类变量

类变量具有类范围的范围，它们可以在类中的任何位置声明。当使用 @@ 作为前缀时，变量将被视为类变量

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"

类变量在相关类之间共享，可以从子类中重写

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

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

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

这种行为在大多数情况下都是不需要的，可以通过使用类级实例变量来规避。

模块内定义的类变量不会覆盖其包含类的类变量：

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!"