运行时动态修改成员/函数定义

#   E5.4-1.rb    
class Person    
  def talk
    puts "Today is Saturday. "    
  end  
end

p1=Person.new
p1.talk

class Person    
  def talk
    puts "Today is #{@date}. "    
  end  
 attr_accessor :date
end
p1.date="Sunday"
p1.talk

=begin    
Today is Saturday.
Today is Sunday.
=end