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# Structs
A `struct`, short for "structure", gives us the ability to 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. `struct` and `enum` (that we will talk about in the next chapter)
are the building blocks you can use in Rust to create new types in your
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.
Let’s write a program which calculates the distance between two points.
We’ll start off with single variable bindings, and then refactor it to
use `struct`s instead.
One way of thinking about structs is that they are similar to tuples that 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.
Let’s make a new project with Cargo called `points`. Here’s a short program
which calculates the distance between two points to put into this project's
`src/main.rs`:
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, a struct to store information about a user account might look like:
```rust
struct User {
username: String,
email: String,
sign_in_count: u64,
active: bool,
}
```
To use a struct, we create an *instance* of that struct by specifying concrete
values for each of the fields. Creating an instance is done by declaring a
binding with `let`, 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:
```rust,ignore
let user1 = User {
email: "someone@example.com",
username: "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`.
## 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 variable
bindings, then refactor our program until we're using `struct`s 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. Here’s a short program that has one way of doing just
that to put into our project's `src/main.rs`:
Filename: src/main.rs
```rust
fn main() {
let x1 = 0.0;
let y1 = 5.0;
let length1 = 50;
let width1 = 30;
let x2 = 12.0;
let y2 = 0.0;
let answer = distance(x1, y1, x2, y2);
println!("Point 1: ({}, {})", x1, y1);
println!("Point 2: ({}, {})", x2, y2);
println!("Distance: {}", answer);
println!("The area of the rectangle is {}", area(length1, width1));
}
fn distance(x1: f64, y1: f64, x2: f64, y2: f64) -> f64 {
let x_squared = f64::powi(x2 - x1, 2);
let y_squared = f64::powi(y2 - y1, 2);
f64::sqrt(x_squared + y_squared)
fn area(length: u32, width: u32) -> u32 {
length * width
}
```
Let's try running this program with `cargo run`:
```bash
$ cargo run
Compiling points v0.1.0 (file:///projects/points)
Running `target/debug/points`
Point 1: (0, 5)
Point 2: (12, 0)
Distance: 13
The area of the rectangle is 1500
```
Let's take a quick look at `distance()` before we move forward. To find the
distance between two points, we can use the Pythagorean Theorem. The theorem is
named after Pythagoras, who was the first person to mathematically prove this
formula. The details aren't that important; just know the theorem says that the
formula for the distance between two points is equal to:
### Refactoring with Tuples
- squaring the distance between the points horizontally (the "x" direction)
- squaring the distance between the points vertically (the "y" direction)
- adding those together
- and taking the square root of that.
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.
So that's what we're implementing here.
The issue with this method is evident in the signature of `area`:
```rust,ignore
f64::powi(2.0, 3)
fn area(length: u32, width: u32) -> u32 {
```
The double colon (`::`) here is a namespace operator. We haven’t talked about
modules and namespaces in depth yet, but you can think of the `powi()` function
as being scoped inside of another name. In this case, the name is `f64`, the
same as the type. The `powi()` function takes two arguments: the first is a
number, and the second is the power that it raises that number to. In this
case, the second number is an integer, hence the `i` in its name. Similarly,
`sqrt()` is a function under the `f64` module, which takes the square root of
its argument.
The area function is supposed to calculate the area of one rectangle, but our
function takes two arguments. The arguments 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.
## Why `struct`s?
Our little program is okay, but we can do better. The key to seeing this is in
the signature of `distance()`:
```rust,ignore
fn distance(x1: f64, y1: f64, x2: f64, y2: f64) -> f64 {
```
The distance function is supposed to calculate the distance between two points.
But our distance function calculates some distance between four numbers. The
first two and last two arguments are related, but that’s not expressed anywhere
in our program itself. It would be nicer if we had a way to group `(x1, y1)`
and `(x2, y2)` together.
