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 and Chapter
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 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:
Filename: src/main.rs
#[derive(Debug)] struct Rectangle { width: u32, height: u32, } impl Rectangle { fn width(&self) -> bool { self.width > 0 } } fn main() { let rect1 = Rectangle { width: 30, height: 50, }; if rect1.width() { println!("The rectangle has a nonzero width; it is {}", rect1.width); } }
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.
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, ifobjectis 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*soobjectmatches the signature of the method. In other words, the following are the same:#![allow(unused)] 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 (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.
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
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 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
#[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 } } 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 5-15: 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
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:
Filename: src/main.rs
#[derive(Debug)] struct Rectangle { width: u32, height: u32, } impl Rectangle { fn square(size: u32) -> Self { Self { width: size, height: size, } } } fn main() { let sq = Rectangle::square(3); }
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.
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.
#[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 } } 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 5-16: 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. 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.