snippet.rb

class Pop  
    
  def initialize(&block)
    instance_eval &block if block
    puts default_doop
  end
  
  def default_doop
    self.class.doop
  end
  
  def self.doop
    @doop
  end
  
  def self.default_doop(str)
    @doop ||= str
  end
  
  default_doop "Not default"
end

Pop.new do
  @doop = "Inside instance"
  Pop.new
end