1 //! A pointer type for heap allocation.
3 //! [`Box<T>`], casually referred to as a 'box', provides the simplest form of
4 //! heap allocation in Rust. Boxes provide ownership for this allocation, and
5 //! drop their contents when they go out of scope. Boxes also ensure that they
6 //! never allocate more than `isize::MAX` bytes.
10 //! Move a value from the stack to the heap by creating a [`Box`]:
14 //! let boxed: Box<u8> = Box::new(val);
17 //! Move a value from a [`Box`] back to the stack by [dereferencing]:
20 //! let boxed: Box<u8> = Box::new(5);
21 //! let val: u8 = *boxed;
24 //! Creating a recursive data structure:
29 //! Cons(T, Box<List<T>>),
33 //! let list: List<i32> = List::Cons(1, Box::new(List::Cons(2, Box::new(List::Nil))));
34 //! println!("{:?}", list);
37 //! This will print `Cons(1, Cons(2, Nil))`.
39 //! Recursive structures must be boxed, because if the definition of `Cons`
42 //! ```compile_fail,E0072
48 //! It wouldn't work. This is because the size of a `List` depends on how many
49 //! elements are in the list, and so we don't know how much memory to allocate
50 //! for a `Cons`. By introducing a [`Box<T>`], which has a defined size, we know how
51 //! big `Cons` needs to be.
55 //! For non-zero-sized values, a [`Box`] will use the [`Global`] allocator for
56 //! its allocation. It is valid to convert both ways between a [`Box`] and a
57 //! raw pointer allocated with the [`Global`] allocator, given that the
58 //! [`Layout`] used with the allocator is correct for the type. More precisely,
59 //! a `value: *mut T` that has been allocated with the [`Global`] allocator
60 //! with `Layout::for_value(&*value)` may be converted into a box using
61 //! [`Box::<T>::from_raw(value)`]. Conversely, the memory backing a `value: *mut
62 //! T` obtained from [`Box::<T>::into_raw`] may be deallocated using the
63 //! [`Global`] allocator with [`Layout::for_value(&*value)`].
65 //! For zero-sized values, the `Box` pointer still has to be [valid] for reads
66 //! and writes and sufficiently aligned. In particular, casting any aligned
67 //! non-zero integer literal to a raw pointer produces a valid pointer, but a
68 //! pointer pointing into previously allocated memory that since got freed is
69 //! not valid. The recommended way to build a Box to a ZST if `Box::new` cannot
70 //! be used is to use [`ptr::NonNull::dangling`].
72 //! So long as `T: Sized`, a `Box<T>` is guaranteed to be represented
73 //! as a single pointer and is also ABI-compatible with C pointers
74 //! (i.e. the C type `T*`). This means that if you have extern "C"
75 //! Rust functions that will be called from C, you can define those
76 //! Rust functions using `Box<T>` types, and use `T*` as corresponding
77 //! type on the C side. As an example, consider this C header which
78 //! declares functions that create and destroy some kind of `Foo`
84 //! /* Returns ownership to the caller */
85 //! struct Foo* foo_new(void);
87 //! /* Takes ownership from the caller; no-op when invoked with null */
88 //! void foo_delete(struct Foo*);
91 //! These two functions might be implemented in Rust as follows. Here, the
92 //! `struct Foo*` type from C is translated to `Box<Foo>`, which captures
93 //! the ownership constraints. Note also that the nullable argument to
94 //! `foo_delete` is represented in Rust as `Option<Box<Foo>>`, since `Box<Foo>`
102 //! pub extern "C" fn foo_new() -> Box<Foo> {
107 //! pub extern "C" fn foo_delete(_: Option<Box<Foo>>) {}
110 //! Even though `Box<T>` has the same representation and C ABI as a C pointer,
111 //! this does not mean that you can convert an arbitrary `T*` into a `Box<T>`
112 //! and expect things to work. `Box<T>` values will always be fully aligned,
113 //! non-null pointers. Moreover, the destructor for `Box<T>` will attempt to
114 //! free the value with the global allocator. In general, the best practice
115 //! is to only use `Box<T>` for pointers that originated from the global
118 //! **Important.** At least at present, you should avoid using
119 //! `Box<T>` types for functions that are defined in C but invoked
120 //! from Rust. In those cases, you should directly mirror the C types
121 //! as closely as possible. Using types like `Box<T>` where the C
122 //! definition is just using `T*` can lead to undefined behavior, as
123 //! described in [rust-lang/unsafe-code-guidelines#198][ucg#198].
125 //! [ucg#198]: https://github.com/rust-lang/unsafe-code-guidelines/issues/198
126 //! [dereferencing]: core::ops::Deref
127 //! [`Box::<T>::from_raw(value)`]: Box::from_raw
128 //! [`Global`]: crate::alloc::Global
129 //! [`Layout`]: crate::alloc::Layout
130 //! [`Layout::for_value(&*value)`]: crate::alloc::Layout::for_value
131 //! [valid]: ptr#safety
133 #![stable(feature = "rust1", since = "1.0.0")]
137 use core::cmp::Ordering;
138 use core::convert::{From, TryFrom};
140 use core::future::Future;
141 use core::hash::{Hash, Hasher};
142 #[cfg(not(no_global_oom_handling))]
143 use core::iter::FromIterator;
144 use core::iter::{FusedIterator, Iterator};
145 use core::marker::{Unpin, Unsize};
148 CoerceUnsized, Deref, DerefMut, DispatchFromDyn, Generator, GeneratorState, Receiver,
151 use core::ptr::{self, Unique};
152 use core::stream::Stream;
153 use core::task::{Context, Poll};
155 #[cfg(not(no_global_oom_handling))]
156 use crate::alloc::{handle_alloc_error, WriteCloneIntoRaw};
157 use crate::alloc::{AllocError, Allocator, Global, Layout};
158 #[cfg(not(no_global_oom_handling))]
159 use crate::borrow::Cow;
160 use crate::raw_vec::RawVec;
161 #[cfg(not(no_global_oom_handling))]
162 use crate::str::from_boxed_utf8_unchecked;
163 #[cfg(not(no_global_oom_handling))]
166 /// A pointer type for heap allocation.
168 /// See the [module-level documentation](../../std/boxed/index.html) for more.
169 #[lang = "owned_box"]
171 #[stable(feature = "rust1", since = "1.0.0")]
172 // The declaration of the `Box` struct must be kept in sync with the
173 // `alloc::alloc::box_free` function or ICEs will happen. See the comment
174 // on `box_free` for more details.
177 #[unstable(feature = "allocator_api", issue = "32838")] A: Allocator = Global,
181 /// Allocates memory on the heap and then places `x` into it.
183 /// This doesn't actually allocate if `T` is zero-sized.
188 /// let five = Box::new(5);
190 #[cfg(not(no_global_oom_handling))]
192 #[stable(feature = "rust1", since = "1.0.0")]
194 pub fn new(x: T) -> Self {
198 /// Constructs a new box with uninitialized contents.
203 /// #![feature(new_uninit)]
205 /// let mut five = Box::<u32>::new_uninit();
207 /// let five = unsafe {
208 /// // Deferred initialization:
209 /// five.as_mut_ptr().write(5);
211 /// five.assume_init()
214 /// assert_eq!(*five, 5)
216 #[cfg(not(no_global_oom_handling))]
217 #[unstable(feature = "new_uninit", issue = "63291")]
220 pub fn new_uninit() -> Box<mem::MaybeUninit<T>> {
221 Self::new_uninit_in(Global)
224 /// Constructs a new `Box` with uninitialized contents, with the memory
225 /// being filled with `0` bytes.
227 /// See [`MaybeUninit::zeroed`][zeroed] for examples of correct and incorrect usage
233 /// #![feature(new_uninit)]
235 /// let zero = Box::<u32>::new_zeroed();
236 /// let zero = unsafe { zero.assume_init() };
238 /// assert_eq!(*zero, 0)
241 /// [zeroed]: mem::MaybeUninit::zeroed
242 #[cfg(not(no_global_oom_handling))]
244 #[unstable(feature = "new_uninit", issue = "63291")]
246 pub fn new_zeroed() -> Box<mem::MaybeUninit<T>> {
247 Self::new_zeroed_in(Global)
250 /// Constructs a new `Pin<Box<T>>`. If `T` does not implement `Unpin`, then
251 /// `x` will be pinned in memory and unable to be moved.
