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.
9 //! Move a value from the stack to the heap by creating a [`Box`]:
13 //! let boxed: Box<u8> = Box::new(val);
16 //! Move a value from a [`Box`] back to the stack by [dereferencing]:
19 //! let boxed: Box<u8> = Box::new(5);
20 //! let val: u8 = *boxed;
23 //! Creating a recursive data structure:
28 //! 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);
38 //! This will print `Cons(1, Cons(2, Nil))`.
40 //! Recursive structures must be boxed, because if the definition of `Cons`
43 //! ```compile_fail,E0072
49 //! It wouldn't work. This is because the size of a `List` depends on how many
50 //! elements are in the list, and so we don't know how much memory to allocate
51 //! for a `Cons`. By introducing a [`Box<T>`], which has a defined size, we know how
52 //! big `Cons` needs to be.
56 //! For non-zero-sized values, a [`Box`] will use the [`Global`] allocator for
57 //! its allocation. It is valid to convert both ways between a [`Box`] and a
58 //! raw pointer allocated with the [`Global`] allocator, given that the
59 //! [`Layout`] used with the allocator is correct for the type. More precisely,
60 //! a `value: *mut T` that has been allocated with the [`Global`] allocator
61 //! with `Layout::for_value(&*value)` may be converted into a box using
62 //! [`Box::<T>::from_raw(value)`]. Conversely, the memory backing a `value: *mut
63 //! T` obtained from [`Box::<T>::into_raw`] may be deallocated using the
64 //! [`Global`] allocator with [`Layout::for_value(&*value)`].
67 //! [dereferencing]: ../../std/ops/trait.Deref.html
68 //! [`Box`]: struct.Box.html
69 //! [`Box<T>`]: struct.Box.html
70 //! [`Box::<T>::from_raw(value)`]: struct.Box.html#method.from_raw
71 //! [`Box::<T>::into_raw`]: struct.Box.html#method.into_raw
72 //! [`Global`]: ../alloc/struct.Global.html
73 //! [`Layout`]: ../alloc/struct.Layout.html
74 //! [`Layout::for_value(&*value)`]: ../alloc/struct.Layout.html#method.for_value
76 #![stable(feature = "rust1", since = "1.0.0")]
79 use core::array::LengthAtMost32;
81 use core::cmp::Ordering;
82 use core::convert::{From, TryFrom};
84 use core::future::Future;
85 use core::hash::{Hash, Hasher};
86 use core::iter::{Iterator, FromIterator, FusedIterator};
87 use core::marker::{Unpin, Unsize};
91 CoerceUnsized, DispatchFromDyn, Deref, DerefMut, Receiver, Generator, GeneratorState
93 use core::ptr::{self, NonNull, Unique};
95 use core::task::{Context, Poll};
97 use crate::alloc::{self, Global, Alloc};
99 use crate::raw_vec::RawVec;
100 use crate::str::from_boxed_utf8_unchecked;
102 /// A pointer type for heap allocation.
104 /// See the [module-level documentation](../../std/boxed/index.html) for more.
105 #[lang = "owned_box"]
107 #[stable(feature = "rust1", since = "1.0.0")]
108 pub struct Box<T: ?Sized>(Unique<T>);
111 /// Allocates memory on the heap and then places `x` into it.
113 /// This doesn't actually allocate if `T` is zero-sized.
118 /// let five = Box::new(5);
120 #[stable(feature = "rust1", since = "1.0.0")]
122 pub fn new(x: T) -> Box<T> {
126 /// Constructs a new box with uninitialized contents.
131 /// #![feature(new_uninit)]
133 /// let mut five = Box::<u32>::new_uninit();
135 /// let five = unsafe {
136 /// // Deferred initialization:
137 /// five.as_mut_ptr().write(5);
139 /// five.assume_init()
142 /// assert_eq!(*five, 5)
144 #[unstable(feature = "new_uninit", issue = "63291")]
145 pub fn new_uninit() -> Box<mem::MaybeUninit<T>> {
146 let layout = alloc::Layout::new::<mem::MaybeUninit<T>>();
149 .unwrap_or_else(|_| alloc::handle_alloc_error(layout))
151 Box(ptr.cast().into())
154 /// Constructs a new `Pin<Box<T>>`. If `T` does not implement `Unpin`, then
155 /// `x` will be pinned in memory and unable to be moved.
