alloc::boxed::Box - Rust
Description: API documentation for the Rust `Box` struct in crate `alloc`.
Keywords: rust, rustlang, rust-lang, Box

# Struct alloc::boxed::Box 1.0.0 [−] [src]

    #[lang = "owned_box"]
    pub struct Box<T: ?Sized>(_);

A pointer type for heap allocation.

See the module-level documentation for more.

## Methods

### `impl<T> Box<T>`
[src]

#### `fn new(x: T) -> Box<T>`

Allocates memory on the heap and then places `x` into it.

This doesn't actually allocate if `T` is zero-sized.

# Examples

    let five = Box::new(5);

### `impl<T: ?Sized> Box<T>`
[src]

#### `unsafe fn from_raw(raw: *mut T) -> Self`
1.4.0

Constructs a box from a raw pointer.

After calling this function, the raw pointer is owned by the resulting `Box`. Specifically, the `Box` destructor will call the destructor of `T` and free the allocated memory. Since the way `Box` allocates and releases memory is unspecified, the only valid pointer to pass to this function is the one taken from another `Box` via the `Box::into_raw` function.

This function is unsafe because improper use may lead to memory problems. For example, a double-free may occur if the function is called twice on the same raw pointer.

# Examples

    let x = Box::new(5);
    let ptr = Box::into_raw(x);
    let x = unsafe { Box::from_raw(ptr) };

#### `fn into_raw(b: Box<T>) -> *mut T`
1.4.0

Consumes the `Box`, returning the wrapped raw pointer.

After calling this function, the caller is responsible for the memory previously managed by the `Box`. In particular, the caller should properly destroy `T` and release the memory. The proper way to do so is to convert the raw pointer back into a `Box` with the `Box::from_raw` function.

Note: this is an associated function, which means that you have to call it as `Box::into_raw(b)` instead of `b.into_raw()`. This is so that there is no conflict with a method on the inner type.

# Examples

    let x = Box::new(5);
    let ptr = Box::into_raw(x);

### `impl Box<Any>`
[src]

#### `fn downcast<T: Any>(self) -> Result<Box<T>, Box<Any>>`

Attempt to downcast the box to a concrete type.

# Examples

    use std::any::Any;

    fn print_if_string(value: Box<Any>) {
        if let Ok(string) = value.downcast::<String>() {
            println!("String ({}): {}", string.len(), string);
        }
    }

    fn main() {
        let my_string = "Hello World".to_string();
        print_if_string(Box::new(my_string));
        print_if_string(Box::new(0i8));
    }

### `impl Box<Any + Send>`
[src]

#### `fn downcast<T: Any>(self) -> Result<Box<T>, Box<Any + Send>>`

Attempt to downcast the box to a concrete type.

# Examples

    use std::any::Any;

    fn print_if_string(value: Box<Any + Send>) {
        if let Ok(string) = value.downcast::<String>() {
            println!("String ({}): {}", string.len(), string);
        }
    }

    fn main() {
        let my_string = "Hello World".to_string();
        print_if_string(Box::new(my_string));
        print_if_string(Box::new(0i8));
    }

## Trait Implementations

### `impl<T> Boxed for Box<T>`
[src]

#### `type Data = T`

🔬 This is a nightly-only experimental API. (`placement_new_protocol` #27779)

The kind of data that is stored in this kind of box.

#### `type Place = IntermediateBox<T>`

🔬 This is a nightly-only experimental API. (`placement_new_protocol` #27779)

The place that will negotiate the storage of the data.

#### `unsafe fn finalize(b: IntermediateBox<T>) -> Box<T>`

🔬 This is a nightly-only experimental API. (`placement_new_protocol` #27779)

Converts filled place into final owning value, shifting deallocation/cleanup responsibilities (if any remain), over to returned instance of `Self` and forgetting `filled`. Read more

### `impl<T: ?Sized> Drop for Box<T>`
[src]

#### `fn drop(&mut self)`

A method called when the value goes out of scope. Read more

### `impl<T: Default> Default for Box<T>`
[src]

#### `fn default() -> Box<T>`

Creates a `Box<T>`, with the `Default` value for T.

### `impl<T> Default for Box<[T]>`
[src]

#### `fn default() -> Box<[T]>`

Returns the "default value" for a type. Read more

### `impl Default for Box<str>`
1.17.0
[src]

#### `fn default() -> Box<str>`

Returns the "default value" for a type. Read more

### `impl<T: Clone> Clone for Box<T>`
[src]

#### `fn clone(&self) -> Box<T>`

Returns a new box with a `clone()` of this box's contents.

