Structs
A struct, short for structure, is a custom data type that lets us name and
package together multiple related values that make up a meaningful group. If
you come from an object-oriented language, a struct is like an object’s data
attributes. In the next section of this chapter, we’ll talk about how to define
methods on our structs; methods are how you specify the behavior that goes
along with a struct’s data. The struct and enum (that we will talk about in
Chapter 6) concepts are the building blocks for creating new types in your
program’s domain in order to take full advantage of Rust’s compile-time type
checking.
One way of thinking about structs is that they are similar to tuples, which we talked about in Chapter 3. Like tuples, the pieces of a struct can be different types. Unlike tuples, we name each piece of data so that it’s clearer what the values mean. Structs are more flexible as a result of these names: we don’t have to rely on the order of the data to specify or access the values of an instance.
To define a struct, we enter the keyword struct and give the whole struct a
name. A struct’s name should describe what the significance is of these pieces
of data being grouped together. Then, inside curly braces, we define the names
of the pieces of data, which we call fields, and specify each field’s type.
For example, Listing 5-1 shows a struct to store information about a user
account:
struct User {
username: String,
email: String,
sign_in_count: u64,
active: bool,
}
Listing 5-1: A User struct definition
To use a struct once we've defined it, we create an instance of that struct
by specifying concrete values for each of the fields. Creating an instance is
done by stating the name of the struct, then curly braces with key: value
pairs inside it where the keys are the names of the fields and the values are
the data we want to store in those fields. The fields don’t have to be
specified in the same order in which the struct declared them. In other words,
the struct definition is like a general template for the type, and instances
fill in that template with particular data to create values of the type. For
example, we can declare a particular user like this:
# struct User {
# username: String,
# email: String,
# sign_in_count: u64,
# active: bool,
# }
#
let user1 = User {
email: String::from("[email protected]"),
username: String::from("someusername123"),
active: true,
sign_in_count: 1,
};
To get a particular value out of a struct, we can use dot notation. If we
wanted just this user’s email address, we can say user1.email.
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 is possible for structs to store references to data owned by something else, but to do so requires the use of lifetimes, a feature of Rust that we'll discuss in Chapter 10. Lifetimes ensure that the data a struct references is valid for as long as the struct is. If you try to store a reference in a struct without specifying lifetimes, like this:
Filename: src/main.rs
struct User {
username: &str,
email: &str,
sign_in_count: u64,
active: bool,
}
fn main() {
let user1 = User {
email: "[email protected]",
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
We will talk about how to fix these errors in order to store references in
structs in Chapter 10, but for now, fix errors like these by switching to owned
types like String instead of references like &str.
An Example Program
To understand when we might want to use structs, let’s write a program that calculates the area of a rectangle. We’ll start off with single variables, then refactor our program until we’re using structs instead.
Let’s make a new binary project with Cargo called rectangles that will take the length and width of a rectangle specified in pixels and will calculate the area of the rectangle. Listing 5-2 has a short program with one way of doing just that in our project’s src/main.rs:
Filename: src/main.rs
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 5-2: Calculating the area of a rectangle specified by its length and width in separate variables
Let’s try running this program with cargo run:
The area of the rectangle is 1500 square pixels.
Refactoring with Tuples
Our little program works okay; it figures out the area of the rectangle by
calling the area function with each dimension. But we can do better. The
length and the width are related to each other since together they describe one
rectangle.
The issue with this method is evident in the signature of area:
fn area(length: u32, width: u32) -> u32 {
The area function is supposed to calculate the area of one rectangle, but our
function has two parameters. The parameters are related, but that’s not
expressed anywhere in our program itself. It would be more readable and more
manageable to group length and width together.
We’ve already discussed one way we might do that in Chapter 3: tuples. Listing 5-3 has a version of our program which uses tuples:
Filename: src/main.rs
fn main() {
let rect1 = (50, 30);
println!(
"The area of the rectangle is {} square pixels.",
area(rect1)
);
}
fn area(dimensions: (u32, u32)) -> u32 {
dimensions.0 * dimensions.1
}
Listing 5-3: Specifying the length and width of the rectangle with a tuple
In one way, this is a little better. Tuples let us add a bit of structure, and
we’re now passing just one argument when we call area. But in another way
this method is less clear: tuples don’t give names to their elements, so our
calculation has gotten more confusing because we have to index into the parts
of the tuple:
dimensions.0 * dimensions.1
It doesn’t matter if we mix up length and width for the area calculation, but
if we were to draw the rectangle on the screen it would matter! We would have
to remember that length was the tuple index 0 and width was the tuple
index 1. If someone else was to work on this code, they would have to figure
this out and remember it as well. It would be easy to forget or mix these
values up and cause errors, since we haven’t conveyed the meaning of our data
in our code.
