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

#[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]

Important traits for Box<I>
pub fn new(x: T) -> Box<T>[src]

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);

Important traits for Box<I>
pub fn new_uninit() -> Box<MaybeUninit<T>>[src]

🔬 This is a nightly-only experimental API. (new_uninit #63291)

Constructs a new box with uninitialized contents.

Examples

#![feature(new_uninit)]

let mut five = Box::<u32>::new_uninit();

let five = unsafe {
    // Deferred initialization:
    five.as_mut_ptr().write(5);

    five.assume_init()
};

assert_eq!(*five, 5)

pub fn pin(x: T) -> Pin<Box<T>>1.33.0[src]

Constructs a new Pin<Box<T>>. If T does not implement Unpin, then x will be pinned in memory and unable to be moved.

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

Important traits for Box<I>
pub fn new_uninit_slice(len: usize) -> Box<[MaybeUninit<T>]>[src]

🔬 This is a nightly-only experimental API. (new_uninit #63291)

Constructs a new boxed slice with uninitialized contents.

Examples

#![feature(new_uninit)]

let mut values = Box::<[u32]>::new_uninit_slice(3);

let values = unsafe {
    // Deferred initialization:
    values[0].as_mut_ptr().write(1);
    values[1].as_mut_ptr().write(2);
    values[2].as_mut_ptr().write(3);

    values.assume_init()
};

assert_eq!(*values, [1, 2, 3])

impl<T> Box<MaybeUninit<T>>[src]

Important traits for Box<I>
pub unsafe fn assume_init(self) -> Box<T>[src]

🔬 This is a nightly-only experimental API. (new_uninit #63291)

Converts to Box<T>.

Safety

As with MaybeUninit::assume_init, it is up to the caller to guarantee that the value really is in an initialized state. Calling this when the content is not yet fully initialized causes immediate undefined behavior.

Examples

#![feature(new_uninit)]

let mut five = Box::<u32>::new_uninit();

let five: Box<u32> = unsafe {
    // Deferred initialization:
    five.as_mut_ptr().write(5);

    five.assume_init()
};

assert_eq!(*five, 5)

impl<T> Box<[MaybeUninit<T>]>[src]

Important traits for Box<I>
pub unsafe fn assume_init(self) -> Box<[T]>[src]

🔬 This is a nightly-only experimental API. (new_uninit #63291)

Converts to Box<[T]>.

Safety

As with MaybeUninit::assume_init, it is up to the caller to guarantee that the values really are in an initialized state. Calling this when the content is not yet fully initialized causes immediate undefined behavior.

Examples

#![feature(new_uninit)]

let mut values = Box::<[u32]>::new_uninit_slice(3);

let values = unsafe {
    // Deferred initialization:
    values[0].as_mut_ptr().write(1);
    values[1].as_mut_ptr().write(2);
    values[2].as_mut_ptr().write(3);

    values.assume_init()
};

assert_eq!(*values, [1, 2, 3])

impl<T: ?Sized> Box<T>[src]

pub unsafe fn from_raw(raw: *mut T) -> Self1.4.0[src]

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. For this to be safe, the memory must have been allocated in accordance with the memory layout used by Box .

Safety

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

Recreate a Box which was previously converted to a raw pointer using Box::into_raw:

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

Manually create a Box from scratch by using the global allocator:

use std::alloc::{alloc, Layout};

unsafe {
    let ptr = alloc(Layout::new::<i32>()) as *mut i32;
    *ptr = 5;
    let x = Box::from_raw(ptr);
}

pub fn into_raw(b: Box<T>) -> *mut T1.4.0[src]

Consumes the Box, returning a wrapped raw pointer.

The pointer will be properly aligned and non-null.

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, taking into account the memory layout used by Box. The easiest way to do this is to convert the raw pointer back into a Box with the Box::from_raw function, allowing the Box destructor to perform the cleanup.

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

Converting the raw pointer back into a Box with Box::from_raw for automatic cleanup:

let x = Box::new(String::from("Hello"));
let ptr = Box::into_raw(x);
let x = unsafe { Box::from_raw(ptr) };

Manual cleanup by explicitly running the destructor and deallocating the memory:

use std::alloc::{dealloc, Layout};
use std::ptr;

let x = Box::new(String::from("Hello"));
let p = Box::into_raw(x);
unsafe {
    ptr::drop_in_place(p);
    dealloc(p as *mut u8, Layout::new::<String>());
}

pub fn into_raw_non_null(b: Box<T>) -> NonNull<T>[src]

🔬 This is a nightly-only experimental API. (box_into_raw_non_null #47336)

Consumes the Box, returning the wrapped pointer as NonNull<T>.

