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.
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 as 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.
Method Calls are Syntactic Sugar for Function Calls
Using the concepts we've discussed so far, we can now see how method calls are syntactic sugar for function calls. For example, let's say we have a rectangle struct with an area method and a set_width method:
struct Rectangle {
width: u32,
height: u32,
}
impl Rectangle {
fn area(&self) -> u32 {
self.width * self.height
}
fn set_width(&mut self, width: u32) {
self.width = width;
}
}And let's say we have a rectangle r. Then the method calls r.area() and r.set_width(2) are equivalent to this:
struct Rectangle { width: u32, height: u32, } impl Rectangle { fn area(&self) -> u32 { self.width * self.height } fn set_width(&mut self, width: u32) { self.width = width; } } fn main() { let mut r = Rectangle { width: 1, height: 2 }; let area1 = r.area(); let area2 = Rectangle::area(&r); assert_eq!(area1, area2); r.set_width(2); Rectangle::set_width(&mut r, 2); }
The method call r.area() becomes Rectangle::area(&r). The function name is the associated function Rectangle::area. The function argument is the &self parameter. Rust automatically inserts the borrowing operator &.
Note: if you are familiar with C or C++, you are used to two different syntaxes for method calls:
r.area()andr->area(). Rust does not have an equivalent to the arrow operator->. Rust will automatically reference and dereference the method receiver when you use the dot operator.
The method call r.set_width(2) similarly becomes Rectangle::set_width(&mut r, 2). This method expects &mut self, so the first argument is a mutable borrow &mut r. The second argument is exactly the same, the number 2.
As we described in Chapter 4.3 "Dereferencing a Pointer Accesses Its Data", Rust will insert as many references and dereferences as needed to make the types match up for the self parameter. For example, here are two equivalent calls to area for a mutable reference to a boxed rectangle:
struct Rectangle { width: u32, height: u32, } impl Rectangle { fn area(&self) -> u32 { self.width * self.height } fn set_width(&mut self, width: u32) { self.width = width; } } fn main() { let r = &mut Box::new(Rectangle { width: 1, height: 2 }); let area1 = r.area(); let area2 = Rectangle::area(&**r); assert_eq!(area1, area2); }
Rust will add two dereferences (once for the mutable reference, once for the box) and then one immutable borrow because area expects &Rectangle. Note that this is also a situation where a mutable reference is "downgraded" into a shared reference, like we discussed in Chapter 4.2. Conversely, you would not be allowed to call set_width on a value of type &Rectangle or &Box<Rectangle>.
Methods and Ownership
Like we discussed in Chapter 4.2 "References and Borrowing", methods must be called on structs that have the necessary permissions. As a running example, we will use these three methods that take &self, &mut self, and self, respectively.
impl Rectangle {
fn area(&self) -> u32 {
self.width * self.height
}
fn set_width(&mut self, width: u32) {
self.width = width;
}
fn max(self, other: Rectangle) -> Rectangle {
Rectangle {
width: self.width.max(other.width),
height: self.height.max(other.height),
}
}
}Reads and Writes with &self and &mut self
If we make an owned rectangle with let rect = Rectangle { ... }, then rect has R and O permissions. With those permissions, it is permissible to call the area and max methods:
However, if we try to call set_width, we are missing the W permission:
Rust will reject this program with the corresponding error:
error[E0596]: cannot borrow `rect` as mutable, as it is not declared as mutable
--> test.rs:28:1
|
24 | let rect = Rectangle {
| ---- help: consider changing this to be mutable: `mut rect`
...
28 | rect.set_width(0);
| ^^^^^^^^^^^^^^^^^ cannot borrow as mutable
We will get a similar error if we try to call set_width on an immutable reference to a Rectangle, even if the underlying rectangle is mutable:
Moves with self
Calling a method that expects self will move the input struct (unless the struct implements Copy). For example, we cannot use a Rectangle after passing it to max:
Once we call rect.max(..), we move rect and so lose all permissions on it. Trying to compile this program would give us the following error:
error[E0382]: borrow of moved value: `rect`
--> test.rs:33:16
|
24 | let rect = Rectangle {
| ---- move occurs because `rect` has type `Rectangle`, which does not implement the `Copy` trait
...
32 | let max_rect = rect.max(other_rect);
| --------------- `rect` moved due to this method call
33 | println!("{}", rect.area());
| ^^^^^^^^^^^ value borrowed here after move
A similar situation arises if we try to call a self method on a reference. For instance, say we tried to make a method set_to_max that assigns self to the output of self.max(..):
Then we can see that self is missing O permissions in the operation self.max(..). Rust therefore rejects this program with the following error:
error[E0507]: cannot move out of `*self` which is behind a mutable reference
--> test.rs:23:17
|
23 | *self = self.max(other);
| ^^^^^----------
| | |
| | `*self` moved due to this method call
| move occurs because `*self` has type `Rectangle`, which does not implement the `Copy` trait
|
This is the same kind of error we discussed in Chapter 4.3 "Copying vs. Moving Out of a Collection".
Good Moves and Bad Moves
You might wonder: why does it matter if we move out of *self? In fact, for the case of Rectangle, it actually is safe to move out of *self, even though Rust doesn't let you do it. For example, if we simulate a program that calls the rejected set_to_max, you can see how nothing unsafe occurs:
The reason it's safe to move out of *self is because Rectangle does not own any heap data.
In fact, we can actually get Rust to compile set_to_max by simply adding #[derive(Copy, Clone)] to the definition of Rectangle:
Notice that unlike before, self.max(other) no longer requires the O permission on *self or other. Remember that self.max(other) desugars to Rectangle::max(*self, other). The dereference *self does not require ownership over *self if Rectangle is copyable.
You might wonder: why doesn't Rust automatically derive Copy for Rectangle? Rust does not auto-derive Copy for stability across API changes. Imagine that the author of the Rectangle type decided to add a name: String field. Then all client code that relies on Rectangle being Copy would suddenly get rejected by the compiler. To avoid that issue, API authors must explicitly add #[derive(Copy)] to indicate that they expect their struct to always be Copy.
To better understand the issue, let's run a simulation. Say we added name: String to Rectangle. What would happen if Rust allowed set_to_max to compile?
In this program, we call set_to_max with two rectangles r1 and r2. self is a mutable reference to r1 and other is a move of r2. After calling self.max(other), the max method consumes ownership of both rectangles. When max returns, Rust deallocates both strings "r1" and "r2" in the heap. Notice the problem: at the location L2, *self is supposed to be readable and writable. However, (*self).name (actually r1.name) has been deallocated.
Therefore when we do *self = max, we encounter undefined behavior. When we overwrite *self, Rust will implicitly drop the data previously in *self. To make that behavior explicit, we have added drop(*self). After calling drop(*self), Rust attempts to free (*self).name a second time. That action is a double-free, which is undefined behavior.
So remember: when you see an error like "cannot move out of *self", that's usually because you're trying to call a self method on a reference like &self or &mut self. Rust is protecting you from a double-free.
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.