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
67 //! as a single pointer and is also ABI-compatible with C pointers
68 //! (i.e. the C type `T*`). This means that if you have extern "C"
69 //! Rust functions that will be called from C, you can define those
70 //! Rust functions using `Box<T>` types, and use `T*` as corresponding
71 //! type on the C side. As an example, consider this C header which
72 //! declares functions that create and destroy some kind of `Foo`
78 //! /* Returns ownership to the caller */
79 //! struct Foo* foo_new(void);
81 //! /* Takes ownership from the caller; no-op when invoked with NULL */
82 //! void foo_delete(struct Foo*);
85 //! These two functions might be implemented in Rust as follows. Here, the
86 //! `struct Foo*` type from C is translated to `Box<Foo>`, which captures
87 //! the ownership constraints. Note also that the nullable argument to
88 //! `foo_delete` is represented in Rust as `Option<Box<Foo>>`, since `Box<Foo>`
96 //! pub extern "C" fn foo_new() -> Box<Foo> {
101 //! pub extern "C" fn foo_delete(_: Option<Box<Foo>>) {}
104 //! Even though `Box<T>` has the same representation and C ABI as a C pointer,
105 //! this does not mean that you can convert an arbitrary `T*` into a `Box<T>`
106 //! and expect things to work. `Box<T>` values will always be fully aligned,
107 //! non-null pointers. Moreover, the destructor for `Box<T>` will attempt to
108 //! free the value with the global allocator. In general, the best practice
109 //! is to only use `Box<T>` for pointers that originated from the global
112 //! **Important.** At least at present, you should avoid using
113 //! `Box<T>` types for functions that are defined in C but invoked
114 //! from Rust. In those cases, you should directly mirror the C types
115 //! as closely as possible. Using types like `Box<T>` where the C
116 //! definition is just using `T*` can lead to undefined behavior, as
117 //! described in [rust-lang/unsafe-code-guidelines#198][ucg#198].
119 //! [ucg#198]: https://github.com/rust-lang/unsafe-code-guidelines/issues/198
120 //! [dereferencing]: ../../std/ops/trait.Deref.html
121 //! [`Box`]: struct.Box.html
122 //! [`Global`]: ../alloc/struct.Global.html
123 //! [`Layout`]: ../alloc/struct.Layout.html
125 #![stable(feature = "rust1", since = "1.0.0")]
129 use core::cmp::Ordering;
130 use core::convert::From;
132 use core::future::Future;
133 use core::hash::{Hash, Hasher};
134 use core::iter::{Iterator, FromIterator, FusedIterator};
135 use core::marker::{Unpin, Unsize};
139 CoerceUnsized, DispatchFromDyn, Deref, DerefMut, Receiver, Generator, GeneratorState
141 use core::ptr::{self, NonNull, Unique};
142 use core::task::{Context, Poll};
145 use crate::raw_vec::RawVec;
146 use crate::str::from_boxed_utf8_unchecked;
148 /// A pointer type for heap allocation.
150 /// See the [module-level documentation](../../std/boxed/index.html) for more.
151 #[lang = "owned_box"]
153 #[stable(feature = "rust1", since = "1.0.0")]
154 pub struct Box<T: ?Sized>(Unique<T>);
157 /// Allocates memory on the heap and then places `x` into it.
159 /// This doesn't actually allocate if `T` is zero-sized.
164 /// let five = Box::new(5);
166 #[stable(feature = "rust1", since = "1.0.0")]
168 pub fn new(x: T) -> Box<T> {
172 /// Constructs a new `Pin<Box<T>>`. If `T` does not implement `Unpin`, then
173 /// `x` will be pinned in memory and unable to be moved.
174 #[stable(feature = "pin", since = "1.33.0")]
176 pub fn pin(x: T) -> Pin<Box<T>> {
181 impl<T: ?Sized> Box<T> {
182 /// Constructs a box from a raw pointer.
184 /// After calling this function, the raw pointer is owned by the
185 /// resulting `Box`. Specifically, the `Box` destructor will call
186 /// the destructor of `T` and free the allocated memory. For this
187 /// to be safe, the memory must have been allocated in accordance
188 /// with the [memory layout] used by `Box` .
