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>>();
199 if layout.size() == 0 {
200 return Box(NonNull::dangling().into());
203 unsafe { Global.alloc(layout).unwrap_or_else(|_| alloc::handle_alloc_error(layout)) };
204 Box(ptr.cast().into())
207 /// Constructs a new `Box` with uninitialized contents, with the memory
208 /// being filled with `0` bytes.
210 /// See [`MaybeUninit::zeroed`][zeroed] for examples of correct and incorrect usage
216 /// #![feature(new_uninit)]
218 /// let zero = Box::<u32>::new_zeroed();
219 /// let zero = unsafe { zero.assume_init() };
221 /// assert_eq!(*zero, 0)
224 /// [zeroed]: ../../std/mem/union.MaybeUninit.html#method.zeroed
225 #[unstable(feature = "new_uninit", issue = "63291")]
226 pub fn new_zeroed() -> Box<mem::MaybeUninit<T>> {
228 let mut uninit = Self::new_uninit();
229 ptr::write_bytes::<T>(uninit.as_mut_ptr(), 0, 1);
234 /// Constructs a new `Pin<Box<T>>`. If `T` does not implement `Unpin`, then
235 /// `x` will be pinned in memory and unable to be moved.
236 #[stable(feature = "pin", since = "1.33.0")]
238 pub fn pin(x: T) -> Pin<Box<T>> {
244 /// Constructs a new boxed slice with uninitialized contents.
249 /// #![feature(new_uninit)]
251 /// let mut values = Box::<[u32]>::new_uninit_slice(3);
253 /// let values = unsafe {
254 /// // Deferred initialization:
255 /// values[0].as_mut_ptr().write(1);
256 /// values[1].as_mut_ptr().write(2);
257 /// values[2].as_mut_ptr().write(3);
259 /// values.assume_init()
262 /// assert_eq!(*values, [1, 2, 3])
264 #[unstable(feature = "new_uninit", issue = "63291")]
265 pub fn new_uninit_slice(len: usize) -> Box<[mem::MaybeUninit<T>]> {
266 let layout = alloc::Layout::array::<mem::MaybeUninit<T>>(len).unwrap();
267 let ptr = if layout.size() == 0 {
271 Global.alloc(layout).unwrap_or_else(|_| alloc::handle_alloc_error(layout)).cast()
274 let slice = unsafe { slice::from_raw_parts_mut(ptr.as_ptr(), len) };
275 Box(Unique::from(slice))
279 impl<T> Box<mem::MaybeUninit<T>> {
280 /// Converts to `Box<T>`.
284 /// As with [`MaybeUninit::assume_init`],
285 /// it is up to the caller to guarantee that the value
286 /// really is in an initialized state.
287 /// Calling this when the content is not yet fully initialized
288 /// causes immediate undefined behavior.
290 /// [`MaybeUninit::assume_init`]: ../../std/mem/union.MaybeUninit.html#method.assume_init
295 /// #![feature(new_uninit)]
297 /// let mut five = Box::<u32>::new_uninit();
299 /// let five: Box<u32> = unsafe {
300 /// // Deferred initialization:
301 /// five.as_mut_ptr().write(5);
303 /// five.assume_init()
306 /// assert_eq!(*five, 5)
308 #[unstable(feature = "new_uninit", issue = "63291")]
310 pub unsafe fn assume_init(self) -> Box<T> {
311 Box(Box::into_unique(self).cast())
315 impl<T> Box<[mem::MaybeUninit<T>]> {
316 /// Converts to `Box<[T]>`.
320 /// As with [`MaybeUninit::assume_init`],
321 /// it is up to the caller to guarantee that the values
322 /// really are in an initialized state.
323 /// Calling this when the content is not yet fully initialized
324 /// causes immediate undefined behavior.
326 /// [`MaybeUninit::assume_init`]: ../../std/mem/union.MaybeUninit.html#method.assume_init
331 /// #![feature(new_uninit)]
333 /// let mut values = Box::<[u32]>::new_uninit_slice(3);
335 /// let values = unsafe {
336 /// // Deferred initialization:
337 /// values[0].as_mut_ptr().write(1);
338 /// values[1].as_mut_ptr().write(2);
339 /// values[2].as_mut_ptr().write(3);
341 /// values.assume_init()
344 /// assert_eq!(*values, [1, 2, 3])
346 #[unstable(feature = "new_uninit", issue = "63291")]
348 pub unsafe fn assume_init(self) -> Box<[T]> {
349 Box(Unique::new_unchecked(Box::into_raw(self) as _))
353 impl<T: ?Sized> Box<T> {
354 /// Constructs a box from a raw pointer.
