Undefining class at runtime

June 16, 2009 at 6:58 am (Model) (, , , , , , , )

This post explains undefining the class at runtime.

Im not gonna explain the live-cases where this might be used. Ask Uncle Google for that.

Lets jump in at d code directly.

When u create a new class, say “Person”, two things happen:

  1. A new object of type(class) Class is instantiated.
  2. A new constant called Person will be instantiated and will hold the value of the class-object created above.

Proof: Class Person;end;

Try, Person.class #=> Class. This means that the Person holds a value which is of class Class. (1 is proved).

Try, Person = 123 #=> Curses saying, Person is already initialized, which means Person is a constant. (2 is proved).

Now, All the constants(Person) and other are silently ducked into the class Object.

Proof: Object.class_eval{Person} #=> Person Class

So, we should understand by now that the newly created class Person resides as a constant of the class Object.

So if we could somehow remove that constant, then the class Person will also be removed.

There is a method called remove_const for the class Object.

So, try Object.remove_const : Person #=> Curses, NoMethodError: private method `remove_const’ called for Object:Class

This is becoz, remove_const is a private method of Object class.

So the worry now is, how to invoke a private method.

We have two solutions:

  1. Object.send(:remove_const , : Person)
  2. Object.class_eval{remove_const : Person}

Now try, Person.new. # Curses, uninitialized constant Person

Voila, job done. Very rarely does an ERROR MESSAGE makes us feel happy. Well, kind of an article.

You can learn much about class_eval from this very good article by Khaled.


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