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Ruby Programming
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Scope of Constants and Variables

Constants defined within a class or module may be accessed unadorned anywhere within the class or module. Outside the class or module, they may be accessed using the scope operator, ``::'' prefixed by an expression that returns the appropriate class or module object. Constants defined outside any class or module may be accessed unadorned or by using the scope operator ``::'' with no prefix. Constants may not be defined in methods.

OUTER_CONST = 99
class Const
  def getConst
    CONST
  end
  CONST = OUTER_CONST + 1
end
Const.new.getConst 100
Const::CONST 100
::OUTER_CONST 99

Global variables are available throughout a program. Every reference to a particular global name returns the same object. Referencing an uninitialized global variable returns nil.

Class variables are available throughout a class or module body. Class variables must be initialized before use. A class variable is shared among all instances of a class and is available within the class itself.

class Song
  @@count = 0
  def initialize
    @@count += 1
  end
  def Song.getCount
    @@count
  end
end

Class variables belong to the innermost enclosing class or module. Class variables used at the top level are defined in Object, and behave like global variables. Class variables defined within singleton methods belong to the receiver if the receiver is a class or a module; otherwise, they belong to the class of the receiver.

class Holder
  @@var = 99
  def Holder.var=(val)
    @@var = val
  end
end

a = Holder.new def a.var   @@var end


Ruby Programming
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