156 #[stable(feature = "pin", since = "1.33.0")]
158 pub fn pin(x: T) -> Pin<Box<T>> {
164 /// Constructs a new boxed slice with uninitialized contents.
169 /// #![feature(new_uninit)]
171 /// let mut values = Box::<[u32]>::new_uninit_slice(3);
173 /// let values = unsafe {
174 /// // Deferred initialization:
175 /// values[0].as_mut_ptr().write(1);
176 /// values[1].as_mut_ptr().write(2);
177 /// values[2].as_mut_ptr().write(3);
179 /// values.assume_init()
182 /// assert_eq!(*values, [1, 2, 3])
184 #[unstable(feature = "new_uninit", issue = "63291")]
185 pub fn new_uninit_slice(len: usize) -> Box<[mem::MaybeUninit<T>]> {
186 let layout = alloc::Layout::array::<mem::MaybeUninit<T>>(len).unwrap();
187 let ptr = unsafe { alloc::alloc(layout) };
188 let unique = Unique::new(ptr).unwrap_or_else(|| alloc::handle_alloc_error(layout));
189 let slice = unsafe { slice::from_raw_parts_mut(unique.cast().as_ptr(), len) };
190 Box(Unique::from(slice))
194 impl<T> Box<mem::MaybeUninit<T>> {
195 /// Converts to `Box<T>`.
199 /// As with [`MaybeUninit::assume_init`],
200 /// it is up to the caller to guarantee that the value
201 /// really is in an initialized state.
202 /// Calling this when the content is not yet fully initialized
203 /// causes immediate undefined behavior.
205 /// [`MaybeUninit::assume_init`]: ../../std/mem/union.MaybeUninit.html#method.assume_init
210 /// #![feature(new_uninit)]
212 /// let mut five = Box::<u32>::new_uninit();
214 /// let five: Box<u32> = unsafe {
215 /// // Deferred initialization:
216 /// five.as_mut_ptr().write(5);
218 /// five.assume_init()
221 /// assert_eq!(*five, 5)
223 #[unstable(feature = "new_uninit", issue = "63291")]
225 pub unsafe fn assume_init(self) -> Box<T> {
226 Box(Box::into_unique(self).cast())
230 impl<T> Box<[mem::MaybeUninit<T>]> {
231 /// Converts to `Box<[T]>`.
235 /// As with [`MaybeUninit::assume_init`],
236 /// it is up to the caller to guarantee that the values
237 /// really are in an initialized state.
238 /// Calling this when the content is not yet fully initialized
239 /// causes immediate undefined behavior.
241 /// [`MaybeUninit::assume_init`]: ../../std/mem/union.MaybeUninit.html#method.assume_init
246 /// #![feature(new_uninit)]
248 /// let mut values = Box::<[u32]>::new_uninit_slice(3);
250 /// let values = unsafe {
251 /// // Deferred initialization:
252 /// values[0].as_mut_ptr().write(1);
253 /// values[1].as_mut_ptr().write(2);
254 /// values[2].as_mut_ptr().write(3);
256 /// values.assume_init()
259 /// assert_eq!(*values, [1, 2, 3])
261 #[unstable(feature = "new_uninit", issue = "63291")]
263 pub unsafe fn assume_init(self) -> Box<[T]> {
264 Box(Unique::new_unchecked(Box::into_raw(self) as _))
268 impl<T: ?Sized> Box<T> {
269 /// Constructs a box from a raw pointer.
271 /// After calling this function, the raw pointer is owned by the
272 /// resulting `Box`. Specifically, the `Box` destructor will call
273 /// the destructor of `T` and free the allocated memory. For this
274 /// to be safe, the memory must have been allocated in accordance
275 /// with the [memory layout] used by `Box` .
279 /// This function is unsafe because improper use may lead to
280 /// memory problems. For example, a double-free may occur if the
281 /// function is called twice on the same raw pointer.