252 #[cfg(not(no_global_oom_handling))]
253 #[stable(feature = "pin", since = "1.33.0")]
256 pub fn pin(x: T) -> Pin<Box<T>> {
260 /// Allocates memory on the heap then places `x` into it,
261 /// returning an error if the allocation fails
263 /// This doesn't actually allocate if `T` is zero-sized.
268 /// #![feature(allocator_api)]
270 /// let five = Box::try_new(5)?;
271 /// # Ok::<(), std::alloc::AllocError>(())
273 #[unstable(feature = "allocator_api", issue = "32838")]
275 pub fn try_new(x: T) -> Result<Self, AllocError> {
276 Self::try_new_in(x, Global)
279 /// Constructs a new box with uninitialized contents on the heap,
280 /// returning an error if the allocation fails
285 /// #![feature(allocator_api, new_uninit)]
287 /// let mut five = Box::<u32>::try_new_uninit()?;
289 /// let five = unsafe {
290 /// // Deferred initialization:
291 /// five.as_mut_ptr().write(5);
293 /// five.assume_init()
296 /// assert_eq!(*five, 5);
297 /// # Ok::<(), std::alloc::AllocError>(())
299 #[unstable(feature = "allocator_api", issue = "32838")]
300 // #[unstable(feature = "new_uninit", issue = "63291")]
302 pub fn try_new_uninit() -> Result<Box<mem::MaybeUninit<T>>, AllocError> {
303 Box::try_new_uninit_in(Global)
306 /// Constructs a new `Box` with uninitialized contents, with the memory
307 /// being filled with `0` bytes on the heap
309 /// See [`MaybeUninit::zeroed`][zeroed] for examples of correct and incorrect usage
315 /// #![feature(allocator_api, new_uninit)]
317 /// let zero = Box::<u32>::try_new_zeroed()?;
318 /// let zero = unsafe { zero.assume_init() };
320 /// assert_eq!(*zero, 0);
321 /// # Ok::<(), std::alloc::AllocError>(())
324 /// [zeroed]: mem::MaybeUninit::zeroed
325 #[unstable(feature = "allocator_api", issue = "32838")]
326 // #[unstable(feature = "new_uninit", issue = "63291")]
328 pub fn try_new_zeroed() -> Result<Box<mem::MaybeUninit<T>>, AllocError> {
329 Box::try_new_zeroed_in(Global)
333 impl<T, A: Allocator> Box<T, A> {
334 /// Allocates memory in the given allocator then places `x` into it.
336 /// This doesn't actually allocate if `T` is zero-sized.
341 /// #![feature(allocator_api)]
343 /// use std::alloc::System;
345 /// let five = Box::new_in(5, System);
347 #[cfg(not(no_global_oom_handling))]
348 #[unstable(feature = "allocator_api", issue = "32838")]
349 #[rustc_const_unstable(feature = "const_box", issue = "92521")]
352 pub const fn new_in(x: T, alloc: A) -> Self
354 A: ~const Allocator + ~const Drop,
356 let mut boxed = Self::new_uninit_in(alloc);
358 boxed.as_mut_ptr().write(x);
363 /// Allocates memory in the given allocator then places `x` into it,
364 /// returning an error if the allocation fails
366 /// This doesn't actually allocate if `T` is zero-sized.
371 /// #![feature(allocator_api)]
373 /// use std::alloc::System;
375 /// let five = Box::try_new_in(5, System)?;
376 /// # Ok::<(), std::alloc::AllocError>(())
378 #[unstable(feature = "allocator_api", issue = "32838")]
379 #[rustc_const_unstable(feature = "const_box", issue = "92521")]
381 pub const fn try_new_in(x: T, alloc: A) -> Result<Self, AllocError>
384 A: ~const Allocator + ~const Drop,
386 let mut boxed = Self::try_new_uninit_in(alloc)?;
388 boxed.as_mut_ptr().write(x);
389 Ok(boxed.assume_init())
393 /// Constructs a new box with uninitialized contents in the provided allocator.
398 /// #![feature(allocator_api, new_uninit)]
400 /// use std::alloc::System;
402 /// let mut five = Box::<u32, _>::new_uninit_in(System);
404 /// let five = unsafe {
405 /// // Deferred initialization:
406 /// five.as_mut_ptr().write(5);
408 /// five.assume_init()
411 /// assert_eq!(*five, 5)
413 #[unstable(feature = "allocator_api", issue = "32838")]
414 #[rustc_const_unstable(feature = "const_box", issue = "92521")]
415 #[cfg(not(no_global_oom_handling))]
417 // #[unstable(feature = "new_uninit", issue = "63291")]
418 pub const fn new_uninit_in(alloc: A) -> Box<mem::MaybeUninit<T>, A>
420 A: ~const Allocator + ~const Drop,
422 let layout = Layout::new::<mem::MaybeUninit<T>>();
423 // NOTE: Prefer match over unwrap_or_else since closure sometimes not inlineable.
424 // That would make code size bigger.
425 match Box::try_new_uninit_in(alloc) {
427 Err(_) => handle_alloc_error(layout),
431 /// Constructs a new box with uninitialized contents in the provided allocator,
432 /// returning an error if the allocation fails
437 /// #![feature(allocator_api, new_uninit)]
439 /// use std::alloc::System;
441 /// let mut five = Box::<u32, _>::try_new_uninit_in(System)?;
443 /// let five = unsafe {
444 /// // Deferred initialization:
445 /// five.as_mut_ptr().write(5);
447 /// five.assume_init()
450 /// assert_eq!(*five, 5);
451 /// # Ok::<(), std::alloc::AllocError>(())
453 #[unstable(feature = "allocator_api", issue = "32838")]
454 // #[unstable(feature = "new_uninit", issue = "63291")]
455 #[rustc_const_unstable(feature = "const_box", issue = "92521")]
456 pub const fn try_new_uninit_in(alloc: A) -> Result<Box<mem::MaybeUninit<T>, A>, AllocError>
458 A: ~const Allocator + ~const Drop,
460 let layout = Layout::new::<mem::MaybeUninit<T>>();
461 let ptr = alloc.allocate(layout)?.cast();
462 unsafe { Ok(Box::from_raw_in(ptr.as_ptr(), alloc)) }
465 /// Constructs a new `Box` with uninitialized contents, with the memory
466 /// being filled with `0` bytes in the provided allocator.
468 /// See [`MaybeUninit::zeroed`][zeroed] for examples of correct and incorrect usage
474 /// #![feature(allocator_api, new_uninit)]
476 /// use std::alloc::System;
478 /// let zero = Box::<u32, _>::new_zeroed_in(System);
479 /// let zero = unsafe { zero.assume_init() };
481 /// assert_eq!(*zero, 0)
484 /// [zeroed]: mem::MaybeUninit::zeroed
485 #[unstable(feature = "allocator_api", issue = "32838")]
486 #[rustc_const_unstable(feature = "const_box", issue = "92521")]
487 #[cfg(not(no_global_oom_handling))]
488 // #[unstable(feature = "new_uninit", issue = "63291")]
490 pub const fn new_zeroed_in(alloc: A) -> Box<mem::MaybeUninit<T>, A>
492 A: ~const Allocator + ~const Drop,
494 let layout = Layout::new::<mem::MaybeUninit<T>>();
495 // NOTE: Prefer match over unwrap_or_else since closure sometimes not inlineable.
496 // That would make code size bigger.
