# Modify instance variable in a method mixed in from external

**URL:** <https://www.ruby-forum.com/t/modify-instance-variable-in-a-method-mixed-in-from-external/72822>\
**Category:** Ruby\
**Created:** [October 19, 2006, 5:43am UTC](https://www.ruby-forum.com/t/modify-instance-variable-in-a-method-mixed-in-from-external/72822 "2006-10-19T05:43:39Z")\
**Posts on this page:** 3\
**Page:** 1

<div class="post-metadata">

**Author:** ![gnufied](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/g/ea666f/32.png) [@gnufied](https://www.ruby-forum.com/u/gnufied)\
**Post date:** [October 19, 2006, 5:43am UTC](https://www.ruby-forum.com/t/modify-instance-variable-in-a-method-mixed-in-from-external/72822/1 "2006-10-19T05:43:39Z")

</div>

I have this gem of a parser that does:

def parse\_data data  
data.chomp!  
while data && data.length \> 0  
data\_array = data.split(/\r?\n/,2)  
@chunked\_data \<\< data\_array[0]  
data = data\_array[1]

```
  if @chunked_data =~ /(\d{3})(.*)<(.*)>##(.*)##/
      dispatch_data
    @chunked_data = ""
  end
end

```

end

That is used in several classes as instance methods. Now, I want to  
include this method in a module to be DRY. And make this method  
available to classes that mixin that module.  
The only trouble is, it uses a instance\_variable.  
Of course…i can keep this method as it is. assuming all the consumer  
classes will have their this particular instance variable with same  
name.

The other approach could be, as suggested by Eric H. on IRC, is to  
have some kind of include, that hookes correct instance variable to  
the module upon inclusion in the class.

And this bit…almost escaped me.

If i use set\_instance\_variable or something like that, then still i  
won’t be able to know the name of that particular instance variable  
,right?

if i pass the @chunked\_data from the any class to the parse\_data  
method, i would recieve it as a local variable in method and any  
changes to the varible will be lost.  
Why can’t we have something like C++ reference, where if i modify the  
variable in mixed in method, i would effectively change the instance  
variable?

Not even that, although in Ruby, AFAIK(which is very little, to tell  
the truth) values are passed by reference, even if we pass a local  
variable to the parse\_data  
method and do some modification and try to read back the data in  
calling method, changes will be lost.

---

<div class="post-metadata">

**Author:** ![gnufied](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/g/ea666f/32.png) [@gnufied](https://www.ruby-forum.com/u/gnufied)\
**Post date:** [October 19, 2006, 4:51pm UTC](https://www.ruby-forum.com/t/modify-instance-variable-in-a-method-mixed-in-from-external/72822/2 "2006-10-19T16:51:56Z")

</div>

On 10/18/06, hemant [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote:

> ```
> dispatch_data
> 
> ```
> 
> Of course…i can keep this method as it is. assuming all the consumer  
> classes will have their this particular instance variable with same  
> name.

Well, if the receiver of parse\_data doesn’t have the instance  
variable, it will after this method is executed.

In Ruby, instance variables aren’t declared/defined in classes, they  
are dynamically added to instances as needed.

I’m not clear on just why you need @chunked\_data to persist across  
invocations, but the only issue I see with guaranteeing that an  
instance of the class has this iv is that it’s not guaranteed to be  
initialized, so in the first iteration of your while loop you might be  
doing

```
nil << data_array[0]

```

which won’t work.

This can be fixed by adding:

@chunked\_data ||= “”

before the while loop.

Now one danger is that a class might have an iv called @chunked\_data  
which it’s using for something else. This seems unlikely but you never  
know.

If you do this you need to document the use of @chunked\_data.

There are likely better ways to design this if you step back a bit,  
but we don’t know enough about the context to help.

> The other approach could be, as suggested by Eric H. on IRC, is to  
> have some kind of include, that hookes correct instance variable to  
> the module upon inclusion in the class.

I’m not sure what this means, so I can’t comment.

> variable in mixed in method, i would effectively change the instance  
> variable?

> Not even that, although in Ruby, AFAIK(which is very little, to tell  
> the truth) values are passed by reference, even if we pass a local  
> variable to the parse\_data  
> method and do some modification and try to read back the data in  
> calling method, changes will be lost.

In Ruby ALL variables are just references, if you mutate the object  
referenced by the variable the object will be changed:

irb(main):005:0\> def change(arg, value)  
irb(main):006:1\> arg \<\< value  
irb(main):007:1\> end  
=\> nil  
irb(main):008:0\> a = “hello”  
=\> “hello”  
irb(main):009:0\> change(a, " there.")  
=\> “hello there.”  
irb(main):010:0\> a  
=\> “hello there.”  
irb(main):011:0\>

Any mutating methods invoked against the parameter will change the  
underlying object. So rather than

```
arg = ""

```

you could use

```
arg.replace("")

```

Note that the value of a method is the value of the last expression  
executed in the method. So this method returns the value of

arg \<\< value

So you could have parse return the chunked data and ask clients to  
pass it back on subsequent calls.

I’m not saying that this is a good design, but you seem to need some  
understanding of how variables and objects relate.

Another note on variables and references which trips up folks  
accustomed to C-like variables:

irb(main):011:0\> b = a  
=\> “hello there.”  
irb(main):012:0\> change(a, " Again!")  
=\> “hello there. Again!”  
irb(main):013:0\> a  
=\> “hello there. Again!”  
irb(main):014:0\> b  
=\> “hello there. Again!”

Understand that and you should be further along in understanding the  
relationship between variables and object in Ruby.

–  
Rick DeNatale

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

---

<div class="post-metadata">

**Author:** ![gnufied](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/g/ea666f/32.png) [@gnufied](https://www.ruby-forum.com/u/gnufied)\
**Post date:** [October 20, 2006, 4:06pm UTC](https://www.ruby-forum.com/t/modify-instance-variable-in-a-method-mixed-in-from-external/72822/3 "2006-10-20T16:06:30Z")

</div>

On 10/19/06, Rick DeNatale [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote:

> relationship between variables and object in Ruby.  
> Thanks Rick…