src/ch05-03-method-syntax.md

## Method Syntax

_Methods_ are similar to functions: we declare them with the `fn` keyword and a
name, they can have parameters and a return value, and they contain some code
that’s run when the method is called from somewhere else. Unlike functions,
methods are defined within the context of a struct (or an enum or a trait
object, which we cover in [Chapter 6][enums]<!-- ignore --> and
[Chapter 17][trait-objects]<!-- ignore -->, respectively), and their first
parameter is always `self`, which represents the instance of the struct the
method is being called on.

### Defining Methods

Let’s change the `area` function that has a `Rectangle` instance as a parameter
and instead make an `area` method defined on the `Rectangle` struct, as shown in
Listing 5-13.

<Listing number="5-13" file-name="src/main.rs" caption="Defining an `area` method on the `Rectangle` struct">

```rust
{{#rustdoc_include ../listings/ch05-using-structs-to-structure-related-data/listing-05-13/src/main.rs}}
```

</Listing>

To define the function within the context of `Rectangle`, we start an `impl`
(implementation) block for `Rectangle`. Everything within this `impl` block will
be associated with the `Rectangle` type. Then we move the `area` function within
the `impl` curly brackets and change the first (and in this case, only)
parameter to be `self` in the signature and everywhere within the body. In
`main`, where we called the `area` function and passed `rect1` as an argument,
we can instead use _method syntax_ to call the `area` method on our `Rectangle`
instance. The method syntax goes after an instance: we add a dot followed by the
method name, parentheses, and any arguments.

In the signature for `area`, we use `&self` instead of `rectangle: &Rectangle`.
The `&self` is actually short for `self: &Self`. Within an `impl` block, the
type `Self` is an alias for the type that the `impl` block is for. Methods must
have a parameter named `self` of type `Self` for their first parameter, so Rust
lets you abbreviate this with only the name `self` in the first parameter spot.
Note that we still need to use the `&` in front of the `self` shorthand to
indicate that this method borrows the `Self` instance, just as we did in
`rectangle: &Rectangle`. Methods can take ownership of `self`, borrow `self`
immutably, as we’ve done here, or borrow `self` mutably, just as they can any
other parameter.

We chose `&self` here for the same reason we used `&Rectangle` in the function
version: we don’t want to take ownership, and we just want to read the data in
the struct, not write to it. If we wanted to change the instance that we’ve
called the method on as part of what the method does, we’d use `&mut self` as
the first parameter. Having a method that takes ownership of the instance by
using just `self` as the first parameter is rare; this technique is usually used
when the method transforms `self` into something else and you want to prevent
the caller from using the original instance after the transformation.

The main reason for using methods instead of functions, in addition to providing
method syntax and not having to repeat the type of `self` in every method’s
signature, is for organization. We’ve put all the things we can do with an
instance of a type in one `impl` block rather than making future users of our
code search for capabilities of `Rectangle` in various places in the library we
provide.

Note that we can choose to give a method the same name as one of the struct’s
fields. For example, we can define a method on `Rectangle` that is also named
`width`:

<Listing file-name="src/main.rs">

```rust
{{#rustdoc_include ../listings/ch05-using-structs-to-structure-related-data/no-listing-06-method-field-interaction/src/main.rs:here}}
```

</Listing>

Here, we’re choosing to make the `width` method return `true` if the value in
the instance’s `width` field is greater than `0` and `false` if the value is
`0`: we can use a field within a method of the same name for any purpose. In
`main`, when we follow `rect1.width` with parentheses, Rust knows we mean the
method `width`. When we don’t use parentheses, Rust knows we mean the field
`width`.

Often, but not always, when we give a method the same name as a field we want it
to only return the value in the field and do nothing else. Methods like this are
called _getters_, and Rust does not implement them automatically for struct
fields as some other languages do. Getters are useful because you can make the
field private but the method public, and thus enable read-only access to that
field as part of the type’s public API. We will discuss what public and private
are and how to designate a field or method as public or private in
[Chapter 7][public]<!-- ignore -->.

> ### Where’s the `->` Operator?
>
> In C and C++, two different operators are used for calling methods: you use
> `.` if you’re calling a method on the object directly and `->` if you’re
> calling the method on a pointer to the object and need to dereference the
> pointer first. In other words, if `object` is a pointer, `object->something()`
> is similar to `(*object).something()`.
>
> Rust doesn’t have an equivalent to the `->` operator; instead, Rust has a
> feature called _automatic referencing and dereferencing_. Calling methods is
> one of the few places in Rust that has this behavior.
>
> Here’s how it works: when you call a method with `object.something()`, Rust
> automatically adds in `&`, `&mut`, or `*` so `object` matches the signature of
> the method. In other words, the following are the same:
>
> <!-- CAN'T EXTRACT SEE BUG https://github.com/rust-lang/mdBook/issues/1127 -->
>
> ```rust
> # #[derive(Debug,Copy,Clone)]
> # struct Point {
> #     x: f64,
> #     y: f64,
> # }
> #
> # impl Point {
> #    fn distance(&self, other: &Point) -> f64 {
> #        let x_squared = f64::powi(other.x - self.x, 2);
> #        let y_squared = f64::powi(other.y - self.y, 2);
> #
> #        f64::sqrt(x_squared + y_squared)
> #    }
> # }
> # let p1 = Point { x: 0.0, y: 0.0 };
> # let p2 = Point { x: 5.0, y: 6.5 };
> p1.distance(&p2);
> (&p1).distance(&p2);
> ```
>
> The first one looks much cleaner. This automatic referencing behavior works
> because methods have a clear receiver—the type of `self`. Given the receiver
> and name of a method, Rust can figure out definitively whether the method is
> reading (`&self`), mutating (`&mut self`), or consuming (`self`). The fact
> that Rust makes borrowing implicit for method receivers is a big part of
> making ownership ergonomic in practice.

