Fetch all the classes in the Object namespace...

class_detector.rb

def blacklist
  [
    :Object, :Module, :Class, :BasicObject, :Kernel, :NilClass, :NIL, :Data,
    :TrueClass, :TRUE, :FalseClass, :FALSE, :Encoding, :Comparable, :Method,
    :String, :Symbol, :Exception, :SystemExit, :SignalException, :Interrupt,
    :StandardError, :TypeError, :ArgumentError, :IndexError, :KeyError, :Dir,
    :RangeError, :ScriptError, :SyntaxError, :LoadError, :NotImplementedError,
    :NameError, :NoMethodError, :RuntimeError, :SecurityError, :NoMemoryError,
    :EncodingError, :SystemCallError, :Errno, :ZeroDivisionError, :Enumerable,
    :FloatDomainError, :Numeric, :Integer, :Fixnum, :Float, :Bignum, :Array,
    :Hash, :ENV, :Struct, :RegexpError, :Regexp, :MatchData, :Marshal, :Range,
    :IOError, :EOFError, :IO, :STDIN, :STDOUT, :STDERR, :ARGF, :FileTest, :File,
    :Time, :Random, :Signal, :Process, :Proc, :LocalJumpError, :UnboundMethod,
    :Binding, :Math, :GC, :ObjectSpace, :Enumerator, :StopIteration, :RubyVM,
    :Thread, :TOPLEVEL_BINDING, :ThreadGroup, :Mutex, :ThreadError, :Fiber,
    :FiberError, :Rational, :Complex, :RUBY_RELEASE_DATE, :RUBY_PLATFORM, :ARGV,
    :RUBY_PATCHLEVEL, :RUBY_REVISION, :RUBY_DESCRIPTION, :RUBY_COPYRIGHT, :Gem,
    :RUBY_ENGINE, :TracePoint, :RbConfig, :Config, :CROSS_COMPILING, :Date, :IRB,
    :Exception2MessageMapper, :RubyToken, :RubyLex, :Readline, :SystemStackError
  ] + ENV.keys.map(&:to_sym)
end

def constants_for name
  (name.constants - blacklist).map { |constant| name.const_get constant }
end

def classes_for constant
  if constant.is_a? Class
    [constant]
  elsif constant.is_a? Module
    constants_for(constant).inject([]) do |array,constant|
      array << classes_for(constant)
      array
    end
  else
    []
  end
end

def constants
  constants_for(Object)
end
def classes
  constants.map { |const| classes_for const }.flatten
end

module A
  class B
  end
end
class C
end

puts "Consts  => #{constants}"
puts "Classes => #{classes}"

It also might be because I setup the blacklist based on 2.0.0 constants, just add the extra 1.9.2 ones into the black list...