284 /// Recreate a `Box` which was previously converted to a raw pointer
285 /// using [`Box::into_raw`]:
287 /// let x = Box::new(5);
288 /// let ptr = Box::into_raw(x);
289 /// let x = unsafe { Box::from_raw(ptr) };
291 /// Manually create a `Box` from scratch by using the global allocator:
293 /// use std::alloc::{alloc, Layout};
296 /// let ptr = alloc(Layout::new::<i32>()) as *mut i32;
298 /// let x = Box::from_raw(ptr);
302 /// [memory layout]: index.html#memory-layout
303 /// [`Layout`]: ../alloc/struct.Layout.html
304 /// [`Box::into_raw`]: struct.Box.html#method.into_raw
305 #[stable(feature = "box_raw", since = "1.4.0")]
307 pub unsafe fn from_raw(raw: *mut T) -> Self {
308 Box(Unique::new_unchecked(raw))
311 /// Consumes the `Box`, returning a wrapped raw pointer.
313 /// The pointer will be properly aligned and non-null.
315 /// After calling this function, the caller is responsible for the
316 /// memory previously managed by the `Box`. In particular, the
317 /// caller should properly destroy `T` and release the memory, taking
318 /// into account the [memory layout] used by `Box`. The easiest way to
319 /// do this is to convert the raw pointer back into a `Box` with the
320 /// [`Box::from_raw`] function, allowing the `Box` destructor to perform
323 /// Note: this is an associated function, which means that you have
324 /// to call it as `Box::into_raw(b)` instead of `b.into_raw()`. This
325 /// is so that there is no conflict with a method on the inner type.
328 /// Converting the raw pointer back into a `Box` with [`Box::from_raw`]
329 /// for automatic cleanup:
331 /// let x = Box::new(String::from("Hello"));
332 /// let ptr = Box::into_raw(x);
333 /// let x = unsafe { Box::from_raw(ptr) };
335 /// Manual cleanup by explicitly running the destructor and deallocating
338 /// use std::alloc::{dealloc, Layout};
341 /// let x = Box::new(String::from("Hello"));
342 /// let p = Box::into_raw(x);
344 /// ptr::drop_in_place(p);
345 /// dealloc(p as *mut u8, Layout::new::<String>());
349 /// [memory layout]: index.html#memory-layout
350 /// [`Box::from_raw`]: struct.Box.html#method.from_raw
351 #[stable(feature = "box_raw", since = "1.4.0")]
353 pub fn into_raw(b: Box<T>) -> *mut T {
354 Box::into_raw_non_null(b).as_ptr()
357 /// Consumes the `Box`, returning the wrapped pointer as `NonNull<T>`.
359 /// After calling this function, the caller is responsible for the
360 /// memory previously managed by the `Box`. In particular, the
361 /// caller should properly destroy `T` and release the memory. The
362 /// easiest way to do so is to convert the `NonNull<T>` pointer
363 /// into a raw pointer and back into a `Box` with the [`Box::from_raw`]
366 /// Note: this is an associated function, which means that you have
367 /// to call it as `Box::into_raw_non_null(b)`
368 /// instead of `b.into_raw_non_null()`. This
369 /// is so that there is no conflict with a method on the inner type.
371 /// [`Box::from_raw`]: struct.Box.html#method.from_raw
376 /// #![feature(box_into_raw_non_null)]
379 /// let x = Box::new(5);
380 /// let ptr = Box::into_raw_non_null(x);
382 /// // Clean up the memory by converting the NonNull pointer back
383 /// // into a Box and letting the Box be dropped.
384 /// let x = unsafe { Box::from_raw(ptr.as_ptr()) };
387 #[unstable(feature = "box_into_raw_non_null", issue = "47336")]
389 pub fn into_raw_non_null(b: Box<T>) -> NonNull<T> {
390 Box::into_unique(b).into()
393 #[unstable(feature = "ptr_internals", issue = "0", reason = "use into_raw_non_null instead")]
396 pub fn into_unique(b: Box<T>) -> Unique<T> {
397 let mut unique = b.0;
399 // Box is kind-of a library type, but recognized as a "unique pointer" by
400 // Stacked Borrows. This function here corresponds to "reborrowing to
401 // a raw pointer", but there is no actual reborrow here -- so
402 // without some care, the pointer we are returning here still carries
403 // the tag of `b`, with `Unique` permission.