192 /// This function is unsafe because improper use may lead to
193 /// memory problems. For example, a double-free may occur if the
194 /// function is called twice on the same raw pointer.
197 /// Recreate a `Box` which was previously converted to a raw pointer
198 /// using [`Box::into_raw`]:
200 /// let x = Box::new(5);
201 /// let ptr = Box::into_raw(x);
202 /// let x = unsafe { Box::from_raw(ptr) };
204 /// Manually create a `Box` from scratch by using the global allocator:
206 /// use std::alloc::{alloc, Layout};
209 /// let ptr = alloc(Layout::new::<i32>()) as *mut i32;
211 /// let x = Box::from_raw(ptr);
215 /// [memory layout]: index.html#memory-layout
216 /// [`Layout`]: ../alloc/struct.Layout.html
217 /// [`Box::into_raw`]: struct.Box.html#method.into_raw
218 #[stable(feature = "box_raw", since = "1.4.0")]
220 pub unsafe fn from_raw(raw: *mut T) -> Self {
221 Box(Unique::new_unchecked(raw))
224 /// Consumes the `Box`, returning a wrapped raw pointer.
226 /// The pointer will be properly aligned and non-null.
228 /// After calling this function, the caller is responsible for the
229 /// memory previously managed by the `Box`. In particular, the
230 /// caller should properly destroy `T` and release the memory, taking
231 /// into account the [memory layout] used by `Box`. The easiest way to
232 /// do this is to convert the raw pointer back into a `Box` with the
233 /// [`Box::from_raw`] function, allowing the `Box` destructor to perform
236 /// Note: this is an associated function, which means that you have
237 /// to call it as `Box::into_raw(b)` instead of `b.into_raw()`. This
238 /// is so that there is no conflict with a method on the inner type.
241 /// Converting the raw pointer back into a `Box` with [`Box::from_raw`]
242 /// for automatic cleanup:
244 /// let x = Box::new(String::from("Hello"));
245 /// let ptr = Box::into_raw(x);
246 /// let x = unsafe { Box::from_raw(ptr) };
248 /// Manual cleanup by explicitly running the destructor and deallocating
251 /// use std::alloc::{dealloc, Layout};
254 /// let x = Box::new(String::from("Hello"));
255 /// let p = Box::into_raw(x);
257 /// ptr::drop_in_place(p);
258 /// dealloc(p as *mut u8, Layout::new::<String>());
262 /// [memory layout]: index.html#memory-layout
263 /// [`Box::from_raw`]: struct.Box.html#method.from_raw
264 #[stable(feature = "box_raw", since = "1.4.0")]
266 pub fn into_raw(b: Box<T>) -> *mut T {
267 Box::into_raw_non_null(b).as_ptr()
270 /// Consumes the `Box`, returning the wrapped pointer as `NonNull<T>`.
272 /// After calling this function, the caller is responsible for the
273 /// memory previously managed by the `Box`. In particular, the
274 /// caller should properly destroy `T` and release the memory. The
275 /// easiest way to do so is to convert the `NonNull<T>` pointer
276 /// into a raw pointer and back into a `Box` with the [`Box::from_raw`]
279 /// Note: this is an associated function, which means that you have
280 /// to call it as `Box::into_raw_non_null(b)`
281 /// instead of `b.into_raw_non_null()`. This
282 /// is so that there is no conflict with a method on the inner type.
284 /// [`Box::from_raw`]: struct.Box.html#method.from_raw
289 /// #![feature(box_into_raw_non_null)]
292 /// let x = Box::new(5);
293 /// let ptr = Box::into_raw_non_null(x);
295 /// // Clean up the memory by converting the NonNull pointer back
296 /// // into a Box and letting the Box be dropped.
