Method Syntax

Methods are similar to functions: they’re declared with the fn keyword and their name, they can have parameters and return values, and they contain some code that gets run when they’re called from somewhere else. Methods are different from functions, however, because they’re defined within the context of a struct (or an enum or a trait object, which we will cover in Chapters 6 and 17, respectively), and their first parameter is always self, which represents the instance of the struct that the method is being called on.

Defining Methods

Let’s change our 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 5-7:

Filename: src/main.rs

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

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

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

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

Listing 5-7: Defining an area method on the Rectangle struct

In order to make the function be defined within the context of Rectangle, we start an impl block (impl is short for implementation). Then we move the function within the impl curly braces, and change the first (and in this case, only) parameter to be self in the signature and everywhere within the body. Then 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. Method syntax is taking an instance and adding a dot followed by the method name, parentheses, and any arguments.

In the signature for area, we get to use &self instead of rectangle: &Rectangle because Rust knows the type of self is Rectangle due to this method being inside the impl Rectangle context. Note we still need to have the & before self, just like we had &Rectangle. Methods can choose to take ownership of self, borrow self immutably as we’ve done here, or borrow self mutably, just like 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 be able to read the data in the struct, not write to it. If we wanted to be able to change the instance that we’ve called the method on as part of what the method does, we’d put &mut self as the first parameter instead. Having a method that takes ownership of the instance by having just self as the first parameter is rarer; this is usually used when the method transforms self into something else and we want to prevent the caller from using the original instance after the transformation.

The main benefit of using methods over functions, in addition to getting to use 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 together in one impl block, rather than make future users of our code search for capabilities of Rectangle all over the place.

Where’s the -> operator?

In languages like C++, there are two different operators for calling methods: . if you’re calling a method on the object directly, and -> if you’re calling the method on a pointer to the object and thus need to dereference the pointer first. In other words, if object is a pointer, object->something() is like (*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 behavior like this.

Here’s how it works: when you call a method with object.something(), Rust will automatically add in &, &mut, or * so that object matches the signature of the method. In other words, these are the same:

# #[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, 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 just reading (so needs &self), mutating (so &mut self), or consuming (so 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 some more with methods by implementing a second method on our Rectangle struct. This time, we’d like for an instance of Rectangle to take another instance of Rectangle and return true if the second rectangle could fit completely within self and false if it would not. That is, if we run the code in Listing 5-8, once we've defined the can_hold method:

Filename: src/main.rs

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

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

Listing 5-8: Demonstration of using the as-yet-unwritten can_hold method

We want to see this output, since both of rect2’s dimensions are smaller than rect1’s, 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 a call site: rect1.can_hold(&rect2) passes in &rect2, which is an immutable borrow to rect2, an instance of Rectangle. This makes sense, since we only need to read rect2 (rather than write, which would mean we’d need a mutable borrow) and we want main to keep ownership of rect2 so that we could use it again after calling this method. The return value of can_hold will be a boolean, and the implementation will check to see if self’s length and width are both greater than the length and width of the other Rectangle, respectively. Let’s add this new method to the impl block from Listing 5-7, shown in Listing 5-9:

If we run this with the main from Listing 5-8, we will get our desired output! Methods can have multiple parameters that we add to the signature after the self parameter, and those parameters work just like parameters in functions do.

Associated Functions

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

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 length and width, thus making it easier to create a square Rectangle rather than having to specify the same value twice:

Filename: src/main.rs

# #[derive(Debug)]
# struct Rectangle {
#     length: u32,
#     width: u32,
# }
#
impl Rectangle {
    fn square(size: u32) -> Rectangle {
        Rectangle { length: size, width: size }
    }
}

To call this associated function, we use the :: syntax with the struct name: let sq = Rectangle::square(3);, for example. This function is namespaced by the struct: the :: syntax is used for both associated functions and namespaces created by modules, which we’ll learn about in Chapter 7.

Summary

Structs let us create custom types that are meaningful for our domain. By using structs, we can keep associated pieces of data connected to each other and name each piece to make our code clear. Methods let us specify the behavior that instances of our structs have, and associated functions let us namespace functionality that is particular to our struct without having an instance available.

Structs aren’t the only way we can create custom types, though; let’s turn to the enum feature of Rust and add another tool to our toolbox.