Class variable using attr_accessor inside class << self

Question

Can someone explain why self.foo= works outside of the class << self, while foo= does not work within the class << self.

class A
  class << self
    attr_accessor :foo
    foo = 'foo'
  end
end

p A.foo # => "nil"

class A
  class << self
    attr_accessor :foo
  end
  self.foo = 'foo'
end

p A.foo # => "foo"

This is not the same question as When to use 'self' in Ruby

To be clear, I am not asking when to use self. I am asking why I cannot set the class variable inside a 'class << self' block, but I can outside of it.

Answer 1

your second example isn't a class variable, it's a class instance variable to this class the proof is that if you inherited like this you'll get nil

class A
  class << self
    attr_accessor :foo
  end
  self.foo = 'foo'
end

class B < A
end

B.foo # => nil

So if you want to add a class variable in class << self you can use @@

class A

  class << self
  @@foo = :foo
    def foo
      @@foo
    end
  end
end