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 `Box` with uninitialized contents, with the memory
156 /// being filled with `0` bytes.
158 /// See [`MaybeUninit::zeroed`][zeroed] for examples of correct and incorrect usage
164 /// #![feature(new_uninit)]
166 /// let zero = Box::<u32>::new_zeroed();
167 /// let zero = unsafe { zero.assume_init() };
169 /// assert_eq!(*zero, 0)
172 /// [zeroed]: ../../std/mem/union.MaybeUninit.html#method.zeroed
173 #[unstable(feature = "new_uninit", issue = "63291")]
174 pub fn new_zeroed() -> Box<mem::MaybeUninit<T>> {
176 let mut uninit = Self::new_uninit();
177 ptr::write_bytes::<T>(uninit.as_mut_ptr(), 0, 1);
182 /// Constructs a new `Pin<Box<T>>`. If `T` does not implement `Unpin`, then
183 /// `x` will be pinned in memory and unable to be moved.
184 #[stable(feature = "pin", since = "1.33.0")]
186 pub fn pin(x: T) -> Pin<Box<T>> {
192 /// Constructs a new boxed slice with uninitialized contents.
197 /// #![feature(new_uninit)]
199 /// let mut values = Box::<[u32]>::new_uninit_slice(3);
201 /// let values = unsafe {
202 /// // Deferred initialization:
203 /// values[0].as_mut_ptr().write(1);
204 /// values[1].as_mut_ptr().write(2);
205 /// values[2].as_mut_ptr().write(3);
207 /// values.assume_init()
210 /// assert_eq!(*values, [1, 2, 3])
212 #[unstable(feature = "new_uninit", issue = "63291")]
213 pub fn new_uninit_slice(len: usize) -> Box<[mem::MaybeUninit<T>]> {
214 let layout = alloc::Layout::array::<mem::MaybeUninit<T>>(len).unwrap();
215 let ptr = if layout.size() == 0 {
220 .unwrap_or_else(|_| alloc::handle_alloc_error(layout))
224 let slice = unsafe { slice::from_raw_parts_mut(ptr.as_ptr(), len) };
225 Box(Unique::from(slice))
229 impl<T> Box<mem::MaybeUninit<T>> {
230 /// Converts to `Box<T>`.
234 /// As with [`MaybeUninit::assume_init`],
235 /// it is up to the caller to guarantee that the value
236 /// really is in an initialized state.
237 /// Calling this when the content is not yet fully initialized
238 /// causes immediate undefined behavior.
240 /// [`MaybeUninit::assume_init`]: ../../std/mem/union.MaybeUninit.html#method.assume_init
245 /// #![feature(new_uninit)]
247 /// let mut five = Box::<u32>::new_uninit();
249 /// let five: Box<u32> = unsafe {
250 /// // Deferred initialization:
251 /// five.as_mut_ptr().write(5);
253 /// five.assume_init()
256 /// assert_eq!(*five, 5)
258 #[unstable(feature = "new_uninit", issue = "63291")]
260 pub unsafe fn assume_init(self) -> Box<T> {
261 Box(Box::into_unique(self).cast())
265 impl<T> Box<[mem::MaybeUninit<T>]> {
266 /// Converts to `Box<[T]>`.
270 /// As with [`MaybeUninit::assume_init`],
271 /// it is up to the caller to guarantee that the values
272 /// really are in an initialized state.
273 /// Calling this when the content is not yet fully initialized
274 /// causes immediate undefined behavior.
276 /// [`MaybeUninit::assume_init`]: ../../std/mem/union.MaybeUninit.html#method.assume_init
281 /// #![feature(new_uninit)]
283 /// let mut values = Box::<[u32]>::new_uninit_slice(3);
285 /// let values = unsafe {
286 /// // Deferred initialization:
287 /// values[0].as_mut_ptr().write(1);
288 /// values[1].as_mut_ptr().write(2);
289 /// values[2].as_mut_ptr().write(3);
291 /// values.assume_init()
294 /// assert_eq!(*values, [1, 2, 3])
296 #[unstable(feature = "new_uninit", issue = "63291")]
298 pub unsafe fn assume_init(self) -> Box<[T]> {
299 Box(Unique::new_unchecked(Box::into_raw(self) as _))
303 impl<T: ?Sized> Box<T> {
304 /// Constructs a box from a raw pointer.
