Attempts 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);
    }
}

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

Downcasts the box to a concrete type.

For a safe alternative see downcast.

§Examples

#![feature(downcast_unchecked)]

use std::any::Any;

let x: Box<dyn Any> = Box::new(1_usize);

unsafe {
    assert_eq!(*x.downcast_unchecked::<usize>(), 1);
}

§Safety

The contained value must be of type T. Calling this method with the incorrect type is undefined behavior.

Attempts 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);
    }
}

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

Downcasts the box to a concrete type.

For a safe alternative see downcast.

§Examples

#![feature(downcast_unchecked)]

use std::any::Any;

let x: Box<dyn Any + Send> = Box::new(1_usize);

unsafe {
    assert_eq!(*x.downcast_unchecked::<usize>(), 1);
}

§Safety

The contained value must be of type T. Calling this method with the incorrect type is undefined behavior.

Attempts to downcast the box to a concrete type.

§Examples

use std::any::Any;

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

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

Downcasts the box to a concrete type.

For a safe alternative see downcast.

§Examples

#![feature(downcast_unchecked)]

use std::any::Any;

let x: Box<dyn Any + Send + Sync> = Box::new(1_usize);

unsafe {
    assert_eq!(*x.downcast_unchecked::<usize>(), 1);
}

§Safety

The contained value must be of type T. Calling this method with the incorrect type is undefined behavior.

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

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

§Examples

Constructs a new box with uninitialized contents.

§Examples

let mut five = Box::<u32>::new_uninit();
// Deferred initialization:
five.write(5);
let five = unsafe { five.assume_init() };

assert_eq!(*five, 5)

Constructs a new Box with uninitialized contents, with the memory being filled with 0 bytes.

See MaybeUninit::zeroed for examples of correct and incorrect usage of this method.

§Examples

let zero = Box::<u32>::new_zeroed();
let zero = unsafe { zero.assume_init() };

assert_eq!(*zero, 0)

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

Constructing and pinning of the Box can also be done in two steps: Box::pin(x) does the same as Box::into_pin(Box::new(x)). Consider using into_pin if you already have a Box<T>, or if you want to construct a (pinned) Box in a different way than with Box::new.

Allocates memory on the heap then places x into it, returning an error if the allocation fails

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

§Examples

#![feature(allocator_api)]

let five = Box::try_new(5)?;

Constructs a new box with uninitialized contents on the heap, returning an error if the allocation fails

§Examples

#![feature(allocator_api)]

let mut five = Box::<u32>::try_new_uninit()?;
// Deferred initialization:
five.write(5);
let five = unsafe { five.assume_init() };

assert_eq!(*five, 5);

Constructs a new Box with uninitialized contents, with the memory being filled with 0 bytes on the heap

See MaybeUninit::zeroed for examples of correct and incorrect usage of this method.

§Examples

#![feature(allocator_api)]

let zero = Box::<u32>::try_new_zeroed()?;
let zero = unsafe { zero.assume_init() };

assert_eq!(*zero, 0);

Maps the value in a box, reusing the allocation if possible.

f is called on the value in the box, and the result is returned, also boxed.

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

§Examples

#![feature(smart_pointer_try_map)]

let b = Box::new(7);
let new = Box::map(b, |i| i + 7);
assert_eq!(*new, 14);

Attempts to map the value in a box, reusing the allocation if possible.

f is called on the value in the box, and if the operation succeeds, the result is returned, also boxed.

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

§Examples

#![feature(smart_pointer_try_map)]

let b = Box::new(7);
let new = Box::try_map(b, u32::try_from).unwrap();
assert_eq!(*new, 7);

Allocates memory in the given allocator then places x into it.

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

§Examples

#![feature(allocator_api)]

use std::alloc::System;

let five = Box::new_in(5, System);

Allocates memory in the given allocator then places x into it, returning an error if the allocation fails

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

§Examples

#![feature(allocator_api)]

use std::alloc::System;

let five = Box::try_new_in(5, System)?;

Constructs a new box with uninitialized contents in the provided allocator.

