Example Program Using Structs

Let’s build a simple program that calculates the area of rectangles to see how structs work in practice. We’ll compare Rust and JavaScript implementations.

Starting with Simple Values

Let’s begin with a simple function to calculate the area of a rectangle.

In JavaScript, we might write:

function calculateArea(width, height) {
  return width * height;
}

const rectWidth = 30;
const rectHeight = 50;
const area = calculateArea(rectWidth, rectHeight);
console.log(`The area of the rectangle is ${area} square pixels.`);

In Rust, the equivalent would be:

fn calculate_area(width: u32, height: u32) -> u32 {
    width * height
}

fn main() {
    let rect_width = 30;
    let rect_height = 50;
    let area = calculate_area(rect_width, rect_height);
    println!("The area of the rectangle is {} square pixels.", area);
}

Both examples work fine, but they have limitations. The width and height are separate variables that aren’t clearly connected. Let’s refactor to group them.

Using Tuples

We can use tuples to group the width and height together:

In JavaScript:

function calculateArea([width, height]) {
  return width * height;
}

const rect = [30, 50];
const area = calculateArea(rect);
console.log(`The area of the rectangle is ${area} square pixels.`);

In Rust:

fn calculate_area(dimensions: (u32, u32)) -> u32 {
    dimensions.0 * dimensions.1
}

fn main() {
    let rect = (30, 50);
    let area = calculate_area(rect);
    println!("The area of the rectangle is {} square pixels.", area);
}

This approach groups our data together, but it lacks clarity. When we access dimensions.0 and dimensions.1, it’s not immediately clear which is width and which is height.

Using Objects/Structs for Better Semantics

In JavaScript, we can use objects to make our code more clear:

function calculateArea({ width, height }) {
  return width * height;
}

const rect = {
  width: 30,
  height: 50
};

const area = calculateArea(rect);
console.log(`The area of the rectangle is ${area} square pixels.`);

In Rust, we’ll use a struct:

struct Rectangle {
    width: u32,
    height: u32,
}

fn calculate_area(rectangle: &Rectangle) -> u32 {
    rectangle.width * rectangle.height
}

fn main() {
    let rect = Rectangle {
        width: 30,
        height: 50,
    };

    let area = calculate_area(&rect);
    println!("The area of the rectangle is {} square pixels.", area);
}

Notice these key differences:

In Rust, we define the Rectangle struct with explicit types
We pass a reference (&Rectangle) to avoid transferring ownership
The syntax for accessing properties is the same in both languages (rectangle.width)

Adding Debug Output

If we want to print our rectangle for debugging:

In JavaScript, this is straightforward:

console.log("Rectangle:", rect);  // Rectangle: { width: 30, height: 50 }

In Rust, we need to opt-in to the Debug trait:

#[derive(Debug)]
struct Rectangle {
    width: u32,
    height: u32,
}

fn main() {
    let rect = Rectangle {
        width: 30,
        height: 50,
    };

    println!("Rectangle: {:?}", rect);  // Rectangle: Rectangle { width: 30, height: 50 }

    // Pretty print
    println!("Rectangle:\n{:#?}", rect);
    // Rectangle:
    // Rectangle {
    //     width: 30,
    //     height: 50,
    // }
}

The #[derive(Debug)] is an attribute that automatically implements the Debug trait for our struct, which allows it to be formatted with {:?} and {:#?} (pretty print).

Adding Methods to Your Types

In JavaScript, we might make a Rectangle class with methods:

class Rectangle {
  constructor(width, height) {
    this.width = width;
    this.height = height;
  }

  area() {
    return this.width * this.height;
  }

  isSquare() {
    return this.width === this.height;
  }
}

const rect = new Rectangle(30, 50);
console.log(`Area: ${rect.area()}`);
console.log(`Is square: ${rect.isSquare()}`);

In Rust, we’d use an impl block to add methods to our struct:

#[derive(Debug)]
struct Rectangle {
    width: u32,
    height: u32,
}

impl Rectangle {
    fn area(&self) -> u32 {
        self.width * self.height
    }

    fn is_square(&self) -> bool {
        self.width == self.height
    }
}

fn main() {
    let rect = Rectangle {
        width: 30,
        height: 50,
    };

    println!("Area: {}", rect.area());
    println!("Is square: {}", rect.is_square());
}

The methods are called in the same way in both languages, but there are important differences in how they’re defined:

In Rust, methods are defined separately from the struct in an impl block
The first parameter in Rust methods is typically &self, which is similar to JavaScript’s implicit this
Rust requires type annotations for return values

Adding Associated Functions (Static Methods)

In JavaScript, we might add static methods to a class:

class Rectangle {
  constructor(width, height) {
    this.width = width;
    this.height = height;
  }

  // Instance method
  area() {
    return this.width * this.height;
  }

  // Static method
  static square(size) {
    return new Rectangle(size, size);
  }
}

const square = Rectangle.square(20);
console.log(`Area of square: ${square.area()}`);

In Rust, we can add associated functions (similar to static methods):