We’ve already discussed one way to do that: tuples. Here’s a version of our
program which uses tuples:
We’ve already discussed one way we might do that in Chapter 3: tuples. Here’s a
version of our program which uses tuples:
Filename: src/main.rs
```rust
fn main() {
let p1 = (0.0, 5.0);
let rect1 = (50, 30);
let p2 = (12.0, 0.0);
let answer = distance(p1, p2);
println!("Point 1: {:?}", p1);
println!("Point 2: {:?}", p2);
println!("Distance: {}", answer);
println!("The area of the rectangle is {}", area(rect1));
}
fn distance(p1: (f64, f64), p2: (f64, f64)) -> f64 {
let x_squared = f64::powi(p2.0 - p1.0, 2);
let y_squared = f64::powi(p2.1 - p1.1, 2);
f64::sqrt(x_squared + y_squared)
fn area(dimensions: (u32, u32)) -> u32 {
dimensions.0 * dimensions.1
}
```
This is a little better, for sure. Tuples let us add a little bit of structure.
We’re now passing two arguments, so that’s more clear. But it’s also worse:
tuples don’t give names to their elements, so our calculation has gotten more
confusing:
In one way, this is a little better. Tuples let us add a little bit of
structure, and we’re now passing just one argument. But in another way this
method less clear: tuples don’t give names to their elements, so our
calculation has gotten more confusing because we have to use the tuple's index:
```rust,ignore
p2.0 - p1.0
p2.1 - p1.1
dimensions.0 * dimensions.1
```
When writing this example, your authors almost got it wrong themselves! Distance
is all about `x` and `y` points, but our code is talking about `0` and `1`.
This isn’t great.
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.
Enter `struct`s. We can transform our tuples into something with a name for the
whole as well as names for the parts:
### Refactoring with Structs: Adding More Meaning
```rust,ignore
let p1 = (0.0, 5.0);
struct Point {
x: f64,
y: f64,
}
let p1 = Point { x: 0.0, y: 5.0 };
```
Here we've defined a `struct` and given it the name `Point`. The parts inside
`{}` are defining the _fields_ of the struct. We can have as many or as few of
them as we'd like, and we give them a name and specify their type. Here we have
two fields named `x` and `y`, and they both hold `f64`s.
We can access the field of a struct in the same way we access an element of
a tuple, except we use its name:
```rust,ignore
let p1 = (0.0, 5.0);
let x = p1.0;
struct Point {
x: f64,
y: f64,
}
let p1 = Point { x: 0.0, y: 5.0 };
let x = p1.x;
```
Let’s convert our program to use our `Point` `struct`. Here’s what it looks
like now:
Here is where we bring in `struct`s. We can transform our tuple into a data
type with a name for the whole as well as names for the parts:
Filename: src/main.rs
```rust
#[derive(Debug,Copy,Clone)]
struct Point {
x: f64,
y: f64,
struct Rectangle {
length: u32,
width: u32,
}
fn main() {
let p1 = Point { x: 0.0, y: 5.0};
let rect1 = Rectangle { length: 50, width: 30 };
let p2 = Point { x: 12.0, y: 0.0};
let answer = distance(p1, p2);
println!("Point 1: {:?}", p1);
println!("Point 2: {:?}", p2);
println!("Distance: {}", answer);
println!("The area of the rectangle is {}", area(&rect1));
}
fn distance(p1: Point, p2: Point) -> f64 {
let x_squared = f64::powi(p2.x - p1.x, 2);
let y_squared = f64::powi(p2.y - p1.y, 2);
f64::sqrt(x_squared + y_squared)
fn area(rectangle: &Rectangle) -> u32 {
rectangle.length * rectangle.width
}
```
Our function signature for `distance()` now says exactly what we mean: it
calculates the distance between two `Point`s. And rather than `0` and `1`,
we’ve got back our `x` and `y`. This is a win for clarity.
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.
## Derived Traits
Our `area` function now takes one argument 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.
There’s one other thing that’s a bit strange here, this stuff above the
`struct` declaration:
The `area` function accesses the `length` and `width` fields of the `Rectangle`
instance it got as an argument. 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 index
values of `0` and `1`. This is a win for clarity.
