# Changing class of object foobar

**URL:** <https://www.ruby-forum.com/t/changing-class-of-object-foobar/114062>\
**Category:** Ruby\
**Created:** [September 7, 2007, 2:04pm UTC](https://www.ruby-forum.com/t/changing-class-of-object-foobar/114062 "2007-09-07T14:04:35Z")\
**Posts on this page:** 9\
**Page:** 1

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**Author:** ![quik77](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/q/dec6dc/32.png) [@quik77](https://www.ruby-forum.com/u/quik77)\
**Post date:** [September 7, 2007, 2:04pm UTC](https://www.ruby-forum.com/t/changing-class-of-object-foobar/114062/1 "2007-09-07T14:04:35Z")

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Hi

Please no “redesign” proposal as in “do not try to do this  
at all” - I really want to know if the following is possible,  
and if it is, how. And if it is not, I can’t try anyway  
and have to rethink/redesign the approach to it _anyway_ 🙂  
Thanks.

I spent a little time digging through ruby-prototype, dl/struct,  
evil.rb but given my lack of knowledge, ended up more  
confused than enlightened.

A somewhat lengthy explanation follows, but please don’t tie  
this too much to my specific problem at hand, in actual  
practice I am more interested in how to have an object  
of class X become an object of Class Y instead

* * *

I have a general class, a button. This class will be responsible for  
providing all required datasets to “become” a “specialized” button.

With “specialized”, I really mean a Button for i.e. a specific widget  
set, like Qt. So that the user can work with a Qt button, or a  
gtk button, or a FXRuby button and so on, but without the need to  
care about the internals and widget-specific problems.  
This is just to help the user focus on one language aspect (describing  
his widget once, and only once) and then have code help him to  
realize the rest. (In reality i’d later plan to separate the  
layout even more to use “classes” similar to Cascading Style Sheets  
for “grouped” styling, but ignore this for now please)

The moment the user needs this “prototype widget” to be specific (our  
button in this case, but there will be more abstract widget sets in the  
future, if this works) he “converts” it to a certain toolset.  
For example, this button becomes FXButton.new now.

The reason for the abstraction is because it makes no sense  
to restrict yourself to one specific widget set, if the logic itself  
is pretty much the same in all toolsets (Clicking, mouse moving,  
actions,  
especially colours and padding etc…) and should be easily  
accessible to a human reader too without the need to pick a  
widget-specific solution, like gtk rc\_files.

Enough pretext, now the question! 🙂

How can I convert my object “foobar” of “class Button”

foobar = Button.new

to become a “real” Gtk::Button for instance?

Inside class Button I can not modify self, so I wonder if  
there is any option at all to convert to the Gtk::Button.  
I can create a new Gtk::Button inside class Button easily,  
with all the required data, but it seems my created object  
is never able to leave its class Button state. At best  
it can only do a “return Gtk::Button” in the end.

I am glad for any ideas and help.

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<div class="post-metadata">

**Author:** ![quik77](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/q/dec6dc/32.png) [@quik77](https://www.ruby-forum.com/u/quik77)\
**Post date:** [September 7, 2007, 2:15pm UTC](https://www.ruby-forum.com/t/changing-class-of-object-foobar/114062/2 "2007-09-07T14:15:53Z")

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On 9/7/07, Marc H. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote:

> confused than enlightened.
> 
> The moment the user needs this “prototype widget” to be specific (our  
> widget-specific solution, like gtk rc\_files.  
> there is any option at all to convert to the Gtk::Button.  
> I can create a new Gtk::Button inside class Button easily,  
> with all the required data, but it seems my created object  
> is never able to leave its class Button state. At best  
> it can only do a “return Gtk::Button” in the end.
> 
> ## I am glad for any ideas and help.
> 
> Posted via [http://www.ruby-forum.com/](http://www.ruby-forum.com/).

Changing the class of the Button object won’t work. I would suggest  
just storing the new object in an instance variable in the Button  
object.

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<div class="post-metadata">

**Author:** ![quik77](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/q/dec6dc/32.png) [@quik77](https://www.ruby-forum.com/u/quik77)\
**Post date:** [September 7, 2007, 3:06pm UTC](https://www.ruby-forum.com/t/changing-class-of-object-foobar/114062/3 "2007-09-07T15:06:45Z")

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On 9/7/07, Marc H. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote:

> I am glad for any ideas and help.  
> There is a way to do this. It’s in a lib, and the name of that lib is  
> evil.rb. It has that name for a reason.

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**Author:** ![quik77](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/q/dec6dc/32.png) [@quik77](https://www.ruby-forum.com/u/quik77)\
**Post date:** [September 7, 2007, 6:46pm UTC](https://www.ruby-forum.com/t/changing-class-of-object-foobar/114062/4 "2007-09-07T18:46:20Z")

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On Sep 7, 8:04 am, Marc H. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote:

> confused than enlightened.
> 
> The moment the user needs this “prototype widget” to be specific (our  
> widget-specific solution, like gtk rc\_files.  
> there is any option at all to convert to the Gtk::Button.  
> I can create a new Gtk::Button inside class Button easily,  
> with all the required data, but it seems my created object  
> is never able to leave its class Button state. At best  
> it can only do a “return Gtk::Button” in the end.
> 
> ## I am glad for any ideas and help.
> 
> Posted viahttp://www.ruby-forum.com/.

Why convert when you can just lie? Does the user ever have to know  
what toolset they’re using?

