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. Boxes also ensure that they
6 //! never allocate more than `isize::MAX` bytes.
10 //! Move a value from the stack to the heap by creating a [`Box`]:
14 //! let boxed: Box<u8> = Box::new(val);
17 //! Move a value from a [`Box`] back to the stack by [dereferencing]:
20 //! let boxed: Box<u8> = Box::new(5);
21 //! let val: u8 = *boxed;
24 //! Creating a recursive data structure:
29 //! 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);
37 //! This will print `Cons(1, Cons(2, Nil))`.
39 //! Recursive structures must be boxed, because if the definition of `Cons`
42 //! ```compile_fail,E0072
48 //! It wouldn't work. This is because the size of a `List` depends on how many
49 //! elements are in the list, and so we don't know how much memory to allocate
50 //! for a `Cons`. By introducing a [`Box<T>`], which has a defined size, we know how
51 //! big `Cons` needs to be.
55 //! For non-zero-sized values, a [`Box`] will use the [`Global`] allocator for
56 //! its allocation. It is valid to convert both ways between a [`Box`] and a
57 //! raw pointer allocated with the [`Global`] allocator, given that the
58 //! [`Layout`] used with the allocator is correct for the type. More precisely,
59 //! a `value: *mut T` that has been allocated with the [`Global`] allocator
60 //! with `Layout::for_value(&*value)` may be converted into a box using
61 //! [`Box::<T>::from_raw(value)`]. Conversely, the memory backing a `value: *mut
62 //! T` obtained from [`Box::<T>::into_raw`] may be deallocated using the
63 //! [`Global`] allocator with [`Layout::for_value(&*value)`].
65 //! So long as `T: Sized`, a `Box<T>` is guaranteed to be represented
66 //! as a single pointer and is also ABI-compatible with C pointers
67 //! (i.e. the C type `T*`). This means that if you have extern "C"
68 //! Rust functions that will be called from C, you can define those
69 //! Rust functions using `Box<T>` types, and use `T*` as corresponding
70 //! type on the C side. As an example, consider this C header which
71 //! declares functions that create and destroy some kind of `Foo`
77 //! /* Returns ownership to the caller */
78 //! struct Foo* foo_new(void);
80 //! /* Takes ownership from the caller; no-op when invoked with NULL */
81 //! void foo_delete(struct Foo*);
84 //! These two functions might be implemented in Rust as follows. Here, the
85 //! `struct Foo*` type from C is translated to `Box<Foo>`, which captures
86 //! the ownership constraints. Note also that the nullable argument to
87 //! `foo_delete` is represented in Rust as `Option<Box<Foo>>`, since `Box<Foo>`
95 //! pub extern "C" fn foo_new() -> Box<Foo> {
100 //! pub extern "C" fn foo_delete(_: Option<Box<Foo>>) {}
103 //! Even though `Box<T>` has the same representation and C ABI as a C pointer,
104 //! this does not mean that you can convert an arbitrary `T*` into a `Box<T>`
105 //! and expect things to work. `Box<T>` values will always be fully aligned,
106 //! non-null pointers. Moreover, the destructor for `Box<T>` will attempt to
107 //! free the value with the global allocator. In general, the best practice
108 //! is to only use `Box<T>` for pointers that originated from the global
111 //! **Important.** At least at present, you should avoid using
112 //! `Box<T>` types for functions that are defined in C but invoked
113 //! from Rust. In those cases, you should directly mirror the C types
114 //! as closely as possible. Using types like `Box<T>` where the C
115 //! definition is just using `T*` can lead to undefined behavior, as
116 //! described in [rust-lang/unsafe-code-guidelines#198][ucg#198].
118 //! [ucg#198]: https://github.com/rust-lang/unsafe-code-guidelines/issues/198
119 //! [dereferencing]: ../../std/ops/trait.Deref.html
120 //! [`Box`]: struct.Box.html
121 //! [`Box<T>`]: struct.Box.html
122 //! [`Box::<T>::from_raw(value)`]: struct.Box.html#method.from_raw
123 //! [`Box::<T>::into_raw`]: struct.Box.html#method.into_raw
124 //! [`Global`]: ../alloc/struct.Global.html
125 //! [`Layout`]: ../alloc/struct.Layout.html
126 //! [`Layout::for_value(&*value)`]: ../alloc/struct.Layout.html#method.for_value
128 #![stable(feature = "rust1", since = "1.0.0")]
131 use core::array::LengthAtMost32;
133 use core::cmp::Ordering;
134 use core::convert::{From, TryFrom};
136 use core::future::Future;
137 use core::hash::{Hash, Hasher};
138 use core::iter::{FromIterator, FusedIterator, Iterator};
139 use core::marker::{Unpin, Unsize};
142 CoerceUnsized, Deref, DerefMut, DispatchFromDyn, Generator, GeneratorState, Receiver,
145 use core::ptr::{self, NonNull, Unique};
147 use core::task::{Context, Poll};
149 use crate::alloc::{self, AllocRef, Global};
150 use crate::raw_vec::RawVec;
151 use crate::str::from_boxed_utf8_unchecked;
154 /// A pointer type for heap allocation.
