn pin(x: T) -> Pin<Box<T>> {
287 /// Allocates memory on the heap then places `x` into it,
288 /// returning an error if the allocation fails
290 /// This doesn't actually allocate if `T` is zero-sized.
295 /// #![feature(allocator_api)]
297 /// let five = Box::try_new(5)?;
298 /// # Ok::<(), std::alloc::AllocError>(())
300 #[unstable(feature = "allocator_api", issue = "32838")]
302 pub fn try_new(x: T) -> Result<Self, AllocError> {
303 Self::try_new_in(x, Global)
306 /// Constructs a new box with uninitialized contents on the heap,
307 /// returning an error if the allocation fails
312 /// #![feature(allocator_api, new_uninit)]
314 /// let mut five = Box::<u32>::try_new_uninit()?;
316 /// let five = unsafe {
317 /// // Deferred initialization:
318 /// five.as_mut_ptr().write(5);
320 /// five.assume_init()
323 /// assert_eq!(*five, 5);
324 /// # Ok::<(), std::alloc::AllocError>(())
326 #[unstable(feature = "allocator_api", issue = "32838")]
327 // #[unstable(feature = "new_uninit", issue = "63291")]
329 pub fn try_new_uninit() -> Result<Box<mem::MaybeUninit<T>>, AllocError> {
330 Box::try_new_uninit_in(Global)
333 /// Constructs a new `Box` with uninitialized contents, with the memory
334 /// being filled with `0` bytes on the heap
336 /// See [`MaybeUninit::zeroed`][zeroed] for examples of correct and incorrect usage
342 /// #![feature(allocator_api, new_uninit)]
344 /// let zero = Box::<u32>::try_new_zeroed()?;
345 /// let zero = unsafe { zero.assume_init() };
347 /// assert_eq!(*zero, 0);
348 /// # Ok::<(), std::alloc::AllocError>(())
351 /// [zeroed]: mem::MaybeUninit::zeroed
352 #[unstable(feature = "allocator_api", issue = "32838")]
353 // #[unstable(feature = "new_uninit", issue = "63291")]
355 pub fn try_new_zeroed() -> Result<Box<mem::MaybeUninit<T>>, AllocError> {
356 Box::try_new_zeroed_in(Global)
360 impl<T, A: Allocator> Box<T, A> {
361 /// Allocates memory in the given allocator then places `x` into it.
363 /// This doesn't actually allocate if `T` is zero-sized.
368 /// #![feature(allocator_api)]
370 /// use std::alloc::System;
372 /// let five = Box::new_in(5, System);
374 #[cfg(not(no_global_oom_handling))]
375 #[unstable(feature = "allocator_api", issue = "32838")]
376 #[rustc_const_unstable(feature = "const_box", issue = "92521")]
379 pub const fn new_in(x: T, alloc: A) -> Self
381 A: ~const Allocator + ~const Destruct,
383 let mut boxed = Self::new_uninit_in(alloc);
385 boxed.as_mut_ptr().write(x);
390 /// Allocates memory in the given allocator then places `x` into it,
391 /// returning an error if the allocation fails
393 /// This doesn't actually allocate if `T` is zero-sized.
398 /// #![feature(allocator_api)]
400 /// use std::alloc::System;
402 /// let five = Box::try_new_in(5, System)?;
403 /// # Ok::<(), std::alloc::AllocError>(())
405 #[unstable(feature = "allocator_api", issue = "32838")]
406 #[rustc_const_unstable(feature = "const_box", issue = "92521")]
408 pub const fn try_new_in(x: T, alloc: A) -> Result<Self, AllocError>
411 A: ~const Allocator + ~const Destruct,
413 let mut boxed = Self::try_new_uninit_in(alloc)?;
415 boxed.as_mut_ptr().write(x);
416 Ok(boxed.assume_init())
420 /// Constructs a new box with uninitialized contents in the provided allocator.
425 /// #![feature(allocator_api, new_uninit)]
427 /// use std::alloc::System;
429 /// let mut five = Box::<u32, _>::new_uninit_in(System);
431 /// let five = unsafe {
432 /// // Deferred initialization:
433 /// five.as_mut_ptr().write(5);
435 /// five.assume_init()
438 /// assert_eq!(*five, 5)
440 #[unstable(feature = "allocator_api", issue = "32838")]
441 #[rustc_const_unstable(feature = "const_box", issue = "92521")]
442 #[cfg(not(no_global_oom_handling))]
444 // #[unstable(feature = "new_uninit", issue = "63291")]
445 pub const fn new_uninit_in(alloc: A) -> Box<mem::MaybeUninit<T>, A>
447 A: ~const Allocator + ~const Destruct,
449 let layout = Layout::new::<mem::MaybeUninit<T>>();
450 // NOTE: Prefer match over unwrap_or_else since closure sometimes not inlineable.
451 // That would make code size bigger.
452 match Box::try_new_uninit_in(alloc) {
454 Err(_) => handle_alloc_error(layout),
458 /// Constructs a new box with uninitialized contents in the provided allocator,
459 /// returning an error if the allocation fails
464 /// #![feature(allocator_api, new_uninit)]
466 /// use std::alloc::System;
468 /// let mut five = Box::<u32, _>::try_new_uninit_in(System)?;
470 /// let five = unsafe {
471 /// // Deferred initialization:
472 /// five.as_mut_ptr().write(5);
474 /// five.assume_init()
477 /// assert_eq!(*five, 5);
478 /// # Ok::<(), std::alloc::AllocError>(())
480 #[unstable(feature = "allocator_api", issue = "32838")]
481 // #[unstable(feature = "new_uninit", issue = "63291")]
482 #[rustc_const_unstable(feature = "const_box", issue = "92521")]
483 pub const fn try_new_uninit_in(alloc: A) -> Result<Box<mem::MaybeUninit<T>, A>, AllocError>
485 A: ~const Allocator + ~const Destruct,
487 let layout = Layout::new::<mem::MaybeUninit<T>>();
488 let ptr = alloc.allocate(layout)?.cast();
489 unsafe { Ok(Box::from_raw_in(ptr.as_ptr(), alloc)) }
492 /// Constructs a new `Box` with uninitialized contents, with the memory
493 /// being filled with `0` bytes in the provided allocator.
