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`, 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)`.
66 //! So long as `T: Sized`, a `Box<T>` is guaranteed to be represented as a
67 //! single pointer and is also ABI-compatible with C pointers (i.e. the C type
68 //! `T*`). This means that you have Rust code which passes ownership of a
69 //! `Box<T>` to C code by using `Box<T>` as the type on the Rust side, and
70 //! `T*` as the corresponding type on the C side. As an example, consider this
71 //! C header which declares functions that create and destroy some kind of
76 //! struct Foo* foo_new(void); /* Returns ownership to the caller */
77 //! void foo_delete(struct Foo*); /* Takes ownership from the caller */
80 //! These two functions might be implemented in Rust as follows. Here, the
81 //! `struct Foo*` type from C is translated to `Box<Foo>`, which captures
82 //! the ownership constraints. Note also that the nullable argument to
83 //! `foo_delete` is represented in Rust as `Option<Box<Foo>>`, since `Box<Foo>`
91 //! pub extern "C" fn foo_new() -> Box<Foo> {
96 //! pub extern "C" fn foo_delete(_: Option<Box<Foo>>) {}
99 //! Even though `Box<T>` has the same representation and C ABI as a C pointer,
100 //! this does not mean that you can convert an arbitrary `T*` into a `Box<T>`
101 //! and expect things to work. `Box<T>` values will always be fully aligned,
102 //! non-null pointers. Moreover, the destructor for `Box<T>` will attempt to
103 //! free the value with the global allocator. In general, the best practice
104 //! is to only use `Box<T>` for pointers that originated from the global
107 //! [dereferencing]: ../../std/ops/trait.Deref.html
108 //! [`Box`]: struct.Box.html
109 //! [`Global`]: ../alloc/struct.Global.html
110 //! [`Layout`]: ../alloc/struct.Layout.html
112 #![stable(feature = "rust1", since = "1.0.0")]
116 use core::cmp::Ordering;
117 use core::convert::From;
119 use core::future::Future;
120 use core::hash::{Hash, Hasher};
121 use core::iter::{Iterator, FromIterator, FusedIterator};
122 use core::marker::{Unpin, Unsize};
126 CoerceUnsized, DispatchFromDyn, Deref, DerefMut, Receiver, Generator, GeneratorState
128 use core::ptr::{self, NonNull, Unique};
129 use core::task::{Context, Poll};
132 use crate::raw_vec::RawVec;
133 use crate::str::from_boxed_utf8_unchecked;
135 /// A pointer type for heap allocation.
137 /// See the [module-level documentation](../../std/boxed/index.html) for more.
138 #[lang = "owned_box"]
140 #[stable(feature = "rust1", since = "1.0.0")]
141 pub struct Box<T: ?Sized>(Unique<T>);
144 /// Allocates memory on the heap and then places `x` into it.
146 /// This doesn't actually allocate if `T` is zero-sized.
151 /// let five = Box::new(5);
153 #[stable(feature = "rust1", since = "1.0.0")]
155 pub fn new(x: T) -> Box<T> {
159 /// Constructs a new `Pin<Box<T>>`. If `T` does not implement `Unpin`, then
160 /// `x` will be pinned in memory and unable to be moved.
161 #[stable(feature = "pin", since = "1.33.0")]
163 pub fn pin(x: T) -> Pin<Box<T>> {
168 impl<T: ?Sized> Box<T> {
169 /// Constructs a box from a raw pointer.
171 /// After calling this function, the raw pointer is owned by the
172 /// resulting `Box`. Specifically, the `Box` destructor will call
173 /// the destructor of `T` and free the allocated memory. For this
174 /// to be safe, the memory must have been allocated in accordance
175 /// with the [memory layout] used by `Box` .
179 /// This function is unsafe because improper use may lead to
180 /// memory problems. For example, a double-free may occur if the
181 /// function is called twice on the same raw pointer.
