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")]
61 use core::cmp::Ordering;
63 use core::hash::{self, Hash};
64 use core::marker::{self, Unsize};
66 use core::ops::{CoerceUnsized, Deref, DerefMut};
67 use core::ops::{Placer, Boxed, Place, InPlace, BoxPlace};
68 use core::ptr::{self, Unique};
69 use core::raw::TraitObject;
71 /// A value that represents the heap. This is the default place that the `box`
72 /// keyword allocates into when no place is supplied.
74 /// The following two examples are equivalent:
77 /// #![feature(box_heap)]
79 /// #![feature(box_syntax, placement_in_syntax)]
80 /// use std::boxed::HEAP;
83 /// let foo: Box<i32> = in HEAP { 5 };
87 #[unstable(feature = "box_heap",
88 reason = "may be renamed; uncertain about custom allocator design",
90 pub const HEAP: ExchangeHeapSingleton =
91 ExchangeHeapSingleton { _force_singleton: () };
93 /// This the singleton type used solely for `boxed::HEAP`.
94 #[unstable(feature = "box_heap",
95 reason = "may be renamed; uncertain about custom allocator design",
97 #[derive(Copy, Clone)]
98 pub struct ExchangeHeapSingleton {
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",
129 reason = "placement box design is still being worked out.",
131 pub struct IntermediateBox<T: ?Sized> {
135 marker: marker::PhantomData<*mut T>,
138 impl<T> Place<T> for IntermediateBox<T> {
139 fn pointer(&mut self) -> *mut T {
140 unsafe { ::core::mem::transmute(self.ptr) }
144 unsafe fn finalize<T>(b: IntermediateBox<T>) -> Box<T> {
145 let p = b.ptr as *mut T;
150 fn make_place<T>() -> IntermediateBox<T> {
151 let size = mem::size_of::<T>();
152 let align = mem::align_of::<T>();
154 let p = if size == 0 {
155 heap::EMPTY as *mut u8
157 let p = unsafe { 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> {
173 impl<T> InPlace<T> for IntermediateBox<T> {
175 unsafe fn finalize(self) -> Box<T> {
180 impl<T> Boxed for Box<T> {
182 type Place = IntermediateBox<T>;
183 unsafe fn finalize(b: IntermediateBox<T>) -> Box<T> {
188 impl<T> Placer<T> for ExchangeHeapSingleton {
189 type Place = IntermediateBox<T>;
191 fn make_place(self) -> IntermediateBox<T> {
196 impl<T: ?Sized> Drop for IntermediateBox<T> {
199 unsafe { heap::deallocate(self.ptr, self.size, self.align) }
205 /// Allocates memory on the heap and then moves `x` into it.
210 /// let x = Box::new(5);
212 #[stable(feature = "rust1", since = "1.0.0")]
214 pub fn new(x: T) -> Box<T> {
219 impl<T : ?Sized> Box<T> {
220 /// Constructs a box from the raw pointer.
222 /// After this function call, pointer is owned by resulting box.
223 /// In particular, it means that `Box` destructor calls destructor
224 /// of `T` and releases memory. Since the way `Box` allocates and
225 /// releases memory is unspecified, the only valid pointer to pass
226 /// to this function is the one taken from another `Box` with
227 /// `Box::into_raw` function.
229 /// Function is unsafe, because improper use of this function may
230 /// lead to memory problems like double-free, for example if the
231 /// function is called twice on the same raw pointer.
232 #[stable(feature = "box_raw", since = "1.4.0")]
234 pub unsafe fn from_raw(raw: *mut T) -> Self {
238 /// Consumes the `Box`, returning the wrapped raw pointer.
240 /// After call to this function, caller is responsible for the memory
241 /// previously managed by `Box`, in particular caller should properly
242 /// destroy `T` and release memory. The proper way to do it is to
243 /// convert pointer back to `Box` with `Box::from_raw` function, because
244 /// `Box` does not specify, how memory is allocated.
249 /// let seventeen = Box::new(17u32);
250 /// let raw = Box::into_raw(seventeen);
251 /// let boxed_again = unsafe { Box::from_raw(raw) };
253 #[stable(feature = "box_raw", since = "1.4.0")]
255 pub fn into_raw(b: Box<T>) -> *mut T {
256 unsafe { mem::transmute(b) }
260 #[stable(feature = "rust1", since = "1.0.0")]
261 impl<T: Default> Default for Box<T> {
262 #[stable(feature = "rust1", since = "1.0.0")]
263 fn default() -> Box<T> {
264 box Default::default()
268 #[stable(feature = "rust1", since = "1.0.0")]
269 impl<T> Default for Box<[T]> {
270 #[stable(feature = "rust1", since = "1.0.0")]
271 fn default() -> Box<[T]> {
272 Box::<[T; 0]>::new([])
276 #[stable(feature = "rust1", since = "1.0.0")]
277 impl<T: Clone> Clone for Box<T> {
278 /// Returns a new box with a `clone()` of this box's contents.
