Ruby-类常量

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一个常量由大写字母开头.它应最多被赋值一次.在Ruby的当前版本中,常量的再赋值只会产生警告而不是错误(non-ANSI版的eval.rb不会报告这一警告) 

ruby>fluid=30

  30

ruby>fluid=31

  31

ruby>Solid=32

  32

ruby>Solid=33

  (eval):1: warning: already initialized constant Solid

  33  

常量可以定义在类里,但不像实变量,它们可以在类的外部访问.

ruby> class ConstClass

   |   C1=101

   |   C2=102

   |   C3=103

   |   def show

   |     print C1," ",C2," ",C3,"\n"

   |   end

   | end

  nil

ruby> C1

ERR: (eval):1: uninitialized constant C1

ruby> ConstClass::C1

  101

ruby> ConstClass.new.show

101 102 103

  nil  

常量也可以定义在模块里.

ruby> module ConstModule

   |   C1=101

   |   C2=102

   |   C3=103

   |   def showConstants

   |     print C1," ",C2," ",C3,"\n"

   |   end

   | end

  nil

ruby> C1

ERR: (eval):1: uninitialized constant C1

ruby> include ConstModule

  Object

ruby> C1

  101

ruby> showConstants

101 102 103

  nil

ruby> C1=99  # not really a good idea

  99

ruby> C1

  99

ruby> ConstModule::C1  # the module's constant is undisturbed ...

  101

ruby> ConstModule::C1=99 

ERR: (eval):1: compile error

(eval):1: parse error

ConstModule::C1=99

               ^

ruby> ConstModule::C1  # .. regardless of how we tamper with it.

  101