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>>();
147 .unwrap_or_else(|_| alloc::handle_alloc_error(layout))
149 Box(ptr.cast().into())
152 /// Constructs a new `Pin<Box<T>>`. If `T` does not implement `Unpin`, then
153 /// `x` will be pinned in memory and unable to be moved.
154 #[stable(feature = "pin", since = "1.33.0")]
156 pub fn pin(x: T) -> Pin<Box<T>> {
162 /// Constructs a new boxed slice with uninitialized contents.
167 /// #![feature(new_uninit)]
169 /// let mut values = Box::<[u32]>::new_uninit_slice(3);
171 /// let values = unsafe {
172 /// // Deferred initialization:
173 /// values[0].as_mut_ptr().write(1);
174 /// values[1].as_mut_ptr().write(2);
175 /// values[2].as_mut_ptr().write(3);
177 /// values.assume_init()
180 /// assert_eq!(*values, [1, 2, 3])
182 #[unstable(feature = "new_uninit", issue = "63291")]
183 pub fn new_uninit_slice(len: usize) -> Box<[mem::MaybeUninit<T>]> {
184 let layout = alloc::Layout::array::<mem::MaybeUninit<T>>(len).unwrap();
185 let ptr = unsafe { alloc::alloc(layout) };
186 let unique = Unique::new(ptr).unwrap_or_else(|| alloc::handle_alloc_error(layout));
187 let slice = unsafe { slice::from_raw_parts_mut(unique.cast().as_ptr(), len) };
188 Box(Unique::from(slice))
192 impl<T> Box<mem::MaybeUninit<T>> {
193 /// Converts to `Box<T>`.
197 /// As with [`MaybeUninit::assume_init`],
198 /// it is up to the caller to guarantee that the value
199 /// really is in an initialized state.
200 /// Calling this when the content is not yet fully initialized
201 /// causes immediate undefined behavior.
203 /// [`MaybeUninit::assume_init`]: ../../std/mem/union.MaybeUninit.html#method.assume_init
208 /// #![feature(new_uninit)]
210 /// let mut five = Box::<u32>::new_uninit();
212 /// let five: Box<u32> = unsafe {
213 /// // Deferred initialization:
214 /// five.as_mut_ptr().write(5);
216 /// five.assume_init()
219 /// assert_eq!(*five, 5)
221 #[unstable(feature = "new_uninit", issue = "63291")]
223 pub unsafe fn assume_init(self) -> Box<T> {
224 Box(Box::into_unique(self).cast())
228 impl<T> Box<[mem::MaybeUninit<T>]> {
229 /// Converts to `Box<[T]>`.
233 /// As with [`MaybeUninit::assume_init`],
234 /// it is up to the caller to guarantee that the values
235 /// really are in an initialized state.
236 /// Calling this when the content is not yet fully initialized
237 /// causes immediate undefined behavior.
239 /// [`MaybeUninit::assume_init`]: ../../std/mem/union.MaybeUninit.html#method.assume_init
244 /// #![feature(new_uninit)]
246 /// let mut values = Box::<[u32]>::new_uninit_slice(3);
248 /// let values = unsafe {
249 /// // Deferred initialization:
250 /// values[0].as_mut_ptr().write(1);
251 /// values[1].as_mut_ptr().write(2);
252 /// values[2].as_mut_ptr().write(3);
254 /// values.assume_init()
257 /// assert_eq!(*values, [1, 2, 3])
259 #[unstable(feature = "new_uninit", issue = "63291")]
261 pub unsafe fn assume_init(self) -> Box<[T]> {
262 Box(Unique::new_unchecked(Box::into_raw(self) as _))
266 impl<T: ?Sized> Box<T> {
267 /// Constructs a box from a raw pointer.
269 /// After calling this function, the raw pointer is owned by the
270 /// resulting `Box`. Specifically, the `Box` destructor will call
271 /// the destructor of `T` and free the allocated memory. For this
272 /// to be safe, the memory must have been allocated in accordance
273 /// with the [memory layout] used by `Box` .
277 /// This function is unsafe because improper use may lead to
278 /// memory problems. For example, a double-free may occur if the
279 /// function is called twice on the same raw pointer.
