類變數

類變數具有類範圍的範圍，它們可以在類中的任何位置宣告。當使用 @@ 作為字首時，變數將被視為類變數

class Dinosaur
    @@classification = "Like a Reptile, but like a bird"
    
    def self.classification
        @@classification
    end

    def classification
        @@classification
    end
end

dino = Dinosaur.new
dino.classification
# => "Like a Reptile, but like a bird"
  
Dinosaur.classification
# => "Like a Reptile, but like a bird"

類變數在相關類之間共享，可以從子類中重寫

class TRex < Dinosaur
    @@classification = "Big teeth bird!"
end

TRex.classification
# => "Big teeth bird!"

Dinosaur.classification
# => "Big teeth bird!"

這種行為在大多數情況下都是不需要的，可以通過使用類級例項變數來規避。

模組內定義的類變數不會覆蓋其包含類的類變數：

module SomethingStrange
    @@classification = "Something Strange"
end

class DuckDinosaur < Dinosaur
    include SomethingStrange
end

DuckDinosaur.class_variables
# => [:@@classification]
SomethingStrange.class_variables
# => [:@@classification]

DuckDinosaur.classification
# => "Big teeth bird!"