The `Deref` Trait Allows Access to the Data Through a Reference - The Rust Programming Language

## The `Deref` Trait Allows Access to the Data Through a Reference

The first important smart pointer-related trait is `Deref`, which allows us to override `*`, the dereference operator (as opposed to the multiplication operator or the glob operator). Overriding `*` for smart pointers makes accessing the data behind the smart pointer convenient, and we’ll talk about what we mean by convenient when we get to deref coercions later in this section.

We briefly mentioned the dereference operator in Chapter 8, in the hash map section titled “Update a Value Based on the Old Value”. We had a mutable reference, and we wanted to change the value that the reference was pointing to. In order to do that, first we had to dereference the reference. Here’s another example using references to `i32` values:

    # #![allow(unused_variables)]
    #fn main() {
    let mut x = 5;
    {
        let y = &mut x;

        *y += 1
    }

    assert_eq!(6, x);

    #}

We use `*y` to access the data that the mutable reference in `y` refers to, rather than the mutable reference itself. We can then modify that data, in this case by adding 1.

With references that aren’t smart pointers, there’s only one value that the reference is pointing to, so the dereference operation is straightforward. Smart pointers can also store metadata about the pointer or the data. When dereferencing a smart pointer, we only want the data, not the metadata, since dereferencing a regular reference only gives us data and not metadata. We want to be able to use smart pointers in the same places that we can use regular references. To enable that, we can override the behavior of the `*` operator by implementing the `Deref` trait.

Listing 15-7 has an example of overriding `*` using `Deref` on a struct we’ve defined to hold mp3 data and metadata. `Mp3` is, in a sense, a smart pointer: it owns the `Vec<u8>` data containing the audio. In addition, it holds some optional metadata, in this case the artist and title of the song in the audio data. We want to be able to conveniently access the audio data, not the metadata, so we implement the `Deref` trait to return the audio data. Implementing the `Deref` trait requires implementing one method named `deref` that borrows `self` and returns the inner data:

Filename: src/main.rs

    use std::ops::Deref;

    struct Mp3 {
        audio: Vec<u8>,
        artist: Option<String>,
        title: Option<String>,
    }

    impl Deref for Mp3 {
        type Target = Vec<u8>;

        fn deref(&self) -> &Vec<u8> {
            &self.audio
        }
    }

    fn main() {
        let my_favorite_song = Mp3 {
            // we would read the actual audio data from an mp3 file
            audio: vec![1, 2, 3],
            artist: Some(String::from("Nirvana")),
            title: Some(String::from("Smells Like Teen Spirit")),
        };

        assert_eq!(vec![1, 2, 3], *my_favorite_song);
    }

Listing 15-7: An implementation of the `Deref` trait on a struct that holds mp3 file data and metadata

Most of this should look familiar: a struct, a trait implementation, and a main function that creates an instance of the struct. There is one part we haven’t explained thoroughly yet: similarly to Chapter 13 when we looked at the Iterator trait with the `type Item`, the `type Target = T;` syntax is defining an associated type, which is covered in more detail in Chapter 19. Don’t worry about that part of the example too much; it is a slightly different way of declaring a generic parameter.

In the `assert_eq!`, we’re verifying that `vec![1, 2, 3]` is the result we get when dereferencing the `Mp3` instance with `*my_favorite_song`, which is what happens since we implemented the `deref` method to return the audio data. If we hadn’t implemented the `Deref` trait for `Mp3`, Rust wouldn’t compile the code `*my_favorite_song`: we’d get an error saying type `Mp3` cannot be dereferenced.

