# Q: how to undefined a class after declaration?

**URL:** <https://www.ruby-forum.com/t/q-how-to-undefined-a-class-after-declaration/58925>\
**Category:** Ruby\
**Created:** [April 27, 2006, 6:47am UTC](https://www.ruby-forum.com/t/q-how-to-undefined-a-class-after-declaration/58925 "2006-04-27T06:47:05Z")\
**Posts on this page:** 4\
**Page:** 1

<div class="post-metadata">

**Author:** ![mikec](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/m/977dab/32.png) [@mikec](https://www.ruby-forum.com/u/mikec)\
**Post date:** [April 27, 2006, 6:47am UTC](https://www.ruby-forum.com/t/q-how-to-undefined-a-class-after-declaration/58925/1 "2006-04-27T06:47:05Z")

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E.g. using irb, I created class A and then class B, later I realized  
what I wanted was class B to be a subclass of A like ‘class B \< A’. irb  
would not allow that now that B exists. How do I undefined B so that I  
can create a new class B with inheritance to A. Is there an undef for  
class ?

Thanks in advance.  
Mike C

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<div class="post-metadata">

**Author:** ![mikec](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/m/977dab/32.png) [@mikec](https://www.ruby-forum.com/u/mikec)\
**Post date:** [April 27, 2006, 6:58am UTC](https://www.ruby-forum.com/t/q-how-to-undefined-a-class-after-declaration/58925/2 "2006-04-27T06:58:13Z")

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On Apr 27, 2006, at 12:47 AM, mike chang wrote:

> –  
> Posted via [http://www.ruby-forum.com/](http://www.ruby-forum.com/).

irb(main):001:0\> class A  
irb(main):002:1\> end  
=\> nil  
irb(main):003:0\> class B  
irb(main):004:1\> end  
=\> nil  
irb(main):005:0\> class B \< A  
irb(main):006:1\> end  
TypeError: superclass mismatch for class B  
from (irb):5  
irb(main):007:0\> class Object  
irb(main):008:1\> remove\_const ‘B’  
irb(main):009:1\> end  
=\> B  
irb(main):010:0\> class B \< A  
irb(main):011:1\> end  
=\> nil

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<div class="post-metadata">

**Author:** ![mikec](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/m/977dab/32.png) [@mikec](https://www.ruby-forum.com/u/mikec)\
**Post date:** [April 27, 2006, 7:24am UTC](https://www.ruby-forum.com/t/q-how-to-undefined-a-class-after-declaration/58925/3 "2006-04-27T07:24:29Z")

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Thanks Logan C… It works.  
But … can you help me explain why my instance object ‘b’ and its  
methods are still reachable after remove\_const on class ‘B’?

E.g.  
irb(main):001:0\> class A  
irb(main):002:1\> def show  
irb(main):003:2\> p “i am A”  
irb(main):004:2\> end  
irb(main):005:1\> end  
=\> nil  
irb(main):006:0\> class B  
irb(main):007:1\> def show  
irb(main):008:2\> p " i am B"  
irb(main):009:2\> end  
irb(main):010:1\> end  
=\> nil  
irb(main):011:0\> b = B.new  
=\> #\<B:0x3866ad8\>  
irb(main):012:0\> b.show  
" i am B"  
=\> nil  
irb(main):013:0\> class Object  
irb(main):014:1\> remove\_const ‘B’  
irb(main):015:1\> end  
=\> B  
irb(main):016:0\> class B \< A  
irb(main):017:1\> def show  
irb(main):018:2\> p “i am B, child of A”  
irb(main):019:2\> end  
irb(main):020:1\> end  
=\> nil  
irb(main):021:0\> b2 = B.new  
=\> #\<B:0x38456e8\>  
irb(main):022:0\> b2.show  
“i am B, child of A”  
=\> nil  
irb(main):023:0\> b.show  
" i am B"  
=\> nil  
irb(main):024:0\>

Thanks again.

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<div class="post-metadata">

**Author:** ![mikec](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/m/977dab/32.png) [@mikec](https://www.ruby-forum.com/u/mikec)\
**Post date:** [April 27, 2006, 7:44am UTC](https://www.ruby-forum.com/t/q-how-to-undefined-a-class-after-declaration/58925/4 "2006-04-27T07:44:29Z")

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On Apr 27, 2006, at 1:24 AM, mike chang wrote:

> =\> nil  
> =\> nil  
> irb(main):021:0\> b2 = B.new
> 
> –  
> Posted via [http://www.ruby-forum.com/](http://www.ruby-forum.com/).

remove\_const just removes the constant named ‘B’. Your instances  
still have a pointer to the same object that the constant B pointed to.

Pretend if you will that constants are in a hash.

constants = { “B” =\> # }

 

class B # look up constants[‘B’], if it doesn’t exist, then do  
constants[‘B’] = Class.new. Regardless add the following methods to it.  
end

 

b = B.new # Construct an object that kinda looks like #\<some object  
@class=# \>

 

class Object; remove\_const ‘B’; end # does something akin to  
constants.delete(‘B’)

 

Now b still has an instance variable (of sorts) that points to the  
actual class information. A constant like any variable in ruby is  
just a label, remove\_const removes the label, not the actual object.