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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add attribute at runtime

June 16, 2009 at 6:23 am (Model) (, , , , , , )

I’m all excited abt this post.

Coz this is something i learnt just now, and consider it my best learning till now in RUBY.

Q: How to add attributes to a class at runtime?


class Class
def new_attr (attr_name)

class_eval {attr_accessor :"#{attr_name}"}

class_eval : Evaluates a string or a block in the context of the receiver, here the class.


1) We defined a new method to the CLASS called new_attr, which is intended to add new attributes at runtime.

2) This method needs the name of the new attribute to be passed.

3) In the definition, it calls class_eval, which means “DO SOMETHING AT THE CLASS LEVEL”

4) What to do is specified in the block after the class_eval, which says : attr_accessor “#{attr_name}”, which will become attr_accessor :name when we call new_attr(‘name’).

5) We know that attr_accessor is rails syntactic sugar to create the a specific instance variable, and its set/get (read/write) methods. So here, when we call new_attr(‘name’), its creating a new instance variable (@name) and 2 new methods ‘name” and “name=”.

Try this;

class Person; end;

This creates a bare class Person.

p = Person.new

Try, p.name, it throws an error cursing “Undefined method name”.

Its fairly expected coz, Person has no attribute called name.

Now, say Person.new_attr(‘name’)

now try, p.name, it will not curse you.

To check, try Person.singleton_methods.include?(“name”)

This says TRUE.

Hope this article comes handy.

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