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`
49 //! It wouldn't work. This is because the size of a `List` depends on how many
50 //! elements are in the list, and so we don't know how much memory to allocate
51 //! for a `Cons`. By introducing a `Box`, which has a defined size, we know how
52 //! big `Cons` needs to be.
54 #![stable(feature = "rust1", since = "1.0.0")]
57 use core::prelude::v1::*;
63 use core::cmp::Ordering;
65 use core::hash::{self, Hash};
66 use core::marker::{self, Unsize};
68 use core::ops::{CoerceUnsized, Deref, DerefMut};
69 use core::ops::{Placer, Boxed, Place, InPlace, BoxPlace};
70 use core::ptr::{self, Unique};
71 use core::raw::{TraitObject};
73 /// A value that represents the heap. This is the default place that the `box`
74 /// keyword allocates into when no place is supplied.
76 /// The following two examples are equivalent:
79 /// #![feature(box_heap)]
81 /// #![feature(box_syntax, placement_in_syntax)]
82 /// use std::boxed::HEAP;
85 /// let foo = box(HEAP) 5;
89 #[lang = "exchange_heap"]
90 #[unstable(feature = "box_heap",
91 reason = "may be renamed; uncertain about custom allocator design")]
93 pub const HEAP: ExchangeHeapSingleton =
94 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")]
99 #[derive(Copy, Clone)]
100 pub struct ExchangeHeapSingleton { _force_singleton: () }
102 /// A pointer type for heap allocation.
104 /// See the [module-level documentation](../../std/boxed/index.html) for more.
105 #[lang = "owned_box"]
106 #[stable(feature = "rust1", since = "1.0.0")]
108 pub struct Box<T: ?Sized>(Unique<T>);
110 /// `IntermediateBox` represents uninitialized backing storage for `Box`.
112 /// FIXME (pnkfelix): Ideally we would just reuse `Box<T>` instead of
113 /// introducing a separate `IntermediateBox<T>`; but then you hit
114 /// issues when you e.g. attempt to destructure an instance of `Box`,
115 /// since it is a lang item and so it gets special handling by the
116 /// compiler. Easier just to make this parallel type for now.
118 /// FIXME (pnkfelix): Currently the `box` protocol only supports
119 /// creating instances of sized types. This IntermediateBox is
120 /// designed to be forward-compatible with a future protocol that
121 /// supports creating instances of unsized types; that is why the type
122 /// parameter has the `?Sized` generalization marker, and is also why
123 /// this carries an explicit size. However, it probably does not need
124 /// to carry the explicit alignment; that is just a work-around for
125 /// the fact that the `align_of` intrinsic currently requires the
126 /// input type to be Sized (which I do not think is strictly
128 #[unstable(feature = "placement_in", reason = "placement box design is still being worked out.")]
129 pub struct IntermediateBox<T: ?Sized>{
133 marker: marker::PhantomData<*mut T>,
136 impl<T> Place<T> for IntermediateBox<T> {
137 fn pointer(&mut self) -> *mut T {
138 unsafe { ::core::mem::transmute(self.ptr) }
142 unsafe fn finalize<T>(b: IntermediateBox<T>) -> Box<T> {
143 let p = b.ptr as *mut T;
148 fn make_place<T>() -> IntermediateBox<T> {
149 let size = mem::size_of::<T>();
150 let align = mem::align_of::<T>();
152 let p = if size == 0 {
153 heap::EMPTY as *mut u8
156 heap::allocate(size, align)
159 panic!("Box make_place allocation failure.");
164 IntermediateBox { ptr: p, size: size, align: align, marker: marker::PhantomData }
167 impl<T> BoxPlace<T> for IntermediateBox<T> {
168 fn make_place() -> IntermediateBox<T> { make_place() }
171 impl<T> InPlace<T> for IntermediateBox<T> {
173 unsafe fn finalize(self) -> Box<T> { finalize(self) }
176 impl<T> Boxed for Box<T> {
178 type Place = IntermediateBox<T>;
179 unsafe fn finalize(b: IntermediateBox<T>) -> Box<T> { finalize(b) }
182 impl<T> Placer<T> for ExchangeHeapSingleton {
183 type Place = IntermediateBox<T>;
185 fn make_place(self) -> IntermediateBox<T> {
190 impl<T: ?Sized> Drop for IntermediateBox<T> {
194 heap::deallocate(self.ptr, self.size, self.align)
201 /// Allocates memory on the heap and then moves `x` into it.
