Private classes in Ruby

One of the most common way to make some part of your code more understandable and explicit is to extract a class. However, many times this class is not intended for public usage. It’s an implementation detail of a bigger unit. It should not be used be anyone else but the module in which it is defined.

So how do we hide such class so that others are not tempted to use it? So that it is clear that it is an implementation detail?

I recently noticed that many people don’t know that since Ruby 1.9.3 you can make a constant private. And that’s your answer to how.

class Person
  class Secret
    def to_s
      "1234vW74X&"
    end
  end
  private_constant :Secret

  def show_secret
    Secret.new.to_s
  end
end

The Person class can use Secret freely:

Person.new.show_secret
# => 1234vW74X&

But others cannot access it.

Person::Secret.new.to_s
# NameError: private constant Person::Secret referenced

So Person is the public API that you expose to other parts of the system and Person::Secret is just an implementation detail.

You should probably not test Person::Secret directly as well but rather through the public Person API that your clients are going to use. That way your tests won’t be brittle and depended on implementation.

Summary

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