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>>),
32 //! let list: List<i32> = List::Cons(1, Box::new(List::Cons(2, Box::new(List::Nil))));
33 //! println!("{:?}", list);
36 //! This will print `Cons(1, Cons(2, Nil))`.
38 //! Recursive structures must be boxed, because if the definition of `Cons`
41 //! ```compile_fail,E0072
47 //! It wouldn't work. This is because the size of a `List` depends on how many
48 //! elements are in the list, and so we don't know how much memory to allocate
49 //! for a `Cons`. By introducing a [`Box<T>`], which has a defined size, we know how
50 //! big `Cons` needs to be.
54 //! For non-zero-sized values, a [`Box`] will use the [`Global`] allocator for
55 //! its allocation. It is valid to convert both ways between a [`Box`] and a
56 //! raw pointer allocated with the [`Global`] allocator, given that the
57 //! [`Layout`] used with the allocator is correct for the type. More precisely,
58 //! a `value: *mut T` that has been allocated with the [`Global`] allocator
59 //! with `Layout::for_value(&*value)` may be converted into a box using
60 //! [`Box::<T>::from_raw(value)`]. Conversely, the memory backing a `value: *mut
61 //! T` obtained from [`Box::<T>::into_raw`] may be deallocated using the
62 //! [`Global`] allocator with [`Layout::for_value(&*value)`].
65 //! [dereferencing]: ../../std/ops/trait.Deref.html
66 //! [`Box`]: struct.Box.html
67 //! [`Box<T>`]: struct.Box.html
68 //! [`Box::<T>::from_raw(value)`]: struct.Box.html#method.from_raw
69 //! [`Box::<T>::into_raw`]: struct.Box.html#method.into_raw
70 //! [`Global`]: ../alloc/struct.Global.html
71 //! [`Layout`]: ../alloc/struct.Layout.html
72 //! [`Layout::for_value(&*value)`]: ../alloc/struct.Layout.html#method.for_value
74 #![stable(feature = "rust1", since = "1.0.0")]
77 use core::array::LengthAtMost32;
79 use core::cmp::Ordering;
80 use core::convert::{From, TryFrom};
82 use core::future::Future;
83 use core::hash::{Hash, Hasher};
84 use core::iter::{Iterator, FromIterator, FusedIterator};
85 use core::marker::{Unpin, Unsize};
89 CoerceUnsized, DispatchFromDyn, Deref, DerefMut, Receiver, Generator, GeneratorState
91 use core::ptr::{self, NonNull, Unique};
93 use core::task::{Context, Poll};
95 use crate::alloc::{self, Global, Alloc};
97 use crate::raw_vec::RawVec;
98 use crate::str::from_boxed_utf8_unchecked;
100 /// A pointer type for heap allocation.
102 /// See the [module-level documentation](../../std/boxed/index.html) for more.
103 #[lang = "owned_box"]
105 #[stable(feature = "rust1", since = "1.0.0")]
106 pub struct Box<T: ?Sized>(Unique<T>);
109 /// Allocates memory on the heap and then places `x` into it.
111 /// This doesn't actually allocate if `T` is zero-sized.
116 /// let five = Box::new(5);
118 #[stable(feature = "rust1", since = "1.0.0")]
120 pub fn new(x: T) -> Box<T> {
124 /// Constructs a new box with uninitialized contents.
129 /// #![feature(new_uninit)]
131 /// let mut five = Box::<u32>::new_uninit();
133 /// let five = unsafe {
134 /// // Deferred initialization:
135 /// five.as_mut_ptr().write(5);
137 /// five.assume_init()
140 /// assert_eq!(*five, 5)
142 #[unstable(feature = "new_uninit", issue = "63291")]
143 pub fn new_uninit() -> Box<mem::MaybeUninit<T>> {
144 let layout = alloc::Layout::new::<mem::MaybeUninit<T>>();
145 if layout.size() == 0 {
146 return Box(NonNull::dangling().into())
150 .unwrap_or_else(|_| alloc::handle_alloc_error(layout))
152 Box(ptr.cast().into())
155 /// Constructs a new `Pin<Box<T>>`. If `T` does not implement `Unpin`, then
156 /// `x` will be pinned in memory and unable to be moved.