495 /// See [`MaybeUninit::zeroed`][zeroed] for examples of correct and incorrect usage
501 /// #![feature(allocator_api, new_uninit)]
503 /// use std::alloc::System;
505 /// let zero = Box::<u32, _>::new_zeroed_in(System);
506 /// let zero = unsafe { zero.assume_init() };
508 /// assert_eq!(*zero, 0)
511 /// [zeroed]: mem::MaybeUninit::zeroed
512 #[unstable(feature = "allocator_api", issue = "32838")]
513 #[rustc_const_unstable(feature = "const_box", issue = "92521")]
514 #[cfg(not(no_global_oom_handling))]
515 // #[unstable(feature = "new_uninit", issue = "63291")]
517 pub const fn new_zeroed_in(alloc: A) -> Box<mem::MaybeUninit<T>, A>
519 A: ~const Allocator + ~const Destruct,
521 let layout = Layout::new::<mem::MaybeUninit<T>>();
522 // NOTE: Prefer match over unwrap_or_else since closure sometimes not inlineable.
523 // That would make code size bigger.
524 match Box::try_new_zeroed_in(alloc) {
526 Err(_) => handle_alloc_error(layout),
530 /// Constructs a new `Box` with uninitialized contents, with the memory
531 /// being filled with `0` bytes in the provided allocator,
532 /// returning an error if the allocation fails,
534 /// See [`MaybeUninit::zeroed`][zeroed] for examples of correct and incorrect usage
540 /// #![feature(allocator_api, new_uninit)]
542 /// use std::alloc::System;
544 /// let zero = Box::<u32, _>::try_new_zeroed_in(System)?;
545 /// let zero = unsafe { zero.assume_init() };
547 /// assert_eq!(*zero, 0);
548 /// # Ok::<(), std::alloc::AllocError>(())
551 /// [zeroed]: mem::MaybeUninit::zeroed
552 #[unstable(feature = "allocator_api", issue = "32838")]
553 // #[unstable(feature = "new_uninit", issue = "63291")]
554 #[rustc_const_unstable(feature = "const_box", issue = "92521")]
555 pub const fn try_new_zeroed_in(alloc: A) -> Result<Box<mem::MaybeUninit<T>, A>, AllocError>
557 A: ~const Allocator + ~const Destruct,
559 let layout = Layout::new::<mem::MaybeUninit<T>>();
560 let ptr = alloc.allocate_zeroed(layout)?.cast();
561 unsafe { Ok(Box::from_raw_in(ptr.as_ptr(), alloc)) }
564 /// Constructs a new `Pin<Box<T, A>>`. If `T` does not implement [`Unpin`], then
565 /// `x` will be pinned in memory and unable to be moved.
567 /// Constructing and pinning of the `Box` can also be done in two steps: `Box::pin_in(x, alloc)`
568 /// does the same as <code>[Box::into_pin]\([Box::new_in]\(x, alloc))</code>. Consider using
569 /// [`into_pin`](Box::into_pin) if you already have a `Box<T, A>`, or if you want to
570 /// construct a (pinned) `Box` in a different way than with [`Box::new_in`].
571 #[cfg(not(no_global_oom_handling))]
572 #[unstable(feature = "allocator_api", issue = "32838")]
573 #[rustc_const_unstable(feature = "const_box", issue = "92521")]
576 pub const fn pin_in(x: T, alloc: A) -> Pin<Self>
578 A: 'static + ~const Allocator + ~const Destruct,
580 Self::into_pin(Self::new_in(x, alloc))
583 /// Converts a `Box<T>` into a `Box<[T]>`
585 /// This conversion does not allocate on the heap and happens in place.
586 #[unstable(feature = "box_into_boxed_slice", issue = "71582")]
587 #[rustc_const_unstable(feature = "const_box", issue = "92521")]
588 pub const fn into_boxed_slice(boxed: Self) -> Box<[T], A> {
589 let (raw, alloc) = Box::into_raw_with_allocator(boxed);
590 unsafe { Box::from_raw_in(raw as *mut [T; 1], alloc) }
593 /// Consumes the `Box`, returning the wrapped value.
598 /// #![feature(box_into_inner)]
600 /// let c = Box::new(5);
602 /// assert_eq!(Box::into_inner(c), 5);
604 #[unstable(feature = "box_into_inner", issue = "80437")]
605 #[rustc_const_unstable(feature = "const_box", issue = "92521")]
607 pub const fn into_inner(boxed: Self) -> T
609 Self: ~const Destruct,
616 /// Constructs a new boxed slice with uninitialized contents.
621 /// #![feature(new_uninit)]
623 /// let mut values = Box::<[u32]>::new_uninit_slice(3);
625 /// let values = unsafe {
626 /// // Deferred initialization:
627 /// values[0].as_mut_ptr().write(1);
628 /// values[1].as_mut_ptr().write(2);
629 /// values[2].as_mut_ptr().write(3);
631 /// values.assume_init()
634 /// assert_eq!(*values, [1, 2, 3])
636 #[cfg(not(no_global_oom_handling))]
637 #[unstable(feature = "new_uninit", issue = "63291")]
639 pub fn new_uninit_slice(len: usize) -> Box<[mem::MaybeUninit<T>]> {
640 unsafe { RawVec::with_capacity(len).into_box(len) }
643 /// Constructs a new boxed slice with uninitialized contents, with the memory
644 /// being filled with `0` bytes.
