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]: core::ops::Deref
123 //! [`Box::<T>::from_raw(value)`]: Box::from_raw
124 //! [`Box::<T>::into_raw`]: Box::into_raw
125 //! [`Global`]: crate::alloc::Global
126 //! [`Layout`]: crate::alloc::Layout
127 //! [`Layout::for_value(&*value)`]: crate::alloc::Layout::for_value
129 #![stable(feature = "rust1", since = "1.0.0")]
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, Unique};
146 use core::task::{Context, Poll};
148 use crate::alloc::{self, AllocRef, Global};
149 use crate::borrow::Cow;
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 let ptr = Global.alloc(layout).unwrap_or_else(|_| alloc::handle_alloc_error(layout)).cast();
200 unsafe { Box::from_raw(ptr.as_ptr()) }
203 /// Constructs a new `Box` with uninitialized contents, with the memory
204 /// being filled with `0` bytes.
206 /// See [`MaybeUninit::zeroed`][zeroed] for examples of correct and incorrect usage
212 /// #![feature(new_uninit)]
214 /// let zero = Box::<u32>::new_zeroed();
215 /// let zero = unsafe { zero.assume_init() };
217 /// assert_eq!(*zero, 0)
220 /// [zeroed]: ../../std/mem/union.MaybeUninit.html#method.zeroed
221 #[unstable(feature = "new_uninit", issue = "63291")]
222 pub fn new_zeroed() -> Box<mem::MaybeUninit<T>> {
223 let layout = alloc::Layout::new::<mem::MaybeUninit<T>>();
225 .alloc_zeroed(layout)
226 .unwrap_or_else(|_| alloc::handle_alloc_error(layout))
228 unsafe { Box::from_raw(ptr.as_ptr()) }
231 /// Constructs a new `Pin<Box<T>>`. If `T` does not implement `Unpin`, then
232 /// `x` will be pinned in memory and unable to be moved.
233 #[stable(feature = "pin", since = "1.33.0")]
235 pub fn pin(x: T) -> Pin<Box<T>> {
239 /// Converts a `Box<T>` into a `Box<[T]>`
241 /// This conversion does not allocate on the heap and happens in place.
242 #[unstable(feature = "box_into_boxed_slice", issue = "71582")]
243 pub fn into_boxed_slice(boxed: Box<T>) -> Box<[T]> {
244 // *mut T and *mut [T; 1] have the same size and alignment
245 unsafe { Box::from_raw(Box::into_raw(boxed) as *mut [T; 1]) }
250 /// Constructs a new boxed slice with uninitialized contents.
255 /// #![feature(new_uninit)]
257 /// let mut values = Box::<[u32]>::new_uninit_slice(3);
259 /// let values = unsafe {
260 /// // Deferred initialization:
261 /// values[0].as_mut_ptr().write(1);
262 /// values[1].as_mut_ptr().write(2);
263 /// values[2].as_mut_ptr().write(3);
265 /// values.assume_init()
268 /// assert_eq!(*values, [1, 2, 3])
270 #[unstable(feature = "new_uninit", issue = "63291")]
271 pub fn new_uninit_slice(len: usize) -> Box<[mem::MaybeUninit<T>]> {
272 unsafe { RawVec::with_capacity(len).into_box(len) }
275 /// Constructs a new boxed slice with uninitialized contents, with the memory
276 /// being filled with `0` bytes.
278 /// See [`MaybeUninit::zeroed`][zeroed] for examples of correct and incorrect usage
284 /// #![feature(new_uninit)]
286 /// let values = Box::<[u32]>::new_zeroed_slice(3);
287 /// let values = unsafe { values.assume_init() };
289 /// assert_eq!(*values, [0, 0, 0])
292 /// [zeroed]: ../../std/mem/union.MaybeUninit.html#method.zeroed
293 #[unstable(feature = "new_uninit", issue = "63291")]
294 pub fn new_zeroed_slice(len: usize) -> Box<[mem::MaybeUninit<T>]> {
295 unsafe { RawVec::with_capacity_zeroed(len).into_box(len) }
299 impl<T> Box<mem::MaybeUninit<T>> {
300 /// Converts to `Box<T>`.