157 #[stable(feature = "pin", since = "1.33.0")]
159 pub fn pin(x: T) -> Pin<Box<T>> {
165 /// Constructs a new boxed slice with uninitialized contents.
170 /// #![feature(new_uninit)]
172 /// let mut values = Box::<[u32]>::new_uninit_slice(3);
174 /// let values = unsafe {
175 /// // Deferred initialization:
176 /// values[0].as_mut_ptr().write(1);
177 /// values[1].as_mut_ptr().write(2);
178 /// values[2].as_mut_ptr().write(3);
180 /// values.assume_init()
183 /// assert_eq!(*values, [1, 2, 3])
185 #[unstable(feature = "new_uninit", issue = "63291")]
186 pub fn new_uninit_slice(len: usize) -> Box<[mem::MaybeUninit<T>]> {
187 let layout = alloc::Layout::array::<mem::MaybeUninit<T>>(len).unwrap();
188 let ptr = if layout.size() == 0 {
193 .unwrap_or_else(|_| alloc::handle_alloc_error(layout))
197 let slice = unsafe { slice::from_raw_parts_mut(ptr.as_ptr(), len) };
198 Box(Unique::from(slice))
202 impl<T> Box<mem::MaybeUninit<T>> {
203 /// Converts to `Box<T>`.
207 /// As with [`MaybeUninit::assume_init`],
208 /// it is up to the caller to guarantee that the value
209 /// really is in an initialized state.
210 /// Calling this when the content is not yet fully initialized
211 /// causes immediate undefined behavior.
213 /// [`MaybeUninit::assume_init`]: ../../std/mem/union.MaybeUninit.html#method.assume_init
218 /// #![feature(new_uninit)]
220 /// let mut five = Box::<u32>::new_uninit();
222 /// let five: Box<u32> = unsafe {
223 /// // Deferred initialization:
224 /// five.as_mut_ptr().write(5);
226 /// five.assume_init()
229 /// assert_eq!(*five, 5)
231 #[unstable(feature = "new_uninit", issue = "63291")]
233 pub unsafe fn assume_init(self) -> Box<T> {
234 Box(Box::into_unique(self).cast())
238 impl<T> Box<[mem::MaybeUninit<T>]> {
239 /// Converts to `Box<[T]>`.
243 /// As with [`MaybeUninit::assume_init`],
244 /// it is up to the caller to guarantee that the values
245 /// really are in an initialized state.
246 /// Calling this when the content is not yet fully initialized
247 /// causes immediate undefined behavior.
249 /// [`MaybeUninit::assume_init`]: ../../std/mem/union.MaybeUninit.html#method.assume_init
254 /// #![feature(new_uninit)]
256 /// let mut values = Box::<[u32]>::new_uninit_slice(3);
258 /// let values = unsafe {
259 /// // Deferred initialization:
260 /// values[0].as_mut_ptr().write(1);
261 /// values[1].as_mut_ptr().write(2);
262 /// values[2].as_mut_ptr().write(3);
264 /// values.assume_init()
267 /// assert_eq!(*values, [1, 2, 3])
269 #[unstable(feature = "new_uninit", issue = "63291")]
271 pub unsafe fn assume_init(self) -> Box<[T]> {
272 Box(Unique::new_unchecked(Box::into_raw(self) as _))
276 impl<T: ?Sized> Box<T> {
277 /// Constructs a box from a raw pointer.
279 /// After calling this function, the raw pointer is owned by the
280 /// resulting `Box`. Specifically, the `Box` destructor will call
281 /// the destructor of `T` and free the allocated memory. For this
282 /// to be safe, the memory must have been allocated in accordance
283 /// with the [memory layout] used by `Box` .
287 /// This function is unsafe because improper use may lead to
288 /// memory problems. For example, a double-free may occur if the
289 /// function is called twice on the same raw pointer.
