How to access class attribute, which was set by class_exec()

Question

This is what I'm trying to do:

class Foo; end

Foo.class_exec do
  class_variable_set(:@@x, 42)
  def get
    @@x # runtime error here!
  end
end

puts Foo.new.get

I'm getting (Ruby 3.2.1):

$ ruby a.rb
a.rb:6:in `get': class variable access from toplevel (RuntimeError)
    from a.rb:10:in `<main>'

How can I read this @@x, which I just set?

Answer 1

There's class_variable_get, but since it's a method of Module, you have to call it on the receiver's class or singleton_class:

Foo.class_exec do
  class_variable_set(:@@x, 42)
  def get
    self.class.class_variable_get(:@@x)
    # or
    # singleton_class_variable_get(:@@x)
  end
end

Alternatively, it's perfectly fine in Ruby to re-open a class:

class Foo; end

class Foo
  @@x = 42

  def get
    @@x
  end
end

Note that it's usually discouraged to use class variables, due to their unexpected inheritance behavior see Why is using a class variable in Ruby considered a 'code smell'?