304 /// As with [`MaybeUninit::assume_init`],
305 /// it is up to the caller to guarantee that the value
306 /// really is in an initialized state.
307 /// Calling this when the content is not yet fully initialized
308 /// causes immediate undefined behavior.
310 /// [`MaybeUninit::assume_init`]: ../../std/mem/union.MaybeUninit.html#method.assume_init
315 /// #![feature(new_uninit)]
317 /// let mut five = Box::<u32>::new_uninit();
319 /// let five: Box<u32> = unsafe {
320 /// // Deferred initialization:
321 /// five.as_mut_ptr().write(5);
323 /// five.assume_init()
326 /// assert_eq!(*five, 5)
328 #[unstable(feature = "new_uninit", issue = "63291")]
330 pub unsafe fn assume_init(self) -> Box<T> {
331 unsafe { Box::from_raw(Box::into_raw(self) as *mut T) }
335 impl<T> Box<[mem::MaybeUninit<T>]> {
336 /// Converts to `Box<[T]>`.
340 /// As with [`MaybeUninit::assume_init`],
341 /// it is up to the caller to guarantee that the values
342 /// really are in an initialized state.
343 /// Calling this when the content is not yet fully initialized
344 /// causes immediate undefined behavior.
346 /// [`MaybeUninit::assume_init`]: ../../std/mem/union.MaybeUninit.html#method.assume_init
351 /// #![feature(new_uninit)]
353 /// let mut values = Box::<[u32]>::new_uninit_slice(3);
355 /// let values = unsafe {
356 /// // Deferred initialization:
357 /// values[0].as_mut_ptr().write(1);
358 /// values[1].as_mut_ptr().write(2);
359 /// values[2].as_mut_ptr().write(3);
361 /// values.assume_init()
364 /// assert_eq!(*values, [1, 2, 3])
366 #[unstable(feature = "new_uninit", issue = "63291")]
368 pub unsafe fn assume_init(self) -> Box<[T]> {
369 unsafe { Box::from_raw(Box::into_raw(self) as *mut [T]) }
373 impl<T: ?Sized> Box<T> {
374 /// Constructs a box from a raw pointer.
376 /// After calling this function, the raw pointer is owned by the
377 /// resulting `Box`. Specifically, the `Box` destructor will call
378 /// the destructor of `T` and free the allocated memory. For this
379 /// to be safe, the memory must have been allocated in accordance
380 /// with the [memory layout] used by `Box` .
384 /// This function is unsafe because improper use may lead to
385 /// memory problems. For example, a double-free may occur if the
386 /// function is called twice on the same raw pointer.
389 /// Recreate a `Box` which was previously converted to a raw pointer
390 /// using [`Box::into_raw`]:
392 /// let x = Box::new(5);
393 /// let ptr = Box::into_raw(x);
394 /// let x = unsafe { Box::from_raw(ptr) };
396 /// Manually create a `Box` from scratch by using the global allocator:
398 /// use std::alloc::{alloc, Layout};
401 /// let ptr = alloc(Layout::new::<i32>()) as *mut i32;
402 /// // In general .write is required to avoid attempting to destruct
403 /// // the (uninitialized) previous contents of `ptr`, though for this
404 /// // simple example `*ptr = 5` would have worked as well.
406 /// let x = Box::from_raw(ptr);
410 /// [memory layout]: self#memory-layout
411 /// [`Layout`]: crate::Layout
412 #[stable(feature = "box_raw", since = "1.4.0")]
414 pub unsafe fn from_raw(raw: *mut T) -> Self {
415 Box(unsafe { Unique::new_unchecked(raw) })
418 /// Consumes the `Box`, returning a wrapped raw pointer.
420 /// The pointer will be properly aligned and non-null.
