src/ch15-02-deref.md

## Treating Smart Pointers Like Regular References with `Deref`

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Implementing the `Deref` trait allows you to customize the behavior of the
_dereference operator_ `*` (not to be confused with 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.

Let’s first look at how the dereference operator works with regular references.
Then we’ll try to define a custom type that behaves like `Box<T>`, and see why
the dereference operator doesn’t work like a reference on our newly defined
type. We’ll explore how implementing the `Deref` trait makes it possible for
smart pointers to work in ways similar to references. Then we’ll look at
Rust’s _deref coercion_ feature and how it lets us work with either references
or smart pointers.

> Note: There’s one big difference between the `MyBox<T>` type we’re about to
> build and the real `Box<T>`: our version will not store its data on the heap.
> We are focusing this example on `Deref`, so where the data is actually stored
> is less important than the pointer-like behavior.

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### Following the Pointer to the Value

A regular reference is a type of pointer, and one way to think of a pointer is
as an arrow to a value stored somewhere else. In Listing 15-6, we create a
reference to an `i32` value and then use the dereference operator to follow the
reference to the value.

<Listing number="15-6" file-name="src/main.rs" caption="Using the dereference operator to follow a reference to an `i32` value">

```rust
{{#rustdoc_include ../listings/ch15-smart-pointers/listing-15-06/src/main.rs}}
```

</Listing>

The variable `x` holds an `i32` value `5`. We set `y` equal to a reference to
`x`. We can assert that `x` is equal to `5`. However, if we want to make an
assertion about the value in `y`, we have to use `*y` to follow the reference
to the value it’s pointing to (hence _dereference_) so the compiler can compare
the actual value. Once we dereference `y`, we have access to the integer value
`y` is pointing to that we can compare with `5`.

If we tried to write `assert_eq!(5, y);` instead, we would get this compilation
error:

```console
{{#include ../listings/ch15-smart-pointers/output-only-01-comparing-to-reference/output.txt}}
```

Comparing a number and a reference to a number isn’t allowed because they’re
different types. We must use the dereference operator to follow the reference
to the value it’s pointing to.

### Using `Box<T>` Like a Reference

We can rewrite the code in Listing 15-6 to use a `Box<T>` instead of a
reference; the dereference operator used on the `Box<T>` in Listing 15-7
functions in the same way as the dereference operator used on the reference in
Listing 15-6:

<Listing number="15-7" file-name="src/main.rs" caption="Using the dereference operator on a `Box<i32>`">

```rust
{{#rustdoc_include ../listings/ch15-smart-pointers/listing-15-07/src/main.rs}}
```

</Listing>

The main difference between Listing 15-7 and Listing 15-6 is that here we set
`y` to be an instance of a box pointing to a copied value of `x` rather than a
reference pointing to the value of `x`. In the last assertion, we can use the
dereference operator to follow the box’s pointer in the same way that we did
when `y` was a reference. Next, we’ll explore what is special about `Box<T>`
that enables us to use the dereference operator by defining our own type.

### Defining Our Own Smart Pointer

Let’s build a smart pointer similar to the `Box<T>` type provided by the
standard library to experience how smart pointers behave differently from
references by default. Then we’ll look at how to add the ability to use the
dereference operator.

The `Box<T>` type is ultimately defined as a tuple struct with one element, so
Listing 15-8 defines a `MyBox<T>` type in the same way. We’ll also define a
`new` function to match the `new` function defined on `Box<T>`.

<Listing number="15-8" file-name="src/main.rs" caption="Defining a `MyBox<T>` type">

```rust
{{#rustdoc_include ../listings/ch15-smart-pointers/listing-15-08/src/main.rs:here}}
```

</Listing>

We define a struct named `MyBox` and declare a generic parameter `T` because
we want our type to hold values of any type. The `MyBox` type is a tuple struct
with one element of type `T`. The `MyBox::new` function takes one parameter of
type `T` and returns a `MyBox` instance that holds the value passed in.