# Examples

    let x = Box::new(5);
    let y = x.clone();

#### `fn clone_from(&mut self, source: &Box<T>)`

Copies `source`'s contents into `self` without creating a new allocation.

# Examples

    let x = Box::new(5);
    let mut y = Box::new(10);

    y.clone_from(&x);

    assert_eq!(*y, 5);

### `impl Clone for Box<str>`
1.3.0
[src]

#### `fn clone(&self) -> Self`

Returns a copy of the value. Read more

#### `fn clone_from(&mut self, source: &Self)`

Performs copy-assignment from `source`. Read more

### `impl<T: ?Sized + PartialEq> PartialEq for Box<T>`
[src]

#### `fn eq(&self, other: &Box<T>) -> bool`

This method tests for `self` and `other` values to be equal, and is used by `==`. Read more

#### `fn ne(&self, other: &Box<T>) -> bool`

This method tests for `!=`.

### `impl<T: ?Sized + PartialOrd> PartialOrd for Box<T>`
[src]

#### `fn partial_cmp(&self, other: &Box<T>) -> Option<Ordering>`

This method returns an ordering between `self` and `other` values if one exists. Read more

#### `fn lt(&self, other: &Box<T>) -> bool`

This method tests less than (for `self` and `other`) and is used by the `<` operator. Read more

#### `fn le(&self, other: &Box<T>) -> bool`

This method tests less than or equal to (for `self` and `other`) and is used by the `<=` operator. Read more

#### `fn ge(&self, other: &Box<T>) -> bool`

This method tests greater than or equal to (for `self` and `other`) and is used by the `>=` operator. Read more

#### `fn gt(&self, other: &Box<T>) -> bool`

This method tests greater than (for `self` and `other`) and is used by the `>` operator. Read more

### `impl<T: ?Sized + Ord> Ord for Box<T>`
[src]

#### `fn cmp(&self, other: &Box<T>) -> Ordering`

This method returns an `Ordering` between `self` and `other`. Read more

#### `fn max(self, other: Self) -> Self`

🔬 This is a nightly-only experimental API. (`ord_max_min` #25663)

Compares and returns the maximum of two values. Read more

#### `fn min(self, other: Self) -> Self`

🔬 This is a nightly-only experimental API. (`ord_max_min` #25663)

Compares and returns the minimum of two values. Read more

### `impl<T: ?Sized + Eq> Eq for Box<T>`
[src]

### `impl<T: ?Sized + Hash> Hash for Box<T>`
[src]

#### `fn hash<H: Hasher>(&self, state: &mut H)`

Feeds this value into the given [`Hasher`]. Read more

#### `fn hash_slice<H>(data: &[Self], state: &mut H) where
H: Hasher,`
1.3.0

Feeds a slice of this type into the given [`Hasher`]. Read more

### `impl<T> From<T> for Box<T>`
1.6.0
[src]

#### `fn from(t: T) -> Self`

Performs the conversion.

### `impl<'a, T: Copy> From<&'a [T]> for Box<[T]>`
1.17.0
[src]

#### `fn from(slice: &'a [T]) -> Box<[T]>`

Performs the conversion.

### `impl<'a> From<&'a str> for Box<str>`
1.17.0
[src]

#### `fn from(s: &'a str) -> Box<str>`

Performs the conversion.

### `impl From<Box<str>> for Box<[u8]>`
1.19.0
[src]

#### `fn from(s: Box<str>) -> Self`

Performs the conversion.

### `impl<T: Display + ?Sized> Display for Box<T>`
[src]

#### `fn fmt(&self, f: &mut Formatter) -> Result`

Formats the value using the given formatter. Read more

### `impl<T: Debug + ?Sized> Debug for Box<T>`
[src]

#### `fn fmt(&self, f: &mut Formatter) -> Result`

Formats the value using the given formatter.

### `impl<T: ?Sized> Pointer for Box<T>`
[src]

#### `fn fmt(&self, f: &mut Formatter) -> Result`

Formats the value using the given formatter.

### `impl<T: ?Sized> Deref for Box<T>`
[src]

#### `type Target = T`

The resulting type after dereferencing

#### `fn deref(&self) -> &T`

The method called to dereference a value

### `impl<T: ?Sized> DerefMut for Box<T>`
[src]

#### `fn deref_mut(&mut self) -> &mut T`

The method called to mutably dereference a value

### `impl<I: Iterator + ?Sized> Iterator for Box<I>`
[src]

#### `type Item = I::Item`

The type of the elements being iterated over.

