gistfile1.rb

class Class
  def attr_with_default(attr_name, default_value)
    attr_writer attr_name
    define_method(attr_name) do
      instance_variable_get("@#{attr_name}") || default_value
    end
  end
end

class Person
  attr_with_default :age, 3
end

person = Person.new
p person.age # => 3
person.age = 30
p person.age # => 30

class Class
  def defaults(hash)
    hash.each do |attribute_name, default_value|
      define_method(attribute_name) do
        instance_variable_get("@#{attribute_name}") || instance_variable_set("@#{attribute_name}", default_value)
      end
    end
  end
end

class Video
  attr_accessor :width, :height
  defaults :width => 400, :height => 300
end

v = Video.new
p v.width
p v.height
v.width = 123
p v.width