Why is `instance_eval`/`class_eval` not able to access class variables?

Question

class MySelf
  @@name = 'jonathan'
  def self.name
    @@name
  end
end

MySelf.instance_eval {@@name}
MySelf.class_eval {@@name}

both throw:

NameError: uninitialized class variable @@collection in Object

but

MySelf.instance_eval {name}
MySelf.class_eval {name}

both work.

How can I access the static var @@name with instance_eval / class_eval , or how can I assign a value from outside the class?

Answer 1

The error thrown is because MySelf.instance_eval('@@name') correctly throws an error. This is not an instance variable, it's a class variable. You'll want to have MySelf.class_eval('@@name') on it's own, and then it'll work.

Check the repl here: https://repl.it/Be0U/0

To set the class variable, use class_variable_set like so:

 MySelf.class_variable_set('@@name', 'graham')

Answer 2

I think you have to use class instance variable .

class MySelf
  @name = 'jonathan'

  def self.name
    @name
  end
end

MySelf.instance_eval { @name } # => "jonathan"
MySelf.class_eval { @name }    # => "jonathan"

Try it, plz.