ruby 類常量 解析

一個常量由大寫字母開頭.它應最多被賦值一次.在Ruby的目前的版本中,常量的再賦值只會產生警告而不是錯誤(non-ANSI版的eval.rb不會報告這一警告)

ruby>fluid=30
30
ruby>fluid=31
31
ruby>Solid=32
32
ruby>Solid=33
(eval):1: warning: already initialized constant Solid
33

常量可以定義在類裡,但不像實變數,它們可以在類的外部存取.

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

常量也可以定義在模組裡.

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