297 /// let x = unsafe { Box::from_raw(ptr.as_ptr()) };
300 #[unstable(feature = "box_into_raw_non_null", issue = "47336")]
302 pub fn into_raw_non_null(b: Box<T>) -> NonNull<T> {
303 Box::into_unique(b).into()
306 #[unstable(feature = "ptr_internals", issue = "0", reason = "use into_raw_non_null instead")]
309 pub fn into_unique(b: Box<T>) -> Unique<T> {
310 let mut unique = b.0;
312 // Box is kind-of a library type, but recognized as a "unique pointer" by
313 // Stacked Borrows. This function here corresponds to "reborrowing to
314 // a raw pointer", but there is no actual reborrow here -- so
315 // without some care, the pointer we are returning here still carries
316 // the tag of `b`, with `Unique` permission.
317 // We round-trip through a mutable reference to avoid that.
318 unsafe { Unique::new_unchecked(unique.as_mut() as *mut T) }
321 /// Consumes and leaks the `Box`, returning a mutable reference,
322 /// `&'a mut T`. Note that the type `T` must outlive the chosen lifetime
323 /// `'a`. If the type has only static references, or none at all, then this
324 /// may be chosen to be `'static`.
326 /// This function is mainly useful for data that lives for the remainder of
327 /// the program's life. Dropping the returned reference will cause a memory
328 /// leak. If this is not acceptable, the reference should first be wrapped
329 /// with the [`Box::from_raw`] function producing a `Box`. This `Box` can
330 /// then be dropped which will properly destroy `T` and release the
331 /// allocated memory.
333 /// Note: this is an associated function, which means that you have
334 /// to call it as `Box::leak(b)` instead of `b.leak()`. This
335 /// is so that there is no conflict with a method on the inner type.
337 /// [`Box::from_raw`]: struct.Box.html#method.from_raw
345 /// let x = Box::new(41);
346 /// let static_ref: &'static mut usize = Box::leak(x);
347 /// *static_ref += 1;
348 /// assert_eq!(*static_ref, 42);
356 /// let x = vec![1, 2, 3].into_boxed_slice();
357 /// let static_ref = Box::leak(x);
358 /// static_ref[0] = 4;
359 /// assert_eq!(*static_ref, [4, 2, 3]);
362 #[stable(feature = "box_leak", since = "1.26.0")]
364 pub fn leak<'a>(b: Box<T>) -> &'a mut T
366 T: 'a // Technically not needed, but kept to be explicit.
368 unsafe { &mut *Box::into_raw(b) }
371 /// Converts a `Box<T>` into a `Pin<Box<T>>`
373 /// This conversion does not allocate on the heap and happens in place.
375 /// This is also available via [`From`].
376 #[unstable(feature = "box_into_pin", issue = "62370")]
377 pub fn into_pin(boxed: Box<T>) -> Pin<Box<T>> {
378 // It's not possible to move or replace the insides of a `Pin<Box<T>>`
379 // when `T: !Unpin`, so it's safe to pin it directly without any
380 // additional requirements.
381 unsafe { Pin::new_unchecked(boxed) }
385 #[stable(feature = "rust1", since = "1.0.0")]
386 unsafe impl<#[may_dangle] T: ?Sized> Drop for Box<T> {
388 // FIXME: Do nothing, drop is currently performed by compiler.
392 #[stable(feature = "rust1", since = "1.0.0")]
393 impl<T: Default> Default for Box<T> {
394 /// Creates a `Box<T>`, with the `Default` value for T.
395 fn default() -> Box<T> {
396 box Default::default()
400 #[stable(feature = "rust1", since = "1.0.0")]
401 impl<T> Default for Box<[T]> {
402 fn default() -> Box<[T]> {
403 Box::<[T; 0]>::new([])
407 #[stable(feature = "default_box_extra", since = "1.17.0")]
408 impl Default for Box<str> {
409 fn default() -> Box<str> {
410 unsafe { from_boxed_utf8_unchecked(Default::default()) }
414 #[stable(feature = "rust1", since = "1.0.0")]
415 impl<T: Clone> Clone for Box<T> {
416 /// Returns a new box with a `clone()` of this box's contents.
421 /// let x = Box::new(5);
422 /// let y = x.clone();
424 /// // The value is the same
425 /// assert_eq!(x, y);
427 /// // But they are unique objects
428 /// assert_ne!(&*x as *const i32, &*y as *const i32);
432 fn clone(&self) -> Box<T> {
433 box { (**self).clone() }
436 /// Copies `source`'s contents into `self` without creating a new allocation.