497 match Box::try_new_zeroed_in(alloc) {
499 Err(_) => handle_alloc_error(layout),
503 /// Constructs a new `Box` with uninitialized contents, with the memory
504 /// being filled with `0` bytes in the provided allocator,
505 /// returning an error if the allocation fails,
507 /// See [`MaybeUninit::zeroed`][zeroed] for examples of correct and incorrect usage
513 /// #![feature(allocator_api, new_uninit)]
515 /// use std::alloc::System;
517 /// let zero = Box::<u32, _>::try_new_zeroed_in(System)?;
518 /// let zero = unsafe { zero.assume_init() };
520 /// assert_eq!(*zero, 0);
521 /// # Ok::<(), std::alloc::AllocError>(())
524 /// [zeroed]: mem::MaybeUninit::zeroed
525 #[unstable(feature = "allocator_api", issue = "32838")]
526 // #[unstable(feature = "new_uninit", issue = "63291")]
527 #[rustc_const_unstable(feature = "const_box", issue = "92521")]
528 pub const fn try_new_zeroed_in(alloc: A) -> Result<Box<mem::MaybeUninit<T>, A>, AllocError>
530 A: ~const Allocator + ~const Drop,
532 let layout = Layout::new::<mem::MaybeUninit<T>>();
533 let ptr = alloc.allocate_zeroed(layout)?.cast();
534 unsafe { Ok(Box::from_raw_in(ptr.as_ptr(), alloc)) }
537 /// Constructs a new `Pin<Box<T, A>>`. If `T` does not implement `Unpin`, then
538 /// `x` will be pinned in memory and unable to be moved.
539 #[cfg(not(no_global_oom_handling))]
540 #[unstable(feature = "allocator_api", issue = "32838")]
541 #[rustc_const_unstable(feature = "const_box", issue = "92521")]
544 pub const fn pin_in(x: T, alloc: A) -> Pin<Self>
546 A: 'static + ~const Allocator + ~const Drop,
548 Self::into_pin(Self::new_in(x, alloc))
551 /// Converts a `Box<T>` into a `Box<[T]>`
553 /// This conversion does not allocate on the heap and happens in place.
554 #[unstable(feature = "box_into_boxed_slice", issue = "71582")]
555 #[rustc_const_unstable(feature = "const_box", issue = "92521")]
556 pub const fn into_boxed_slice(boxed: Self) -> Box<[T], A> {
557 let (raw, alloc) = Box::into_raw_with_allocator(boxed);
558 unsafe { Box::from_raw_in(raw as *mut [T; 1], alloc) }
561 /// Consumes the `Box`, returning the wrapped value.
566 /// #![feature(box_into_inner)]
568 /// let c = Box::new(5);
570 /// assert_eq!(Box::into_inner(c), 5);
572 #[unstable(feature = "box_into_inner", issue = "80437")]
573 #[rustc_const_unstable(feature = "const_box", issue = "92521")]
575 pub const fn into_inner(boxed: Self) -> T
584 /// Constructs a new boxed slice with uninitialized contents.
589 /// #![feature(new_uninit)]
591 /// let mut values = Box::<[u32]>::new_uninit_slice(3);
593 /// let values = unsafe {
594 /// // Deferred initialization:
595 /// values[0].as_mut_ptr().write(1);
596 /// values[1].as_mut_ptr().write(2);
597 /// values[2].as_mut_ptr().write(3);
599 /// values.assume_init()
602 /// assert_eq!(*values, [1, 2, 3])
604 #[cfg(not(no_global_oom_handling))]
605 #[unstable(feature = "new_uninit", issue = "63291")]
607 pub fn new_uninit_slice(len: usize) -> Box<[mem::MaybeUninit<T>]> {
608 unsafe { RawVec::with_capacity(len).into_box(len) }
611 /// Constructs a new boxed slice with uninitialized contents, with the memory
612 /// being filled with `0` bytes.
614 /// See [`MaybeUninit::zeroed`][zeroed] for examples of correct and incorrect usage
620 /// #![feature(new_uninit)]
622 /// let values = Box::<[u32]>::new_zeroed_slice(3);
623 /// let values = unsafe { values.assume_init() };
625 /// assert_eq!(*values, [0, 0, 0])
628 /// [zeroed]: mem::MaybeUninit::zeroed
629 #[cfg(not(no_global_oom_handling))]
630 #[unstable(feature = "new_uninit", issue = "63291")]
632 pub fn new_zeroed_slice(len: usize) -> Box<[mem::MaybeUninit<T>]> {
633 unsafe { RawVec::with_capacity_zeroed(len).into_box(len) }
636 /// Constructs a new boxed slice with uninitialized contents. Returns an error if
637 /// the allocation fails
642 /// #![feature(allocator_api, new_uninit)]
644 /// let mut values = Box::<[u32]>::try_new_uninit_slice(3)?;
645 /// let values = unsafe {
646 /// // Deferred initialization:
647 /// values[0].as_mut_ptr().write(1);
648 /// values[1].as_mut_ptr().write(2);
649 /// values[2].as_mut_ptr().write(3);
650 /// values.assume_init()
653 /// assert_eq!(*values, [1, 2, 3]);
654 /// # Ok::<(), std::alloc::AllocError>(())
656 #[unstable(feature = "allocator_api", issue = "32838")]
658 pub fn try_new_uninit_slice(len: usize) -> Result<Box<[mem::MaybeUninit<T>]>, AllocError> {
660 let layout = match Layout::array::<mem::MaybeUninit<T>>(len) {
662 Err(_) => return Err(AllocError),
664 let ptr = Global.allocate(layout)?;
665 Ok(RawVec::from_raw_parts_in(ptr.as_mut_ptr() as *mut _, len, Global).into_box(len))
669 /// Constructs a new boxed slice with uninitialized contents, with the memory
670 /// being filled with `0` bytes. Returns an error if the allocation fails
672 /// See [`MaybeUninit::zeroed`][zeroed] for examples of correct and incorrect usage
678 /// #![feature(allocator_api, new_uninit)]
680 /// let values = Box::<[u32]>::try_new_zeroed_slice(3)?;
681 /// let values = unsafe { values.assume_init() };
683 /// assert_eq!(*values, [0, 0, 0]);
684 /// # Ok::<(), std::alloc::AllocError>(())
687 /// [zeroed]: mem::MaybeUninit::zeroed
688 #[unstable(feature = "allocator_api", issue = "32838")]
690 pub fn try_new_zeroed_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_zeroed(layout)?;
697 Ok(RawVec::from_raw_parts_in(ptr.as_mut_ptr() as *mut _, len, Global).into_box(len))
702 impl<T, A: Allocator> Box<[T], A> {
703 /// Constructs a new boxed slice with uninitialized contents in the provided allocator.
708 /// #![feature(allocator_api, new_uninit)]
710 /// use std::alloc::System;
712 /// let mut values = Box::<[u32], _>::new_uninit_slice_in(3, System);
714 /// let values = unsafe {
715 /// // Deferred initialization:
716 /// values[0].as_mut_ptr().write(1);
717 /// values[1].as_mut_ptr().write(2);
718 /// values[2].as_mut_ptr().write(3);
720 /// values.assume_init()
723 /// assert_eq!(*values, [1, 2, 3])
725 #[cfg(not(no_global_oom_handling))]
726 #[unstable(feature = "allocator_api", issue = "32838")]
727 // #[unstable(feature = "new_uninit", issue = "63291")]
729 pub fn new_uninit_slice_in(len: usize, alloc: A) -> Box<[mem::MaybeUninit<T>], A> {
730 unsafe { RawVec::with_capacity_in(len, alloc).into_box(len) }
733 /// Constructs a new boxed slice with uninitialized contents in the provided allocator,
734 /// with the memory being filled with `0` bytes.
736 /// See [`MaybeUninit::zeroed`][zeroed] for examples of correct and incorrect usage
742 /// #![feature(allocator_api, new_uninit)]
744 /// use std::alloc::System;
746 /// let values = Box::<[u32], _>::new_zeroed_slice_in(3, System);
747 /// let values = unsafe { values.assume_init() };
749 /// assert_eq!(*values, [0, 0, 0])
752 /// [zeroed]: mem::MaybeUninit::zeroed
753 #[cfg(not(no_global_oom_handling))]
754 #[unstable(feature = "allocator_api", issue = "32838")]
755 // #[unstable(feature = "new_uninit", issue = "63291")]
757 pub fn new_zeroed_slice_in(len: usize, alloc: A) -> Box<[mem::MaybeUninit<T>], A> {
758 unsafe { RawVec::with_capacity_zeroed_in(len, alloc).into_box(len) }
762 impl<T, A: Allocator> Box<mem::MaybeUninit<T>, A> {
763 /// Converts to `Box<T, A>`.
767 /// As with [`MaybeUninit::assume_init`],
768 /// it is up to the caller to guarantee that the value
769 /// really is in an initialized state.
770 /// Calling this when the content is not yet fully initialized
771 /// causes immediate undefined behavior.
