Why should @@class_variables be avoided in Ruby?

Question

I know that some say that class variables (e.g. @@class_var) should be avoid in Ruby and should use the an instance variable (e.g. @instance_var) in the class scope instead:

def MyClass
  @@foo = 'bar' # Should not do this.
  @foo = 'bar'  # Should do this.
end

Why is the use of class variables frowned upon in Ruby?

Answer 1

Class variables are often maligned because of their sometimes confusing behavior regarding inheritance:

class Foo
  @@foo = 42

  def self.foo
    @@foo
  end
end

class Bar < Foo
  @@foo = 23
end

Foo.foo #=> 23
Bar.foo #=> 23

If you use class instance variables instead, you get:

class Foo
  @foo = 42

  def self.foo
    @foo
  end
end

class Bar < Foo
  @foo = 23
end

Foo.foo #=> 42
Bar.foo #=> 23

This is often more useful.

Answer 2

Be careful; class @@variables and instance @variables are not the same thing.

Essentially, when you declare a class variable in a base class, it’s shared with all subclasses. Changing its value in a subclass will affect the base class and all of its subclasses all the way down the inheritance tree. This behavior is often exactly what’s desired. But equally often, this behavior is not what was intended by the programmer, and it leads to bugs, especially if the programmer did not originally expect for the class to be subclassed by someone else.

From: http://sporkmonger.com/2007/2/19/instance-variables-class-variables-and-inheritance-in-ruby