283 /// let x = Box::new(5);
284 /// let y = x.clone();
288 fn clone(&self) -> Box<T> {
289 box { (**self).clone() }
291 /// Copies `source`'s contents into `self` without creating a new allocation.
296 /// let x = Box::new(5);
297 /// let mut y = Box::new(10);
299 /// y.clone_from(&x);
301 /// assert_eq!(*y, 5);
304 fn clone_from(&mut self, source: &Box<T>) {
305 (**self).clone_from(&(**source));
310 #[stable(feature = "box_slice_clone", since = "1.3.0")]
311 impl Clone for Box<str> {
312 fn clone(&self) -> Self {
313 let len = self.len();
314 let buf = RawVec::with_capacity(len);
316 ptr::copy_nonoverlapping(self.as_ptr(), buf.ptr(), len);
317 mem::transmute(buf.into_box()) // bytes to str ~magic
322 #[stable(feature = "rust1", since = "1.0.0")]
323 impl<T: ?Sized + PartialEq> PartialEq for Box<T> {
325 fn eq(&self, other: &Box<T>) -> bool {
326 PartialEq::eq(&**self, &**other)
329 fn ne(&self, other: &Box<T>) -> bool {
330 PartialEq::ne(&**self, &**other)
333 #[stable(feature = "rust1", since = "1.0.0")]
334 impl<T: ?Sized + PartialOrd> PartialOrd for Box<T> {
336 fn partial_cmp(&self, other: &Box<T>) -> Option<Ordering> {
337 PartialOrd::partial_cmp(&**self, &**other)
340 fn lt(&self, other: &Box<T>) -> bool {
341 PartialOrd::lt(&**self, &**other)
344 fn le(&self, other: &Box<T>) -> bool {
345 PartialOrd::le(&**self, &**other)
348 fn ge(&self, other: &Box<T>) -> bool {
349 PartialOrd::ge(&**self, &**other)
352 fn gt(&self, other: &Box<T>) -> bool {
353 PartialOrd::gt(&**self, &**other)
356 #[stable(feature = "rust1", since = "1.0.0")]
357 impl<T: ?Sized + Ord> Ord for Box<T> {
359 fn cmp(&self, other: &Box<T>) -> Ordering {
360 Ord::cmp(&**self, &**other)
363 #[stable(feature = "rust1", since = "1.0.0")]
364 impl<T: ?Sized + Eq> Eq for Box<T> {}
366 #[stable(feature = "rust1", since = "1.0.0")]
367 impl<T: ?Sized + Hash> Hash for Box<T> {
368 fn hash<H: hash::Hasher>(&self, state: &mut H) {
369 (**self).hash(state);
375 #[stable(feature = "rust1", since = "1.0.0")]
376 /// Attempt to downcast the box to a concrete type.
377 pub fn downcast<T: Any>(self) -> Result<Box<T>, Box<Any>> {
380 // Get the raw representation of the trait object
381 let raw = Box::into_raw(self);
382 let to: TraitObject = mem::transmute::<*mut Any, TraitObject>(raw);
384 // Extract the data pointer
385 Ok(Box::from_raw(to.data as *mut T))
393 impl Box<Any + Send> {
395 #[stable(feature = "rust1", since = "1.0.0")]
396 /// Attempt to downcast the box to a concrete type.