422 /// After calling this function, the caller is responsible for the
423 /// memory previously managed by the `Box`. In particular, the
424 /// caller should properly destroy `T` and release the memory, taking
425 /// into account the [memory layout] used by `Box`. The easiest way to
426 /// do this is to convert the raw pointer back into a `Box` with the
427 /// [`Box::from_raw`] function, allowing the `Box` destructor to perform
430 /// Note: this is an associated function, which means that you have
431 /// to call it as `Box::into_raw(b)` instead of `b.into_raw()`. This
432 /// is so that there is no conflict with a method on the inner type.
435 /// Converting the raw pointer back into a `Box` with [`Box::from_raw`]
436 /// for automatic cleanup:
438 /// let x = Box::new(String::from("Hello"));
439 /// let ptr = Box::into_raw(x);
440 /// let x = unsafe { Box::from_raw(ptr) };
442 /// Manual cleanup by explicitly running the destructor and deallocating
445 /// use std::alloc::{dealloc, Layout};
448 /// let x = Box::new(String::from("Hello"));
449 /// let p = Box::into_raw(x);
451 /// ptr::drop_in_place(p);
452 /// dealloc(p as *mut u8, Layout::new::<String>());
456 /// [memory layout]: self#memory-layout
457 #[stable(feature = "box_raw", since = "1.4.0")]
459 pub fn into_raw(b: Box<T>) -> *mut T {
460 // Box is recognized as a "unique pointer" by Stacked Borrows, but internally it is a
461 // raw pointer for the type system. Turning it directly into a raw pointer would not be
462 // recognized as "releasing" the unique pointer to permit aliased raw accesses,
463 // so all raw pointer methods go through `leak` which creates a (unique)
464 // mutable reference. Turning *that* to a raw pointer behaves correctly.
465 Box::leak(b) as *mut T
469 feature = "ptr_internals",
471 reason = "use `Box::leak(b).into()` or `Unique::from(Box::leak(b))` instead"
475 pub fn into_unique(b: Box<T>) -> Unique<T> {
476 // Box is recognized as a "unique pointer" by Stacked Borrows, but internally it is a
477 // raw pointer for the type system. Turning it directly into a raw pointer would not be
478 // recognized as "releasing" the unique pointer to permit aliased raw accesses,
479 // so all raw pointer methods go through `leak` which creates a (unique)
480 // mutable reference. Turning *that* to a raw pointer behaves correctly.
484 /// Consumes and leaks the `Box`, returning a mutable reference,
485 /// `&'a mut T`. Note that the type `T` must outlive the chosen lifetime
486 /// `'a`. If the type has only static references, or none at all, then this
487 /// may be chosen to be `'static`.
489 /// This function is mainly useful for data that lives for the remainder of
490 /// the program's life. Dropping the returned reference will cause a memory
491 /// leak. If this is not acceptable, the reference should first be wrapped
492 /// with the [`Box::from_raw`] function producing a `Box`. This `Box` can
493 /// then be dropped which will properly destroy `T` and release the
494 /// allocated memory.
496 /// Note: this is an associated function, which means that you have
497 /// to call it as `Box::leak(b)` instead of `b.leak()`. This
498 /// is so that there is no conflict with a method on the inner type.
505 /// let x = Box::new(41);
506 /// let static_ref: &'static mut usize = Box::leak(x);
507 /// *static_ref += 1;
508 /// assert_eq!(*static_ref, 42);
514 /// let x = vec![1, 2, 3].into_boxed_slice();
515 /// let static_ref = Box::leak(x);
516 /// static_ref[0] = 4;
517 /// assert_eq!(*static_ref, [4, 2, 3]);
519 #[stable(feature = "box_leak", since = "1.26.0")]
521 pub fn leak<'a>(b: Box<T>) -> &'a mut T
523 T: 'a, // Technically not needed, but kept to be explicit.
525 unsafe { &mut *mem::ManuallyDrop::new(b).0.as_ptr() }
528 /// Converts a `Box<T>` into a `Pin<Box<T>>`
530 /// This conversion does not allocate on the heap and happens in place.