773 /// [`MaybeUninit::assume_init`]: mem::MaybeUninit::assume_init
778 /// #![feature(new_uninit)]
780 /// let mut five = Box::<u32>::new_uninit();
782 /// let five: Box<u32> = unsafe {
783 /// // Deferred initialization:
784 /// five.as_mut_ptr().write(5);
786 /// five.assume_init()
789 /// assert_eq!(*five, 5)
791 #[unstable(feature = "new_uninit", issue = "63291")]
792 #[rustc_const_unstable(feature = "const_box", issue = "92521")]
794 pub const unsafe fn assume_init(self) -> Box<T, A> {
795 let (raw, alloc) = Box::into_raw_with_allocator(self);
796 unsafe { Box::from_raw_in(raw as *mut T, alloc) }
799 /// Writes the value and converts to `Box<T, A>`.
801 /// This method converts the box similarly to [`Box::assume_init`] but
802 /// writes `value` into it before conversion thus guaranteeing safety.
803 /// In some scenarios use of this method may improve performance because
804 /// the compiler may be able to optimize copying from stack.
809 /// #![feature(new_uninit)]
811 /// let big_box = Box::<[usize; 1024]>::new_uninit();
813 /// let mut array = [0; 1024];
814 /// for (i, place) in array.iter_mut().enumerate() {
818 /// // The optimizer may be able to elide this copy, so previous code writes
819 /// // to heap directly.
820 /// let big_box = Box::write(big_box, array);
822 /// for (i, x) in big_box.iter().enumerate() {
823 /// assert_eq!(*x, i);
826 #[unstable(feature = "new_uninit", issue = "63291")]
827 #[rustc_const_unstable(feature = "const_box", issue = "92521")]
829 pub const fn write(mut boxed: Self, value: T) -> Box<T, A> {
831 (*boxed).write(value);
837 impl<T, A: Allocator> Box<[mem::MaybeUninit<T>], A> {
838 /// Converts to `Box<[T], A>`.
842 /// As with [`MaybeUninit::assume_init`],
843 /// it is up to the caller to guarantee that the values
844 /// really are in an initialized state.
845 /// Calling this when the content is not yet fully initialized
846 /// causes immediate undefined behavior.
848 /// [`MaybeUninit::assume_init`]: mem::MaybeUninit::assume_init
853 /// #![feature(new_uninit)]
855 /// let mut values = Box::<[u32]>::new_uninit_slice(3);
857 /// let values = unsafe {
858 /// // Deferred initialization:
859 /// values[0].as_mut_ptr().write(1);
860 /// values[1].as_mut_ptr().write(2);
861 /// values[2].as_mut_ptr().write(3);
863 /// values.assume_init()
866 /// assert_eq!(*values, [1, 2, 3])
868 #[unstable(feature = "new_uninit", issue = "63291")]
870 pub unsafe fn assume_init(self) -> Box<[T], A> {
871 let (raw, alloc) = Box::into_raw_with_allocator(self);
872 unsafe { Box::from_raw_in(raw as *mut [T], alloc) }
876 impl<T: ?Sized> Box<T> {
877 /// Constructs a box from a raw pointer.
879 /// After calling this function, the raw pointer is owned by the
880 /// resulting `Box`. Specifically, the `Box` destructor will call
881 /// the destructor of `T` and free the allocated memory. For this
882 /// to be safe, the memory must have been allocated in accordance
883 /// with the [memory layout] used by `Box` .
887 /// This function is unsafe because improper use may lead to
888 /// memory problems. For example, a double-free may occur if the
889 /// function is called twice on the same raw pointer.
891 /// The safety conditions are described in the [memory layout] section.
895 /// Recreate a `Box` which was previously converted to a raw pointer
896 /// using [`Box::into_raw`]:
898 /// let x = Box::new(5);
899 /// let ptr = Box::into_raw(x);
900 /// let x = unsafe { Box::from_raw(ptr) };
902 /// Manually create a `Box` from scratch by using the global allocator:
904 /// use std::alloc::{alloc, Layout};
907 /// let ptr = alloc(Layout::new::<i32>()) as *mut i32;
908 /// // In general .write is required to avoid attempting to destruct
909 /// // the (uninitialized) previous contents of `ptr`, though for this
910 /// // simple example `*ptr = 5` would have worked as well.
912 /// let x = Box::from_raw(ptr);
916 /// [memory layout]: self#memory-layout
917 /// [`Layout`]: crate::Layout
918 #[stable(feature = "box_raw", since = "1.4.0")]
920 pub unsafe fn from_raw(raw: *mut T) -> Self {
921 unsafe { Self::from_raw_in(raw, Global) }
925 impl<T: ?Sized, A: Allocator> Box<T, A> {
926 /// Constructs a box from a raw pointer in the given allocator.
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.
943 /// Recreate a `Box` which was previously converted to a raw pointer
944 /// using [`Box::into_raw_with_allocator`]:
946 /// #![feature(allocator_api)]
948 /// use std::alloc::System;
950 /// let x = Box::new_in(5, System);
951 /// let (ptr, alloc) = Box::into_raw_with_allocator(x);
952 /// let x = unsafe { Box::from_raw_in(ptr, alloc) };
954 /// Manually create a `Box` from scratch by using the system allocator:
956 /// #![feature(allocator_api, slice_ptr_get)]
958 /// use std::alloc::{Allocator, Layout, System};
961 /// let ptr = System.allocate(Layout::new::<i32>())?.as_mut_ptr() as *mut i32;
962 /// // In general .write is required to avoid attempting to destruct
963 /// // the (uninitialized) previous contents of `ptr`, though for this
964 /// // simple example `*ptr = 5` would have worked as well.
966 /// let x = Box::from_raw_in(ptr, System);
968 /// # Ok::<(), std::alloc::AllocError>(())
971 /// [memory layout]: self#memory-layout
972 /// [`Layout`]: crate::Layout
973 #[unstable(feature = "allocator_api", issue = "32838")]
974 #[rustc_const_unstable(feature = "const_box", issue = "92521")]
976 pub const unsafe fn from_raw_in(raw: *mut T, alloc: A) -> Self {
977 Box(unsafe { Unique::new_unchecked(raw) }, alloc)
980 /// Consumes the `Box`, returning a wrapped raw pointer.
982 /// The pointer will be properly aligned and non-null.
984 /// After calling this function, the caller is responsible for the
985 /// memory previously managed by the `Box`. In particular, the
986 /// caller should properly destroy `T` and release the memory, taking
987 /// into account the [memory layout] used by `Box`. The easiest way to
988 /// do this is to convert the raw pointer back into a `Box` with the
989 /// [`Box::from_raw`] function, allowing the `Box` destructor to perform
992 /// Note: this is an associated function, which means that you have
993 /// to call it as `Box::into_raw(b)` instead of `b.into_raw()`. This
994 /// is so that there is no conflict with a method on the inner type.
997 /// Converting the raw pointer back into a `Box` with [`Box::from_raw`]
998 /// for automatic cleanup:
1000 /// let x = Box::new(String::from("Hello"));
1001 /// let ptr = Box::into_raw(x);
1002 /// let x = unsafe { Box::from_raw(ptr) };
1004 /// Manual cleanup by explicitly running the destructor and deallocating
1007 /// use std::alloc::{dealloc, Layout};
1010 /// let x = Box::new(String::from("Hello"));
1011 /// let p = Box::into_raw(x);
1013 /// ptr::drop_in_place(p);
1014 /// dealloc(p as *mut u8, Layout::new::<String>());
1018 /// [memory layout]: self#memory-layout
1019 #[stable(feature = "box_raw", since = "1.4.0")]
1021 pub fn into_raw(b: Self) -> *mut T {
1022 Self::into_raw_with_allocator(b).0
1025 /// Consumes the `Box`, returning a wrapped raw pointer and the allocator.
1027 /// The pointer will be properly aligned and non-null.
1029 /// After calling this function, the caller is responsible for the
1030 /// memory previously managed by the `Box`. In particular, the
1031 /// caller should properly destroy `T` and release the memory, taking
1032 /// into account the [memory layout] used by `Box`. The easiest way to
1033 /// do this is to convert the raw pointer back into a `Box` with the
1034 /// [`Box::from_raw_in`] function, allowing the `Box` destructor to perform
1037 /// Note: this is an associated function, which means that you have
1038 /// to call it as `Box::into_raw_with_allocator(b)` instead of `b.into_raw_with_allocator()`. This
1039 /// is so that there is no conflict with a method on the inner type.
