Both class and module names are constants in Ruby. We can get the list of all
classes and modules by viewing the all defined constants using
Module.constants.
{{code:php $result = get_declared_classes(); var_export($result); // => array(0 => 'stdClass', 1 => 'Exception', ...) }}
{{code:ruby p Module.constants # => ["TrueClass", "ObjectSpace", ...] }}
{{related: classobj/class_exists classobj/get_declared_interfaces classobj/get_defined_functions }}