# 1.0.0[−][src]Module alloc::boxed

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.

# Examples

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:

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.

# Memory layout

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)`

.

## Structs

Box | A pointer type for heap allocation. |