306 /// After calling this function, the raw pointer is owned by the
307 /// resulting `Box`. Specifically, the `Box` destructor will call
308 /// the destructor of `T` and free the allocated memory. For this
309 /// to be safe, the memory must have been allocated in accordance
310 /// with the [memory layout] used by `Box` .
314 /// This function is unsafe because improper use may lead to
315 /// memory problems. For example, a double-free may occur if the
316 /// function is called twice on the same raw pointer.
319 /// Recreate a `Box` which was previously converted to a raw pointer
320 /// using [`Box::into_raw`]:
322 /// let x = Box::new(5);
323 /// let ptr = Box::into_raw(x);
324 /// let x = unsafe { Box::from_raw(ptr) };
326 /// Manually create a `Box` from scratch by using the global allocator:
328 /// use std::alloc::{alloc, Layout};
331 /// let ptr = alloc(Layout::new::<i32>()) as *mut i32;
333 /// let x = Box::from_raw(ptr);
337 /// [memory layout]: index.html#memory-layout
338 /// [`Layout`]: ../alloc/struct.Layout.html
339 /// [`Box::into_raw`]: struct.Box.html#method.into_raw
340 #[stable(feature = "box_raw", since = "1.4.0")]
342 pub unsafe fn from_raw(raw: *mut T) -> Self {
343 Box(Unique::new_unchecked(raw))
346 /// Consumes the `Box`, returning a wrapped raw pointer.
348 /// The pointer will be properly aligned and non-null.
350 /// After calling this function, the caller is responsible for the
351 /// memory previously managed by the `Box`. In particular, the
352 /// caller should properly destroy `T` and release the memory, taking
353 /// into account the [memory layout] used by `Box`. The easiest way to
354 /// do this is to convert the raw pointer back into a `Box` with the
355 /// [`Box::from_raw`] function, allowing the `Box` destructor to perform
358 /// Note: this is an associated function, which means that you have
359 /// to call it as `Box::into_raw(b)` instead of `b.into_raw()`. This
360 /// is so that there is no conflict with a method on the inner type.
363 /// Converting the raw pointer back into a `Box` with [`Box::from_raw`]
364 /// for automatic cleanup:
366 /// let x = Box::new(String::from("Hello"));
367 /// let ptr = Box::into_raw(x);
368 /// let x = unsafe { Box::from_raw(ptr) };
370 /// Manual cleanup by explicitly running the destructor and deallocating
373 /// use std::alloc::{dealloc, Layout};
376 /// let x = Box::new(String::from("Hello"));
377 /// let p = Box::into_raw(x);
379 /// ptr::drop_in_place(p);
380 /// dealloc(p as *mut u8, Layout::new::<String>());
384 /// [memory layout]: index.html#memory-layout
385 /// [`Box::from_raw`]: struct.Box.html#method.from_raw
386 #[stable(feature = "box_raw", since = "1.4.0")]
388 pub fn into_raw(b: Box<T>) -> *mut T {
389 Box::into_raw_non_null(b).as_ptr()
392 /// Consumes the `Box`, returning the wrapped pointer as `NonNull<T>`.
394 /// After calling this function, the caller is responsible for the
395 /// memory previously managed by the `Box`. In particular, the
396 /// caller should properly destroy `T` and release the memory. The
397 /// easiest way to do so is to convert the `NonNull<T>` pointer
398 /// into a raw pointer and back into a `Box` with the [`Box::from_raw`]
401 /// Note: this is an associated function, which means that you have
402 /// to call it as `Box::into_raw_non_null(b)`
403 /// instead of `b.into_raw_non_null()`. This
404 /// is so that there is no conflict with a method on the inner type.
406 /// [`Box::from_raw`]: struct.Box.html#method.from_raw
411 /// #![feature(box_into_raw_non_null)]
413 /// let x = Box::new(5);
414 /// let ptr = Box::into_raw_non_null(x);
416 /// // Clean up the memory by converting the NonNull pointer back
417 /// // into a Box and letting the Box be dropped.
