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 //! #[allow(improper_ctypes_definitions)]
96 //! pub extern "C" fn foo_new() -> Box<Foo> {
101 //! #[allow(improper_ctypes_definitions)]
102 //! pub extern "C" fn foo_delete(_: Option<Box<Foo>>) {}
105 //! Even though `Box<T>` has the same representation and C ABI as a C pointer,
106 //! this does not mean that you can convert an arbitrary `T*` into a `Box<T>`
107 //! and expect things to work. `Box<T>` values will always be fully aligned,
108 //! non-null pointers. Moreover, the destructor for `Box<T>` will attempt to
109 //! free the value with the global allocator. In general, the best practice
110 //! is to only use `Box<T>` for pointers that originated from the global
113 //! **Important.** At least at present, you should avoid using
114 //! `Box<T>` types for functions that are defined in C but invoked
115 //! from Rust. In those cases, you should directly mirror the C types
116 //! as closely as possible. Using types like `Box<T>` where the C
117 //! definition is just using `T*` can lead to undefined behavior, as
118 //! described in [rust-lang/unsafe-code-guidelines#198][ucg#198].
120 //! [ucg#198]: https://github.com/rust-lang/unsafe-code-guidelines/issues/198
121 //! [dereferencing]: ../../std/ops/trait.Deref.html
122 //! [`Box`]: struct.Box.html
123 //! [`Box<T>`]: struct.Box.html
124 //! [`Box::<T>::from_raw(value)`]: struct.Box.html#method.from_raw
125 //! [`Box::<T>::into_raw`]: struct.Box.html#method.into_raw
126 //! [`Global`]: ../alloc/struct.Global.html
127 //! [`Layout`]: ../alloc/struct.Layout.html
128 //! [`Layout::for_value(&*value)`]: ../alloc/struct.Layout.html#method.for_value
130 #![stable(feature = "rust1", since = "1.0.0")]
134 use core::cmp::Ordering;
135 use core::convert::{From, TryFrom};
137 use core::future::Future;
138 use core::hash::{Hash, Hasher};
139 use core::iter::{FromIterator, FusedIterator, Iterator};
140 use core::marker::{Unpin, Unsize};
143 CoerceUnsized, Deref, DerefMut, DispatchFromDyn, Generator, GeneratorState, Receiver,
146 use core::ptr::{self, Unique};
147 use core::task::{Context, Poll};
149 use crate::alloc::{self, AllocRef, Global};
150 use crate::borrow::Cow;
151 use crate::raw_vec::RawVec;
152 use crate::str::from_boxed_utf8_unchecked;
155 /// A pointer type for heap allocation.
157 /// See the [module-level documentation](../../std/boxed/index.html) for more.
158 #[lang = "owned_box"]
160 #[stable(feature = "rust1", since = "1.0.0")]
161 pub struct Box<T: ?Sized>(Unique<T>);
164 /// Allocates memory on the heap and then places `x` into it.
166 /// This doesn't actually allocate if `T` is zero-sized.
171 /// let five = Box::new(5);
173 #[stable(feature = "rust1", since = "1.0.0")]
175 pub fn new(x: T) -> Box<T> {
179 /// Constructs a new box with uninitialized contents.
184 /// #![feature(new_uninit)]
186 /// let mut five = Box::<u32>::new_uninit();
188 /// let five = unsafe {
189 /// // Deferred initialization:
190 /// five.as_mut_ptr().write(5);
192 /// five.assume_init()
195 /// assert_eq!(*five, 5)
197 #[unstable(feature = "new_uninit", issue = "63291")]
198 pub fn new_uninit() -> Box<mem::MaybeUninit<T>> {
199 let layout = alloc::Layout::new::<mem::MaybeUninit<T>>();
200 let ptr = Global.alloc(layout).unwrap_or_else(|_| alloc::handle_alloc_error(layout)).cast();
201 unsafe { Box::from_raw(ptr.as_ptr()) }
204 /// Constructs a new `Box` with uninitialized contents, with the memory
205 /// being filled with `0` bytes.
