Haxe Code Cookbook
Haxe programming cookbookBeginnerDeclare classes using @:structInit

Declare classes using @:structInit

Reading time: 1 minute

As an alternative to the traditional way of instantiating classes using the new keyword - which is how it's done in most object oriented languages such as Java or C# - you can use the @:structInit metadata when you declare the class.

A class can be annotated with @:structInit metadata:

@:structInit class User {
    final name:String;
    var age:Int = 30;

    public function new(name:String, age:Int) {
        this.name = name;
        this.age = age;

    public function greet()
        trace( 'Hello, I\'m $name, and I\'m $age years old!');

Then it can be instantiated traditionally, with the new keyword:

var bob:User = new User('Bob', 32);

But it can also be instantiated using a compatible object structure, like the following:

var bob:User = {name: 'Bob', age: 32};

If the class is a simple data object, without any need for initial method calls when instantiated, it can be declared even more compactly by removing the constructor:

@:structInit class User {
    final name:String;
    var age:Int = 30;

    public function greet()
        trace('Hello, I\'m $name, and I\'m $age years old!');

Note that if the constructor is removed as above, new cannot be used for instantiaton:

var bob:User = {name: 'Bob', age: 32}; // works!
var bob:User = {name: 'Bob'}; // defaults Bob's age to 30 years

var bob:User = new User('Bob', 32);// gives error "User does not have a constructor" 

Why use @:structInit?

Simple typedef-like syntax but with full class power

Typedefs might be the most common way to declare simple data objects, and @:structInit gives you a way to use the same simple syntax - but you also get all the bells and whistles of real classes: you can use public or private fields and methods, getters and setters etc. just like in any class.

Performance gain on static targets

Haxe might perform better on static targets when using classes compared to using typedefs or anonymous objects. As a consequence, replacing typedefs with @:structInit classes might slightly increase the runtime performance.

Last modified:
Category:  Beginner