§Examples

#![feature(allocator_api)]

use std::alloc::System;

let mut five = Box::<u32, _>::new_uninit_in(System);
// Deferred initialization:
five.write(5);
let five = unsafe { five.assume_init() };

assert_eq!(*five, 5)

Constructs a new box with uninitialized contents in the provided allocator, returning an error if the allocation fails

§Examples

#![feature(allocator_api)]

use std::alloc::System;

let mut five = Box::<u32, _>::try_new_uninit_in(System)?;
// Deferred initialization:
five.write(5);
let five = unsafe { five.assume_init() };

assert_eq!(*five, 5);

Constructs a new Box with uninitialized contents, with the memory being filled with 0 bytes in the provided allocator.

See MaybeUninit::zeroed for examples of correct and incorrect usage of this method.

§Examples

#![feature(allocator_api)]

use std::alloc::System;

let zero = Box::<u32, _>::new_zeroed_in(System);
let zero = unsafe { zero.assume_init() };

assert_eq!(*zero, 0)

Constructs a new Box with uninitialized contents, with the memory being filled with 0 bytes in the provided allocator, returning an error if the allocation fails,

See MaybeUninit::zeroed for examples of correct and incorrect usage of this method.

§Examples

#![feature(allocator_api)]

use std::alloc::System;

let zero = Box::<u32, _>::try_new_zeroed_in(System)?;
let zero = unsafe { zero.assume_init() };

assert_eq!(*zero, 0);

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

Constructing and pinning of the Box can also be done in two steps: Box::pin_in(x, alloc) does the same as Box::into_pin(Box::new_in(x, alloc)). Consider using into_pin if you already have a Box<T, A>, or if you want to construct a (pinned) Box in a different way than with Box::new_in.

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

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

Consumes the Box, returning the wrapped value.

§Examples

#![feature(box_into_inner)]

let c = Box::new(5);

assert_eq!(Box::into_inner(c), 5);

Consumes the Box without consuming its allocation, returning the wrapped value and a Box to the uninitialized memory where the wrapped value used to live.

This can be used together with write to reuse the allocation for multiple boxed values.

§Examples

#![feature(box_take)]

let c = Box::new(5);

// take the value out of the box
let (value, uninit) = Box::take(c);
assert_eq!(value, 5);

// reuse the box for a second value
let c = Box::write(uninit, 6);
assert_eq!(*c, 6);

Allocates memory on the heap then clones src into it.

This doesn’t actually allocate if src is zero-sized.

§Examples

#![feature(clone_from_ref)]

let hello: Box<str> = Box::clone_from_ref("hello");

Allocates memory on the heap then clones src into it, returning an error if allocation fails.

This doesn’t actually allocate if src is zero-sized.

§Examples

#![feature(clone_from_ref)]
#![feature(allocator_api)]

let hello: Box<str> = Box::try_clone_from_ref("hello")?;

Allocates memory in the given allocator then clones src into it.

This doesn’t actually allocate if src is zero-sized.

§Examples

#![feature(clone_from_ref)]
#![feature(allocator_api)]

use std::alloc::System;

let hello: Box<str, System> = Box::clone_from_ref_in("hello", System);

Allocates memory in the given allocator then clones src into it, returning an error if allocation fails.

This doesn’t actually allocate if src is zero-sized.

§Examples

#![feature(clone_from_ref)]
#![feature(allocator_api)]

use std::alloc::System;

let hello: Box<str, System> = Box::try_clone_from_ref_in("hello", System)?;

Constructs a new boxed slice with uninitialized contents.

§Examples

let mut values = Box::<[u32]>::new_uninit_slice(3);
// Deferred initialization:
values[0].write(1);
values[1].write(2);
values[2].write(3);
let values = unsafe { values.assume_init() };

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

Constructs a new boxed slice with uninitialized contents, with the memory being filled with 0 bytes.