292 /// Recreate a `Box` which was previously converted to a raw pointer
293 /// using [`Box::into_raw`]:
295 /// let x = Box::new(5);
296 /// let ptr = Box::into_raw(x);
297 /// let x = unsafe { Box::from_raw(ptr) };
299 /// Manually create a `Box` from scratch by using the global allocator:
301 /// use std::alloc::{alloc, Layout};
304 /// let ptr = alloc(Layout::new::<i32>()) as *mut i32;
306 /// let x = Box::from_raw(ptr);
310 /// [memory layout]: index.html#memory-layout
311 /// [`Layout`]: ../alloc/struct.Layout.html
312 /// [`Box::into_raw`]: struct.Box.html#method.into_raw
313 #[stable(feature = "box_raw", since = "1.4.0")]
315 pub unsafe fn from_raw(raw: *mut T) -> Self {
316 Box(Unique::new_unchecked(raw))
319 /// Consumes the `Box`, returning a wrapped raw pointer.
321 /// The pointer will be properly aligned and non-null.
323 /// After calling this function, the caller is responsible for the
324 /// memory previously managed by the `Box`. In particular, the
325 /// caller should properly destroy `T` and release the memory, taking
326 /// into account the [memory layout] used by `Box`. The easiest way to
327 /// do this is to convert the raw pointer back into a `Box` with the
328 /// [`Box::from_raw`] function, allowing the `Box` destructor to perform
331 /// Note: this is an associated function, which means that you have
332 /// to call it as `Box::into_raw(b)` instead of `b.into_raw()`. This
333 /// is so that there is no conflict with a method on the inner type.
336 /// Converting the raw pointer back into a `Box` with [`Box::from_raw`]
337 /// for automatic cleanup:
339 /// let x = Box::new(String::from("Hello"));
340 /// let ptr = Box::into_raw(x);
341 /// let x = unsafe { Box::from_raw(ptr) };
343 /// Manual cleanup by explicitly running the destructor and deallocating
346 /// use std::alloc::{dealloc, Layout};
349 /// let x = Box::new(String::from("Hello"));
350 /// let p = Box::into_raw(x);
352 /// ptr::drop_in_place(p);
353 /// dealloc(p as *mut u8, Layout::new::<String>());
357 /// [memory layout]: index.html#memory-layout
358 /// [`Box::from_raw`]: struct.Box.html#method.from_raw
359 #[stable(feature = "box_raw", since = "1.4.0")]
361 pub fn into_raw(b: Box<T>) -> *mut T {
362 Box::into_raw_non_null(b).as_ptr()
365 /// Consumes the `Box`, returning the wrapped pointer as `NonNull<T>`.
367 /// After calling this function, the caller is responsible for the
368 /// memory previously managed by the `Box`. In particular, the
369 /// caller should properly destroy `T` and release the memory. The
370 /// easiest way to do so is to convert the `NonNull<T>` pointer
371 /// into a raw pointer and back into a `Box` with the [`Box::from_raw`]
374 /// Note: this is an associated function, which means that you have
375 /// to call it as `Box::into_raw_non_null(b)`
376 /// instead of `b.into_raw_non_null()`. This
377 /// is so that there is no conflict with a method on the inner type.
379 /// [`Box::from_raw`]: struct.Box.html#method.from_raw
384 /// #![feature(box_into_raw_non_null)]
386 /// let x = Box::new(5);
387 /// let ptr = Box::into_raw_non_null(x);
389 /// // Clean up the memory by converting the NonNull pointer back
390 /// // into a Box and letting the Box be dropped.
391 /// let x = unsafe { Box::from_raw(ptr.as_ptr()) };
393 #[unstable(feature = "box_into_raw_non_null", issue = "47336")]
395 pub fn into_raw_non_null(b: Box<T>) -> NonNull<T> {
396 Box::into_unique(b).into()
399 #[unstable(feature = "ptr_internals", issue = "0", reason = "use into_raw_non_null instead")]
402 pub fn into_unique(b: Box<T>) -> Unique<T> {
403 let mut unique = b.0;
405 // Box is kind-of a library type, but recognized as a "unique pointer" by
406 // Stacked Borrows. This function here corresponds to "reborrowing to
407 // a raw pointer", but there is no actual reborrow here -- so
408 // without some care, the pointer we are returning here still carries
409 // the tag of `b`, with `Unique` permission.