#### `fn next(&mut self) -> Option<I::Item>`

Advances the iterator and returns the next value. Read more

#### `fn size_hint(&self) -> (usize, Option<usize>)`

Returns the bounds on the remaining length of the iterator. Read more

#### `fn nth(&mut self, n: usize) -> Option<I::Item>`

Returns the `n`th element of the iterator. Read more

#### `fn count(self) -> usize`

Consumes the iterator, counting the number of iterations and returning it. Read more

#### `fn last(self) -> Option<Self::Item>`

Consumes the iterator, returning the last element. Read more

#### `fn step_by(self, step: usize) -> StepBy<Self>`

🔬 This is a nightly-only experimental API. (`iterator_step_by` #27741)

unstable replacement of Range::step_by

Creates an iterator starting at the same point, but stepping by the given amount at each iteration. Read more

#### `fn chain<U>(self, other: U) -> Chain<Self, <U as IntoIterator>::IntoIter> where
U: IntoIterator<Item = Self::Item>,`

Takes two iterators and creates a new iterator over both in sequence. Read more

#### `fn zip<U>(self, other: U) -> Zip<Self, <U as IntoIterator>::IntoIter> where
U: IntoIterator,`

'Zips up' two iterators into a single iterator of pairs. Read more

#### `fn map<B, F>(self, f: F) -> Map<Self, F> where
F: FnMut(Self::Item) -> B,`

Takes a closure and creates an iterator which calls that closure on each element. Read more

#### `fn for_each<F>(self, f: F) where
F: FnMut(Self::Item) -> (),`

🔬 This is a nightly-only experimental API. (`iterator_for_each` #42986)

Calls a closure on each element of an iterator. Read more

#### `fn filter<P>(self, predicate: P) -> Filter<Self, P> where
P: FnMut(&Self::Item) -> bool,`

Creates an iterator which uses a closure to determine if an element should be yielded. Read more

#### `fn filter_map<B, F>(self, f: F) -> FilterMap<Self, F> where
F: FnMut(Self::Item) -> Option<B>,`

Creates an iterator that both filters and maps. Read more

#### `fn enumerate(self) -> Enumerate<Self>`

Creates an iterator which gives the current iteration count as well as the next value. Read more

#### `fn peekable(self) -> Peekable<Self>`

Creates an iterator which can use `peek` to look at the next element of the iterator without consuming it. Read more

#### `fn skip_while<P>(self, predicate: P) -> SkipWhile<Self, P> where
P: FnMut(&Self::Item) -> bool,`

Creates an iterator that [`skip`]s elements based on a predicate. Read more

#### `fn take_while<P>(self, predicate: P) -> TakeWhile<Self, P> where
P: FnMut(&Self::Item) -> bool,`

Creates an iterator that yields elements based on a predicate. Read more

#### `fn skip(self, n: usize) -> Skip<Self>`

Creates an iterator that skips the first `n` elements. Read more

#### `fn take(self, n: usize) -> Take<Self>`

Creates an iterator that yields its first `n` elements. Read more

#### `fn scan<St, B, F>(self, initial_state: St, f: F) -> Scan<Self, St, F> where
F: FnMut(&mut St, Self::Item) -> Option<B>,`

An iterator adaptor similar to [`fold`] that holds internal state and produces a new iterator. Read more

#### `fn flat_map<U, F>(self, f: F) -> FlatMap<Self, U, F> where
F: FnMut(Self::Item) -> U,
U: IntoIterator,`

Creates an iterator that works like map, but flattens nested structure. Read more

#### `fn fuse(self) -> Fuse<Self>`

Creates an iterator which ends after the first [`None`]. Read more

#### `fn inspect<F>(self, f: F) -> Inspect<Self, F> where
F: FnMut(&Self::Item) -> (),`

Do something with each element of an iterator, passing the value on. Read more

#### `fn by_ref(&mut self) -> &mut Self`

Borrows an iterator, rather than consuming it. Read more

#### `fn collect<B>(self) -> B where
B: FromIterator<Self::Item>,`

Transforms an iterator into a collection. Read more

#### `fn partition<B, F>(self, f: F) -> (B, B) where
B: Default + Extend<Self::Item>,
F: FnMut(&Self::Item) -> bool,`

Consumes an iterator, creating two collections from it. Read more

#### `fn fold<B, F>(self, init: B, f: F) -> B where
F: FnMut(B, Self::Item) -> B,`

An iterator adaptor that applies a function, producing a single, final value. Read more

