Ruby gives you three levels of access protection, public, protected and private. The public methods can be called by anyone. In this article, we will look at some examples to learn about the access control and the subtle difference between private and protected access control.

Internal Visibility

class Foo
  def testing_public
  end

  protected
  def testing_protected
  end

  private
  def testing_private
  end

  def ok
    testing_public
    testing_protected
    testing_private
  end
end

f = Foo.new
f.ok

This will throw the exception.

NoMethodError: private method ‘ok’ called for Foo.

External Visibility

Only the public method can be called outside the Foo class.

f = Foo.new
f.testing_public

will work. The methods testing_protected and testing_private will not work.

Difference between Private and Protected

The difference between private and protected can be demonstrated using the example below. The Foo class remains the same as shown above.

class Bar < Foo
  def do_something(parent)
    parent.testing_public
    parent.testing_protected
    parent.testing_private
  end
end

b = Bar.new
b.do_something(Foo.new)

We can call the protected method but we cannot call the private method. We will get the error:

NoMethodError: private method ‘testing_private’ called for Foo.

This article is based on the blog post by Aaron Patterson.

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