410 // We round-trip through a mutable reference to avoid that.
411 unsafe { Unique::new_unchecked(unique.as_mut() as *mut T) }
414 /// Consumes and leaks the `Box`, returning a mutable reference,
415 /// `&'a mut T`. Note that the type `T` must outlive the chosen lifetime
416 /// `'a`. If the type has only static references, or none at all, then this
417 /// may be chosen to be `'static`.
419 /// This function is mainly useful for data that lives for the remainder of
420 /// the program's life. Dropping the returned reference will cause a memory
421 /// leak. If this is not acceptable, the reference should first be wrapped
422 /// with the [`Box::from_raw`] function producing a `Box`. This `Box` can
423 /// then be dropped which will properly destroy `T` and release the
424 /// allocated memory.
426 /// Note: this is an associated function, which means that you have
427 /// to call it as `Box::leak(b)` instead of `b.leak()`. This
428 /// is so that there is no conflict with a method on the inner type.
430 /// [`Box::from_raw`]: struct.Box.html#method.from_raw
437 /// let x = Box::new(41);
438 /// let static_ref: &'static mut usize = Box::leak(x);
439 /// *static_ref += 1;
440 /// assert_eq!(*static_ref, 42);
446 /// let x = vec![1, 2, 3].into_boxed_slice();
447 /// let static_ref = Box::leak(x);
448 /// static_ref[0] = 4;
449 /// assert_eq!(*static_ref, [4, 2, 3]);
451 #[stable(feature = "box_leak", since = "1.26.0")]
453 pub fn leak<'a>(b: Box<T>) -> &'a mut T
455 T: 'a // Technically not needed, but kept to be explicit.
457 unsafe { &mut *Box::into_raw(b) }
460 /// Converts a `Box<T>` into a `Pin<Box<T>>`
462 /// This conversion does not allocate on the heap and happens in place.
464 /// This is also available via [`From`].
465 #[unstable(feature = "box_into_pin", issue = "62370")]
466 pub fn into_pin(boxed: Box<T>) -> Pin<Box<T>> {
467 // It's not possible to move or replace the insides of a `Pin<Box<T>>`
468 // when `T: !Unpin`, so it's safe to pin it directly without any
469 // additional requirements.
470 unsafe { Pin::new_unchecked(boxed) }
474 #[stable(feature = "rust1", since = "1.0.0")]
475 unsafe impl<#[may_dangle] T: ?Sized> Drop for Box<T> {
477 // FIXME: Do nothing, drop is currently performed by compiler.
481 #[stable(feature = "rust1", since = "1.0.0")]
482 impl<T: Default> Default for Box<T> {
483 /// Creates a `Box<T>`, with the `Default` value for T.
484 fn default() -> Box<T> {
485 box Default::default()
489 #[stable(feature = "rust1", since = "1.0.0")]
490 impl<T> Default for Box<[T]> {
491 fn default() -> Box<[T]> {
492 Box::<[T; 0]>::new([])
496 #[stable(feature = "default_box_extra", since = "1.17.0")]
497 impl Default for Box<str> {
498 fn default() -> Box<str> {
499 unsafe { from_boxed_utf8_unchecked(Default::default()) }
503 #[stable(feature = "rust1", since = "1.0.0")]
504 impl<T: Clone> Clone for Box<T> {
505 /// Returns a new box with a `clone()` of this box's contents.
510 /// let x = Box::new(5);
511 /// let y = x.clone();
513 /// // The value is the same
514 /// assert_eq!(x, y);
516 /// // But they are unique objects
517 /// assert_ne!(&*x as *const i32, &*y as *const i32);
521 fn clone(&self) -> Box<T> {
522 box { (**self).clone() }
525 /// Copies `source`'s contents into `self` without creating a new allocation.