404 // We round-trip through a mutable reference to avoid that.
405 unsafe { Unique::new_unchecked(unique.as_mut() as *mut T) }
408 /// Consumes and leaks the `Box`, returning a mutable reference,
409 /// `&'a mut T`. Note that the type `T` must outlive the chosen lifetime
410 /// `'a`. If the type has only static references, or none at all, then this
411 /// may be chosen to be `'static`.
413 /// This function is mainly useful for data that lives for the remainder of
414 /// the program's life. Dropping the returned reference will cause a memory
415 /// leak. If this is not acceptable, the reference should first be wrapped
416 /// with the [`Box::from_raw`] function producing a `Box`. This `Box` can
417 /// then be dropped which will properly destroy `T` and release the
418 /// allocated memory.
420 /// Note: this is an associated function, which means that you have
421 /// to call it as `Box::leak(b)` instead of `b.leak()`. This
422 /// is so that there is no conflict with a method on the inner type.
424 /// [`Box::from_raw`]: struct.Box.html#method.from_raw
432 /// let x = Box::new(41);
433 /// let static_ref: &'static mut usize = Box::leak(x);
434 /// *static_ref += 1;
435 /// assert_eq!(*static_ref, 42);
443 /// let x = vec![1, 2, 3].into_boxed_slice();
444 /// let static_ref = Box::leak(x);
445 /// static_ref[0] = 4;
446 /// assert_eq!(*static_ref, [4, 2, 3]);
449 #[stable(feature = "box_leak", since = "1.26.0")]
451 pub fn leak<'a>(b: Box<T>) -> &'a mut T
453 T: 'a // Technically not needed, but kept to be explicit.
455 unsafe { &mut *Box::into_raw(b) }
458 /// Converts a `Box<T>` into a `Pin<Box<T>>`
460 /// This conversion does not allocate on the heap and happens in place.
462 /// This is also available via [`From`].
463 #[unstable(feature = "box_into_pin", issue = "62370")]
464 pub fn into_pin(boxed: Box<T>) -> Pin<Box<T>> {
465 // It's not possible to move or replace the insides of a `Pin<Box<T>>`
466 // when `T: !Unpin`, so it's safe to pin it directly without any
467 // additional requirements.
468 unsafe { Pin::new_unchecked(boxed) }
472 #[stable(feature = "rust1", since = "1.0.0")]
473 unsafe impl<#[may_dangle] T: ?Sized> Drop for Box<T> {
475 // FIXME: Do nothing, drop is currently performed by compiler.
479 #[stable(feature = "rust1", since = "1.0.0")]
480 impl<T: Default> Default for Box<T> {
481 /// Creates a `Box<T>`, with the `Default` value for T.
482 fn default() -> Box<T> {
483 box Default::default()
487 #[stable(feature = "rust1", since = "1.0.0")]
488 impl<T> Default for Box<[T]> {
489 fn default() -> Box<[T]> {
490 Box::<[T; 0]>::new([])
494 #[stable(feature = "default_box_extra", since = "1.17.0")]
495 impl Default for Box<str> {
496 fn default() -> Box<str> {
497 unsafe { from_boxed_utf8_unchecked(Default::default()) }
501 #[stable(feature = "rust1", since = "1.0.0")]
502 impl<T: Clone> Clone for Box<T> {
503 /// Returns a new box with a `clone()` of this box's contents.
508 /// let x = Box::new(5);
509 /// let y = x.clone();
511 /// // The value is the same
512 /// assert_eq!(x, y);
514 /// // But they are unique objects
515 /// assert_ne!(&*x as *const i32, &*y as *const i32);
519 fn clone(&self) -> Box<T> {
520 box { (**self).clone() }
523 /// Copies `source`'s contents into `self` without creating a new allocation.
