const_missing.rb

class TestConstMissing
  # default value
  DEFAULT_VALUE = 0
  
  # self because constants defined on that level
  # and not in instances
  def self.const_missing(name)
    puts "[log] Nope! '#{name}'"

    self::DEFAULT_VALUE
  end
end

class MyStuff < TestConstMissing
  # override default value
  DEFAULT_VALUE = 1
  def do
    puts "do '#{D}'"
  end
end

stuff = MyStuff.new
stuff.do

puts "Outside: '#{MyStuff::D}'"
puts "Parent: '#{TestConstMissing::D}'"

# Output:
# ======
# [log] Nope! 'D'
# do '1'
# [log] Nope! 'D'
# Outside: '1'
# [log] Nope! 'D'
# Parent: '0'