Ruby 中有好几种不同类型的标识符

• constants
• global variables
• instance variables
• class variables
• local variables

# 变量名的首字母是大写的
Pi = 3.14
module Math # module name is capitalized
  PI = 3.1415926 # constant is capitalized
  class DoubleFloating # class name is capitalized
    @@num_objs = 0
    def initialize
      @@num_objs += 1
      @value = 0.0
    end
  end
end

内建变量

nil, true, false, self