# Adding an attribute to an object

**URL:** <https://www.ruby-forum.com/t/adding-an-attribute-to-an-object/120243>\
**Category:** Ruby\
**Created:** [November 15, 2007, 3:10am UTC](https://www.ruby-forum.com/t/adding-an-attribute-to-an-object/120243 "2007-11-15T03:10:56Z")\
**Posts on this page:** 4\
**Page:** 1

<div class="post-metadata">

**Author:** ![markcasio](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/m/e9bcb4/32.png) [@markcasio](https://www.ruby-forum.com/u/markcasio)\
**Post date:** [November 15, 2007, 3:10am UTC](https://www.ruby-forum.com/t/adding-an-attribute-to-an-object/120243/1 "2007-11-15T03:10:56Z")

</div>

I know how to add new methods to an existing object, but is there a way  
to add an attribute?  
I know I can do it if the object was created with ObjectStruct, but what  
if it is a “normal” Ruby object?

R. Mark V.  
Object Computing, Inc.

---

<div class="post-metadata">

**Author:** ![markcasio](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/m/e9bcb4/32.png) [@markcasio](https://www.ruby-forum.com/u/markcasio)\
**Post date:** [November 15, 2007, 3:27am UTC](https://www.ruby-forum.com/t/adding-an-attribute-to-an-object/120243/2 "2007-11-15T03:27:18Z")

</div>

On Nov 14, 2007 6:10 PM, Mark V. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote:

> I know how to add new methods to an existing object, but is there a way to add an attribute?  
> I know I can do it if the object was created with ObjectStruct, but what if it is a “normal” Ruby object?
> 
> R. Mark V.  
> Object Computing, Inc.

Not quite sure what you mean…but anyway, maybe this irb session will  
help you along:

irb(main):001:0\> class Dog; end  
=\> nil  
irb(main):002:0\> d = Dog.new  
=\> #Dog:0x2063cc  
irb(main):003:0\> Dog.class\_eval { attr\_accessor :name }  
=\> nil  
irb(main):004:0\> d.name = “Fido”  
=\> “Fido”  
irb(main):005:0\> (class \<\< d; self; end).class\_eval { attr\_accessor :age  
}  
=\> nil  
irb(main):006:0\> d.age = 12  
=\> 12  
irb(main):007:0\> d2 = Dog.new  
=\> #Dog:0x265afc  
irb(main):008:0\> d2.name = “Spike”  
=\> “Spike”  
irb(main):009:0\> d2.age = 7  
NoMethodError: undefined method `age=’ for #\<Dog:0x265afc @name=“Spike”\>  
from (irb):9  
from :0

First we call attr\_accessor for Dog, which defines #name and #name=  
for all dogs. Then we define #age and #age= for only the one dog, by  
calling attr\_accessor on that object’s singleton class.

However, there’s not really a concept of “attributes” in Ruby (afaik).  
attr\_\* are just macros that define methods that look like

class Dog  
def age=(val)  
@age = val  
end

def age  
@age  
end  
end

You can also dig in to an object with instance\_eval and do your work:

irb(main):001:0\> class Dog  
irb(main):002:1\> attr\_reader :name  
irb(main):003:1\> end  
=\> nil  
irb(main):004:0\> d = Dog.new  
=\> #Dog:0x287eb8  
irb(main):005:0\> d.name  
=\> nil  
irb(main):006:0\> d.instance\_eval { @name = “Spot” }  
=\> “Spot”  
irb(main):007:0\> d.name  
=\> “Spot”

hth

Pat

---

<div class="post-metadata">

**Author:** ![markcasio](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/m/e9bcb4/32.png) [@markcasio](https://www.ruby-forum.com/u/markcasio)\
**Post date:** [November 15, 2007, 4:28am UTC](https://www.ruby-forum.com/t/adding-an-attribute-to-an-object/120243/3 "2007-11-15T04:28:40Z")

</div>

On Nov 14, 2007, at 7:10 PM, Mark V. wrote:

> I know how to add new methods to an existing object, but is there a  
> way to add an attribute?  
> I know I can do it if the object was created with ObjectStruct, but  
> what if it is a “normal” Ruby object?
> 
> R. Mark V.  
> Object Computing, Inc.

cfp:~ \> cat a.rb  
require ‘attributes’ ### gem install attributes

o = Object.new

o.instance\_eval{ attribute ‘foobar’ =\> 42 }

p o.foobar #=\> 42

o.foobar = ‘forty-two’

p o.foobar #=\> “forty-two”

p o.foobar? #=\> “forty-two” (true)

o.foobar 42.0

p o.foobar #=\> 42.0

cfp:~ \> ruby a.rb  
42  
“forty-two”  
“forty-two”  
42.0

a @ [http://codeforpeople.com/](http://codeforpeople.com/)

---

<div class="post-metadata">

**Author:** ![markcasio](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/m/e9bcb4/32.png) [@markcasio](https://www.ruby-forum.com/u/markcasio)\
**Post date:** [November 15, 2007, 4:46am UTC](https://www.ruby-forum.com/t/adding-an-attribute-to-an-object/120243/4 "2007-11-15T04:46:13Z")

</div>

On Nov 14, 7:10 pm, Mark V. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote:

> I know how to add new methods to an existing object, but is there a way to add an attribute?  
> I know I can do it if the object was created with ObjectStruct, but what if it is a “normal” Ruby object?

As Pat pointed out, there are no ‘attributes’ in Ruby, just methods.  
(Those methods may or may not have a 1-to-1 correspondence to instance  
variables; that’s all the convenience methods like attr\_accessor do,  
is set up a simple method.)

irb(main):001:0\> o = Object.new  
=\> #Object:0x1dbbf4  
irb(main):002:0\> def o.duration=( seconds )  
irb(main):003:1\> @length = seconds  
irb(main):004:1\> end  
=\> nil  
irb(main):005:0\> def o.duration; @length; end  
=\> nil  
irb(main):006:0\> o.duration = 120  
=\> 120  
irb(main):007:0\> puts o.duration  
120  
=\> nil  
irb(main):008:0\> def o.duration\_in\_minutes; @length / 60; end  
=\> nil  
irb(main):009:0\> puts o.duration\_in\_minutes  
2  
=\> nil