Another way of creating instance of `struct` in Rust

Till now, I knew only one way of creating instance of struct in Rust , which is mentioned in Rust’s official programming book, by adding curly bracket after the name of the struct and add key-value pairs inside curly brackets like below example.

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 structs attributes that will create instance of struct.

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

  • Add 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 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
        }
    );
}

Since I am from 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 #[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 ex: author, geners, pages), whose values have been set as default, will be removed from new function. You can create instance of Book only from title, as you can see in above example.

Now what will happen, if you pass all parameters in 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 traditional way, if you want to create 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 struct instance with default values. I hope you enjoy reading this blog. Thanks !


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Written by 

Ayush is the Sr. Lead Consultant @ Knoldus Software LLP. In his 10 years of experience he has become a developer with proven experience in architecting and developing web applications. Ayush has a Masters in Computer Application from U.P. Technical University, Ayush is a strong-willed and self-motivated professional who takes deep care in adhering to quality norms within projects. He is capable of managing challenging projects with remarkable deadline sensitivity without compromising code quality.

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