646 /// See [`MaybeUninit::zeroed`][zeroed] for examples of correct and incorrect usage
652 /// #![feature(new_uninit)]
654 /// let values = Box::<[u32]>::new_zeroed_slice(3);
655 /// let values = unsafe { values.assume_init() };
657 /// assert_eq!(*values, [0, 0, 0])
660 /// [zeroed]: mem::MaybeUninit::zeroed
661 #[cfg(not(no_global_oom_handling))]
662 #[unstable(feature = "new_uninit", issue = "63291")]
664 pub fn new_zeroed_slice(len: usize) -> Box<[mem::MaybeUninit<T>]> {
665 unsafe { RawVec::with_capacity_zeroed(len).into_box(len) }
668 /// Constructs a new boxed slice with uninitialized contents. Returns an error if
669 /// the allocation fails
674 /// #![feature(allocator_api, new_uninit)]
676 /// let mut values = Box::<[u32]>::try_new_uninit_slice(3)?;
677 /// let values = unsafe {
678 /// // Deferred initialization:
679 /// values[0].as_mut_ptr().write(1);
680 /// values[1].as_mut_ptr().write(2);
681 /// values[2].as_mut_ptr().write(3);
682 /// values.assume_init()
685 /// assert_eq!(*values, [1, 2, 3]);
686 /// # Ok::<(), std::alloc::AllocError>(())
688 #[unstable(feature = "allocator_api", issue = "32838")]
690 pub fn try_new_uninit_slice(len: usize) -> Result<Box<[mem::MaybeUninit<T>]>, AllocError> {
692 let layout = match Layout::array::<mem::MaybeUninit<T>>(len) {
694 Err(_) => return Err(AllocError),
696 let ptr = Global.allocate(layout)?;
697 Ok(RawVec::from_raw_parts_in(ptr.as_mut_ptr() as *mut _, len, Global).into_box(len))
701 /// Constructs a new boxed slice with uninitialized contents, with the memory
702 /// being filled with `0` bytes. Returns an error if the allocation fails
704 /// See [`MaybeUninit::zeroed`][zeroed] for examples of correct and incorrect usage
710 /// #![feature(allocator_api, new_uninit)]
712 /// let values = Box::<[u32]>::try_new_zeroed_slice(3)?;
713 /// let values = unsafe { values.assume_init() };
715 /// assert_eq!(*values, [0, 0, 0]);
716 /// # Ok::<(), std::alloc::AllocError>(())
719 /// [zeroed]: mem::MaybeUninit::zeroed
720 #[unstable(feature = "allocator_api", issue = "32838")]
722 pub fn try_new_zeroed_slice(len: usize) -> Result<Box<[mem::MaybeUninit<T>]>, AllocError> {
724 let layout = match Layout::array::<mem::MaybeUninit<T>>(len) {
726 Err(_) => return Err(AllocError),
728 let ptr = Global.allocate_zeroed(layout)?;
729 Ok(RawVec::from_raw_parts_in(ptr.as_mut_ptr() as *mut _, len, Global).into_box(len))
734 impl<T, A: Allocator> Box<[T], A> {
735 /// Constructs a new boxed slice with uninitialized contents in the provided allocator.
740 /// #![feature(allocator_api, new_uninit)]
742 /// use std::alloc::System;
744 /// let mut values = Box::<[u32], _>::new_uninit_slice_in(3, System);
746 /// let values = unsafe {
747 /// // Deferred initialization:
748 /// values[0].as_mut_ptr().write(1);
749 /// values[1].as_mut_ptr().write(2);
750 /// values[2].as_mut_ptr().write(3);
752 /// values.assume_init()
755 /// assert_eq!(*values, [1, 2, 3])
757 #[cfg(not(no_global_oom_handling))]
758 #[unstable(feature = "allocator_api", issue = "32838")]
759 // #[unstable(feature = "new_uninit", issue = "63291")]
761 pub fn new_uninit_slice_in(len: usize, alloc: A) -> Box<[mem::MaybeUninit<T>], A> {
762 unsafe { RawVec::with_capacity_in(len, alloc).into_box(len) }
765 /// Constructs a new boxed slice with uninitialized contents in the provided allocator,
766 /// with the memory being filled with `0` bytes.
768 /// See [`MaybeUninit::zeroed`][zeroed] for examples of correct and incorrect usage
774 /// #![feature(allocator_api, new_uninit)]
776 /// use std::alloc::System;
778 /// let values = Box::<[u32], _>::new_zeroed_slice_in(3, System);
779 /// let values = unsafe { values.assume_init() };
781 /// assert_eq!(*values, [0, 0, 0])
784 /// [zeroed]: mem::MaybeUninit::zeroed
785 #[cfg(not(no_global_oom_handling))]
786 #[unstable(feature = "allocator_api", issue = "32838")]
787 // #[unstable(feature = "new_uninit", issue = "63291")]
789 pub fn new_zeroed_slice_in(len: usize, alloc: A) -> Box<[mem::MaybeUninit<T>], A> {
790 unsafe { RawVec::with_capacity_zeroed_in(len, alloc).into_box(len) }
794 impl<T, A: Allocator> Box<mem::MaybeUninit<T>, A> {
795 /// Converts to `Box<T, A>`.
799 /// As with [`MaybeUninit::assume_init`],
800 /// it is up to the caller to guarantee that the value
801 /// really is in an initialized state.
802 /// Calling this when the content is not yet fully initialized
803 /// causes immediate undefined behavior.
805 /// [`MaybeUninit::assume_init`]: mem::MaybeUninit::assume_init
810 /// #![feature(new_uninit)]
812 /// let mut five = Box::<u32>::new_uninit();
814 /// let five: Box<u32> = unsafe {
815 /// // Deferred initialization:
816 /// five.as_mut_ptr().write(5);
818 /// five.assume_init()
821 /// assert_eq!(*five, 5)
823 #[unstable(feature = "new_uninit", issue = "63291")]
824 #[rustc_const_unstable(feature = "const_box", issue = "92521")]
826 pub const unsafe fn assume_init(self) -> Box<T, A> {
827 let (raw, alloc) = Box::into_raw_with_allocator(self);
828 unsafe { Box::from_raw_in(raw as *mut T, alloc) }
831 /// Writes the value and converts to `Box<T, A>`.
833 /// This method converts the box similarly to [`Box::assume_init`] but
834 /// writes `value` into it before conversion thus guaranteeing safety.
835 /// In some scenarios use of this method may improve performance because
836 /// the compiler may be able to optimize copying from stack.
841 /// #![feature(new_uninit)]
843 /// let big_box = Box::<[usize; 1024]>::new_uninit();
845 /// let mut array = [0; 1024];
846 /// for (i, place) in array.iter_mut().enumerate() {
850 /// // The optimizer may be able to elide this copy, so previous code writes
851 /// // to heap directly.
852 /// let big_box = Box::write(big_box, array);
854 /// for (i, x) in big_box.iter().enumerate() {
855 /// assert_eq!(*x, i);
858 #[unstable(feature = "new_uninit", issue = "63291")]
859 #[rustc_const_unstable(feature = "const_box", issue = "92521")]
861 pub const fn write(mut boxed: Self, value: T) -> Box<T, A> {
863 (*boxed).write(value);
869 impl<T, A: Allocator> Box<[mem::MaybeUninit<T>], A> {
870 /// Converts to `Box<[T], A>`.
874 /// As with [`MaybeUninit::assume_init`],
875 /// it is up to the caller to guarantee that the values
876 /// really are in an initialized state.
877 /// Calling this when the content is not yet fully initialized
878 /// causes immediate undefined behavior.
880 /// [`MaybeUninit::assume_init`]: mem::MaybeUninit::assume_init
885 /// #![feature(new_uninit)]
887 /// let mut values = Box::<[u32]>::new_uninit_slice(3);
889 /// let values = unsafe {
890 /// // Deferred initialization:
891 /// values[0].as_mut_ptr().write(1);
892 /// values[1].as_mut_ptr().write(2);
893 /// values[2].as_mut_ptr().write(3);
895 /// values.assume_init()
898 /// assert_eq!(*values, [1, 2, 3])
900 #[unstable(feature = "new_uninit", issue = "63291")]
902 pub unsafe fn assume_init(self) -> Box<[T], A> {
903 let (raw, alloc) = Box::into_raw_with_allocator(self);
904 unsafe { Box::from_raw_in(raw as *mut [T], alloc) }
908 impl<T: ?Sized> Box<T> {
909 /// Constructs a box from a raw pointer.