282 /// Recreate a `Box` which was previously converted to a raw pointer
283 /// using [`Box::into_raw`]:
285 /// let x = Box::new(5);
286 /// let ptr = Box::into_raw(x);
287 /// let x = unsafe { Box::from_raw(ptr) };
289 /// Manually create a `Box` from scratch by using the global allocator:
291 /// use std::alloc::{alloc, Layout};
294 /// let ptr = alloc(Layout::new::<i32>()) as *mut i32;
296 /// let x = Box::from_raw(ptr);
300 /// [memory layout]: index.html#memory-layout
301 /// [`Layout`]: ../alloc/struct.Layout.html
302 /// [`Box::into_raw`]: struct.Box.html#method.into_raw
303 #[stable(feature = "box_raw", since = "1.4.0")]
305 pub unsafe fn from_raw(raw: *mut T) -> Self {
306 Box(Unique::new_unchecked(raw))
309 /// Consumes the `Box`, returning a wrapped raw pointer.
311 /// The pointer will be properly aligned and non-null.
313 /// After calling this function, the caller is responsible for the
314 /// memory previously managed by the `Box`. In particular, the
315 /// caller should properly destroy `T` and release the memory, taking
316 /// into account the [memory layout] used by `Box`. The easiest way to
317 /// do this is to convert the raw pointer back into a `Box` with the
318 /// [`Box::from_raw`] function, allowing the `Box` destructor to perform
321 /// Note: this is an associated function, which means that you have
322 /// to call it as `Box::into_raw(b)` instead of `b.into_raw()`. This
323 /// is so that there is no conflict with a method on the inner type.
326 /// Converting the raw pointer back into a `Box` with [`Box::from_raw`]
327 /// for automatic cleanup:
329 /// let x = Box::new(String::from("Hello"));
330 /// let ptr = Box::into_raw(x);
331 /// let x = unsafe { Box::from_raw(ptr) };
333 /// Manual cleanup by explicitly running the destructor and deallocating
336 /// use std::alloc::{dealloc, Layout};
339 /// let x = Box::new(String::from("Hello"));
340 /// let p = Box::into_raw(x);
342 /// ptr::drop_in_place(p);
343 /// dealloc(p as *mut u8, Layout::new::<String>());
347 /// [memory layout]: index.html#memory-layout
348 /// [`Box::from_raw`]: struct.Box.html#method.from_raw
349 #[stable(feature = "box_raw", since = "1.4.0")]
351 pub fn into_raw(b: Box<T>) -> *mut T {
352 Box::into_raw_non_null(b).as_ptr()
355 /// Consumes the `Box`, returning the wrapped pointer as `NonNull<T>`.
357 /// After calling this function, the caller is responsible for the
358 /// memory previously managed by the `Box`. In particular, the
359 /// caller should properly destroy `T` and release the memory. The
360 /// easiest way to do so is to convert the `NonNull<T>` pointer
361 /// into a raw pointer and back into a `Box` with the [`Box::from_raw`]
364 /// Note: this is an associated function, which means that you have
365 /// to call it as `Box::into_raw_non_null(b)`
366 /// instead of `b.into_raw_non_null()`. This
367 /// is so that there is no conflict with a method on the inner type.
369 /// [`Box::from_raw`]: struct.Box.html#method.from_raw
374 /// #![feature(box_into_raw_non_null)]
376 /// let x = Box::new(5);
377 /// let ptr = Box::into_raw_non_null(x);
379 /// // Clean up the memory by converting the NonNull pointer back
380 /// // into a Box and letting the Box be dropped.
381 /// let x = unsafe { Box::from_raw(ptr.as_ptr()) };
383 #[unstable(feature = "box_into_raw_non_null", issue = "47336")]
385 pub fn into_raw_non_null(b: Box<T>) -> NonNull<T> {
386 Box::into_unique(b).into()
389 #[unstable(feature = "ptr_internals", issue = "0", reason = "use into_raw_non_null instead")]
392 pub fn into_unique(b: Box<T>) -> Unique<T> {
393 let mut unique = b.0;
395 // Box is kind-of a library type, but recognized as a "unique pointer" by
396 // Stacked Borrows. This function here corresponds to "reborrowing to
397 // a raw pointer", but there is no actual reborrow here -- so
398 // without some care, the pointer we are returning here still carries
399 // the tag of `b`, with `Unique` permission.