184 /// Recreate a `Box` which was previously converted to a raw pointer
185 /// using [`Box::into_raw`]:
187 /// let x = Box::new(5);
188 /// let ptr = Box::into_raw(x);
189 /// let x = unsafe { Box::from_raw(ptr) };
191 /// Manually create a `Box` from scratch by using the global allocator:
193 /// use std::alloc::{alloc, Layout};
196 /// let ptr = alloc(Layout::new::<i32>()) as *mut i32;
198 /// let x = Box::from_raw(ptr);
202 /// [memory layout]: index.html#memory-layout
203 /// [`Layout`]: ../alloc/struct.Layout.html
204 /// [`Box::into_raw`]: struct.Box.html#method.into_raw
205 #[stable(feature = "box_raw", since = "1.4.0")]
207 pub unsafe fn from_raw(raw: *mut T) -> Self {
208 Box(Unique::new_unchecked(raw))
211 /// Consumes the `Box`, returning a wrapped raw pointer.
213 /// The pointer will be properly aligned and non-null.
215 /// After calling this function, the caller is responsible for the
216 /// memory previously managed by the `Box`. In particular, the
217 /// caller should properly destroy `T` and release the memory, taking
218 /// into account the [memory layout] used by `Box`. The easiest way to
219 /// do this is to convert the raw pointer back into a `Box` with the
220 /// [`Box::from_raw`] function, allowing the `Box` destructor to perform
223 /// Note: this is an associated function, which means that you have
224 /// to call it as `Box::into_raw(b)` instead of `b.into_raw()`. This
225 /// is so that there is no conflict with a method on the inner type.
228 /// Converting the raw pointer back into a `Box` with [`Box::from_raw`]
229 /// for automatic cleanup:
231 /// let x = Box::new(String::from("Hello"));
232 /// let ptr = Box::into_raw(x);
233 /// let x = unsafe { Box::from_raw(ptr) };
235 /// Manual cleanup by explicitly running the destructor and deallocating
238 /// use std::alloc::{dealloc, Layout};
241 /// let x = Box::new(String::from("Hello"));
242 /// let p = Box::into_raw(x);
244 /// ptr::drop_in_place(p);
245 /// dealloc(p as *mut u8, Layout::new::<String>());
249 /// [memory layout]: index.html#memory-layout
250 /// [`Box::from_raw`]: struct.Box.html#method.from_raw
251 #[stable(feature = "box_raw", since = "1.4.0")]
253 pub fn into_raw(b: Box<T>) -> *mut T {
254 Box::into_raw_non_null(b).as_ptr()
257 /// Consumes the `Box`, returning the wrapped pointer as `NonNull<T>`.
259 /// After calling this function, the caller is responsible for the
260 /// memory previously managed by the `Box`. In particular, the
261 /// caller should properly destroy `T` and release the memory. The
262 /// easiest way to do so is to convert the `NonNull<T>` pointer
263 /// into a raw pointer and back into a `Box` with the [`Box::from_raw`]
266 /// Note: this is an associated function, which means that you have
267 /// to call it as `Box::into_raw_non_null(b)`
268 /// instead of `b.into_raw_non_null()`. This
269 /// is so that there is no conflict with a method on the inner type.
271 /// [`Box::from_raw`]: struct.Box.html#method.from_raw
276 /// #![feature(box_into_raw_non_null)]
279 /// let x = Box::new(5);
280 /// let ptr = Box::into_raw_non_null(x);
282 /// // Clean up the memory by converting the NonNull pointer back
283 /// // into a Box and letting the Box be dropped.
284 /// let x = unsafe { Box::from_raw(ptr.as_ptr()) };
287 #[unstable(feature = "box_into_raw_non_null", issue = "47336")]
289 pub fn into_raw_non_null(b: Box<T>) -> NonNull<T> {
290 Box::into_unique(b).into()
293 #[unstable(feature = "ptr_internals", issue = "0", reason = "use into_raw_non_null instead")]
296 pub fn into_unique(b: Box<T>) -> Unique<T> {
297 let mut unique = b.0;
299 // Box is kind-of a library type, but recognized as a "unique pointer" by
300 // Stacked Borrows. This function here corresponds to "reborrowing to
301 // a raw pointer", but there is no actual reborrow here -- so
302 // without some care, the pointer we are returning here still carries
303 // the tag of `b`, with `Unique` permission.
