Box, stack and heap
All values in Rust are stack allocated by default. Values can be boxed
(allocated in 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 in 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 in 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 in the stack",
mem::size_of_val(&point));
println!("Rectangle occupies {} bytes in the stack",
mem::size_of_val(&rectangle));
// box size = pointer size
println!("Boxed point occupies {} bytes in the stack",
mem::size_of_val(&boxed_point));
println!("Boxed rectangle occupies {} bytes in the stack",
mem::size_of_val(&boxed_rectangle));
println!("Boxed box occupies {} bytes in 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 in the stack",
mem::size_of_val(&unboxed_point));
}