441 /// let x = Box::new(5);
442 /// let mut y = Box::new(10);
443 /// let yp: *const i32 = &*y;
445 /// y.clone_from(&x);
447 /// // The value is the same
448 /// assert_eq!(x, y);
450 /// // And no allocation occurred
451 /// assert_eq!(yp, &*y);
454 fn clone_from(&mut self, source: &Box<T>) {
455 (**self).clone_from(&(**source));
460 #[stable(feature = "box_slice_clone", since = "1.3.0")]
461 impl Clone for Box<str> {
462 fn clone(&self) -> Self {
463 // this makes a copy of the data
464 let buf: Box<[u8]> = self.as_bytes().into();
466 from_boxed_utf8_unchecked(buf)
471 #[stable(feature = "rust1", since = "1.0.0")]
472 impl<T: ?Sized + PartialEq> PartialEq for Box<T> {
474 fn eq(&self, other: &Box<T>) -> bool {
475 PartialEq::eq(&**self, &**other)
478 fn ne(&self, other: &Box<T>) -> bool {
479 PartialEq::ne(&**self, &**other)
482 #[stable(feature = "rust1", since = "1.0.0")]
483 impl<T: ?Sized + PartialOrd> PartialOrd for Box<T> {
485 fn partial_cmp(&self, other: &Box<T>) -> Option<Ordering> {
486 PartialOrd::partial_cmp(&**self, &**other)
489 fn lt(&self, other: &Box<T>) -> bool {
490 PartialOrd::lt(&**self, &**other)
493 fn le(&self, other: &Box<T>) -> bool {
494 PartialOrd::le(&**self, &**other)
497 fn ge(&self, other: &Box<T>) -> bool {
498 PartialOrd::ge(&**self, &**other)
501 fn gt(&self, other: &Box<T>) -> bool {
502 PartialOrd::gt(&**self, &**other)
505 #[stable(feature = "rust1", since = "1.0.0")]
506 impl<T: ?Sized + Ord> Ord for Box<T> {
508 fn cmp(&self, other: &Box<T>) -> Ordering {
509 Ord::cmp(&**self, &**other)
512 #[stable(feature = "rust1", since = "1.0.0")]
513 impl<T: ?Sized + Eq> Eq for Box<T> {}
515 #[stable(feature = "rust1", since = "1.0.0")]
516 impl<T: ?Sized + Hash> Hash for Box<T> {
517 fn hash<H: Hasher>(&self, state: &mut H) {
518 (**self).hash(state);
522 #[stable(feature = "indirect_hasher_impl", since = "1.22.0")]
523 impl<T: ?Sized + Hasher> Hasher for Box<T> {
524 fn finish(&self) -> u64 {
527 fn write(&mut self, bytes: &[u8]) {
528 (**self).write(bytes)
530 fn write_u8(&mut self, i: u8) {
533 fn write_u16(&mut self, i: u16) {
534 (**self).write_u16(i)
536 fn write_u32(&mut self, i: u32) {
537 (**self).write_u32(i)
539 fn write_u64(&mut self, i: u64) {
540 (**self).write_u64(i)
542 fn write_u128(&mut self, i: u128) {
543 (**self).write_u128(i)
545 fn write_usize(&mut self, i: usize) {
546 (**self).write_usize(i)
548 fn write_i8(&mut self, i: i8) {
551 fn write_i16(&mut self, i: i16) {
552 (**self).write_i16(i)
554 fn write_i32(&mut self, i: i32) {
555 (**self).write_i32(i)
557 fn write_i64(&mut self, i: i64) {
558 (**self).write_i64(i)
560 fn write_i128(&mut self, i: i128) {
561 (**self).write_i128(i)
563 fn write_isize(&mut self, i: isize) {
564 (**self).write_isize(i)
568 #[stable(feature = "from_for_ptrs", since = "1.6.0")]
569 impl<T> From<T> for Box<T> {
570 /// Converts a generic type `T` into a `Box<T>`
572 /// The conversion allocates on the heap and moves `t`
573 /// from the stack into it.
