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 nonzerosized 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. 
Traits
FnBox  Deprecation plannedExperimental
