?一個常量由大寫字母開頭.它應最多被賦值一次.在Ruby的當前版本中,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.html" target="_blank">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,但不像實變量,它們可以在類的外部訪問.

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???

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