对于那些有c++背景的人来说，你可能会对与c++的比较感兴趣:

class S
{
private: // this is not quite true, in Ruby you can still access these
  static int    k = 23;
  int           s = 15;

public:
  int get_s() { return s; }
  static int get_k() { return k; }

};

std::cerr << S::k() << "\n";

S instance;
std::cerr << instance.s() << "\n";
std::cerr << instance.k() << "\n";

正如我们所看到的，k是一个静态变量。这完全像一个全局变量，除了它由类拥有(范围是正确的)。这样可以更容易地避免名称相似的变量之间的冲突。像任何全局变量一样，该变量只有一个实例，对它的修改总是对所有人可见。

On the other hand, s is an object specific value. Each object has its own instance of the value. In C++, you must create an instance to have access to that variable. In Ruby, the class definition is itself an instance of the class (in JavaScript, this is called a prototype), therefore you can access s from the class without additional instantiation. The class instance can be modified, but modification of s is going to be specific to each instance (each object of type S). So modifying one will not change the value in another.

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

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。

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] 

对于那些有c++背景的人来说，你可能会对与c++的比较感兴趣:

class S
{
private: // this is not quite true, in Ruby you can still access these
  static int    k = 23;
  int           s = 15;

public:
  int get_s() { return s; }
  static int get_k() { return k; }

};

std::cerr << S::k() << "\n";

S instance;
std::cerr << instance.s() << "\n";
std::cerr << instance.k() << "\n";

正如我们所看到的，k是一个静态变量。这完全像一个全局变量，除了它由类拥有(范围是正确的)。这样可以更容易地避免名称相似的变量之间的冲突。像任何全局变量一样，该变量只有一个实例，对它的修改总是对所有人可见。

On the other hand, s is an object specific value. Each object has its own instance of the value. In C++, you must create an instance to have access to that variable. In Ruby, the class definition is itself an instance of the class (in JavaScript, this is called a prototype), therefore you can access s from the class without additional instantiation. The class instance can be modified, but modification of s is going to be specific to each instance (each object of type S). So modifying one will not change the value in another.

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"