528 /// let x = Box::new(5);
529 /// let mut y = Box::new(10);
530 /// let yp: *const i32 = &*y;
532 /// y.clone_from(&x);
534 /// // The value is the same
535 /// assert_eq!(x, y);
537 /// // And no allocation occurred
538 /// assert_eq!(yp, &*y);
541 fn clone_from(&mut self, source: &Box<T>) {
542 (**self).clone_from(&(**source));
547 #[stable(feature = "box_slice_clone", since = "1.3.0")]
548 impl Clone for Box<str> {
549 fn clone(&self) -> Self {
550 // this makes a copy of the data
551 let buf: Box<[u8]> = self.as_bytes().into();
553 from_boxed_utf8_unchecked(buf)
558 #[stable(feature = "rust1", since = "1.0.0")]
559 impl<T: ?Sized + PartialEq> PartialEq for Box<T> {
561 fn eq(&self, other: &Box<T>) -> bool {
562 PartialEq::eq(&**self, &**other)
565 fn ne(&self, other: &Box<T>) -> bool {
566 PartialEq::ne(&**self, &**other)
569 #[stable(feature = "rust1", since = "1.0.0")]
570 impl<T: ?Sized + PartialOrd> PartialOrd for Box<T> {
572 fn partial_cmp(&self, other: &Box<T>) -> Option<Ordering> {
573 PartialOrd::partial_cmp(&**self, &**other)
576 fn lt(&self, other: &Box<T>) -> bool {
577 PartialOrd::lt(&**self, &**other)
580 fn le(&self, other: &Box<T>) -> bool {
581 PartialOrd::le(&**self, &**other)
584 fn ge(&self, other: &Box<T>) -> bool {
585 PartialOrd::ge(&**self, &**other)
588 fn gt(&self, other: &Box<T>) -> bool {
589 PartialOrd::gt(&**self, &**other)
592 #[stable(feature = "rust1", since = "1.0.0")]
593 impl<T: ?Sized + Ord> Ord for Box<T> {
595 fn cmp(&self, other: &Box<T>) -> Ordering {
596 Ord::cmp(&**self, &**other)
599 #[stable(feature = "rust1", since = "1.0.0")]
600 impl<T: ?Sized + Eq> Eq for Box<T> {}
602 #[stable(feature = "rust1", since = "1.0.0")]
603 impl<T: ?Sized + Hash> Hash for Box<T> {
604 fn hash<H: Hasher>(&self, state: &mut H) {
605 (**self).hash(state);
609 #[stable(feature = "indirect_hasher_impl", since = "1.22.0")]
610 impl<T: ?Sized + Hasher> Hasher for Box<T> {
611 fn finish(&self) -> u64 {
614 fn write(&mut self, bytes: &[u8]) {
615 (**self).write(bytes)
617 fn write_u8(&mut self, i: u8) {
620 fn write_u16(&mut self, i: u16) {
621 (**self).write_u16(i)
623 fn write_u32(&mut self, i: u32) {
624 (**self).write_u32(i)
626 fn write_u64(&mut self, i: u64) {
627 (**self).write_u64(i)
629 fn write_u128(&mut self, i: u128) {
630 (**self).write_u128(i)
632 fn write_usize(&mut self, i: usize) {
633 (**self).write_usize(i)
635 fn write_i8(&mut self, i: i8) {
638 fn write_i16(&mut self, i: i16) {
639 (**self).write_i16(i)
641 fn write_i32(&mut self, i: i32) {
642 (**self).write_i32(i)
644 fn write_i64(&mut self, i: i64) {
645 (**self).write_i64(i)
647 fn write_i128(&mut self, i: i128) {
648 (**self).write_i128(i)
650 fn write_isize(&mut self, i: isize) {
651 (**self).write_isize(i)
655 #[stable(feature = "from_for_ptrs", since = "1.6.0")]
656 impl<T> From<T> for Box<T> {
657 /// Converts a generic type `T` into a `Box<T>`
659 /// The conversion allocates on the heap and moves `t`
660 /// from the stack into it.
665 /// let boxed = Box::new(5);
667 /// assert_eq!(Box::from(x), boxed);
669 fn from(t: T) -> Self {
674 #[stable(feature = "pin", since = "1.33.0")]
675 impl<T: ?Sized> From<Box<T>> for Pin<Box<T>> {
676 /// Converts a `Box<T>` into a `Pin<Box<T>>`
678 /// This conversion does not allocate on the heap and happens in place.