207 /// See [`MaybeUninit::zeroed`][zeroed] for examples of correct and incorrect usage
213 /// #![feature(new_uninit)]
215 /// let zero = Box::<u32>::new_zeroed();
216 /// let zero = unsafe { zero.assume_init() };
218 /// assert_eq!(*zero, 0)
221 /// [zeroed]: ../../std/mem/union.MaybeUninit.html#method.zeroed
222 #[unstable(feature = "new_uninit", issue = "63291")]
223 pub fn new_zeroed() -> Box<mem::MaybeUninit<T>> {
224 let layout = alloc::Layout::new::<mem::MaybeUninit<T>>();
226 .alloc_zeroed(layout)
227 .unwrap_or_else(|_| alloc::handle_alloc_error(layout))
229 unsafe { Box::from_raw(ptr.as_ptr()) }
232 /// Constructs a new `Pin<Box<T>>`. If `T` does not implement `Unpin`, then
233 /// `x` will be pinned in memory and unable to be moved.
234 #[stable(feature = "pin", since = "1.33.0")]
236 pub fn pin(x: T) -> Pin<Box<T>> {
240 /// Converts a `Box<T>` into a `Box<[T]>`
242 /// This conversion does not allocate on the heap and happens in place.
244 #[unstable(feature = "box_into_boxed_slice", issue = "71582")]
245 pub fn into_boxed_slice(boxed: Box<T>) -> Box<[T]> {
246 // *mut T and *mut [T; 1] have the same size and alignment
247 unsafe { Box::from_raw(Box::into_raw(boxed) as *mut [T; 1]) }
252 /// Constructs a new boxed slice with uninitialized contents.
257 /// #![feature(new_uninit)]
259 /// let mut values = Box::<[u32]>::new_uninit_slice(3);
261 /// let values = unsafe {
262 /// // Deferred initialization:
263 /// values[0].as_mut_ptr().write(1);
264 /// values[1].as_mut_ptr().write(2);
265 /// values[2].as_mut_ptr().write(3);
267 /// values.assume_init()
270 /// assert_eq!(*values, [1, 2, 3])
272 #[unstable(feature = "new_uninit", issue = "63291")]
273 pub fn new_uninit_slice(len: usize) -> Box<[mem::MaybeUninit<T>]> {
274 unsafe { RawVec::with_capacity(len).into_box(len) }
278 impl<T> Box<mem::MaybeUninit<T>> {
279 /// Converts to `Box<T>`.
283 /// As with [`MaybeUninit::assume_init`],
284 /// it is up to the caller to guarantee that the value
285 /// really is in an initialized state.
286 /// Calling this when the content is not yet fully initialized
287 /// causes immediate undefined behavior.
289 /// [`MaybeUninit::assume_init`]: ../../std/mem/union.MaybeUninit.html#method.assume_init
294 /// #![feature(new_uninit)]
296 /// let mut five = Box::<u32>::new_uninit();
298 /// let five: Box<u32> = unsafe {
299 /// // Deferred initialization:
300 /// five.as_mut_ptr().write(5);
302 /// five.assume_init()
305 /// assert_eq!(*five, 5)
307 #[unstable(feature = "new_uninit", issue = "63291")]
309 pub unsafe fn assume_init(self) -> Box<T> {
310 unsafe { Box::from_raw(Box::into_raw(self) as *mut T) }
314 impl<T> Box<[mem::MaybeUninit<T>]> {
315 /// Converts to `Box<[T]>`.
319 /// As with [`MaybeUninit::assume_init`],
320 /// it is up to the caller to guarantee that the values
321 /// really are in an initialized state.
322 /// Calling this when the content is not yet fully initialized
323 /// causes immediate undefined behavior.
325 /// [`MaybeUninit::assume_init`]: ../../std/mem/union.MaybeUninit.html#method.assume_init
330 /// #![feature(new_uninit)]
332 /// let mut values = Box::<[u32]>::new_uninit_slice(3);
334 /// let values = unsafe {
335 /// // Deferred initialization:
336 /// values[0].as_mut_ptr().write(1);
337 /// values[1].as_mut_ptr().write(2);
338 /// values[2].as_mut_ptr().write(3);
340 /// values.assume_init()
343 /// assert_eq!(*values, [1, 2, 3])
345 #[unstable(feature = "new_uninit", issue = "63291")]
347 pub unsafe fn assume_init(self) -> Box<[T]> {
348 unsafe { Box::from_raw(Box::into_raw(self) as *mut [T]) }
352 impl<T: ?Sized> Box<T> {
353 /// Constructs a box from a raw pointer.
