What is the difference between passing a value to a function by reference and passing it by Box?

Question

What is the difference between passing a value to a function by reference and passing it \"by Box\":

fn main() {
    let mut stack_a = 3;
    let mut heap_a          


        

Answer 1

When you pass a boxed value, you are moving the value completely. You no longer own it, the thing you passed it to does. It is so for any type that is not Copy (plain old data that can just be memcpy’d, which a heap allocation certainly can’t be). This is how Rust’s ownership model works: each object is owned in exactly one place.

If you wish to mutate the contents of the box, you should pass in a &mut i32 rather than the whole Box.

Really, Box is only useful for recursive data structures (so that they can be represented rather than being of infinite size) and for the very occasional performance optimisation on large types (which you shouldn’t try doing without measurements).

To get &mut i32 out of a Box, take a mutable reference to the dereferenced box, i.e. &mut *heap_a.