1042 /// Converting the raw pointer back into a `Box` with [`Box::from_raw_in`]
1043 /// for automatic cleanup:
1045 /// #![feature(allocator_api)]
1047 /// use std::alloc::System;
1049 /// let x = Box::new_in(String::from("Hello"), System);
1050 /// let (ptr, alloc) = Box::into_raw_with_allocator(x);
1051 /// let x = unsafe { Box::from_raw_in(ptr, alloc) };
1053 /// Manual cleanup by explicitly running the destructor and deallocating
1056 /// #![feature(allocator_api)]
1058 /// use std::alloc::{Allocator, Layout, System};
1059 /// use std::ptr::{self, NonNull};
1061 /// let x = Box::new_in(String::from("Hello"), System);
1062 /// let (ptr, alloc) = Box::into_raw_with_allocator(x);
1064 /// ptr::drop_in_place(ptr);
1065 /// let non_null = NonNull::new_unchecked(ptr);
1066 /// alloc.deallocate(non_null.cast(), Layout::new::<String>());
1070 /// [memory layout]: self#memory-layout
1071 #[unstable(feature = "allocator_api", issue = "32838")]
1072 #[rustc_const_unstable(feature = "const_box", issue = "92521")]
1074 pub const fn into_raw_with_allocator(b: Self) -> (*mut T, A) {
1075 let (leaked, alloc) = Box::into_unique(b);
1076 (leaked.as_ptr(), alloc)
1080 feature = "ptr_internals",
1082 reason = "use `Box::leak(b).into()` or `Unique::from(Box::leak(b))` instead"
1084 #[rustc_const_unstable(feature = "const_box", issue = "92521")]
1087 pub const fn into_unique(b: Self) -> (Unique<T>, A) {
1088 // Box is recognized as a "unique pointer" by Stacked Borrows, but internally it is a
1089 // raw pointer for the type system. Turning it directly into a raw pointer would not be
1090 // recognized as "releasing" the unique pointer to permit aliased raw accesses,
1091 // so all raw pointer methods have to go through `Box::leak`. Turning *that* to a raw pointer
1092 // behaves correctly.
1093 let alloc = unsafe { ptr::read(&b.1) };
1094 (Unique::from(Box::leak(b)), alloc)
1097 /// Returns a reference to the underlying allocator.
1099 /// Note: this is an associated function, which means that you have
1100 /// to call it as `Box::allocator(&b)` instead of `b.allocator()`. This
1101 /// is so that there is no conflict with a method on the inner type.
1102 #[unstable(feature = "allocator_api", issue = "32838")]
1103 #[rustc_const_unstable(feature = "const_box", issue = "92521")]
1105 pub const fn allocator(b: &Self) -> &A {
1109 /// Consumes and leaks the `Box`, returning a mutable reference,
1110 /// `&'a mut T`. Note that the type `T` must outlive the chosen lifetime
1111 /// `'a`. If the type has only static references, or none at all, then this
1112 /// may be chosen to be `'static`.
1114 /// This function is mainly useful for data that lives for the remainder of
1115 /// the program's life. Dropping the returned reference will cause a memory
1116 /// leak. If this is not acceptable, the reference should first be wrapped
1117 /// with the [`Box::from_raw`] function producing a `Box`. This `Box` can
1118 /// then be dropped which will properly destroy `T` and release the
1119 /// allocated memory.
1121 /// Note: this is an associated function, which means that you have
1122 /// to call it as `Box::leak(b)` instead of `b.leak()`. This
1123 /// is so that there is no conflict with a method on the inner type.
1130 /// let x = Box::new(41);
1131 /// let static_ref: &'static mut usize = Box::leak(x);
1132 /// *static_ref += 1;
1133 /// assert_eq!(*static_ref, 42);
1139 /// let x = vec![1, 2, 3].into_boxed_slice();
1140 /// let static_ref = Box::leak(x);
1141 /// static_ref[0] = 4;
1142 /// assert_eq!(*static_ref, [4, 2, 3]);
1144 #[stable(feature = "box_leak", since = "1.26.0")]
1145 #[rustc_const_unstable(feature = "const_box", issue = "92521")]
1147 pub const fn leak<'a>(b: Self) -> &'a mut T
1151 unsafe { &mut *mem::ManuallyDrop::new(b).0.as_ptr() }
1154 /// Converts a `Box<T>` into a `Pin<Box<T>>`
1156 /// This conversion does not allocate on the heap and happens in place.
1158 /// This is also available via [`From`].
1159 #[unstable(feature = "box_into_pin", issue = "62370")]
1160 #[rustc_const_unstable(feature = "const_box", issue = "92521")]
1161 pub const fn into_pin(boxed: Self) -> Pin<Self>
1165 // It's not possible to move or replace the insides of a `Pin<Box<T>>`
1166 // when `T: !Unpin`, so it's safe to pin it directly without any
1167 // additional requirements.
1168 unsafe { Pin::new_unchecked(boxed) }
1172 #[stable(feature = "rust1", since = "1.0.0")]
1173 #[rustc_const_unstable(feature = "const_box", issue = "92521")]
1174 unsafe impl<#[may_dangle] T: ?Sized, A: Allocator> const Drop for Box<T, A> {
1175 fn drop(&mut self) {
1176 // FIXME: Do nothing, drop is currently performed by compiler.
1180 #[cfg(not(no_global_oom_handling))]
1181 #[stable(feature = "rust1", since = "1.0.0")]
1182 impl<T: Default> Default for Box<T> {
1183 /// Creates a `Box<T>`, with the `Default` value for T.
1184 fn default() -> Self {
1189 #[cfg(not(no_global_oom_handling))]
1190 #[stable(feature = "rust1", since = "1.0.0")]
1191 impl<T> Default for Box<[T]> {
1192 fn default() -> Self {
1193 Box::<[T; 0]>::new([])
1197 #[cfg(not(no_global_oom_handling))]
1198 #[stable(feature = "default_box_extra", since = "1.17.0")]
1199 impl Default for Box<str> {
1200 fn default() -> Self {
1201 unsafe { from_boxed_utf8_unchecked(Default::default()) }
1205 #[cfg(not(no_global_oom_handling))]
1206 #[stable(feature = "rust1", since = "1.0.0")]
1207 impl<T: Clone, A: Allocator + Clone> Clone for Box<T, A> {
1208 /// Returns a new box with a `clone()` of this box's contents.
1213 /// let x = Box::new(5);
1214 /// let y = x.clone();
1216 /// // The value is the same
1217 /// assert_eq!(x, y);
1219 /// // But they are unique objects
1220 /// assert_ne!(&*x as *const i32, &*y as *const i32);
1223 fn clone(&self) -> Self {
1224 // Pre-allocate memory to allow writing the cloned value directly.
1225 let mut boxed = Self::new_uninit_in(self.1.clone());
1227 (**self).write_clone_into_raw(boxed.as_mut_ptr());
1232 /// Copies `source`'s contents into `self` without creating a new allocation.
