Question

BasicObject.class_eval do
    def instance(ins)
        eval "@#{ins}=#{ins}", binding
    end
end

Is the code in question. What I want it to do is in the following code, create a new instance variable, bar, inside class Foo.
What I get after running this code:

class Foo
  bar = 3
end
Foo.instance(:bar)

is:

 NameError: undefined local variable or method `bar for Foo:Class
        from /Users/Solomon/Desktop/Ruby/instance.rb:3:in `instance'
        from /Users/Solomon/Desktop/Ruby/instance.rb:3:in `eval'
        from /Users/Solomon/Desktop/Ruby/instance.rb:3:in `instance'

Why is this happening.

Answer 1

Couple things here….You have defined an instance method instance for BasicObject. You are then calling this instance method on the Foo object. The Foo object is a class. You have not set any instance variables for the Foo object. You set an instance variable with the @ sign.
All that the instance method does is set the instance variable @ins to itself.

The , binding is not needed here either as binding is a top level method that returns the current variable bindings. You only need to save a binding if you need to pass around a saved “environment”. Having , binding does not hurt anything but is redundant.

 BasicObject.class_eval do
    def instance(ins)
       eval "@#{ins}= @#{ins}", binding      # @ after the '='
    end
 end



class Foo
  @bar = 3     # @ here
end
Foo.instance(:bar)
puts Foo.instance_variable_get("@bar")    # Shows the instance variable @bar for Foo object