See MaybeUninit::zeroed for examples of correct and incorrect usage of this method.

§Examples

let values = Box::<[u32]>::new_zeroed_slice(3);
let values = unsafe { values.assume_init() };

assert_eq!(*values, [0, 0, 0])

Constructs a new boxed slice with uninitialized contents. Returns an error if the allocation fails.

§Examples

#![feature(allocator_api)]

let mut values = Box::<[u32]>::try_new_uninit_slice(3)?;
// Deferred initialization:
values[0].write(1);
values[1].write(2);
values[2].write(3);
let values = unsafe { values.assume_init() };

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

Constructs a new boxed slice with uninitialized contents, with the memory being filled with 0 bytes. Returns an error if the allocation fails.

See MaybeUninit::zeroed for examples of correct and incorrect usage of this method.

§Examples

#![feature(allocator_api)]

let values = Box::<[u32]>::try_new_zeroed_slice(3)?;
let values = unsafe { values.assume_init() };

assert_eq!(*values, [0, 0, 0]);

Converts the boxed slice into a boxed array.

This operation does not reallocate; the underlying array of the slice is simply reinterpreted as an array type.

If N is not exactly equal to the length of self, then this method returns None.

Constructs a new boxed slice with uninitialized contents in the provided allocator.

§Examples

#![feature(allocator_api)]

use std::alloc::System;

let mut values = Box::<[u32], _>::new_uninit_slice_in(3, System);
// Deferred initialization:
values[0].write(1);
values[1].write(2);
values[2].write(3);
let values = unsafe { values.assume_init() };

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

Constructs a new boxed slice with uninitialized contents in the provided allocator, with the memory being filled with 0 bytes.

See MaybeUninit::zeroed for examples of correct and incorrect usage of this method.

§Examples

#![feature(allocator_api)]

use std::alloc::System;

let values = Box::<[u32], _>::new_zeroed_slice_in(3, System);
let values = unsafe { values.assume_init() };

assert_eq!(*values, [0, 0, 0])

Constructs a new boxed slice with uninitialized contents in the provided allocator. Returns an error if the allocation fails.

§Examples

#![feature(allocator_api)]

use std::alloc::System;

let mut values = Box::<[u32], _>::try_new_uninit_slice_in(3, System)?;
// Deferred initialization:
values[0].write(1);
values[1].write(2);
values[2].write(3);
let values = unsafe { values.assume_init() };

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

Constructs a new boxed slice with uninitialized contents in the provided allocator, with the memory being filled with 0 bytes. Returns an error if the allocation fails.

See MaybeUninit::zeroed for examples of correct and incorrect usage of this method.

§Examples

#![feature(allocator_api)]

use std::alloc::System;

let values = Box::<[u32], _>::try_new_zeroed_slice_in(3, System)?;
let values = unsafe { values.assume_init() };

assert_eq!(*values, [0, 0, 0]);

Converts to Box<T, A>.

§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

let mut five = Box::<u32>::new_uninit();
// Deferred initialization:
five.write(5);
let five: Box<u32> = unsafe { five.assume_init() };

assert_eq!(*five, 5)

Writes the value and converts to Box<T, A>.

This method converts the box similarly to Box::assume_init but writes value into it before conversion thus guaranteeing safety. In some scenarios use of this method may improve performance because the compiler may be able to optimize copying from stack.

§Examples

let big_box = Box::<[usize; 1024]>::new_uninit();

let mut array = [0; 1024];
for (i, place) in array.iter_mut().enumerate() {
    *place = i;
}

// The optimizer may be able to elide this copy, so previous code writes
// to heap directly.
let big_box = Box::write(big_box, array);

for (i, x) in big_box.iter().enumerate() {
    assert_eq!(*x, i);
}

Converts to Box<[T], A>.

§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

let mut values = Box::<[u32]>::new_uninit_slice(3);
// Deferred initialization:
values[0].write(1);
values[1].write(2);
values[2].write(3);
let values = unsafe { values.assume_init() };

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

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.

