RUST : 5. Using structs to structure related data
A struct, or structure, is a custom data type that lets you name and package together multiple related values that make up a meaningful group.
Defining and Instantiating Structs
Structs are similar to tuples, which were discussed in Chapter 3. Like tuples, the pieces of a struct can be different types. Unlike with tuples, you’ll name each piece of data so it’s clear what the values mean. As a result of these names, structs are more flexible than tuples.
To define a struct, we enter the keyword struct and name the entire struct
struct User {
username: String,
email: String,
sign_in_count: u64,
active: bool,
}
Listing 5-1: A User struct definition
To use a struct after we’ve defined it, we create an instance of that struct by specifying concrete values for each of the fields.
We create an instance by stating the name of the struct and then add curly brackets containing key: value pairs, where the keys are the names of the fields and the values are the data we want to store in those fields. We don’t have to specify the fields in the same order in which we declared them in the struct.
let user1 = User {
email: String::from("someone@example.com"),
username: String::from("someusername123"),
active: true,
sign_in_count: 1,
};
Listing 5-2: Creating an instance of the User struct
To get a specific value from a struct, we can use dot(.) notation. If we wanted just this user’s email address, we could use user1.email wherever we wanted to use this value. If the instance is mutable, we can change a value by using the dot(.) notation and assigning into a particular field. Listing 5-3 shows how to change the value in the email field of a mutable User instance.
let mut user1 = User {
email: String::from("someone@example.com"),
username: String::from("someusername123"),
active: true,
sign_in_count: 1,
};
user1.email = String::from("anotheremail@example.com");
Listing 5-3: Changing the value in the email field of a User instance
Listing 5-4 shows a build_user function that returns a User instance with the given email and username. The active field gets the value of true, and the sign_in_count gets a value of 1.
// JHS) maybe this function similar constructor in C++
fn build_user(email: String, username: String) -> User {
User {
email: email,
username: username,
active: true,
sign_in_count: 1,
}
}
Listing 5-4: A build_user function that takes an email and username and returns a User instance
Using the Field Init Shorthand when Variables and Fields Have the Same Name
Because the parameter names and the struct field names are exactly the same in Listing 5-4, we can use the field init shorthand syntax to rewrite build_user so that it behaves exactly the same but doesn’t have the repetition of email and username, as shown in Listing 5-5.
fn build_user(email: String, username: String) -> User {
User {
email,
username,
active: true,
sign_in_count: 1,
}
}
Listing 5-5: A build_user function that uses field init shorthand because the email and username parameters have the same name as struct fields
Creating Instances From Other Instances With Struct Update Syntax
It’s often useful to create a new instance of a struct that uses most of an old instance’s values but changes some. You’ll do this using struct update syntax.
First, Listing 5-6 shows how we create a new User instance in user2 without the update syntax. We set new values for email and username but otherwise use the same values from user1 that we created in Listing 5-2.
let user2 = User {
email: String::from("another@example.com"),
username: String::from("anotherusername567"),
active: user1.active,
sign_in_count: user1.sign_in_count,
};
Listing 5-6: Creating a new User instance using some of the values from user1
Using struct update syntax, we can achieve the same effect with less code, as shown in Listing 5-7. The syntax .. specifies that the remaining fields not explicitly set should have the same value as the fields in the given instance. The code in Listing 5-7 also creates an instance in user2 that has a different value for email and username but has the same values for the active and sign_in_count fields from user1.
let user2 = User {
email: String::from("another@example.com"),
username: String::from("anotherusername567"),
..user1
};
Listing 5-7: Using struct update syntax to set new email and username values for a User instance but use the rest of the values from the fields of the instance in the user1 variable
Using Tuple Structs without Named Fields to Create Different Types
You can also define structs that look similar to tuples, called tuple structs. Tuple structs have the added meaning the struct name provides but don’t have names associated with their fields; rather, they just have the types of the fields. Tuple structs are useful when you want to give the whole tuple a name and make the tuple be a different type from other tuples, and naming each field as in a regular struct would be verbose or redundant.
To define a tuple struct, start with the struct keyword and the struct name followed by the types in the tuple.
struct Color(i32, i32, i32);
struct Point(i32, i32, i32);
let black = Color(0, 0, 0);
let origin = Point(0, 0, 0);
Tuple struct instances behave like tuples: you can destructure them into their individual pieces, you can use a . followed by the index to access an individual value, and so on.
