Rust by Example

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. 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,

}

// A Rectangle can be specified by where its top left and bottom right

// corners are in space

#[allow(dead_code)]

struct Rectangle {

top_left: Point,

bottom_right: Point,

}

fn origin() -> Point {

Point { x: 0.0, y: 0.0 }

}

fn boxed_origin() -> Box {

// 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 {

top_left: origin(),

bottom_right: Point { x: 3.0, y: -4.0 }

};

// Heap allocated rectangle

let boxed_rectangle: Box = Box::new(Rectangle {

top_left: origin(),

bottom_right: Point { x: 3.0, y: -4.0 },

});

// The output of functions can be boxed

let boxed_point: Box = Box::new(origin());

// Double indirection

let box_in_a_box: Box> = 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));

}

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