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.
17 //! Boxes are useful in two situations: recursive data structures, and
18 //! occasionally when returning data. [The Pointer chapter of the
19 //! Book](../../../book/pointers.html#best-practices-1) explains these cases in
27 //! let x = Box::new(5);
30 //! Creating a recursive data structure:
35 //! Cons(T, Box<List<T>>),
40 //! let list: List<i32> = List::Cons(1, Box::new(List::Cons(2, Box::new(List::Nil))));
41 //! println!("{:?}", list);
45 //! This will print `Cons(1, Box(Cons(2, Box(Nil))))`.
47 #![stable(feature = "rust1", since = "1.0.0")]
52 use core::cmp::Ordering;
53 use core::default::Default;
54 use core::error::{Error, FromError};
56 use core::hash::{self, Hash};
58 use core::ops::{Deref, DerefMut};
59 use core::ptr::{self, Unique};
60 use core::raw::{TraitObject, Slice};
64 /// A value that represents the heap. This is the default place that the `box`
65 /// keyword allocates into when no place is supplied.
67 /// The following two examples are equivalent:
70 /// # #![feature(alloc)]
71 /// #![feature(box_syntax)]
72 /// use std::boxed::HEAP;
75 /// let foo = box(HEAP) 5;
79 #[lang = "exchange_heap"]
80 #[unstable(feature = "alloc",
81 reason = "may be renamed; uncertain about custom allocator design")]
82 pub static HEAP: () = ();
84 /// A pointer type for heap allocation.
86 /// See the [module-level documentation](../../std/boxed/index.html) for more.
88 #[stable(feature = "rust1", since = "1.0.0")]
89 pub struct Box<T>(Unique<T>);
92 /// Allocates memory on the heap and then moves `x` into it.
97 /// let x = Box::new(5);
99 #[stable(feature = "rust1", since = "1.0.0")]
101 pub fn new(x: T) -> Box<T> {
106 impl<T : ?Sized> Box<T> {
107 /// Constructs a box from the raw pointer.
109 /// After this function call, pointer is owned by resulting box.
110 /// In particular, it means that `Box` destructor calls destructor
111 /// of `T` and releases memory. Since the way `Box` allocates and
112 /// releases memory is unspecified, the only valid pointer to pass
113 /// to this function is the one taken from another `Box` with
114 /// `boxed::into_raw` function.
116 /// Function is unsafe, because improper use of this function may
117 /// lead to memory problems like double-free, for example if the
118 /// function is called twice on the same raw pointer.
119 #[unstable(feature = "alloc",
120 reason = "may be renamed or moved out of Box scope")]
122 pub unsafe fn from_raw(raw: *mut T) -> Self {
127 /// Consumes the `Box`, returning the wrapped raw pointer.
129 /// After call to this function, caller is responsible for the memory
130 /// previously managed by `Box`, in particular caller should properly
131 /// destroy `T` and release memory. The proper way to do it is to
132 /// convert pointer back to `Box` with `Box::from_raw` function, because
133 /// `Box` does not specify, how memory is allocated.
135 /// Function is unsafe, because result of this function is no longer
136 /// automatically managed that may lead to memory or other resource
141 /// # #![feature(alloc)]
144 /// let seventeen = Box::new(17u32);
145 /// let raw = unsafe { boxed::into_raw(seventeen) };
146 /// let boxed_again = unsafe { Box::from_raw(raw) };
148 #[unstable(feature = "alloc",
149 reason = "may be renamed")]
151 pub unsafe fn into_raw<T : ?Sized>(b: Box<T>) -> *mut T {
155 #[stable(feature = "rust1", since = "1.0.0")]
156 impl<T: Default> Default for Box<T> {
157 #[stable(feature = "rust1", since = "1.0.0")]
158 fn default() -> Box<T> { box Default::default() }
161 #[stable(feature = "rust1", since = "1.0.0")]
162 impl<T> Default for Box<[T]> {
163 #[stable(feature = "rust1", since = "1.0.0")]
164 fn default() -> Box<[T]> { Box::<[T; 0]>::new([]) }
167 #[stable(feature = "rust1", since = "1.0.0")]
168 impl<T: Clone> Clone for Box<T> {
169 /// Returns a new box with a `clone()` of this box's contents.
174 /// let x = Box::new(5);
175 /// let y = x.clone();
178 fn clone(&self) -> Box<T> { box {(**self).clone()} }
180 /// Copies `source`'s contents into `self` without creating a new allocation.