Unit-Like Structs Without Any Fields
You can also define structs that don’t have any fields! These are called unit-like structs because they behave similarly to (), the unit type. Unit-like structs can be useful in situations in which you need to implement a trait on some type but don’t have any data that you want to store in the type itself. We’ll discuss traits in Chapter 10.
Ownership of Struct Data
In the User struct definition in Listing 5-1, we used the owned String type rather than the &str string slice type. This is a deliberate choice because we want instances of this struct to own all of its data and for that data to be valid for as long as the entire struct is valid.
It’s possible for structs to store references to data owned by something else, but to do so requires the use of lifetimes, a Rust feature that we’ll discuss in Chapter 10. Lifetimes ensure that the data referenced by a struct is valid for as long as the struct is.
Let’s say you try to store a reference in a struct without specifying lifetimes, like this, which won’t work:
In Chapter 10, we’ll discuss how to fix these errors so you can store references in structs, but for now, we’ll fix errors like these using owned types like String instead of references like &str.
// This code does not compile!
struct User {
username: &str,
email: &str,
sign_in_count: u64,
active: bool,
}
fn main() {
let user1 = User {
email: "someone@example.com",
username: "someusername123",
active: true,
sign_in_count: 1,
};
}
// The compiler will complain that it needs lifetime specifiers:
error[E0106]: missing lifetime specifier
-->
|
2 | username: &str,
| ^ expected lifetime parameter
error[E0106]: missing lifetime specifier
-->
|
3 | email: &str,
| ^ expected lifetime parameter
An Example Program Using Structs
Let’s write a program that calculates the area of a rectangle. We’ll start with single variables, and then refactor the program until we’re using structs instead.
fn main() {
let length1 = 50;
let width1 = 30;
println!(
"The area of the rectangle is {} square pixels.",
area(length1, width1)
);
}
fn area(length: u32, width: u32) -> u32 {
length * width
}
Listing 5-8: Calculating the area of a rectangle specified by separate width and height variables
The issue with this code is evident in the signature of area:
Even though Listing 5-8 works and figures out the area of the rectangle by calling the area function with each dimension, we can do better.
fn area(width: u32, height: u32) -> u32 {
The area function is supposed to calculate the area of one rectangle, but the function we wrote has two parameters. The parameters are related, but that’s not expressed anywhere in our program. It would be more readable and more manageable to group width and height together. We’ve already discussed one way we might do that in “The Tuple Type” section of Chapter 3: by using tuples.
Refactoring with Tuples
Listing 5-9 shows another version of our program that uses tuples.
fn main() {
let rect1 = (30, 50);
println!(
"The area of the rectangle is {} square pixels.",
area(rect1)
);
}
fn area(dimensions: (u32, u32)) -> u32 {
dimensions.0 * dimensions.1
}
Listing 5-9: Specifying the width and height of the rectangle with a tuple
In one way, this program is better. Tuples let us add a bit of structure, and we’re now passing just one argument. But in another way, this version is less clear: tuples don’t name their elements, so our calculation has become more confusing because we have to index into the parts of the tuple.
It doesn’t matter if we mix up width and height for the area calculation, but if we want to draw the rectangle on the screen, it would matter! We would have to keep in mind that width is the tuple index 0 and height is the tuple index 1. If someone else worked on this code, they would have to figure this out and keep it in mind as well. It would be easy to forget or mix up these values and cause errors, because we haven’t conveyed the meaning of our data in our code.
Refactoring with Structs: Adding More Meaning
We use structs to add meaning by labeling the data. We can transform the tuple we’re using into a data type with a name for the whole as well as names for the parts, as shown in Listing 5-10.
struct Rectangle {
width: u32,
height: u32,
}
fn main() {
let rect1 = Rectangle { width: 30, height: 50 };
println!(
"The area of the rectangle is {} square pixels.",
area(&rect1)
);
}
fn area(rectangle: &Rectangle) -> u32 {
rectangle.width * rectangle.height
}
Listing 5-10: Defining a Rectangle struct
Here we’ve defined a struct and named it Rectangle. Inside the curly brackets, we defined the fields as width and height, both of which have type u32. Then in main, we created a particular instance of Rectangle that has a width of 30 and a height of 50.