Without the `Deref` trait, the compiler can only dereference `&` references, which `my_favorite_song` is not (it is an `Mp3` struct). With the `Deref` trait, the compiler knows that types implementing the `Deref` trait have a `deref` method that returns a reference (in this case, `&self.audio` because of our definition of `deref` in Listing 15-7). So in order to get a `&` reference that `*` can dereference, the compiler expands `*my_favorite_song` to this:

    *(my_favorite_song.deref())

The result is the value in `self.audio`. The reason `deref` returns a reference that we then have to dereference, rather than just returning a value directly, is because of ownership: if the `deref` method directly returned the value instead of a reference to it, the value would be moved out of `self`. We don’t want to take ownership of `my_favorite_song.audio` in this case and most cases where we use the dereference operator.

Note that replacing `*` with a call to the `deref` method and then a call to `*` happens once, each time the `*` is used. The substitution of `*` does not recurse infinitely. That’s how we end up with data of type `Vec<u8>`, which matches the `vec![1, 2, 3]` in the `assert_eq!` in Listing 15-7.

### Implicit Deref Coercions with Functions and Methods

Rust tends to favor explicitness over implicitness, but one case where this does not hold true is deref coercions of arguments to functions and methods. A deref coercion will automatically convert a reference to any pointer into a reference to that pointer’s contents. A deref coercion happens when the reference type of the argument passed into the function differs from the reference type of the parameter defined in that function’s signature. Deref coercion was added to Rust to make calling functions and methods not need as many explicit references and dereferences with `&` and `*`.

Using our `Mp3` struct from Listing 15-7, here’s the signature of a function to compress mp3 audio data that takes a slice of `u8`:

    fn compress_mp3(audio: &[u8]) -> Vec<u8> {
        // the actual implementation would go here
    }

If Rust didn’t have deref coercion, in order to call this function with the audio data in `my_favorite_song`, we’d have to write:

    compress_mp3(my_favorite_song.audio.as_slice())

That is, we’d have to explicitly say that we want the data in the `audio` field of `my_favorite_song` and that we want a slice referring to the whole `Vec<u8>`. If there were a lot of places where we’d want to process the `audio` data in a similar manner, `.audio.as_slice()` would be wordy and repetitive.

However, because of deref coercion and our implementation of the `Deref` trait on `Mp3`, we can call this function with the data in `my_favorite_song` by using this code:

    let result = compress_mp3(&my_favorite_song);

Just an `&` and the instance, nice! We can treat our smart pointer as if it was a regular reference. Deref coercion means that Rust can use its knowledge of our `Deref` implementation, namely: Rust knows that `Mp3` implements the `Deref` trait and returns `&Vec<u8>` from the `deref` method. Rust also knows the standard library implements the `Deref` trait on `Vec<T>` to return `&[T]` from the `deref` method (and we can find that out too by looking at the API documentation for `Vec<T>`). So, at compile time, Rust will see that it can use `Deref::deref` twice to turn `&Mp3` into `&Vec<u8>` and then into `&[T]` to match the signature of `compress_mp3`. That means we get to do less typing! Rust will analyze types through `Deref::deref` as many times as it needs to in order to get a reference to match the parameter’s type, when the `Deref` trait is defined for the types involved. This indirection is resolved at compile time, so there is no run-time penalty for taking advantage of deref coercion!

Similar to how we use the `Deref` trait to override `*` on `&T`s, there is also a `DerefMut` trait for overriding `*` on `&mut T`.

Rust does deref coercion when it finds types and trait implementations in three cases:

- From `&T` to `&U` when `T: Deref<Target=U>`.
- From `&mut T` to `&mut U` when `T: DerefMut<Target=U>`.
- From `&mut T` to `&U` when `T: Deref<Target=U>`.

The first two are the same, except for mutability: if you have a `&T`, and `T` implements `Deref` to some type `U`, you can get a `&U` transparently. Same for mutable references. The last one is more tricky: if you have a mutable reference, it will also coerce to an immutable one. The other case is not possible though: immutable references will never coerce to mutable ones.

The reason that the `Deref` trait is important to the smart pointer pattern is that smart pointers can then be treated like regular references and used in places that expect regular references. We don’t have to redefine methods and functions to take smart pointers explicitly, for example.