532 /// This is also available via [`From`].
533 #[unstable(feature = "box_into_pin", issue = "62370")]
534 pub fn into_pin(boxed: Box<T>) -> Pin<Box<T>> {
535 // It's not possible to move or replace the insides of a `Pin<Box<T>>`
536 // when `T: !Unpin`, so it's safe to pin it directly without any
537 // additional requirements.
538 unsafe { Pin::new_unchecked(boxed) }
542 #[stable(feature = "rust1", since = "1.0.0")]
543 unsafe impl<#[may_dangle] T: ?Sized> Drop for Box<T> {
545 // FIXME: Do nothing, drop is currently performed by compiler.
549 #[stable(feature = "rust1", since = "1.0.0")]
550 impl<T: Default> Default for Box<T> {
551 /// Creates a `Box<T>`, with the `Default` value for T.
552 fn default() -> Box<T> {
553 box Default::default()
557 #[stable(feature = "rust1", since = "1.0.0")]
558 impl<T> Default for Box<[T]> {
559 fn default() -> Box<[T]> {
560 Box::<[T; 0]>::new([])
564 #[stable(feature = "default_box_extra", since = "1.17.0")]
565 impl Default for Box<str> {
566 fn default() -> Box<str> {
567 unsafe { from_boxed_utf8_unchecked(Default::default()) }
571 #[stable(feature = "rust1", since = "1.0.0")]
572 impl<T: Clone> Clone for Box<T> {
573 /// Returns a new box with a `clone()` of this box's contents.
578 /// let x = Box::new(5);
579 /// let y = x.clone();
581 /// // The value is the same
582 /// assert_eq!(x, y);
584 /// // But they are unique objects
585 /// assert_ne!(&*x as *const i32, &*y as *const i32);
589 fn clone(&self) -> Box<T> {
590 box { (**self).clone() }
593 /// Copies `source`'s contents into `self` without creating a new allocation.
598 /// let x = Box::new(5);
599 /// let mut y = Box::new(10);
600 /// let yp: *const i32 = &*y;
602 /// y.clone_from(&x);
604 /// // The value is the same
605 /// assert_eq!(x, y);
607 /// // And no allocation occurred
608 /// assert_eq!(yp, &*y);
611 fn clone_from(&mut self, source: &Box<T>) {
612 (**self).clone_from(&(**source));
616 #[stable(feature = "box_slice_clone", since = "1.3.0")]
617 impl Clone for Box<str> {
618 fn clone(&self) -> Self {
619 // this makes a copy of the data
620 let buf: Box<[u8]> = self.as_bytes().into();
621 unsafe { from_boxed_utf8_unchecked(buf) }
625 #[stable(feature = "rust1", since = "1.0.0")]
626 impl<T: ?Sized + PartialEq> PartialEq for Box<T> {
628 fn eq(&self, other: &Box<T>) -> bool {
629 PartialEq::eq(&**self, &**other)
632 fn ne(&self, other: &Box<T>) -> bool {
633 PartialEq::ne(&**self, &**other)
636 #[stable(feature = "rust1", since = "1.0.0")]
637 impl<T: ?Sized + PartialOrd> PartialOrd for Box<T> {
639 fn partial_cmp(&self, other: &Box<T>) -> Option<Ordering> {
640 PartialOrd::partial_cmp(&**self, &**other)
643 fn lt(&self, other: &Box<T>) -> bool {
644 PartialOrd::lt(&**self, &**other)
647 fn le(&self, other: &Box<T>) -> bool {
648 PartialOrd::le(&**self, &**other)
651 fn ge(&self, other: &Box<T>) -> bool {
652 PartialOrd::ge(&**self, &**other)
655 fn gt(&self, other: &Box<T>) -> bool {
656 PartialOrd::gt(&**self, &**other)
659 #[stable(feature = "rust1", since = "1.0.0")]