578 /// let boxed = Box::new(5);
580 /// assert_eq!(Box::from(x), boxed);
582 fn from(t: T) -> Self {
587 #[stable(feature = "pin", since = "1.33.0")]
588 impl<T: ?Sized> From<Box<T>> for Pin<Box<T>> {
589 /// Converts a `Box<T>` into a `Pin<Box<T>>`
591 /// This conversion does not allocate on the heap and happens in place.
592 fn from(boxed: Box<T>) -> Self {
597 #[stable(feature = "box_from_slice", since = "1.17.0")]
598 impl<T: Copy> From<&[T]> for Box<[T]> {
599 /// Converts a `&[T]` into a `Box<[T]>`
601 /// This conversion allocates on the heap
602 /// and performs a copy of `slice`.
606 /// // create a &[u8] which will be used to create a Box<[u8]>
607 /// let slice: &[u8] = &[104, 101, 108, 108, 111];
608 /// let boxed_slice: Box<[u8]> = Box::from(slice);
610 /// println!("{:?}", boxed_slice);
612 fn from(slice: &[T]) -> Box<[T]> {
613 let len = slice.len();
614 let buf = RawVec::with_capacity(len);
616 ptr::copy_nonoverlapping(slice.as_ptr(), buf.ptr(), len);
622 #[stable(feature = "box_from_slice", since = "1.17.0")]
623 impl From<&str> for Box<str> {
624 /// Converts a `&str` into a `Box<str>`
626 /// This conversion allocates on the heap
627 /// and performs a copy of `s`.
631 /// let boxed: Box<str> = Box::from("hello");
632 /// println!("{}", boxed);
635 fn from(s: &str) -> Box<str> {
636 unsafe { from_boxed_utf8_unchecked(Box::from(s.as_bytes())) }
640 #[stable(feature = "boxed_str_conv", since = "1.19.0")]
641 impl From<Box<str>> for Box<[u8]> {
642 /// Converts a `Box<str>>` into a `Box<[u8]>`
644 /// This conversion does not allocate on the heap and happens in place.
648 /// // create a Box<str> which will be used to create a Box<[u8]>
649 /// let boxed: Box<str> = Box::from("hello");
650 /// let boxed_str: Box<[u8]> = Box::from(boxed);
652 /// // create a &[u8] which will be used to create a Box<[u8]>
653 /// let slice: &[u8] = &[104, 101, 108, 108, 111];
654 /// let boxed_slice = Box::from(slice);
656 /// assert_eq!(boxed_slice, boxed_str);
659 fn from(s: Box<str>) -> Self {
660 unsafe { Box::from_raw(Box::into_raw(s) as *mut [u8]) }
666 #[stable(feature = "rust1", since = "1.0.0")]
667 /// Attempt to downcast the box to a concrete type.
672 /// use std::any::Any;
674 /// fn print_if_string(value: Box<dyn Any>) {
675 /// if let Ok(string) = value.downcast::<String>() {
676 /// println!("String ({}): {}", string.len(), string);
681 /// let my_string = "Hello World".to_string();
682 /// print_if_string(Box::new(my_string));
683 /// print_if_string(Box::new(0i8));
686 pub fn downcast<T: Any>(self) -> Result<Box<T>, Box<dyn Any>> {
689 let raw: *mut dyn Any = Box::into_raw(self);
690 Ok(Box::from_raw(raw as *mut T))
698 impl Box<dyn Any + Send> {
700 #[stable(feature = "rust1", since = "1.0.0")]
701 /// Attempt to downcast the box to a concrete type.
