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

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

    #[lang = "owned_box"]
    pub struct Box<T>(_) where 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.

# Examples

    let five = Box::new(5);Run

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

#### `unsafe fn from_raw(raw: *mut T) -> Box<T>`
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) };Run

#### `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);Run

### `impl Box<Any + 'static>`
[src]

#### `fn downcast<T>(self) -> Result<Box<T>, Box<Any + 'static>> where T: 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));
    }Run

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

#### `fn downcast<T>(self) -> Result<Box<T>, Box<Any + 'static + Send>> where T: Any`

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));
    }Run

## Trait Implementations

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

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

Mutably borrows from an owned value. Read more

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

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

Formats the value using the given formatter.

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

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

The method called to mutably dereference a value

### `impl<T> PartialOrd<Box<T>> for Box<T> where T: PartialOrd<T> + ?Sized`
[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<I> Iterator for Box<I> where I: Iterator + ?Sized`
[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 chain<U>(self, other: U) -> Chain<Self, U::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::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 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::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::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::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::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::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::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::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> FusedIterator for Box<I> where I: FusedIterator + ?Sized`
[src]

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

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

Feeds this value into the state given, updating the hasher as necessary.

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

Feeds a slice of this type into the state provided.

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

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

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

Formats the value using the given formatter.

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

#### `fn from(t: T) -> Box<T>`

Performs the conversion.

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

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

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

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

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

Performs the conversion.

### `impl<T> Clone for Box<T> where T: Clone`
[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();Run

#### `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);Run

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

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

Returns a copy of the value. Read more

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

Performs copy-assignment from `source`. Read more

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

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

Returns a copy of the value. Read more

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

Performs copy-assignment from `source`. Read more

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

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

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

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

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

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

Formats the value using the given formatter.

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

#### `type Target = T`

The resulting type after dereferencing

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

The method called to dereference a value

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

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

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

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

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

Immutably borrows from an owned value. Read more

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

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

Performs the conversion.

### `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> + 'a + Send>`
[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, E: Error + 'a> From<E> for Box<Error + 'a>`
[src]

#### `fn from(err: E) -> Box<Error + 'a>`

Performs the conversion.

### `impl<'a, E: Error + Send + Sync + 'a> From<E> for Box<Error + Send + Sync + 'a>`
[src]

#### `fn from(err: E) -> Box<Error + Send + Sync + 'a>`

Performs the conversion.

### `impl From<String> for Box<Error + Send + Sync>`
[src]

#### `fn from(err: String) -> Box<Error + Send + Sync>`

Performs the conversion.

### `impl From<String> for Box<Error>`
1.7.0
[src]

#### `fn from(str_err: String) -> Box<Error>`

Performs the conversion.

### `impl<'a, 'b> From<&'b str> for Box<Error + Send + Sync + 'a>`
[src]

#### `fn from(err: &'b str) -> Box<Error + Send + Sync + 'a>`

Performs the conversion.

### `impl<'a> From<&'a str> for Box<Error>`
1.7.0
[src]

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

Performs the conversion.

### `impl<T: Error> Error for Box<T>`
1.7.0
[src]

#### `fn description(&self) -> &str`

A short description of the error. Read more

#### `fn cause(&self) -> Option<&Error>`

The lower-level cause of this error, if any. Read more

### `impl<R: Read + ?Sized> Read for Box<R>`
[src]

#### `fn read(&mut self, buf: &mut [u8]) -> Result<usize>`

Pull some bytes from this source into the specified buffer, returning how many bytes were read. Read more

#### `fn read_to_end(&mut self, buf: &mut Vec<u8>) -> Result<usize>`

Read all bytes until EOF in this source, placing them into `buf`. Read more

#### `fn read_to_string(&mut self, buf: &mut String) -> Result<usize>`

Read all bytes until EOF in this source, placing them into `buf`. Read more

#### `fn read_exact(&mut self, buf: &mut [u8]) -> Result<()>`

Read the exact number of bytes required to fill `buf`. Read more

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

Creates a "by reference" adaptor for this instance of `Read`. Read more

#### `fn bytes(self) -> Bytes<Self> where Self: Sized`

Transforms this `Read` instance to an `Iterator` over its bytes. Read more

#### `fn chars(self) -> Chars<Self> where Self: Sized`

🔬 This is a nightly-only experimental API. (`io` #27802)

the semantics of a partial read/write of where errors happen is currently unclear and may change

Transforms this `Read` instance to an `Iterator` over `char`s. Read more

#### `fn chain<R: Read>(self, next: R) -> Chain<Self, R> where Self: Sized`

Creates an adaptor which will chain this stream with another. Read more

#### `fn take(self, limit: u64) -> Take<Self> where Self: Sized`

Creates an adaptor which will read at most `limit` bytes from it. Read more

### `impl<W: Write + ?Sized> Write for Box<W>`
[src]

#### `fn write(&mut self, buf: &[u8]) -> Result<usize>`

Write a buffer into this object, returning how many bytes were written. Read more

#### `fn flush(&mut self) -> Result<()>`

Flush this output stream, ensuring that all intermediately buffered contents reach their destination. Read more

#### `fn write_all(&mut self, buf: &[u8]) -> Result<()>`

Attempts to write an entire buffer into this write. Read more

#### `fn write_fmt(&mut self, fmt: Arguments) -> Result<()>`

Writes a formatted string into this writer, returning any error encountered. Read more

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

Creates a "by reference" adaptor for this instance of `Write`. Read more

### `impl<S: Seek + ?Sized> Seek for Box<S>`
[src]

#### `fn seek(&mut self, pos: SeekFrom) -> Result<u64>`

Seek to an offset, in bytes, in a stream. Read more

### `impl<B: BufRead + ?Sized> BufRead for Box<B>`
[src]

#### `fn fill_buf(&mut self) -> Result<&[u8]>`

Fills the internal buffer of this object, returning the buffer contents. Read more

#### `fn consume(&mut self, amt: usize)`

Tells this buffer that `amt` bytes have been consumed from the buffer, so they should no longer be returned in calls to `read`. Read more

#### `fn read_until(&mut self, byte: u8, buf: &mut Vec<u8>) -> Result<usize>`

Read all bytes into `buf` until the delimiter `byte` or EOF is reached. Read more

#### `fn read_line(&mut self, buf: &mut String) -> Result<usize>`

Read all bytes until a newline (the 0xA byte) is reached, and append them to the provided buffer. Read more

#### `fn split(self, byte: u8) -> Split<Self> where Self: Sized`

Returns an iterator over the contents of this reader split on the byte `byte`. Read more

#### `fn lines(self) -> Lines<Self> where Self: Sized`

Returns an iterator over the lines of this reader. Read more

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