660 impl<T: ?Sized + Ord> Ord for Box<T> {
662 fn cmp(&self, other: &Box<T>) -> Ordering {
663 Ord::cmp(&**self, &**other)
666 #[stable(feature = "rust1", since = "1.0.0")]
667 impl<T: ?Sized + Eq> Eq for Box<T> {}
669 #[stable(feature = "rust1", since = "1.0.0")]
670 impl<T: ?Sized + Hash> Hash for Box<T> {
671 fn hash<H: Hasher>(&self, state: &mut H) {
672 (**self).hash(state);
676 #[stable(feature = "indirect_hasher_impl", since = "1.22.0")]
677 impl<T: ?Sized + Hasher> Hasher for Box<T> {
678 fn finish(&self) -> u64 {
681 fn write(&mut self, bytes: &[u8]) {
682 (**self).write(bytes)
684 fn write_u8(&mut self, i: u8) {
687 fn write_u16(&mut self, i: u16) {
688 (**self).write_u16(i)
690 fn write_u32(&mut self, i: u32) {
691 (**self).write_u32(i)
693 fn write_u64(&mut self, i: u64) {
694 (**self).write_u64(i)
696 fn write_u128(&mut self, i: u128) {
697 (**self).write_u128(i)
699 fn write_usize(&mut self, i: usize) {
700 (**self).write_usize(i)
702 fn write_i8(&mut self, i: i8) {
705 fn write_i16(&mut self, i: i16) {
706 (**self).write_i16(i)
708 fn write_i32(&mut self, i: i32) {
709 (**self).write_i32(i)
711 fn write_i64(&mut self, i: i64) {
712 (**self).write_i64(i)
714 fn write_i128(&mut self, i: i128) {
715 (**self).write_i128(i)
717 fn write_isize(&mut self, i: isize) {
718 (**self).write_isize(i)
722 #[stable(feature = "from_for_ptrs", since = "1.6.0")]
723 impl<T> From<T> for Box<T> {
724 /// Converts a generic type `T` into a `Box<T>`
726 /// The conversion allocates on the heap and moves `t`
727 /// from the stack into it.
732 /// let boxed = Box::new(5);
734 /// assert_eq!(Box::from(x), boxed);
736 fn from(t: T) -> Self {
741 #[stable(feature = "pin", since = "1.33.0")]
742 impl<T: ?Sized> From<Box<T>> for Pin<Box<T>> {
743 /// Converts a `Box<T>` into a `Pin<Box<T>>`
745 /// This conversion does not allocate on the heap and happens in place.
746 fn from(boxed: Box<T>) -> Self {
751 #[stable(feature = "box_from_slice", since = "1.17.0")]
752 impl<T: Copy> From<&[T]> for Box<[T]> {
753 /// Converts a `&[T]` into a `Box<[T]>`
755 /// This conversion allocates on the heap
756 /// and performs a copy of `slice`.
760 /// // create a &[u8] which will be used to create a Box<[u8]>
761 /// let slice: &[u8] = &[104, 101, 108, 108, 111];
762 /// let boxed_slice: Box<[u8]> = Box::from(slice);
764 /// println!("{:?}", boxed_slice);
766 fn from(slice: &[T]) -> Box<[T]> {
767 let len = slice.len();
768 let buf = RawVec::with_capacity(len);
770 ptr::copy_nonoverlapping(slice.as_ptr(), buf.ptr(), len);
771 buf.into_box(slice.len()).assume_init()
776 #[stable(feature = "box_from_cow", since = "1.45.0")]
777 impl<T: Copy> From<Cow<'_, [T]>> for Box<[T]> {
779 fn from(cow: Cow<'_, [T]>) -> Box<[T]> {
781 Cow::Borrowed(slice) => Box::from(slice),
782 Cow::Owned(slice) => Box::from(slice),
787 #[stable(feature = "box_from_slice", since = "1.17.0")]
788 impl From<&str> for Box<str> {
789 /// Converts a `&str` into a `Box<str>`
791 /// This conversion allocates on the heap
792 /// and performs a copy of `s`.