304 // We round-trip through a mutable reference to avoid that.
305 unsafe { Unique::new_unchecked(unique.as_mut() as *mut T) }
308 /// Consumes and leaks the `Box`, returning a mutable reference,
309 /// `&'a mut T`. Note that the type `T` must outlive the chosen lifetime
310 /// `'a`. If the type has only static references, or none at all, then this
311 /// may be chosen to be `'static`.
313 /// This function is mainly useful for data that lives for the remainder of
314 /// the program's life. Dropping the returned reference will cause a memory
315 /// leak. If this is not acceptable, the reference should first be wrapped
316 /// with the [`Box::from_raw`] function producing a `Box`. This `Box` can
317 /// then be dropped which will properly destroy `T` and release the
318 /// allocated memory.
320 /// Note: this is an associated function, which means that you have
321 /// to call it as `Box::leak(b)` instead of `b.leak()`. This
322 /// is so that there is no conflict with a method on the inner type.
324 /// [`Box::from_raw`]: struct.Box.html#method.from_raw
332 /// let x = Box::new(41);
333 /// let static_ref: &'static mut usize = Box::leak(x);
334 /// *static_ref += 1;
335 /// assert_eq!(*static_ref, 42);
343 /// let x = vec![1, 2, 3].into_boxed_slice();
344 /// let static_ref = Box::leak(x);
345 /// static_ref[0] = 4;
346 /// assert_eq!(*static_ref, [4, 2, 3]);
349 #[stable(feature = "box_leak", since = "1.26.0")]
351 pub fn leak<'a>(b: Box<T>) -> &'a mut T
353 T: 'a // Technically not needed, but kept to be explicit.
355 unsafe { &mut *Box::into_raw(b) }
358 /// Converts a `Box<T>` into a `Pin<Box<T>>`
360 /// This conversion does not allocate on the heap and happens in place.
362 /// This is also available via [`From`].
363 #[unstable(feature = "box_into_pin", issue = "62370")]
364 pub fn into_pin(boxed: Box<T>) -> Pin<Box<T>> {
365 // It's not possible to move or replace the insides of a `Pin<Box<T>>`
366 // when `T: !Unpin`, so it's safe to pin it directly without any
367 // additional requirements.
368 unsafe { Pin::new_unchecked(boxed) }
372 #[stable(feature = "rust1", since = "1.0.0")]
373 unsafe impl<#[may_dangle] T: ?Sized> Drop for Box<T> {
375 // FIXME: Do nothing, drop is currently performed by compiler.
379 #[stable(feature = "rust1", since = "1.0.0")]
380 impl<T: Default> Default for Box<T> {
381 /// Creates a `Box<T>`, with the `Default` value for T.
382 fn default() -> Box<T> {
383 box Default::default()
387 #[stable(feature = "rust1", since = "1.0.0")]
388 impl<T> Default for Box<[T]> {
389 fn default() -> Box<[T]> {
390 Box::<[T; 0]>::new([])
394 #[stable(feature = "default_box_extra", since = "1.17.0")]
395 impl Default for Box<str> {
396 fn default() -> Box<str> {
397 unsafe { from_boxed_utf8_unchecked(Default::default()) }
401 #[stable(feature = "rust1", since = "1.0.0")]
402 impl<T: Clone> Clone for Box<T> {
403 /// Returns a new box with a `clone()` of this box's contents.
408 /// let x = Box::new(5);
409 /// let y = x.clone();
411 /// // The value is the same
412 /// assert_eq!(x, y);
414 /// // But they are unique objects
415 /// assert_ne!(&*x as *const i32, &*y as *const i32);
419 fn clone(&self) -> Box<T> {
420 box { (**self).clone() }
423 /// Copies `source`'s contents into `self` without creating a new allocation.