679 fn from(boxed: Box<T>) -> Self {
684 #[stable(feature = "box_from_slice", since = "1.17.0")]
685 impl<T: Copy> From<&[T]> for Box<[T]> {
686 /// Converts a `&[T]` into a `Box<[T]>`
688 /// This conversion allocates on the heap
689 /// and performs a copy of `slice`.
693 /// // create a &[u8] which will be used to create a Box<[u8]>
694 /// let slice: &[u8] = &[104, 101, 108, 108, 111];
695 /// let boxed_slice: Box<[u8]> = Box::from(slice);
697 /// println!("{:?}", boxed_slice);
699 fn from(slice: &[T]) -> Box<[T]> {
700 let len = slice.len();
701 let buf = RawVec::with_capacity(len);
703 ptr::copy_nonoverlapping(slice.as_ptr(), buf.ptr(), len);
709 #[stable(feature = "box_from_slice", since = "1.17.0")]
710 impl From<&str> for Box<str> {
711 /// Converts a `&str` into a `Box<str>`
713 /// This conversion allocates on the heap
714 /// and performs a copy of `s`.
718 /// let boxed: Box<str> = Box::from("hello");
719 /// println!("{}", boxed);
722 fn from(s: &str) -> Box<str> {
723 unsafe { from_boxed_utf8_unchecked(Box::from(s.as_bytes())) }
727 #[stable(feature = "boxed_str_conv", since = "1.19.0")]
728 impl From<Box<str>> for Box<[u8]> {
729 /// Converts a `Box<str>>` into a `Box<[u8]>`
731 /// This conversion does not allocate on the heap and happens in place.
735 /// // create a Box<str> which will be used to create a Box<[u8]>
736 /// let boxed: Box<str> = Box::from("hello");
737 /// let boxed_str: Box<[u8]> = Box::from(boxed);
739 /// // create a &[u8] which will be used to create a Box<[u8]>
740 /// let slice: &[u8] = &[104, 101, 108, 108, 111];
741 /// let boxed_slice = Box::from(slice);
743 /// assert_eq!(boxed_slice, boxed_str);
746 fn from(s: Box<str>) -> Self {
747 unsafe { Box::from_raw(Box::into_raw(s) as *mut [u8]) }
751 #[unstable(feature = "boxed_slice_try_from", issue = "0")]
752 impl<T, const N: usize> TryFrom<Box<[T]>> for Box<[T; N]>
754 [T; N]: LengthAtMost32,
756 type Error = Box<[T]>;
758 fn try_from(boxed_slice: Box<[T]>) -> Result<Self, Self::Error> {
759 if boxed_slice.len() == N {
760 Ok(unsafe { Box::from_raw(Box::into_raw(boxed_slice) as *mut [T; N]) })
769 #[stable(feature = "rust1", since = "1.0.0")]
770 /// Attempt to downcast the box to a concrete type.
775 /// use std::any::Any;
777 /// fn print_if_string(value: Box<dyn Any>) {
778 /// if let Ok(string) = value.downcast::<String>() {
779 /// println!("String ({}): {}", string.len(), string);
784 /// let my_string = "Hello World".to_string();
785 /// print_if_string(Box::new(my_string));
786 /// print_if_string(Box::new(0i8));
789 pub fn downcast<T: Any>(self) -> Result<Box<T>, Box<dyn Any>> {
792 let raw: *mut dyn Any = Box::into_raw(self);
793 Ok(Box::from_raw(raw as *mut T))
801 impl Box<dyn Any + Send> {
803 #[stable(feature = "rust1", since = "1.0.0")]
804 /// Attempt to downcast the box to a concrete type.