206 /// let x = Box::new(5);
208 #[stable(feature = "rust1", since = "1.0.0")]
210 pub fn new(x: T) -> Box<T> {
215 impl<T : ?Sized> Box<T> {
216 /// Constructs a box from the raw pointer.
218 /// After this function call, pointer is owned by resulting box.
219 /// In particular, it means that `Box` destructor calls destructor
220 /// of `T` and releases memory. Since the way `Box` allocates and
221 /// releases memory is unspecified, the only valid pointer to pass
222 /// to this function is the one taken from another `Box` with
223 /// `Box::into_raw` function.
225 /// Function is unsafe, because improper use of this function may
226 /// lead to memory problems like double-free, for example if the
227 /// function is called twice on the same raw pointer.
228 #[unstable(feature = "box_raw",
229 reason = "may be renamed or moved out of Box scope")]
231 // NB: may want to be called from_ptr, see comments on CStr::from_ptr
232 pub unsafe fn from_raw(raw: *mut T) -> Self {
236 /// Consumes the `Box`, returning the wrapped raw pointer.
238 /// After call to this function, caller is responsible for the memory
239 /// previously managed by `Box`, in particular caller should properly
240 /// destroy `T` and release memory. The proper way to do it is to
241 /// convert pointer back to `Box` with `Box::from_raw` function, because
242 /// `Box` does not specify, how memory is allocated.
246 /// #![feature(box_raw)]
248 /// let seventeen = Box::new(17u32);
249 /// let raw = Box::into_raw(seventeen);
250 /// let boxed_again = unsafe { Box::from_raw(raw) };
252 #[unstable(feature = "box_raw", reason = "may be renamed")]
254 // NB: may want to be called into_ptr, see comments on CStr::from_ptr
255 pub fn into_raw(b: Box<T>) -> *mut T {
256 unsafe { mem::transmute(b) }
260 /// Consumes the `Box`, returning the wrapped raw pointer.
262 /// After call to this function, caller is responsible for the memory
263 /// previously managed by `Box`, in particular caller should properly
264 /// destroy `T` and release memory. The proper way to do it is to
265 /// convert pointer back to `Box` with `Box::from_raw` function, because
266 /// `Box` does not specify, how memory is allocated.
270 /// #![feature(box_raw)]
274 /// let seventeen = Box::new(17u32);
275 /// let raw = boxed::into_raw(seventeen);
276 /// let boxed_again = unsafe { Box::from_raw(raw) };
278 #[unstable(feature = "box_raw", reason = "may be renamed")]
279 #[deprecated(since = "1.2.0", reason = "renamed to Box::into_raw")]
281 pub fn into_raw<T : ?Sized>(b: Box<T>) -> *mut T {
285 #[stable(feature = "rust1", since = "1.0.0")]
286 impl<T: Default> Default for Box<T> {
287 #[stable(feature = "rust1", since = "1.0.0")]
288 fn default() -> Box<T> { box Default::default() }
291 #[stable(feature = "rust1", since = "1.0.0")]
292 impl<T> Default for Box<[T]> {
293 #[stable(feature = "rust1", since = "1.0.0")]
294 fn default() -> Box<[T]> { Box::<[T; 0]>::new([]) }
297 #[stable(feature = "rust1", since = "1.0.0")]
298 impl<T: Clone> Clone for Box<T> {
299 /// Returns a new box with a `clone()` of this box's contents.
304 /// let x = Box::new(5);
305 /// let y = x.clone();
308 fn clone(&self) -> Box<T> { box {(**self).clone()} }
309 /// Copies `source`'s contents into `self` without creating a new allocation.