400 // We round-trip through a mutable reference to avoid that.
401 unsafe { Unique::new_unchecked(unique.as_mut() as *mut T) }
404 /// Consumes and leaks the `Box`, returning a mutable reference,
405 /// `&'a mut T`. Note that the type `T` must outlive the chosen lifetime
406 /// `'a`. If the type has only static references, or none at all, then this
407 /// may be chosen to be `'static`.
409 /// This function is mainly useful for data that lives for the remainder of
410 /// the program's life. Dropping the returned reference will cause a memory
411 /// leak. If this is not acceptable, the reference should first be wrapped
412 /// with the [`Box::from_raw`] function producing a `Box`. This `Box` can
413 /// then be dropped which will properly destroy `T` and release the
414 /// allocated memory.
416 /// Note: this is an associated function, which means that you have
417 /// to call it as `Box::leak(b)` instead of `b.leak()`. This
418 /// is so that there is no conflict with a method on the inner type.
420 /// [`Box::from_raw`]: struct.Box.html#method.from_raw
427 /// let x = Box::new(41);
428 /// let static_ref: &'static mut usize = Box::leak(x);
429 /// *static_ref += 1;
430 /// assert_eq!(*static_ref, 42);
436 /// let x = vec![1, 2, 3].into_boxed_slice();
437 /// let static_ref = Box::leak(x);
438 /// static_ref[0] = 4;
439 /// assert_eq!(*static_ref, [4, 2, 3]);
441 #[stable(feature = "box_leak", since = "1.26.0")]
443 pub fn leak<'a>(b: Box<T>) -> &'a mut T
445 T: 'a // Technically not needed, but kept to be explicit.
447 unsafe { &mut *Box::into_raw(b) }
450 /// Converts a `Box<T>` into a `Pin<Box<T>>`
452 /// This conversion does not allocate on the heap and happens in place.
454 /// This is also available via [`From`].
455 #[unstable(feature = "box_into_pin", issue = "62370")]
456 pub fn into_pin(boxed: Box<T>) -> Pin<Box<T>> {
457 // It's not possible to move or replace the insides of a `Pin<Box<T>>`
458 // when `T: !Unpin`, so it's safe to pin it directly without any
459 // additional requirements.
460 unsafe { Pin::new_unchecked(boxed) }
464 #[stable(feature = "rust1", since = "1.0.0")]
465 unsafe impl<#[may_dangle] T: ?Sized> Drop for Box<T> {
467 // FIXME: Do nothing, drop is currently performed by compiler.
471 #[stable(feature = "rust1", since = "1.0.0")]
472 impl<T: Default> Default for Box<T> {
473 /// Creates a `Box<T>`, with the `Default` value for T.
474 fn default() -> Box<T> {
475 box Default::default()
479 #[stable(feature = "rust1", since = "1.0.0")]
480 impl<T> Default for Box<[T]> {
481 fn default() -> Box<[T]> {
482 Box::<[T; 0]>::new([])
486 #[stable(feature = "default_box_extra", since = "1.17.0")]
487 impl Default for Box<str> {
488 fn default() -> Box<str> {
489 unsafe { from_boxed_utf8_unchecked(Default::default()) }
493 #[stable(feature = "rust1", since = "1.0.0")]
494 impl<T: Clone> Clone for Box<T> {
495 /// Returns a new box with a `clone()` of this box's contents.
500 /// let x = Box::new(5);
501 /// let y = x.clone();
503 /// // The value is the same
504 /// assert_eq!(x, y);
506 /// // But they are unique objects
507 /// assert_ne!(&*x as *const i32, &*y as *const i32);
511 fn clone(&self) -> Box<T> {
512 box { (**self).clone() }
515 /// Copies `source`'s contents into `self` without creating a new allocation.