Let’s try adding the `main` function in Listing 15-7 to Listing 15-8 and
changing it to use the `MyBox<T>` type we’ve defined instead of `Box<T>`. The
code in Listing 15-9 won’t compile because Rust doesn’t know how to dereference
`MyBox`.

<Listing number="15-9" file-name="src/main.rs" caption="Attempting to use `MyBox<T>` in the same way we used references and `Box<T>`">

```rust,ignore,does_not_compile
{{#rustdoc_include ../listings/ch15-smart-pointers/listing-15-09/src/main.rs:here}}
```

</Listing>

Here’s the resultant compilation error:

```console
{{#include ../listings/ch15-smart-pointers/listing-15-09/output.txt}}
```

Our `MyBox<T>` type can’t be dereferenced because we haven’t implemented that
ability on our type. To enable dereferencing with the `*` operator, we
implement the `Deref` trait.

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### Implementing the `Deref` Trait

As discussed in [“Implementing a Trait on a Type”][impl-trait]<!-- ignore --> in
Chapter 10, to implement a trait, we need to provide implementations for the
trait’s required methods. The `Deref` trait, provided by the standard library,
requires us to implement one method named `deref` that borrows `self` and
returns a reference to the inner data. Listing 15-10 contains an implementation
of `Deref` to add to the definition of `MyBox<T>`.

<Listing number="15-10" file-name="src/main.rs" caption="Implementing `Deref` on `MyBox<T>`">

```rust
{{#rustdoc_include ../listings/ch15-smart-pointers/listing-15-10/src/main.rs:here}}
```

</Listing>

The `type Target = T;` syntax defines an associated type for the `Deref`
trait to use. Associated types are a slightly different way of declaring a
generic parameter, but you don’t need to worry about them for now; we’ll cover
them in more detail in Chapter 20.

We fill in the body of the `deref` method with `&self.0` so `deref` returns a
reference to the value we want to access with the `*` operator; recall from
[“Using Tuple Structs Without Named Fields to Create Different
Types”][tuple-structs]<!-- ignore --> in Chapter 5 that `.0` accesses the first
value in a tuple struct. The `main` function in Listing 15-9 that calls `*` on
the `MyBox<T>` value now compiles, and the assertions pass!

Without the `Deref` trait, the compiler can only dereference `&` references.
The `deref` method gives the compiler the ability to take a value of any type
that implements `Deref` and call the `deref` method to get an `&` reference that
it knows how to dereference.

When we entered `*y` in Listing 15-9, behind the scenes Rust actually ran this
code:

```rust,ignore
*(y.deref())
```

Rust substitutes the `*` operator with a call to the `deref` method and then a
plain dereference so we don’t have to think about whether or not we need to
call the `deref` method. This Rust feature lets us write code that functions
identically whether we have a regular reference or a type that implements
`Deref`.

The reason the `deref` method returns a reference to a value, and that the
plain dereference outside the parentheses in `*(y.deref())` is still necessary,
has to do with the ownership system. If the `deref` method returned the value
directly instead of a reference to the value, the value would be moved out of
`self`. We don’t want to take ownership of the inner value inside `MyBox<T>` in
this case or in most cases where we use the dereference operator.

Note that the `*` operator is replaced with a call to the `deref` method and
then a call to the `*` operator just once, each time we use a `*` in our code.
Because the substitution of the `*` operator does not recurse infinitely, we
end up with data of type `i32`, which matches the `5` in `assert_eq!` in
Listing 15-9.

### Implicit Deref Coercions with Functions and Methods

_Deref coercion_ converts a reference to a type that implements the `Deref`
trait 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 is a convenience Rust performs on arguments to
functions and methods, and works only on types that implement the `Deref`
trait. It 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.

