1 // Copyright 2012-2015 The Rust Project Developers. See the COPYRIGHT
2 // file at the top-level directory of this distribution and at
3 // http://rust-lang.org/COPYRIGHT.
5 // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
6 // http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
7 // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
8 // option. This file may not be copied, modified, or distributed
9 // except according to those terms.
11 //! A pointer type for heap allocation.
13 //! `Box<T>`, casually referred to as a 'box', provides the simplest form of
14 //! heap allocation in Rust. Boxes provide ownership for this allocation, and
15 //! drop their contents when they go out of scope.
22 //! let x = Box::new(5);
25 //! Creating a recursive data structure:
30 //! Cons(T, Box<List<T>>),
35 //! let list: List<i32> = List::Cons(1, Box::new(List::Cons(2, Box::new(List::Nil))));
36 //! println!("{:?}", list);
40 //! This will print `Cons(1, Cons(2, Nil))`.
42 //! Recursive structures must be boxed, because if the definition of `Cons`
45 //! ```compile_fail,E0072
51 //! It wouldn't work. This is because the size of a `List` depends on how many
52 //! elements are in the list, and so we don't know how much memory to allocate
53 //! for a `Cons`. By introducing a `Box`, which has a defined size, we know how
54 //! big `Cons` needs to be.
56 #![stable(feature = "rust1", since = "1.0.0")]
58 use heap::{Heap, Layout, Alloc};
63 use core::cmp::Ordering;
65 use core::hash::{self, Hash, Hasher};
66 use core::iter::FusedIterator;
67 use core::marker::{self, Unsize};
69 use core::ops::{CoerceUnsized, Deref, DerefMut, Generator, GeneratorState};
70 use core::ops::{BoxPlace, Boxed, InPlace, Place, Placer};
71 use core::ptr::{self, Unique};
72 use core::convert::From;
73 use str::from_boxed_utf8_unchecked;
75 /// A value that represents the heap. This is the default place that the `box`
76 /// keyword allocates into when no place is supplied.
78 /// The following two examples are equivalent:
81 /// #![feature(box_heap)]
83 /// #![feature(box_syntax, placement_in_syntax)]
84 /// use std::boxed::HEAP;
87 /// let foo: Box<i32> = in HEAP { 5 };
91 #[unstable(feature = "box_heap",
92 reason = "may be renamed; uncertain about custom allocator design",
94 pub const HEAP: ExchangeHeapSingleton = ExchangeHeapSingleton { _force_singleton: () };
96 /// This the singleton type used solely for `boxed::HEAP`.
97 #[unstable(feature = "box_heap",
98 reason = "may be renamed; uncertain about custom allocator design",
100 #[allow(missing_debug_implementations)]
101 #[derive(Copy, Clone)]
102 pub struct ExchangeHeapSingleton {
103 _force_singleton: (),
106 /// A pointer type for heap allocation.
108 /// See the [module-level documentation](../../std/boxed/index.html) for more.
109 #[lang = "owned_box"]
111 #[stable(feature = "rust1", since = "1.0.0")]
112 pub struct Box<T: ?Sized>(Unique<T>);
114 /// `IntermediateBox` represents uninitialized backing storage for `Box`.
116 /// FIXME (pnkfelix): Ideally we would just reuse `Box<T>` instead of
117 /// introducing a separate `IntermediateBox<T>`; but then you hit
118 /// issues when you e.g. attempt to destructure an instance of `Box`,
119 /// since it is a lang item and so it gets special handling by the
120 /// compiler. Easier just to make this parallel type for now.
