wiki/content/20200929141711-deref_trait.md

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

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

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

# Box

A [Box\<T\>](20200929135609-box_t) can also be used:

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

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

# Deref Trait

``` rust
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.

``` rust
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);
}
```