Ruby: private class level methods

ruby_private_class_level_methods.rb

class Foo
  class << self
    def bar
      p "im public"
    end
    
    private
    def baz
      p "im private"
    end
  end
end

Foo.bar # => im public
Foo.baz # => NoMethodError: private method `baz' called for Foo:Class

this_is_not_the_same.rb

class Foo
  def self.bar
    p "im public"
  end
  
  private
  def self.baz
    p "im private"
  end
end

Foo.bar # => im public
Foo.baz # => im private

It has been suggested that private is actually a method (not a directive or special syntax) and so the private method only changes the visibility of instance methods. Class methods on the other hand are instance methods of the Eigenclass.

In the first example (class << self) the private method goes through the Eigenclass (so the class methods technically become instance methods on the Eigenclass and so the private directive is enforced) but in the second example the private method goes through the Foo class (and its class level methods aren't affected by the private method).

You can get the second example (def self.baz) to work by using private_class_method :baz...

class Foo
  def self.bar
    p "im public"
  end

  def self.baz
    p "im private"
  end
  private_class_method :baz
end

Foo.bar # => im public
Foo.baz # => NoMethodError: private method `baz' called for Foo:Class

See also https://stackoverflow.com/a/12925407/322020 and other answers there.