Constructor in Rust 1.55

In Rust, constructors are known as associated functions. They are used to create new instances of a struct or an enum. To define an associated function in Rust, you use the impl block and the ::new notation. Here's an example of a simple struct named Rectangle:

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

To create a constructor for this struct, you need to define an associated function that returns a new instance of the struct. Here's an example of a constructor for the Rectangle struct:

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

In the above example, new is the name of the associated function. It takes two parameters length and width, both of type u32. The function then creates and returns a new instance of the Rectangle struct.

To use the constructor, you can call it like this:

let rect = Rectangle::new(10, 20);

This will create a new instance of the Rectangle struct with a length of 10 and a width of 20.

Here's another example that demonstrates the use of constructors with an enum:

enum Color {
    Red(u8, u8, u8),
    Green(u8, u8, u8),
    Blue(u8, u8, u8),
}
impl Color {

    fn new_red(r: u8, g: u8, b: u8) -> Color {

        Color::Red(r, g, b)

    }

    

    fn new_green(r: u8, g: u8, b: u8) -> Color {

        Color::Green(r, g, b)

    }

    

    fn new_blue(r: u8, g: u8, b: u8) -> Color {

        Color::Blue(r, g, b)

    }

}

In the above example, Color is an enum with three variants: Red, Green, and Blue. Each variant takes three parameters of type u8, representing the RGB values for the corresponding color. The Color struct also has three associated functions that serve as constructors for the Red, Green, and Blue variants. These constructors simply create a new instance of the Color enum and return it with the appropriate variant.

You can use these constructors like this:

let red = Color::new_red(255, 0, 0);
let green = Color::new_green(0, 255, 0);
let blue = Color::new_blue(0, 0, 255);

Each of these calls creates a new instance of the Color enum with the appropriate variant and RGB values.