520 /// let x = Box::new(5);
521 /// let mut y = Box::new(10);
522 /// let yp: *const i32 = &*y;
524 /// y.clone_from(&x);
526 /// // The value is the same
527 /// assert_eq!(x, y);
529 /// // And no allocation occurred
530 /// assert_eq!(yp, &*y);
533 fn clone_from(&mut self, source: &Box<T>) {
534 (**self).clone_from(&(**source));
539 #[stable(feature = "box_slice_clone", since = "1.3.0")]
540 impl Clone for Box<str> {
541 fn clone(&self) -> Self {
542 // this makes a copy of the data
543 let buf: Box<[u8]> = self.as_bytes().into();
545 from_boxed_utf8_unchecked(buf)
550 #[stable(feature = "rust1", since = "1.0.0")]
551 impl<T: ?Sized + PartialEq> PartialEq for Box<T> {
553 fn eq(&self, other: &Box<T>) -> bool {
554 PartialEq::eq(&**self, &**other)
557 fn ne(&self, other: &Box<T>) -> bool {
558 PartialEq::ne(&**self, &**other)
561 #[stable(feature = "rust1", since = "1.0.0")]
562 impl<T: ?Sized + PartialOrd> PartialOrd for Box<T> {
564 fn partial_cmp(&self, other: &Box<T>) -> Option<Ordering> {
565 PartialOrd::partial_cmp(&**self, &**other)
568 fn lt(&self, other: &Box<T>) -> bool {
569 PartialOrd::lt(&**self, &**other)
572 fn le(&self, other: &Box<T>) -> bool {
573 PartialOrd::le(&**self, &**other)
576 fn ge(&self, other: &Box<T>) -> bool {
577 PartialOrd::ge(&**self, &**other)
580 fn gt(&self, other: &Box<T>) -> bool {
581 PartialOrd::gt(&**self, &**other)
584 #[stable(feature = "rust1", since = "1.0.0")]
585 impl<T: ?Sized + Ord> Ord for Box<T> {
587 fn cmp(&self, other: &Box<T>) -> Ordering {
588 Ord::cmp(&**self, &**other)
591 #[stable(feature = "rust1", since = "1.0.0")]
592 impl<T: ?Sized + Eq> Eq for Box<T> {}
594 #[stable(feature = "rust1", since = "1.0.0")]
595 impl<T: ?Sized + Hash> Hash for Box<T> {
596 fn hash<H: Hasher>(&self, state: &mut H) {
597 (**self).hash(state);
601 #[stable(feature = "indirect_hasher_impl", since = "1.22.0")]
602 impl<T: ?Sized + Hasher> Hasher for Box<T> {
603 fn finish(&self) -> u64 {
606 fn write(&mut self, bytes: &[u8]) {
607 (**self).write(bytes)
609 fn write_u8(&mut self, i: u8) {
612 fn write_u16(&mut self, i: u16) {
613 (**self).write_u16(i)
615 fn write_u32(&mut self, i: u32) {
616 (**self).write_u32(i)
618 fn write_u64(&mut self, i: u64) {
619 (**self).write_u64(i)
621 fn write_u128(&mut self, i: u128) {
622 (**self).write_u128(i)
624 fn write_usize(&mut self, i: usize) {
625 (**self).write_usize(i)
627 fn write_i8(&mut self, i: i8) {
630 fn write_i16(&mut self, i: i16) {
631 (**self).write_i16(i)
633 fn write_i32(&mut self, i: i32) {
634 (**self).write_i32(i)
636 fn write_i64(&mut self, i: i64) {
637 (**self).write_i64(i)
639 fn write_i128(&mut self, i: i128) {
640 (**self).write_i128(i)
642 fn write_isize(&mut self, i: isize) {
643 (**self).write_isize(i)
647 #[stable(feature = "from_for_ptrs", since = "1.6.0")]
648 impl<T> From<T> for Box<T> {
649 /// Converts a generic type `T` into a `Box<T>`
651 /// The conversion allocates on the heap and moves `t`
652 /// from the stack into it.
657 /// let boxed = Box::new(5);
659 /// assert_eq!(Box::from(x), boxed);
661 fn from(t: T) -> Self {
666 #[stable(feature = "pin", since = "1.33.0")]
667 impl<T: ?Sized> From<Box<T>> for Pin<Box<T>> {
668 /// Converts a `Box<T>` into a `Pin<Box<T>>`
670 /// This conversion does not allocate on the heap and happens in place.
