# Re: How to find out object type

**URL:** <https://www.ruby-forum.com/t/re-how-to-find-out-object-type/86138>\
**Category:** Ruby\
**Created:** [February 20, 2007, 6:01am UTC](https://www.ruby-forum.com/t/re-how-to-find-out-object-type/86138 "2007-02-20T06:01:32Z")\
**Posts on this page:** 3\
**Page:** 1

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**Author:** ![Phy\_P](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/p/a587f6/32.png) [@Phy\_P](https://www.ruby-forum.com/u/Phy_P)\
**Post date:** [February 20, 2007, 6:01am UTC](https://www.ruby-forum.com/t/re-how-to-find-out-object-type/86138/1 "2007-02-20T06:01:32Z")

</div>

ah, thats better. So now for the real nubie question, I am lost as to  
why this does not work:

irb(main):001:0\> o = 123  
=\> 123  
irb(main):002:0\> #{o} = Array.new  
irb(main):003:0\* o.class  
=\> Fixnum  
irb(main):004:0\> #{o}.class  
irb(main):005:0\*  
irb(main):006:0\* ^C  
irb(main):006:0\>

I was hoping to be create an array from the numb stored as “o”, but this  
must be wrong, because trying to get the class of the object does not  
work. Any help?

----- Original Message ----  
From: Gary W. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid)  
To: ruby-talk ML [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid)  
Sent: Monday, February 19, 2007 8:51:18 PM  
Subject: Re: How to find out object type

On Feb 19, 2007, at 11:38 PM, Phy P. wrote:

> I am curious, is there a way to find out an object type?

You can find out what class gave ‘birth’ to the object via #class:

[1,2].class # Array  
3.class # Fixnum

Note that #class does _not_ return a string (e.g., “Array”) but instead  
returns a reference to the actual class object:

a = [1,2].class.new # allocates a brand new array  
p a # []

It is important to realize that the ‘class’ of an object is really just  
a hint at how an object will respond to methods. It might be a really  
good hint, but a hint nevertheless since objects are free to override  
method definitions provided by the class or even to add methods that  
the class doesn’t provide:

b = [10,20,30]  
def b.penultimate  
self[-2]  
end  
b.penultimate # 20

Gary W.

---

<div class="post-metadata">

**Author:** ![Phy\_P](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/p/a587f6/32.png) [@Phy\_P](https://www.ruby-forum.com/u/Phy_P)\
**Post date:** [February 20, 2007, 6:10am UTC](https://www.ruby-forum.com/t/re-how-to-find-out-object-type/86138/2 "2007-02-20T06:10:32Z")

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On 2/20/07, Phy P. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote:

> irb(main):006:0\>
> 
> I was hoping to be create an array from the numb stored as “o”, but this must be wrong,  
> because trying to get the class of the object does not work. Any help?

irb(main):001:0\> o = 42  
=\> 42  
irb(main):002:0\> the\_o\_array = [o]  
=\> [42]  
irb(main):003:0\> the\_o\_array.class  
=\> Array

hth,  
-Harold

---

<div class="post-metadata">

**Author:** ![Phy\_P](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/p/a587f6/32.png) [@Phy\_P](https://www.ruby-forum.com/u/Phy_P)\
**Post date:** [February 20, 2007, 6:12am UTC](https://www.ruby-forum.com/t/re-how-to-find-out-object-type/86138/3 "2007-02-20T06:12:32Z")

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On Feb 20, 2007, at 12:01 AM, Phy P. wrote:

> ah, thats better. So now for the real nubie question, I am lost as  
> to why this does not work:
> 
> irb(main):001:0\> o = 123  
> =\> 123  
> irb(main):002:0\> #{o} = Array.new  
> irb(main):003:0\* o.class  
> =\> Fixnum  
> irb(main):004:0\> #{o}.class

The #{} notation is only applicable inside a string: “#{o}”. The  
way you are using it make the line look like a comment.  
Everything after the # is simply discarded.

Maybe you are looking for something like:

a = 123  
b = [o]  
p b # [123]

Gary W.