setting_reader.rb

class SettingReader
  def initialize hash
    # You may want to consider doing a deep copy of the hash and then #freeze-ing it
    @hash = hash.nil? ? {} : hash
    raise ArgumentError, "must pass a hash or nil" unless @hash.kind_of? Hash
  end 
  def get key, default = nil
    h = @hash
    for subkey in key.split(".")
      h = h[subkey]
      return default if h.nil?
    end
    # You may want to #freeze or #dup this here if you didn't in #initialize
    return h
  end
  alias [] get
end

sr = SettingReader.new nil

sr["a.b.c"] == nil or raise
sr["a"] == nil or raise

sr = SettingReader.new({"a" => { "b" => { "c" => 1 } }, "d" => 2})

sr["a"].kind_of?(Hash) or raise
sr["d"] == 2 or raise
sr["a.b.c"] == 1 or raise