530 /// let x = Box::new(5);
531 /// let mut y = Box::new(10);
532 /// let yp: *const i32 = &*y;
534 /// y.clone_from(&x);
536 /// // The value is the same
537 /// assert_eq!(x, y);
539 /// // And no allocation occurred
540 /// assert_eq!(yp, &*y);
543 fn clone_from(&mut self, source: &Box<T>) {
544 (**self).clone_from(&(**source));
549 #[stable(feature = "box_slice_clone", since = "1.3.0")]
550 impl Clone for Box<str> {
551 fn clone(&self) -> Self {
552 // this makes a copy of the data
553 let buf: Box<[u8]> = self.as_bytes().into();
555 from_boxed_utf8_unchecked(buf)
560 #[stable(feature = "rust1", since = "1.0.0")]
561 impl<T: ?Sized + PartialEq> PartialEq for Box<T> {
563 fn eq(&self, other: &Box<T>) -> bool {
564 PartialEq::eq(&**self, &**other)
567 fn ne(&self, other: &Box<T>) -> bool {
568 PartialEq::ne(&**self, &**other)
571 #[stable(feature = "rust1", since = "1.0.0")]
572 impl<T: ?Sized + PartialOrd> PartialOrd for Box<T> {
574 fn partial_cmp(&self, other: &Box<T>) -> Option<Ordering> {
575 PartialOrd::partial_cmp(&**self, &**other)
578 fn lt(&self, other: &Box<T>) -> bool {
579 PartialOrd::lt(&**self, &**other)
582 fn le(&self, other: &Box<T>) -> bool {
583 PartialOrd::le(&**self, &**other)
586 fn ge(&self, other: &Box<T>) -> bool {
587 PartialOrd::ge(&**self, &**other)
590 fn gt(&self, other: &Box<T>) -> bool {
591 PartialOrd::gt(&**self, &**other)
594 #[stable(feature = "rust1", since = "1.0.0")]
595 impl<T: ?Sized + Ord> Ord for Box<T> {
597 fn cmp(&self, other: &Box<T>) -> Ordering {
598 Ord::cmp(&**self, &**other)
601 #[stable(feature = "rust1", since = "1.0.0")]
602 impl<T: ?Sized + Eq> Eq for Box<T> {}
604 #[stable(feature = "rust1", since = "1.0.0")]
605 impl<T: ?Sized + Hash> Hash for Box<T> {
606 fn hash<H: Hasher>(&self, state: &mut H) {
607 (**self).hash(state);
611 #[stable(feature = "indirect_hasher_impl", since = "1.22.0")]
612 impl<T: ?Sized + Hasher> Hasher for Box<T> {
613 fn finish(&self) -> u64 {
616 fn write(&mut self, bytes: &[u8]) {
617 (**self).write(bytes)
619 fn write_u8(&mut self, i: u8) {
622 fn write_u16(&mut self, i: u16) {
623 (**self).write_u16(i)
625 fn write_u32(&mut self, i: u32) {
626 (**self).write_u32(i)
628 fn write_u64(&mut self, i: u64) {
629 (**self).write_u64(i)
631 fn write_u128(&mut self, i: u128) {
632 (**self).write_u128(i)
634 fn write_usize(&mut self, i: usize) {
635 (**self).write_usize(i)
637 fn write_i8(&mut self, i: i8) {
640 fn write_i16(&mut self, i: i16) {
641 (**self).write_i16(i)
643 fn write_i32(&mut self, i: i32) {
644 (**self).write_i32(i)
646 fn write_i64(&mut self, i: i64) {
647 (**self).write_i64(i)
649 fn write_i128(&mut self, i: i128) {
650 (**self).write_i128(i)
652 fn write_isize(&mut self, i: isize) {
653 (**self).write_isize(i)
657 #[stable(feature = "from_for_ptrs", since = "1.6.0")]
658 impl<T> From<T> for Box<T> {
659 /// Converts a generic type `T` into a `Box<T>`
661 /// The conversion allocates on the heap and moves `t`
662 /// from the stack into it.
667 /// let boxed = Box::new(5);
669 /// assert_eq!(Box::from(x), boxed);
671 fn from(t: T) -> Self {
676 #[stable(feature = "pin", since = "1.33.0")]
677 impl<T: ?Sized> From<Box<T>> for Pin<Box<T>> {
678 /// Converts a `Box<T>` into a `Pin<Box<T>>`
680 /// This conversion does not allocate on the heap and happens in place.