671 fn from(boxed: Box<T>) -> Self {
676 #[stable(feature = "box_from_slice", since = "1.17.0")]
677 impl<T: Copy> From<&[T]> for Box<[T]> {
678 /// Converts a `&[T]` into a `Box<[T]>`
680 /// This conversion allocates on the heap
681 /// and performs a copy of `slice`.
685 /// // create a &[u8] which will be used to create a Box<[u8]>
686 /// let slice: &[u8] = &[104, 101, 108, 108, 111];
687 /// let boxed_slice: Box<[u8]> = Box::from(slice);
689 /// println!("{:?}", boxed_slice);
691 fn from(slice: &[T]) -> Box<[T]> {
692 let len = slice.len();
693 let buf = RawVec::with_capacity(len);
695 ptr::copy_nonoverlapping(slice.as_ptr(), buf.ptr(), len);
701 #[stable(feature = "box_from_slice", since = "1.17.0")]
702 impl From<&str> for Box<str> {
703 /// Converts a `&str` into a `Box<str>`
705 /// This conversion allocates on the heap
706 /// and performs a copy of `s`.
710 /// let boxed: Box<str> = Box::from("hello");
711 /// println!("{}", boxed);
714 fn from(s: &str) -> Box<str> {
715 unsafe { from_boxed_utf8_unchecked(Box::from(s.as_bytes())) }
719 #[stable(feature = "boxed_str_conv", since = "1.19.0")]
720 impl From<Box<str>> for Box<[u8]> {
721 /// Converts a `Box<str>>` into a `Box<[u8]>`
723 /// This conversion does not allocate on the heap and happens in place.
727 /// // create a Box<str> which will be used to create a Box<[u8]>
728 /// let boxed: Box<str> = Box::from("hello");
729 /// let boxed_str: Box<[u8]> = Box::from(boxed);
731 /// // create a &[u8] which will be used to create a Box<[u8]>
732 /// let slice: &[u8] = &[104, 101, 108, 108, 111];
733 /// let boxed_slice = Box::from(slice);
735 /// assert_eq!(boxed_slice, boxed_str);
738 fn from(s: Box<str>) -> Self {
739 unsafe { Box::from_raw(Box::into_raw(s) as *mut [u8]) }
743 #[unstable(feature = "boxed_slice_try_from", issue = "0")]
744 impl<T, const N: usize> TryFrom<Box<[T]>> for Box<[T; N]>
746 [T; N]: LengthAtMost32,
748 type Error = Box<[T]>;
750 fn try_from(boxed_slice: Box<[T]>) -> Result<Self, Self::Error> {
751 if boxed_slice.len() == N {
752 Ok(unsafe { Box::from_raw(Box::into_raw(boxed_slice) as *mut [T; N]) })
761 #[stable(feature = "rust1", since = "1.0.0")]
762 /// Attempt to downcast the box to a concrete type.
767 /// use std::any::Any;
769 /// fn print_if_string(value: Box<dyn Any>) {
770 /// if let Ok(string) = value.downcast::<String>() {
771 /// println!("String ({}): {}", string.len(), string);
775 /// let my_string = "Hello World".to_string();
776 /// print_if_string(Box::new(my_string));
777 /// print_if_string(Box::new(0i8));
779 pub fn downcast<T: Any>(self) -> Result<Box<T>, Box<dyn Any>> {
782 let raw: *mut dyn Any = Box::into_raw(self);
783 Ok(Box::from_raw(raw as *mut T))
791 impl Box<dyn Any + Send> {
793 #[stable(feature = "rust1", since = "1.0.0")]
794 /// Attempt to downcast the box to a concrete type.
