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[rustc.git] / src / doc / book / src / ch05-03-method-syntax.md

## Method Syntax

*Methods* are similar to functions: they’re declared with the `fn` keyword and
their name, they can have parameters and a return value, and they contain some
code that is run when they’re called from somewhere else. However, methods are
different from functions in that they’re defined within the context of a struct
(or an enum or a trait object, which we cover in Chapters 6 and 17,
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.

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

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

<span class="caption">Listing 5-13: Defining an `area` method on the
`Rectangle` struct</span>

To define the function within the context of `Rectangle`, we start an `impl`
(implementation) block. 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`
because Rust knows the type of `self` is `Rectangle` due to this method’s being
inside the `impl Rectangle` context. Note that we still need to use the `&`
before `self`, just as we did in `&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’ve chosen `&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 benefit of using methods instead of functions, in addition to using
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.

> ### 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`; otherwise it should return `false`. That is, we want to be able
to write the program shown in Listing 5-14, once we’ve defined the `can_hold`
method.

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

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

<span class="caption">Listing 5-14: Using the as-yet-unwritten `can_hold`
method</span>

And 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 both 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.

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

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

<span class="caption">Listing 5-15: Implementing the `can_hold` method on
`Rectangle` that takes another `Rectangle` instance as a parameter</span>

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

Another useful feature of `impl` blocks is that we’re allowed to define
functions within `impl` blocks that *don’t* take `self` as a parameter. These
are called *associated functions* because they’re associated with the struct.
They’re still functions, not methods, because they don’t have an instance of
the struct to work with. You’ve already used the `String::from` associated
function.

Associated functions are often used for constructors that will return a new
instance of the struct. For example, we could provide an associated function
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}}
```

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.

### 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.

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

<span class="caption">Listing 5-16: Rewriting Listing 5-15 using multiple `impl`
blocks</span>

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. Methods let you specify the
behavior that instances of your structs have, and associated functions let you
namespace functionality that is particular to your struct without having an
instance available.

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.