The raw pointer must point to a block of memory allocated by the global allocator.

The safety conditions are described in the memory layout section.

§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;
    // In general .write is required to avoid attempting to destruct
    // the (uninitialized) previous contents of `ptr`, though for this
    // simple example `*ptr = 5` would have worked as well.
    ptr.write(5);
    let x = Box::from_raw(ptr);
}

Constructs a box from a NonNull pointer.

After calling this function, the NonNull 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 NonNull pointer.

The non-null pointer must point to a block of memory allocated by the global allocator.

The safety conditions are described in the memory layout section.

§Examples

Recreate a Box which was previously converted to a NonNull pointer using Box::into_non_null:

#![feature(box_vec_non_null)]

let x = Box::new(5);
let non_null = Box::into_non_null(x);
let x = unsafe { Box::from_non_null(non_null) };

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

#![feature(box_vec_non_null)]

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

unsafe {
    let non_null = NonNull::new(alloc(Layout::new::<i32>()).cast::<i32>())
        .expect("allocation failed");
    // In general .write is required to avoid attempting to destruct
    // the (uninitialized) previous contents of `non_null`.
    non_null.write(5);
    let x = Box::from_non_null(non_null);
}

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

Note: This is equivalent to the following:

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

Consumes the Box, returning a wrapped NonNull pointer.

The pointer will be properly aligned.

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 NonNull pointer back into a Box with the Box::from_non_null 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_non_null(b) instead of b.into_non_null(). This is so that there is no conflict with a method on the inner type.

§Examples

Converting the NonNull pointer back into a Box with Box::from_non_null for automatic cleanup:

#![feature(box_vec_non_null)]

let x = Box::new(String::from("Hello"));
let non_null = Box::into_non_null(x);
let x = unsafe { Box::from_non_null(non_null) };

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

#![feature(box_vec_non_null)]

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

let x = Box::new(String::from("Hello"));
let non_null = Box::into_non_null(x);
unsafe {
    non_null.drop_in_place();
    dealloc(non_null.as_ptr().cast::<u8>(), Layout::new::<String>());
}

Note: This is equivalent to the following:

#![feature(box_vec_non_null)]

let x = Box::new(String::from("Hello"));
let non_null = Box::into_non_null(x);
unsafe {
    drop(Box::from_non_null(non_null));
}

Constructs a box from a raw pointer in the given allocator.

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.

The raw pointer must point to a block of memory allocated by alloc.

§Examples

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

#![feature(allocator_api)]

use std::alloc::System;

let x = Box::new_in(5, System);
let (ptr, alloc) = Box::into_raw_with_allocator(x);
let x = unsafe { Box::from_raw_in(ptr, alloc) };

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

#![feature(allocator_api, slice_ptr_get)]

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

unsafe {
    let ptr = System.allocate(Layout::new::<i32>())?.as_mut_ptr() as *mut i32;
    // In general .write is required to avoid attempting to destruct
    // the (uninitialized) previous contents of `ptr`, though for this
    // simple example `*ptr = 5` would have worked as well.
    ptr.write(5);
    let x = Box::from_raw_in(ptr, System);
}

Constructs a box from a NonNull pointer in the given allocator.

After calling this function, the NonNull 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.

The non-null pointer must point to a block of memory allocated by alloc.

§Examples

Recreate a Box which was previously converted to a NonNull pointer using Box::into_non_null_with_allocator:

#![feature(allocator_api, box_vec_non_null)]

use std::alloc::System;

let x = Box::new_in(5, System);
let (non_null, alloc) = Box::into_non_null_with_allocator(x);
let x = unsafe { Box::from_non_null_in(non_null, alloc) };

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

#![feature(allocator_api, box_vec_non_null, slice_ptr_get)]

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

unsafe {
    let non_null = System.allocate(Layout::new::<i32>())?.cast::<i32>();
    // In general .write is required to avoid attempting to destruct
    // the (uninitialized) previous contents of `non_null`.
    non_null.write(5);
    let x = Box::from_non_null_in(non_null, System);
}

Consumes the Box, returning a wrapped raw pointer and the allocator.

