We use Structs in Ruby to create simple classes with constructor and some instance variables.

Look at this simple User class:

{% highlight ruby %} class User attr_accessor :first_name, :last_name, :age

def initialize(first_name, last_name, age) @first_name = first_name @last_name = last_name @age = age end end {% endhighlight %}

Using Struct, you can simply use a single line instead of declaring attr_accessors and constructor and the class will have exactly the same API:

{% highlight ruby %} User = Struct.new(:first_name, :last_name, :age) {% endhighlight %}

But when it comes to a class with internal resources like constants, you may get a warning:

{% highlight ruby %} User = Struct.new(:first_name, :last_name, :age) do MIN_AGE = 18

some methods dealing with user

end

Admin = Struct.new(:first_name, :last_name, :age) do MIN_AGE = 21 end

=> warning: already initialized constant MIN_AGE

=> warning: previous definition of MIN_AGE was here

{% endhighlight %}

What happends here? We assumed that MIN_AGE was declared inside User and Admin, but actually it was declared on the top level:

{% highlight ruby %}

Object::MIN_AGE => 18 {% endhighlight %}

And warnings were printed.

But wait, there is actually a proper way to subclass from Struct:

{% highlight ruby %} class User < Struct.new(:first_name, :last_name, :age) LIMIT = 1 end

class Admin < Struct.new(:first_name, :last_name, :age) MIN_AGE = 21 end {% endhighlight %}

No warnings here!

I also recommend you to check it the post by Steve Klabnik, where he describes all power of Struct.