Hello! In this lesson, we will explore how to add functionality to your structs using methods in Rust. Just as you use functions to encapsulate behavior in your programs, methods allow you to encapsulate behavior within a struct. This lesson will cover defining methods, creating constructors, and using mutable methods to add functionality to structs.

Let's get started!

Methods in Rust are similar to functions, but they are associated with an instance of a struct and can operate on its data. Think of methods as actions that instances of structs can perform. Let's use a Rectangle struct as an analogy to understand this concept better.

Imagine we have a Rectangle struct, and we want this rectangle to perform certain actions, such as calculating its area or changing its dimensions. Methods allow us to define these actions directly so the rectangle "knows" how to perform these actions on its own.

In Rust, methods are defined within an impl block, connecting them to a specific struct. This enables us to associate behavior with the struct. The first parameter of a method must always be &self. The self keyword refers to the instance of the struct. We use & to prevent the method from taking ownership of the instance. After defining a method, call it using dot syntax on the desired instance of the struct. The instance of the struct automatically gets passed as the self parameter. Let's take a look:

Rust
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1// Define a struct
2struct Rectangle {
3    width: f32,
4    height: f32,
5}
6
7// Implement methods for the struct
8impl Rectangle {
9
10    // Method to calculate the area
11    fn area(&self) -> f32 {
12        self.width * self.height
13    }
14}
15
16fn main() {
17    // Create an instance of Rectangle
18    let rect = Rectangle {
19        width: 30.5,
20        height: 50.1,
21    };
22
23    // Call the area method
24    let rect_area = rect.area();
25    println!("Area: {}", rect_area); // Prints: Area: 1528.0499
26}

In this example:

• We defined a struct named Rectangle with two fields: width and height, both of type f32.
• Inside the impl block, we defined a method area that borrows the instance of the struct using &self. This method calculates and returns the area of the rectangle.
• We then created an instance of Rectangle, called rect
• We use rect.area() to call the area method. rect automatically gets passed as the self parameter.

Associated functions in Rust are functions that are defined within the impl block but are not associated with an instance of the struct. These functions do not take &self as a parameter. This is because they are called on the struct data type itself rather than an instance of the struct.

Associated functions are often used to create constructors for a struct. The take in the necessary data to instantiate the struct and return an instance of the struct containing this data. Let's take a look at the syntax to create a constructor for our Rectangle struct.

Rust
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1// Define a struct
2struct Rectangle {
3    width: f32,
4    height: f32,
5}
6
7impl Rectangle {
8
9    // Method to calculate the area
10    fn area(&self) -> f32 {
11        self.width * self.height
12    }
13
14    // Associated function to create a new Rectangle instance
15    fn new(width: f32, height: f32) -> Rectangle {
16        Rectangle { width: width, height: height }
17    }
18}
19
20fn main() {
21    // Create an instance using the associated function
22    let rect = Rectangle::new(30.5, 50.1);
23
24    // Call methods on the struct
25    println!("Area: {}", rect.area()); // Prints: Area: 1528.0499
26}

In this example:

• We added an associated function new that takes width and height as parameters and returns a new instance of Rectangle.
• In the main function, we used Rectangle::new to create a new instance, passing the width and height as parameters.
• We then call the area method and print the result.

Sometimes, we need to modify the fields of a struct. In such cases, methods can be defined to take a mutable reference to self. Let's add a method called change_dims to change the dimensions of our rectangle.

Rust
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1// Define a struct
2struct Rectangle {
3    width: f32,
4    height: f32,
5}
6
7impl Rectangle {
8
9    // Method to calculate the area
10    fn area(&self) -> f32 {
11        self.width * self.height
12    }
13
14    // Associated function to create a new Rectangle instance
15    fn new(width: f32, height: f32) -> Rectangle {
16        Rectangle { width: width, height: height }
17    }
18
19    // Method to change the dimensions of the rectangle
20    fn change_dims(&mut self, width: f32, height: f32) {
21        self.width = width;
22        self.height = height;
23    }
24}
25
26fn main() {
27    // Create an instance using the associated function
28    let mut rect = Rectangle::new(30.5, 50.1);
29
30    // Call methods on the struct
31    println!("Area: {}", rect.area()); // Prints: Area: 1528.0499
32
33    // Change dimensions and print the new area
34    rect.change_dims(20.0, 30.3);
35    println!("New Area: {}", rect.area()); // Prints: New Area: 606
36}

In this example:

• We added a mutable method change_dims that takes &mut self, width, and height.
• The method updates the width and height fields using self.width and self.height.
• In the main function, we created a mutable instance of Rectangle and called change_dims to modify its dimensions.
• We then calculated and printed the new area of the rectangle.

Excellent work! Today, you've learned how to define methods to add functionality to your structs, create constructors, and use mutable methods to modify struct fields. These skills enhance the utility and maintainability of your code by encapsulating behavior within the structs themselves.

Understanding how to integrate behavior with data using methods is crucial for writing clean and reusable code. Now, it's time to practice what you've learned. Dive into the exercises to reinforce your understanding and see the power of methods in action.

Let's get coding!