122 /// FIXME (pnkfelix): Currently the `box` protocol only supports
123 /// creating instances of sized types. This IntermediateBox is
124 /// designed to be forward-compatible with a future protocol that
125 /// supports creating instances of unsized types; that is why the type
126 /// parameter has the `?Sized` generalization marker, and is also why
127 /// this carries an explicit size. However, it probably does not need
128 /// to carry the explicit alignment; that is just a work-around for
129 /// the fact that the `align_of` intrinsic currently requires the
130 /// input type to be Sized (which I do not think is strictly
132 #[unstable(feature = "placement_in",
133 reason = "placement box design is still being worked out.",
135 #[allow(missing_debug_implementations)]
136 pub struct IntermediateBox<T: ?Sized> {
139 marker: marker::PhantomData<*mut T>,
142 #[unstable(feature = "placement_in",
143 reason = "placement box design is still being worked out.",
145 impl<T> Place<T> for IntermediateBox<T> {
146 fn pointer(&mut self) -> *mut T {
151 unsafe fn finalize<T>(b: IntermediateBox<T>) -> Box<T> {
152 let p = b.ptr as *mut T;
157 fn make_place<T>() -> IntermediateBox<T> {
158 let layout = Layout::new::<T>();
160 let p = if layout.size() == 0 {
161 mem::align_of::<T>() as *mut u8
164 Heap.alloc(layout.clone()).unwrap_or_else(|err| {
173 marker: marker::PhantomData,
177 #[unstable(feature = "placement_in",
178 reason = "placement box design is still being worked out.",
180 impl<T> BoxPlace<T> for IntermediateBox<T> {
181 fn make_place() -> IntermediateBox<T> {
186 #[unstable(feature = "placement_in",
187 reason = "placement box design is still being worked out.",
189 impl<T> InPlace<T> for IntermediateBox<T> {
191 unsafe fn finalize(self) -> Box<T> {
196 #[unstable(feature = "placement_new_protocol", issue = "27779")]
197 impl<T> Boxed for Box<T> {
199 type Place = IntermediateBox<T>;
200 unsafe fn finalize(b: IntermediateBox<T>) -> Box<T> {
205 #[unstable(feature = "placement_in",
206 reason = "placement box design is still being worked out.",
208 impl<T> Placer<T> for ExchangeHeapSingleton {
209 type Place = IntermediateBox<T>;
211 fn make_place(self) -> IntermediateBox<T> {
216 #[unstable(feature = "placement_in",
217 reason = "placement box design is still being worked out.",
219 impl<T: ?Sized> Drop for IntermediateBox<T> {
221 if self.layout.size() > 0 {
223 Heap.dealloc(self.ptr, self.layout.clone())
230 /// Allocates memory on the heap and then places `x` into it.
232 /// This doesn't actually allocate if `T` is zero-sized.
237 /// let five = Box::new(5);
239 #[stable(feature = "rust1", since = "1.0.0")]
241 pub fn new(x: T) -> Box<T> {
246 impl<T: ?Sized> Box<T> {
247 /// Constructs a box from a raw pointer.
249 /// After calling this function, the raw pointer is owned by the
250 /// resulting `Box`. Specifically, the `Box` destructor will call
251 /// the destructor of `T` and free the allocated memory. Since the
252 /// way `Box` allocates and releases memory is unspecified, the
253 /// only valid pointer to pass to this function is the one taken
254 /// from another `Box` via the [`Box::into_raw`] function.
256 /// This function is unsafe because improper use may lead to
257 /// memory problems. For example, a double-free may occur if the
258 /// function is called twice on the same raw pointer.
260 /// [`Box::into_raw`]: struct.Box.html#method.into_raw
265 /// let x = Box::new(5);
266 /// let ptr = Box::into_raw(x);
267 /// let x = unsafe { Box::from_raw(ptr) };
269 #[stable(feature = "box_raw", since = "1.4.0")]
271 pub unsafe fn from_raw(raw: *mut T) -> Self {
275 /// Consumes the `Box`, returning the wrapped raw pointer.
277 /// After calling this function, the caller is responsible for the
278 /// memory previously managed by the `Box`. In particular, the
279 /// caller should properly destroy `T` and release the memory. The
280 /// proper way to do so is to convert the raw pointer back into a
281 /// `Box` with the [`Box::from_raw`] function.