#[derive(Debug)]
struct Rectangle {
    width: u32,
    height: u32,
}

impl Rectangle {
    // Regular method (takes &self)
    fn area(&self) -> u32 {
        self.width * self.height
    }

    // Associated function (no self parameter)
    fn square(size: u32) -> Rectangle {
        Rectangle {
            width: size,
            height: size,
        }
    }
}

fn main() {
    // Call the associated function with ::
    let square = Rectangle::square(20);
    println!("Area of square: {}", square.area());
}

Key differences:

In Rust, associated functions don’t take self as a parameter
Rust uses the :: syntax to call associated functions (similar to static methods in JavaScript)

Multiple `impl` Blocks

In Rust, you can have multiple impl blocks for the same struct, which is a way to organize your code:

#[derive(Debug)]
struct Rectangle {
    width: u32,
    height: u32,
}

// First impl block with basic methods
impl Rectangle {
    fn area(&self) -> u32 {
        self.width * self.height
    }

    fn perimeter(&self) -> u32 {
        2 * (self.width + self.height)
    }
}

// Second impl block with factory methods
impl Rectangle {
    fn square(size: u32) -> Rectangle {
        Rectangle {
            width: size,
            height: size,
        }
    }

    fn new(width: u32, height: u32) -> Rectangle {
        Rectangle { width, height }
    }
}

This is useful for organizing code, especially when implementing traits (coming up in a later section).

A Complete Rectangle Program

Here’s a more complete example in Rust showcasing various features:

#[derive(Debug)]
struct Rectangle {
    width: u32,
    height: u32,
}

impl Rectangle {
    // Constructor
    fn new(width: u32, height: u32) -> Rectangle {
        Rectangle { width, height }
    }

    // Factory method for squares
    fn square(size: u32) -> Rectangle {
        Rectangle::new(size, size)
    }

    // Regular methods
    fn area(&self) -> u32 {
        self.width * self.height
    }

    fn perimeter(&self) -> u32 {
        2 * (self.width + self.height)
    }

    fn is_square(&self) -> bool {
        self.width == self.height
    }

    // Method that modifies the rectangle
    fn scale(&mut self, factor: u32) {
        self.width *= factor;
        self.height *= factor;
    }

    // Method that checks if this rectangle can contain another
    fn can_hold(&self, other: &Rectangle) -> bool {
        self.width > other.width && self.height > other.height
    }
}

fn main() {
    // Create rectangles
    let mut rect1 = Rectangle::new(30, 50);
    let rect2 = Rectangle::square(20);

    // Use methods
    println!("Rectangle 1: {:?}", rect1);
    println!("Area: {}", rect1.area());
    println!("Perimeter: {}", rect1.perimeter());
    println!("Is square: {}", rect1.is_square());

    println!("\nRectangle 2: {:?}", rect2);
    println!("Area: {}", rect2.area());
    println!("Is square: {}", rect2.is_square());

    // Check containment
    println!("\nCan rect1 hold rect2? {}", rect1.can_hold(&rect2));

    // Modify rect1
    rect1.scale(2);
    println!("\nAfter scaling rect1: {:?}", rect1);
    println!("New area: {}", rect1.area());
}

The equivalent in JavaScript:

class Rectangle {
  constructor(width, height) {
    this.width = width;
    this.height = height;
  }

  // Static factory method for squares
  static square(size) {
    return new Rectangle(size, size);
  }

  // Regular methods
  area() {
    return this.width * this.height;
  }

  perimeter() {
    return 2 * (this.width + this.height);
  }

  isSquare() {
    return this.width === this.height;
  }

  // Method that modifies the rectangle
  scale(factor) {
    this.width *= factor;
    this.height *= factor;
    return this;  // for method chaining
  }

  // Method that checks if this rectangle can contain another
  canHold(other) {
    return this.width > other.width && this.height > other.height;
  }
}

// Create rectangles
const rect1 = new Rectangle(30, 50);
const rect2 = Rectangle.square(20);

// Use methods
console.log("Rectangle 1:", rect1);
console.log("Area:", rect1.area());
console.log("Perimeter:", rect1.perimeter());
console.log("Is square:", rect1.isSquare());

console.log("\nRectangle 2:", rect2);
console.log("Area:", rect2.area());
console.log("Is square:", rect2.isSquare());

// Check containment
console.log("\nCan rect1 hold rect2?", rect1.canHold(rect2));

// Modify rect1
rect1.scale(2);
console.log("\nAfter scaling rect1:", rect1);
console.log("New area:", rect1.area());

Summary

Through this example, we’ve seen how to:

Define a struct to group related data
Add methods to a struct using impl blocks
Create constructor and factory methods (associated functions)
Create methods that borrow the instance immutably (&self) or mutably (&mut self)
Add debug printing capabilities with #[derive(Debug)]

Structs in Rust provide a powerful way to create custom types that bundle data with behavior in a way that will feel familiar to JavaScript developers who use classes, but with Rust’s added benefits of memory safety and performance.

In the next section, we’ll dive deeper into method syntax and explore how to implement even more functionality on our structs.