156 /// See the [module-level documentation](../../std/boxed/index.html) for more.
157 #[lang = "owned_box"]
159 #[stable(feature = "rust1", since = "1.0.0")]
160 pub struct Box<T: ?Sized>(Unique<T>);
163 /// Allocates memory on the heap and then places `x` into it.
165 /// This doesn't actually allocate if `T` is zero-sized.
170 /// let five = Box::new(5);
172 #[stable(feature = "rust1", since = "1.0.0")]
174 pub fn new(x: T) -> Box<T> {
178 /// Constructs a new box with uninitialized contents.
183 /// #![feature(new_uninit)]
185 /// let mut five = Box::<u32>::new_uninit();
187 /// let five = unsafe {
188 /// // Deferred initialization:
189 /// five.as_mut_ptr().write(5);
191 /// five.assume_init()
194 /// assert_eq!(*five, 5)
196 #[unstable(feature = "new_uninit", issue = "63291")]
197 pub fn new_uninit() -> Box<mem::MaybeUninit<T>> {
198 let layout = alloc::Layout::new::<mem::MaybeUninit<T>>();
200 let ptr = if layout.size() == 0 {
203 Global.alloc(layout).unwrap_or_else(|_| alloc::handle_alloc_error(layout)).cast()
205 Box::from_raw(ptr.as_ptr())
209 /// Constructs a new `Box` with uninitialized contents, with the memory
210 /// being filled with `0` bytes.
212 /// See [`MaybeUninit::zeroed`][zeroed] for examples of correct and incorrect usage
218 /// #![feature(new_uninit)]
220 /// let zero = Box::<u32>::new_zeroed();
221 /// let zero = unsafe { zero.assume_init() };
223 /// assert_eq!(*zero, 0)
226 /// [zeroed]: ../../std/mem/union.MaybeUninit.html#method.zeroed
227 #[unstable(feature = "new_uninit", issue = "63291")]
228 pub fn new_zeroed() -> Box<mem::MaybeUninit<T>> {
230 let mut uninit = Self::new_uninit();
231 ptr::write_bytes::<T>(uninit.as_mut_ptr(), 0, 1);
236 /// Constructs a new `Pin<Box<T>>`. If `T` does not implement `Unpin`, then
237 /// `x` will be pinned in memory and unable to be moved.
238 #[stable(feature = "pin", since = "1.33.0")]
240 pub fn pin(x: T) -> Pin<Box<T>> {
246 /// Constructs a new boxed slice with uninitialized contents.
251 /// #![feature(new_uninit)]
253 /// let mut values = Box::<[u32]>::new_uninit_slice(3);
255 /// let values = unsafe {
256 /// // Deferred initialization:
257 /// values[0].as_mut_ptr().write(1);
258 /// values[1].as_mut_ptr().write(2);
259 /// values[2].as_mut_ptr().write(3);
261 /// values.assume_init()
264 /// assert_eq!(*values, [1, 2, 3])
266 #[unstable(feature = "new_uninit", issue = "63291")]
267 pub fn new_uninit_slice(len: usize) -> Box<[mem::MaybeUninit<T>]> {
268 let layout = alloc::Layout::array::<mem::MaybeUninit<T>>(len).unwrap();
270 let ptr = if layout.size() == 0 {
273 Global.alloc(layout).unwrap_or_else(|_| alloc::handle_alloc_error(layout)).cast()
275 Box::from_raw(slice::from_raw_parts_mut(ptr.as_ptr(), len))
280 impl<T> Box<mem::MaybeUninit<T>> {
281 /// Converts to `Box<T>`.
285 /// As with [`MaybeUninit::assume_init`],
286 /// it is up to the caller to guarantee that the value
287 /// really is in an initialized state.
288 /// Calling this when the content is not yet fully initialized
289 /// causes immediate undefined behavior.