1237 /// let x = Box::new(5);
1238 /// let mut y = Box::new(10);
1239 /// let yp: *const i32 = &*y;
1241 /// y.clone_from(&x);
1243 /// // The value is the same
1244 /// assert_eq!(x, y);
1246 /// // And no allocation occurred
1247 /// assert_eq!(yp, &*y);
1250 fn clone_from(&mut self, source: &Self) {
1251 (**self).clone_from(&(**source));
1255 #[cfg(not(no_global_oom_handling))]
1256 #[stable(feature = "box_slice_clone", since = "1.3.0")]
1257 impl Clone for Box<str> {
1258 fn clone(&self) -> Self {
1259 // this makes a copy of the data
1260 let buf: Box<[u8]> = self.as_bytes().into();
1261 unsafe { from_boxed_utf8_unchecked(buf) }
1265 #[stable(feature = "rust1", since = "1.0.0")]
1266 impl<T: ?Sized + PartialEq, A: Allocator> PartialEq for Box<T, A> {
1268 fn eq(&self, other: &Self) -> bool {
1269 PartialEq::eq(&**self, &**other)
1272 fn ne(&self, other: &Self) -> bool {
1273 PartialEq::ne(&**self, &**other)
1276 #[stable(feature = "rust1", since = "1.0.0")]
1277 impl<T: ?Sized + PartialOrd, A: Allocator> PartialOrd for Box<T, A> {
1279 fn partial_cmp(&self, other: &Self) -> Option<Ordering> {
1280 PartialOrd::partial_cmp(&**self, &**other)
1283 fn lt(&self, other: &Self) -> bool {
1284 PartialOrd::lt(&**self, &**other)
1287 fn le(&self, other: &Self) -> bool {
1288 PartialOrd::le(&**self, &**other)
1291 fn ge(&self, other: &Self) -> bool {
1292 PartialOrd::ge(&**self, &**other)
1295 fn gt(&self, other: &Self) -> bool {
1296 PartialOrd::gt(&**self, &**other)
1299 #[stable(feature = "rust1", since = "1.0.0")]
1300 impl<T: ?Sized + Ord, A: Allocator> Ord for Box<T, A> {
1302 fn cmp(&self, other: &Self) -> Ordering {
1303 Ord::cmp(&**self, &**other)
1306 #[stable(feature = "rust1", since = "1.0.0")]
1307 impl<T: ?Sized + Eq, A: Allocator> Eq for Box<T, A> {}
1309 #[stable(feature = "rust1", since = "1.0.0")]
1310 impl<T: ?Sized + Hash, A: Allocator> Hash for Box<T, A> {
1311 fn hash<H: Hasher>(&self, state: &mut H) {
1312 (**self).hash(state);
1316 #[stable(feature = "indirect_hasher_impl", since = "1.22.0")]
1317 impl<T: ?Sized + Hasher, A: Allocator> Hasher for Box<T, A> {
1318 fn finish(&self) -> u64 {
1321 fn write(&mut self, bytes: &[u8]) {
1322 (**self).write(bytes)
1324 fn write_u8(&mut self, i: u8) {
1325 (**self).write_u8(i)
1327 fn write_u16(&mut self, i: u16) {
1328 (**self).write_u16(i)
1330 fn write_u32(&mut self, i: u32) {
1331 (**self).write_u32(i)
1333 fn write_u64(&mut self, i: u64) {
1334 (**self).write_u64(i)
1336 fn write_u128(&mut self, i: u128) {
1337 (**self).write_u128(i)
1339 fn write_usize(&mut self, i: usize) {
1340 (**self).write_usize(i)
1342 fn write_i8(&mut self, i: i8) {
1343 (**self).write_i8(i)
1345 fn write_i16(&mut self, i: i16) {
1346 (**self).write_i16(i)
1348 fn write_i32(&mut self, i: i32) {
1349 (**self).write_i32(i)
1351 fn write_i64(&mut self, i: i64) {
1352 (**self).write_i64(i)
1354 fn write_i128(&mut self, i: i128) {
1355 (**self).write_i128(i)
1357 fn write_isize(&mut self, i: isize) {
1358 (**self).write_isize(i)
1362 #[cfg(not(no_global_oom_handling))]
1363 #[stable(feature = "from_for_ptrs", since = "1.6.0")]
1364 impl<T> From<T> for Box<T> {
1365 /// Converts a `T` into a `Box<T>`
1367 /// The conversion allocates on the heap and moves `t`
1368 /// from the stack into it.
1374 /// let boxed = Box::new(5);
1376 /// assert_eq!(Box::from(x), boxed);
1378 fn from(t: T) -> Self {
1383 #[stable(feature = "pin", since = "1.33.0")]
1384 #[rustc_const_unstable(feature = "const_box", issue = "92521")]
1385 impl<T: ?Sized, A: Allocator> const From<Box<T, A>> for Pin<Box<T, A>>
1389 /// Converts a `Box<T>` into a `Pin<Box<T>>`
1391 /// This conversion does not allocate on the heap and happens in place.
1392 fn from(boxed: Box<T, A>) -> Self {
1393 Box::into_pin(boxed)
1397 #[cfg(not(no_global_oom_handling))]
1398 #[stable(feature = "box_from_slice", since = "1.17.0")]
1399 impl<T: Copy> From<&[T]> for Box<[T]> {
1400 /// Converts a `&[T]` into a `Box<[T]>`
1402 /// This conversion allocates on the heap
1403 /// and performs a copy of `slice`.
1407 /// // create a &[u8] which will be used to create a Box<[u8]>
1408 /// let slice: &[u8] = &[104, 101, 108, 108, 111];
1409 /// let boxed_slice: Box<[u8]> = Box::from(slice);
1411 /// println!("{:?}", boxed_slice);
1413 fn from(slice: &[T]) -> Box<[T]> {
1414 let len = slice.len();
1415 let buf = RawVec::with_capacity(len);
1417 ptr::copy_nonoverlapping(slice.as_ptr(), buf.ptr(), len);
1418 buf.into_box(slice.len()).assume_init()
1423 #[cfg(not(no_global_oom_handling))]
1424 #[stable(feature = "box_from_cow", since = "1.45.0")]
1425 impl<T: Copy> From<Cow<'_, [T]>> for Box<[T]> {
1426 /// Converts a `Cow<'_, [T]>` into a `Box<[T]>`
1428 /// When `cow` is the `Cow::Borrowed` variant, this
1429 /// conversion allocates on the heap and copies the
1430 /// underlying slice. Otherwise, it will try to reuse the owned
1431 /// `Vec`'s allocation.
1433 fn from(cow: Cow<'_, [T]>) -> Box<[T]> {
1435 Cow::Borrowed(slice) => Box::from(slice),
1436 Cow::Owned(slice) => Box::from(slice),
1441 #[cfg(not(no_global_oom_handling))]
1442 #[stable(feature = "box_from_slice", since = "1.17.0")]
1443 impl From<&str> for Box<str> {
1444 /// Converts a `&str` into a `Box<str>`
1446 /// This conversion allocates on the heap
1447 /// and performs a copy of `s`.
1452 /// let boxed: Box<str> = Box::from("hello");
1453 /// println!("{}", boxed);
1456 fn from(s: &str) -> Box<str> {
1457 unsafe { from_boxed_utf8_unchecked(Box::from(s.as_bytes())) }
1461 #[cfg(not(no_global_oom_handling))]
1462 #[stable(feature = "box_from_cow", since = "1.45.0")]
1463 impl From<Cow<'_, str>> for Box<str> {
1464 /// Converts a `Cow<'_, str>` into a `Box<str>`
1466 /// When `cow` is the `Cow::Borrowed` variant, this
1467 /// conversion allocates on the heap and copies the
1468 /// underlying `str`. Otherwise, it will try to reuse the owned
1469 /// `String`'s allocation.
1474 /// use std::borrow::Cow;
1476 /// let unboxed = Cow::Borrowed("hello");
1477 /// let boxed: Box<str> = Box::from(unboxed);
1478 /// println!("{}", boxed);
1482 /// # use std::borrow::Cow;
1483 /// let unboxed = Cow::Owned("hello".to_string());
1484 /// let boxed: Box<str> = Box::from(unboxed);
1485 /// println!("{}", boxed);
1488 fn from(cow: Cow<'_, str>) -> Box<str> {
1490 Cow::Borrowed(s) => Box::from(s),
1491 Cow::Owned(s) => Box::from(s),
1496 #[stable(feature = "boxed_str_conv", since = "1.19.0")]
1497 impl<A: Allocator> From<Box<str, A>> for Box<[u8], A> {
1498 /// Converts a `Box<str>` into a `Box<[u8]>`
1500 /// This conversion does not allocate on the heap and happens in place.
1504 /// // create a Box<str> which will be used to create a Box<[u8]>
1505 /// let boxed: Box<str> = Box::from("hello");
1506 /// let boxed_str: Box<[u8]> = Box::from(boxed);
1508 /// // create a &[u8] which will be used to create a Box<[u8]>
1509 /// let slice: &[u8] = &[104, 101, 108, 108, 111];
1510 /// let boxed_slice = Box::from(slice);
1512 /// assert_eq!(boxed_slice, boxed_str);
1515 fn from(s: Box<str, A>) -> Self {
1516 let (raw, alloc) = Box::into_raw_with_allocator(s);
1517 unsafe { Box::from_raw_in(raw as *mut [u8], alloc) }
1521 #[cfg(not(no_global_oom_handling))]
1522 #[stable(feature = "box_from_array", since = "1.45.0")]
1523 impl<T, const N: usize> From<[T; N]> for Box<[T]> {
1524 /// Converts a `[T; N]` into a `Box<[T]>`
1526 /// This conversion moves the array to newly heap-allocated memory.