681 fn from(boxed: Box<T>) -> Self {
686 #[stable(feature = "box_from_slice", since = "1.17.0")]
687 impl<T: Copy> From<&[T]> for Box<[T]> {
688 /// Converts a `&[T]` into a `Box<[T]>`
690 /// This conversion allocates on the heap
691 /// and performs a copy of `slice`.
695 /// // create a &[u8] which will be used to create a Box<[u8]>
696 /// let slice: &[u8] = &[104, 101, 108, 108, 111];
697 /// let boxed_slice: Box<[u8]> = Box::from(slice);
699 /// println!("{:?}", boxed_slice);
701 fn from(slice: &[T]) -> Box<[T]> {
702 let len = slice.len();
703 let buf = RawVec::with_capacity(len);
705 ptr::copy_nonoverlapping(slice.as_ptr(), buf.ptr(), len);
711 #[stable(feature = "box_from_slice", since = "1.17.0")]
712 impl From<&str> for Box<str> {
713 /// Converts a `&str` into a `Box<str>`
715 /// This conversion allocates on the heap
716 /// and performs a copy of `s`.
720 /// let boxed: Box<str> = Box::from("hello");
721 /// println!("{}", boxed);
724 fn from(s: &str) -> Box<str> {
725 unsafe { from_boxed_utf8_unchecked(Box::from(s.as_bytes())) }
729 #[stable(feature = "boxed_str_conv", since = "1.19.0")]
730 impl From<Box<str>> for Box<[u8]> {
731 /// Converts a `Box<str>>` into a `Box<[u8]>`
733 /// This conversion does not allocate on the heap and happens in place.
737 /// // create a Box<str> which will be used to create a Box<[u8]>
738 /// let boxed: Box<str> = Box::from("hello");
739 /// let boxed_str: Box<[u8]> = Box::from(boxed);
741 /// // create a &[u8] which will be used to create a Box<[u8]>
742 /// let slice: &[u8] = &[104, 101, 108, 108, 111];
743 /// let boxed_slice = Box::from(slice);
745 /// assert_eq!(boxed_slice, boxed_str);
748 fn from(s: Box<str>) -> Self {
749 unsafe { Box::from_raw(Box::into_raw(s) as *mut [u8]) }
753 #[unstable(feature = "boxed_slice_try_from", issue = "0")]
754 impl<T, const N: usize> TryFrom<Box<[T]>> for Box<[T; N]>
756 [T; N]: LengthAtMost32,
758 type Error = Box<[T]>;
760 fn try_from(boxed_slice: Box<[T]>) -> Result<Self, Self::Error> {
761 if boxed_slice.len() == N {
762 Ok(unsafe { Box::from_raw(Box::into_raw(boxed_slice) as *mut [T; N]) })
771 #[stable(feature = "rust1", since = "1.0.0")]
772 /// Attempt to downcast the box to a concrete type.