291 /// [`MaybeUninit::assume_init`]: ../../std/mem/union.MaybeUninit.html#method.assume_init
296 /// #![feature(new_uninit)]
298 /// let mut five = Box::<u32>::new_uninit();
300 /// let five: Box<u32> = unsafe {
301 /// // Deferred initialization:
302 /// five.as_mut_ptr().write(5);
304 /// five.assume_init()
307 /// assert_eq!(*five, 5)
309 #[unstable(feature = "new_uninit", issue = "63291")]
311 pub unsafe fn assume_init(self) -> Box<T> {
312 Box::from_raw(Box::into_raw(self) as *mut T)
316 impl<T> Box<[mem::MaybeUninit<T>]> {
317 /// Converts to `Box<[T]>`.
321 /// As with [`MaybeUninit::assume_init`],
322 /// it is up to the caller to guarantee that the values
323 /// really are in an initialized state.
324 /// Calling this when the content is not yet fully initialized
325 /// causes immediate undefined behavior.
327 /// [`MaybeUninit::assume_init`]: ../../std/mem/union.MaybeUninit.html#method.assume_init
332 /// #![feature(new_uninit)]
334 /// let mut values = Box::<[u32]>::new_uninit_slice(3);
336 /// let values = unsafe {
337 /// // Deferred initialization:
338 /// values[0].as_mut_ptr().write(1);
339 /// values[1].as_mut_ptr().write(2);
340 /// values[2].as_mut_ptr().write(3);
342 /// values.assume_init()
345 /// assert_eq!(*values, [1, 2, 3])
347 #[unstable(feature = "new_uninit", issue = "63291")]
349 pub unsafe fn assume_init(self) -> Box<[T]> {
350 Box::from_raw(Box::into_raw(self) as *mut [T])
354 impl<T: ?Sized> Box<T> {
355 /// Constructs a box from a raw pointer.
357 /// After calling this function, the raw pointer is owned by the
358 /// resulting `Box`. Specifically, the `Box` destructor will call
359 /// the destructor of `T` and free the allocated memory. For this
360 /// to be safe, the memory must have been allocated in accordance
361 /// with the [memory layout] used by `Box` .
365 /// This function is unsafe because improper use may lead to
366 /// memory problems. For example, a double-free may occur if the
367 /// function is called twice on the same raw pointer.
370 /// Recreate a `Box` which was previously converted to a raw pointer
371 /// using [`Box::into_raw`]:
373 /// let x = Box::new(5);
374 /// let ptr = Box::into_raw(x);
375 /// let x = unsafe { Box::from_raw(ptr) };
377 /// Manually create a `Box` from scratch by using the global allocator:
379 /// use std::alloc::{alloc, Layout};
382 /// let ptr = alloc(Layout::new::<i32>()) as *mut i32;
384 /// let x = Box::from_raw(ptr);
388 /// [memory layout]: index.html#memory-layout
389 /// [`Layout`]: ../alloc/struct.Layout.html
390 /// [`Box::into_raw`]: struct.Box.html#method.into_raw
391 #[stable(feature = "box_raw", since = "1.4.0")]
393 pub unsafe fn from_raw(raw: *mut T) -> Self {
394 Box(Unique::new_unchecked(raw))
397 /// Consumes the `Box`, returning a wrapped raw pointer.
399 /// The pointer will be properly aligned and non-null.
401 /// After calling this function, the caller is responsible for the
402 /// memory previously managed by the `Box`. In particular, the
403 /// caller should properly destroy `T` and release the memory, taking
404 /// into account the [memory layout] used by `Box`. The easiest way to
405 /// do this is to convert the raw pointer back into a `Box` with the
406 /// [`Box::from_raw`] function, allowing the `Box` destructor to perform
409 /// Note: this is an associated function, which means that you have
410 /// to call it as `Box::into_raw(b)` instead of `b.into_raw()`. This
411 /// is so that there is no conflict with a method on the inner type.
414 /// Converting the raw pointer back into a `Box` with [`Box::from_raw`]
415 /// for automatic cleanup:
417 /// let x = Box::new(String::from("Hello"));
418 /// let ptr = Box::into_raw(x);
419 /// let x = unsafe { Box::from_raw(ptr) };
421 /// Manual cleanup by explicitly running the destructor and deallocating
424 /// use std::alloc::{dealloc, Layout};
427 /// let x = Box::new(String::from("Hello"));
428 /// let p = Box::into_raw(x);
430 /// ptr::drop_in_place(p);
431 /// dealloc(p as *mut u8, Layout::new::<String>());
435 /// [memory layout]: index.html#memory-layout
436 /// [`Box::from_raw`]: struct.Box.html#method.from_raw
437 #[stable(feature = "box_raw", since = "1.4.0")]
439 pub fn into_raw(b: Box<T>) -> *mut T {
440 Box::into_raw_non_null(b).as_ptr()
443 /// Consumes the `Box`, returning the wrapped pointer as `NonNull<T>`.
