Deref Coercions

Motivation

Why does the following work?

struct Point {
    x: i32,
    y: i32
}

fn main() {
    let boxed_p = Box::new(Point { x: 1, y: 2 });
    println!("{}", boxed_p.x);
}

Box doesn't have a field named "x"!

Auto-Dereferencing

Rust automatically dereferences in certain cases. Like everything else, it must be explicitly requested:

• Through a call or field access using the . operator
• By explicitly dereferencing through *
• When borrowing through &
• This sometimes leads to the ugly &*-Pattern

This makes wrapper types very ergonomic and easy to use!

Dereferencing is described by the Deref and DerefMut-Traits.

impl<T> std::ops::Deref for Box<T> {
    type Target = T;

    fn deref(&self) -> &T {
        todo!()
    }
}

This call is introduced when dereferencing is requested.

Important deref behaviours

• String -> &str
• Vec -> &[T]

Functions that don't modify the lengths of a String or a Vector should accept a slice instead. The memory layout is chosen so that this is cost free.

fn print_me(message: &str) { println!("{}", message); }

fn main() {
    print_me("Foo");
    let a_string = String::from("Bar");
    print_me(&a_string);
    print_me(a_string.as_str())
}