Class eval passing the class varibale by reference

Question

The following code snippet:

class A
end
    @class = eval("A")
    @class.class_eval do
        @@attr = 100
        def self.get_attr
            @@attr
        end
        def self.set_attr(_x)
            @@attr = _x
        end
    end

    class B
    end

    @class = eval("B")
    @class.class_eval do
        @@attr = 100
        def self.get_attr
            @@attr
        end
        def self.set_attr(_x)
            @@attr = _x
        end
    end
    a = A.new
    b = B.new
    A.set_attr(103)
    B.set_attr(222)
    puts A.get_attr
    puts B.get_attr

Class variable is getting shared.

Result:

222 222

Answer 1

It's because you set not the variable you think you do.

class A; end

@class = eval("A")
@class.class_eval do
  class_variable_set :@@attr, 100

  def self.get_attr
    class_variable_get :@@attr
  end

  def self.set_attr(_x)
    class_variable_set :@@attr, _x
  end
end

class B
end

@class = eval("B")
@class.class_eval do
  class_variable_set :@@attr, 100

  def self.get_attr
    class_variable_get :@@attr
  end
  def self.set_attr(_x)
    class_variable_set :@@attr, _x
  end
end

A.set_attr(103)
B.set_attr(222)
puts A.get_attr
puts B.get_attr

# >> 103
# >> 222

When I run your code, it gives several warnings of "Access to class variable from toplevel". So, apparently, you're setting and reading class variables of main object, not of your classes.