evalで動的にメソッドを定義

7-9.rb

class AttrClass
   def initialize
	  @attr = "attr"
   end

   def add_reader(instance_val_name)
	  eval <<-END_OF_DEF
        def #{instance_val_name}
          @#{instance_val_name}
        end
      END_OF_DEF
   end

   def add_writer(instance_val_name)
	  eval <<-END_OF_DEF
        def #{instance_val_name}=(val)
          @#{instance_val_name} = val
        end
      END_OF_DEF
   end
end
	  
attr_obj = AttrClass.new
attr_obj.respond_to? "attr"		# => false

attr_obj.add_reader "attr"

attr_obj.respond_to? "attr"		# => true
attr_obj.attr 					# => "attr"

attr_obj.respond_to? "attr="	# => false

attr_obj.add_writer "attr"

attr_obj.respond_to? "attr="	# => true
attr_obj.attr = "new val"		# => "new val"
attr_obj.attr					# => "new val"