418 /// let x = unsafe { Box::from_raw(ptr.as_ptr()) };
420 #[unstable(feature = "box_into_raw_non_null", issue = "47336")]
422 pub fn into_raw_non_null(b: Box<T>) -> NonNull<T> {
423 Box::into_unique(b).into()
426 #[unstable(feature = "ptr_internals", issue = "0", reason = "use into_raw_non_null instead")]
429 pub fn into_unique(b: Box<T>) -> Unique<T> {
430 let mut unique = b.0;
432 // Box is kind-of a library type, but recognized as a "unique pointer" by
433 // Stacked Borrows. This function here corresponds to "reborrowing to
434 // a raw pointer", but there is no actual reborrow here -- so
435 // without some care, the pointer we are returning here still carries
436 // the tag of `b`, with `Unique` permission.
437 // We round-trip through a mutable reference to avoid that.
438 unsafe { Unique::new_unchecked(unique.as_mut() as *mut T) }
441 /// Consumes and leaks the `Box`, returning a mutable reference,
442 /// `&'a mut T`. Note that the type `T` must outlive the chosen lifetime
443 /// `'a`. If the type has only static references, or none at all, then this
444 /// may be chosen to be `'static`.
446 /// This function is mainly useful for data that lives for the remainder of
447 /// the program's life. Dropping the returned reference will cause a memory
448 /// leak. If this is not acceptable, the reference should first be wrapped
449 /// with the [`Box::from_raw`] function producing a `Box`. This `Box` can
450 /// then be dropped which will properly destroy `T` and release the
451 /// allocated memory.
453 /// Note: this is an associated function, which means that you have
454 /// to call it as `Box::leak(b)` instead of `b.leak()`. This
455 /// is so that there is no conflict with a method on the inner type.
457 /// [`Box::from_raw`]: struct.Box.html#method.from_raw
464 /// let x = Box::new(41);
465 /// let static_ref: &'static mut usize = Box::leak(x);
466 /// *static_ref += 1;
467 /// assert_eq!(*static_ref, 42);
473 /// let x = vec![1, 2, 3].into_boxed_slice();
474 /// let static_ref = Box::leak(x);
475 /// static_ref[0] = 4;
476 /// assert_eq!(*static_ref, [4, 2, 3]);
478 #[stable(feature = "box_leak", since = "1.26.0")]
480 pub fn leak<'a>(b: Box<T>) -> &'a mut T
482 T: 'a // Technically not needed, but kept to be explicit.
484 unsafe { &mut *Box::into_raw(b) }
487 /// Converts a `Box<T>` into a `Pin<Box<T>>`
489 /// This conversion does not allocate on the heap and happens in place.
491 /// This is also available via [`From`].
492 #[unstable(feature = "box_into_pin", issue = "62370")]
493 pub fn into_pin(boxed: Box<T>) -> Pin<Box<T>> {
494 // It's not possible to move or replace the insides of a `Pin<Box<T>>`
495 // when `T: !Unpin`, so it's safe to pin it directly without any
496 // additional requirements.
497 unsafe { Pin::new_unchecked(boxed) }
501 #[stable(feature = "rust1", since = "1.0.0")]
502 unsafe impl<#[may_dangle] T: ?Sized> Drop for Box<T> {
504 // FIXME: Do nothing, drop is currently performed by compiler.
508 #[stable(feature = "rust1", since = "1.0.0")]
509 impl<T: Default> Default for Box<T> {
510 /// Creates a `Box<T>`, with the `Default` value for T.
511 fn default() -> Box<T> {
512 box Default::default()
516 #[stable(feature = "rust1", since = "1.0.0")]
517 impl<T> Default for Box<[T]> {
518 fn default() -> Box<[T]> {
519 Box::<[T; 0]>::new([])
523 #[stable(feature = "default_box_extra", since = "1.17.0")]
524 impl Default for Box<str> {
525 fn default() -> Box<str> {
526 unsafe { from_boxed_utf8_unchecked(Default::default()) }
530 #[stable(feature = "rust1", since = "1.0.0")]
531 impl<T: Clone> Clone for Box<T> {
532 /// Returns a new box with a `clone()` of this box's contents.
537 /// let x = Box::new(5);
538 /// let y = x.clone();
540 /// // The value is the same
541 /// assert_eq!(x, y);
543 /// // But they are unique objects
544 /// assert_ne!(&*x as *const i32, &*y as *const i32);
548 fn clone(&self) -> Box<T> {
549 box { (**self).clone() }
552 /// Copies `source`'s contents into `self` without creating a new allocation.
