Ruby Private Class Method

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The following is just so I can remember it… You need not read all this.

In Java, if you need a class whose constructor is private – so you can’t instantiate it from anywhere else, you’d do:

public class AClass {
  private int a;
  private AClass(int a){
    this.a = a;

In Ruby, it’s a bit different. The why will come later (dynamic language), but just hiding the initialize method is not enough. You can still create an object coz you are actually calling the new method:

irb(main):001:0> class AClass
irb(main):002:1>   attr_reader :a
irb(main):003:1>   private
irb(main):004:1>   def initialize(a)
irb(main):005:2>     @a = a
irb(main):006:2>     end
irb(main):007:1>   def some_other_method
irb(main):008:2>     "ha"
irb(main):009:2>     end
irb(main):010:1>   end
=> nil
irb(main):011:0> obj =
=> #<aclass :0xb75768c4 @a=100>
irb(main):012:0> obj.a
=> 100
irb(main):013:0> obj.some_other_method
NoMethodError: private method `some_other_method&#039; called for #</aclass><aclass :0xb75768c4 @a=100>
        from (irb):13
        from :0

You’d have to hide the new method:

class AClass
  attr_reader :a
  def initialize(a)
    @a = a
  def self.test
  private_class_method :test

puts AClass.test

That shows:

test.rb:12: private method `test&#039; called for AClass:Class (NoMethodError)