445 /// After calling this function, the caller is responsible for the
446 /// memory previously managed by the `Box`. In particular, the
447 /// caller should properly destroy `T` and release the memory. The
448 /// easiest way to do so is to convert the `NonNull<T>` pointer
449 /// into a raw pointer and back into a `Box` with the [`Box::from_raw`]
452 /// Note: this is an associated function, which means that you have
453 /// to call it as `Box::into_raw_non_null(b)`
454 /// instead of `b.into_raw_non_null()`. 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
462 /// #![feature(box_into_raw_non_null)]
464 /// let x = Box::new(5);
465 /// let ptr = Box::into_raw_non_null(x);
467 /// // Clean up the memory by converting the NonNull pointer back
468 /// // into a Box and letting the Box be dropped.
469 /// let x = unsafe { Box::from_raw(ptr.as_ptr()) };
471 #[unstable(feature = "box_into_raw_non_null", issue = "47336")]
473 pub fn into_raw_non_null(b: Box<T>) -> NonNull<T> {
474 Box::into_unique(b).into()
477 #[unstable(feature = "ptr_internals", issue = "none", reason = "use into_raw_non_null instead")]
480 pub fn into_unique(b: Box<T>) -> Unique<T> {
481 let mut unique = b.0;
483 // Box is kind-of a library type, but recognized as a "unique pointer" by
484 // Stacked Borrows. This function here corresponds to "reborrowing to
485 // a raw pointer", but there is no actual reborrow here -- so
486 // without some care, the pointer we are returning here still carries
487 // the tag of `b`, with `Unique` permission.
488 // We round-trip through a mutable reference to avoid that.
489 unsafe { Unique::new_unchecked(unique.as_mut() as *mut T) }
492 /// Consumes and leaks the `Box`, returning a mutable reference,
493 /// `&'a mut T`. Note that the type `T` must outlive the chosen lifetime
494 /// `'a`. If the type has only static references, or none at all, then this
495 /// may be chosen to be `'static`.
497 /// This function is mainly useful for data that lives for the remainder of
498 /// the program's life. Dropping the returned reference will cause a memory
499 /// leak. If this is not acceptable, the reference should first be wrapped
500 /// with the [`Box::from_raw`] function producing a `Box`. This `Box` can
501 /// then be dropped which will properly destroy `T` and release the
502 /// allocated memory.
504 /// Note: this is an associated function, which means that you have
505 /// to call it as `Box::leak(b)` instead of `b.leak()`. This
506 /// is so that there is no conflict with a method on the inner type.
508 /// [`Box::from_raw`]: struct.Box.html#method.from_raw
515 /// let x = Box::new(41);
516 /// let static_ref: &'static mut usize = Box::leak(x);
517 /// *static_ref += 1;
518 /// assert_eq!(*static_ref, 42);
524 /// let x = vec![1, 2, 3].into_boxed_slice();
525 /// let static_ref = Box::leak(x);
526 /// static_ref[0] = 4;
527 /// assert_eq!(*static_ref, [4, 2, 3]);
529 #[stable(feature = "box_leak", since = "1.26.0")]
531 pub fn leak<'a>(b: Box<T>) -> &'a mut T
533 T: 'a, // Technically not needed, but kept to be explicit.
535 unsafe { &mut *Box::into_raw(b) }
538 /// Converts a `Box<T>` into a `Pin<Box<T>>`
540 /// This conversion does not allocate on the heap and happens in place.
542 /// This is also available via [`From`].
543 #[unstable(feature = "box_into_pin", issue = "62370")]
544 pub fn into_pin(boxed: Box<T>) -> Pin<Box<T>> {
545 // It's not possible to move or replace the insides of a `Pin<Box<T>>`
546 // when `T: !Unpin`, so it's safe to pin it directly without any
547 // additional requirements.
548 unsafe { Pin::new_unchecked(boxed) }
552 #[stable(feature = "rust1", since = "1.0.0")]
553 unsafe impl<#[may_dangle] T: ?Sized> Drop for Box<T> {
555 // FIXME: Do nothing, drop is currently performed by compiler.
559 #[stable(feature = "rust1", since = "1.0.0")]
560 impl<T: Default> Default for Box<T> {
561 /// Creates a `Box<T>`, with the `Default` value for T.
