Constructor

Sometimes a Rust program creates structs in many places throughout its code. This can lead to duplicate code, and some difficulty with validating how structs are created.

By adding a method that returns Self, we can create a constructor method. This returns an instance of a struct based on some arguments. We can even validate arguments to ensure program correctness.

Example

For demonstration, this program has a Bird struct that includes 4 separate fields. The fields are not important, but there are enough of them to make creating Bird structs cumbersome.

Part 1 We create a Bird struct by calling from_name. In from_name we return Self—a new Bird struct is returned.

Part 2 We use from_color, which is similar to from_name but validates its argument, and returns a Bird with the specified color.

Part 3 We call from_color with an invalid argument, and the program panics. The panic helps ensure programs are correct.

struct Bird {
    name: String,
    color: String,
    size: usize,
    index: usize,
}

impl Bird {
    fn from_name(name: &str) -> Self {
        Bird {
            name: name.to_string(),
            color: String::new(),
            size: usize::MAX,
            index: usize::MAX,
        }
    }
    fn from_color(color: &str) -> Self {
        // It is possible to validate arguments within a function.
        if color != "blue" {
            panic!("Color is invalid");
        }
        Bird {
            name: String::new(),
            color: color.to_string(),
            size: usize::MAX,
            index: usize::MAX,
        }
    }
}

fn main() {
    // Part 1: create Bird struct with just 1 value.
    let bird1 = Bird::from_name("parrot");
    println!("{}", bird1.name);

    // Part 2: use the from_color constructor.
    let bird2 = Bird::from_color("blue");
    println!("{}", bird2.color);

    // Part 3: our constructor validates its argument, so this will panic.
    let bird3 = Bird::from_color("tree");
}parrot
blue
thread 'main' panicked at src\main.rs:20:13:
Color is invalid

Struct notes

It is also possible in Rust to create structs based on existing structs, even const structs. This can also simplify code that must create many structs.

However To have a validation step, we must use a constructor method. This can be useful to ensure our code has the highest quality.

While Rust does not have special syntax for constructor methods, it does allow methods to return Self. This is done when implementing traits like From as well.