356 /// After calling this function, the raw pointer is owned by the
357 /// resulting `Box`. Specifically, the `Box` destructor will call
358 /// the destructor of `T` and free the allocated memory. For this
359 /// to be safe, the memory must have been allocated in accordance
360 /// with the [memory layout] used by `Box` .
364 /// This function is unsafe because improper use may lead to
365 /// memory problems. For example, a double-free may occur if the
366 /// function is called twice on the same raw pointer.
369 /// Recreate a `Box` which was previously converted to a raw pointer
370 /// using [`Box::into_raw`]:
372 /// let x = Box::new(5);
373 /// let ptr = Box::into_raw(x);
374 /// let x = unsafe { Box::from_raw(ptr) };
376 /// Manually create a `Box` from scratch by using the global allocator:
378 /// use std::alloc::{alloc, Layout};
381 /// let ptr = alloc(Layout::new::<i32>()) as *mut i32;
383 /// let x = Box::from_raw(ptr);
387 /// [memory layout]: index.html#memory-layout
388 /// [`Layout`]: ../alloc/struct.Layout.html
389 /// [`Box::into_raw`]: struct.Box.html#method.into_raw
390 #[stable(feature = "box_raw", since = "1.4.0")]
392 pub unsafe fn from_raw(raw: *mut T) -> Self {
393 Box(Unique::new_unchecked(raw))
396 /// Consumes the `Box`, returning a wrapped raw pointer.
398 /// The pointer will be properly aligned and non-null.
400 /// After calling this function, the caller is responsible for the
401 /// memory previously managed by the `Box`. In particular, the
402 /// caller should properly destroy `T` and release the memory, taking
403 /// into account the [memory layout] used by `Box`. The easiest way to
404 /// do this is to convert the raw pointer back into a `Box` with the
405 /// [`Box::from_raw`] function, allowing the `Box` destructor to perform
408 /// Note: this is an associated function, which means that you have
409 /// to call it as `Box::into_raw(b)` instead of `b.into_raw()`. This
410 /// is so that there is no conflict with a method on the inner type.
413 /// Converting the raw pointer back into a `Box` with [`Box::from_raw`]
414 /// for automatic cleanup:
416 /// let x = Box::new(String::from("Hello"));
417 /// let ptr = Box::into_raw(x);
418 /// let x = unsafe { Box::from_raw(ptr) };
420 /// Manual cleanup by explicitly running the destructor and deallocating
423 /// use std::alloc::{dealloc, Layout};
426 /// let x = Box::new(String::from("Hello"));
427 /// let p = Box::into_raw(x);
429 /// ptr::drop_in_place(p);
430 /// dealloc(p as *mut u8, Layout::new::<String>());
434 /// [memory layout]: index.html#memory-layout
435 /// [`Box::from_raw`]: struct.Box.html#method.from_raw
436 #[stable(feature = "box_raw", since = "1.4.0")]
438 pub fn into_raw(b: Box<T>) -> *mut T {
439 Box::into_raw_non_null(b).as_ptr()
442 /// Consumes the `Box`, returning the wrapped pointer as `NonNull<T>`.
444 /// After calling this function, the caller is responsible for the
445 /// memory previously managed by the `Box`. In particular, the
446 /// caller should properly destroy `T` and release the memory. The
447 /// easiest way to do so is to convert the `NonNull<T>` pointer
448 /// into a raw pointer and back into a `Box` with the [`Box::from_raw`]
451 /// Note: this is an associated function, which means that you have
452 /// to call it as `Box::into_raw_non_null(b)`
453 /// instead of `b.into_raw_non_null()`. This
454 /// is so that there is no conflict with a method on the inner type.
456 /// [`Box::from_raw`]: struct.Box.html#method.from_raw
461 /// #![feature(box_into_raw_non_null)]
463 /// let x = Box::new(5);
464 /// let ptr = Box::into_raw_non_null(x);
466 /// // Clean up the memory by converting the NonNull pointer back
467 /// // into a Box and letting the Box be dropped.
