A pointer type for heap allocation.

`Box<T>`

, casually referred to as a 'box', provides the simplest form of heap allocation in Rust. Boxes provide ownership for this allocation, and drop their contents when they go out of scope.

Move a value from the stack to the heap by creating a `Box`

:

let val: u8 = 5; let boxed: Box<u8> = Box::new(val);

Move a value from a `Box`

back to the stack by dereferencing:

let boxed: Box<u8> = Box::new(5); let val: u8 = *boxed;

Creating a recursive data structure:

#[derive(Debug)] enum List<T> { Cons(T, Box<List<T>>), Nil, } fn main() { let list: List<i32> = List::Cons(1, Box::new(List::Cons(2, Box::new(List::Nil)))); println!("{:?}", list); }

This will print `Cons(1, Cons(2, Nil))`

.

Recursive structures must be boxed, because if the definition of `Cons`

looked like this:

ⓘThis example deliberately fails to compile

Cons(T, List<T>),

It wouldn't work. This is because the size of a `List`

depends on how many elements are in the list, and so we don't know how much memory to allocate for a `Cons`

. By introducing a `Box`

, which has a defined size, we know how big `Cons`

needs to be.

For non-zero-sized values, a `Box`

will use the `Global`

allocator for its allocation. It is valid to convert both ways between a `Box`

and a raw pointer allocated with the `Global`

allocator, given that the `Layout`

used with the allocator is correct for the type. More precisely, a `value: *mut T`

that has been allocated with the `Global`

allocator with `Layout::for_value(&*value)`

may be converted into a box using `Box::<T>::from_raw(value)`

. Conversely, the memory backing a `value: *mut T`

obtained from `Box::<T>::into_raw`

may be deallocated using the `Global`

allocator with `Layout::for_value(&*value)`

.

Box |
A pointer type for heap allocation. |

© 2010 The Rust Project Developers

Licensed under the Apache License, Version 2.0 or the MIT license, at your option.

https://doc.rust-lang.org/std/boxed/index.html