Method Syntax - The Rust Programming Language

## 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:

Filename: src/main.rs

    #[derive(Debug)]
    struct Rectangle {
        width: u32,
        height: u32,
    }

    impl Rectangle {
        fn area(&self) -> u32 {
            self.width * self.height
        }
    }

    fn main() {
        let rect1 = Rectangle { width: 30, height: 50 };

        println!(
            "The area of the rectangle is {} square pixels.",
            rect1.area()
        );
    }

Listing 5-13: Defining an `area` method on the `Rectangle` struct

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:

>     # #![allow(unused_variables)]
>     #fn main() {
>     # #[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:

Filename: src/main.rs

    fn main() {
        let rect1 = Rectangle { width: 30, height: 50 };
        let rect2 = Rectangle { width: 10, height: 40 };
        let rect3 = Rectangle { width: 60, height: 45 };

        println!("Can rect1 hold rect2? {}", rect1.can_hold(&rect2));
        println!("Can rect1 hold rect3? {}", rect1.can_hold(&rect3));
    }

Listing 5-14: Using the as-yet-unwritten `can_hold` method

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`:

    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:

Filename: src/main.rs

    # #![allow(unused_variables)]
    #fn main() {
    # #[derive(Debug)]
    # struct Rectangle {
    #     width: u32,
    #     height: u32,
    # }
    #
    impl Rectangle {
        fn area(&self) -> u32 {
            self.width * self.height
        }

        fn can_hold(&self, other: &Rectangle) -> bool {
            self.width > other.width && self.height > other.height
        }
    }
    #}

Listing 5-15: Implementing the `can_hold` method on `Rectangle` that takes another `Rectangle` instance as a parameter

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:

Filename: src/main.rs

    # #![allow(unused_variables)]
    #fn main() {
    # #[derive(Debug)]
    # struct Rectangle {
    #     width: u32,
    #     height: u32,
    # }
    #
    impl Rectangle {
        fn square(size: u32) -> Rectangle {
            Rectangle { width: size, height: size }
        }
    }
    #}

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:

    # #![allow(unused_variables)]
    #fn main() {
    # #[derive(Debug)]
    # struct Rectangle {
    #     width: u32,
    #     height: u32,
    # }
    #
    impl Rectangle {
        fn area(&self) -> u32 {
            self.width * self.height
        }
    }

    impl Rectangle {
        fn can_hold(&self, other: &Rectangle) -> bool {
            self.width > other.width && self.height > other.height
        }
    }
    #}

Listing 5-16: Rewriting Listing 5-15 using multiple `impl` blocks

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.