777 /// use std::any::Any;
779 /// fn print_if_string(value: Box<dyn Any>) {
780 /// if let Ok(string) = value.downcast::<String>() {
781 /// println!("String ({}): {}", string.len(), string);
785 /// let my_string = "Hello World".to_string();
786 /// print_if_string(Box::new(my_string));
787 /// 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);
817 /// let my_string = "Hello World".to_string();
818 /// print_if_string(Box::new(my_string));
819 /// print_if_string(Box::new(0i8));
821 pub fn downcast<T: Any>(self) -> Result<Box<T>, Box<dyn Any + Send>> {
822 <Box<dyn Any>>::downcast(self).map_err(|s| unsafe {
823 // reapply the Send marker
824 Box::from_raw(Box::into_raw(s) as *mut (dyn Any + Send))
829 #[stable(feature = "rust1", since = "1.0.0")]
830 impl<T: fmt::Display + ?Sized> fmt::Display for Box<T> {
831 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
832 fmt::Display::fmt(&**self, f)
836 #[stable(feature = "rust1", since = "1.0.0")]
837 impl<T: fmt::Debug + ?Sized> fmt::Debug for Box<T> {
838 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
839 fmt::Debug::fmt(&**self, f)
843 #[stable(feature = "rust1", since = "1.0.0")]
844 impl<T: ?Sized> fmt::Pointer for Box<T> {
845 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
846 // It's not possible to extract the inner Uniq directly from the Box,
847 // instead we cast it to a *const which aliases the Unique
848 let ptr: *const T = &**self;
849 fmt::Pointer::fmt(&ptr, f)
853 #[stable(feature = "rust1", since = "1.0.0")]
854 impl<T: ?Sized> Deref for Box<T> {
857 fn deref(&self) -> &T {
862 #[stable(feature = "rust1", since = "1.0.0")]
863 impl<T: ?Sized> DerefMut for Box<T> {
864 fn deref_mut(&mut self) -> &mut T {
869 #[unstable(feature = "receiver_trait", issue = "0")]
870 impl<T: ?Sized> Receiver for Box<T> {}
872 #[stable(feature = "rust1", since = "1.0.0")]
873 impl<I: Iterator + ?Sized> Iterator for Box<I> {
875 fn next(&mut self) -> Option<I::Item> {
878 fn size_hint(&self) -> (usize, Option<usize>) {
881 fn nth(&mut self, n: usize) -> Option<I::Item> {
884 fn last(self) -> Option<I::Item> {
891 fn last(self) -> Option<Self::Item>;
894 impl<I: Iterator + ?Sized> BoxIter for Box<I> {
896 default fn last(self) -> Option<I::Item> {
898 fn some<T>(_: Option<T>, x: T) -> Option<T> {
902 self.fold(None, some)
906 /// Specialization for sized `I`s that uses `I`s implementation of `last()`
907 /// instead of the default.
908 #[stable(feature = "rust1", since = "1.0.0")]
909 impl<I: Iterator> BoxIter for Box<I> {
910 fn last(self) -> Option<I::Item> {
915 #[stable(feature = "rust1", since = "1.0.0")]
916 impl<I: DoubleEndedIterator + ?Sized> DoubleEndedIterator for Box<I> {
917 fn next_back(&mut self) -> Option<I::Item> {
920 fn nth_back(&mut self, n: usize) -> Option<I::Item> {
924 #[stable(feature = "rust1", since = "1.0.0")]
925 impl<I: ExactSizeIterator + ?Sized> ExactSizeIterator for Box<I> {
926 fn len(&self) -> usize {
929 fn is_empty(&self) -> bool {
934 #[stable(feature = "fused", since = "1.26.0")]
935 impl<I: FusedIterator + ?Sized> FusedIterator for Box<I> {}
937 #[stable(feature = "boxed_closure_impls", since = "1.35.0")]
938 impl<A, F: FnOnce<A> + ?Sized> FnOnce<A> for Box<F> {
939 type Output = <F as FnOnce<A>>::Output;
941 extern "rust-call" fn call_once(self, args: A) -> Self::Output {
942 <F as FnOnce<A>>::call_once(*self, args)
946 #[stable(feature = "boxed_closure_impls", since = "1.35.0")]
947 impl<A, F: FnMut<A> + ?Sized> FnMut<A> for Box<F> {
948 extern "rust-call" fn call_mut(&mut self, args: A) -> Self::Output {
949 <F as FnMut<A>>::call_mut(self, args)
953 #[stable(feature = "boxed_closure_impls", since = "1.35.0")]
954 impl<A, F: Fn<A> + ?Sized> Fn<A> for Box<F> {
955 extern "rust-call" fn call(&self, args: A) -> Self::Output {
956 <F as Fn<A>>::call(self, args)
960 #[unstable(feature = "coerce_unsized", issue = "27732")]
961 impl<T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<Box<U>> for Box<T> {}
963 #[unstable(feature = "dispatch_from_dyn", issue = "0")]
964 impl<T: ?Sized + Unsize<U>, U: ?Sized> DispatchFromDyn<Box<U>> for Box<T> {}
966 #[stable(feature = "boxed_slice_from_iter", since = "1.32.0")]
967 impl<A> FromIterator<A> for Box<[A]> {
968 fn from_iter<T: IntoIterator<Item = A>>(iter: T) -> Self {
969 iter.into_iter().collect::<Vec<_>>().into_boxed_slice()
973 #[stable(feature = "box_slice_clone", since = "1.3.0")]
974 impl<T: Clone> Clone for Box<[T]> {
975 fn clone(&self) -> Self {
976 let mut new = BoxBuilder {
977 data: RawVec::with_capacity(self.len()),
981 let mut target = new.data.ptr();
983 for item in self.iter() {
985 ptr::write(target, item.clone());
986 target = target.offset(1);
992 return unsafe { new.into_box() };
994 // Helper type for responding to panics correctly.
