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.


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:

enum List<T> {
    Cons(T, Box<List<T>>),

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.



A pointer type for heap allocation.



FnBox is a version of the FnOnce intended for use with boxed closure objects. The idea is that where one would normally store a Box<dyn FnOnce()> in a data structure, you should use Box<dyn FnBox()>. The two traits behave essentially the same, except that a FnBox closure can only be called if it is boxed. (Note that FnBox may be deprecated in the future if Box<dyn FnOnce()> closures become directly usable.)