428 /// let x = Box::new(5);
429 /// let mut y = Box::new(10);
430 /// let yp: *const i32 = &*y;
432 /// y.clone_from(&x);
434 /// // The value is the same
435 /// assert_eq!(x, y);
437 /// // And no allocation occurred
438 /// assert_eq!(yp, &*y);
441 fn clone_from(&mut self, source: &Box<T>) {
442 (**self).clone_from(&(**source));
447 #[stable(feature = "box_slice_clone", since = "1.3.0")]
448 impl Clone for Box<str> {
449 fn clone(&self) -> Self {
450 // this makes a copy of the data
451 let buf: Box<[u8]> = self.as_bytes().into();
453 from_boxed_utf8_unchecked(buf)
458 #[stable(feature = "rust1", since = "1.0.0")]
459 impl<T: ?Sized + PartialEq> PartialEq for Box<T> {
461 fn eq(&self, other: &Box<T>) -> bool {
462 PartialEq::eq(&**self, &**other)
465 fn ne(&self, other: &Box<T>) -> bool {
466 PartialEq::ne(&**self, &**other)
469 #[stable(feature = "rust1", since = "1.0.0")]
470 impl<T: ?Sized + PartialOrd> PartialOrd for Box<T> {
472 fn partial_cmp(&self, other: &Box<T>) -> Option<Ordering> {
473 PartialOrd::partial_cmp(&**self, &**other)
476 fn lt(&self, other: &Box<T>) -> bool {
477 PartialOrd::lt(&**self, &**other)
480 fn le(&self, other: &Box<T>) -> bool {
481 PartialOrd::le(&**self, &**other)
484 fn ge(&self, other: &Box<T>) -> bool {
485 PartialOrd::ge(&**self, &**other)
488 fn gt(&self, other: &Box<T>) -> bool {
489 PartialOrd::gt(&**self, &**other)
492 #[stable(feature = "rust1", since = "1.0.0")]
493 impl<T: ?Sized + Ord> Ord for Box<T> {
495 fn cmp(&self, other: &Box<T>) -> Ordering {
496 Ord::cmp(&**self, &**other)
499 #[stable(feature = "rust1", since = "1.0.0")]
500 impl<T: ?Sized + Eq> Eq for Box<T> {}
502 #[stable(feature = "rust1", since = "1.0.0")]
503 impl<T: ?Sized + Hash> Hash for Box<T> {
504 fn hash<H: Hasher>(&self, state: &mut H) {
505 (**self).hash(state);
509 #[stable(feature = "indirect_hasher_impl", since = "1.22.0")]
510 impl<T: ?Sized + Hasher> Hasher for Box<T> {
511 fn finish(&self) -> u64 {
514 fn write(&mut self, bytes: &[u8]) {
515 (**self).write(bytes)
517 fn write_u8(&mut self, i: u8) {
520 fn write_u16(&mut self, i: u16) {
521 (**self).write_u16(i)
523 fn write_u32(&mut self, i: u32) {
524 (**self).write_u32(i)
526 fn write_u64(&mut self, i: u64) {
527 (**self).write_u64(i)
529 fn write_u128(&mut self, i: u128) {
530 (**self).write_u128(i)
532 fn write_usize(&mut self, i: usize) {
533 (**self).write_usize(i)
535 fn write_i8(&mut self, i: i8) {
538 fn write_i16(&mut self, i: i16) {
539 (**self).write_i16(i)
541 fn write_i32(&mut self, i: i32) {
542 (**self).write_i32(i)
544 fn write_i64(&mut self, i: i64) {
545 (**self).write_i64(i)
547 fn write_i128(&mut self, i: i128) {
548 (**self).write_i128(i)
550 fn write_isize(&mut self, i: isize) {
551 (**self).write_isize(i)
555 #[stable(feature = "from_for_ptrs", since = "1.6.0")]
556 impl<T> From<T> for Box<T> {
557 /// Converts a generic type `T` into a `Box<T>`
559 /// The conversion allocates on the heap and moves `t`
560 /// from the stack into it.