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 easiest way to do so is to convert the NonNull<T> pointer into a raw pointer and 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_non_null(b) instead of b.into_raw_non_null(). This is so that there is no conflict with a method on the inner type.

Examples

#![feature(box_into_raw_non_null)]

fn main() {
    let x = Box::new(5);
    let ptr = Box::into_raw_non_null(x);

    // Clean up the memory by converting the NonNull pointer back
    // into a Box and letting the Box be dropped.
    let x = unsafe { Box::from_raw(ptr.as_ptr()) };
}

pub fn leak<'a>(b: Box<T>) -> &'a mut T where
    T: 'a, 
1.26.0[src]

Consumes and leaks the Box, returning a mutable reference, &'a mut T. Note that the type T must outlive the chosen lifetime 'a. If the type has only static references, or none at all, then this may be chosen to be 'static.

This function is mainly useful for data that lives for the remainder of the program's life. Dropping the returned reference will cause a memory leak. If this is not acceptable, the reference should first be wrapped with the Box::from_raw function producing a Box. This Box can then be dropped which will properly destroy T and release the allocated memory.

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

Examples

Simple usage:

fn main() {
    let x = Box::new(41);
    let static_ref: &'static mut usize = Box::leak(x);
    *static_ref += 1;
    assert_eq!(*static_ref, 42);
}

Unsized data:

fn main() {
    let x = vec![1, 2, 3].into_boxed_slice();
    let static_ref = Box::leak(x);
    static_ref[0] = 4;
    assert_eq!(*static_ref, [4, 2, 3]);
}

pub fn into_pin(boxed: Box<T>) -> Pin<Box<T>>[src]

🔬 This is a nightly-only experimental API. (box_into_pin #62370)

Converts a Box<T> into a Pin<Box<T>>

This conversion does not allocate on the heap and happens in place.

This is also available via From.

impl Box<dyn Any>[src]

pub fn downcast<T: Any>(self) -> Result<Box<T>, Box<dyn Any>>[src]

Attempt to downcast the box to a concrete type.

Examples

use std::any::Any;

fn print_if_string(value: Box<dyn 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<dyn Any + Send>[src]

pub fn downcast<T: Any>(self) -> Result<Box<T>, Box<dyn Any + Send>>[src]

Attempt to downcast the box to a concrete type.

Examples

use std::any::Any;

fn print_if_string(value: Box<dyn 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: ?Sized> DerefMut for Box<T>[src]

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

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

fn from(t: T) -> Self[src]

Converts a generic type T into a Box<T>

The conversion allocates on the heap and moves t from the stack into it.

Examples

let x = 5;
let boxed = Box::new(5);

assert_eq!(Box::from(x), boxed);

impl<T: ?Sized> From<Box<T>> for Pin<Box<T>>1.33.0[src]

fn from(boxed: Box<T>) -> Self[src]

Converts a Box<T> into a Pin<Box<T>>

This conversion does not allocate on the heap and happens in place.

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

Important traits for Box<I>
fn from(slice: &[T]) -> Box<[T]>[src]

Converts a &[T] into a Box<[T]>

This conversion allocates on the heap and performs a copy of slice.

Examples

// create a &[u8] which will be used to create a Box<[u8]>
let slice: &[u8] = &[104, 101, 108, 108, 111];
let boxed_slice: Box<[u8]> = Box::from(slice);

println!("{:?}", boxed_slice);

impl<'_> From<&'_ str> for Box<str>1.17.0[src]

Important traits for Box<I>
fn from(s: &str) -> Box<str>[src]

Converts a &str into a Box<str>

This conversion allocates on the heap and performs a copy of s.

Examples

let boxed: Box<str> = Box::from("hello");
println!("{}", boxed);

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

fn from(s: Box<str>) -> Self[src]

Converts a Box<str>> into a Box<[u8]>

This conversion does not allocate on the heap and happens in place.

Examples

// create a Box<str> which will be used to create a Box<[u8]>
let boxed: Box<str> = Box::from("hello");
let boxed_str: Box<[u8]> = Box::from(boxed);

// create a &[u8] which will be used to create a Box<[u8]>
let slice: &[u8] = &[104, 101, 108, 108, 111];
let boxed_slice = Box::from(slice);

assert_eq!(boxed_slice, boxed_str);

impl<T: ?Sized> From<Box<T>> for Arc<T>1.21.0[src]

impl<T: ?Sized> From<Box<T>> for Rc<T>1.21.0[src]

impl From<Box<str>> for String1.18.0[src]

fn from(s: Box<str>) -> String[src]

Converts the given boxed str slice to a String. It is notable that the str slice is owned.