468 /// let x = unsafe { Box::from_raw(ptr.as_ptr()) };
470 #[unstable(feature = "box_into_raw_non_null", issue = "47336")]
472 pub fn into_raw_non_null(b: Box<T>) -> NonNull<T> {
473 Box::into_unique(b).into()
476 #[unstable(feature = "ptr_internals", issue = "none", reason = "use into_raw_non_null instead")]
479 pub fn into_unique(b: Box<T>) -> Unique<T> {
480 let mut unique = b.0;
482 // Box is kind-of a library type, but recognized as a "unique pointer" by
483 // Stacked Borrows. This function here corresponds to "reborrowing to
484 // a raw pointer", but there is no actual reborrow here -- so
485 // without some care, the pointer we are returning here still carries
486 // the tag of `b`, with `Unique` permission.
487 // We round-trip through a mutable reference to avoid that.
488 unsafe { Unique::new_unchecked(unique.as_mut() as *mut T) }
491 /// Consumes and leaks the `Box`, returning a mutable reference,
492 /// `&'a mut T`. Note that the type `T` must outlive the chosen lifetime
493 /// `'a`. If the type has only static references, or none at all, then this
494 /// may be chosen to be `'static`.
496 /// This function is mainly useful for data that lives for the remainder of
497 /// the program's life. Dropping the returned reference will cause a memory
498 /// leak. If this is not acceptable, the reference should first be wrapped
499 /// with the [`Box::from_raw`] function producing a `Box`. This `Box` can
500 /// then be dropped which will properly destroy `T` and release the
501 /// allocated memory.
503 /// Note: this is an associated function, which means that you have
504 /// to call it as `Box::leak(b)` instead of `b.leak()`. This
505 /// is so that there is no conflict with a method on the inner type.
507 /// [`Box::from_raw`]: struct.Box.html#method.from_raw
514 /// let x = Box::new(41);
515 /// let static_ref: &'static mut usize = Box::leak(x);
516 /// *static_ref += 1;
517 /// assert_eq!(*static_ref, 42);
523 /// let x = vec![1, 2, 3].into_boxed_slice();
524 /// let static_ref = Box::leak(x);
525 /// static_ref[0] = 4;
526 /// assert_eq!(*static_ref, [4, 2, 3]);
528 #[stable(feature = "box_leak", since = "1.26.0")]
530 pub fn leak<'a>(b: Box<T>) -> &'a mut T
532 T: 'a, // Technically not needed, but kept to be explicit.
534 unsafe { &mut *Box::into_raw(b) }
537 /// Converts a `Box<T>` into a `Pin<Box<T>>`
539 /// This conversion does not allocate on the heap and happens in place.
541 /// This is also available via [`From`].
542 #[unstable(feature = "box_into_pin", issue = "62370")]
543 pub fn into_pin(boxed: Box<T>) -> Pin<Box<T>> {
544 // It's not possible to move or replace the insides of a `Pin<Box<T>>`
545 // when `T: !Unpin`, so it's safe to pin it directly without any
546 // additional requirements.
547 unsafe { Pin::new_unchecked(boxed) }
551 #[stable(feature = "rust1", since = "1.0.0")]
552 unsafe impl<#[may_dangle] T: ?Sized> Drop for Box<T> {
554 // FIXME: Do nothing, drop is currently performed by compiler.
558 #[stable(feature = "rust1", since = "1.0.0")]
559 impl<T: Default> Default for Box<T> {
560 /// Creates a `Box<T>`, with the `Default` value for T.
561 fn default() -> Box<T> {
562 box Default::default()
566 #[stable(feature = "rust1", since = "1.0.0")]
567 impl<T> Default for Box<[T]> {
568 fn default() -> Box<[T]> {
569 Box::<[T; 0]>::new([])
573 #[stable(feature = "default_box_extra", since = "1.17.0")]
574 impl Default for Box<str> {
575 fn default() -> Box<str> {
576 unsafe { from_boxed_utf8_unchecked(Default::default()) }
580 #[stable(feature = "rust1", since = "1.0.0")]
581 impl<T: Clone> Clone for Box<T> {
582 /// Returns a new box with a `clone()` of this box's contents.
587 /// let x = Box::new(5);
588 /// let y = x.clone();
590 /// // The value is the same
591 /// assert_eq!(x, y);
593 /// // But they are unique objects
594 /// assert_ne!(&*x as *const i32, &*y as *const i32);
598 fn clone(&self) -> Box<T> {
599 box { (**self).clone() }
602 /// Copies `source`'s contents into `self` without creating a new allocation.
