Another Way of Creating struct Instances in Rust

Until now, I knew of only one way to create an instance of struct, which is mentioned in Rust’s official programming book — by adding curly brackets after the name of the struct and adding key-value pairs inside the curly brackets like below.

struct Book {
    title: String,
    author: String,
    genres: String,
    pages: u32,
}
let book = Book {
        title: String::from("Hey "),
        author: String::from("abc"),
        genres: "Fiction".to_string(),
        pages: 150,
    };

I explored another way of doing this using derive-new. This will add an impl fn new(…) -> Self method generated from the struct's attribute, which will create an instance of the struct.

Let’s create the above struct Book using derive-new. To use this,

• Add a dependency in Cargo.toml

[dependencies]
derive-new = "0.5"

• Include the macro

#[macro_use]
extern crate derive_new;

#[derive(new)]
struct Book {
    title: String,
    author: String,
    genres: String,
    pages: u32,
}

The #[derive(new)] line invokes a procedural macro in that crate, which generates code like:

impl Book {
    fn new(title_value: String, author_value: String, genres_value: String, pages_value: u32) -> Bar {
        Book { title: title_value, author: author_value, genres: genres_value, pages: pages_value }
    }
}

You can create an instance of Book.

#[test]
fn test_book() {
    let common_book = Book::new(
        String::from("Common"),
        String::new(),
        String::from("Programming"),
        100,
    );
    assert_eq!(
        common_book,
        Book {
            title: String::from("Common"),
            author: String::new(),
            genres: "Programming".to_string(),
            pages: 100
        }
    );
}

I am from a Scala background and I think of Rust’s struct as Scala’s case class, so another advantage of using #[derive(new)] is that we can set default values for struct variables. Default values can be specified either using the #[new(default)] attribute, which removes the argument from the constructor, or #[new(value = "..")]. Let’s see an example.

#[macro_use]
extern crate derive_new;

#[derive(Debug, new, PartialEq)]
struct Book {
    title: String,
    #[new(default)]
    author: String,
    #[new(value = r#""Programming".to_string()"#)]
    genres: String,
    #[new(value = "100")]
    pages: u32,
}

#[test]
fn test_book() {
    let common_book = Book::new(String::from("Common"));
    assert_eq!(
        common_book,
        Book {
            title: String::from("Common"),
            author: String::new(),
            genres: "Programming".to_string(),
            pages: 100
        }
    );
}

Now all those arguments (for example: author, genres, pages), whose values have been set as default, will be removed from the new function. You can create the instance of Book only from titles, as you can see in the above example.

Now what will happen if you pass all the parameters in a new function? Let’s see:

let another_book = Book::new(String::from("Hey"), String::from("abc"), "Fiction".to_string(), 150);

You will get a compilation error.

 error[E0061]: this function takes 1 parameter but 4 parameters were supplied
  --> src/main.rs:51:24
   |
4  | #[derive(Debug, new, PartialEq)]
   |                 --- defined here
...
51 |     let another_book = Book::new(String::from("Hey"), String::from("abc"), "Fiction".to_string(), 150);
   |                        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected 1 parameter

error: aborting due to previous error

For more information about this error, try `rustc --explain E0061`.
error: Could not compile `variables`.

You can now use the traditional way if you want to create a Book instance with different parameters.

    let another_book = Book {
        title: String::from("Hey "),
        author: String::from("abc"),
        genres: "Fiction".to_string(),
        pages: 150,
    };

derive-new is helpful when you want to create a struct instance with default values. I hope you enjoyed reading this blog.