Our area function is now defined with one parameter, which we’ve named rectangle, whose type is an immutable borrow of a struct Rectangle instance. As mentioned in Chapter 4, we want to borrow the struct rather than take ownership of it. This way, main retains its ownership and can continue using rect1, which is the reason we use the & in the function signature and where we call the function.
The area function accesses the width and height fields of the Rectangle instance. Our function signature for area now says exactly what we mean: calculate the area of Rectangle, using its width and height fields. This conveys that the width and height are related to each other, and it gives descriptive names to the values rather than using the tuple index values of 0 and 1. This is a win for clarity.
Adding Useful Functionality with Derived Traits
It’d be nice to be able to print an instance of Rectangle while we’re debugging our program and see the values for all its fields. Listing 5-11 tries using the println! macro as we have used in previous chapters. This won’t work, however.
struct Rectangle {
length: u32,
width: u32,
}
fn main() {
let rect1 = Rectangle { length: 50, width: 30 };
println!("rect1 is {}", rect1);
// When we compile this code, we get an error with this core message:
// error[E0277]: `Rectangle` doesn't implement `std::fmt::Display`
}
Listing 5-11: Attempting to print a Rectangle instance
The println! macro can do many kinds of formatting, and by default, the curly brackets tell println! to use formatting known as Display: output intended for direct end user consumption. The primitive types we’ve seen so far implement Display by default, because there’s only one way you’d want to show a 1 or any other primitive type to a user. But with structs, the way println! should format the output is less clear because there are more display possibilities: Do you want commas or not? Do you want to print the curly brackets? Should all the fields be shown? Due to this ambiguity, Rust doesn’t try to guess what we want, and structs don’t have a provided implementation of Display.
= help: the trait `std::fmt::Display` is not implemented for `Rectangle`
= note: in format strings you may be able to use `{:?}`
(or {:#?} for pretty-print) instead
Let’s try it! The println! macro call will now look like println!("rect1 is {:?}", rect1);. Putting the specifier :? inside the curly brackets tells println! we want to use an output format called Debug. The Debug trait enables us to print our struct in a way that is useful for developers so we can see its value while we’re debugging our code.
Compile the code with this change. Drat! We still get an error:
error[E0277]: `Rectangle` doesn't implement `std::fmt::Debug`
But again, the compiler gives us a helpful note:
= help: the trait `std::fmt::Debug` is not implemented for `Rectangle`
= note: add `#[derive(Debug)]` or manually implement `std::fmt::Debug`
Rust does include functionality to print out debugging information, but we have to explicitly opt in to make that functionality available for our struct. To do that, we add the annotation #[derive(Debug)] just before the struct definition, as shown in Listing 5-12.
#[derive(Debug)]
struct Rectangle {
width: u32,
height: u32,
}
fn main() {
let rect1 = Rectangle { width: 30, height: 50 };
println!("rect1 is {:?}", rect1);
}
Listing 5-12: Adding the annotation to derive the Debug trait and printing the Rectangle instance using debug formatting
Now when we run the program, we won’t get any errors, and we’ll see the following output:
rect1 is Rectangle { width: 30, height: 50 }
Nice! It’s not the prettiest output, but it shows the values of all the fields for this instance, which would definitely help during debugging. When we have larger structs, it’s useful to have output that’s a bit easier to read; in those cases, we can use {:#?}** instead of {:?} in the println! string. When we use the {:#?} style in the example, the output will look like this:
rect1 is Rectangle {
width: 30,
height: 50
}
Rust has provided a number of traits for us to use with the derive annotation that can add useful behavior to our custom types. Those traits and their behaviors are listed in Appendix C. We’ll cover how to implement these traits with custom behavior as well as how to create your own traits in Chapter 10.
Our area function is very specific: it only computes the area of rectangles. It would be helpful to tie this behavior more closely to our Rectangle struct, because it won’t work with any other type. Let’s look at how we can continue to refactor this code by turning the area function into an area method defined on our Rectangle type.
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.
#[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:
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.
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 5-14: 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.
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
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.
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); 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.
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.