#### `fn all<F>(&mut self, f: F) -> bool where
F: FnMut(Self::Item) -> bool,`

Tests if every element of the iterator matches a predicate. Read more

#### `fn any<F>(&mut self, f: F) -> bool where
F: FnMut(Self::Item) -> bool,`

Tests if any element of the iterator matches a predicate. Read more

#### `fn find<P>(&mut self, predicate: P) -> Option<Self::Item> where
P: FnMut(&Self::Item) -> bool,`

Searches for an element of an iterator that satisfies a predicate. Read more

#### `fn position<P>(&mut self, predicate: P) -> Option<usize> where
P: FnMut(Self::Item) -> bool,`

Searches for an element in an iterator, returning its index. Read more

#### `fn rposition<P>(&mut self, predicate: P) -> Option<usize> where
P: FnMut(Self::Item) -> bool,
Self: ExactSizeIterator + DoubleEndedIterator,`

Searches for an element in an iterator from the right, returning its index. Read more

#### `fn max(self) -> Option<Self::Item> where
Self::Item: Ord,`

Returns the maximum element of an iterator. Read more

#### `fn min(self) -> Option<Self::Item> where
Self::Item: Ord,`

Returns the minimum element of an iterator. Read more

#### `fn max_by_key<B, F>(self, f: F) -> Option<Self::Item> where
B: Ord,
F: FnMut(&Self::Item) -> B,`
1.6.0

Returns the element that gives the maximum value from the specified function. Read more

#### `fn max_by<F>(self, compare: F) -> Option<Self::Item> where
F: FnMut(&Self::Item, &Self::Item) -> Ordering,`
1.15.0

Returns the element that gives the maximum value with respect to the specified comparison function. Read more

#### `fn min_by_key<B, F>(self, f: F) -> Option<Self::Item> where
B: Ord,
F: FnMut(&Self::Item) -> B,`
1.6.0

Returns the element that gives the minimum value from the specified function. Read more

#### `fn min_by<F>(self, compare: F) -> Option<Self::Item> where
F: FnMut(&Self::Item, &Self::Item) -> Ordering,`
1.15.0

Returns the element that gives the minimum value with respect to the specified comparison function. Read more

#### `fn rev(self) -> Rev<Self> where
Self: DoubleEndedIterator,`

Reverses an iterator's direction. Read more

#### `fn unzip<A, B, FromA, FromB>(self) -> (FromA, FromB) where
FromA: Default + Extend<A>,
FromB: Default + Extend<B>,
Self: Iterator<Item = (A, B)>,`

Converts an iterator of pairs into a pair of containers. Read more

#### `fn cloned<'a, T>(self) -> Cloned<Self> where
Self: Iterator<Item = &'a T>,
T: 'a + Clone,`

Creates an iterator which [`clone`]s all of its elements. Read more

#### `fn cycle(self) -> Cycle<Self> where
Self: Clone,`

Repeats an iterator endlessly. Read more

#### `fn sum<S>(self) -> S where
S: Sum<Self::Item>,`
1.11.0

Sums the elements of an iterator. Read more

#### `fn product<P>(self) -> P where
P: Product<Self::Item>,`
1.11.0

Iterates over the entire iterator, multiplying all the elements Read more

#### `fn cmp<I>(self, other: I) -> Ordering where
I: IntoIterator<Item = Self::Item>,
Self::Item: Ord,`
1.5.0

Lexicographically compares the elements of this `Iterator` with those of another. Read more

#### `fn partial_cmp<I>(self, other: I) -> Option<Ordering> where
I: IntoIterator,
Self::Item: PartialOrd<<I as IntoIterator>::Item>,`
1.5.0

Lexicographically compares the elements of this `Iterator` with those of another. Read more

#### `fn eq<I>(self, other: I) -> bool where
I: IntoIterator,
Self::Item: PartialEq<<I as IntoIterator>::Item>,`
1.5.0

Determines if the elements of this `Iterator` are equal to those of another. Read more

#### `fn ne<I>(self, other: I) -> bool where
I: IntoIterator,
Self::Item: PartialEq<<I as IntoIterator>::Item>,`
1.5.0

Determines if the elements of this `Iterator` are unequal to those of another. Read more

#### `fn lt<I>(self, other: I) -> bool where
I: IntoIterator,
Self::Item: PartialOrd<<I as IntoIterator>::Item>,`
1.5.0

Determines if the elements of this `Iterator` are lexicographically less than those of another. Read more