### Methods with More Parameters

Let’s practice using methods by implementing a second method on the `Rectangle`
struct. This time we want an instance of `Rectangle` to take another instance of
`Rectangle` and return `true` if the second `Rectangle` can fit completely
within `self` (the first `Rectangle`); otherwise, it should return `false`. That
is, once we’ve defined the `can_hold` method, we want to be able to write the
program shown in Listing 5-14.

<Listing number="5-14" file-name="src/main.rs" caption="Using the as-yet-unwritten `can_hold` method">

```rust,ignore
{{#rustdoc_include ../listings/ch05-using-structs-to-structure-related-data/listing-05-14/src/main.rs}}
```

</Listing>

The expected output would look like the following because both dimensions of
`rect2` are smaller than the dimensions of `rect1`, but `rect3` is wider than
`rect1`:

```text
Can rect1 hold rect2? true
Can rect1 hold rect3? false
```

We know we want to define a method, so it will be within the `impl Rectangle`
block. The method name will be `can_hold`, and it will take an immutable borrow
of another `Rectangle` as a parameter. We can tell what the type of the
parameter will be by looking at the code that calls the method:
`rect1.can_hold(&rect2)` passes in `&rect2`, which is an immutable borrow to
`rect2`, an instance of `Rectangle`. This makes sense because we only need to
read `rect2` (rather than write, which would mean we’d need a mutable borrow),
and we want `main` to retain ownership of `rect2` so we can use it again after
calling the `can_hold` method. The return value of `can_hold` will be a Boolean,
and the implementation will check whether the width and height of `self` are
greater than the width and height of the other `Rectangle`, respectively. Let’s
add the new `can_hold` method to the `impl` block from Listing 5-13, shown in
Listing 5-15.

<Listing number="5-15" file-name="src/main.rs" caption="Implementing the `can_hold` method on `Rectangle` that takes another `Rectangle` instance as a parameter">

```rust
{{#rustdoc_include ../listings/ch05-using-structs-to-structure-related-data/listing-05-15/src/main.rs:here}}
```

</Listing>

When we run this code with the `main` function in Listing 5-14, we’ll get our
desired output. Methods can take multiple parameters that we add to the
signature after the `self` parameter, and those parameters work just like
parameters in functions.

### Associated Functions

All functions defined within an `impl` block are called _associated functions_
because they’re associated with the type named after the `impl`. We can define
associated functions that don’t have `self` as their first parameter (and thus
are not methods) because they don’t need an instance of the type to work with.
We’ve already used one function like this: the `String::from` function that’s
defined on the `String` type.

Associated functions that aren’t methods are often used for constructors that
will return a new instance of the struct. These are often called `new`, but
`new` isn’t a special name and isn’t built into the language. For example, we
could choose to provide an associated function named `square` that would have
one dimension parameter and use that as both width and height, thus making it
easier to create a square `Rectangle` rather than having to specify the same
value twice:

<span class="filename">Filename: src/main.rs</span>

```rust
{{#rustdoc_include ../listings/ch05-using-structs-to-structure-related-data/no-listing-03-associated-functions/src/main.rs:here}}
```

The `Self` keywords in the return type and in the body of the function are
aliases for the type that appears after the `impl` keyword, which in this case
is `Rectangle`.

To call this associated function, we use the `::` syntax with the struct name;
`let sq = Rectangle::square(3);` is an example. This function is namespaced by
the struct: the `::` syntax is used for both associated functions and namespaces
created by modules. We’ll discuss modules in
[Chapter 7][modules]<!-- ignore -->.

### Multiple `impl` Blocks

Each struct is allowed to have multiple `impl` blocks. For example, Listing 5-15
is equivalent to the code shown in Listing 5-16, which has each method in its
own `impl` block.

<Listing number="5-16" caption="Rewriting Listing 5-15 using multiple `impl` blocks">

```rust
{{#rustdoc_include ../listings/ch05-using-structs-to-structure-related-data/listing-05-16/src/main.rs:here}}
```

</Listing>

There’s no reason to separate these methods into multiple `impl` blocks here,
but this is valid syntax. We’ll see a case in which multiple `impl` blocks are
useful in Chapter 10, where we discuss generic types and traits.

## Summary

Structs let you create custom types that are meaningful for your domain. By
using structs, you can keep associated pieces of data connected to each other
and name each piece to make your code clear. In `impl` blocks, you can define
functions that are associated with your type, and methods are a kind of
associated function that let you specify the behavior that instances of your
structs have.

But structs aren’t the only way you can create custom types: let’s turn to
Rust’s enum feature to add another tool to your toolbox.

[enums]: ch06-00-enums.html
[trait-objects]: ch18-02-trait-objects.md
[public]: ch07-03-paths-for-referring-to-an-item-in-the-module-tree.html#exposing-paths-with-the-pub-keyword
[modules]: ch07-02-defining-modules-to-control-scope-and-privacy.html