397 pub fn downcast<T: Any>(self) -> Result<Box<T>, Box<Any + Send>> {
398 <Box<Any>>::downcast(self).map_err(|s| unsafe {
399 // reapply the Send marker
400 mem::transmute::<Box<Any>, Box<Any + Send>>(s)
405 #[stable(feature = "rust1", since = "1.0.0")]
406 impl<T: fmt::Display + ?Sized> fmt::Display for Box<T> {
407 fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
408 fmt::Display::fmt(&**self, f)
412 #[stable(feature = "rust1", since = "1.0.0")]
413 impl<T: fmt::Debug + ?Sized> fmt::Debug for Box<T> {
414 fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
415 fmt::Debug::fmt(&**self, f)
419 #[stable(feature = "rust1", since = "1.0.0")]
420 impl<T> fmt::Pointer for Box<T> {
421 fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
422 // It's not possible to extract the inner Uniq directly from the Box,
423 // instead we cast it to a *const which aliases the Unique
424 let ptr: *const T = &**self;
425 fmt::Pointer::fmt(&ptr, f)
429 #[stable(feature = "rust1", since = "1.0.0")]
430 impl<T: ?Sized> Deref for Box<T> {
433 fn deref(&self) -> &T {
438 #[stable(feature = "rust1", since = "1.0.0")]
439 impl<T: ?Sized> DerefMut for Box<T> {
440 fn deref_mut(&mut self) -> &mut T {
445 #[stable(feature = "rust1", since = "1.0.0")]
446 impl<I: Iterator + ?Sized> Iterator for Box<I> {
448 fn next(&mut self) -> Option<I::Item> {
451 fn size_hint(&self) -> (usize, Option<usize>) {
455 #[stable(feature = "rust1", since = "1.0.0")]
456 impl<I: DoubleEndedIterator + ?Sized> DoubleEndedIterator for Box<I> {
457 fn next_back(&mut self) -> Option<I::Item> {
461 #[stable(feature = "rust1", since = "1.0.0")]
462 impl<I: ExactSizeIterator + ?Sized> ExactSizeIterator for Box<I> {}
465 /// `FnBox` is a version of the `FnOnce` intended for use with boxed
466 /// closure objects. The idea is that where one would normally store a
467 /// `Box<FnOnce()>` in a data structure, you should use
468 /// `Box<FnBox()>`. The two traits behave essentially the same, except
469 /// that a `FnBox` closure can only be called if it is boxed. (Note
470 /// that `FnBox` may be deprecated in the future if `Box<FnOnce()>`
471 /// closures become directly usable.)
475 /// Here is a snippet of code which creates a hashmap full of boxed
476 /// once closures and then removes them one by one, calling each
477 /// closure as it is removed. Note that the type of the closures
478 /// stored in the map is `Box<FnBox() -> i32>` and not `Box<FnOnce()
482 /// #![feature(fnbox)]
484 /// use std::boxed::FnBox;
485 /// use std::collections::HashMap;
487 /// fn make_map() -> HashMap<i32, Box<FnBox() -> i32>> {
488 /// let mut map: HashMap<i32, Box<FnBox() -> i32>> = HashMap::new();
489 /// map.insert(1, Box::new(|| 22));
490 /// map.insert(2, Box::new(|| 44));
495 /// let mut map = make_map();
496 /// for i in &[1, 2] {
497 /// let f = map.remove(&i).unwrap();
498 /// assert_eq!(f(), i * 22);
503 #[unstable(feature = "fnbox", reason = "Newly introduced", issue = "0")]
507 fn call_box(self: Box<Self>, args: A) -> Self::Output;
510 impl<A,F> FnBox<A> for F
513 type Output = F::Output;
515 fn call_box(self: Box<F>, args: A) -> F::Output {
520 impl<'a,A,R> FnOnce<A> for Box<FnBox<A,Output=R>+'a> {
523 extern "rust-call" fn call_once(self, args: A) -> R {
528 impl<'a,A,R> FnOnce<A> for Box<FnBox<A,Output=R>+Send+'a> {
531 extern "rust-call" fn call_once(self, args: A) -> R {
536 impl<T: ?Sized+Unsize<U>, U: ?Sized> CoerceUnsized<Box<U>> for Box<T> {}
538 #[stable(feature = "box_slice_clone", since = "1.3.0")]
539 impl<T: Clone> Clone for Box<[T]> {
540 fn clone(&self) -> Self {
541 let mut new = BoxBuilder { data: RawVec::with_capacity(self.len()), len: 0 };
543 let mut target = new.data.ptr();
545 for item in self.iter() {
547 ptr::write(target, item.clone());
548 target = target.offset(1);
554 return unsafe { new.into_box() };
556 // Helper type for responding to panics correctly.
557 struct BoxBuilder<T> {
562 impl<T> BoxBuilder<T> {
563 unsafe fn into_box(self) -> Box<[T]> {
564 let raw = ptr::read(&self.data);
570 impl<T> Drop for BoxBuilder<T> {
572 let mut data = self.data.ptr();
573 let max = unsafe { data.offset(self.len as isize) };
578 data = data.offset(1);
586 impl<T: ?Sized> borrow::Borrow<T> for Box<T> {
587 fn borrow(&self) -> &T {
592 impl<T: ?Sized> borrow::BorrowMut<T> for Box<T> {
593 fn borrow_mut(&mut self) -> &mut T {