One way of lying would be to have your ‘abstract’ Button just be a  
delegator that delegates to what ever  
Toolset’s button is actually being used. See ‘ri SimpleDelegator’ and  
other such documentation.

Or, if you don’t think you’re going to dynamically change Toolsets  
that often, you could have your Button::new method ‘lie’ by  
overwriting it to return an instance of whatever’s Toolkit’s button  
you actually want to use. Or heck, you wouldn’t need to go that far  
even. You could just have the user use a Toolset that you declare,  
something like:

require ‘toolset’  
include Toolset  
button = Button.new

# …

And then, have toolset be something like:

#Toolset = Gtk  
Toolset = Qt

For more dynamic scenarios (you want to make sure their code runs  
against a set of toolsets), you could pass them the toolset in another  
manner.

I’m honestly not saying “Please don’t do this at all”; I’m actually  
shooting for something more like “Does this make it so you don’t need  
to?”

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**Author:** ![quik77](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/q/dec6dc/32.png) [@quik77](https://www.ruby-forum.com/u/quik77)\
**Post date:** [September 7, 2007, 7:42pm UTC](https://www.ruby-forum.com/t/changing-class-of-object-foobar/114062/5 "2007-09-07T19:42:25Z")

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On 9/7/07, Marc H. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote:

> How can I convert my object “foobar” of “class Button”
> 
> foobar = Button.new
> 
> to become a “real” Gtk::Button for instance?

foobar = Gtk::Button.new

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**Author:** ![quik77](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/q/dec6dc/32.png) [@quik77](https://www.ruby-forum.com/u/quik77)\
**Post date:** [September 8, 2007, 1:20am UTC](https://www.ruby-forum.com/t/changing-class-of-object-foobar/114062/6 "2007-09-08T01:20:01Z")

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On 9/7/07, Wilson B. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote:

> On 9/7/07, Marc H. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote:
> 
> > How can I convert my object “foobar” of “class Button”
> > 
> > foobar = Button.new
> > 
> > to become a “real” Gtk::Button for instance?
> 
> foobar = Gtk::Button.new

This only binds the VARIABLE foobar in the current scope to a  
different object, it DOESN’T change the OBJECT which foobar pointed to  
originally. So for example

def dub\_gtk\_button(button)  
button = Gtk::Button.new  
puts “in dub\_gtk #{button.class}”  
end

button = Button.new  
puts “before #{button.class}”  
dub\_gtk(button)  
puts “before #{button.class}”

will print:

before Button  
in dub\_gtk Gtk::Button  
after Button

If you want to change defer determining the behavior of a ruby object  
until sometime after it’s instantiated, there are better ways to do it  
than changing the class.

1. As others have pointed out you can have the original object  
delegate to a reall implementation obuect perhaps using delegator or  
forwardable from the standard library.

2. You can treat the original class of the object as a superclass, and  
add the particular behavior either by adding singleton methods, or by  
mixing one or more modules into the instance using the Object#extend  
method.

The second approach might seem cooler and might perform slightly  
better although if you want to change the behavior more than once it  
can more be problematical than when using the first.

–  
Rick DeNatale

My blog on Ruby  
[http://talklikeaduck.denhaven2.com/](http://talklikeaduck.denhaven2.com/)

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**Author:** ![quik77](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/q/dec6dc/32.png) [@quik77](https://www.ruby-forum.com/u/quik77)\
**Post date:** [November 10, 2007, 10:50am UTC](https://www.ruby-forum.com/t/changing-class-of-object-foobar/114062/7 "2007-11-10T10:50:25Z")

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Marc H. wrote:

> How can I convert my object “foobar” of “class Button”
> 
> foobar = Button.new
> 
> to become a “real” Gtk::Button for instance?

foobar = Button.new  
foobar.kind\_of?(Gtk::Button) # =\> true

How?  
class Button  
def initialize  
extend Gtk::Button  
end  
end

It does not really change the class, and if your Gtk::Button is a  
preexisting class it won’t work (because this idea requires Gtk::Button  
to be a module). But it changes the ancestry and methods of Gtk::Button  
are invoked before those of Button.

HTH, regards  
Stefan

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**Author:** ![quik77](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/q/dec6dc/32.png) [@quik77](https://www.ruby-forum.com/u/quik77)\
**Post date:** [November 10, 2007, 3:32am UTC](https://www.ruby-forum.com/t/changing-class-of-object-foobar/114062/8 "2007-11-10T03:32:26Z")

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I think i will go with evil.rb, at least digging into it as much as  
possible.

I rather want to try and adjust the code to my thinking, not the other  
way around by “delegating” or adding complexity to it, when I just want  
to keep it simple in essence. I don’t want to lie, I want to convert. 😉

Thanks.

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**Author:** ![quik77](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/q/dec6dc/32.png) [@quik77](https://www.ruby-forum.com/u/quik77)\
**Post date:** [November 10, 2007, 10:57am UTC](https://www.ruby-forum.com/t/changing-class-of-object-foobar/114062/9 "2007-11-10T10:57:22Z")

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Hi –

On Sat, 10 Nov 2007, Marc H. wrote:

> I think i will go with evil.rb, at least digging into it as much as  
> possible.
> 
> I rather want to try and adjust the code to my thinking, not the other  
> way around by “delegating” or adding complexity to it, when I just want  
> to keep it simple in essence. I don’t want to lie, I want to convert. 😉

Then you’re adding rather enormous complexity where the language  
doesn’t natively, or simply, support it. It’s fine if you want to do  
it, but it certainly isn’t the simplest strategy.

David