565 /// let boxed = Box::new(5);
567 /// assert_eq!(Box::from(x), boxed);
569 fn from(t: T) -> Self {
574 #[stable(feature = "pin", since = "1.33.0")]
575 impl<T: ?Sized> From<Box<T>> for Pin<Box<T>> {
576 /// Converts a `Box<T>` into a `Pin<Box<T>>`
578 /// This conversion does not allocate on the heap and happens in place.
579 fn from(boxed: Box<T>) -> Self {
584 #[stable(feature = "box_from_slice", since = "1.17.0")]
585 impl<T: Copy> From<&[T]> for Box<[T]> {
586 /// Converts a `&[T]` into a `Box<[T]>`
588 /// This conversion allocates on the heap
589 /// and performs a copy of `slice`.
593 /// // create a &[u8] which will be used to create a Box<[u8]>
594 /// let slice: &[u8] = &[104, 101, 108, 108, 111];
595 /// let boxed_slice: Box<[u8]> = Box::from(slice);
597 /// println!("{:?}", boxed_slice);
599 fn from(slice: &[T]) -> Box<[T]> {
600 let len = slice.len();
601 let buf = RawVec::with_capacity(len);
603 ptr::copy_nonoverlapping(slice.as_ptr(), buf.ptr(), len);
609 #[stable(feature = "box_from_slice", since = "1.17.0")]
610 impl From<&str> for Box<str> {
611 /// Converts a `&str` into a `Box<str>`
613 /// This conversion allocates on the heap
614 /// and performs a copy of `s`.
618 /// let boxed: Box<str> = Box::from("hello");
619 /// println!("{}", boxed);
622 fn from(s: &str) -> Box<str> {
623 unsafe { from_boxed_utf8_unchecked(Box::from(s.as_bytes())) }
627 #[stable(feature = "boxed_str_conv", since = "1.19.0")]
628 impl From<Box<str>> for Box<[u8]> {
629 /// Converts a `Box<str>>` into a `Box<[u8]>`
631 /// This conversion does not allocate on the heap and happens in place.
635 /// // create a Box<str> which will be used to create a Box<[u8]>
636 /// let boxed: Box<str> = Box::from("hello");
637 /// let boxed_str: Box<[u8]> = Box::from(boxed);
639 /// // create a &[u8] which will be used to create a Box<[u8]>
640 /// let slice: &[u8] = &[104, 101, 108, 108, 111];
641 /// let boxed_slice = Box::from(slice);
643 /// assert_eq!(boxed_slice, boxed_str);
646 fn from(s: Box<str>) -> Self {
647 unsafe { Box::from_raw(Box::into_raw(s) as *mut [u8]) }
653 #[stable(feature = "rust1", since = "1.0.0")]
654 /// Attempt to downcast the box to a concrete type.
659 /// use std::any::Any;
661 /// fn print_if_string(value: Box<dyn Any>) {
662 /// if let Ok(string) = value.downcast::<String>() {
663 /// println!("String ({}): {}", string.len(), string);
668 /// let my_string = "Hello World".to_string();
669 /// print_if_string(Box::new(my_string));
670 /// print_if_string(Box::new(0i8));
673 pub fn downcast<T: Any>(self) -> Result<Box<T>, Box<dyn Any>> {
676 let raw: *mut dyn Any = Box::into_raw(self);
677 Ok(Box::from_raw(raw as *mut T))
685 impl Box<dyn Any + Send> {
687 #[stable(feature = "rust1", since = "1.0.0")]
688 /// Attempt to downcast the box to a concrete type.