185 /// # #![feature(alloc, core)]
186 /// let x = Box::new(5);
187 /// let mut y = Box::new(10);
189 /// y.clone_from(&x);
191 /// assert_eq!(*y, 5);
194 fn clone_from(&mut self, source: &Box<T>) {
195 (**self).clone_from(&(**source));
199 #[stable(feature = "rust1", since = "1.0.0")]
200 impl<T: ?Sized + PartialEq> PartialEq for Box<T> {
202 fn eq(&self, other: &Box<T>) -> bool { PartialEq::eq(&**self, &**other) }
204 fn ne(&self, other: &Box<T>) -> bool { PartialEq::ne(&**self, &**other) }
206 #[stable(feature = "rust1", since = "1.0.0")]
207 impl<T: ?Sized + PartialOrd> PartialOrd for Box<T> {
209 fn partial_cmp(&self, other: &Box<T>) -> Option<Ordering> {
210 PartialOrd::partial_cmp(&**self, &**other)
213 fn lt(&self, other: &Box<T>) -> bool { PartialOrd::lt(&**self, &**other) }
215 fn le(&self, other: &Box<T>) -> bool { PartialOrd::le(&**self, &**other) }
217 fn ge(&self, other: &Box<T>) -> bool { PartialOrd::ge(&**self, &**other) }
219 fn gt(&self, other: &Box<T>) -> bool { PartialOrd::gt(&**self, &**other) }
221 #[stable(feature = "rust1", since = "1.0.0")]
222 impl<T: ?Sized + Ord> Ord for Box<T> {
224 fn cmp(&self, other: &Box<T>) -> Ordering {
225 Ord::cmp(&**self, &**other)
228 #[stable(feature = "rust1", since = "1.0.0")]
229 impl<T: ?Sized + Eq> Eq for Box<T> {}
231 #[stable(feature = "rust1", since = "1.0.0")]
232 impl<T: ?Sized + Hash> Hash for Box<T> {
233 fn hash<H: hash::Hasher>(&self, state: &mut H) {
234 (**self).hash(state);
238 /// Extension methods for an owning `Any` trait object.
239 #[unstable(feature = "alloc",
240 reason = "this trait will likely disappear once compiler bugs blocking \
241 a direct impl on `Box<Any>` have been fixed ")]
242 // FIXME(#18737): this should be a direct impl on `Box<Any>`. If you're
243 // removing this please make sure that you can downcase on
244 // `Box<Any + Send>` as well as `Box<Any>`
246 /// Returns the boxed value if it is of type `T`, or
247 /// `Err(Self)` if it isn't.
248 #[stable(feature = "rust1", since = "1.0.0")]
249 fn downcast<T: Any>(self) -> Result<Box<T>, Box<Any>>;
252 #[stable(feature = "rust1", since = "1.0.0")]
253 impl BoxAny for Box<Any> {
255 fn downcast<T: Any>(self) -> Result<Box<T>, Box<Any>> {
258 // Get the raw representation of the trait object
259 let raw = into_raw(self);
260 let to: TraitObject =
261 mem::transmute::<*mut Any, TraitObject>(raw);
263 // Extract the data pointer
264 Ok(Box::from_raw(to.data as *mut T))
272 #[stable(feature = "rust1", since = "1.0.0")]
273 impl BoxAny for Box<Any+Send> {
275 fn downcast<T: Any>(self) -> Result<Box<T>, Box<Any>> {
276 <Box<Any>>::downcast(self)
280 #[stable(feature = "rust1", since = "1.0.0")]
281 impl<T: fmt::Display + ?Sized> fmt::Display for Box<T> {
282 fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
283 fmt::Display::fmt(&**self, f)
287 #[stable(feature = "rust1", since = "1.0.0")]
288 impl<T: fmt::Debug + ?Sized> fmt::Debug for Box<T> {
289 fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
290 fmt::Debug::fmt(&**self, f)
294 #[stable(feature = "rust1", since = "1.0.0")]
295 impl fmt::Debug for Box<Any> {
296 fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
301 #[stable(feature = "rust1", since = "1.0.0")]
302 impl<T: ?Sized> Deref for Box<T> {
305 fn deref(&self) -> &T { &**self }
308 #[stable(feature = "rust1", since = "1.0.0")]
309 impl<T: ?Sized> DerefMut for Box<T> {
310 fn deref_mut(&mut self) -> &mut T { &mut **self }
313 #[stable(feature = "rust1", since = "1.0.0")]
314 impl<I: Iterator + ?Sized> Iterator for Box<I> {
316 fn next(&mut self) -> Option<I::Item> { (**self).next() }
317 fn size_hint(&self) -> (usize, Option<usize>) { (**self).size_hint() }
319 #[stable(feature = "rust1", since = "1.0.0")]
320 impl<I: DoubleEndedIterator + ?Sized> DoubleEndedIterator for Box<I> {
321 fn next_back(&mut self) -> Option<I::Item> { (**self).next_back() }
323 #[stable(feature = "rust1", since = "1.0.0")]
324 impl<I: ExactSizeIterator + ?Sized> ExactSizeIterator for Box<I> {}
326 #[stable(feature = "rust1", since = "1.0.0")]
327 impl<'a, E: Error + 'a> FromError<E> for Box<Error + 'a> {
328 fn from_error(err: E) -> Box<Error + 'a> {
333 #[stable(feature = "rust1", since = "1.0.0")]
334 impl<'a, E: Error + Send + 'a> From<E> for Box<Error + Send + 'a> {
335 fn from(err: E) -> Box<Error + Send + 'a> {
340 #[stable(feature = "rust1", since = "1.0.0")]
341 impl<'a, 'b> From<&'b str> for Box<Error + Send + 'a> {
342 fn from(err: &'b str) -> Box<Error + Send + 'a> {
344 struct StringError(Box<str>);
345 impl Error for StringError {
346 fn description(&self) -> &str { &self.0 }
348 impl fmt::Display for StringError {
349 fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
354 // Unfortunately `String` is located in libcollections, so we construct
355 // a `Box<str>` manually here.
357 let alloc = if err.len() == 0 {
360 let ptr = heap::allocate(err.len(), 1);
361 if ptr.is_null() { ::oom(); }
364 ptr::copy(err.as_bytes().as_ptr(), alloc, err.len());
365 Box::new(StringError(mem::transmute(Slice {