ruby - Difference between class variables and class instance variables? -

Can anybody tell me about the differences between class variables and square instance variables?

between a square variable ( @@ ) and all its descendants The class instance variables are shared ( @ ) by the descendants of the class.

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Class variables ( @@ )

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

and a derivative category:

  class bar & lt; Foo End  

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

  p Foo .i # = & gt; 1 p bar.i # = & gt; 1  

and changing it to @@ i it is converted to both:

  bar.i = 2p Foo # = & gt; 2p bar.i # = & gt; 2  

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Class example variable ( @ )

Let's have a simple class with a square To create an example, type the variable @i and @i to read and write:

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

and a derived class:

  class bar & lt; Foo End  

We see that though the accessory code for the bar was not found by @ i , it is not @i :

p foo.i # = & gt; 1 p bar.i # = & gt; Zero

We can set the bar without affecting the @i of @i :

< Pre> bar = I = 2 pu FU # => 1 p bar.i # = & gt; 2