355 /// After calling this function, the raw pointer is owned by the
356 /// resulting `Box`. Specifically, the `Box` destructor will call
357 /// the destructor of `T` and free the allocated memory. For this
358 /// to be safe, the memory must have been allocated in accordance
359 /// with the [memory layout] used by `Box` .
363 /// This function is unsafe because improper use may lead to
364 /// memory problems. For example, a double-free may occur if the
365 /// function is called twice on the same raw pointer.
368 /// Recreate a `Box` which was previously converted to a raw pointer
369 /// using [`Box::into_raw`]:
371 /// let x = Box::new(5);
372 /// let ptr = Box::into_raw(x);
373 /// let x = unsafe { Box::from_raw(ptr) };
375 /// Manually create a `Box` from scratch by using the global allocator:
377 /// use std::alloc::{alloc, Layout};
380 /// let ptr = alloc(Layout::new::<i32>()) as *mut i32;
381 /// // In general .write is required to avoid attempting to destruct
382 /// // the (uninitialized) previous contents of `ptr`, though for this
383 /// // simple example `*ptr = 5` would have worked as well.
385 /// let x = Box::from_raw(ptr);
389 /// [memory layout]: index.html#memory-layout
390 /// [`Layout`]: ../alloc/struct.Layout.html
391 /// [`Box::into_raw`]: struct.Box.html#method.into_raw
392 #[stable(feature = "box_raw", since = "1.4.0")]
394 pub unsafe fn from_raw(raw: *mut T) -> Self {
395 Box(unsafe { Unique::new_unchecked(raw) })
398 /// Consumes the `Box`, returning a wrapped raw pointer.
400 /// The pointer will be properly aligned and non-null.
402 /// After calling this function, the caller is responsible for the
403 /// memory previously managed by the `Box`. In particular, the
404 /// caller should properly destroy `T` and release the memory, taking
405 /// into account the [memory layout] used by `Box`. The easiest way to
406 /// do this is to convert the raw pointer back into a `Box` with the
407 /// [`Box::from_raw`] function, allowing the `Box` destructor to perform
410 /// Note: this is an associated function, which means that you have
411 /// to call it as `Box::into_raw(b)` instead of `b.into_raw()`. This
412 /// is so that there is no conflict with a method on the inner type.
415 /// Converting the raw pointer back into a `Box` with [`Box::from_raw`]
416 /// for automatic cleanup:
418 /// let x = Box::new(String::from("Hello"));
419 /// let ptr = Box::into_raw(x);
420 /// let x = unsafe { Box::from_raw(ptr) };
422 /// Manual cleanup by explicitly running the destructor and deallocating
425 /// use std::alloc::{dealloc, Layout};
428 /// let x = Box::new(String::from("Hello"));
429 /// let p = Box::into_raw(x);
431 /// ptr::drop_in_place(p);
432 /// dealloc(p as *mut u8, Layout::new::<String>());
436 /// [memory layout]: index.html#memory-layout
437 /// [`Box::from_raw`]: struct.Box.html#method.from_raw
438 #[stable(feature = "box_raw", since = "1.4.0")]
440 pub fn into_raw(b: Box<T>) -> *mut T {
441 // Box is recognized as a "unique pointer" by Stacked Borrows, but internally it is a
442 // raw pointer for the type system. Turning it directly into a raw pointer would not be
443 // recognized as "releasing" the unique pointer to permit aliased raw accesses,
444 // so all raw pointer methods go through `leak` which creates a (unique)
445 // mutable reference. Turning *that* to a raw pointer behaves correctly.
446 Box::leak(b) as *mut T
450 feature = "ptr_internals",
452 reason = "use `Box::leak(b).into()` or `Unique::from(Box::leak(b))` instead"
456 pub fn into_unique(b: Box<T>) -> Unique<T> {
457 // Box is recognized as a "unique pointer" by Stacked Borrows, but internally it is a
458 // raw pointer for the type system. Turning it directly into a raw pointer would not be
459 // recognized as "releasing" the unique pointer to permit aliased raw accesses,
460 // so all raw pointer methods go through `leak` which creates a (unique)
461 // mutable reference. Turning *that* to a raw pointer behaves correctly.
465 /// Consumes and leaks the `Box`, returning a mutable reference,
466 /// `&'a mut T`. Note that the type `T` must outlive the chosen lifetime
467 /// `'a`. If the type has only static references, or none at all, then this
468 /// may be chosen to be `'static`.