693 /// use std::any::Any;
695 /// fn print_if_string(value: Box<dyn Any + Send>) {
696 /// if let Ok(string) = value.downcast::<String>() {
697 /// println!("String ({}): {}", string.len(), string);
702 /// let my_string = "Hello World".to_string();
703 /// print_if_string(Box::new(my_string));
704 /// print_if_string(Box::new(0i8));
707 pub fn downcast<T: Any>(self) -> Result<Box<T>, Box<dyn Any + Send>> {
708 <Box<dyn Any>>::downcast(self).map_err(|s| unsafe {
709 // reapply the Send marker
710 Box::from_raw(Box::into_raw(s) as *mut (dyn Any + Send))
715 #[stable(feature = "rust1", since = "1.0.0")]
716 impl<T: fmt::Display + ?Sized> fmt::Display for Box<T> {
717 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
718 fmt::Display::fmt(&**self, f)
722 #[stable(feature = "rust1", since = "1.0.0")]
723 impl<T: fmt::Debug + ?Sized> fmt::Debug for Box<T> {
724 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
725 fmt::Debug::fmt(&**self, f)
729 #[stable(feature = "rust1", since = "1.0.0")]
730 impl<T: ?Sized> fmt::Pointer for Box<T> {
731 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
732 // It's not possible to extract the inner Uniq directly from the Box,
733 // instead we cast it to a *const which aliases the Unique
734 let ptr: *const T = &**self;
735 fmt::Pointer::fmt(&ptr, f)
739 #[stable(feature = "rust1", since = "1.0.0")]
740 impl<T: ?Sized> Deref for Box<T> {
743 fn deref(&self) -> &T {
748 #[stable(feature = "rust1", since = "1.0.0")]
749 impl<T: ?Sized> DerefMut for Box<T> {
750 fn deref_mut(&mut self) -> &mut T {
755 #[unstable(feature = "receiver_trait", issue = "0")]
756 impl<T: ?Sized> Receiver for Box<T> {}
758 #[stable(feature = "rust1", since = "1.0.0")]
759 impl<I: Iterator + ?Sized> Iterator for Box<I> {
761 fn next(&mut self) -> Option<I::Item> {
764 fn size_hint(&self) -> (usize, Option<usize>) {
767 fn nth(&mut self, n: usize) -> Option<I::Item> {
772 #[stable(feature = "rust1", since = "1.0.0")]
773 impl<I: Iterator + Sized> Iterator for Box<I> {
774 fn last(self) -> Option<I::Item> where I: Sized {
779 #[stable(feature = "rust1", since = "1.0.0")]
780 impl<I: DoubleEndedIterator + ?Sized> DoubleEndedIterator for Box<I> {
781 fn next_back(&mut self) -> Option<I::Item> {
784 fn nth_back(&mut self, n: usize) -> Option<I::Item> {
788 #[stable(feature = "rust1", since = "1.0.0")]
789 impl<I: ExactSizeIterator + ?Sized> ExactSizeIterator for Box<I> {
790 fn len(&self) -> usize {
793 fn is_empty(&self) -> bool {
798 #[stable(feature = "fused", since = "1.26.0")]
799 impl<I: FusedIterator + ?Sized> FusedIterator for Box<I> {}
801 #[stable(feature = "boxed_closure_impls", since = "1.35.0")]
802 impl<A, F: FnOnce<A> + ?Sized> FnOnce<A> for Box<F> {
803 type Output = <F as FnOnce<A>>::Output;
805 extern "rust-call" fn call_once(self, args: A) -> Self::Output {
806 <F as FnOnce<A>>::call_once(*self, args)
810 #[stable(feature = "boxed_closure_impls", since = "1.35.0")]
811 impl<A, F: FnMut<A> + ?Sized> FnMut<A> for Box<F> {
812 extern "rust-call" fn call_mut(&mut self, args: A) -> Self::Output {
813 <F as FnMut<A>>::call_mut(self, args)
817 #[stable(feature = "boxed_closure_impls", since = "1.35.0")]
818 impl<A, F: Fn<A> + ?Sized> Fn<A> for Box<F> {
819 extern "rust-call" fn call(&self, args: A) -> Self::Output {
820 <F as Fn<A>>::call(self, args)
824 #[unstable(feature = "coerce_unsized", issue = "27732")]
825 impl<T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<Box<U>> for Box<T> {}
827 #[unstable(feature = "dispatch_from_dyn", issue = "0")]
828 impl<T: ?Sized + Unsize<U>, U: ?Sized> DispatchFromDyn<Box<U>> for Box<T> {}
830 #[stable(feature = "boxed_slice_from_iter", since = "1.32.0")]
831 impl<A> FromIterator<A> for Box<[A]> {
832 fn from_iter<T: IntoIterator<Item = A>>(iter: T) -> Self {
833 iter.into_iter().collect::<Vec<_>>().into_boxed_slice()
837 #[stable(feature = "box_slice_clone", since = "1.3.0")]
838 impl<T: Clone> Clone for Box<[T]> {
839 fn clone(&self) -> Self {
840 let mut new = BoxBuilder {
841 data: RawVec::with_capacity(self.len()),
845 let mut target = new.data.ptr();
847 for item in self.iter() {
849 ptr::write(target, item.clone());
850 target = target.offset(1);
856 return unsafe { new.into_box() };
858 // Helper type for responding to panics correctly.