809 /// use std::any::Any;
811 /// fn print_if_string(value: Box<dyn Any + Send>) {
812 /// if let Ok(string) = value.downcast::<String>() {
813 /// println!("String ({}): {}", string.len(), string);
818 /// let my_string = "Hello World".to_string();
819 /// print_if_string(Box::new(my_string));
820 /// print_if_string(Box::new(0i8));
823 pub fn downcast<T: Any>(self) -> Result<Box<T>, Box<dyn Any + Send>> {
824 <Box<dyn Any>>::downcast(self).map_err(|s| unsafe {
825 // reapply the Send marker
826 Box::from_raw(Box::into_raw(s) as *mut (dyn Any + Send))
831 #[stable(feature = "rust1", since = "1.0.0")]
832 impl<T: fmt::Display + ?Sized> fmt::Display for Box<T> {
833 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
834 fmt::Display::fmt(&**self, f)
838 #[stable(feature = "rust1", since = "1.0.0")]
839 impl<T: fmt::Debug + ?Sized> fmt::Debug for Box<T> {
840 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
841 fmt::Debug::fmt(&**self, f)
845 #[stable(feature = "rust1", since = "1.0.0")]
846 impl<T: ?Sized> fmt::Pointer for Box<T> {
847 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
848 // It's not possible to extract the inner Uniq directly from the Box,
849 // instead we cast it to a *const which aliases the Unique
850 let ptr: *const T = &**self;
851 fmt::Pointer::fmt(&ptr, f)
855 #[stable(feature = "rust1", since = "1.0.0")]
856 impl<T: ?Sized> Deref for Box<T> {
859 fn deref(&self) -> &T {
864 #[stable(feature = "rust1", since = "1.0.0")]
865 impl<T: ?Sized> DerefMut for Box<T> {
866 fn deref_mut(&mut self) -> &mut T {
871 #[unstable(feature = "receiver_trait", issue = "0")]
872 impl<T: ?Sized> Receiver for Box<T> {}
874 #[stable(feature = "rust1", since = "1.0.0")]
875 impl<I: Iterator + ?Sized> Iterator for Box<I> {
877 fn next(&mut self) -> Option<I::Item> {
880 fn size_hint(&self) -> (usize, Option<usize>) {
883 fn nth(&mut self, n: usize) -> Option<I::Item> {
888 #[stable(feature = "rust1", since = "1.0.0")]
889 impl<I: Iterator + Sized> Iterator for Box<I> {
890 fn last(self) -> Option<I::Item> where I: Sized {
895 #[stable(feature = "rust1", since = "1.0.0")]
896 impl<I: DoubleEndedIterator + ?Sized> DoubleEndedIterator for Box<I> {
897 fn next_back(&mut self) -> Option<I::Item> {
900 fn nth_back(&mut self, n: usize) -> Option<I::Item> {
904 #[stable(feature = "rust1", since = "1.0.0")]
905 impl<I: ExactSizeIterator + ?Sized> ExactSizeIterator for Box<I> {
906 fn len(&self) -> usize {
909 fn is_empty(&self) -> bool {
914 #[stable(feature = "fused", since = "1.26.0")]
915 impl<I: FusedIterator + ?Sized> FusedIterator for Box<I> {}
917 #[stable(feature = "boxed_closure_impls", since = "1.35.0")]
918 impl<A, F: FnOnce<A> + ?Sized> FnOnce<A> for Box<F> {
919 type Output = <F as FnOnce<A>>::Output;
921 extern "rust-call" fn call_once(self, args: A) -> Self::Output {
922 <F as FnOnce<A>>::call_once(*self, args)
926 #[stable(feature = "boxed_closure_impls", since = "1.35.0")]
927 impl<A, F: FnMut<A> + ?Sized> FnMut<A> for Box<F> {
928 extern "rust-call" fn call_mut(&mut self, args: A) -> Self::Output {
929 <F as FnMut<A>>::call_mut(self, args)
933 #[stable(feature = "boxed_closure_impls", since = "1.35.0")]
934 impl<A, F: Fn<A> + ?Sized> Fn<A> for Box<F> {
935 extern "rust-call" fn call(&self, args: A) -> Self::Output {
936 <F as Fn<A>>::call(self, args)
940 #[unstable(feature = "coerce_unsized", issue = "27732")]
941 impl<T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<Box<U>> for Box<T> {}
943 #[unstable(feature = "dispatch_from_dyn", issue = "0")]
944 impl<T: ?Sized + Unsize<U>, U: ?Sized> DispatchFromDyn<Box<U>> for Box<T> {}
946 #[stable(feature = "boxed_slice_from_iter", since = "1.32.0")]
947 impl<A> FromIterator<A> for Box<[A]> {
948 fn from_iter<T: IntoIterator<Item = A>>(iter: T) -> Self {
949 iter.into_iter().collect::<Vec<_>>().into_boxed_slice()
953 #[stable(feature = "box_slice_clone", since = "1.3.0")]
954 impl<T: Clone> Clone for Box<[T]> {
955 fn clone(&self) -> Self {
956 let mut new = BoxBuilder {
957 data: RawVec::with_capacity(self.len()),
961 let mut target = new.data.ptr();
963 for item in self.iter() {
965 ptr::write(target, item.clone());
966 target = target.offset(1);
972 return unsafe { new.into_box() };
974 // Helper type for responding to panics correctly.
