20.1 Traits

A trait represents a functionality. It defines a list of one or more methods that a type must implement in order to provide this functionality.

In Rust, almost any data type can implement a trait:

• Structs.

• Enums.

• Primitive types.

• Tuples.

Inside the standard library:

• Many traits are defined (Clone, Copy, Debug, PartialEq, ...).

• Most of them declare only one method.

• Most of the provided types (structs, primitive types, ...) implement multiple traits.

A list of the most common traits of the standard library is available in Table 1. Among them are the comparison operators from the cmp module. Other operators (*, /, +, %, ...) are defined inside the ops module. They are listed inside the ops traits section.

20.1.1Defining a trait¶

As an example we want to create a trait Surface that defines a method for computing the area. We will use this trait for a structure Rectangle.

Here is the Rectangle structure:

:clear

struct Rectangle {
  width: f64,
  height: f64,
}

And an instance of that structure:

let rect = Rectangle {width: 7.2, height: 9.5};

To define a custom trait, we use the trait keyword. At least one method must be defined, and only its declaration (i.e.: function’s header, without a body) is necessary. In the following example, we define the trait Surface with its method area():

trait Surface {
  fn area(&self) -> f64;
}

To implement the trait for our structure, we use the impl keyword in its form impl <trait> for <struct>:

impl Surface for Rectangle {
  fn area(&self) -> f64 {
    self.width * self.height
  }
}

The area() method is now available for our rect object:

rect.area()