557 /// let x = Box::new(5);
558 /// let mut y = Box::new(10);
559 /// let yp: *const i32 = &*y;
561 /// y.clone_from(&x);
563 /// // The value is the same
564 /// assert_eq!(x, y);
566 /// // And no allocation occurred
567 /// assert_eq!(yp, &*y);
570 fn clone_from(&mut self, source: &Box<T>) {
571 (**self).clone_from(&(**source));
576 #[stable(feature = "box_slice_clone", since = "1.3.0")]
577 impl Clone for Box<str> {
578 fn clone(&self) -> Self {
579 // this makes a copy of the data
580 let buf: Box<[u8]> = self.as_bytes().into();
582 from_boxed_utf8_unchecked(buf)
587 #[stable(feature = "rust1", since = "1.0.0")]
588 impl<T: ?Sized + PartialEq> PartialEq for Box<T> {
590 fn eq(&self, other: &Box<T>) -> bool {
591 PartialEq::eq(&**self, &**other)
594 fn ne(&self, other: &Box<T>) -> bool {
595 PartialEq::ne(&**self, &**other)
598 #[stable(feature = "rust1", since = "1.0.0")]
599 impl<T: ?Sized + PartialOrd> PartialOrd for Box<T> {
601 fn partial_cmp(&self, other: &Box<T>) -> Option<Ordering> {
602 PartialOrd::partial_cmp(&**self, &**other)
605 fn lt(&self, other: &Box<T>) -> bool {
606 PartialOrd::lt(&**self, &**other)
609 fn le(&self, other: &Box<T>) -> bool {
610 PartialOrd::le(&**self, &**other)
613 fn ge(&self, other: &Box<T>) -> bool {
614 PartialOrd::ge(&**self, &**other)
617 fn gt(&self, other: &Box<T>) -> bool {
618 PartialOrd::gt(&**self, &**other)
621 #[stable(feature = "rust1", since = "1.0.0")]
622 impl<T: ?Sized + Ord> Ord for Box<T> {
624 fn cmp(&self, other: &Box<T>) -> Ordering {
625 Ord::cmp(&**self, &**other)
628 #[stable(feature = "rust1", since = "1.0.0")]
629 impl<T: ?Sized + Eq> Eq for Box<T> {}
631 #[stable(feature = "rust1", since = "1.0.0")]
632 impl<T: ?Sized + Hash> Hash for Box<T> {
633 fn hash<H: Hasher>(&self, state: &mut H) {
634 (**self).hash(state);
638 #[stable(feature = "indirect_hasher_impl", since = "1.22.0")]
639 impl<T: ?Sized + Hasher> Hasher for Box<T> {
640 fn finish(&self) -> u64 {
643 fn write(&mut self, bytes: &[u8]) {
644 (**self).write(bytes)
646 fn write_u8(&mut self, i: u8) {
649 fn write_u16(&mut self, i: u16) {
650 (**self).write_u16(i)
652 fn write_u32(&mut self, i: u32) {
653 (**self).write_u32(i)
655 fn write_u64(&mut self, i: u64) {
656 (**self).write_u64(i)
658 fn write_u128(&mut self, i: u128) {
659 (**self).write_u128(i)
661 fn write_usize(&mut self, i: usize) {
662 (**self).write_usize(i)
664 fn write_i8(&mut self, i: i8) {
667 fn write_i16(&mut self, i: i16) {
668 (**self).write_i16(i)
670 fn write_i32(&mut self, i: i32) {
671 (**self).write_i32(i)
673 fn write_i64(&mut self, i: i64) {
674 (**self).write_i64(i)
676 fn write_i128(&mut self, i: i128) {
677 (**self).write_i128(i)
679 fn write_isize(&mut self, i: isize) {
680 (**self).write_isize(i)
684 #[stable(feature = "from_for_ptrs", since = "1.6.0")]
685 impl<T> From<T> for Box<T> {
686 /// Converts a generic type `T` into a `Box<T>`
688 /// The conversion allocates on the heap and moves `t`
689 /// from the stack into it.
694 /// let boxed = Box::new(5);
696 /// assert_eq!(Box::from(x), boxed);
698 fn from(t: T) -> Self {
703 #[stable(feature = "pin", since = "1.33.0")]
704 impl<T: ?Sized> From<Box<T>> for Pin<Box<T>> {
705 /// Converts a `Box<T>` into a `Pin<Box<T>>`
707 /// This conversion does not allocate on the heap and happens in place.