```rust,ignore
#[derive(Debug,Copy,Clone)]
struct Point {
```
### Adding Useful Functionality with Derived Traits
This is an annotation that tells the compiler our struct should get some
default behavior for the `Debug`, `Copy`, and `Clone` traits. We talked about
marking that types can be `Copy` and `Clone`-able in Chapter XX when we
discussed ownership. `Debug` is the trait that enables us to print out our
struct so that we can see its value while we are debugging our code.
So far, we’ve been printing values using `{}` in a `println!` macro. If we try
that with a struct, however, by default, we'll get an error. Say we have the
following program:
It'd be nice to be able to print out an instance of our `Rectangle` while we're
debugging our program and be able to see the values for all its fields. Let's
try using the `println!` macro as we have been and see what happens:
Filename: src/main.rs
```rust,ignore
struct Point {
x: f64,
y: f64,
```rust
struct Rectangle {
length: u32,
width: u32,
}
fn main() {
let p1 = Point { x: 0.0, y: 5.0};
println!("Point 1: {}", p1);
let rect1 = Rectangle { length: 50, width: 30 };
println!("The rectangle is {}", rect1);
}
```
This code tries to print the `p1` point directly, which may seem innocuous. But
running it produces the following output:
If we run this, we get an error with the core message ``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 a kind
of 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 options: 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`.
```bash
$ cargo run
Compiling points v0.1.0 (file:///projects/points)
error: the trait bound `Point: std::fmt::Display` is not satisfied [--explain E0277]
--> src/main.rs:8:29
8 |> println!("Point 1: {}", p1);
|> ^^
<std macros>:2:27: 2:58: note: in this expansion of format_args!
<std macros>:3:1: 3:54: note: in this expansion of print! (defined in <std macros>)
src/main.rs:8:5: 8:33: note: in this expansion of println! (defined in <std macros>)
note: `Point` cannot be formatted with the default formatter; try using `:?` instead if you are using a format string
note: required by `std::fmt::Display::fmt`
```
If we keep reading the error messages, though, we'll find ``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!("The rectangle 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.
Whew! The core of the error is this part: *the trait bound `Point:
std::fmt::Display` is not satisfied*. `println!` can do many kinds of
formatting. By default, `{}` implements a kind of formatting known as
`Display`: output intended for direct end-user consumption. The primitive types
we’ve seen implement `Display`, as there’s only one way you’d show a `1` to a
user. But with structs, the output is less clear. Do you want commas or not?
What about the `{}`s? Should all the fields be shown?
Let's try running with this change and... drat. We still get an error: ``the
trait bound `Rectangle: std::fmt::Debug` is not satisfied``. Again, though, the
compliler 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``.
More complex types in the standard library and that are defined by the
programmer do not automatically implement `Display` formatting. Standard
library types implement `Debug` formatting, which is intended for the
programmer to see. The `#[derive(Debug)]` annotation lets us use a default
implementation of `Debug` formatting to easily get this ability for types we've
defined. To ask `println!` to use `Debug` formatting with our `Point`, we add
the annotation to derive the trait and include `:?` in the print string, like
this:
Filename: src/main.rs
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, so now our program looks like this:
```rust
#[derive(Debug)]
struct Point {
x: f64,
y: f64,
struct Rectangle {
length: u32,
width: u32,
}
fn main() {
let p1 = Point { x: 0.0, y: 5.0};
println!("Point 1: {:?}", p1);
let rect1 = Rectangle { length: 50, width: 30 };
println!("The rectangle is {:?}", rect1);
}
```
If you run this, it should print the values of each field in the `Point` struct
as desired:
*Now* if we run this program, we won't get any errors and we'll see the
following output:
```bash
$ cargo run
Compiling points v0.1.0 (file:///projects/points)
Running `target/debug/points`
Point 1: Point { x: 0, y: 5 }
The rectangle is Rectangle { length: 50, width: 30 }
```
You’ll see this repeated later with other types. We’ll cover traits fully in
Chapter XX.
Neat! It's not the prettiest output, but it shows the values of all the fields
for this instance, which would definitely help during debugging.
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 XX. We'll be covering how to implement
these traits with different 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.