911 /// After calling this function, the raw pointer is owned by the
912 /// resulting `Box`. Specifically, the `Box` destructor will call
913 /// the destructor of `T` and free the allocated memory. For this
914 /// to be safe, the memory must have been allocated in accordance
915 /// with the [memory layout] used by `Box` .
919 /// This function is unsafe because improper use may lead to
920 /// memory problems. For example, a double-free may occur if the
921 /// function is called twice on the same raw pointer.
923 /// The safety conditions are described in the [memory layout] section.
927 /// Recreate a `Box` which was previously converted to a raw pointer
928 /// using [`Box::into_raw`]:
930 /// let x = Box::new(5);
931 /// let ptr = Box::into_raw(x);
932 /// let x = unsafe { Box::from_raw(ptr) };
934 /// Manually create a `Box` from scratch by using the global allocator:
936 /// use std::alloc::{alloc, Layout};
939 /// let ptr = alloc(Layout::new::<i32>()) as *mut i32;
940 /// // In general .write is required to avoid attempting to destruct
941 /// // the (uninitialized) previous contents of `ptr`, though for this
942 /// // simple example `*ptr = 5` would have worked as well.
944 /// let x = Box::from_raw(ptr);
948 /// [memory layout]: self#memory-layout
949 /// [`Layout`]: crate::Layout
950 #[stable(feature = "box_raw", since = "1.4.0")]
952 #[must_use = "call `drop(from_raw(ptr))` if you intend to drop the `Box`"]
953 pub unsafe fn from_raw(raw: *mut T) -> Self {
954 unsafe { Self::from_raw_in(raw, Global) }
958 impl<T: ?Sized, A: Allocator> Box<T, A> {
959 /// Constructs a box from a raw pointer in the given allocator.
961 /// After calling this function, the raw pointer is owned by the
962 /// resulting `Box`. Specifically, the `Box` destructor will call
963 /// the destructor of `T` and free the allocated memory. For this
964 /// to be safe, the memory must have been allocated in accordance
965 /// with the [memory layout] used by `Box` .
969 /// This function is unsafe because improper use may lead to
970 /// memory problems. For example, a double-free may occur if the
971 /// function is called twice on the same raw pointer.
976 /// Recreate a `Box` which was previously converted to a raw pointer
977 /// using [`Box::into_raw_with_allocator`]:
979 /// #![feature(allocator_api)]
981 /// use std::alloc::System;
983 /// let x = Box::new_in(5, System);
984 /// let (ptr, alloc) = Box::into_raw_with_allocator(x);
985 /// let x = unsafe { Box::from_raw_in(ptr, alloc) };
987 /// Manually create a `Box` from scratch by using the system allocator:
989 /// #![feature(allocator_api, slice_ptr_get)]
991 /// use std::alloc::{Allocator, Layout, System};
994 /// let ptr = System.allocate(Layout::new::<i32>())?.as_mut_ptr() as *mut i32;
995 /// // In general .write is required to avoid attempting to destruct
996 /// // the (uninitialized) previous contents of `ptr`, though for this
997 /// // simple example `*ptr = 5` would have worked as well.
999 /// let x = Box::from_raw_in(ptr, System);
1001 /// # Ok::<(), std::alloc::AllocError>(())
1004 /// [memory layout]: self#memory-layout
1005 /// [`Layout`]: crate::Layout
1006 #[unstable(feature = "allocator_api", issue = "32838")]
1007 #[rustc_const_unstable(feature = "const_box", issue = "92521")]
1009 pub const unsafe fn from_raw_in(raw: *mut T, alloc: A) -> Self {
1010 Box(unsafe { Unique::new_unchecked(raw) }, alloc)
1013 /// Consumes the `Box`, returning a wrapped raw pointer.
1015 /// The pointer will be properly aligned and non-null.
1017 /// After calling this function, the caller is responsible for the
1018 /// memory previously managed by the `Box`. In particular, the
1019 /// caller should properly destroy `T` and release the memory, taking
1020 /// into account the [memory layout] used by `Box`. The easiest way to
1021 /// do this is to convert the raw pointer back into a `Box` with the
1022 /// [`Box::from_raw`] function, allowing the `Box` destructor to perform
1025 /// Note: this is an associated function, which means that you have
1026 /// to call it as `Box::into_raw(b)` instead of `b.into_raw()`. This
1027 /// is so that there is no conflict with a method on the inner type.
1030 /// Converting the raw pointer back into a `Box` with [`Box::from_raw`]
1031 /// for automatic cleanup:
1033 /// let x = Box::new(String::from("Hello"));
1034 /// let ptr = Box::into_raw(x);
1035 /// let x = unsafe { Box::from_raw(ptr) };
1037 /// Manual cleanup by explicitly running the destructor and deallocating
1040 /// use std::alloc::{dealloc, Layout};
1043 /// let x = Box::new(String::from("Hello"));
1044 /// let p = Box::into_raw(x);
1046 /// ptr::drop_in_place(p);
1047 /// dealloc(p as *mut u8, Layout::new::<String>());
1051 /// [memory layout]: self#memory-layout
1052 #[stable(feature = "box_raw", since = "1.4.0")]
1054 pub fn into_raw(b: Self) -> *mut T {
1055 Self::into_raw_with_allocator(b).0
1058 /// Consumes the `Box`, returning a wrapped raw pointer and the allocator.
1060 /// The pointer will be properly aligned and non-null.
1062 /// After calling this function, the caller is responsible for the
1063 /// memory previously managed by the `Box`. In particular, the
1064 /// caller should properly destroy `T` and release the memory, taking
1065 /// into account the [memory layout] used by `Box`. The easiest way to
1066 /// do this is to convert the raw pointer back into a `Box` with the
1067 /// [`Box::from_raw_in`] function, allowing the `Box` destructor to perform
1070 /// Note: this is an associated function, which means that you have
1071 /// to call it as `Box::into_raw_with_allocator(b)` instead of `b.into_raw_with_allocator()`. This
1072 /// is so that there is no conflict with a method on the inner type.