799 /// use std::any::Any;
801 /// fn print_if_string(value: Box<dyn Any + Send>) {
802 /// if let Ok(string) = value.downcast::<String>() {
803 /// println!("String ({}): {}", string.len(), string);
807 /// let my_string = "Hello World".to_string();
808 /// print_if_string(Box::new(my_string));
809 /// print_if_string(Box::new(0i8));
811 pub fn downcast<T: Any>(self) -> Result<Box<T>, Box<dyn Any + Send>> {
812 <Box<dyn Any>>::downcast(self).map_err(|s| unsafe {
813 // reapply the Send marker
814 Box::from_raw(Box::into_raw(s) as *mut (dyn Any + Send))
819 #[stable(feature = "rust1", since = "1.0.0")]
820 impl<T: fmt::Display + ?Sized> fmt::Display for Box<T> {
821 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
822 fmt::Display::fmt(&**self, f)
826 #[stable(feature = "rust1", since = "1.0.0")]
827 impl<T: fmt::Debug + ?Sized> fmt::Debug for Box<T> {
828 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
829 fmt::Debug::fmt(&**self, f)
833 #[stable(feature = "rust1", since = "1.0.0")]
834 impl<T: ?Sized> fmt::Pointer for Box<T> {
835 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
836 // It's not possible to extract the inner Uniq directly from the Box,
837 // instead we cast it to a *const which aliases the Unique
838 let ptr: *const T = &**self;
839 fmt::Pointer::fmt(&ptr, f)
843 #[stable(feature = "rust1", since = "1.0.0")]
844 impl<T: ?Sized> Deref for Box<T> {
847 fn deref(&self) -> &T {
852 #[stable(feature = "rust1", since = "1.0.0")]
853 impl<T: ?Sized> DerefMut for Box<T> {
854 fn deref_mut(&mut self) -> &mut T {
859 #[unstable(feature = "receiver_trait", issue = "0")]
860 impl<T: ?Sized> Receiver for Box<T> {}
862 #[stable(feature = "rust1", since = "1.0.0")]
863 impl<I: Iterator + ?Sized> Iterator for Box<I> {
865 fn next(&mut self) -> Option<I::Item> {
868 fn size_hint(&self) -> (usize, Option<usize>) {
871 fn nth(&mut self, n: usize) -> Option<I::Item> {
874 fn last(self) -> Option<I::Item> {
881 fn last(self) -> Option<Self::Item>;
884 impl<I: Iterator + ?Sized> BoxIter for Box<I> {
886 default fn last(self) -> Option<I::Item> {
888 fn some<T>(_: Option<T>, x: T) -> Option<T> {
892 self.fold(None, some)
896 /// Specialization for sized `I`s that uses `I`s implementation of `last()`
897 /// instead of the default.
898 #[stable(feature = "rust1", since = "1.0.0")]
899 impl<I: Iterator> BoxIter for Box<I> {
900 fn last(self) -> Option<I::Item> {
905 #[stable(feature = "rust1", since = "1.0.0")]
906 impl<I: DoubleEndedIterator + ?Sized> DoubleEndedIterator for Box<I> {
907 fn next_back(&mut self) -> Option<I::Item> {
910 fn nth_back(&mut self, n: usize) -> Option<I::Item> {
914 #[stable(feature = "rust1", since = "1.0.0")]
915 impl<I: ExactSizeIterator + ?Sized> ExactSizeIterator for Box<I> {
916 fn len(&self) -> usize {
919 fn is_empty(&self) -> bool {
924 #[stable(feature = "fused", since = "1.26.0")]
925 impl<I: FusedIterator + ?Sized> FusedIterator for Box<I> {}
927 #[stable(feature = "boxed_closure_impls", since = "1.35.0")]
928 impl<A, F: FnOnce<A> + ?Sized> FnOnce<A> for Box<F> {
929 type Output = <F as FnOnce<A>>::Output;
931 extern "rust-call" fn call_once(self, args: A) -> Self::Output {
932 <F as FnOnce<A>>::call_once(*self, args)
936 #[stable(feature = "boxed_closure_impls", since = "1.35.0")]
937 impl<A, F: FnMut<A> + ?Sized> FnMut<A> for Box<F> {
938 extern "rust-call" fn call_mut(&mut self, args: A) -> Self::Output {
939 <F as FnMut<A>>::call_mut(self, args)
943 #[stable(feature = "boxed_closure_impls", since = "1.35.0")]
944 impl<A, F: Fn<A> + ?Sized> Fn<A> for Box<F> {
945 extern "rust-call" fn call(&self, args: A) -> Self::Output {
946 <F as Fn<A>>::call(self, args)
950 #[unstable(feature = "coerce_unsized", issue = "27732")]
951 impl<T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<Box<U>> for Box<T> {}
953 #[unstable(feature = "dispatch_from_dyn", issue = "0")]
954 impl<T: ?Sized + Unsize<U>, U: ?Sized> DispatchFromDyn<Box<U>> for Box<T> {}
956 #[stable(feature = "boxed_slice_from_iter", since = "1.32.0")]
957 impl<A> FromIterator<A> for Box<[A]> {
958 fn from_iter<T: IntoIterator<Item = A>>(iter: T) -> Self {
959 iter.into_iter().collect::<Vec<_>>().into_boxed_slice()
963 #[stable(feature = "box_slice_clone", since = "1.3.0")]
964 impl<T: Clone> Clone for Box<[T]> {
965 fn clone(&self) -> Self {
966 let mut new = BoxBuilder {
967 data: RawVec::with_capacity(self.len()),
971 let mut target = new.data.ptr();
973 for item in self.iter() {
975 ptr::write(target, item.clone());
976 target = target.offset(1);
982 return unsafe { new.into_box() };
984 // Helper type for responding to panics correctly.
