Difference between class variables and class instance variables?

Question

Can anyone tell me about the difference between class variables and class instance variables?

Answer 1

A class variable (@@) is shared among the class and all of its descendants. A class instance variable (@) is not shared by the class's descendants.

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Class variable (@@)

Let's have a class Foo with a class variable @@i, and accessors for reading and writing @@i:

class Foo    @@i = 1    def self.i     @@i   end    def self.i=(value)     @@i = value   end  end 

And a derived class:

class Bar < Foo end 

We see that Foo and Bar have the same value for @@i:

p Foo.i    # => 1 p Bar.i    # => 1 

And changing @@i in one changes it in both:

Bar.i = 2 p Foo.i    # => 2 p Bar.i    # => 2 

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Class instance variable (@)

Let's make a simple class with a class instance variable @i and accessors for reading and writing @i:

class Foo    @i = 1    def self.i     @i   end    def self.i=(value)     @i = value   end  end 

And a derived class:

class Bar < Foo end 

We see that although Bar inherits the accessors for @i, it does not inherit @i itself:

p Foo.i    # => 1 p Bar.i    # => nil 

We can set Bar's @i without affecting Foo's @i:

Bar.i = 2 p Foo.i    # => 1 p Bar.i    # => 2