607 /// let x = Box::new(5);
608 /// let mut y = Box::new(10);
609 /// let yp: *const i32 = &*y;
611 /// y.clone_from(&x);
613 /// // The value is the same
614 /// assert_eq!(x, y);
616 /// // And no allocation occurred
617 /// assert_eq!(yp, &*y);
620 fn clone_from(&mut self, source: &Box<T>) {
621 (**self).clone_from(&(**source));
625 #[stable(feature = "box_slice_clone", since = "1.3.0")]
626 impl Clone for Box<str> {
627 fn clone(&self) -> Self {
628 // this makes a copy of the data
629 let buf: Box<[u8]> = self.as_bytes().into();
630 unsafe { from_boxed_utf8_unchecked(buf) }
634 #[stable(feature = "rust1", since = "1.0.0")]
635 impl<T: ?Sized + PartialEq> PartialEq for Box<T> {
637 fn eq(&self, other: &Box<T>) -> bool {
638 PartialEq::eq(&**self, &**other)
641 fn ne(&self, other: &Box<T>) -> bool {
642 PartialEq::ne(&**self, &**other)
645 #[stable(feature = "rust1", since = "1.0.0")]
646 impl<T: ?Sized + PartialOrd> PartialOrd for Box<T> {
648 fn partial_cmp(&self, other: &Box<T>) -> Option<Ordering> {
649 PartialOrd::partial_cmp(&**self, &**other)
652 fn lt(&self, other: &Box<T>) -> bool {
653 PartialOrd::lt(&**self, &**other)
656 fn le(&self, other: &Box<T>) -> bool {
657 PartialOrd::le(&**self, &**other)
660 fn ge(&self, other: &Box<T>) -> bool {
661 PartialOrd::ge(&**self, &**other)
664 fn gt(&self, other: &Box<T>) -> bool {
665 PartialOrd::gt(&**self, &**other)
668 #[stable(feature = "rust1", since = "1.0.0")]
669 impl<T: ?Sized + Ord> Ord for Box<T> {
671 fn cmp(&self, other: &Box<T>) -> Ordering {
672 Ord::cmp(&**self, &**other)
675 #[stable(feature = "rust1", since = "1.0.0")]
676 impl<T: ?Sized + Eq> Eq for Box<T> {}
678 #[stable(feature = "rust1", since = "1.0.0")]
679 impl<T: ?Sized + Hash> Hash for Box<T> {
680 fn hash<H: Hasher>(&self, state: &mut H) {
681 (**self).hash(state);
685 #[stable(feature = "indirect_hasher_impl", since = "1.22.0")]
686 impl<T: ?Sized + Hasher> Hasher for Box<T> {
687 fn finish(&self) -> u64 {
690 fn write(&mut self, bytes: &[u8]) {
691 (**self).write(bytes)
693 fn write_u8(&mut self, i: u8) {
696 fn write_u16(&mut self, i: u16) {
697 (**self).write_u16(i)
699 fn write_u32(&mut self, i: u32) {
700 (**self).write_u32(i)
702 fn write_u64(&mut self, i: u64) {
703 (**self).write_u64(i)
705 fn write_u128(&mut self, i: u128) {
706 (**self).write_u128(i)
708 fn write_usize(&mut self, i: usize) {
709 (**self).write_usize(i)
711 fn write_i8(&mut self, i: i8) {
714 fn write_i16(&mut self, i: i16) {
715 (**self).write_i16(i)
717 fn write_i32(&mut self, i: i32) {
718 (**self).write_i32(i)
720 fn write_i64(&mut self, i: i64) {
721 (**self).write_i64(i)
723 fn write_i128(&mut self, i: i128) {
724 (**self).write_i128(i)
726 fn write_isize(&mut self, i: isize) {
727 (**self).write_isize(i)
731 #[stable(feature = "from_for_ptrs", since = "1.6.0")]
732 impl<T> From<T> for Box<T> {
733 /// Converts a generic type `T` into a `Box<T>`
735 /// The conversion allocates on the heap and moves `t`
736 /// from the stack into it.
741 /// let boxed = Box::new(5);
743 /// assert_eq!(Box::from(x), boxed);
745 fn from(t: T) -> Self {
750 #[stable(feature = "pin", since = "1.33.0")]
751 impl<T: ?Sized> From<Box<T>> for Pin<Box<T>> {
752 /// Converts a `Box<T>` into a `Pin<Box<T>>`
754 /// This conversion does not allocate on the heap and happens in place.