975 struct BoxBuilder<T> {
980 impl<T> BoxBuilder<T> {
981 unsafe fn into_box(self) -> Box<[T]> {
982 let raw = ptr::read(&self.data);
988 impl<T> Drop for BoxBuilder<T> {
990 let mut data = self.data.ptr();
991 let max = unsafe { data.add(self.len) };
996 data = data.offset(1);
1004 #[stable(feature = "box_borrow", since = "1.1.0")]
1005 impl<T: ?Sized> borrow::Borrow<T> for Box<T> {
1006 fn borrow(&self) -> &T {
1011 #[stable(feature = "box_borrow", since = "1.1.0")]
1012 impl<T: ?Sized> borrow::BorrowMut<T> for Box<T> {
1013 fn borrow_mut(&mut self) -> &mut T {
1018 #[stable(since = "1.5.0", feature = "smart_ptr_as_ref")]
1019 impl<T: ?Sized> AsRef<T> for Box<T> {
1020 fn as_ref(&self) -> &T {
1025 #[stable(since = "1.5.0", feature = "smart_ptr_as_ref")]
1026 impl<T: ?Sized> AsMut<T> for Box<T> {
1027 fn as_mut(&mut self) -> &mut T {
1034 * We could have chosen not to add this impl, and instead have written a
1035 * function of Pin<Box<T>> to Pin<T>. Such a function would not be sound,
1036 * because Box<T> implements Unpin even when T does not, as a result of
1039 * We chose this API instead of the alternative for a few reasons:
1040 * - Logically, it is helpful to understand pinning in regard to the
1041 * memory region being pointed to. For this reason none of the
1042 * standard library pointer types support projecting through a pin
1043 * (Box<T> is the only pointer type in std for which this would be
1045 * - It is in practice very useful to have Box<T> be unconditionally
1046 * Unpin because of trait objects, for which the structural auto
1047 * trait functionality does not apply (e.g., Box<dyn Foo> would
1048 * otherwise not be Unpin).
1050 * Another type with the same semantics as Box but only a conditional
1051 * implementation of `Unpin` (where `T: Unpin`) would be valid/safe, and
1052 * could have a method to project a Pin<T> from it.
1054 #[stable(feature = "pin", since = "1.33.0")]
1055 impl<T: ?Sized> Unpin for Box<T> { }
1057 #[unstable(feature = "generator_trait", issue = "43122")]
1058 impl<G: ?Sized + Generator + Unpin> Generator for Box<G> {
1059 type Yield = G::Yield;
1060 type Return = G::Return;
1062 fn resume(mut self: Pin<&mut Self>) -> GeneratorState<Self::Yield, Self::Return> {
1063 G::resume(Pin::new(&mut *self))
1067 #[unstable(feature = "generator_trait", issue = "43122")]
1068 impl<G: ?Sized + Generator> Generator for Pin<Box<G>> {
1069 type Yield = G::Yield;
1070 type Return = G::Return;
1072 fn resume(mut self: Pin<&mut Self>) -> GeneratorState<Self::Yield, Self::Return> {
1073 G::resume((*self).as_mut())
1077 #[stable(feature = "futures_api", since = "1.36.0")]
1078 impl<F: ?Sized + Future + Unpin> Future for Box<F> {
1079 type Output = F::Output;
1081 fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
1082 F::poll(Pin::new(&mut *self), cx)