470 /// This function is mainly useful for data that lives for the remainder of
471 /// the program's life. Dropping the returned reference will cause a memory
472 /// leak. If this is not acceptable, the reference should first be wrapped
473 /// with the [`Box::from_raw`] function producing a `Box`. This `Box` can
474 /// then be dropped which will properly destroy `T` and release the
475 /// allocated memory.
477 /// Note: this is an associated function, which means that you have
478 /// to call it as `Box::leak(b)` instead of `b.leak()`. This
479 /// is so that there is no conflict with a method on the inner type.
481 /// [`Box::from_raw`]: struct.Box.html#method.from_raw
488 /// let x = Box::new(41);
489 /// let static_ref: &'static mut usize = Box::leak(x);
490 /// *static_ref += 1;
491 /// assert_eq!(*static_ref, 42);
497 /// let x = vec![1, 2, 3].into_boxed_slice();
498 /// let static_ref = Box::leak(x);
499 /// static_ref[0] = 4;
500 /// assert_eq!(*static_ref, [4, 2, 3]);
502 #[stable(feature = "box_leak", since = "1.26.0")]
504 pub fn leak<'a>(b: Box<T>) -> &'a mut T
506 T: 'a, // Technically not needed, but kept to be explicit.
508 unsafe { &mut *mem::ManuallyDrop::new(b).0.as_ptr() }
511 /// Converts a `Box<T>` into a `Pin<Box<T>>`
513 /// This conversion does not allocate on the heap and happens in place.
515 /// This is also available via [`From`].
516 #[unstable(feature = "box_into_pin", issue = "62370")]
517 pub fn into_pin(boxed: Box<T>) -> Pin<Box<T>> {
518 // It's not possible to move or replace the insides of a `Pin<Box<T>>`
519 // when `T: !Unpin`, so it's safe to pin it directly without any
520 // additional requirements.
521 unsafe { Pin::new_unchecked(boxed) }
525 #[stable(feature = "rust1", since = "1.0.0")]
526 unsafe impl<#[may_dangle] T: ?Sized> Drop for Box<T> {
528 // FIXME: Do nothing, drop is currently performed by compiler.
532 #[stable(feature = "rust1", since = "1.0.0")]
533 impl<T: Default> Default for Box<T> {
534 /// Creates a `Box<T>`, with the `Default` value for T.
535 fn default() -> Box<T> {
536 box Default::default()
540 #[stable(feature = "rust1", since = "1.0.0")]
541 impl<T> Default for Box<[T]> {
542 fn default() -> Box<[T]> {
543 Box::<[T; 0]>::new([])
547 #[stable(feature = "default_box_extra", since = "1.17.0")]
548 impl Default for Box<str> {
549 fn default() -> Box<str> {
550 unsafe { from_boxed_utf8_unchecked(Default::default()) }
554 #[stable(feature = "rust1", since = "1.0.0")]
555 impl<T: Clone> Clone for Box<T> {
556 /// Returns a new box with a `clone()` of this box's contents.
561 /// let x = Box::new(5);
562 /// let y = x.clone();
564 /// // The value is the same
565 /// assert_eq!(x, y);
567 /// // But they are unique objects
568 /// assert_ne!(&*x as *const i32, &*y as *const i32);
572 fn clone(&self) -> Box<T> {
573 box { (**self).clone() }
576 /// Copies `source`'s contents into `self` without creating a new allocation.
