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,}// 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<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 { top_left: origin(), bottom_right: Point { x: 3.0, y: -4.0 } }; // Heap allocated rectangle let boxed_rectangle: Box<Rectangle> = 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<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));}