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_in 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_with_allocator(b) instead of b.into_raw_with_allocator(). 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_in for automatic cleanup:

#![feature(allocator_api)]

use std::alloc::System;

let x = Box::new_in(String::from("Hello"), System);
let (ptr, alloc) = Box::into_raw_with_allocator(x);
let x = unsafe { Box::from_raw_in(ptr, alloc) };

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

#![feature(allocator_api)]

use std::alloc::{Allocator, Layout, System};
use std::ptr::{self, NonNull};

let x = Box::new_in(String::from("Hello"), System);
let (ptr, alloc) = Box::into_raw_with_allocator(x);
unsafe {
    ptr::drop_in_place(ptr);
    let non_null = NonNull::new_unchecked(ptr);
    alloc.deallocate(non_null.cast(), Layout::new::<String>());
}

Consumes the Box, returning a wrapped NonNull pointer and the allocator.

The pointer will be properly aligned.

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 NonNull pointer back into a Box with the Box::from_non_null_in 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_non_null_with_allocator(b) instead of b.into_non_null_with_allocator(). This is so that there is no conflict with a method on the inner type.

§Examples

Converting the NonNull pointer back into a Box with Box::from_non_null_in for automatic cleanup:

#![feature(allocator_api, box_vec_non_null)]

use std::alloc::System;

let x = Box::new_in(String::from("Hello"), System);
let (non_null, alloc) = Box::into_non_null_with_allocator(x);
let x = unsafe { Box::from_non_null_in(non_null, alloc) };

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

#![feature(allocator_api, box_vec_non_null)]

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

let x = Box::new_in(String::from("Hello"), System);
let (non_null, alloc) = Box::into_non_null_with_allocator(x);
unsafe {
    non_null.drop_in_place();
    alloc.deallocate(non_null.cast::<u8>(), Layout::new::<String>());
}

Returns a raw mutable pointer to the Box’s contents.

The caller must ensure that the Box outlives the pointer this function returns, or else it will end up dangling.

This method guarantees that for the purpose of the aliasing model, this method does not materialize a reference to the underlying memory, and thus the returned pointer will remain valid when mixed with other calls to as_ptr and as_mut_ptr. Note that calling other methods that materialize references to the memory may still invalidate this pointer. See the example below for how this guarantee can be used.

§Examples

Due to the aliasing guarantee, the following code is legal:

#![feature(box_as_ptr)]

unsafe {
    let mut b = Box::new(0);
    let ptr1 = Box::as_mut_ptr(&mut b);
    ptr1.write(1);
    let ptr2 = Box::as_mut_ptr(&mut b);
    ptr2.write(2);
    // Notably, the write to `ptr2` did *not* invalidate `ptr1`:
    ptr1.write(3);
}

Returns a raw pointer to the Box’s contents.

The caller must ensure that the Box outlives the pointer this function returns, or else it will end up dangling.

The caller must also ensure that the memory the pointer (non-transitively) points to is never written to (except inside an UnsafeCell) using this pointer or any pointer derived from it. If you need to mutate the contents of the Box, use as_mut_ptr.

This method guarantees that for the purpose of the aliasing model, this method does not materialize a reference to the underlying memory, and thus the returned pointer will remain valid when mixed with other calls to as_ptr and as_mut_ptr. Note that calling other methods that materialize mutable references to the memory, as well as writing to this memory, may still invalidate this pointer. See the example below for how this guarantee can be used.

§Examples

Due to the aliasing guarantee, the following code is legal:

#![feature(box_as_ptr)]

unsafe {
    let mut v = Box::new(0);
    let ptr1 = Box::as_ptr(&v);
    let ptr2 = Box::as_mut_ptr(&mut v);
    let _val = ptr2.read();
    // No write to this memory has happened yet, so `ptr1` is still valid.
    let _val = ptr1.read();
    // However, once we do a write...
    ptr2.write(1);
    // ... `ptr1` is no longer valid.
    // This would be UB: let _val = ptr1.read();
}

Returns a reference to the underlying allocator.