1531 /// let boxed: Box<[u8]> = Box::from([4, 2]);
1532 /// println!("{:?}", boxed);
1534 fn from(array: [T; N]) -> Box<[T]> {
1539 #[stable(feature = "boxed_slice_try_from", since = "1.43.0")]
1540 impl<T, const N: usize> TryFrom<Box<[T]>> for Box<[T; N]> {
1541 type Error = Box<[T]>;
1543 /// Attempts to convert a `Box<[T]>` into a `Box<[T; N]>`.
1545 /// The conversion occurs in-place and does not require a
1546 /// new memory allocation.
1550 /// Returns the old `Box<[T]>` in the `Err` variant if
1551 /// `boxed_slice.len()` does not equal `N`.
1552 fn try_from(boxed_slice: Box<[T]>) -> Result<Self, Self::Error> {
1553 if boxed_slice.len() == N {
1554 Ok(unsafe { Box::from_raw(Box::into_raw(boxed_slice) as *mut [T; N]) })
1561 impl<A: Allocator> Box<dyn Any, A> {
1562 /// Attempt to downcast the box to a concrete type.
1567 /// use std::any::Any;
1569 /// fn print_if_string(value: Box<dyn Any>) {
1570 /// if let Ok(string) = value.downcast::<String>() {
1571 /// println!("String ({}): {}", string.len(), string);
1575 /// let my_string = "Hello World".to_string();
1576 /// print_if_string(Box::new(my_string));
1577 /// print_if_string(Box::new(0i8));
1580 #[stable(feature = "rust1", since = "1.0.0")]
1581 pub fn downcast<T: Any>(self) -> Result<Box<T, A>, Self> {
1582 if self.is::<T>() { unsafe { Ok(self.downcast_unchecked::<T>()) } } else { Err(self) }
1585 /// Downcasts the box to a concrete type.
1587 /// For a safe alternative see [`downcast`].
1592 /// #![feature(downcast_unchecked)]
1594 /// use std::any::Any;
1596 /// let x: Box<dyn Any> = Box::new(1_usize);
1599 /// assert_eq!(*x.downcast_unchecked::<usize>(), 1);
1605 /// The contained value must be of type `T`. Calling this method
1606 /// with the incorrect type is *undefined behavior*.
1608 /// [`downcast`]: Self::downcast
1610 #[unstable(feature = "downcast_unchecked", issue = "90850")]
1611 pub unsafe fn downcast_unchecked<T: Any>(self) -> Box<T, A> {
1612 debug_assert!(self.is::<T>());
1614 let (raw, alloc): (*mut dyn Any, _) = Box::into_raw_with_allocator(self);
1615 Box::from_raw_in(raw as *mut T, alloc)
1620 impl<A: Allocator> Box<dyn Any + Send, A> {
1621 /// Attempt to downcast the box to a concrete type.
1626 /// use std::any::Any;
1628 /// fn print_if_string(value: Box<dyn Any + Send>) {
1629 /// if let Ok(string) = value.downcast::<String>() {
1630 /// println!("String ({}): {}", string.len(), string);
1634 /// let my_string = "Hello World".to_string();
1635 /// print_if_string(Box::new(my_string));
1636 /// print_if_string(Box::new(0i8));
1639 #[stable(feature = "rust1", since = "1.0.0")]
1640 pub fn downcast<T: Any>(self) -> Result<Box<T, A>, Self> {
1641 if self.is::<T>() { unsafe { Ok(self.downcast_unchecked::<T>()) } } else { Err(self) }
1644 /// Downcasts the box to a concrete type.
1646 /// For a safe alternative see [`downcast`].
1651 /// #![feature(downcast_unchecked)]
1653 /// use std::any::Any;
1655 /// let x: Box<dyn Any + Send> = Box::new(1_usize);
1658 /// assert_eq!(*x.downcast_unchecked::<usize>(), 1);
1664 /// The contained value must be of type `T`. Calling this method
1665 /// with the incorrect type is *undefined behavior*.
1667 /// [`downcast`]: Self::downcast
1669 #[unstable(feature = "downcast_unchecked", issue = "90850")]
1670 pub unsafe fn downcast_unchecked<T: Any>(self) -> Box<T, A> {
1671 debug_assert!(self.is::<T>());
1673 let (raw, alloc): (*mut (dyn Any + Send), _) = Box::into_raw_with_allocator(self);
1674 Box::from_raw_in(raw as *mut T, alloc)
1679 impl<A: Allocator> Box<dyn Any + Send + Sync, A> {
1680 /// Attempt to downcast the box to a concrete type.
1685 /// use std::any::Any;
1687 /// fn print_if_string(value: Box<dyn Any + Send + Sync>) {
1688 /// if let Ok(string) = value.downcast::<String>() {
1689 /// println!("String ({}): {}", string.len(), string);
1693 /// let my_string = "Hello World".to_string();
1694 /// print_if_string(Box::new(my_string));
1695 /// print_if_string(Box::new(0i8));
1698 #[stable(feature = "box_send_sync_any_downcast", since = "1.51.0")]
1699 pub fn downcast<T: Any>(self) -> Result<Box<T, A>, Self> {
1700 if self.is::<T>() { unsafe { Ok(self.downcast_unchecked::<T>()) } } else { Err(self) }
1703 /// Downcasts the box to a concrete type.
1705 /// For a safe alternative see [`downcast`].
1710 /// #![feature(downcast_unchecked)]
1712 /// use std::any::Any;
1714 /// let x: Box<dyn Any + Send + Sync> = Box::new(1_usize);
1717 /// assert_eq!(*x.downcast_unchecked::<usize>(), 1);
1723 /// The contained value must be of type `T`. Calling this method
1724 /// with the incorrect type is *undefined behavior*.
1726 /// [`downcast`]: Self::downcast
1728 #[unstable(feature = "downcast_unchecked", issue = "90850")]
1729 pub unsafe fn downcast_unchecked<T: Any>(self) -> Box<T, A> {
1730 debug_assert!(self.is::<T>());
1732 let (raw, alloc): (*mut (dyn Any + Send + Sync), _) =
1733 Box::into_raw_with_allocator(self);
1734 Box::from_raw_in(raw as *mut T, alloc)
1739 #[stable(feature = "rust1", since = "1.0.0")]
1740 impl<T: fmt::Display + ?Sized, A: Allocator> fmt::Display for Box<T, A> {
1741 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
1742 fmt::Display::fmt(&**self, f)
1746 #[stable(feature = "rust1", since = "1.0.0")]
1747 impl<T: fmt::Debug + ?Sized, A: Allocator> fmt::Debug for Box<T, A> {
1748 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
1749 fmt::Debug::fmt(&**self, f)
1753 #[stable(feature = "rust1", since = "1.0.0")]
1754 impl<T: ?Sized, A: Allocator> fmt::Pointer for Box<T, A> {
1755 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
1756 // It's not possible to extract the inner Uniq directly from the Box,
1757 // instead we cast it to a *const which aliases the Unique
1758 let ptr: *const T = &**self;
1759 fmt::Pointer::fmt(&ptr, f)
1763 #[stable(feature = "rust1", since = "1.0.0")]
1764 #[rustc_const_unstable(feature = "const_box", issue = "92521")]
1765 impl<T: ?Sized, A: Allocator> const Deref for Box<T, A> {
1768 fn deref(&self) -> &T {
1773 #[stable(feature = "rust1", since = "1.0.0")]
1774 #[rustc_const_unstable(feature = "const_box", issue = "92521")]
1775 impl<T: ?Sized, A: Allocator> const DerefMut for Box<T, A> {
1776 fn deref_mut(&mut self) -> &mut T {
1781 #[unstable(feature = "receiver_trait", issue = "none")]
1782 impl<T: ?Sized, A: Allocator> Receiver for Box<T, A> {}
1784 #[stable(feature = "rust1", since = "1.0.0")]
1785 impl<I: Iterator + ?Sized, A: Allocator> Iterator for Box<I, A> {
1786 type Item = I::Item;
1787 fn next(&mut self) -> Option<I::Item> {
1790 fn size_hint(&self) -> (usize, Option<usize>) {
1791 (**self).size_hint()
1793 fn nth(&mut self, n: usize) -> Option<I::Item> {
1796 fn last(self) -> Option<I::Item> {
1803 fn last(self) -> Option<Self::Item>;
1806 impl<I: Iterator + ?Sized, A: Allocator> BoxIter for Box<I, A> {
1807 type Item = I::Item;
1808 default fn last(self) -> Option<I::Item> {
1810 fn some<T>(_: Option<T>, x: T) -> Option<T> {
1814 self.fold(None, some)
1818 /// Specialization for sized `I`s that uses `I`s implementation of `last()`
1819 /// instead of the default.
