Module collections::boxed [−] [src]

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

Creating a box:

fn main() { let x = Box::new(5); }

let x = Box::new(5);

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); }

#[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:

fn main() { Cons(T, List<T>), }

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.

Structs

Box	A pointer type for heap allocation.
ExchangeHeapSingleton	[Unstable] This the singleton type used solely for `boxed::HEAP`.
IntermediateBox	[Unstable] `IntermediateBox` represents uninitialized backing storage for `Box`.

Constants

HEAP	[Unstable] A value that represents the heap. This is the default place that the `box` keyword allocates into when no place is supplied.

Traits

FnBox

[Unstable]

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<FnOnce()> in a data structure, you should use Box<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<FnOnce()> closures become directly usable.)