283 /// Note: this is an associated function, which means that you have
284 /// to call it as `Box::into_raw(b)` instead of `b.into_raw()`. This
285 /// is so that there is no conflict with a method on the inner type.
287 /// [`Box::from_raw`]: struct.Box.html#method.from_raw
292 /// let x = Box::new(5);
293 /// let ptr = Box::into_raw(x);
295 #[stable(feature = "box_raw", since = "1.4.0")]
297 pub fn into_raw(b: Box<T>) -> *mut T {
298 unsafe { mem::transmute(b) }
301 /// Consumes the `Box`, returning the wrapped pointer as `Unique<T>`.
303 /// After calling this function, the caller is responsible for the
304 /// memory previously managed by the `Box`. In particular, the
305 /// caller should properly destroy `T` and release the memory. The
306 /// proper way to do so is to convert the raw pointer back into a
307 /// `Box` with the [`Box::from_raw`] function.
309 /// Note: this is an associated function, which means that you have
310 /// to call it as `Box::into_unique(b)` instead of `b.into_unique()`. This
311 /// is so that there is no conflict with a method on the inner type.
313 /// [`Box::from_raw`]: struct.Box.html#method.from_raw
318 /// #![feature(unique)]
321 /// let x = Box::new(5);
322 /// let ptr = Box::into_unique(x);
325 #[unstable(feature = "unique", reason = "needs an RFC to flesh out design",
328 pub fn into_unique(b: Box<T>) -> Unique<T> {
329 unsafe { mem::transmute(b) }
333 #[stable(feature = "rust1", since = "1.0.0")]
334 unsafe impl<#[may_dangle] T: ?Sized> Drop for Box<T> {
336 // FIXME: Do nothing, drop is currently performed by compiler.
340 #[stable(feature = "rust1", since = "1.0.0")]
341 impl<T: Default> Default for Box<T> {
342 /// Creates a `Box<T>`, with the `Default` value for T.
343 fn default() -> Box<T> {
344 box Default::default()
348 #[stable(feature = "rust1", since = "1.0.0")]
349 impl<T> Default for Box<[T]> {
350 fn default() -> Box<[T]> {
351 Box::<[T; 0]>::new([])
355 #[stable(feature = "default_box_extra", since = "1.17.0")]
356 impl Default for Box<str> {
357 fn default() -> Box<str> {
358 unsafe { from_boxed_utf8_unchecked(Default::default()) }
362 #[stable(feature = "rust1", since = "1.0.0")]
363 impl<T: Clone> Clone for Box<T> {
364 /// Returns a new box with a `clone()` of this box's contents.
369 /// let x = Box::new(5);
370 /// let y = x.clone();
374 fn clone(&self) -> Box<T> {
375 box { (**self).clone() }
377 /// Copies `source`'s contents into `self` without creating a new allocation.
382 /// let x = Box::new(5);
383 /// let mut y = Box::new(10);
385 /// y.clone_from(&x);
387 /// assert_eq!(*y, 5);
390 fn clone_from(&mut self, source: &Box<T>) {
391 (**self).clone_from(&(**source));
396 #[stable(feature = "box_slice_clone", since = "1.3.0")]
397 impl Clone for Box<str> {
398 fn clone(&self) -> Self {
399 let len = self.len();
400 let buf = RawVec::with_capacity(len);
402 ptr::copy_nonoverlapping(self.as_ptr(), buf.ptr(), len);
403 from_boxed_utf8_unchecked(buf.into_box())
408 #[stable(feature = "rust1", since = "1.0.0")]
