一個(gè)常量由大寫字母開頭.它應(yīng)最多被賦值一次.在Ruby的當(dāng)前版本中,常量的再賦值只會(huì)產(chǎn)生警告而不是錯(cuò)誤(non-ANSI版的eval.rb不會(huì)報(bào)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
常量可以定義在類里,但不像實(shí)變量,它們可以在類的外部訪問(wèn).
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塊里.
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