796 /// let boxed: Box<str> = Box::from("hello");
797 /// println!("{}", boxed);
800 fn from(s: &str) -> Box<str> {
801 unsafe { from_boxed_utf8_unchecked(Box::from(s.as_bytes())) }
805 #[stable(feature = "box_from_cow", since = "1.45.0")]
806 impl From<Cow<'_, str>> for Box<str> {
808 fn from(cow: Cow<'_, str>) -> Box<str> {
810 Cow::Borrowed(s) => Box::from(s),
811 Cow::Owned(s) => Box::from(s),
816 #[stable(feature = "boxed_str_conv", since = "1.19.0")]
817 impl From<Box<str>> for Box<[u8]> {
818 /// Converts a `Box<str>>` into a `Box<[u8]>`
820 /// This conversion does not allocate on the heap and happens in place.
824 /// // create a Box<str> which will be used to create a Box<[u8]>
825 /// let boxed: Box<str> = Box::from("hello");
826 /// let boxed_str: Box<[u8]> = Box::from(boxed);
828 /// // create a &[u8] which will be used to create a Box<[u8]>
829 /// let slice: &[u8] = &[104, 101, 108, 108, 111];
830 /// let boxed_slice = Box::from(slice);
832 /// assert_eq!(boxed_slice, boxed_str);
835 fn from(s: Box<str>) -> Self {
836 unsafe { Box::from_raw(Box::into_raw(s) as *mut [u8]) }
840 #[stable(feature = "box_from_array", since = "1.45.0")]
841 impl<T, const N: usize> From<[T; N]> for Box<[T]> {
842 /// Converts a `[T; N]` into a `Box<[T]>`
844 /// This conversion moves the array to newly heap-allocated memory.
848 /// let boxed: Box<[u8]> = Box::from([4, 2]);
849 /// println!("{:?}", boxed);
851 fn from(array: [T; N]) -> Box<[T]> {
856 #[stable(feature = "boxed_slice_try_from", since = "1.43.0")]
857 impl<T, const N: usize> TryFrom<Box<[T]>> for Box<[T; N]> {
858 type Error = Box<[T]>;
860 fn try_from(boxed_slice: Box<[T]>) -> Result<Self, Self::Error> {
861 if boxed_slice.len() == N {
862 Ok(unsafe { Box::from_raw(Box::into_raw(boxed_slice) as *mut [T; N]) })
871 #[stable(feature = "rust1", since = "1.0.0")]
872 /// Attempt to downcast the box to a concrete type.
877 /// use std::any::Any;
879 /// fn print_if_string(value: Box<dyn Any>) {
880 /// if let Ok(string) = value.downcast::<String>() {
881 /// println!("String ({}): {}", string.len(), string);
885 /// let my_string = "Hello World".to_string();
886 /// print_if_string(Box::new(my_string));
887 /// print_if_string(Box::new(0i8));
889 pub fn downcast<T: Any>(self) -> Result<Box<T>, Box<dyn Any>> {
892 let raw: *mut dyn Any = Box::into_raw(self);
893 Ok(Box::from_raw(raw as *mut T))
901 impl Box<dyn Any + Send> {
903 #[stable(feature = "rust1", since = "1.0.0")]
904 /// Attempt to downcast the box to a concrete type.