409 impl<T: ?Sized + PartialEq> PartialEq for Box<T> {
411 fn eq(&self, other: &Box<T>) -> bool {
412 PartialEq::eq(&**self, &**other)
415 fn ne(&self, other: &Box<T>) -> bool {
416 PartialEq::ne(&**self, &**other)
419 #[stable(feature = "rust1", since = "1.0.0")]
420 impl<T: ?Sized + PartialOrd> PartialOrd for Box<T> {
422 fn partial_cmp(&self, other: &Box<T>) -> Option<Ordering> {
423 PartialOrd::partial_cmp(&**self, &**other)
426 fn lt(&self, other: &Box<T>) -> bool {
427 PartialOrd::lt(&**self, &**other)
430 fn le(&self, other: &Box<T>) -> bool {
431 PartialOrd::le(&**self, &**other)
434 fn ge(&self, other: &Box<T>) -> bool {
435 PartialOrd::ge(&**self, &**other)
438 fn gt(&self, other: &Box<T>) -> bool {
439 PartialOrd::gt(&**self, &**other)
442 #[stable(feature = "rust1", since = "1.0.0")]
443 impl<T: ?Sized + Ord> Ord for Box<T> {
445 fn cmp(&self, other: &Box<T>) -> Ordering {
446 Ord::cmp(&**self, &**other)
449 #[stable(feature = "rust1", since = "1.0.0")]
450 impl<T: ?Sized + Eq> Eq for Box<T> {}
452 #[stable(feature = "rust1", since = "1.0.0")]
453 impl<T: ?Sized + Hash> Hash for Box<T> {
454 fn hash<H: hash::Hasher>(&self, state: &mut H) {
455 (**self).hash(state);
459 #[stable(feature = "indirect_hasher_impl", since = "1.22.0")]
460 impl<T: ?Sized + Hasher> Hasher for Box<T> {
461 fn finish(&self) -> u64 {
464 fn write(&mut self, bytes: &[u8]) {
465 (**self).write(bytes)
467 fn write_u8(&mut self, i: u8) {
470 fn write_u16(&mut self, i: u16) {
471 (**self).write_u16(i)
473 fn write_u32(&mut self, i: u32) {
474 (**self).write_u32(i)
476 fn write_u64(&mut self, i: u64) {
477 (**self).write_u64(i)
479 fn write_u128(&mut self, i: u128) {
480 (**self).write_u128(i)
482 fn write_usize(&mut self, i: usize) {
483 (**self).write_usize(i)
485 fn write_i8(&mut self, i: i8) {
488 fn write_i16(&mut self, i: i16) {
489 (**self).write_i16(i)
491 fn write_i32(&mut self, i: i32) {
492 (**self).write_i32(i)
494 fn write_i64(&mut self, i: i64) {
495 (**self).write_i64(i)
497 fn write_i128(&mut self, i: i128) {
498 (**self).write_i128(i)
500 fn write_isize(&mut self, i: isize) {
501 (**self).write_isize(i)
505 #[stable(feature = "from_for_ptrs", since = "1.6.0")]
506 impl<T> From<T> for Box<T> {
507 fn from(t: T) -> Self {
512 #[stable(feature = "box_from_slice", since = "1.17.0")]
513 impl<'a, T: Copy> From<&'a [T]> for Box<[T]> {
514 fn from(slice: &'a [T]) -> Box<[T]> {
515 let mut boxed = unsafe { RawVec::with_capacity(slice.len()).into_box() };
516 boxed.copy_from_slice(slice);
521 #[stable(feature = "box_from_slice", since = "1.17.0")]
522 impl<'a> From<&'a str> for Box<str> {
523 fn from(s: &'a str) -> Box<str> {
524 unsafe { from_boxed_utf8_unchecked(Box::from(s.as_bytes())) }
528 #[stable(feature = "boxed_str_conv", since = "1.19.0")]
529 impl From<Box<str>> for Box<[u8]> {
530 fn from(s: Box<str>) -> Self {
531 unsafe { Box::from_raw(Box::into_raw(s) as *mut [u8]) }
537 #[stable(feature = "rust1", since = "1.0.0")]
538 /// Attempt to downcast the box to a concrete type.