#### `fn le<I>(self, other: I) -> bool where
I: IntoIterator,
Self::Item: PartialOrd<<I as IntoIterator>::Item>,`
1.5.0

Determines if the elements of this `Iterator` are lexicographically less or equal to those of another. Read more

#### `fn gt<I>(self, other: I) -> bool where
I: IntoIterator,
Self::Item: PartialOrd<<I as IntoIterator>::Item>,`
1.5.0

Determines if the elements of this `Iterator` are lexicographically greater than those of another. Read more

#### `fn ge<I>(self, other: I) -> bool where
I: IntoIterator,
Self::Item: PartialOrd<<I as IntoIterator>::Item>,`
1.5.0

Determines if the elements of this `Iterator` are lexicographically greater than or equal to those of another. Read more

### `impl<I: DoubleEndedIterator + ?Sized> DoubleEndedIterator for Box<I>`
[src]

#### `fn next_back(&mut self) -> Option<I::Item>`

Removes and returns an element from the end of the iterator. Read more

#### `fn rfind<P>(&mut self, predicate: P) -> Option<Self::Item> where
P: FnMut(&Self::Item) -> bool,`

🔬 This is a nightly-only experimental API. (`iter_rfind` #39480)

Searches for an element of an iterator from the right that satisfies a predicate. Read more

### `impl<I: ExactSizeIterator + ?Sized> ExactSizeIterator for Box<I>`
[src]

#### `fn len(&self) -> usize`

Returns the exact number of times the iterator will iterate. Read more

#### `fn is_empty(&self) -> bool`

🔬 This is a nightly-only experimental API. (`exact_size_is_empty` #35428)

Returns whether the iterator is empty. Read more

### `impl<I: FusedIterator + ?Sized> FusedIterator for Box<I>`
[src]

### `impl<'a, A, R> FnOnce<A> for Box<FnBox<A, Output = R> + 'a>`
[src]

#### `type Output = R`

The returned type after the call operator is used.

#### `extern "rust-call" fn call_once(self, args: A) -> R`

🔬 This is a nightly-only experimental API. (`fn_traits` #29625)

This is called when the call operator is used.

### `impl<'a, A, R> FnOnce<A> for Box<FnBox<A, Output = R> + Send + 'a>`
[src]

#### `type Output = R`

The returned type after the call operator is used.

#### `extern "rust-call" fn call_once(self, args: A) -> R`

🔬 This is a nightly-only experimental API. (`fn_traits` #29625)

This is called when the call operator is used.

### `impl<T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<Box<U>> for Box<T>`
[src]

### `impl<T: Clone> Clone for Box<[T]>`
1.3.0
[src]

#### `fn clone(&self) -> Self`

Returns a copy of the value. Read more

#### `fn clone_from(&mut self, source: &Self)`

Performs copy-assignment from `source`. Read more

### `impl<T: ?Sized> Borrow<T> for Box<T>`
1.1.0
[src]

#### `fn borrow(&self) -> &T`

Immutably borrows from an owned value. Read more

### `impl<T: ?Sized> BorrowMut<T> for Box<T>`
1.1.0
[src]

#### `fn borrow_mut(&mut self) -> &mut T`

Mutably borrows from an owned value. Read more

### `impl<T: ?Sized> AsRef<T> for Box<T>`
1.5.0
[src]

#### `fn as_ref(&self) -> &T`

Performs the conversion.

### `impl<T: ?Sized> AsMut<T> for Box<T>`
1.5.0
[src]

#### `fn as_mut(&mut self) -> &mut T`

Performs the conversion.

### `impl From<String> for Box<str>`
1.20.0
[src]

#### `fn from(s: String) -> Box<str>`

Performs the conversion.

### `impl<T> From<Vec<T>> for Box<[T]>`
1.20.0
[src]

#### `fn from(v: Vec<T>) -> Box<[T]>`

Performs the conversion.

# Help

## Keyboard Shortcuts

?
  Show this help dialog
S
  Focus the search field
⇤
  Move up in search results
⇥
  Move down in search results
⏎
  Go to active search result
+
  Collapse/expand all sections

## Search Tricks

Prefix searches with a type followed by a colon (e.g. `fn:`) to restrict the search to a given type.

Accepted types are: `fn`, `mod`, `struct`, `enum`, `trait`, `type`, `macro`, and `const`.

Search functions by type signature (e.g. `vec -> usize` or `* -> vec`)