类的实例变量:

class Parent
  @things = []
  def self.things
    @things
  end
  def things
    self.class.things
  end
end

class Child < Parent
  @things = []
end

Parent.things << :car
Child.things  << :doll
mom = Parent.new
dad = Parent.new

p Parent.things #=> [:car]
p Child.things  #=> [:doll]
p mom.things    #=> [:car]
p dad.things    #=> [:car]

类变量:

class Parent
  @@things = []
  def self.things
    @@things
  end
  def things
    @@things
  end
end

class Child < Parent
end

Parent.things << :car
Child.things  << :doll

p Parent.things #=> [:car,:doll]
p Child.things  #=> [:car,:doll]

mom = Parent.new
dad = Parent.new
son1 = Child.new
son2 = Child.new
daughter = Child.new

[ mom, dad, son1, son2, daughter ].each{ |person| p person.things }
#=> [:car, :doll]
#=> [:car, :doll]
#=> [:car, :doll]
#=> [:car, :doll]
#=> [:car, :doll]

使用类上的实例变量(而不是该类的实例)，您可以存储该类的公共内容，而无需自动获取子类(反之亦然)。使用类变量，您可以方便地不必从实例对象编写self.class，并且(当需要时)还可以在整个类层次结构中实现自动共享。

将这些合并到一个例子中，还包括实例上的实例变量:

class Parent
  @@family_things = []    # Shared between class and subclasses
  @shared_things  = []    # Specific to this class

  def self.family_things
    @@family_things
  end
  def self.shared_things
    @shared_things
  end

  attr_accessor :my_things
  def initialize
    @my_things = []       # Just for me
  end
  def family_things
    self.class.family_things
  end
  def shared_things
    self.class.shared_things
  end
end

class Child < Parent
  @shared_things = []
end

然后是行动:

mama = Parent.new
papa = Parent.new
joey = Child.new
suzy = Child.new

Parent.family_things << :house
papa.family_things   << :vacuum
mama.shared_things   << :car
papa.shared_things   << :blender
papa.my_things       << :quadcopter
joey.my_things       << :bike
suzy.my_things       << :doll
joey.shared_things   << :puzzle
suzy.shared_things   << :blocks

p Parent.family_things #=> [:house, :vacuum]
p Child.family_things  #=> [:house, :vacuum]
p papa.family_things   #=> [:house, :vacuum]
p mama.family_things   #=> [:house, :vacuum]
p joey.family_things   #=> [:house, :vacuum]
p suzy.family_things   #=> [:house, :vacuum]

p Parent.shared_things #=> [:car, :blender]
p papa.shared_things   #=> [:car, :blender]
p mama.shared_things   #=> [:car, :blender]
p Child.shared_things  #=> [:puzzle, :blocks]  
p joey.shared_things   #=> [:puzzle, :blocks]
p suzy.shared_things   #=> [:puzzle, :blocks]

p papa.my_things       #=> [:quadcopter]
p mama.my_things       #=> []
p joey.my_things       #=> [:bike]
p suzy.my_things       #=> [:doll] 

虽然利用类变量看起来很有用，但由于类变量在子类之间共享，并且它们可以在单例方法和实例方法中引用，因此有一个显著的缺点。它们是共享的，所以子类可以改变类变量的值，基类也会受到这个变化的影响，这通常是不可取的行为:

class C
  @@c = 'c'
  def self.c_val
    @@c
  end
end

C.c_val
 => "c" 

class D < C
end

D.instance_eval do 
  def change_c_val
    @@c = 'd'
  end
end
 => :change_c_val 

D.change_c_val
(irb):12: warning: class variable access from toplevel
 => "d" 

C.c_val
 => "d" 

Rails引入了一个名为class_attribute的方便方法。顾名思义，它声明了一个类级属性，其值可由子类继承。class_attribute值可以在单例方法和实例方法中访问，类变量也是如此。然而，在Rails中使用class_attribute的巨大好处是子类可以改变自己的值，而且不会影响父类。

class C
  class_attribute :c
  self.c = 'c'
end

 C.c
 => "c" 

class D < C
end

D.c = 'd'
 => "d" 

 C.c
 => "c" 

简单示例显示

类变量的继承性 类实例变量的封装

注意:使用类<< self是一种方便，而不是必须在这个块中的所有方法前面加上self。注意:类<< self修改self，因此它指向父类的元类(参见https://stackoverflow.com/a/38041660/960184)

