Ruby class instance variable vs class variable

Question

I read  quot https   stackoverflow com questions 826734 when-do-ruby-instance-variables-get-set quot  but I m of two minds when to use class instance variables  Class variables are shared by all objects of a class  Instance variables belong to one object  There s not much room left to use  class instance variables if we have class variables  Could someone explain the difference between these two and when to use them  Here s a code example  class S     k   23    s   15   def self s      s   end   def self k        k   end  end p S s  15 p S k  23  Update  I understand now  Class Instance Variables are not passed along the inheritance chain

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I believe the main  only   different is inheritance   class T  lt  S end  p T k   gt  23  S k   24 p T k   gt  24  p T s   gt  nil   Class variables are shared by all  class instances   i e  subclasses   whereas class instance variables are specific to only that class  But if you never intend to extend your class  the difference is purely academic

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For those with a C   background  you may be interested in a comparison with the C   equivalent   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  lt  lt  S  k    lt  lt    n    S instance  std  cerr  lt  lt  instance s    lt  lt    n   std  cerr  lt  lt  instance k    lt  lt    n     As we can see  k is a static like variable  This is 100  like a global variable  except that it s owned by the class  scoped to be correct   This makes it easier to avoid clashes between similarly named variables  Like any global variable  there is just one instance of that variable and modifying it is always visible by all   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

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While it may immediately seem useful to utilize class instance variables  since class instance variable are shared among subclasses  and they can be referred to within both singleton and instance methods  there is a singificant drawback  They are shared and so subclasses can change the value of the class instance variable  and the base class will also be affected by the change  which is usually undesirable behavior   class C     c    c    def self c val       c   end end  C c val    gt   c    class D  lt  C end  D instance eval do    def change c val       c    d    end end    gt   change c val   D change c val  irb  12  warning  class variable access from toplevel    gt   d    C c val    gt   d     Rails introduces a handy method called class attribute  As the name implies  it declares a class-level attribute whose value is inheritable by subclasses  The class attribute value can be accessed in both singleton and instance methods  as is the case with the class instance variable  However  the huge benefit with class attribute in Rails is subclasses can change their own value and it will not impact parent class   class C   class attribute  c   self c    c  end   C c    gt   c    class D  lt  C end  D c    d     gt   d     C c    gt   c

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Source  Availability to instance methods   Class instance variables are available only to class methods and not to instance methods  Class variables are available to both instance methods and class methods    Inheritability   Class instance variables are lost in the inheritance chain  Class variables are not    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  lt  Vars   end  VarsChild class method

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Instance variable on a class   class Parent    things        def self things      things   end   def things     self class things   end end  class Child  lt  Parent    things      end  Parent things  lt  lt   car Child things   lt  lt   doll mom   Parent new dad   Parent new  p Parent things    gt    car  p Child things     gt    doll  p mom things       gt    car  p dad things       gt    car    Class variable   class Parent     things        def self things       things   end   def things       things   end end  class Child  lt  Parent end  Parent things  lt  lt   car Child things   lt  lt   doll  p Parent things    gt    car  doll  p Child things     gt    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      gt    car   doll     gt    car   doll     gt    car   doll     gt    car   doll     gt    car   doll    With an instance variable on a class  not on an instance of that class  you can store something common to that class without having sub-classes automatically also get them  and vice-versa   With class variables  you have the convenience of not having to write self class from an instance object  and  when desirable  you also get automatic sharing throughout the class hierarchy     Merging these together into a single example that also covers instance variables on instances   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  lt  Parent    shared things      end   And then in action   mama   Parent new papa   Parent new joey   Child new suzy   Child new  Parent family things  lt  lt   house papa family things    lt  lt   vacuum mama shared things    lt  lt   car papa shared things    lt  lt   blender papa my things        lt  lt   quadcopter joey my things        lt  lt   bike suzy my things        lt  lt   doll joey shared things    lt  lt   puzzle suzy shared things    lt  lt   blocks  p Parent family things    gt    house   vacuum  p Child family things     gt    house   vacuum  p papa family things      gt    house   vacuum  p mama family things      gt    house   vacuum  p joey family things      gt    house   vacuum  p suzy family things      gt    house   vacuum   p Parent shared things    gt    car   blender  p papa shared things      gt    car   blender  p mama shared things      gt    car   blender  p Child shared things     gt    puzzle   blocks    p joey shared things      gt    puzzle   blocks  p suzy shared things      gt    puzzle   blocks   p papa my things          gt    quadcopter  p mama my things          gt     p joey my things          gt    bike  p suzy my things          gt    doll

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As others said  class variables are shared between a given class and its subclasses  Class instance variables belong to exactly one class  its subclasses are separate   Why does this behavior exist  Well  everything in Ruby is an object   even classes  That means that each class has an object of the class Class  or rather  a subclass of Class  corresponding to it    When you say class Foo  you re really declaring a constant Foo and assigning a class object to it   And every Ruby object can have instance variables  so class objects can have instance variables  too   The trouble is  instance variables on class objects don t really behave the way you usually want class variables to behave  You usually want a class variable defined in a superclass to be shared with its subclasses  but that s not how instance variables work   the subclass has its own class object  and that class object has its own instance variables  So they introduced separate class variables with the behavior you re more likely to want   In other words  class instance variables are sort of an accident of Ruby s design  You probably shouldn t use them unless you specifically know they re what you re looking for

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Official Ruby FAQ  What is the difference between class variables and class instance variables   The main difference is the behavior concerning inheritance  class variables are shared between a class and all its subclasses  while class instance variables only belong to one specific class   Class variables in some way can be seen as global variables within the context of an inheritance hierarchy  with all the problems that come with global variables  For instance  a class variable might  accidentally  be reassigned by any of its subclasses  affecting all other classes   class Woof      sound    woof     def self sound       sound   end end  Woof sound      gt   woof   class LoudWoof  lt  Woof     sound    WOOF  end  LoudWoof sound      gt   WOOF  Woof sound          gt   WOOF        Or  an ancestor class might later be reopened and changed  with possibly surprising effects   class Foo      var    foo     def self var       var   end end  Foo var      gt   foo   as expected   class Object     var    object  end  Foo var      gt   object        So  unless you exactly know what you are doing and explicitly need this kind of behavior  you better should use class instance variables

[ruby] Ruby class instance variable vs. class variable

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