Box, stack and heap - Rust By Example
Description: A description

# Box, stack and heap

All values in Rust are stack allocated by default. Values can be boxed (allocated on the heap) by creating a `Box<T>`. A box is a smart pointer to a heap allocated value of type `T`. When a box goes out of scope, its destructor is called, the inner object is destroyed, and the memory on the heap is freed.

Boxed values can be dereferenced using the `*` operator; this removes one layer of indirection.

    use std::mem;

    #[allow(dead_code)]
    #[derive(Debug, Clone, Copy)]
    struct Point {
        x: f64,
        y: f64,
    }

    #[allow(dead_code)]
    struct Rectangle {
        p1: Point,
        p2: Point,
    }

    fn origin() -> Point {
        Point { x: 0.0, y: 0.0 }
    }

    fn boxed_origin() -> Box<Point> {
        // Allocate this point on the heap, and return a pointer to it
        Box::new(Point { x: 0.0, y: 0.0 })
    }

    fn main() {
        // (all the type annotations are superfluous)
        // Stack allocated variables
        let point: Point = origin();
        let rectangle: Rectangle = Rectangle {
            p1: origin(),
            p2: Point { x: 3.0, y: 4.0 }
        };

        // Heap allocated rectangle
        let boxed_rectangle: Box<Rectangle> = Box::new(Rectangle {
            p1: origin(),
            p2: origin()
        });

        // The output of functions can be boxed
        let boxed_point: Box<Point> = Box::new(origin());

        // Double indirection
        let box_in_a_box: Box<Box<Point>> = Box::new(boxed_origin());

        println!("Point occupies {} bytes on the stack",
                 mem::size_of_val(&point));
        println!("Rectangle occupies {} bytes on the stack",
                 mem::size_of_val(&rectangle));

        // box size == pointer size
        println!("Boxed point occupies {} bytes on the stack",
                 mem::size_of_val(&boxed_point));
        println!("Boxed rectangle occupies {} bytes on the stack",
                 mem::size_of_val(&boxed_rectangle));
        println!("Boxed box occupies {} bytes on the stack",
                 mem::size_of_val(&box_in_a_box));

        // Copy the data contained in `boxed_point` into `unboxed_point`
        let unboxed_point: Point = *boxed_point;
        println!("Unboxed point occupies {} bytes on the stack",
                 mem::size_of_val(&unboxed_point));
    }