985 struct BoxBuilder<T> {
990 impl<T> BoxBuilder<T> {
991 unsafe fn into_box(self) -> Box<[T]> {
992 let raw = ptr::read(&self.data);
998 impl<T> Drop for BoxBuilder<T> {
1000 let mut data = self.data.ptr();
1001 let max = unsafe { data.add(self.len) };
1006 data = data.offset(1);
1014 #[stable(feature = "box_borrow", since = "1.1.0")]
1015 impl<T: ?Sized> borrow::Borrow<T> for Box<T> {
1016 fn borrow(&self) -> &T {
1021 #[stable(feature = "box_borrow", since = "1.1.0")]
1022 impl<T: ?Sized> borrow::BorrowMut<T> for Box<T> {
1023 fn borrow_mut(&mut self) -> &mut T {
1028 #[stable(since = "1.5.0", feature = "smart_ptr_as_ref")]
1029 impl<T: ?Sized> AsRef<T> for Box<T> {
1030 fn as_ref(&self) -> &T {
1035 #[stable(since = "1.5.0", feature = "smart_ptr_as_ref")]
1036 impl<T: ?Sized> AsMut<T> for Box<T> {
1037 fn as_mut(&mut self) -> &mut T {
1044 * We could have chosen not to add this impl, and instead have written a
1045 * function of Pin<Box<T>> to Pin<T>. Such a function would not be sound,
1046 * because Box<T> implements Unpin even when T does not, as a result of
1049 * We chose this API instead of the alternative for a few reasons:
1050 * - Logically, it is helpful to understand pinning in regard to the
1051 * memory region being pointed to. For this reason none of the
1052 * standard library pointer types support projecting through a pin
1053 * (Box<T> is the only pointer type in std for which this would be
1055 * - It is in practice very useful to have Box<T> be unconditionally
1056 * Unpin because of trait objects, for which the structural auto
1057 * trait functionality does not apply (e.g., Box<dyn Foo> would
1058 * otherwise not be Unpin).
1060 * Another type with the same semantics as Box but only a conditional
1061 * implementation of `Unpin` (where `T: Unpin`) would be valid/safe, and
1062 * could have a method to project a Pin<T> from it.
1064 #[stable(feature = "pin", since = "1.33.0")]
1065 impl<T: ?Sized> Unpin for Box<T> { }
1067 #[unstable(feature = "generator_trait", issue = "43122")]
1068 impl<G: ?Sized + Generator + Unpin> Generator for 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(Pin::new(&mut *self))
1077 #[unstable(feature = "generator_trait", issue = "43122")]
1078 impl<G: ?Sized + Generator> Generator for Pin<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((*self).as_mut())
1087 #[stable(feature = "futures_api", since = "1.36.0")]
1088 impl<F: ?Sized + Future + Unpin> Future for Box<F> {
1089 type Output = F::Output;
1091 fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
1092 F::poll(Pin::new(&mut *self), cx)