708 fn from(boxed: Box<T>) -> Self {
713 #[stable(feature = "box_from_slice", since = "1.17.0")]
714 impl<T: Copy> From<&[T]> for Box<[T]> {
715 /// Converts a `&[T]` into a `Box<[T]>`
717 /// This conversion allocates on the heap
718 /// and performs a copy of `slice`.
722 /// // create a &[u8] which will be used to create a Box<[u8]>
723 /// let slice: &[u8] = &[104, 101, 108, 108, 111];
724 /// let boxed_slice: Box<[u8]> = Box::from(slice);
726 /// println!("{:?}", boxed_slice);
728 fn from(slice: &[T]) -> Box<[T]> {
729 let len = slice.len();
730 let buf = RawVec::with_capacity(len);
732 ptr::copy_nonoverlapping(slice.as_ptr(), buf.ptr(), len);
738 #[stable(feature = "box_from_slice", since = "1.17.0")]
739 impl From<&str> for Box<str> {
740 /// Converts a `&str` into a `Box<str>`
742 /// This conversion allocates on the heap
743 /// and performs a copy of `s`.
747 /// let boxed: Box<str> = Box::from("hello");
748 /// println!("{}", boxed);
751 fn from(s: &str) -> Box<str> {
752 unsafe { from_boxed_utf8_unchecked(Box::from(s.as_bytes())) }
756 #[stable(feature = "boxed_str_conv", since = "1.19.0")]
757 impl From<Box<str>> for Box<[u8]> {
758 /// Converts a `Box<str>>` into a `Box<[u8]>`
760 /// This conversion does not allocate on the heap and happens in place.
764 /// // create a Box<str> which will be used to create a Box<[u8]>
765 /// let boxed: Box<str> = Box::from("hello");
766 /// let boxed_str: Box<[u8]> = Box::from(boxed);
768 /// // create a &[u8] which will be used to create a Box<[u8]>
769 /// let slice: &[u8] = &[104, 101, 108, 108, 111];
770 /// let boxed_slice = Box::from(slice);
772 /// assert_eq!(boxed_slice, boxed_str);
775 fn from(s: Box<str>) -> Self {
776 unsafe { Box::from_raw(Box::into_raw(s) as *mut [u8]) }
780 #[unstable(feature = "boxed_slice_try_from", issue = "0")]
781 impl<T, const N: usize> TryFrom<Box<[T]>> for Box<[T; N]>
783 [T; N]: LengthAtMost32,
785 type Error = Box<[T]>;
787 fn try_from(boxed_slice: Box<[T]>) -> Result<Self, Self::Error> {
788 if boxed_slice.len() == N {
789 Ok(unsafe { Box::from_raw(Box::into_raw(boxed_slice) as *mut [T; N]) })
798 #[stable(feature = "rust1", since = "1.0.0")]
799 /// Attempt to downcast the box to a concrete type.
804 /// use std::any::Any;
806 /// fn print_if_string(value: Box<dyn Any>) {
807 /// if let Ok(string) = value.downcast::<String>() {
808 /// println!("String ({}): {}", string.len(), string);
812 /// let my_string = "Hello World".to_string();
813 /// print_if_string(Box::new(my_string));
814 /// print_if_string(Box::new(0i8));
816 pub fn downcast<T: Any>(self) -> Result<Box<T>, Box<dyn Any>> {
819 let raw: *mut dyn Any = Box::into_raw(self);
820 Ok(Box::from_raw(raw as *mut T))
828 impl Box<dyn Any + Send> {
830 #[stable(feature = "rust1", since = "1.0.0")]
831 /// Attempt to downcast the box to a concrete type.
