class_map.rb

require "delegate"

class ClassMap < DelegateClass(Hash)
  def [](klass)
    __getobj__.fetch(klass) {
      __getobj__.each { |key, value|
        return value if klass == key.to_s
      }
      nil
    }
  end
end

map = ClassMap.new({
  Integer => "1",
  String => "2"
})

pp map[String]
pp map[Integer]
pp map["String"]
pp map["Integer"]
pp map["foo"]