示例代码

class Parent
  class << self
    attr_reader :class_instance_var

    def class_instance_var=(value)
      @class_instance_var="set by #{self.name} to #{value}"
    end

    def class_var
      @@class_var
    end

    def class_var=(value)
      @@class_var = "set by #{self.name} to #{value}"
    end
  end
end

class Child < Parent
end

# use the instance separately in parent and subclass
puts "\n* Exercising class instance variable setters
* Setting Parent and Child class instance variables differently
* Parent.class_instance_var = 1000\n* Child.class_instance_var = 2000\n\n"

Parent.class_instance_var = 1000
Child.class_instance_var = 2000
puts "Parent.class_instance_var=(#{Parent.class_instance_var})"
puts "Child.class_instance_var=(#{Child.class_instance_var})"

# set class variable in via parent (changes both in parent and subclass)
puts "\n* Exercising Parent class variable setter
* Set class variable value to 3000 using parent, it changes in Child also
* Parent.class_var = 3000\n\n"

Parent.class_var = 3000
puts "Parent.class_var=(#{Parent.class_var})"
puts "Child.class_var=(#{Child.class_var})"

# set class variable in via subclass (changes both in parent and subclass)
puts "\n* Exercising Child class variable setter
* Set class variable value to 5000 using child, it changes in Parent also
* Child.class_var = 5000\n\n"

Child.class_var = 5000
puts "Parent.class_var=(#{Parent.class_var})"
puts "Child.class_var=(#{Child.class_var})"

使用ruby v3.0.2输出

* Exercising class instance variable setters
* Setting Parent and Child class instance variables differently
* Parent.class_instance_var = 1000
* Child.class_instance_var = 2000

Parent.class_instance_var=(set by Parent to 1000)
Child.class_instance_var=(set by Child to 2000)

* Exercising Parent class variable setter
* Set class variable value to 3000 using parent, it changes in Child also
* Parent.class_var = 3000

Parent.class_var=(set by Parent to 3000)
Child.class_var=(set by Parent to 3000)

* Exercising Child class variable setter
* Set class variable value to 5000 using child, it changes in Parent also
* Child.class_var = 5000

Parent.class_var=(set by Child to 5000)
Child.class_var=(set by Child to 5000)

正如其他人所说，类变量在给定的类及其子类之间共享。类实例变量只属于一个类;它的子类是分开的。

为什么会有这种行为?Ruby中的所有东西都是对象——甚至是类。这意味着每个类都有一个class类的对象(或者更确切地说，class的子类)对应于它。(当你说类Foo时，你实际上是在声明一个常量Foo并将一个类对象赋值给它。)每个Ruby对象都可以有实例变量，所以类对象也可以有实例变量。

问题是，类对象上的实例变量的行为并不像您通常希望的类变量的行为那样。通常，您希望在超类中定义的类变量与其子类共享，但实例变量不是这样工作的——子类有自己的类对象，而类对象有自己的实例变量。所以他们引入了独立的类变量来区分你更想要的行为。

换句话说，类实例变量在某种程度上是Ruby设计的一个意外。你可能不应该使用它们，除非你明确知道它们是你要找的东西。

源

实例方法的可用性

类实例变量仅对类方法可用，对实例方法不可用。 类变量对实例方法和类方法都可用。

继承性

类实例变量在继承链中丢失。 类变量不是。

class Vars

  @class_ins_var = "class instance variable value"  #class instance variable
  @@class_var = "class variable value" #class  variable

  def self.class_method
    puts @class_ins_var
    puts @@class_var
  end

  def instance_method
    puts @class_ins_var
    puts @@class_var
  end
end

Vars.class_method

puts "see the difference"

obj = Vars.new

obj.instance_method

class VarsChild < Vars


end

VarsChild.class_method

Ruby官方常见问题:类变量和类实例变量之间有什么区别?

主要的区别是有关继承的行为:类变量在类及其所有子类之间共享，而类实例变量只属于一个特定的类。

在某种程度上，类变量可以被视为继承层次结构上下文中的全局变量，而全局变量会带来各种问题。例如，一个类变量可能(意外地)被它的任何子类重新赋值，影响所有其他类:

class Woof

  @@sound = "woof"

  def self.sound
    @@sound
  end
end

Woof.sound  # => "woof"

class LoudWoof < Woof
  @@sound = "WOOF"
end

LoudWoof.sound  # => "WOOF"
Woof.sound      # => "WOOF" (!)

或者，一个祖先类可能稍后被重新打开并更改，可能会产生令人惊讶的效果:

class Foo

  @@var = "foo"

  def self.var
    @@var
  end
end

Foo.var  # => "foo" (as expected)

class Object
  @@var = "object"
end

Foo.var  # => "object" (!)

因此，除非您确切地知道自己在做什么并且显式地需要这种行为，否则最好使用类实例变量。