859 struct BoxBuilder<T> {
864 impl<T> BoxBuilder<T> {
865 unsafe fn into_box(self) -> Box<[T]> {
866 let raw = ptr::read(&self.data);
872 impl<T> Drop for BoxBuilder<T> {
874 let mut data = self.data.ptr();
875 let max = unsafe { data.add(self.len) };
880 data = data.offset(1);
888 #[stable(feature = "box_borrow", since = "1.1.0")]
889 impl<T: ?Sized> borrow::Borrow<T> for Box<T> {
890 fn borrow(&self) -> &T {
895 #[stable(feature = "box_borrow", since = "1.1.0")]
896 impl<T: ?Sized> borrow::BorrowMut<T> for Box<T> {
897 fn borrow_mut(&mut self) -> &mut T {
902 #[stable(since = "1.5.0", feature = "smart_ptr_as_ref")]
903 impl<T: ?Sized> AsRef<T> for Box<T> {
904 fn as_ref(&self) -> &T {
909 #[stable(since = "1.5.0", feature = "smart_ptr_as_ref")]
910 impl<T: ?Sized> AsMut<T> for Box<T> {
911 fn as_mut(&mut self) -> &mut T {
918 * We could have chosen not to add this impl, and instead have written a
919 * function of Pin<Box<T>> to Pin<T>. Such a function would not be sound,
920 * because Box<T> implements Unpin even when T does not, as a result of
923 * We chose this API instead of the alternative for a few reasons:
924 * - Logically, it is helpful to understand pinning in regard to the
925 * memory region being pointed to. For this reason none of the
926 * standard library pointer types support projecting through a pin
927 * (Box<T> is the only pointer type in std for which this would be
929 * - It is in practice very useful to have Box<T> be unconditionally
930 * Unpin because of trait objects, for which the structural auto
931 * trait functionality does not apply (e.g., Box<dyn Foo> would
932 * otherwise not be Unpin).
934 * Another type with the same semantics as Box but only a conditional
935 * implementation of `Unpin` (where `T: Unpin`) would be valid/safe, and
936 * could have a method to project a Pin<T> from it.
938 #[stable(feature = "pin", since = "1.33.0")]
939 impl<T: ?Sized> Unpin for Box<T> { }
941 #[unstable(feature = "generator_trait", issue = "43122")]
942 impl<G: ?Sized + Generator + Unpin> Generator for Box<G> {
943 type Yield = G::Yield;
944 type Return = G::Return;
946 fn resume(mut self: Pin<&mut Self>) -> GeneratorState<Self::Yield, Self::Return> {
947 G::resume(Pin::new(&mut *self))
951 #[unstable(feature = "generator_trait", issue = "43122")]
952 impl<G: ?Sized + Generator> Generator for Pin<Box<G>> {
953 type Yield = G::Yield;
954 type Return = G::Return;
956 fn resume(mut self: Pin<&mut Self>) -> GeneratorState<Self::Yield, Self::Return> {
957 G::resume((*self).as_mut())
961 #[stable(feature = "futures_api", since = "1.36.0")]
962 impl<F: ?Sized + Future + Unpin> Future for Box<F> {
963 type Output = F::Output;
965 fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
966 F::poll(Pin::new(&mut *self), cx)