836 /// use std::any::Any;
838 /// fn print_if_string(value: Box<dyn Any + Send>) {
839 /// if let Ok(string) = value.downcast::<String>() {
840 /// println!("String ({}): {}", string.len(), string);
844 /// let my_string = "Hello World".to_string();
845 /// print_if_string(Box::new(my_string));
846 /// print_if_string(Box::new(0i8));
848 pub fn downcast<T: Any>(self) -> Result<Box<T>, Box<dyn Any + Send>> {
849 <Box<dyn Any>>::downcast(self).map_err(|s| unsafe {
850 // reapply the Send marker
851 Box::from_raw(Box::into_raw(s) as *mut (dyn Any + Send))
856 #[stable(feature = "rust1", since = "1.0.0")]
857 impl<T: fmt::Display + ?Sized> fmt::Display for Box<T> {
858 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
859 fmt::Display::fmt(&**self, f)
863 #[stable(feature = "rust1", since = "1.0.0")]
864 impl<T: fmt::Debug + ?Sized> fmt::Debug for Box<T> {
865 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
866 fmt::Debug::fmt(&**self, f)
870 #[stable(feature = "rust1", since = "1.0.0")]
871 impl<T: ?Sized> fmt::Pointer for Box<T> {
872 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
873 // It's not possible to extract the inner Uniq directly from the Box,
874 // instead we cast it to a *const which aliases the Unique
875 let ptr: *const T = &**self;
876 fmt::Pointer::fmt(&ptr, f)
880 #[stable(feature = "rust1", since = "1.0.0")]
881 impl<T: ?Sized> Deref for Box<T> {
884 fn deref(&self) -> &T {
889 #[stable(feature = "rust1", since = "1.0.0")]
890 impl<T: ?Sized> DerefMut for Box<T> {
891 fn deref_mut(&mut self) -> &mut T {
896 #[unstable(feature = "receiver_trait", issue = "0")]
897 impl<T: ?Sized> Receiver for Box<T> {}
899 #[stable(feature = "rust1", since = "1.0.0")]
900 impl<I: Iterator + ?Sized> Iterator for Box<I> {
902 fn next(&mut self) -> Option<I::Item> {
905 fn size_hint(&self) -> (usize, Option<usize>) {
908 fn nth(&mut self, n: usize) -> Option<I::Item> {
911 fn last(self) -> Option<I::Item> {
918 fn last(self) -> Option<Self::Item>;
921 impl<I: Iterator + ?Sized> BoxIter for Box<I> {
923 default fn last(self) -> Option<I::Item> {
925 fn some<T>(_: Option<T>, x: T) -> Option<T> {
929 self.fold(None, some)
933 /// Specialization for sized `I`s that uses `I`s implementation of `last()`
934 /// instead of the default.
935 #[stable(feature = "rust1", since = "1.0.0")]
936 impl<I: Iterator> BoxIter for Box<I> {
937 fn last(self) -> Option<I::Item> {
942 #[stable(feature = "rust1", since = "1.0.0")]
943 impl<I: DoubleEndedIterator + ?Sized> DoubleEndedIterator for Box<I> {
944 fn next_back(&mut self) -> Option<I::Item> {
947 fn nth_back(&mut self, n: usize) -> Option<I::Item> {
951 #[stable(feature = "rust1", since = "1.0.0")]
952 impl<I: ExactSizeIterator + ?Sized> ExactSizeIterator for Box<I> {
953 fn len(&self) -> usize {
956 fn is_empty(&self) -> bool {
961 #[stable(feature = "fused", since = "1.26.0")]
962 impl<I: FusedIterator + ?Sized> FusedIterator for Box<I> {}
964 #[stable(feature = "boxed_closure_impls", since = "1.35.0")]
965 impl<A, F: FnOnce<A> + ?Sized> FnOnce<A> for Box<F> {
966 type Output = <F as FnOnce<A>>::Output;
968 extern "rust-call" fn call_once(self, args: A) -> Self::Output {
969 <F as FnOnce<A>>::call_once(*self, args)
973 #[stable(feature = "boxed_closure_impls", since = "1.35.0")]
974 impl<A, F: FnMut<A> + ?Sized> FnMut<A> for Box<F> {
975 extern "rust-call" fn call_mut(&mut self, args: A) -> Self::Output {
976 <F as FnMut<A>>::call_mut(self, args)
980 #[stable(feature = "boxed_closure_impls", since = "1.35.0")]
981 impl<A, F: Fn<A> + ?Sized> Fn<A> for Box<F> {
982 extern "rust-call" fn call(&self, args: A) -> Self::Output {
983 <F as Fn<A>>::call(self, args)
987 #[unstable(feature = "coerce_unsized", issue = "27732")]
988 impl<T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<Box<U>> for Box<T> {}
990 #[unstable(feature = "dispatch_from_dyn", issue = "0")]
991 impl<T: ?Sized + Unsize<U>, U: ?Sized> DispatchFromDyn<Box<U>> for Box<T> {}
993 #[stable(feature = "boxed_slice_from_iter", since = "1.32.0")]
994 impl<A> FromIterator<A> for Box<[A]> {
995 fn from_iter<T: IntoIterator<Item = A>>(iter: T) -> Self {
996 iter.into_iter().collect::<Vec<_>>().into_boxed_slice()
1000 #[stable(feature = "box_slice_clone", since = "1.3.0")]
1001 impl<T: Clone> Clone for Box<[T]> {
1002 fn clone(&self) -> Self {
1003 let mut new = BoxBuilder {
1004 data: RawVec::with_capacity(self.len()),
1008 let mut target = new.data.ptr();
1010 for item in self.iter() {
1012 ptr::write(target, item.clone());
1013 target = target.offset(1);
1019 return unsafe { new.into_box() };
1021 // Helper type for responding to panics correctly.