543 /// use std::any::Any;
545 /// fn print_if_string(value: Box<Any>) {
546 /// if let Ok(string) = value.downcast::<String>() {
547 /// println!("String ({}): {}", string.len(), string);
552 /// let my_string = "Hello World".to_string();
553 /// print_if_string(Box::new(my_string));
554 /// print_if_string(Box::new(0i8));
557 pub fn downcast<T: Any>(self) -> Result<Box<T>, Box<Any>> {
560 let raw: *mut Any = Box::into_raw(self);
561 Ok(Box::from_raw(raw as *mut T))
569 impl Box<Any + Send> {
571 #[stable(feature = "rust1", since = "1.0.0")]
572 /// Attempt to downcast the box to a concrete type.
577 /// use std::any::Any;
579 /// fn print_if_string(value: Box<Any + Send>) {
580 /// if let Ok(string) = value.downcast::<String>() {
581 /// println!("String ({}): {}", string.len(), string);
586 /// let my_string = "Hello World".to_string();
587 /// print_if_string(Box::new(my_string));
588 /// print_if_string(Box::new(0i8));
591 pub fn downcast<T: Any>(self) -> Result<Box<T>, Box<Any + Send>> {
592 <Box<Any>>::downcast(self).map_err(|s| unsafe {
593 // reapply the Send marker
594 mem::transmute::<Box<Any>, Box<Any + Send>>(s)
599 #[stable(feature = "rust1", since = "1.0.0")]
600 impl<T: fmt::Display + ?Sized> fmt::Display for Box<T> {
601 fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
602 fmt::Display::fmt(&**self, f)
606 #[stable(feature = "rust1", since = "1.0.0")]
607 impl<T: fmt::Debug + ?Sized> fmt::Debug for Box<T> {
608 fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
609 fmt::Debug::fmt(&**self, f)
613 #[stable(feature = "rust1", since = "1.0.0")]
614 impl<T: ?Sized> fmt::Pointer for Box<T> {
615 fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
616 // It's not possible to extract the inner Uniq directly from the Box,
617 // instead we cast it to a *const which aliases the Unique
618 let ptr: *const T = &**self;
619 fmt::Pointer::fmt(&ptr, f)
623 #[stable(feature = "rust1", since = "1.0.0")]
624 impl<T: ?Sized> Deref for Box<T> {
627 fn deref(&self) -> &T {
632 #[stable(feature = "rust1", since = "1.0.0")]
633 impl<T: ?Sized> DerefMut for Box<T> {
634 fn deref_mut(&mut self) -> &mut T {
639 #[stable(feature = "rust1", since = "1.0.0")]
640 impl<I: Iterator + ?Sized> Iterator for Box<I> {
642 fn next(&mut self) -> Option<I::Item> {
645 fn size_hint(&self) -> (usize, Option<usize>) {
648 fn nth(&mut self, n: usize) -> Option<I::Item> {
652 #[stable(feature = "rust1", since = "1.0.0")]
653 impl<I: DoubleEndedIterator + ?Sized> DoubleEndedIterator for Box<I> {
654 fn next_back(&mut self) -> Option<I::Item> {
658 #[stable(feature = "rust1", since = "1.0.0")]
659 impl<I: ExactSizeIterator + ?Sized> ExactSizeIterator for Box<I> {
660 fn len(&self) -> usize {
663 fn is_empty(&self) -> bool {
668 #[unstable(feature = "fused", issue = "35602")]
669 impl<I: FusedIterator + ?Sized> FusedIterator for Box<I> {}
672 /// `FnBox` is a version of the `FnOnce` intended for use with boxed
673 /// closure objects. The idea is that where one would normally store a
674 /// `Box<FnOnce()>` in a data structure, you should use
675 /// `Box<FnBox()>`. The two traits behave essentially the same, except
676 /// that a `FnBox` closure can only be called if it is boxed. (Note
677 /// that `FnBox` may be deprecated in the future if `Box<FnOnce()>`
678 /// closures become directly usable.)