562 fn default() -> Box<T> {
563 box Default::default()
567 #[stable(feature = "rust1", since = "1.0.0")]
568 impl<T> Default for Box<[T]> {
569 fn default() -> Box<[T]> {
570 Box::<[T; 0]>::new([])
574 #[stable(feature = "default_box_extra", since = "1.17.0")]
575 impl Default for Box<str> {
576 fn default() -> Box<str> {
577 unsafe { from_boxed_utf8_unchecked(Default::default()) }
581 #[stable(feature = "rust1", since = "1.0.0")]
582 impl<T: Clone> Clone for Box<T> {
583 /// Returns a new box with a `clone()` of this box's contents.
588 /// let x = Box::new(5);
589 /// let y = x.clone();
591 /// // The value is the same
592 /// assert_eq!(x, y);
594 /// // But they are unique objects
595 /// assert_ne!(&*x as *const i32, &*y as *const i32);
599 fn clone(&self) -> Box<T> {
600 box { (**self).clone() }
603 /// Copies `source`'s contents into `self` without creating a new allocation.
608 /// let x = Box::new(5);
609 /// let mut y = Box::new(10);
610 /// let yp: *const i32 = &*y;
612 /// y.clone_from(&x);
614 /// // The value is the same
615 /// assert_eq!(x, y);
617 /// // And no allocation occurred
618 /// assert_eq!(yp, &*y);
621 fn clone_from(&mut self, source: &Box<T>) {
622 (**self).clone_from(&(**source));
626 #[stable(feature = "box_slice_clone", since = "1.3.0")]
627 impl Clone for Box<str> {
628 fn clone(&self) -> Self {
629 // this makes a copy of the data
630 let buf: Box<[u8]> = self.as_bytes().into();
631 unsafe { from_boxed_utf8_unchecked(buf) }
635 #[stable(feature = "rust1", since = "1.0.0")]
636 impl<T: ?Sized + PartialEq> PartialEq for Box<T> {
638 fn eq(&self, other: &Box<T>) -> bool {
639 PartialEq::eq(&**self, &**other)
642 fn ne(&self, other: &Box<T>) -> bool {
643 PartialEq::ne(&**self, &**other)
646 #[stable(feature = "rust1", since = "1.0.0")]
647 impl<T: ?Sized + PartialOrd> PartialOrd for Box<T> {
649 fn partial_cmp(&self, other: &Box<T>) -> Option<Ordering> {
650 PartialOrd::partial_cmp(&**self, &**other)
653 fn lt(&self, other: &Box<T>) -> bool {
654 PartialOrd::lt(&**self, &**other)
657 fn le(&self, other: &Box<T>) -> bool {
658 PartialOrd::le(&**self, &**other)
661 fn ge(&self, other: &Box<T>) -> bool {
662 PartialOrd::ge(&**self, &**other)
665 fn gt(&self, other: &Box<T>) -> bool {
666 PartialOrd::gt(&**self, &**other)
669 #[stable(feature = "rust1", since = "1.0.0")]
670 impl<T: ?Sized + Ord> Ord for Box<T> {
672 fn cmp(&self, other: &Box<T>) -> Ordering {
673 Ord::cmp(&**self, &**other)
676 #[stable(feature = "rust1", since = "1.0.0")]
677 impl<T: ?Sized + Eq> Eq for Box<T> {}
679 #[stable(feature = "rust1", since = "1.0.0")]
680 impl<T: ?Sized + Hash> Hash for Box<T> {
681 fn hash<H: Hasher>(&self, state: &mut H) {
682 (**self).hash(state);
686 #[stable(feature = "indirect_hasher_impl", since = "1.22.0")]
687 impl<T: ?Sized + Hasher> Hasher for Box<T> {
688 fn finish(&self) -> u64 {
691 fn write(&mut self, bytes: &[u8]) {
692 (**self).write(bytes)
694 fn write_u8(&mut self, i: u8) {
697 fn write_u16(&mut self, i: u16) {
698 (**self).write_u16(i)
700 fn write_u32(&mut self, i: u32) {
701 (**self).write_u32(i)
703 fn write_u64(&mut self, i: u64) {
704 (**self).write_u64(i)
706 fn write_u128(&mut self, i: u128) {
707 (**self).write_u128(i)
709 fn write_usize(&mut self, i: usize) {
710 (**self).write_usize(i)
712 fn write_i8(&mut self, i: i8) {
715 fn write_i16(&mut self, i: i16) {
716 (**self).write_i16(i)
718 fn write_i32(&mut self, i: i32) {
719 (**self).write_i32(i)
721 fn write_i64(&mut self, i: i64) {
722 (**self).write_i64(i)
724 fn write_i128(&mut self, i: i128) {
725 (**self).write_i128(i)
727 fn write_isize(&mut self, i: isize) {
728 (**self).write_isize(i)
732 #[stable(feature = "from_for_ptrs", since = "1.6.0")]
733 impl<T> From<T> for Box<T> {
734 /// Converts a generic type `T` into a `Box<T>`
736 /// The conversion allocates on the heap and moves `t`
737 /// from the stack into it.
