メソッドをevalで定義する

7-8.rb

class LoggingInstanceVariable
   logging_instance_val_names = %w(first_val second_val third_val)

   logging_instance_val_names.each do |val_name| 
	  eval <<-END_OF_DEF
        attr_reader :#{val_name}, :before_#{val_name}

        def #{val_name}=(val)
          @before_#{val_name} = @#{val_name}
          @#{val_name} = val
        end
      END_OF_DEF
   end
end

obj = LoggingInstanceVariable.new

obj.first_val = 1
obj.first_val = 2

obj.first_val					# => 2
obj.before_first_val			# => 1

obj.first_val					# => 2
obj.before_first_val			# => 1

obj.third_val = :third_val
obj.third_val = 'third_val'

obj.third_val					# => "third_val"
obj.before_third_val			# => :third_val