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

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:

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

Unsized data:

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]);

Converts a Box<T> into a Pin<Box<T>>. If T does not implement Unpin, then *boxed will be pinned in memory and unable to be moved.

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

This is also available via From.

Constructing and pinning a Box with Box::into_pin(Box::new(x)) can also be written more concisely using Box::pin(x). This into_pin method is useful if you already have a Box<T>, or you are constructing a (pinned) Box in a different way than with Box::new.

§Notes

It’s not recommended that crates add an impl like From<Box<T>> for Pin<T>, as it’ll introduce an ambiguity when calling Pin::from. A demonstration of such a poor impl is shown below.

struct Foo; // A type defined in this crate.
impl From<Box<()>> for Pin<Foo> {
    fn from(_: Box<()>) -> Pin<Foo> {
        Pin::new(Foo)
    }
}

let foo = Box::new(());
let bar = Pin::from(foo);

Copies source’s contents into self without creating a new allocation, so long as the two are of the same length.

§Examples

let x = Box::new([5, 6, 7]);
let mut y = Box::new([8, 9, 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);

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);

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);

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

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

§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:?}");

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

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

§Examples

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

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

Converts a &mut str into a Box<str>

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

§Examples

let mut original = String::from("hello");
let original: &mut str = &mut original;
let boxed: Box<str> = Box::from(original);
println!("{boxed}");

Converts a str into a box of dyn Error.

§Examples

use std::error::Error;

let a_str_error = "a str error";
let a_boxed_error = Box::<dyn Error>::from(a_str_error);
assert!(size_of::<Box<dyn Error>>() == size_of_val(&a_boxed_error))

Converts a str into a box of dyn Error + Send + Sync.

§Examples

use std::error::Error;

let a_str_error = "a str error";
let a_boxed_error = Box::<dyn Error + Send + Sync>::from(a_str_error);
assert!(
    size_of::<Box<dyn Error + Send + Sync>>() == size_of_val(&a_boxed_error))

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}");

Converts a [T; N] into a Box<[T]>

This conversion moves the array to newly heap-allocated memory.

§Examples

let boxed: Box<[u8]> = Box::from([4, 2]);
println!("{boxed:?}");

Converts a boxed slice into a vector by transferring ownership of the existing heap allocation.

§Examples

let b: Box<[i32]> = vec![1, 2, 3].into_boxed_slice();
assert_eq!(Vec::from(b), vec![1, 2, 3]);

Move a boxed object to a new, reference-counted allocation.

§Example

let unique: Box<str> = Box::from("eggplant");
let shared: Arc<str> = Arc::from(unique);
assert_eq!("eggplant", &shared[..]);

Converts a Box<T> into a Pin<Box<T>>. If T does not implement Unpin, then *boxed will be pinned in memory and unable to be moved.

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

This is also available via Box::into_pin.

Constructing and pinning a Box with <Pin<Box<T>>>::from(Box::new(x)) can also be written more concisely using Box::pin(x). This From implementation is useful if you already have a Box<T>, or you are constructing a (pinned) Box in a different way than with Box::new.

Move a boxed object to a new, reference counted, allocation.

§Example

let original: Box<i32> = Box::new(1);
let shared: Rc<i32> = Rc::from(original);
assert_eq!(1, *shared);

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

§Examples

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)

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);

Converts a Cow<'_, [T]> into a Box<[T]>

When cow is the Cow::Borrowed variant, this conversion allocates on the heap and copies the underlying slice. Otherwise, it will try to reuse the owned Vec’s allocation.

Converts a Cow<'_, str> into a Box<str>

When cow is the Cow::Borrowed variant, this conversion allocates on the heap and copies the underlying str. Otherwise, it will try to reuse the owned String’s allocation.