581 /// let x = Box::new(5);
582 /// let mut y = Box::new(10);
583 /// let yp: *const i32 = &*y;
585 /// y.clone_from(&x);
587 /// // The value is the same
588 /// assert_eq!(x, y);
590 /// // And no allocation occurred
591 /// assert_eq!(yp, &*y);
594 fn clone_from(&mut self, source: &Box<T>) {
595 (**self).clone_from(&(**source));
599 #[stable(feature = "box_slice_clone", since = "1.3.0")]
600 impl Clone for Box<str> {
601 fn clone(&self) -> Self {
602 // this makes a copy of the data
603 let buf: Box<[u8]> = self.as_bytes().into();
604 unsafe { from_boxed_utf8_unchecked(buf) }
608 #[stable(feature = "rust1", since = "1.0.0")]
609 impl<T: ?Sized + PartialEq> PartialEq for Box<T> {
611 fn eq(&self, other: &Box<T>) -> bool {
612 PartialEq::eq(&**self, &**other)
615 fn ne(&self, other: &Box<T>) -> bool {
616 PartialEq::ne(&**self, &**other)
619 #[stable(feature = "rust1", since = "1.0.0")]
620 impl<T: ?Sized + PartialOrd> PartialOrd for Box<T> {
622 fn partial_cmp(&self, other: &Box<T>) -> Option<Ordering> {
623 PartialOrd::partial_cmp(&**self, &**other)
626 fn lt(&self, other: &Box<T>) -> bool {
627 PartialOrd::lt(&**self, &**other)
630 fn le(&self, other: &Box<T>) -> bool {
631 PartialOrd::le(&**self, &**other)
634 fn ge(&self, other: &Box<T>) -> bool {
635 PartialOrd::ge(&**self, &**other)
638 fn gt(&self, other: &Box<T>) -> bool {
639 PartialOrd::gt(&**self, &**other)
642 #[stable(feature = "rust1", since = "1.0.0")]
643 impl<T: ?Sized + Ord> Ord for Box<T> {
645 fn cmp(&self, other: &Box<T>) -> Ordering {
646 Ord::cmp(&**self, &**other)
649 #[stable(feature = "rust1", since = "1.0.0")]
650 impl<T: ?Sized + Eq> Eq for Box<T> {}
652 #[stable(feature = "rust1", since = "1.0.0")]
653 impl<T: ?Sized + Hash> Hash for Box<T> {
654 fn hash<H: Hasher>(&self, state: &mut H) {
655 (**self).hash(state);
659 #[stable(feature = "indirect_hasher_impl", since = "1.22.0")]
660 impl<T: ?Sized + Hasher> Hasher for Box<T> {
661 fn finish(&self) -> u64 {
664 fn write(&mut self, bytes: &[u8]) {
665 (**self).write(bytes)
667 fn write_u8(&mut self, i: u8) {
670 fn write_u16(&mut self, i: u16) {
671 (**self).write_u16(i)
673 fn write_u32(&mut self, i: u32) {
674 (**self).write_u32(i)
676 fn write_u64(&mut self, i: u64) {
677 (**self).write_u64(i)
679 fn write_u128(&mut self, i: u128) {
680 (**self).write_u128(i)
682 fn write_usize(&mut self, i: usize) {
683 (**self).write_usize(i)
685 fn write_i8(&mut self, i: i8) {
688 fn write_i16(&mut self, i: i16) {
689 (**self).write_i16(i)
691 fn write_i32(&mut self, i: i32) {
692 (**self).write_i32(i)
694 fn write_i64(&mut self, i: i64) {
695 (**self).write_i64(i)
697 fn write_i128(&mut self, i: i128) {
698 (**self).write_i128(i)
700 fn write_isize(&mut self, i: isize) {
701 (**self).write_isize(i)
705 #[stable(feature = "from_for_ptrs", since = "1.6.0")]
706 impl<T> From<T> for Box<T> {
707 /// Converts a generic type `T` into a `Box<T>`
709 /// The conversion allocates on the heap and moves `t`
710 /// from the stack into it.
715 /// let boxed = Box::new(5);
717 /// assert_eq!(Box::from(x), boxed);
719 fn from(t: T) -> Self {
724 #[stable(feature = "pin", since = "1.33.0")]
725 impl<T: ?Sized> From<Box<T>> for Pin<Box<T>> {
726 /// Converts a `Box<T>` into a `Pin<Box<T>>`
728 /// This conversion does not allocate on the heap and happens in place.
729 fn from(boxed: Box<T>) -> Self {
734 #[stable(feature = "box_from_slice", since = "1.17.0")]
735 impl<T: Copy> From<&[T]> for Box<[T]> {
736 /// Converts a `&[T]` into a `Box<[T]>`
738 /// This conversion allocates on the heap
739 /// and performs a copy of `slice`.