1075 /// Converting the raw pointer back into a `Box` with [`Box::from_raw_in`]
1076 /// for automatic cleanup:
1078 /// #![feature(allocator_api)]
1080 /// use std::alloc::System;
1082 /// let x = Box::new_in(String::from("Hello"), System);
1083 /// let (ptr, alloc) = Box::into_raw_with_allocator(x);
1084 /// let x = unsafe { Box::from_raw_in(ptr, alloc) };
1086 /// Manual cleanup by explicitly running the destructor and deallocating
1089 /// #![feature(allocator_api)]
1091 /// use std::alloc::{Allocator, Layout, System};
1092 /// use std::ptr::{self, NonNull};
1094 /// let x = Box::new_in(String::from("Hello"), System);
1095 /// let (ptr, alloc) = Box::into_raw_with_allocator(x);
1097 /// ptr::drop_in_place(ptr);
1098 /// let non_null = NonNull::new_unchecked(ptr);
1099 /// alloc.deallocate(non_null.cast(), Layout::new::<String>());
1103 /// [memory layout]: self#memory-layout
1104 #[unstable(feature = "allocator_api", issue = "32838")]
1105 #[rustc_const_unstable(feature = "const_box", issue = "92521")]
1107 pub const fn into_raw_with_allocator(b: Self) -> (*mut T, A) {
1108 let (leaked, alloc) = Box::into_unique(b);
1109 (leaked.as_ptr(), alloc)
1113 feature = "ptr_internals",
1115 reason = "use `Box::leak(b).into()` or `Unique::from(Box::leak(b))` instead"
1117 #[rustc_const_unstable(feature = "const_box", issue = "92521")]
1120 pub const fn into_unique(b: Self) -> (Unique<T>, A) {
1121 // Box is recognized as a "unique pointer" by Stacked Borrows, but internally it is a
1122 // raw pointer for the type system. Turning it directly into a raw pointer would not be
1123 // recognized as "releasing" the unique pointer to permit aliased raw accesses,
1124 // so all raw pointer methods have to go through `Box::leak`. Turning *that* to a raw pointer
1125 // behaves correctly.
1126 let alloc = unsafe { ptr::read(&b.1) };
1127 (Unique::from(Box::leak(b)), alloc)
1130 /// Returns a reference to the underlying allocator.
1132 /// Note: this is an associated function, which means that you have
1133 /// to call it as `Box::allocator(&b)` instead of `b.allocator()`. This
1134 /// is so that there is no conflict with a method on the inner type.
1135 #[unstable(feature = "allocator_api", issue = "32838")]
1136 #[rustc_const_unstable(feature = "const_box", issue = "92521")]
1138 pub const fn allocator(b: &Self) -> &A {
1142 /// Consumes and leaks the `Box`, returning a mutable reference,
1143 /// `&'a mut T`. Note that the type `T` must outlive the chosen lifetime
1144 /// `'a`. If the type has only static references, or none at all, then this
1145 /// may be chosen to be `'static`.
1147 /// This function is mainly useful for data that lives for the remainder of
1148 /// the program's life. Dropping the returned reference will cause a memory
1149 /// leak. If this is not acceptable, the reference should first be wrapped
1150 /// with the [`Box::from_raw`] function producing a `Box`. This `Box` can
1151 /// then be dropped which will properly destroy `T` and release the
1152 /// allocated memory.
1154 /// Note: this is an associated function, which means that you have
1155 /// to call it as `Box::leak(b)` instead of `b.leak()`. This
1156 /// is so that there is no conflict with a method on the inner type.
1163 /// let x = Box::new(41);
1164 /// let static_ref: &'static mut usize = Box::leak(x);
1165 /// *static_ref += 1;
1166 /// assert_eq!(*static_ref, 42);
1172 /// let x = vec![1, 2, 3].into_boxed_slice();
1173 /// let static_ref = Box::leak(x);
1174 /// static_ref[0] = 4;
1175 /// assert_eq!(*static_ref, [4, 2, 3]);
1177 #[stable(feature = "box_leak", since = "1.26.0")]
1178 #[rustc_const_unstable(feature = "const_box", issue = "92521")]
1180 pub const fn leak<'a>(b: Self) -> &'a mut T
1184 unsafe { &mut *mem::ManuallyDrop::new(b).0.as_ptr() }
1187 /// Converts a `Box<T>` into a `Pin<Box<T>>`. If `T` does not implement [`Unpin`], then
1188 /// `*boxed` will be pinned in memory and unable to be moved.
1190 /// This conversion does not allocate on the heap and happens in place.
1192 /// This is also available via [`From`].
1194 /// Constructing and pinning a `Box` with <code>Box::into_pin([Box::new]\(x))</code>
1195 /// can also be written more concisely using <code>[Box::pin]\(x)</code>.
1196 /// This `into_pin` method is useful if you already have a `Box<T>`, or you are
1197 /// constructing a (pinned) `Box` in a different way than with [`Box::new`].
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 {
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>>`. If `T` does not implement [`Unpin`], then
1462 /// `*boxed` will be pinned in memory and unable to be moved.
1464 /// This conversion does not allocate on the heap and happens in place.
1466 /// This is also available via [`Box::into_pin`].
1468 /// Constructing and pinning a `Box` with <code><Pin<Box\<T>>>::from([Box::new]\(x))</code>
1469 /// can also be written more concisely using <code>[Box::pin]\(x)</code>.
1470 /// This `From` implementation is useful if you already have a `Box<T>`, or you are
1471 /// constructing a (pinned) `Box` in a different way than with [`Box::new`].
1472 fn from(boxed: Box<T, A>) -> Self {
1473 Box::into_pin(boxed)
1477 #[cfg(not(no_global_oom_handling))]
1478 #[stable(feature = "box_from_slice", since = "1.17.0")]
1479 impl<T: Copy> From<&[T]> for Box<[T]> {
1480 /// Converts a `&[T]` into a `Box<[T]>`
1482 /// This conversion allocates on the heap
1483 /// and performs a copy of `slice`.
1487 /// // create a &[u8] which will be used to create a Box<[u8]>
1488 /// let slice: &[u8] = &[104, 101, 108, 108, 111];
1489 /// let boxed_slice: Box<[u8]> = Box::from(slice);
1491 /// println!("{boxed_slice:?}");
1493 fn from(slice: &[T]) -> Box<[T]> {
1494 let len = slice.len();
1495 let buf = RawVec::with_capacity(len);
1497 ptr::copy_nonoverlapping(slice.as_ptr(), buf.ptr(), len);
1498 buf.into_box(slice.len()).assume_init()
1503 #[cfg(not(no_global_oom_handling))]
1504 #[stable(feature = "box_from_cow", since = "1.45.0")]
1505 impl<T: Copy> From<Cow<'_, [T]>> for Box<[T]> {
1506 /// Converts a `Cow<'_, [T]>` into a `Box<[T]>`
1508 /// When `cow` is the `Cow::Borrowed` variant, this
1509 /// conversion allocates on the heap and copies the
1510 /// underlying slice. Otherwise, it will try to reuse the owned
1511 /// `Vec`'s allocation.