909 /// use std::any::Any;
911 /// fn print_if_string(value: Box<dyn Any + Send>) {
912 /// if let Ok(string) = value.downcast::<String>() {
913 /// println!("String ({}): {}", string.len(), string);
917 /// let my_string = "Hello World".to_string();
918 /// print_if_string(Box::new(my_string));
919 /// print_if_string(Box::new(0i8));
921 pub fn downcast<T: Any>(self) -> Result<Box<T>, Box<dyn Any + Send>> {
922 <Box<dyn Any>>::downcast(self).map_err(|s| unsafe {
923 // reapply the Send marker
924 Box::from_raw(Box::into_raw(s) as *mut (dyn Any + Send))
929 #[stable(feature = "rust1", since = "1.0.0")]
930 impl<T: fmt::Display + ?Sized> fmt::Display for Box<T> {
931 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
932 fmt::Display::fmt(&**self, f)
936 #[stable(feature = "rust1", since = "1.0.0")]
937 impl<T: fmt::Debug + ?Sized> fmt::Debug for Box<T> {
938 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
939 fmt::Debug::fmt(&**self, f)
943 #[stable(feature = "rust1", since = "1.0.0")]
944 impl<T: ?Sized> fmt::Pointer for Box<T> {
945 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
946 // It's not possible to extract the inner Uniq directly from the Box,
947 // instead we cast it to a *const which aliases the Unique
948 let ptr: *const T = &**self;
949 fmt::Pointer::fmt(&ptr, f)
953 #[stable(feature = "rust1", since = "1.0.0")]
954 impl<T: ?Sized> Deref for Box<T> {
957 fn deref(&self) -> &T {
962 #[stable(feature = "rust1", since = "1.0.0")]
963 impl<T: ?Sized> DerefMut for Box<T> {
964 fn deref_mut(&mut self) -> &mut T {
969 #[unstable(feature = "receiver_trait", issue = "none")]
970 impl<T: ?Sized> Receiver for Box<T> {}
972 #[stable(feature = "rust1", since = "1.0.0")]
973 impl<I: Iterator + ?Sized> Iterator for Box<I> {
975 fn next(&mut self) -> Option<I::Item> {
978 fn size_hint(&self) -> (usize, Option<usize>) {
981 fn nth(&mut self, n: usize) -> Option<I::Item> {
984 fn last(self) -> Option<I::Item> {
991 fn last(self) -> Option<Self::Item>;
994 impl<I: Iterator + ?Sized> BoxIter for Box<I> {
996 default fn last(self) -> Option<I::Item> {
998 fn some<T>(_: Option<T>, x: T) -> Option<T> {
1002 self.fold(None, some)
1006 /// Specialization for sized `I`s that uses `I`s implementation of `last()`
1007 /// instead of the default.
1008 #[stable(feature = "rust1", since = "1.0.0")]
1009 impl<I: Iterator> BoxIter for Box<I> {
1010 fn last(self) -> Option<I::Item> {
1015 #[stable(feature = "rust1", since = "1.0.0")]
1016 impl<I: DoubleEndedIterator + ?Sized> DoubleEndedIterator for Box<I> {
1017 fn next_back(&mut self) -> Option<I::Item> {
1018 (**self).next_back()
1020 fn nth_back(&mut self, n: usize) -> Option<I::Item> {
1021 (**self).nth_back(n)
1024 #[stable(feature = "rust1", since = "1.0.0")]
1025 impl<I: ExactSizeIterator + ?Sized> ExactSizeIterator for Box<I> {
1026 fn len(&self) -> usize {
1029 fn is_empty(&self) -> bool {
1034 #[stable(feature = "fused", since = "1.26.0")]
1035 impl<I: FusedIterator + ?Sized> FusedIterator for Box<I> {}
1037 #[stable(feature = "boxed_closure_impls", since = "1.35.0")]
1038 impl<A, F: FnOnce<A> + ?Sized> FnOnce<A> for Box<F> {
1039 type Output = <F as FnOnce<A>>::Output;
1041 extern "rust-call" fn call_once(self, args: A) -> Self::Output {
1042 <F as FnOnce<A>>::call_once(*self, args)
1046 #[stable(feature = "boxed_closure_impls", since = "1.35.0")]
1047 impl<A, F: FnMut<A> + ?Sized> FnMut<A> for Box<F> {
1048 extern "rust-call" fn call_mut(&mut self, args: A) -> Self::Output {
1049 <F as FnMut<A>>::call_mut(self, args)