706 /// use std::any::Any;
708 /// fn print_if_string(value: Box<dyn Any + Send>) {
709 /// if let Ok(string) = value.downcast::<String>() {
710 /// println!("String ({}): {}", string.len(), string);
715 /// let my_string = "Hello World".to_string();
716 /// print_if_string(Box::new(my_string));
717 /// print_if_string(Box::new(0i8));
720 pub fn downcast<T: Any>(self) -> Result<Box<T>, Box<dyn Any + Send>> {
721 <Box<dyn Any>>::downcast(self).map_err(|s| unsafe {
722 // reapply the Send marker
723 Box::from_raw(Box::into_raw(s) as *mut (dyn Any + Send))
728 #[stable(feature = "rust1", since = "1.0.0")]
729 impl<T: fmt::Display + ?Sized> fmt::Display for Box<T> {
730 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
731 fmt::Display::fmt(&**self, f)
735 #[stable(feature = "rust1", since = "1.0.0")]
736 impl<T: fmt::Debug + ?Sized> fmt::Debug for Box<T> {
737 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
738 fmt::Debug::fmt(&**self, f)
742 #[stable(feature = "rust1", since = "1.0.0")]
743 impl<T: ?Sized> fmt::Pointer for Box<T> {
744 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
745 // It's not possible to extract the inner Uniq directly from the Box,
746 // instead we cast it to a *const which aliases the Unique
747 let ptr: *const T = &**self;
748 fmt::Pointer::fmt(&ptr, f)
752 #[stable(feature = "rust1", since = "1.0.0")]
753 impl<T: ?Sized> Deref for Box<T> {
756 fn deref(&self) -> &T {
761 #[stable(feature = "rust1", since = "1.0.0")]
762 impl<T: ?Sized> DerefMut for Box<T> {
763 fn deref_mut(&mut self) -> &mut T {
768 #[unstable(feature = "receiver_trait", issue = "0")]
769 impl<T: ?Sized> Receiver for Box<T> {}
771 #[stable(feature = "rust1", since = "1.0.0")]
772 impl<I: Iterator + ?Sized> Iterator for Box<I> {
774 fn next(&mut self) -> Option<I::Item> {
777 fn size_hint(&self) -> (usize, Option<usize>) {
780 fn nth(&mut self, n: usize) -> Option<I::Item> {
785 #[stable(feature = "rust1", since = "1.0.0")]
786 impl<I: Iterator + Sized> Iterator for Box<I> {
787 fn last(self) -> Option<I::Item> where I: Sized {
792 #[stable(feature = "rust1", since = "1.0.0")]
793 impl<I: DoubleEndedIterator + ?Sized> DoubleEndedIterator for Box<I> {
794 fn next_back(&mut self) -> Option<I::Item> {
797 fn nth_back(&mut self, n: usize) -> Option<I::Item> {
801 #[stable(feature = "rust1", since = "1.0.0")]
802 impl<I: ExactSizeIterator + ?Sized> ExactSizeIterator for Box<I> {
803 fn len(&self) -> usize {
806 fn is_empty(&self) -> bool {
811 #[stable(feature = "fused", since = "1.26.0")]
812 impl<I: FusedIterator + ?Sized> FusedIterator for Box<I> {}
814 #[stable(feature = "boxed_closure_impls", since = "1.35.0")]
815 impl<A, F: FnOnce<A> + ?Sized> FnOnce<A> for Box<F> {
816 type Output = <F as FnOnce<A>>::Output;
818 extern "rust-call" fn call_once(self, args: A) -> Self::Output {
819 <F as FnOnce<A>>::call_once(*self, args)
823 #[stable(feature = "boxed_closure_impls", since = "1.35.0")]
824 impl<A, F: FnMut<A> + ?Sized> FnMut<A> for Box<F> {
825 extern "rust-call" fn call_mut(&mut self, args: A) -> Self::Output {
826 <F as FnMut<A>>::call_mut(self, args)
830 #[stable(feature = "boxed_closure_impls", since = "1.35.0")]
831 impl<A, F: Fn<A> + ?Sized> Fn<A> for Box<F> {
832 extern "rust-call" fn call(&self, args: A) -> Self::Output {
833 <F as Fn<A>>::call(self, args)
837 #[unstable(feature = "coerce_unsized", issue = "27732")]
838 impl<T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<Box<U>> for Box<T> {}
840 #[unstable(feature = "dispatch_from_dyn", issue = "0")]
841 impl<T: ?Sized + Unsize<U>, U: ?Sized> DispatchFromDyn<Box<U>> for Box<T> {}
843 #[stable(feature = "boxed_slice_from_iter", since = "1.32.0")]
844 impl<A> FromIterator<A> for Box<[A]> {
845 fn from_iter<T: IntoIterator<Item = A>>(iter: T) -> Self {
846 iter.into_iter().collect::<Vec<_>>().into_boxed_slice()
850 #[stable(feature = "box_slice_clone", since = "1.3.0")]
851 impl<T: Clone> Clone for Box<[T]> {
852 fn clone(&self) -> Self {
853 let mut new = BoxBuilder {
854 data: RawVec::with_capacity(self.len()),
858 let mut target = new.data.ptr();
860 for item in self.iter() {
862 ptr::write(target, item.clone());
863 target = target.offset(1);
869 return unsafe { new.into_box() };
871 // Helper type for responding to panics correctly.