1820 #[stable(feature = "rust1", since = "1.0.0")]
1821 impl<I: Iterator, A: Allocator> BoxIter for Box<I, A> {
1822 fn last(self) -> Option<I::Item> {
1827 #[stable(feature = "rust1", since = "1.0.0")]
1828 impl<I: DoubleEndedIterator + ?Sized, A: Allocator> DoubleEndedIterator for Box<I, A> {
1829 fn next_back(&mut self) -> Option<I::Item> {
1830 (**self).next_back()
1832 fn nth_back(&mut self, n: usize) -> Option<I::Item> {
1833 (**self).nth_back(n)
1836 #[stable(feature = "rust1", since = "1.0.0")]
1837 impl<I: ExactSizeIterator + ?Sized, A: Allocator> ExactSizeIterator for Box<I, A> {
1838 fn len(&self) -> usize {
1841 fn is_empty(&self) -> bool {
1846 #[stable(feature = "fused", since = "1.26.0")]
1847 impl<I: FusedIterator + ?Sized, A: Allocator> FusedIterator for Box<I, A> {}
1849 #[stable(feature = "boxed_closure_impls", since = "1.35.0")]
1850 impl<Args, F: FnOnce<Args> + ?Sized, A: Allocator> FnOnce<Args> for Box<F, A> {
1851 type Output = <F as FnOnce<Args>>::Output;
1853 extern "rust-call" fn call_once(self, args: Args) -> Self::Output {
1854 <F as FnOnce<Args>>::call_once(*self, args)
1858 #[stable(feature = "boxed_closure_impls", since = "1.35.0")]
1859 impl<Args, F: FnMut<Args> + ?Sized, A: Allocator> FnMut<Args> for Box<F, A> {
1860 extern "rust-call" fn call_mut(&mut self, args: Args) -> Self::Output {
1861 <F as FnMut<Args>>::call_mut(self, args)
1865 #[stable(feature = "boxed_closure_impls", since = "1.35.0")]
1866 impl<Args, F: Fn<Args> + ?Sized, A: Allocator> Fn<Args> for Box<F, A> {
1867 extern "rust-call" fn call(&self, args: Args) -> Self::Output {
1868 <F as Fn<Args>>::call(self, args)
1872 #[unstable(feature = "coerce_unsized", issue = "27732")]
1873 impl<T: ?Sized + Unsize<U>, U: ?Sized, A: Allocator> CoerceUnsized<Box<U, A>> for Box<T, A> {}
1875 #[unstable(feature = "dispatch_from_dyn", issue = "none")]
1876 impl<T: ?Sized + Unsize<U>, U: ?Sized> DispatchFromDyn<Box<U>> for Box<T, Global> {}
1878 #[cfg(not(no_global_oom_handling))]
1879 #[stable(feature = "boxed_slice_from_iter", since = "1.32.0")]
1880 impl<I> FromIterator<I> for Box<[I]> {
1881 fn from_iter<T: IntoIterator<Item = I>>(iter: T) -> Self {
1882 iter.into_iter().collect::<Vec<_>>().into_boxed_slice()
1886 #[cfg(not(no_global_oom_handling))]
1887 #[stable(feature = "box_slice_clone", since = "1.3.0")]
1888 impl<T: Clone, A: Allocator + Clone> Clone for Box<[T], A> {
1889 fn clone(&self) -> Self {
1890 let alloc = Box::allocator(self).clone();
1891 self.to_vec_in(alloc).into_boxed_slice()
1894 fn clone_from(&mut self, other: &Self) {
1895 if self.len() == other.len() {
1896 self.clone_from_slice(&other);
1898 *self = other.clone();
1903 #[stable(feature = "box_borrow", since = "1.1.0")]
1904 impl<T: ?Sized, A: Allocator> borrow::Borrow<T> for Box<T, A> {
1905 fn borrow(&self) -> &T {
1910 #[stable(feature = "box_borrow", since = "1.1.0")]
1911 impl<T: ?Sized, A: Allocator> borrow::BorrowMut<T> for Box<T, A> {
1912 fn borrow_mut(&mut self) -> &mut T {
1917 #[stable(since = "1.5.0", feature = "smart_ptr_as_ref")]
1918 impl<T: ?Sized, A: Allocator> AsRef<T> for Box<T, A> {
1919 fn as_ref(&self) -> &T {
1924 #[stable(since = "1.5.0", feature = "smart_ptr_as_ref")]
1925 impl<T: ?Sized, A: Allocator> AsMut<T> for Box<T, A> {
1926 fn as_mut(&mut self) -> &mut T {
1933 * We could have chosen not to add this impl, and instead have written a
1934 * function of Pin<Box<T>> to Pin<T>. Such a function would not be sound,
1935 * because Box<T> implements Unpin even when T does not, as a result of
1938 * We chose this API instead of the alternative for a few reasons:
1939 * - Logically, it is helpful to understand pinning in regard to the
1940 * memory region being pointed to. For this reason none of the
1941 * standard library pointer types support projecting through a pin
1942 * (Box<T> is the only pointer type in std for which this would be
1944 * - It is in practice very useful to have Box<T> be unconditionally
1945 * Unpin because of trait objects, for which the structural auto
1946 * trait functionality does not apply (e.g., Box<dyn Foo> would
1947 * otherwise not be Unpin).
1949 * Another type with the same semantics as Box but only a conditional
1950 * implementation of `Unpin` (where `T: Unpin`) would be valid/safe, and
1951 * could have a method to project a Pin<T> from it.
1953 #[stable(feature = "pin", since = "1.33.0")]
1954 #[rustc_const_unstable(feature = "const_box", issue = "92521")]
1955 impl<T: ?Sized, A: Allocator> const Unpin for Box<T, A> where A: 'static {}
1957 #[unstable(feature = "generator_trait", issue = "43122")]
1958 impl<G: ?Sized + Generator<R> + Unpin, R, A: Allocator> Generator<R> for Box<G, A>
1962 type Yield = G::Yield;
1963 type Return = G::Return;
1965 fn resume(mut self: Pin<&mut Self>, arg: R) -> GeneratorState<Self::Yield, Self::Return> {
1966 G::resume(Pin::new(&mut *self), arg)
1970 #[unstable(feature = "generator_trait", issue = "43122")]
1971 impl<G: ?Sized + Generator<R>, R, A: Allocator> Generator<R> for Pin<Box<G, A>>
1975 type Yield = G::Yield;
1976 type Return = G::Return;
1978 fn resume(mut self: Pin<&mut Self>, arg: R) -> GeneratorState<Self::Yield, Self::Return> {
1979 G::resume((*self).as_mut(), arg)
1983 #[stable(feature = "futures_api", since = "1.36.0")]
1984 impl<F: ?Sized + Future + Unpin, A: Allocator> Future for Box<F, A>
1988 type Output = F::Output;
1990 fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
1991 F::poll(Pin::new(&mut *self), cx)
1995 #[unstable(feature = "async_stream", issue = "79024")]
1996 impl<S: ?Sized + Stream + Unpin> Stream for Box<S> {
1997 type Item = S::Item;
1999 fn poll_next(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Option<Self::Item>> {
2000 Pin::new(&mut **self).poll_next(cx)
2003 fn size_hint(&self) -> (usize, Option<usize>) {
2004 (**self).size_hint()