742 /// let boxed = Box::new(5);
744 /// assert_eq!(Box::from(x), boxed);
746 fn from(t: T) -> Self {
751 #[stable(feature = "pin", since = "1.33.0")]
752 impl<T: ?Sized> From<Box<T>> for Pin<Box<T>> {
753 /// Converts a `Box<T>` into a `Pin<Box<T>>`
755 /// This conversion does not allocate on the heap and happens in place.
756 fn from(boxed: Box<T>) -> Self {
761 #[stable(feature = "box_from_slice", since = "1.17.0")]
762 impl<T: Copy> From<&[T]> for Box<[T]> {
763 /// Converts a `&[T]` into a `Box<[T]>`
765 /// This conversion allocates on the heap
766 /// and performs a copy of `slice`.
770 /// // create a &[u8] which will be used to create a Box<[u8]>
771 /// let slice: &[u8] = &[104, 101, 108, 108, 111];
772 /// let boxed_slice: Box<[u8]> = Box::from(slice);
774 /// println!("{:?}", boxed_slice);
776 fn from(slice: &[T]) -> Box<[T]> {
777 let len = slice.len();
778 let buf = RawVec::with_capacity(len);
780 ptr::copy_nonoverlapping(slice.as_ptr(), buf.ptr(), len);
786 #[stable(feature = "box_from_slice", since = "1.17.0")]
787 impl From<&str> for Box<str> {
788 /// Converts a `&str` into a `Box<str>`
790 /// This conversion allocates on the heap
791 /// and performs a copy of `s`.
795 /// let boxed: Box<str> = Box::from("hello");
796 /// println!("{}", boxed);
799 fn from(s: &str) -> Box<str> {
800 unsafe { from_boxed_utf8_unchecked(Box::from(s.as_bytes())) }
804 #[stable(feature = "boxed_str_conv", since = "1.19.0")]
805 impl From<Box<str>> for Box<[u8]> {
806 /// Converts a `Box<str>>` into a `Box<[u8]>`
808 /// This conversion does not allocate on the heap and happens in place.
812 /// // create a Box<str> which will be used to create a Box<[u8]>
813 /// let boxed: Box<str> = Box::from("hello");
814 /// let boxed_str: Box<[u8]> = Box::from(boxed);
816 /// // create a &[u8] which will be used to create a Box<[u8]>
817 /// let slice: &[u8] = &[104, 101, 108, 108, 111];
818 /// let boxed_slice = Box::from(slice);
820 /// assert_eq!(boxed_slice, boxed_str);
823 fn from(s: Box<str>) -> Self {
824 unsafe { Box::from_raw(Box::into_raw(s) as *mut [u8]) }
828 #[unstable(feature = "boxed_slice_try_from", issue = "none")]
829 impl<T, const N: usize> TryFrom<Box<[T]>> for Box<[T; N]>
831 [T; N]: LengthAtMost32,
833 type Error = Box<[T]>;
835 fn try_from(boxed_slice: Box<[T]>) -> Result<Self, Self::Error> {
836 if boxed_slice.len() == N {
837 Ok(unsafe { Box::from_raw(Box::into_raw(boxed_slice) as *mut [T; N]) })
846 #[stable(feature = "rust1", since = "1.0.0")]
847 /// Attempt to downcast the box to a concrete type.
852 /// use std::any::Any;
854 /// fn print_if_string(value: Box<dyn Any>) {
855 /// if let Ok(string) = value.downcast::<String>() {
856 /// println!("String ({}): {}", string.len(), string);
860 /// let my_string = "Hello World".to_string();
861 /// print_if_string(Box::new(my_string));
862 /// print_if_string(Box::new(0i8));
864 pub fn downcast<T: Any>(self) -> Result<Box<T>, Box<dyn Any>> {
867 let raw: *mut dyn Any = Box::into_raw(self);
868 Ok(Box::from_raw(raw as *mut T))
876 impl Box<dyn Any + Send> {
878 #[stable(feature = "rust1", since = "1.0.0")]
879 /// Attempt to downcast the box to a concrete type.