755 fn from(boxed: Box<T>) -> Self {
760 #[stable(feature = "box_from_slice", since = "1.17.0")]
761 impl<T: Copy> From<&[T]> for Box<[T]> {
762 /// Converts a `&[T]` into a `Box<[T]>`
764 /// This conversion allocates on the heap
765 /// and performs a copy of `slice`.
769 /// // create a &[u8] which will be used to create a Box<[u8]>
770 /// let slice: &[u8] = &[104, 101, 108, 108, 111];
771 /// let boxed_slice: Box<[u8]> = Box::from(slice);
773 /// println!("{:?}", boxed_slice);
775 fn from(slice: &[T]) -> Box<[T]> {
776 let len = slice.len();
777 let buf = RawVec::with_capacity(len);
779 ptr::copy_nonoverlapping(slice.as_ptr(), buf.ptr(), len);
785 #[stable(feature = "box_from_slice", since = "1.17.0")]
786 impl From<&str> for Box<str> {
787 /// Converts a `&str` into a `Box<str>`
789 /// This conversion allocates on the heap
790 /// and performs a copy of `s`.
794 /// let boxed: Box<str> = Box::from("hello");
795 /// println!("{}", boxed);
798 fn from(s: &str) -> Box<str> {
799 unsafe { from_boxed_utf8_unchecked(Box::from(s.as_bytes())) }
803 #[stable(feature = "boxed_str_conv", since = "1.19.0")]
804 impl From<Box<str>> for Box<[u8]> {
805 /// Converts a `Box<str>>` into a `Box<[u8]>`
807 /// This conversion does not allocate on the heap and happens in place.
811 /// // create a Box<str> which will be used to create a Box<[u8]>
812 /// let boxed: Box<str> = Box::from("hello");
813 /// let boxed_str: Box<[u8]> = Box::from(boxed);
815 /// // create a &[u8] which will be used to create a Box<[u8]>
816 /// let slice: &[u8] = &[104, 101, 108, 108, 111];
817 /// let boxed_slice = Box::from(slice);
819 /// assert_eq!(boxed_slice, boxed_str);
822 fn from(s: Box<str>) -> Self {
823 unsafe { Box::from_raw(Box::into_raw(s) as *mut [u8]) }
827 #[unstable(feature = "boxed_slice_try_from", issue = "none")]
828 impl<T, const N: usize> TryFrom<Box<[T]>> for Box<[T; N]>
830 [T; N]: LengthAtMost32,
832 type Error = Box<[T]>;
834 fn try_from(boxed_slice: Box<[T]>) -> Result<Self, Self::Error> {
835 if boxed_slice.len() == N {
836 Ok(unsafe { Box::from_raw(Box::into_raw(boxed_slice) as *mut [T; N]) })
845 #[stable(feature = "rust1", since = "1.0.0")]
846 /// Attempt to downcast the box to a concrete type.
851 /// use std::any::Any;
853 /// fn print_if_string(value: Box<dyn Any>) {
854 /// if let Ok(string) = value.downcast::<String>() {
855 /// println!("String ({}): {}", string.len(), string);
859 /// let my_string = "Hello World".to_string();
860 /// print_if_string(Box::new(my_string));
861 /// print_if_string(Box::new(0i8));
863 pub fn downcast<T: Any>(self) -> Result<Box<T>, Box<dyn Any>> {
866 let raw: *mut dyn Any = Box::into_raw(self);
867 Ok(Box::from_raw(raw as *mut T))
875 impl Box<dyn Any + Send> {
877 #[stable(feature = "rust1", since = "1.0.0")]
878 /// Attempt to downcast the box to a concrete type.