743 /// // create a &[u8] which will be used to create a Box<[u8]>
744 /// let slice: &[u8] = &[104, 101, 108, 108, 111];
745 /// let boxed_slice: Box<[u8]> = Box::from(slice);
747 /// println!("{:?}", boxed_slice);
749 fn from(slice: &[T]) -> Box<[T]> {
750 let len = slice.len();
751 let buf = RawVec::with_capacity(len);
753 ptr::copy_nonoverlapping(slice.as_ptr(), buf.ptr(), len);
754 buf.into_box(slice.len()).assume_init()
759 #[stable(feature = "box_from_cow", since = "1.45.0")]
760 impl<T: Copy> From<Cow<'_, [T]>> for Box<[T]> {
762 fn from(cow: Cow<'_, [T]>) -> Box<[T]> {
764 Cow::Borrowed(slice) => Box::from(slice),
765 Cow::Owned(slice) => Box::from(slice),
770 #[stable(feature = "box_from_slice", since = "1.17.0")]
771 impl From<&str> for Box<str> {
772 /// Converts a `&str` into a `Box<str>`
774 /// This conversion allocates on the heap
775 /// and performs a copy of `s`.
779 /// let boxed: Box<str> = Box::from("hello");
780 /// println!("{}", boxed);
783 fn from(s: &str) -> Box<str> {
784 unsafe { from_boxed_utf8_unchecked(Box::from(s.as_bytes())) }
788 #[stable(feature = "box_from_cow", since = "1.45.0")]
789 impl From<Cow<'_, str>> for Box<str> {
791 fn from(cow: Cow<'_, str>) -> Box<str> {
793 Cow::Borrowed(s) => Box::from(s),
794 Cow::Owned(s) => Box::from(s),
799 #[stable(feature = "boxed_str_conv", since = "1.19.0")]
800 impl From<Box<str>> for Box<[u8]> {
801 /// Converts a `Box<str>>` into a `Box<[u8]>`
803 /// This conversion does not allocate on the heap and happens in place.
807 /// // create a Box<str> which will be used to create a Box<[u8]>
808 /// let boxed: Box<str> = Box::from("hello");
809 /// let boxed_str: Box<[u8]> = Box::from(boxed);
811 /// // create a &[u8] which will be used to create a Box<[u8]>
812 /// let slice: &[u8] = &[104, 101, 108, 108, 111];
813 /// let boxed_slice = Box::from(slice);
815 /// assert_eq!(boxed_slice, boxed_str);
818 fn from(s: Box<str>) -> Self {
819 unsafe { Box::from_raw(Box::into_raw(s) as *mut [u8]) }
823 #[stable(feature = "box_from_array", since = "1.45.0")]
824 impl<T, const N: usize> From<[T; N]> for Box<[T]> {
825 /// Converts a `[T; N]` into a `Box<[T]>`
827 /// This conversion moves the array to newly heap-allocated memory.
831 /// let boxed: Box<[u8]> = Box::from([4, 2]);
832 /// println!("{:?}", boxed);
834 fn from(array: [T; N]) -> Box<[T]> {
839 #[stable(feature = "boxed_slice_try_from", since = "1.43.0")]
840 impl<T, const N: usize> TryFrom<Box<[T]>> for Box<[T; N]> {
841 type Error = Box<[T]>;
843 fn try_from(boxed_slice: Box<[T]>) -> Result<Self, Self::Error> {
844 if boxed_slice.len() == N {
845 Ok(unsafe { Box::from_raw(Box::into_raw(boxed_slice) as *mut [T; N]) })
854 #[stable(feature = "rust1", since = "1.0.0")]
855 /// Attempt to downcast the box to a concrete type.
860 /// use std::any::Any;
862 /// fn print_if_string(value: Box<dyn Any>) {
863 /// if let Ok(string) = value.downcast::<String>() {
864 /// println!("String ({}): {}", string.len(), string);
868 /// let my_string = "Hello World".to_string();
869 /// print_if_string(Box::new(my_string));
870 /// print_if_string(Box::new(0i8));
872 pub fn downcast<T: Any>(self) -> Result<Box<T>, Box<dyn Any>> {
875 let raw: *mut dyn Any = Box::into_raw(self);
876 Ok(Box::from_raw(raw as *mut T))
884 impl Box<dyn Any + Send> {
886 #[stable(feature = "rust1", since = "1.0.0")]
887 /// Attempt to downcast the box to a concrete type.
