Easy Rust

Structs

With structs, you can create your own type. You will use structs all the time in Rust because they are so convenient. Structs are created with the keyword struct. The name of a struct should be in UpperCamelCase (capital letter for each word, no spaces). If you write a struct in all lowercase, the compiler will tell you.

There are three types of structs. One is a "unit struct". Unit means "doesn't have anything". For a unit struct, you just write the name and a semicolon.

struct FileDirectory;
fn main() {}

The next is a tuple struct, or an unnamed struct. It is "unnamed" because you only need to write the types, not the field names. Tuple structs are good when you need a simple struct and don't need to remember names.

struct Colour(u8, u8, u8);

fn main() {
    let my_colour = Colour(50, 0, 50); // Make a colour out of RGB (red, green, blue)
    println!("The second part of the colour is: {}", my_colour.1);
}

This prints The second part of the colour is: 0.

The third type is the named struct. This is probably the most common struct. In this struct you declare field names and types inside a {} code block. Note that you don't write a semicolon after a named struct, because there is a whole code block after it.

struct Colour(u8, u8, u8); // Declare the same Colour tuple struct

struct SizeAndColour {
    size: u32,
    colour: Colour, // And we put it in our new named struct
}

fn main() {
    let my_colour = Colour(50, 0, 50);

    let size_and_colour = SizeAndColour {
        size: 150,
        colour: my_colour
    };
}

You separate fields by commas in a named struct too. For the last field you can add a comma or not - it's up to you. SizeAndColour had a comma after colour:

struct Colour(u8, u8, u8); // Declare the same Colour tuple struct

struct SizeAndColour {
    size: u32,
    colour: Colour, // And we put it in our new named struct
}

fn main() {}

but you don't need it. But it can be a good idea to always put a comma, because sometimes you will change the order of the fields:

struct Colour(u8, u8, u8); // Declare the same Colour tuple struct

struct SizeAndColour {
    size: u32,
    colour: Colour // No comma here
}

fn main() {}

Then we decide to change the order...

struct SizeAndColour {
    colour: Colour // ⚠️ Whoops! Now this doesn't have a comma.
    size: u32,
}

fn main() {}

But it is not very important either way so you can choose whether to use a comma or not.

Let's create a Country struct to give an example. The Country struct has the fields population, capital, and leader_name.

struct Country {
    population: u32,
    capital: String,
    leader_name: String
}

fn main() {
    let population = 500_000;
    let capital = String::from("Elista");
    let leader_name = String::from("Batu Khasikov");

    let kalmykia = Country {
        population: population,
        capital: capital,
        leader_name: leader_name,
    };
}

Did you notice that we wrote the same thing twice? We wrote population: population, capital: capital, and leader_name: leader_name. Actually, you don't need to do that. If the field name and variable name are the same, you don't have to write it twice.

struct Country {
    population: u32,
    capital: String,
    leader_name: String
}

fn main() {
    let population = 500_000;
    let capital = String::from("Elista");
    let leader_name = String::from("Batu Khasikov");

    let kalmykia = Country {
        population,
        capital,
        leader_name,
    };
}