884 /// use std::any::Any;
886 /// fn print_if_string(value: Box<dyn Any + Send>) {
887 /// if let Ok(string) = value.downcast::<String>() {
888 /// println!("String ({}): {}", string.len(), string);
892 /// let my_string = "Hello World".to_string();
893 /// print_if_string(Box::new(my_string));
894 /// print_if_string(Box::new(0i8));
896 pub fn downcast<T: Any>(self) -> Result<Box<T>, Box<dyn Any + Send>> {
897 <Box<dyn Any>>::downcast(self).map_err(|s| unsafe {
898 // reapply the Send marker
899 Box::from_raw(Box::into_raw(s) as *mut (dyn Any + Send))
904 #[stable(feature = "rust1", since = "1.0.0")]
905 impl<T: fmt::Display + ?Sized> fmt::Display for Box<T> {
906 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
907 fmt::Display::fmt(&**self, f)
911 #[stable(feature = "rust1", since = "1.0.0")]
912 impl<T: fmt::Debug + ?Sized> fmt::Debug for Box<T> {
913 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
914 fmt::Debug::fmt(&**self, f)
918 #[stable(feature = "rust1", since = "1.0.0")]
919 impl<T: ?Sized> fmt::Pointer for Box<T> {
920 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
921 // It's not possible to extract the inner Uniq directly from the Box,
922 // instead we cast it to a *const which aliases the Unique
923 let ptr: *const T = &**self;
924 fmt::Pointer::fmt(&ptr, f)
928 #[stable(feature = "rust1", since = "1.0.0")]
929 impl<T: ?Sized> Deref for Box<T> {
932 fn deref(&self) -> &T {
937 #[stable(feature = "rust1", since = "1.0.0")]
938 impl<T: ?Sized> DerefMut for Box<T> {
939 fn deref_mut(&mut self) -> &mut T {
944 #[unstable(feature = "receiver_trait", issue = "none")]
945 impl<T: ?Sized> Receiver for Box<T> {}
947 #[stable(feature = "rust1", since = "1.0.0")]
948 impl<I: Iterator + ?Sized> Iterator for Box<I> {
950 fn next(&mut self) -> Option<I::Item> {
953 fn size_hint(&self) -> (usize, Option<usize>) {
956 fn nth(&mut self, n: usize) -> Option<I::Item> {
959 fn last(self) -> Option<I::Item> {
966 fn last(self) -> Option<Self::Item>;
969 impl<I: Iterator + ?Sized> BoxIter for Box<I> {
971 default fn last(self) -> Option<I::Item> {
973 fn some<T>(_: Option<T>, x: T) -> Option<T> {
977 self.fold(None, some)
981 /// Specialization for sized `I`s that uses `I`s implementation of `last()`
982 /// instead of the default.
983 #[stable(feature = "rust1", since = "1.0.0")]
984 impl<I: Iterator> BoxIter for Box<I> {
985 fn last(self) -> Option<I::Item> {
990 #[stable(feature = "rust1", since = "1.0.0")]
991 impl<I: DoubleEndedIterator + ?Sized> DoubleEndedIterator for Box<I> {
992 fn next_back(&mut self) -> Option<I::Item> {
995 fn nth_back(&mut self, n: usize) -> Option<I::Item> {
999 #[stable(feature = "rust1", since = "1.0.0")]
1000 impl<I: ExactSizeIterator + ?Sized> ExactSizeIterator for Box<I> {
1001 fn len(&self) -> usize {
1004 fn is_empty(&self) -> bool {
1009 #[stable(feature = "fused", since = "1.26.0")]
1010 impl<I: FusedIterator + ?Sized> FusedIterator for Box<I> {}
1012 #[stable(feature = "boxed_closure_impls", since = "1.35.0")]
1013 impl<A, F: FnOnce<A> + ?Sized> FnOnce<A> for Box<F> {
1014 type Output = <F as FnOnce<A>>::Output;
1016 extern "rust-call" fn call_once(self, args: A) -> Self::Output {
1017 <F as FnOnce<A>>::call_once(*self, args)
1021 #[stable(feature = "boxed_closure_impls", since = "1.35.0")]
1022 impl<A, F: FnMut<A> + ?Sized> FnMut<A> for Box<F> {
1023 extern "rust-call" fn call_mut(&mut self, args: A) -> Self::Output {
1024 <F as FnMut<A>>::call_mut(self, args)
1028 #[stable(feature = "boxed_closure_impls", since = "1.35.0")]
1029 impl<A, F: Fn<A> + ?Sized> Fn<A> for Box<F> {
1030 extern "rust-call" fn call(&self, args: A) -> Self::Output {
1031 <F as Fn<A>>::call(self, args)
1035 #[unstable(feature = "coerce_unsized", issue = "27732")]
1036 impl<T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<Box<U>> for Box<T> {}