Examples

Basic usage:

let s1: String = String::from("hello world");
let s2: Box<str> = s1.into_boxed_str();
let s3: String = String::from(s2);

assert_eq!("hello world", s3)

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

Important traits for Box<I>
fn from(s: String) -> Box<str>[src]

Converts the given String to a boxed str slice that is owned.

Examples

Basic usage:

let s1: String = String::from("hello world");
let s2: Box<str> = Box::from(s1);
let s3: String = String::from(s2);

assert_eq!("hello world", s3)

impl<T> From<Box<[T]>> for Vec<T>1.18.0[src]

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

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

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

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

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

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

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

impl<T: ?Sized + Hasher> Hasher for Box<T>1.22.0[src]

impl<T: ?Sized> Deref for Box<T>[src]

type Target = T

The resulting type after dereferencing.

impl<T: ?Sized> Receiver for Box<T>[src]

impl<T: ?Sized> Drop for Box<T>[src]

impl<A, F: Fn<A> + ?Sized> Fn<A> for Box<F>1.35.0[src]

impl<A, F: FnMut<A> + ?Sized> FnMut<A> for Box<F>1.35.0[src]

impl<A, F: FnOnce<A> + ?Sized> FnOnce<A> for Box<F>1.35.0[src]

type Output = <F as FnOnce<A>>::Output

The returned type after the call operator is used.

impl<G: ?Sized + Generator + Unpin> Generator for Box<G>[src]

type Yield = G::Yield

🔬 This is a nightly-only experimental API. (generator_trait #43122)

The type of value this generator yields. Read more

type Return = G::Return

🔬 This is a nightly-only experimental API. (generator_trait #43122)

The type of value this generator returns. Read more

impl<T: ?Sized> Unpin for Box<T>1.33.0[src]

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

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

impl<T, const N: usize> TryFrom<Box<[T]>> for Box<[T; N]> where
    [T; N]: LengthAtMost32
[src]

type Error = Box<[T]>

The type returned in the event of a conversion error.

impl<I: FusedIterator + ?Sized> FusedIterator for Box<I>1.26.0[src]

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

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

impl<A> FromIterator<A> for Box<[A]>1.32.0[src]

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

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

type Item = I::Item

The type of the elements being iterated over.

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

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

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

impl<T: Clone> Clone for Box<T>[src]

Important traits for Box<I>
fn clone(&self) -> Box<T>[src]

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

Examples

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

// The value is the same
assert_eq!(x, y);

// But they are unique objects
assert_ne!(&*x as *const i32, &*y as *const i32);

fn clone_from(&mut self, source: &Box<T>)[src]

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

Examples

let x = Box::new(5);
let mut y = Box::new(10);
let yp: *const i32 = &*y;

y.clone_from(&x);

// The value is the same
assert_eq!(x, y);

// And no allocation occurred
assert_eq!(yp, &*y);

impl Clone for Box<str>1.3.0[src]

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

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

Important traits for Box<I>
fn default() -> Box<T>[src]

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

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

impl Default for Box<str>1.17.0[src]

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

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

impl<F: ?Sized + Future + Unpin> Future for Box<F>1.36.0[src]

type Output = F::Output

The type of value produced on completion.

Auto Trait Implementations

impl<T: ?Sized> Send for Box<T> where
    T: Send

impl<T: ?Sized> Sync for Box<T> where
    T: Sync

Blanket Implementations

impl<T> ToOwned for T where
    T: Clone
[src]

type Owned = T

The resulting type after obtaining ownership.

impl<T> ToString for T where
    T: Display + ?Sized
[src]

impl<T> From<T> for T[src]

impl<'a, F> Pattern<'a> for F where
    F: FnMut(char) -> bool, 
[src]

type Searcher = CharPredicateSearcher<'a, F>

🔬 This is a nightly-only experimental API. (pattern #27721)

API not fully fleshed out and ready to be stabilized

Associated searcher for this pattern

impl<T, U> TryFrom<U> for T where
    U: Into<T>, 
[src]

type Error = Infallible

The type returned in the event of a conversion error.

impl<T, U> Into<U> for T where
    U: From<T>, 
[src]

impl<T, U> TryInto<U> for T where
    U: TryFrom<T>, 
[src]

type Error = <U as TryFrom<T>>::Error

The type returned in the event of a conversion error.

impl<I> IntoIterator for I where
    I: Iterator
[src]

type Item = <I as Iterator>::Item

The type of the elements being iterated over.

type IntoIter = I

Which kind of iterator are we turning this into?

impl<T> Borrow<T> for T where
    T: ?Sized
[src]

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

impl<T> Any for T where
    T: 'static + ?Sized
[src]