In Rust, the term “derive” typically refers to a procedural macro that automatically implements certain traits for your structs or enums. This feature helps reduce boilerplate code by generating implementations based on the structure of your types.

Common Usage of derive

When you use the #[derive(...)] attribute above a struct or enum, you’re telling the Rust compiler to automatically generate implementations of specified traits. Some common traits that can be derived include:

  • Debug: Enables formatting the struct for debugging.
  • Clone: Allows creating a copy of the struct.
  • Copy: A marker trait for types that can be duplicated simply by copying bits (usually simple types).
  • PartialEq: Enables comparison using == and !=.
  • Eq: A marker trait for types that can be compared for equality.
  • PartialOrd: Allows comparison using <, >, etc.
  • Ord: A trait for types that have a total ordering.

Example

Here’s a simple example of using derive in Rust:

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#[derive(Debug, Clone, PartialEq)]
struct Point {
    x: f64,
    y: f64,
}

fn main() {
    let p1 = Point { x: 1.0, y: 2.0 };
    let p2 = p1.clone(); // Using the derived Clone trait
    println!("{:?}", p1); // Using the derived Debug trait
    assert_eq!(p1, p2); // Using the derived PartialEq trait
}

Custom Derive Macros

You can also create your own custom derive macros to implement traits specific to your application. This involves writing a procedural macro that generates the appropriate code.

Summary

In summary, derive in Rust is a powerful feature that simplifies the implementation of common traits for your types, allowing for cleaner and more maintainable code.

In Rust, if you derive the Default trait for a struct or enum, it allows you to create a default value for that type. The Default trait provides a way to create a “default” instance of a type, which is useful when you want to initialize a struct or enum with default values.

Using derive(Default)

When you use #[derive(Default)], Rust automatically generates an implementation of the Default trait for your type. The default value for each field is determined by its type’s own default.

Example

Here’s a simple example:

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#[derive(Debug, Default)]
struct Point {
    x: f64,
    y: f64,
}

fn main() {
    let default_point = Point::default(); // Creates a Point with default values (x: 0.0, y: 0.0)
    println!("{:?}", default_point);
}

Behavior of Default

  • For primitive types like f64, the default value is 0.0.
  • For bool, the default is false.
  • For String, the default is an empty string ("").
  • For Option<T>, the default is None.

Custom Default Implementation

If you want to specify custom default values, you can implement the Default trait manually instead of deriving it:

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struct Point {
    x: f64,
    y: f64,
}

impl Default for Point {
    fn default() -> Self {
        Point { x: 10.0, y: 20.0 } // Custom default values
    }
}

fn main() {
    let custom_default_point = Point::default();
    println!("{:?}", custom_default_point); // Outputs: Point { x: 10.0, y: 20.0 }
}

Summary

Using #[derive(Default)] simplifies creating default values for your types, while implementing the Default trait manually allows for more control over what those default values are.