883 /// use std::any::Any;
885 /// fn print_if_string(value: Box<dyn Any + Send>) {
886 /// if let Ok(string) = value.downcast::<String>() {
887 /// println!("String ({}): {}", string.len(), string);
891 /// let my_string = "Hello World".to_string();
892 /// print_if_string(Box::new(my_string));
893 /// print_if_string(Box::new(0i8));
895 pub fn downcast<T: Any>(self) -> Result<Box<T>, Box<dyn Any + Send>> {
896 <Box<dyn Any>>::downcast(self).map_err(|s| unsafe {
897 // reapply the Send marker
898 Box::from_raw(Box::into_raw(s) as *mut (dyn Any + Send))
903 #[stable(feature = "rust1", since = "1.0.0")]
904 impl<T: fmt::Display + ?Sized> fmt::Display for Box<T> {
905 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
906 fmt::Display::fmt(&**self, f)
910 #[stable(feature = "rust1", since = "1.0.0")]
911 impl<T: fmt::Debug + ?Sized> fmt::Debug for Box<T> {
912 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
913 fmt::Debug::fmt(&**self, f)
917 #[stable(feature = "rust1", since = "1.0.0")]
918 impl<T: ?Sized> fmt::Pointer for Box<T> {
919 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
920 // It's not possible to extract the inner Uniq directly from the Box,
921 // instead we cast it to a *const which aliases the Unique
922 let ptr: *const T = &**self;
923 fmt::Pointer::fmt(&ptr, f)
927 #[stable(feature = "rust1", since = "1.0.0")]
928 impl<T: ?Sized> Deref for Box<T> {
931 fn deref(&self) -> &T {
936 #[stable(feature = "rust1", since = "1.0.0")]
937 impl<T: ?Sized> DerefMut for Box<T> {
938 fn deref_mut(&mut self) -> &mut T {
943 #[unstable(feature = "receiver_trait", issue = "none")]
944 impl<T: ?Sized> Receiver for Box<T> {}
946 #[stable(feature = "rust1", since = "1.0.0")]
947 impl<I: Iterator + ?Sized> Iterator for Box<I> {
949 fn next(&mut self) -> Option<I::Item> {
952 fn size_hint(&self) -> (usize, Option<usize>) {
955 fn nth(&mut self, n: usize) -> Option<I::Item> {
958 fn last(self) -> Option<I::Item> {
965 fn last(self) -> Option<Self::Item>;
968 impl<I: Iterator + ?Sized> BoxIter for Box<I> {
970 default fn last(self) -> Option<I::Item> {
972 fn some<T>(_: Option<T>, x: T) -> Option<T> {
976 self.fold(None, some)
980 /// Specialization for sized `I`s that uses `I`s implementation of `last()`
981 /// instead of the default.
982 #[stable(feature = "rust1", since = "1.0.0")]
983 impl<I: Iterator> BoxIter for Box<I> {
984 fn last(self) -> Option<I::Item> {
989 #[stable(feature = "rust1", since = "1.0.0")]
990 impl<I: DoubleEndedIterator + ?Sized> DoubleEndedIterator for Box<I> {
991 fn next_back(&mut self) -> Option<I::Item> {
994 fn nth_back(&mut self, n: usize) -> Option<I::Item> {
998 #[stable(feature = "rust1", since = "1.0.0")]
999 impl<I: ExactSizeIterator + ?Sized> ExactSizeIterator for Box<I> {
1000 fn len(&self) -> usize {
1003 fn is_empty(&self) -> bool {
1008 #[stable(feature = "fused", since = "1.26.0")]
1009 impl<I: FusedIterator + ?Sized> FusedIterator for Box<I> {}
1011 #[stable(feature = "boxed_closure_impls", since = "1.35.0")]
1012 impl<A, F: FnOnce<A> + ?Sized> FnOnce<A> for Box<F> {
1013 type Output = <F as FnOnce<A>>::Output;
1015 extern "rust-call" fn call_once(self, args: A) -> Self::Output {
1016 <F as FnOnce<A>>::call_once(*self, args)
1020 #[stable(feature = "boxed_closure_impls", since = "1.35.0")]
1021 impl<A, F: FnMut<A> + ?Sized> FnMut<A> for Box<F> {
1022 extern "rust-call" fn call_mut(&mut self, args: A) -> Self::Output {
1023 <F as FnMut<A>>::call_mut(self, args)
1027 #[stable(feature = "boxed_closure_impls", since = "1.35.0")]
1028 impl<A, F: Fn<A> + ?Sized> Fn<A> for Box<F> {
1029 extern "rust-call" fn call(&self, args: A) -> Self::Output {
1030 <F as Fn<A>>::call(self, args)
1034 #[unstable(feature = "coerce_unsized", issue = "27732")]
1035 impl<T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<Box<U>> for Box<T> {}