682 /// Here is a snippet of code which creates a hashmap full of boxed
683 /// once closures and then removes them one by one, calling each
684 /// closure as it is removed. Note that the type of the closures
685 /// stored in the map is `Box<FnBox() -> i32>` and not `Box<FnOnce()
689 /// #![feature(fnbox)]
691 /// use std::boxed::FnBox;
692 /// use std::collections::HashMap;
694 /// fn make_map() -> HashMap<i32, Box<FnBox() -> i32>> {
695 /// let mut map: HashMap<i32, Box<FnBox() -> i32>> = HashMap::new();
696 /// map.insert(1, Box::new(|| 22));
697 /// map.insert(2, Box::new(|| 44));
702 /// let mut map = make_map();
703 /// for i in &[1, 2] {
704 /// let f = map.remove(&i).unwrap();
705 /// assert_eq!(f(), i * 22);
710 #[unstable(feature = "fnbox",
711 reason = "will be deprecated if and when `Box<FnOnce>` becomes usable", issue = "28796")]
715 fn call_box(self: Box<Self>, args: A) -> Self::Output;
718 #[unstable(feature = "fnbox",
719 reason = "will be deprecated if and when `Box<FnOnce>` becomes usable", issue = "28796")]
720 impl<A, F> FnBox<A> for F
723 type Output = F::Output;
725 fn call_box(self: Box<F>, args: A) -> F::Output {
730 #[unstable(feature = "fnbox",
731 reason = "will be deprecated if and when `Box<FnOnce>` becomes usable", issue = "28796")]
732 impl<'a, A, R> FnOnce<A> for Box<FnBox<A, Output = R> + 'a> {
735 extern "rust-call" fn call_once(self, args: A) -> R {
740 #[unstable(feature = "fnbox",
741 reason = "will be deprecated if and when `Box<FnOnce>` becomes usable", issue = "28796")]
742 impl<'a, A, R> FnOnce<A> for Box<FnBox<A, Output = R> + Send + 'a> {
745 extern "rust-call" fn call_once(self, args: A) -> R {
750 #[unstable(feature = "coerce_unsized", issue = "27732")]
751 impl<T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<Box<U>> for Box<T> {}
753 #[stable(feature = "box_slice_clone", since = "1.3.0")]
754 impl<T: Clone> Clone for Box<[T]> {
755 fn clone(&self) -> Self {
756 let mut new = BoxBuilder {
757 data: RawVec::with_capacity(self.len()),
761 let mut target = new.data.ptr();
763 for item in self.iter() {
765 ptr::write(target, item.clone());
766 target = target.offset(1);
772 return unsafe { new.into_box() };
774 // Helper type for responding to panics correctly.
775 struct BoxBuilder<T> {
780 impl<T> BoxBuilder<T> {
781 unsafe fn into_box(self) -> Box<[T]> {
782 let raw = ptr::read(&self.data);
788 impl<T> Drop for BoxBuilder<T> {
790 let mut data = self.data.ptr();
791 let max = unsafe { data.offset(self.len as isize) };
796 data = data.offset(1);
804 #[stable(feature = "box_borrow", since = "1.1.0")]
805 impl<T: ?Sized> borrow::Borrow<T> for Box<T> {
806 fn borrow(&self) -> &T {
811 #[stable(feature = "box_borrow", since = "1.1.0")]
812 impl<T: ?Sized> borrow::BorrowMut<T> for Box<T> {
813 fn borrow_mut(&mut self) -> &mut T {
818 #[stable(since = "1.5.0", feature = "smart_ptr_as_ref")]
819 impl<T: ?Sized> AsRef<T> for Box<T> {
820 fn as_ref(&self) -> &T {
825 #[stable(since = "1.5.0", feature = "smart_ptr_as_ref")]
826 impl<T: ?Sized> AsMut<T> for Box<T> {
827 fn as_mut(&mut self) -> &mut T {
832 #[unstable(feature = "generator_trait", issue = "43122")]
833 impl<T> Generator for Box<T>
834 where T: Generator + ?Sized
836 type Yield = T::Yield;
837 type Return = T::Return;
838 fn resume(&mut self) -> GeneratorState<Self::Yield, Self::Return> {