314 /// #![feature(box_raw)]
316 /// let x = Box::new(5);
317 /// let mut y = Box::new(10);
319 /// y.clone_from(&x);
321 /// assert_eq!(*y, 5);
324 fn clone_from(&mut self, source: &Box<T>) {
325 (**self).clone_from(&(**source));
330 #[stable(feature = "box_slice_clone", since = "1.3.0")]
331 impl Clone for Box<str> {
332 fn clone(&self) -> Self {
333 let len = self.len();
334 let buf = RawVec::with_capacity(len);
336 ptr::copy_nonoverlapping(self.as_ptr(), buf.ptr(), len);
337 mem::transmute(buf.into_box()) // bytes to str ~magic
342 #[stable(feature = "rust1", since = "1.0.0")]
343 impl<T: ?Sized + PartialEq> PartialEq for Box<T> {
345 fn eq(&self, other: &Box<T>) -> bool { PartialEq::eq(&**self, &**other) }
347 fn ne(&self, other: &Box<T>) -> bool { PartialEq::ne(&**self, &**other) }
349 #[stable(feature = "rust1", since = "1.0.0")]
350 impl<T: ?Sized + PartialOrd> PartialOrd for Box<T> {
352 fn partial_cmp(&self, other: &Box<T>) -> Option<Ordering> {
353 PartialOrd::partial_cmp(&**self, &**other)
356 fn lt(&self, other: &Box<T>) -> bool { PartialOrd::lt(&**self, &**other) }
358 fn le(&self, other: &Box<T>) -> bool { PartialOrd::le(&**self, &**other) }
360 fn ge(&self, other: &Box<T>) -> bool { PartialOrd::ge(&**self, &**other) }
362 fn gt(&self, other: &Box<T>) -> bool { PartialOrd::gt(&**self, &**other) }
364 #[stable(feature = "rust1", since = "1.0.0")]
365 impl<T: ?Sized + Ord> Ord for Box<T> {
367 fn cmp(&self, other: &Box<T>) -> Ordering {
368 Ord::cmp(&**self, &**other)
371 #[stable(feature = "rust1", since = "1.0.0")]
372 impl<T: ?Sized + Eq> Eq for Box<T> {}
374 #[stable(feature = "rust1", since = "1.0.0")]
375 impl<T: ?Sized + Hash> Hash for Box<T> {
376 fn hash<H: hash::Hasher>(&self, state: &mut H) {
377 (**self).hash(state);
383 #[stable(feature = "rust1", since = "1.0.0")]
384 /// Attempt to downcast the box to a concrete type.
385 pub fn downcast<T: Any>(self) -> Result<Box<T>, Box<Any>> {
388 // Get the raw representation of the trait object
389 let raw = Box::into_raw(self);
390 let to: TraitObject =
391 mem::transmute::<*mut Any, TraitObject>(raw);
393 // Extract the data pointer
394 Ok(Box::from_raw(to.data as *mut T))
402 impl Box<Any + Send> {
404 #[stable(feature = "rust1", since = "1.0.0")]
405 /// Attempt to downcast the box to a concrete type.
