Ruby class变量 VS class实例变量 VS实例变量

#!/usr/bin/ruby

class A
    @@a = "a" # a class variable
    @b = "a" #a class instance variable
    
    attr_accessor :c

    def initialize(c)
        @c = c # an instance variable
    end
    
    class << self
        attr_accessor :b
    end

    def self.greeting
        puts @@a
        puts @b
    end

    
end

class B < A
    @@a = "b"
    @b = "b"
end

A.greeting #=> b a
B.greeting #=> b b
puts A.b
puts A.new('c').c

class变量 当前类及其子类共享
class实例变量 所有实例共享
实例变量 class new出的实例独享