wiki/content/20200929141711-deref_trait.md

date	id	title
2020-09-29	d2d028c8-733f-4cdb-b096-89cf8b3de961	Deref Trait

Introduction

Implementing the Deref trait allows you to customize the behavior of the dereference operator, * (as opposed to the multiplication or glob operator). By implementing Deref in such a way that a smart pointer can be treated like a regular reference, you can write code that operates on references and use that code with smart pointers too:

fn main() {
    let x = 5;
    let y = &x;

    assert_eq!(5, x);
    assert_eq!(5, *y);
}

Box

A Box<T> can also be used:

fn main() {
    let x = 5;
    let y = Box::new(x);

    assert_eq!(5, x);
    assert_eq!(5, *y);
}

Deref Trait

use std::ops::Deref;

impl<T> Deref for MyBox<T> {
    type Target = T;

    fn deref(&self) -> &T {
        &self.0
    }
}

struct MyBox<T>(T);

impl<T> MyBox<T> {
    fn new(x: T) -> MyBox<T> {
        MyBox(x)
    }
}

fn main() {
    let x = 5;
    let y = MyBox::new(x);

    assert_eq!(5, x);
    assert_eq!(5, *y);
}

Deref Coercion

Deref coercion is a convenience that Rust performs on arguments to functions and methods. Deref coercion works only on types that implement the Deref trait. Deref coercion converts such a type into a reference to another type. For example, deref coercion can convert &String to &str because String implements the Deref trait such that it returns str. Deref coercion happens automatically when we pass a reference to a particular type’s value as an argument to a function or method that doesn’t match the parameter type in the function or method definition. A sequence of calls to the deref method converts the type we provided into the type the parameter needs.

use std::ops::Deref;

impl<T> Deref for MyBox<T> {
    type Target = T;

    fn deref(&self) -> &T {
        &self.0
    }
}

struct MyBox<T>(T);

impl<T> MyBox<T> {
    fn new(x: T) -> MyBox<T> {
        MyBox(x)
    }
}

fn hello(name: &str) {
    println!("Hello, {}!", name);
}

fn main() {
    let m = MyBox::new(String::from("Rust"));
    hello(&m);
}