995 struct BoxBuilder<T> {
1000 impl<T> BoxBuilder<T> {
1001 unsafe fn into_box(self) -> Box<[T]> {
1002 let raw = ptr::read(&self.data);
1008 impl<T> Drop for BoxBuilder<T> {
1009 fn drop(&mut self) {
1010 let mut data = self.data.ptr();
1011 let max = unsafe { data.add(self.len) };
1016 data = data.offset(1);
1024 #[stable(feature = "box_borrow", since = "1.1.0")]
1025 impl<T: ?Sized> borrow::Borrow<T> for Box<T> {
1026 fn borrow(&self) -> &T {
1031 #[stable(feature = "box_borrow", since = "1.1.0")]
1032 impl<T: ?Sized> borrow::BorrowMut<T> for Box<T> {
1033 fn borrow_mut(&mut self) -> &mut T {
1038 #[stable(since = "1.5.0", feature = "smart_ptr_as_ref")]
1039 impl<T: ?Sized> AsRef<T> for Box<T> {
1040 fn as_ref(&self) -> &T {
1045 #[stable(since = "1.5.0", feature = "smart_ptr_as_ref")]
1046 impl<T: ?Sized> AsMut<T> for Box<T> {
1047 fn as_mut(&mut self) -> &mut T {
1054 * We could have chosen not to add this impl, and instead have written a
1055 * function of Pin<Box<T>> to Pin<T>. Such a function would not be sound,
1056 * because Box<T> implements Unpin even when T does not, as a result of
1059 * We chose this API instead of the alternative for a few reasons:
1060 * - Logically, it is helpful to understand pinning in regard to the
1061 * memory region being pointed to. For this reason none of the
1062 * standard library pointer types support projecting through a pin
1063 * (Box<T> is the only pointer type in std for which this would be
1065 * - It is in practice very useful to have Box<T> be unconditionally
1066 * Unpin because of trait objects, for which the structural auto
1067 * trait functionality does not apply (e.g., Box<dyn Foo> would
1068 * otherwise not be Unpin).
1070 * Another type with the same semantics as Box but only a conditional
1071 * implementation of `Unpin` (where `T: Unpin`) would be valid/safe, and
1072 * could have a method to project a Pin<T> from it.
1074 #[stable(feature = "pin", since = "1.33.0")]
1075 impl<T: ?Sized> Unpin for Box<T> { }
1077 #[unstable(feature = "generator_trait", issue = "43122")]
1078 impl<G: ?Sized + Generator + Unpin> Generator for Box<G> {
1079 type Yield = G::Yield;
1080 type Return = G::Return;
1082 fn resume(mut self: Pin<&mut Self>) -> GeneratorState<Self::Yield, Self::Return> {
1083 G::resume(Pin::new(&mut *self))
1087 #[unstable(feature = "generator_trait", issue = "43122")]
1088 impl<G: ?Sized + Generator> Generator for Pin<Box<G>> {
1089 type Yield = G::Yield;
1090 type Return = G::Return;
1092 fn resume(mut self: Pin<&mut Self>) -> GeneratorState<Self::Yield, Self::Return> {
1093 G::resume((*self).as_mut())
1097 #[stable(feature = "futures_api", since = "1.36.0")]
1098 impl<F: ?Sized + Future + Unpin> Future for Box<F> {
1099 type Output = F::Output;
1101 fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
1102 F::poll(Pin::new(&mut *self), cx)