§Examples

use std::borrow::Cow;

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

let unboxed = Cow::Owned("hello".to_string());
let boxed: Box<str> = Box::from(unboxed);
println!("{boxed}");

Converts a Cow into a box of dyn Error.

§Examples

use std::error::Error;
use std::borrow::Cow;

let a_cow_str_error = Cow::from("a str error");
let a_boxed_error = Box::<dyn Error>::from(a_cow_str_error);
assert!(size_of::<Box<dyn Error>>() == size_of_val(&a_boxed_error))

Converts a Cow into a box of dyn Error + Send + Sync.

§Examples

use std::error::Error;
use std::borrow::Cow;

let a_cow_str_error = Cow::from("a str error");
let a_boxed_error = Box::<dyn Error + Send + Sync>::from(a_cow_str_error);
assert!(
    size_of::<Box<dyn Error + Send + Sync>>() == size_of_val(&a_boxed_error))

Converts a type of Error into a box of dyn Error.

§Examples

use std::error::Error;
use std::fmt;

#[derive(Debug)]
struct AnError;

impl fmt::Display for AnError {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        write!(f, "An error")
    }
}

impl Error for AnError {}

let an_error = AnError;
assert!(0 == size_of_val(&an_error));
let a_boxed_error = Box::<dyn Error>::from(an_error);
assert!(size_of::<Box<dyn Error>>() == size_of_val(&a_boxed_error))

Converts a type of Error + Send + Sync into a box of dyn Error + Send + Sync.

§Examples

use std::error::Error;
use std::fmt;

#[derive(Debug)]
struct AnError;

impl fmt::Display for AnError {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        write!(f, "An error")
    }
}

impl Error for AnError {}

unsafe impl Send for AnError {}

unsafe impl Sync for AnError {}

let an_error = AnError;
assert!(0 == size_of_val(&an_error));
let a_boxed_error = Box::<dyn Error + Send + Sync>::from(an_error);
assert!(
    size_of::<Box<dyn Error + Send + Sync>>() == size_of_val(&a_boxed_error))

Converts a String into a box of dyn Error.

§Examples

use std::error::Error;

let a_string_error = "a string error".to_string();
let a_boxed_error = Box::<dyn Error>::from(a_string_error);
assert!(size_of::<Box<dyn Error>>() == size_of_val(&a_boxed_error))

Converts a String into a box of dyn Error + Send + Sync.

§Examples

use std::error::Error;

let a_string_error = "a string error".to_string();
let a_boxed_error = Box::<dyn Error + Send + Sync>::from(a_string_error);
assert!(
    size_of::<Box<dyn Error + Send + Sync>>() == size_of_val(&a_boxed_error))

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

§Examples

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)

Converts a 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);

Converts a vector into a boxed slice.

Before doing the conversion, this method discards excess capacity like Vec::shrink_to_fit.

§Examples

assert_eq!(Box::from(vec![1, 2, 3]), vec![1, 2, 3].into_boxed_slice());

Any excess capacity is removed:

let mut vec = Vec::with_capacity(10);
vec.extend([1, 2, 3]);

assert_eq!(Box::from(vec), vec![1, 2, 3].into_boxed_slice());

Attempts to convert a Box<[T]> into a Box<[T; N]>.

The conversion occurs in-place and does not require a new memory allocation.

§Errors

Returns the old Box<[T]> in the Err variant if boxed_slice.len() does not equal N.

Attempts to convert a Vec<T> into a Box<[T; N]>.

Like Vec::into_boxed_slice, this is in-place if vec.capacity() == N, but will require a reallocation otherwise.

§Errors

Returns the original Vec<T> in the Err variant if boxed_slice.len() does not equal N.

§Examples

This can be used with vec! to create an array on the heap:

let state: Box<[f32; 100]> = vec![1.0; 100].try_into().unwrap();
assert_eq!(state.len(), 100);

Calls U::from(self).

That is, this conversion is whatever the implementation of From<T> for U chooses to do.