1513 fn from(cow: Cow<'_, [T]>) -> Box<[T]> {
1515 Cow::Borrowed(slice) => Box::from(slice),
1516 Cow::Owned(slice) => Box::from(slice),
1521 #[cfg(not(no_global_oom_handling))]
1522 #[stable(feature = "box_from_slice", since = "1.17.0")]
1523 impl From<&str> for Box<str> {
1524 /// Converts a `&str` into a `Box<str>`
1526 /// This conversion allocates on the heap
1527 /// and performs a copy of `s`.
1532 /// let boxed: Box<str> = Box::from("hello");
1533 /// println!("{boxed}");
1536 fn from(s: &str) -> Box<str> {
1537 unsafe { from_boxed_utf8_unchecked(Box::from(s.as_bytes())) }
1541 #[cfg(not(no_global_oom_handling))]
1542 #[stable(feature = "box_from_cow", since = "1.45.0")]
1543 impl From<Cow<'_, str>> for Box<str> {
1544 /// Converts a `Cow<'_, str>` into a `Box<str>`
1546 /// When `cow` is the `Cow::Borrowed` variant, this
1547 /// conversion allocates on the heap and copies the
1548 /// underlying `str`. Otherwise, it will try to reuse the owned
1549 /// `String`'s allocation.
1554 /// use std::borrow::Cow;
1556 /// let unboxed = Cow::Borrowed("hello");
1557 /// let boxed: Box<str> = Box::from(unboxed);
1558 /// println!("{boxed}");
1562 /// # use std::borrow::Cow;
1563 /// let unboxed = Cow::Owned("hello".to_string());
1564 /// let boxed: Box<str> = Box::from(unboxed);
1565 /// println!("{boxed}");
1568 fn from(cow: Cow<'_, str>) -> Box<str> {
1570 Cow::Borrowed(s) => Box::from(s),
1571 Cow::Owned(s) => Box::from(s),
1576 #[stable(feature = "boxed_str_conv", since = "1.19.0")]
1577 impl<A: Allocator> From<Box<str, A>> for Box<[u8], A> {
1578 /// Converts a `Box<str>` into a `Box<[u8]>`
1580 /// This conversion does not allocate on the heap and happens in place.
1584 /// // create a Box<str> which will be used to create a Box<[u8]>
1585 /// let boxed: Box<str> = Box::from("hello");
1586 /// let boxed_str: Box<[u8]> = Box::from(boxed);
1588 /// // create a &[u8] which will be used to create a Box<[u8]>
1589 /// let slice: &[u8] = &[104, 101, 108, 108, 111];
1590 /// let boxed_slice = Box::from(slice);
1592 /// assert_eq!(boxed_slice, boxed_str);
1595 fn from(s: Box<str, A>) -> Self {
1596 let (raw, alloc) = Box::into_raw_with_allocator(s);
1597 unsafe { Box::from_raw_in(raw as *mut [u8], alloc) }
1601 #[cfg(not(no_global_oom_handling))]
1602 #[stable(feature = "box_from_array", since = "1.45.0")]
1603 impl<T, const N: usize> From<[T; N]> for Box<[T]> {
1604 /// Converts a `[T; N]` into a `Box<[T]>`
1606 /// This conversion moves the array to newly heap-allocated memory.
1611 /// let boxed: Box<[u8]> = Box::from([4, 2]);
1612 /// println!("{boxed:?}");
1614 fn from(array: [T; N]) -> Box<[T]> {
1620 #[stable(feature = "boxed_slice_try_from", since = "1.43.0")]
1621 impl<T, const N: usize> TryFrom<Box<[T]>> for Box<[T; N]> {
1622 type Error = Box<[T]>;
1624 /// Attempts to convert a `Box<[T]>` into a `Box<[T; N]>`.
1626 /// The conversion occurs in-place and does not require a
1627 /// new memory allocation.
1631 /// Returns the old `Box<[T]>` in the `Err` variant if
1632 /// `boxed_slice.len()` does not equal `N`.
1633 fn try_from(boxed_slice: Box<[T]>) -> Result<Self, Self::Error> {
1634 if boxed_slice.len() == N {
1635 Ok(unsafe { Box::from_raw(Box::into_raw(boxed_slice) as *mut [T; N]) })
1642 impl<A: Allocator> Box<dyn Any, A> {
1643 /// Attempt to downcast the box to a concrete type.
1648 /// use std::any::Any;
1650 /// fn print_if_string(value: Box<dyn Any>) {
1651 /// if let Ok(string) = value.downcast::<String>() {
1652 /// println!("String ({}): {}", string.len(), string);
1656 /// let my_string = "Hello World".to_string();
1657 /// print_if_string(Box::new(my_string));
1658 /// print_if_string(Box::new(0i8));
1661 #[stable(feature = "rust1", since = "1.0.0")]
1662 pub fn downcast<T: Any>(self) -> Result<Box<T, A>, Self> {
1663 if self.is::<T>() { unsafe { Ok(self.downcast_unchecked::<T>()) } } else { Err(self) }
1666 /// Downcasts the box to a concrete type.
1668 /// For a safe alternative see [`downcast`].
1673 /// #![feature(downcast_unchecked)]
1675 /// use std::any::Any;
1677 /// let x: Box<dyn Any> = Box::new(1_usize);
1680 /// assert_eq!(*x.downcast_unchecked::<usize>(), 1);
1686 /// The contained value must be of type `T`. Calling this method
1687 /// with the incorrect type is *undefined behavior*.
1689 /// [`downcast`]: Self::downcast
1691 #[unstable(feature = "downcast_unchecked", issue = "90850")]
1692 pub unsafe fn downcast_unchecked<T: Any>(self) -> Box<T, A> {
1693 debug_assert!(self.is::<T>());
1695 let (raw, alloc): (*mut dyn Any, _) = Box::into_raw_with_allocator(self);
1696 Box::from_raw_in(raw as *mut T, alloc)
1701 impl<A: Allocator> Box<dyn Any + Send, A> {
1702 /// Attempt to downcast the box to a concrete type.
