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.