872 struct BoxBuilder<T> {
877 impl<T> BoxBuilder<T> {
878 unsafe fn into_box(self) -> Box<[T]> {
879 let raw = ptr::read(&self.data);
885 impl<T> Drop for BoxBuilder<T> {
887 let mut data = self.data.ptr();
888 let max = unsafe { data.add(self.len) };
893 data = data.offset(1);
901 #[stable(feature = "box_borrow", since = "1.1.0")]
902 impl<T: ?Sized> borrow::Borrow<T> for Box<T> {
903 fn borrow(&self) -> &T {
908 #[stable(feature = "box_borrow", since = "1.1.0")]
909 impl<T: ?Sized> borrow::BorrowMut<T> for Box<T> {
910 fn borrow_mut(&mut self) -> &mut T {
915 #[stable(since = "1.5.0", feature = "smart_ptr_as_ref")]
916 impl<T: ?Sized> AsRef<T> for Box<T> {
917 fn as_ref(&self) -> &T {
922 #[stable(since = "1.5.0", feature = "smart_ptr_as_ref")]
923 impl<T: ?Sized> AsMut<T> for Box<T> {
924 fn as_mut(&mut self) -> &mut T {
931 * We could have chosen not to add this impl, and instead have written a
932 * function of Pin<Box<T>> to Pin<T>. Such a function would not be sound,
933 * because Box<T> implements Unpin even when T does not, as a result of
936 * We chose this API instead of the alternative for a few reasons:
937 * - Logically, it is helpful to understand pinning in regard to the
938 * memory region being pointed to. For this reason none of the
939 * standard library pointer types support projecting through a pin
940 * (Box<T> is the only pointer type in std for which this would be
942 * - It is in practice very useful to have Box<T> be unconditionally
943 * Unpin because of trait objects, for which the structural auto
944 * trait functionality does not apply (e.g., Box<dyn Foo> would
945 * otherwise not be Unpin).
947 * Another type with the same semantics as Box but only a conditional
948 * implementation of `Unpin` (where `T: Unpin`) would be valid/safe, and
949 * could have a method to project a Pin<T> from it.
951 #[stable(feature = "pin", since = "1.33.0")]
952 impl<T: ?Sized> Unpin for Box<T> { }
954 #[unstable(feature = "generator_trait", issue = "43122")]
955 impl<G: ?Sized + Generator + Unpin> Generator for Box<G> {
956 type Yield = G::Yield;
957 type Return = G::Return;
959 fn resume(mut self: Pin<&mut Self>) -> GeneratorState<Self::Yield, Self::Return> {
960 G::resume(Pin::new(&mut *self))
964 #[unstable(feature = "generator_trait", issue = "43122")]
965 impl<G: ?Sized + Generator> Generator for Pin<Box<G>> {
966 type Yield = G::Yield;
967 type Return = G::Return;
969 fn resume(mut self: Pin<&mut Self>) -> GeneratorState<Self::Yield, Self::Return> {
970 G::resume((*self).as_mut())
974 #[stable(feature = "futures_api", since = "1.36.0")]
975 impl<F: ?Sized + Future + Unpin> Future for Box<F> {
976 type Output = F::Output;
978 fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
979 F::poll(Pin::new(&mut *self), cx)