1022 struct BoxBuilder<T> {
1027 impl<T> BoxBuilder<T> {
1028 unsafe fn into_box(self) -> Box<[T]> {
1029 let raw = ptr::read(&self.data);
1035 impl<T> Drop for BoxBuilder<T> {
1036 fn drop(&mut self) {
1037 let mut data = self.data.ptr();
1038 let max = unsafe { data.add(self.len) };
1043 data = data.offset(1);
1051 #[stable(feature = "box_borrow", since = "1.1.0")]
1052 impl<T: ?Sized> borrow::Borrow<T> for Box<T> {
1053 fn borrow(&self) -> &T {
1058 #[stable(feature = "box_borrow", since = "1.1.0")]
1059 impl<T: ?Sized> borrow::BorrowMut<T> for Box<T> {
1060 fn borrow_mut(&mut self) -> &mut T {
1065 #[stable(since = "1.5.0", feature = "smart_ptr_as_ref")]
1066 impl<T: ?Sized> AsRef<T> for Box<T> {
1067 fn as_ref(&self) -> &T {
1072 #[stable(since = "1.5.0", feature = "smart_ptr_as_ref")]
1073 impl<T: ?Sized> AsMut<T> for Box<T> {
1074 fn as_mut(&mut self) -> &mut T {
1081 * We could have chosen not to add this impl, and instead have written a
1082 * function of Pin<Box<T>> to Pin<T>. Such a function would not be sound,
1083 * because Box<T> implements Unpin even when T does not, as a result of
1086 * We chose this API instead of the alternative for a few reasons:
1087 * - Logically, it is helpful to understand pinning in regard to the
1088 * memory region being pointed to. For this reason none of the
1089 * standard library pointer types support projecting through a pin
1090 * (Box<T> is the only pointer type in std for which this would be
1092 * - It is in practice very useful to have Box<T> be unconditionally
1093 * Unpin because of trait objects, for which the structural auto
1094 * trait functionality does not apply (e.g., Box<dyn Foo> would
1095 * otherwise not be Unpin).
1097 * Another type with the same semantics as Box but only a conditional
1098 * implementation of `Unpin` (where `T: Unpin`) would be valid/safe, and
1099 * could have a method to project a Pin<T> from it.
1101 #[stable(feature = "pin", since = "1.33.0")]
1102 impl<T: ?Sized> Unpin for Box<T> { }
1104 #[unstable(feature = "generator_trait", issue = "43122")]
1105 impl<G: ?Sized + Generator + Unpin> Generator for Box<G> {
1106 type Yield = G::Yield;
1107 type Return = G::Return;
1109 fn resume(mut self: Pin<&mut Self>) -> GeneratorState<Self::Yield, Self::Return> {
1110 G::resume(Pin::new(&mut *self))
1114 #[unstable(feature = "generator_trait", issue = "43122")]
1115 impl<G: ?Sized + Generator> Generator for Pin<Box<G>> {
1116 type Yield = G::Yield;
1117 type Return = G::Return;
1119 fn resume(mut self: Pin<&mut Self>) -> GeneratorState<Self::Yield, Self::Return> {
1120 G::resume((*self).as_mut())
1124 #[stable(feature = "futures_api", since = "1.36.0")]
1125 impl<F: ?Sized + Future + Unpin> Future for Box<F> {
1126 type Output = F::Output;
1128 fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
1129 F::poll(Pin::new(&mut *self), cx)