892 /// use std::any::Any;
894 /// fn print_if_string(value: Box<dyn Any + Send>) {
895 /// if let Ok(string) = value.downcast::<String>() {
896 /// println!("String ({}): {}", string.len(), string);
900 /// let my_string = "Hello World".to_string();
901 /// print_if_string(Box::new(my_string));
902 /// print_if_string(Box::new(0i8));
904 pub fn downcast<T: Any>(self) -> Result<Box<T>, Box<dyn Any + Send>> {
905 <Box<dyn Any>>::downcast(self).map_err(|s| unsafe {
906 // reapply the Send marker
907 Box::from_raw(Box::into_raw(s) as *mut (dyn Any + Send))
912 #[stable(feature = "rust1", since = "1.0.0")]
913 impl<T: fmt::Display + ?Sized> fmt::Display for Box<T> {
914 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
915 fmt::Display::fmt(&**self, f)
919 #[stable(feature = "rust1", since = "1.0.0")]
920 impl<T: fmt::Debug + ?Sized> fmt::Debug for Box<T> {
921 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
922 fmt::Debug::fmt(&**self, f)
926 #[stable(feature = "rust1", since = "1.0.0")]
927 impl<T: ?Sized> fmt::Pointer for Box<T> {
928 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
929 // It's not possible to extract the inner Uniq directly from the Box,
930 // instead we cast it to a *const which aliases the Unique
931 let ptr: *const T = &**self;
932 fmt::Pointer::fmt(&ptr, f)
936 #[stable(feature = "rust1", since = "1.0.0")]
937 impl<T: ?Sized> Deref for Box<T> {
940 fn deref(&self) -> &T {
945 #[stable(feature = "rust1", since = "1.0.0")]
946 impl<T: ?Sized> DerefMut for Box<T> {
947 fn deref_mut(&mut self) -> &mut T {
952 #[unstable(feature = "receiver_trait", issue = "none")]
953 impl<T: ?Sized> Receiver for Box<T> {}
955 #[stable(feature = "rust1", since = "1.0.0")]
956 impl<I: Iterator + ?Sized> Iterator for Box<I> {
958 fn next(&mut self) -> Option<I::Item> {
961 fn size_hint(&self) -> (usize, Option<usize>) {
964 fn nth(&mut self, n: usize) -> Option<I::Item> {
967 fn last(self) -> Option<I::Item> {
974 fn last(self) -> Option<Self::Item>;
977 impl<I: Iterator + ?Sized> BoxIter for Box<I> {
979 default fn last(self) -> Option<I::Item> {
981 fn some<T>(_: Option<T>, x: T) -> Option<T> {
985 self.fold(None, some)
989 /// Specialization for sized `I`s that uses `I`s implementation of `last()`
990 /// instead of the default.
991 #[stable(feature = "rust1", since = "1.0.0")]
992 impl<I: Iterator> BoxIter for Box<I> {
993 fn last(self) -> Option<I::Item> {
998 #[stable(feature = "rust1", since = "1.0.0")]
999 impl<I: DoubleEndedIterator + ?Sized> DoubleEndedIterator for Box<I> {
1000 fn next_back(&mut self) -> Option<I::Item> {
1001 (**self).next_back()
1003 fn nth_back(&mut self, n: usize) -> Option<I::Item> {
1004 (**self).nth_back(n)
1007 #[stable(feature = "rust1", since = "1.0.0")]
1008 impl<I: ExactSizeIterator + ?Sized> ExactSizeIterator for Box<I> {
1009 fn len(&self) -> usize {
1012 fn is_empty(&self) -> bool {
1017 #[stable(feature = "fused", since = "1.26.0")]
1018 impl<I: FusedIterator + ?Sized> FusedIterator for Box<I> {}
1020 #[stable(feature = "boxed_closure_impls", since = "1.35.0")]
1021 impl<A, F: FnOnce<A> + ?Sized> FnOnce<A> for Box<F> {
1022 type Output = <F as FnOnce<A>>::Output;
1024 extern "rust-call" fn call_once(self, args: A) -> Self::Output {
1025 <F as FnOnce<A>>::call_once(*self, args)
1029 #[stable(feature = "boxed_closure_impls", since = "1.35.0")]
1030 impl<A, F: FnMut<A> + ?Sized> FnMut<A> for Box<F> {
1031 extern "rust-call" fn call_mut(&mut self, args: A) -> Self::Output {
1032 <F as FnMut<A>>::call_mut(self, args)
1036 #[stable(feature = "boxed_closure_impls", since = "1.35.0")]