1707 /// use std::any::Any;
1709 /// fn print_if_string(value: Box<dyn Any + Send>) {
1710 /// if let Ok(string) = value.downcast::<String>() {
1711 /// println!("String ({}): {}", string.len(), string);
1715 /// let my_string = "Hello World".to_string();
1716 /// print_if_string(Box::new(my_string));
1717 /// print_if_string(Box::new(0i8));
1720 #[stable(feature = "rust1", since = "1.0.0")]
1721 pub fn downcast<T: Any>(self) -> Result<Box<T, A>, Self> {
1722 if self.is::<T>() { unsafe { Ok(self.downcast_unchecked::<T>()) } } else { Err(self) }
1725 /// Downcasts the box to a concrete type.
1727 /// For a safe alternative see [`downcast`].
1732 /// #![feature(downcast_unchecked)]
1734 /// use std::any::Any;
1736 /// let x: Box<dyn Any + Send> = Box::new(1_usize);
1739 /// assert_eq!(*x.downcast_unchecked::<usize>(), 1);
1745 /// The contained value must be of type `T`. Calling this method
1746 /// with the incorrect type is *undefined behavior*.
1748 /// [`downcast`]: Self::downcast
1750 #[unstable(feature = "downcast_unchecked", issue = "90850")]
1751 pub unsafe fn downcast_unchecked<T: Any>(self) -> Box<T, A> {
1752 debug_assert!(self.is::<T>());
1754 let (raw, alloc): (*mut (dyn Any + Send), _) = Box::into_raw_with_allocator(self);
1755 Box::from_raw_in(raw as *mut T, alloc)
1760 impl<A: Allocator> Box<dyn Any + Send + Sync, A> {
1761 /// Attempt to downcast the box to a concrete type.
1766 /// use std::any::Any;
1768 /// fn print_if_string(value: Box<dyn Any + Send + Sync>) {
1769 /// if let Ok(string) = value.downcast::<String>() {
1770 /// println!("String ({}): {}", string.len(), string);
1774 /// let my_string = "Hello World".to_string();
1775 /// print_if_string(Box::new(my_string));
1776 /// print_if_string(Box::new(0i8));
1779 #[stable(feature = "box_send_sync_any_downcast", since = "1.51.0")]
1780 pub fn downcast<T: Any>(self) -> Result<Box<T, A>, Self> {
1781 if self.is::<T>() { unsafe { Ok(self.downcast_unchecked::<T>()) } } else { Err(self) }
1784 /// Downcasts the box to a concrete type.
1786 /// For a safe alternative see [`downcast`].
1791 /// #![feature(downcast_unchecked)]
1793 /// use std::any::Any;
1795 /// let x: Box<dyn Any + Send + Sync> = Box::new(1_usize);
1798 /// assert_eq!(*x.downcast_unchecked::<usize>(), 1);
1804 /// The contained value must be of type `T`. Calling this method
1805 /// with the incorrect type is *undefined behavior*.
1807 /// [`downcast`]: Self::downcast
1809 #[unstable(feature = "downcast_unchecked", issue = "90850")]
1810 pub unsafe fn downcast_unchecked<T: Any>(self) -> Box<T, A> {
1811 debug_assert!(self.is::<T>());
1813 let (raw, alloc): (*mut (dyn Any + Send + Sync), _) =
1814 Box::into_raw_with_allocator(self);
1815 Box::from_raw_in(raw as *mut T, alloc)
1820 #[stable(feature = "rust1", since = "1.0.0")]
1821 impl<T: fmt::Display + ?Sized, A: Allocator> fmt::Display for Box<T, A> {
1822 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
1823 fmt::Display::fmt(&**self, f)
1827 #[stable(feature = "rust1", since = "1.0.0")]
1828 impl<T: fmt::Debug + ?Sized, A: Allocator> fmt::Debug for Box<T, A> {
1829 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
1830 fmt::Debug::fmt(&**self, f)
1834 #[stable(feature = "rust1", since = "1.0.0")]
1835 impl<T: ?Sized, A: Allocator> fmt::Pointer for Box<T, A> {
1836 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
1837 // It's not possible to extract the inner Uniq directly from the Box,
1838 // instead we cast it to a *const which aliases the Unique
1839 let ptr: *const T = &**self;
1840 fmt::Pointer::fmt(&ptr, f)
1844 #[stable(feature = "rust1", since = "1.0.0")]
1845 #[rustc_const_unstable(feature = "const_box", issue = "92521")]
1846 impl<T: ?Sized, A: Allocator> const Deref for Box<T, A> {
1849 fn deref(&self) -> &T {
1854 #[stable(feature = "rust1", since = "1.0.0")]
1855 #[rustc_const_unstable(feature = "const_box", issue = "92521")]
1856 impl<T: ?Sized, A: Allocator> const DerefMut for Box<T, A> {
1857 fn deref_mut(&mut self) -> &mut T {
1862 #[unstable(feature = "receiver_trait", issue = "none")]
1863 impl<T: ?Sized, A: Allocator> Receiver for Box<T, A> {}
1865 #[stable(feature = "rust1", since = "1.0.0")]
1866 impl<I: Iterator + ?Sized, A: Allocator> Iterator for Box<I, A> {
1867 type Item = I::Item;
1868 fn next(&mut self) -> Option<I::Item> {
1871 fn size_hint(&self) -> (usize, Option<usize>) {
1872 (**self).size_hint()
1874 fn nth(&mut self, n: usize) -> Option<I::Item> {
1877 fn last(self) -> Option<I::Item> {
1884 fn last(self) -> Option<Self::Item>;
1887 impl<I: Iterator + ?Sized, A: Allocator> BoxIter for Box<I, A> {
1888 type Item = I::Item;
1889 default fn last(self) -> Option<I::Item> {
1891 fn some<T>(_: Option<T>, x: T) -> Option<T> {
1895 self.fold(None, some)
1899 /// Specialization for sized `I`s that uses `I`s implementation of `last()`
1900 /// instead of the default.
