## An Example Program Using Structs

To understand when we might want to use 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.

Let’s make a new binary project with Cargo called rectangles that will take the width and height of a rectangle specified in pixels and will calculate the area of the rectangle. Listing 5-8 shows a short program with one way of doing just that in our project’s src/main.rs:

Filename: src/main.rs

    fn main() {
        let width1 = 30;
        let height1 = 50;

        println!(
            "The area of the rectangle is {} square pixels.",
            area(width1, height1)
        );
    }

    fn area(width: u32, height: u32) -> u32 {
        width * height
    }

Listing 5-8: Calculating the area of a rectangle specified by its width and height in separate variables

Now, run this program using `cargo run`:

    The area of the rectangle is 1500 square pixels.

### Refactoring with Tuples

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. The width and the height are related to each other because together they describe one rectangle.

The issue with this code is evident in the signature of `area`:

    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 “Grouping Values into Tuples” section of Chapter 3: by using tuples. Listing 5-9 shows another version of our program that uses tuples:

Filename: src/main.rs

    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:

Filename: src/main.rs

    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 `{}` we defined the fields as `width` and `height`, both of which have type `u32`. Then in `main` we create a particular instance of a `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 a `Rectangle`, using its `width` and `height` fields. This conveys that the width and height 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 5-11 tries the `println!` macro as we have used it in Chapters 2, 3, and 4:

Filename: src/main.rs

    struct Rectangle {
        width: u32,
        height: u32,
    }

    fn main() {
        let rect1 = Rectangle { width: 30, height: 50 };

        println!("rect1 is {}", rect1);
    }

Listing 5-11: Attempting to print a `Rectangle` instance

When we run this code, 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, 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`.

If we continue reading the errors, we’ll find this helpful 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!` macro call 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 we can see its value while we’re debugging our code.

Run the code with this change. Drat! We still get an error:

    error[E0277]: the trait bound `Rectangle: std::fmt::Debug` is not satisfied

But again, the compiler gives us a helpful 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 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:

Filename: src/main.rs

    #[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.