1037 impl<A, F: Fn<A> + ?Sized> Fn<A> for Box<F> {
1038 extern "rust-call" fn call(&self, args: A) -> Self::Output {
1039 <F as Fn<A>>::call(self, args)
1043 #[unstable(feature = "coerce_unsized", issue = "27732")]
1044 impl<T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<Box<U>> for Box<T> {}
1046 #[unstable(feature = "dispatch_from_dyn", issue = "none")]
1047 impl<T: ?Sized + Unsize<U>, U: ?Sized> DispatchFromDyn<Box<U>> for Box<T> {}
1049 #[stable(feature = "boxed_slice_from_iter", since = "1.32.0")]
1050 impl<A> FromIterator<A> for Box<[A]> {
1051 fn from_iter<T: IntoIterator<Item = A>>(iter: T) -> Self {
1052 iter.into_iter().collect::<Vec<_>>().into_boxed_slice()
1056 #[stable(feature = "box_slice_clone", since = "1.3.0")]
1057 impl<T: Clone> Clone for Box<[T]> {
1058 fn clone(&self) -> Self {
1059 self.to_vec().into_boxed_slice()
1062 fn clone_from(&mut self, other: &Self) {
1063 if self.len() == other.len() {
1064 self.clone_from_slice(&other);
1066 *self = other.clone();
1071 #[stable(feature = "box_borrow", since = "1.1.0")]
1072 impl<T: ?Sized> borrow::Borrow<T> for Box<T> {
1073 fn borrow(&self) -> &T {
1078 #[stable(feature = "box_borrow", since = "1.1.0")]
1079 impl<T: ?Sized> borrow::BorrowMut<T> for Box<T> {
1080 fn borrow_mut(&mut self) -> &mut T {
1085 #[stable(since = "1.5.0", feature = "smart_ptr_as_ref")]
1086 impl<T: ?Sized> AsRef<T> for Box<T> {
1087 fn as_ref(&self) -> &T {
1092 #[stable(since = "1.5.0", feature = "smart_ptr_as_ref")]
1093 impl<T: ?Sized> AsMut<T> for Box<T> {
1094 fn as_mut(&mut self) -> &mut T {
1101 * We could have chosen not to add this impl, and instead have written a
1102 * function of Pin<Box<T>> to Pin<T>. Such a function would not be sound,
1103 * because Box<T> implements Unpin even when T does not, as a result of
1106 * We chose this API instead of the alternative for a few reasons:
1107 * - Logically, it is helpful to understand pinning in regard to the
1108 * memory region being pointed to. For this reason none of the
1109 * standard library pointer types support projecting through a pin
1110 * (Box<T> is the only pointer type in std for which this would be
1112 * - It is in practice very useful to have Box<T> be unconditionally
1113 * Unpin because of trait objects, for which the structural auto
1114 * trait functionality does not apply (e.g., Box<dyn Foo> would
1115 * otherwise not be Unpin).
1117 * Another type with the same semantics as Box but only a conditional
1118 * implementation of `Unpin` (where `T: Unpin`) would be valid/safe, and
1119 * could have a method to project a Pin<T> from it.
1121 #[stable(feature = "pin", since = "1.33.0")]
1122 impl<T: ?Sized> Unpin for Box<T> {}
1124 #[unstable(feature = "generator_trait", issue = "43122")]
1125 impl<G: ?Sized + Generator<R> + Unpin, R> Generator<R> for Box<G> {
1126 type Yield = G::Yield;
1127 type Return = G::Return;
1129 fn resume(mut self: Pin<&mut Self>, arg: R) -> GeneratorState<Self::Yield, Self::Return> {
1130 G::resume(Pin::new(&mut *self), arg)
1134 #[unstable(feature = "generator_trait", issue = "43122")]
1135 impl<G: ?Sized + Generator<R>, R> Generator<R> for Pin<Box<G>> {
1136 type Yield = G::Yield;
1137 type Return = G::Return;
1139 fn resume(mut self: Pin<&mut Self>, arg: R) -> GeneratorState<Self::Yield, Self::Return> {
1140 G::resume((*self).as_mut(), arg)
1144 #[stable(feature = "futures_api", since = "1.36.0")]
1145 impl<F: ?Sized + Future + Unpin> Future for Box<F> {
1146 type Output = F::Output;
1148 fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
1149 F::poll(Pin::new(&mut *self), cx)