Deref coercion was added to Rust so that programmers writing function and
method calls don’t need to add as many explicit references and dereferences
with `&` and `*`. The deref coercion feature also lets us write more code that
can work for either references or smart pointers.

To see deref coercion in action, let’s use the `MyBox<T>` type we defined in
Listing 15-8 as well as the implementation of `Deref` that we added in Listing
15-10. Listing 15-11 shows the definition of a function that has a string slice
parameter.

<Listing number="15-11" file-name="src/main.rs" caption="A `hello` function that has the parameter `name` of type `&str`">

```rust
{{#rustdoc_include ../listings/ch15-smart-pointers/listing-15-11/src/main.rs:here}}
```

</Listing>

We can call the `hello` function with a string slice as an argument, such as
`hello("Rust");`, for example. Deref coercion makes it possible to call `hello`
with a reference to a value of type `MyBox<String>`, as shown in Listing 15-12.

<Listing number="15-12" file-name="src/main.rs" caption="Calling `hello` with a reference to a `MyBox<String>` value, which works because of deref coercion">

```rust
{{#rustdoc_include ../listings/ch15-smart-pointers/listing-15-12/src/main.rs:here}}
```

</Listing>

Here we’re calling the `hello` function with the argument `&m`, which is a
reference to a `MyBox<String>` value. Because we implemented the `Deref` trait
on `MyBox<T>` in Listing 15-10, Rust can turn `&MyBox<String>` into `&String`
by calling `deref`. The standard library provides an implementation of `Deref`
on `String` that returns a string slice, and this is in the API documentation
for `Deref`. Rust calls `deref` again to turn the `&String` into `&str`, which
matches the `hello` function’s definition.

If Rust didn’t implement deref coercion, we would have to write the code in
Listing 15-13 instead of the code in Listing 15-12 to call `hello` with a value
of type `&MyBox<String>`.

<Listing number="15-13" file-name="src/main.rs" caption="The code we would have to write if Rust didn’t have deref coercion">

```rust
{{#rustdoc_include ../listings/ch15-smart-pointers/listing-15-13/src/main.rs:here}}
```

</Listing>

The `(*m)` dereferences the `MyBox<String>` into a `String`. Then the `&` and
`[..]` take a string slice of the `String` that is equal to the whole string to
match the signature of `hello`. This code without deref coercions is harder to
read, write, and understand with all of these symbols involved. Deref coercion
allows Rust to handle these conversions for us automatically.

When the `Deref` trait is defined for the types involved, Rust will analyze the
types and use `Deref::deref` as many times as necessary to get a reference to
match the parameter’s type. The number of times that `Deref::deref` needs to be
inserted is resolved at compile time, so there is no runtime penalty for taking
advantage of deref coercion!

### How Deref Coercion Interacts with Mutability

Similar to how you use the `Deref` trait to override the `*` operator on
immutable references, you can use the `DerefMut` trait to override the `*`
operator on mutable references.

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

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

The first two cases are the same except that the second implements mutability.
The first case states that if you have a `&T`, and `T` implements `Deref` to
some type `U`, you can get a `&U` transparently. The second case states that the
same deref coercion happens for mutable references.

The third case is trickier: Rust will also coerce a mutable reference to an
immutable one. But the reverse is _not_ possible: immutable references will
never coerce to mutable references. Because of the borrowing rules, if you have
a mutable reference, that mutable reference must be the only reference to that
data (otherwise, the program wouldn’t compile). Converting one mutable
reference to one immutable reference will never break the borrowing rules.
Converting an immutable reference to a mutable reference would require that the
initial immutable reference is the only immutable reference to that data, but
the borrowing rules don’t guarantee that. Therefore, Rust can’t make the
assumption that converting an immutable reference to a mutable reference is
possible.

[impl-trait]: ch10-02-traits.html#implementing-a-trait-on-a-type
[tuple-structs]: ch05-01-defining-structs.html#using-tuple-structs-without-named-fields-to-create-different-types