Refactoring with Structs: Adding More Meaning
Here is where we bring in structs. We can transform our tuple into a data type with a name for the whole as well as names for the parts, as shown in Listing 5-4:
Filename: src/main.rs
struct Rectangle {
length: u32,
width: u32,
}
fn main() {
let rect1 = Rectangle { length: 50, width: 30 };
println!(
"The area of the rectangle is {} square pixels.",
area(&rect1)
);
}
fn area(rectangle: &Rectangle) -> u32 {
rectangle.length * rectangle.width
}
Listing 5-4: Defining a Rectangle struct
Here we’ve defined a struct and given it the name Rectangle. Inside the {}
we defined the fields to be length and width, both of which have type
u32. Then in main, we create a particular instance of a Rectangle that
has a length of 50 and a width of 30.
Our area function is now defined with one parameter that we’ve named
rectangle whose type is an immutable borrow of a struct Rectangle instance.
As we covered in Chapter 4, we want to borrow the struct rather than take
ownership of it so that main keeps its ownership and can continue using
rect1, so that’s why we have the & in the function signature and at the
call site.
The area function accesses the length and width fields of the
Rectangle. Our function signature for area now says exactly what we mean:
calculate the area of a Rectangle, using its length and width fields.
This conveys that the length and width are related to each other, and 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 out an instance of our Rectangle while we’re
debugging our program and see the values for all its fields. Listing 5-5 tries
using the println! macro as we have been:
Filename: src/main.rs
struct Rectangle {
length: u32,
width: u32,
}
fn main() {
let rect1 = Rectangle { length: 50, width: 30 };
println!("rect1 is {}", rect1);
}
Listing 5-5: Attempting to print a Rectangle
instance
If we run this, we get an error with this core message:
error[E0277]: the trait bound `Rectangle: std::fmt::Display` is not satisfied
The println! macro can do many kinds of formatting, and by default, {}
tells 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, as 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 as there are more display possibilities: Do you
want commas or not? Do you want to print the struct {}s? Should all the
fields be shown? Because of this ambiguity, Rust doesn’t try to guess what we
want and structs do not have a provided implementation of Display.
If we keep reading the errors, though, we’ll find this helpful note:
note: `Rectangle` cannot be formatted with the default formatter; try using
`:?` instead if you are using a format string
Let’s try it! The println! will now look like
println!("rect1 is {:?}", rect1);. Putting the specifier :? inside
the {} tells println! we want to use an output format called Debug.
Debug is a trait that enables us to print out our struct in a way that is
useful for developers so that we can see its value while we are debugging our
code.
Let’s try running with this change and… drat. We still get an error:
error: the trait bound `Rectangle: std::fmt::Debug` is not satisfied
Again, though, the compiler has given us a helpful note!
note: `Rectangle` cannot be formatted using `:?`; if it is defined in your
crate, add `#[derive(Debug)]` or manually implement it
Rust does include functionality to print out debugging information, but we
have to explicitly opt-in to having that functionality be available for our
struct. To do that, we add the annotation #[derive(Debug)] just before our
struct definition, as shown in Listing 5-6:
#[derive(Debug)]
struct Rectangle {
length: u32,
width: u32,
}
fn main() {
let rect1 = Rectangle { length: 50, width: 30 };
println!("rect1 is {:?}", rect1);
}
Listing 5-6: Adding the annotation to derive the Debug
trait and printing the Rectangle instance using debug formatting
At this point, if we run this program, we won’t get any errors and we’ll see the following output:
rect1 is Rectangle { length: 50, width: 30 }
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. If we want
output that is a bit prettier and easier to read, which can be helpful with
larger structs, we can use {:#?} in place of {:?} in the println! string.
If we use the pretty debug style in this example, the output will look like:
rect1 is Rectangle {
length: 50,
width: 30
}
There are a number of traits Rust has provided 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 be covering how to implement
these traits with custom behavior, as well as creating your own traits, in
Chapter 10.
Our area function is pretty specific—it only computes the area of rectangles.
It would be nice to tie this behavior together more closely with our
Rectangle struct, since it’s behavior that our Rectangle type has
specifically. Let’s now look at how we can continue to refactor this code by
turning the area function into an area method defined on our Rectangle
type.