1037 #[unstable(feature = "dispatch_from_dyn", issue = "none")]
1038 impl<T: ?Sized + Unsize<U>, U: ?Sized> DispatchFromDyn<Box<U>> for Box<T> {}
1040 #[stable(feature = "boxed_slice_from_iter", since = "1.32.0")]
1041 impl<A> FromIterator<A> for Box<[A]> {
1042 fn from_iter<T: IntoIterator<Item = A>>(iter: T) -> Self {
1043 iter.into_iter().collect::<Vec<_>>().into_boxed_slice()
1047 #[stable(feature = "box_slice_clone", since = "1.3.0")]
1048 impl<T: Clone> Clone for Box<[T]> {
1049 fn clone(&self) -> Self {
1050 self.to_vec().into_boxed_slice()
1054 #[stable(feature = "box_borrow", since = "1.1.0")]
1055 impl<T: ?Sized> borrow::Borrow<T> for Box<T> {
1056 fn borrow(&self) -> &T {
1061 #[stable(feature = "box_borrow", since = "1.1.0")]
1062 impl<T: ?Sized> borrow::BorrowMut<T> for Box<T> {
1063 fn borrow_mut(&mut self) -> &mut T {
1068 #[stable(since = "1.5.0", feature = "smart_ptr_as_ref")]
1069 impl<T: ?Sized> AsRef<T> for Box<T> {
1070 fn as_ref(&self) -> &T {
1075 #[stable(since = "1.5.0", feature = "smart_ptr_as_ref")]
1076 impl<T: ?Sized> AsMut<T> for Box<T> {
1077 fn as_mut(&mut self) -> &mut T {
1084 * We could have chosen not to add this impl, and instead have written a
1085 * function of Pin<Box<T>> to Pin<T>. Such a function would not be sound,
1086 * because Box<T> implements Unpin even when T does not, as a result of
1089 * We chose this API instead of the alternative for a few reasons:
1090 * - Logically, it is helpful to understand pinning in regard to the
1091 * memory region being pointed to. For this reason none of the
1092 * standard library pointer types support projecting through a pin
1093 * (Box<T> is the only pointer type in std for which this would be
1095 * - It is in practice very useful to have Box<T> be unconditionally
1096 * Unpin because of trait objects, for which the structural auto
1097 * trait functionality does not apply (e.g., Box<dyn Foo> would
1098 * otherwise not be Unpin).
1100 * Another type with the same semantics as Box but only a conditional
1101 * implementation of `Unpin` (where `T: Unpin`) would be valid/safe, and
1102 * could have a method to project a Pin<T> from it.
1104 #[stable(feature = "pin", since = "1.33.0")]
1105 impl<T: ?Sized> Unpin for Box<T> {}
1108 #[unstable(feature = "generator_trait", issue = "43122")]
1109 impl<G: ?Sized + Generator + Unpin> Generator for Box<G> {
1110 type Yield = G::Yield;
1111 type Return = G::Return;
1113 fn resume(mut self: Pin<&mut Self>) -> GeneratorState<Self::Yield, Self::Return> {
1114 G::resume(Pin::new(&mut *self))
1119 #[unstable(feature = "generator_trait", issue = "43122")]
1120 impl<G: ?Sized + Generator> Generator for Pin<Box<G>> {
1121 type Yield = G::Yield;
1122 type Return = G::Return;
1124 fn resume(mut self: Pin<&mut Self>) -> GeneratorState<Self::Yield, Self::Return> {
1125 G::resume((*self).as_mut())
1129 #[cfg(not(bootstrap))]
1130 #[unstable(feature = "generator_trait", issue = "43122")]
1131 impl<G: ?Sized + Generator<R> + Unpin, R> Generator<R> for Box<G> {
1132 type Yield = G::Yield;
1133 type Return = G::Return;
1135 fn resume(mut self: Pin<&mut Self>, arg: R) -> GeneratorState<Self::Yield, Self::Return> {
1136 G::resume(Pin::new(&mut *self), arg)
1140 #[cfg(not(bootstrap))]
1141 #[unstable(feature = "generator_trait", issue = "43122")]
1142 impl<G: ?Sized + Generator<R>, R> Generator<R> for Pin<Box<G>> {
1143 type Yield = G::Yield;
1144 type Return = G::Return;
1146 fn resume(mut self: Pin<&mut Self>, arg: R) -> GeneratorState<Self::Yield, Self::Return> {
1147 G::resume((*self).as_mut(), arg)
1151 #[stable(feature = "futures_api", since = "1.36.0")]
1152 impl<F: ?Sized + Future + Unpin> Future for Box<F> {
1153 type Output = F::Output;
1155 fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
1156 F::poll(Pin::new(&mut *self), cx)