attr-with-default.rb

class Module
  def attr_with_default(name, default = nil, &init)
    n = "@#{name}"

    define_method name do
      instance_variable_get(n) || (init ? init.call(self) : default)
    end

    attr_writer name
  end
end


class Foo
  attr_with_default "bar", 123
  attr_with_default "baz" do |obj| obj.object_id end
end

f = Foo.new

printf "%-10s %p\n", 'bar', f.bar
f.bar = 99
printf "%-10s %p\n", 'bar', f.bar

printf "%-10s %p\n", 'baz', f.baz
f.baz = 11
printf "%-10s %p\n", 'baz', f.baz