1901 #[stable(feature = "rust1", since = "1.0.0")]
1902 impl<I: Iterator, A: Allocator> BoxIter for Box<I, A> {
1903 fn last(self) -> Option<I::Item> {
1908 #[stable(feature = "rust1", since = "1.0.0")]
1909 impl<I: DoubleEndedIterator + ?Sized, A: Allocator> DoubleEndedIterator for Box<I, A> {
1910 fn next_back(&mut self) -> Option<I::Item> {
1911 (**self).next_back()
1913 fn nth_back(&mut self, n: usize) -> Option<I::Item> {
1914 (**self).nth_back(n)
1917 #[stable(feature = "rust1", since = "1.0.0")]
1918 impl<I: ExactSizeIterator + ?Sized, A: Allocator> ExactSizeIterator for Box<I, A> {
1919 fn len(&self) -> usize {
1922 fn is_empty(&self) -> bool {
1927 #[stable(feature = "fused", since = "1.26.0")]
1928 impl<I: FusedIterator + ?Sized, A: Allocator> FusedIterator for Box<I, A> {}
1930 #[stable(feature = "boxed_closure_impls", since = "1.35.0")]
1931 impl<Args, F: FnOnce<Args> + ?Sized, A: Allocator> FnOnce<Args> for Box<F, A> {
1932 type Output = <F as FnOnce<Args>>::Output;
1934 extern "rust-call" fn call_once(self, args: Args) -> Self::Output {
1935 <F as FnOnce<Args>>::call_once(*self, args)
1939 #[stable(feature = "boxed_closure_impls", since = "1.35.0")]
1940 impl<Args, F: FnMut<Args> + ?Sized, A: Allocator> FnMut<Args> for Box<F, A> {
1941 extern "rust-call" fn call_mut(&mut self, args: Args) -> Self::Output {
1942 <F as FnMut<Args>>::call_mut(self, args)
1946 #[stable(feature = "boxed_closure_impls", since = "1.35.0")]
1947 impl<Args, F: Fn<Args> + ?Sized, A: Allocator> Fn<Args> for Box<F, A> {
1948 extern "rust-call" fn call(&self, args: Args) -> Self::Output {
1949 <F as Fn<Args>>::call(self, args)
1953 #[unstable(feature = "coerce_unsized", issue = "27732")]
1954 impl<T: ?Sized + Unsize<U>, U: ?Sized, A: Allocator> CoerceUnsized<Box<U, A>> for Box<T, A> {}
1956 #[unstable(feature = "dispatch_from_dyn", issue = "none")]
1957 impl<T: ?Sized + Unsize<U>, U: ?Sized> DispatchFromDyn<Box<U>> for Box<T, Global> {}
1959 #[cfg(not(no_global_oom_handling))]
1960 #[stable(feature = "boxed_slice_from_iter", since = "1.32.0")]
1961 impl<I> FromIterator<I> for Box<[I]> {
1962 fn from_iter<T: IntoIterator<Item = I>>(iter: T) -> Self {
1963 iter.into_iter().collect::<Vec<_>>().into_boxed_slice()
1967 #[cfg(not(no_global_oom_handling))]
1968 #[stable(feature = "box_slice_clone", since = "1.3.0")]
1969 impl<T: Clone, A: Allocator + Clone> Clone for Box<[T], A> {
1970 fn clone(&self) -> Self {
1971 let alloc = Box::allocator(self).clone();
1972 self.to_vec_in(alloc).into_boxed_slice()
1975 fn clone_from(&mut self, other: &Self) {
1976 if self.len() == other.len() {
1977 self.clone_from_slice(&other);
1979 *self = other.clone();
1984 #[stable(feature = "box_borrow", since = "1.1.0")]
1985 impl<T: ?Sized, A: Allocator> borrow::Borrow<T> for Box<T, A> {
1986 fn borrow(&self) -> &T {
1991 #[stable(feature = "box_borrow", since = "1.1.0")]
1992 impl<T: ?Sized, A: Allocator> borrow::BorrowMut<T> for Box<T, A> {
1993 fn borrow_mut(&mut self) -> &mut T {
1998 #[stable(since = "1.5.0", feature = "smart_ptr_as_ref")]
1999 impl<T: ?Sized, A: Allocator> AsRef<T> for Box<T, A> {
2000 fn as_ref(&self) -> &T {
2005 #[stable(since = "1.5.0", feature = "smart_ptr_as_ref")]
2006 impl<T: ?Sized, A: Allocator> AsMut<T> for Box<T, A> {
2007 fn as_mut(&mut self) -> &mut T {
2014 * We could have chosen not to add this impl, and instead have written a
2015 * function of Pin<Box<T>> to Pin<T>. Such a function would not be sound,
2016 * because Box<T> implements Unpin even when T does not, as a result of
2019 * We chose this API instead of the alternative for a few reasons:
2020 * - Logically, it is helpful to understand pinning in regard to the
2021 * memory region being pointed to. For this reason none of the
2022 * standard library pointer types support projecting through a pin
2023 * (Box<T> is the only pointer type in std for which this would be
2025 * - It is in practice very useful to have Box<T> be unconditionally
2026 * Unpin because of trait objects, for which the structural auto
2027 * trait functionality does not apply (e.g., Box<dyn Foo> would
2028 * otherwise not be Unpin).
2030 * Another type with the same semantics as Box but only a conditional
2031 * implementation of `Unpin` (where `T: Unpin`) would be valid/safe, and
2032 * could have a method to project a Pin<T> from it.
2034 #[stable(feature = "pin", since = "1.33.0")]
2035 #[rustc_const_unstable(feature = "const_box", issue = "92521")]
2036 impl<T: ?Sized, A: Allocator> const Unpin for Box<T, A> where A: 'static {}
2038 #[unstable(feature = "generator_trait", issue = "43122")]
2039 impl<G: ?Sized + Generator<R> + Unpin, R, A: Allocator> Generator<R> for Box<G, A>
2043 type Yield = G::Yield;
2044 type Return = G::Return;
2046 fn resume(mut self: Pin<&mut Self>, arg: R) -> GeneratorState<Self::Yield, Self::Return> {
2047 G::resume(Pin::new(&mut *self), arg)
2051 #[unstable(feature = "generator_trait", issue = "43122")]
2052 impl<G: ?Sized + Generator<R>, R, A: Allocator> Generator<R> for Pin<Box<G, A>>
2056 type Yield = G::Yield;
2057 type Return = G::Return;
2059 fn resume(mut self: Pin<&mut Self>, arg: R) -> GeneratorState<Self::Yield, Self::Return> {
2060 G::resume((*self).as_mut(), arg)
2064 #[stable(feature = "futures_api", since = "1.36.0")]
2065 impl<F: ?Sized + Future + Unpin, A: Allocator> Future for Box<F, A>
2069 type Output = F::Output;
2071 fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
2072 F::poll(Pin::new(&mut *self), cx)
2076 #[unstable(feature = "async_iterator", issue = "79024")]
2077 impl<S: ?Sized + AsyncIterator + Unpin> AsyncIterator for Box<S> {
2078 type Item = S::Item;
2080 fn poll_next(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Option<Self::Item>> {
2081 Pin::new(&mut **self).poll_next(cx)
2084 fn size_hint(&self) -> (usize, Option<usize>) {
2085 (**self).size_hint()