1053 #[stable(feature = "boxed_closure_impls", since = "1.35.0")]
1054 impl<A, F: Fn<A> + ?Sized> Fn<A> for Box<F> {
1055 extern "rust-call" fn call(&self, args: A) -> Self::Output {
1056 <F as Fn<A>>::call(self, args)
1060 #[unstable(feature = "coerce_unsized", issue = "27732")]
1061 impl<T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<Box<U>> for Box<T> {}
1063 #[unstable(feature = "dispatch_from_dyn", issue = "none")]
1064 impl<T: ?Sized + Unsize<U>, U: ?Sized> DispatchFromDyn<Box<U>> for Box<T> {}
1066 #[stable(feature = "boxed_slice_from_iter", since = "1.32.0")]
1067 impl<A> FromIterator<A> for Box<[A]> {
1068 fn from_iter<T: IntoIterator<Item = A>>(iter: T) -> Self {
1069 iter.into_iter().collect::<Vec<_>>().into_boxed_slice()
1073 #[stable(feature = "box_slice_clone", since = "1.3.0")]
1074 impl<T: Clone> Clone for Box<[T]> {
1075 fn clone(&self) -> Self {
1076 self.to_vec().into_boxed_slice()
1079 fn clone_from(&mut self, other: &Self) {
1080 if self.len() == other.len() {
1081 self.clone_from_slice(&other);
1083 *self = other.clone();
1088 #[stable(feature = "box_borrow", since = "1.1.0")]
1089 impl<T: ?Sized> borrow::Borrow<T> for Box<T> {
1090 fn borrow(&self) -> &T {
1095 #[stable(feature = "box_borrow", since = "1.1.0")]
1096 impl<T: ?Sized> borrow::BorrowMut<T> for Box<T> {
1097 fn borrow_mut(&mut self) -> &mut T {
1102 #[stable(since = "1.5.0", feature = "smart_ptr_as_ref")]
1103 impl<T: ?Sized> AsRef<T> for Box<T> {
1104 fn as_ref(&self) -> &T {
1109 #[stable(since = "1.5.0", feature = "smart_ptr_as_ref")]
1110 impl<T: ?Sized> AsMut<T> for Box<T> {
1111 fn as_mut(&mut self) -> &mut T {
1118 * We could have chosen not to add this impl, and instead have written a
1119 * function of Pin<Box<T>> to Pin<T>. Such a function would not be sound,
1120 * because Box<T> implements Unpin even when T does not, as a result of
1123 * We chose this API instead of the alternative for a few reasons:
1124 * - Logically, it is helpful to understand pinning in regard to the
1125 * memory region being pointed to. For this reason none of the
1126 * standard library pointer types support projecting through a pin
1127 * (Box<T> is the only pointer type in std for which this would be
1129 * - It is in practice very useful to have Box<T> be unconditionally
1130 * Unpin because of trait objects, for which the structural auto
1131 * trait functionality does not apply (e.g., Box<dyn Foo> would
1132 * otherwise not be Unpin).
1134 * Another type with the same semantics as Box but only a conditional
1135 * implementation of `Unpin` (where `T: Unpin`) would be valid/safe, and
1136 * could have a method to project a Pin<T> from it.
1138 #[stable(feature = "pin", since = "1.33.0")]
1139 impl<T: ?Sized> Unpin for Box<T> {}
1141 #[unstable(feature = "generator_trait", issue = "43122")]
1142 impl<G: ?Sized + Generator<R> + Unpin, R> Generator<R> for 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(Pin::new(&mut *self), arg)
1151 #[unstable(feature = "generator_trait", issue = "43122")]
1152 impl<G: ?Sized + Generator<R>, R> Generator<R> for Pin<Box<G>> {
1153 type Yield = G::Yield;
1154 type Return = G::Return;
1156 fn resume(mut self: Pin<&mut Self>, arg: R) -> GeneratorState<Self::Yield, Self::Return> {
1157 G::resume((*self).as_mut(), arg)
1161 #[stable(feature = "futures_api", since = "1.36.0")]
1162 impl<F: ?Sized + Future + Unpin> Future for Box<F> {
1163 type Output = F::Output;
1165 fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
1166 F::poll(Pin::new(&mut *self), cx)