1038 #[unstable(feature = "dispatch_from_dyn", issue = "none")]
1039 impl<T: ?Sized + Unsize<U>, U: ?Sized> DispatchFromDyn<Box<U>> for Box<T> {}
1041 #[stable(feature = "boxed_slice_from_iter", since = "1.32.0")]
1042 impl<A> FromIterator<A> for Box<[A]> {
1043 fn from_iter<T: IntoIterator<Item = A>>(iter: T) -> Self {
1044 iter.into_iter().collect::<Vec<_>>().into_boxed_slice()
1048 #[stable(feature = "box_slice_clone", since = "1.3.0")]
1049 impl<T: Clone> Clone for Box<[T]> {
1050 fn clone(&self) -> Self {
1051 self.to_vec().into_boxed_slice()
1055 #[stable(feature = "box_borrow", since = "1.1.0")]
1056 impl<T: ?Sized> borrow::Borrow<T> for Box<T> {
1057 fn borrow(&self) -> &T {
1062 #[stable(feature = "box_borrow", since = "1.1.0")]
1063 impl<T: ?Sized> borrow::BorrowMut<T> for Box<T> {
1064 fn borrow_mut(&mut self) -> &mut T {
1069 #[stable(since = "1.5.0", feature = "smart_ptr_as_ref")]
1070 impl<T: ?Sized> AsRef<T> for Box<T> {
1071 fn as_ref(&self) -> &T {
1076 #[stable(since = "1.5.0", feature = "smart_ptr_as_ref")]
1077 impl<T: ?Sized> AsMut<T> for Box<T> {
1078 fn as_mut(&mut self) -> &mut T {
1085 * We could have chosen not to add this impl, and instead have written a
1086 * function of Pin<Box<T>> to Pin<T>. Such a function would not be sound,
1087 * because Box<T> implements Unpin even when T does not, as a result of
1090 * We chose this API instead of the alternative for a few reasons:
1091 * - Logically, it is helpful to understand pinning in regard to the
1092 * memory region being pointed to. For this reason none of the
1093 * standard library pointer types support projecting through a pin
1094 * (Box<T> is the only pointer type in std for which this would be
1096 * - It is in practice very useful to have Box<T> be unconditionally
1097 * Unpin because of trait objects, for which the structural auto
1098 * trait functionality does not apply (e.g., Box<dyn Foo> would
1099 * otherwise not be Unpin).
1101 * Another type with the same semantics as Box but only a conditional
1102 * implementation of `Unpin` (where `T: Unpin`) would be valid/safe, and
1103 * could have a method to project a Pin<T> from it.
1105 #[stable(feature = "pin", since = "1.33.0")]
1106 impl<T: ?Sized> Unpin for Box<T> {}
1109 #[unstable(feature = "generator_trait", issue = "43122")]
1110 impl<G: ?Sized + Generator + Unpin> Generator for Box<G> {
1111 type Yield = G::Yield;
1112 type Return = G::Return;
1114 fn resume(mut self: Pin<&mut Self>) -> GeneratorState<Self::Yield, Self::Return> {
1115 G::resume(Pin::new(&mut *self))
1120 #[unstable(feature = "generator_trait", issue = "43122")]
1121 impl<G: ?Sized + Generator> Generator for Pin<Box<G>> {
1122 type Yield = G::Yield;
1123 type Return = G::Return;
1125 fn resume(mut self: Pin<&mut Self>) -> GeneratorState<Self::Yield, Self::Return> {
1126 G::resume((*self).as_mut())
1130 #[cfg(not(bootstrap))]
1131 #[unstable(feature = "generator_trait", issue = "43122")]
1132 impl<G: ?Sized + Generator<R> + Unpin, R> Generator<R> for Box<G> {
1133 type Yield = G::Yield;
1134 type Return = G::Return;
1136 fn resume(mut self: Pin<&mut Self>, arg: R) -> GeneratorState<Self::Yield, Self::Return> {
1137 G::resume(Pin::new(&mut *self), arg)
1141 #[cfg(not(bootstrap))]
1142 #[unstable(feature = "generator_trait", issue = "43122")]
1143 impl<G: ?Sized + Generator<R>, R> Generator<R> for Pin<Box<G>> {
1144 type Yield = G::Yield;
1145 type Return = G::Return;
1147 fn resume(mut self: Pin<&mut Self>, arg: R) -> GeneratorState<Self::Yield, Self::Return> {
1148 G::resume((*self).as_mut(), arg)
1152 #[stable(feature = "futures_api", since = "1.36.0")]
1153 impl<F: ?Sized + Future + Unpin> Future for Box<F> {
1154 type Output = F::Output;
1156 fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
1157 F::poll(Pin::new(&mut *self), cx)