Private vs protected in Ruby

subclass_test.rb

class A
  protected

  def a_protected
    'can call protected'
  end

  private

  def a_private
    'can call private'
  end
end

class B < A
  def call_a_protected_with_explicit_self
    self.a_protected
  end

  def call_a_protected_with_implicit_self
    a_protected
  end

  def call_a_private_with_explicit_self
    self.a_private
  end

  def call_a_private_with_implicit_self
    a_private
  end

  def call_a_protected_on_a_instance(a)
    a.a_protected
  end
end

b = B.new

b.call_a_protected_with_explicit_self
# => "can call protected"

b.call_a_protected_with_implicit_self
# => "can call protected"

b.call_a_private_with_explicit_self
# NoMethodError: private method `a_private' called for #<B:0x007fb1ed9cc240>

b.call_a_private_with_implicit_self
# => "can call private"

A.new.a_protected
# NoMethodError: protected method `a_protected' called for #<A:0x007fae9881df00>

b.call_a_protected_on_a_instance(A.new)
# => "can call protected"