406 pub fn downcast<T: Any>(self) -> Result<Box<T>, Box<Any + Send>> {
407 <Box<Any>>::downcast(self).map_err(|s| unsafe {
408 // reapply the Send marker
409 mem::transmute::<Box<Any>, Box<Any + Send>>(s)
414 #[stable(feature = "rust1", since = "1.0.0")]
415 impl<T: fmt::Display + ?Sized> fmt::Display for Box<T> {
416 fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
417 fmt::Display::fmt(&**self, f)
421 #[stable(feature = "rust1", since = "1.0.0")]
422 impl<T: fmt::Debug + ?Sized> fmt::Debug for Box<T> {
423 fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
424 fmt::Debug::fmt(&**self, f)
428 #[stable(feature = "rust1", since = "1.0.0")]
429 impl<T> fmt::Pointer for Box<T> {
430 fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
431 // It's not possible to extract the inner Uniq directly from the Box,
432 // instead we cast it to a *const which aliases the Unique
433 let ptr: *const T = &**self;
434 fmt::Pointer::fmt(&ptr, f)
438 #[stable(feature = "rust1", since = "1.0.0")]
439 impl<T: ?Sized> Deref for Box<T> {
442 fn deref(&self) -> &T { &**self }
445 #[stable(feature = "rust1", since = "1.0.0")]
446 impl<T: ?Sized> DerefMut for Box<T> {
447 fn deref_mut(&mut self) -> &mut T { &mut **self }
450 #[stable(feature = "rust1", since = "1.0.0")]
451 impl<I: Iterator + ?Sized> Iterator for Box<I> {
453 fn next(&mut self) -> Option<I::Item> { (**self).next() }
454 fn size_hint(&self) -> (usize, Option<usize>) { (**self).size_hint() }
456 #[stable(feature = "rust1", since = "1.0.0")]
457 impl<I: DoubleEndedIterator + ?Sized> DoubleEndedIterator for Box<I> {
458 fn next_back(&mut self) -> Option<I::Item> { (**self).next_back() }
460 #[stable(feature = "rust1", since = "1.0.0")]
461 impl<I: ExactSizeIterator + ?Sized> ExactSizeIterator for Box<I> {}
464 /// `FnBox` is a version of the `FnOnce` intended for use with boxed
465 /// closure objects. The idea is that where one would normally store a
466 /// `Box<FnOnce()>` in a data structure, you should use
467 /// `Box<FnBox()>`. The two traits behave essentially the same, except
468 /// that a `FnBox` closure can only be called if it is boxed. (Note
469 /// that `FnBox` may be deprecated in the future if `Box<FnOnce()>`
470 /// closures become directly usable.)
474 /// Here is a snippet of code which creates a hashmap full of boxed
475 /// once closures and then removes them one by one, calling each
476 /// closure as it is removed. Note that the type of the closures
477 /// stored in the map is `Box<FnBox() -> i32>` and not `Box<FnOnce()
481 /// #![feature(fnbox)]
483 /// use std::boxed::FnBox;
484 /// use std::collections::HashMap;
486 /// fn make_map() -> HashMap<i32, Box<FnBox() -> i32>> {
487 /// let mut map: HashMap<i32, Box<FnBox() -> i32>> = HashMap::new();
488 /// map.insert(1, Box::new(|| 22));
489 /// map.insert(2, Box::new(|| 44));
494 /// let mut map = make_map();
495 /// for i in &[1, 2] {
496 /// let f = map.remove(&i).unwrap();
497 /// assert_eq!(f(), i * 22);
502 #[unstable(feature = "fnbox", reason = "Newly introduced")]
506 fn call_box(self: Box<Self>, args: A) -> Self::Output;
509 impl<A,F> FnBox<A> for F
512 type Output = F::Output;
514 fn call_box(self: Box<F>, args: A) -> F::Output {
519 impl<'a,A,R> FnOnce<A> for Box<FnBox<A,Output=R>+'a> {
522 extern "rust-call" fn call_once(self, args: A) -> R {
527 impl<'a,A,R> FnOnce<A> for Box<FnBox<A,Output=R>+Send+'a> {
530 extern "rust-call" fn call_once(self, args: A) -> R {
535 impl<T: ?Sized+Unsize<U>, U: ?Sized> CoerceUnsized<Box<U>> for Box<T> {}
537 #[stable(feature = "box_slice_clone", since = "1.3.0")]
538 impl<T: Clone> Clone for Box<[T]> {
539 fn clone(&self) -> Self {
540 let mut new = BoxBuilder {
541 data: RawVec::with_capacity(self.len()),
545 let mut target = new.data.ptr();
547 for item in self.iter() {
549 ptr::write(target, item.clone());
550 target = target.offset(1);
556 return unsafe { new.into_box() };
558 // Helper type for responding to panics correctly.
559 struct BoxBuilder<T> {
564 impl<T> BoxBuilder<T> {
565 unsafe fn into_box(self) -> Box<[T]> {
566 let raw = ptr::read(&self.data);
572 impl<T> Drop for BoxBuilder<T> {
574 let mut data = self.data.ptr();
575 let max = unsafe { data.offset(self.len as isize) };
580 data = data.offset(1);