# Class in a Class problem

**URL:** <https://www.ruby-forum.com/t/class-in-a-class-problem/164613>\
**Category:** Ruby\
**Created:** [April 3, 2009, 3:35am UTC](https://www.ruby-forum.com/t/class-in-a-class-problem/164613 "2009-04-03T03:35:50Z")\
**Posts on this page:** 9\
**Page:** 1

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**Author:** ![unknown](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/u/4bbf92/32.png) [@unknown](https://www.ruby-forum.com/u/unknown)\
**Post date:** [April 3, 2009, 3:35am UTC](https://www.ruby-forum.com/t/class-in-a-class-problem/164613/1 "2009-04-03T03:35:50Z")

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My intent is to create a class that has access to call a method in an  
instantiation of another class

However my knowledge of Ruby classes (Ok classes in general) is  
somewhat lacking

The example below is a LOT simplified But I need class B to call a  
method within an instantiation of class A’s.

one alternative is to seperate the classes and then just do “b=B.new  
(a)” but I think that looks really messy and

- I have then to check a bunch of stuff about the parameter a
- I would have a world of more hurt trying to ensure that only one B  
instantiation pointed to each a

Can anyone help? Or tell me that I am doing this all wrong.

Paul

* * *

class A  
class B  
def initialize()  
puts “b init”  
print\_stuff()  
end  
end

def B  
return B  
end

def print\_stuff()  
puts “here”  
end  
end

a = A.new()  
b = a.B.new()

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<div class="post-metadata">

**Author:** ![unknown](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/u/4bbf92/32.png) [@unknown](https://www.ruby-forum.com/u/unknown)\
**Post date:** [April 3, 2009, 4:01am UTC](https://www.ruby-forum.com/t/class-in-a-class-problem/164613/2 "2009-04-03T04:01:02Z")

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removed\_email\_address@domain.invalid wrote:

> - I have then to check a bunch of stuff about the parameter a
> - I would have a world of more hurt trying to ensure that only one B  
> instantiation pointed to each a

> end  
> def B(\*args)  
> @b ||= B.new(self, \*args)  
> end

> def print\_stuff()  
> puts “here”  
> end  
> end

a = A.new()  
b = a.B(my\_args)

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<div class="post-metadata">

**Author:** ![unknown](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/u/4bbf92/32.png) [@unknown](https://www.ruby-forum.com/u/unknown)\
**Post date:** [April 3, 2009, 4:05am UTC](https://www.ruby-forum.com/t/class-in-a-class-problem/164613/3 "2009-04-03T04:05:43Z")

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removed\_email\_address@domain.invalid wrote:

> - I would have a world of more hurt trying…

You have only a _leetle_ bit more to learn about Ruby, Young  
Grasshopper!

(-:

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**Author:** ![unknown](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/u/4bbf92/32.png) [@unknown](https://www.ruby-forum.com/u/unknown)\
**Post date:** [April 3, 2009, 6:05am UTC](https://www.ruby-forum.com/t/class-in-a-class-problem/164613/4 "2009-04-03T06:05:47Z")

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[removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote:

> (a)" but I think that looks really messy and
> 
> end
> 
> def print\_stuff()  
> puts “here”  
> end  
> end
> 
> a = A.new()  
> b = a.B.new()

Maybe I’m reading your question wrong, but the concept “has access to  
call” is misleading, it seems to me, since there is never lack of access  
in Ruby. So what you’re really asking, I’m guessing, is that when a B  
instance is produced by an A instance, it should point to that A  
instance. We can do that without even giving the B class a name:

class A  
def b  
Class.new do  
def initialize(a)  
@a = a  
end  
attr :a  
end.new(self)  
end

# other stuff that an A knows how to do

end

So now you say:

a = A.new  
b = a.b  
puts (b.a == a) #=\> true

So we see that b’s a is indeed pointing at the correct instance. So now  
you can say

b.a.print\_stuff

or anything else you like. Is that the spirit of what you’re after?

m.

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**Author:** ![unknown](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/u/4bbf92/32.png) [@unknown](https://www.ruby-forum.com/u/unknown)\
**Post date:** [April 3, 2009, 8:10pm UTC](https://www.ruby-forum.com/t/class-in-a-class-problem/164613/5 "2009-04-03T20:10:47Z")

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Robert K. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote:

> > end
> 
> You are creating new classes all the time. I don’t think this is a good  
> idea.

I’m not sure what you mean about new _classes_. It is certainly true  
that I didn’t do anything prevent the user from calling “b” twice on the  
same A instance, thus getting two different objects pointing at that  
instance:

a = A.new  
b = a.b # b is an instance whose “@a” points at “a”  
bb = a.b # bb is a different instance whose “@a” points at “a”

But that’s easy to solve if desired. Obviously the real problem here is  
that I couldn’t fathom what the OP was actually trying to _do_. m.

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**Author:** ![unknown](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/u/4bbf92/32.png) [@unknown](https://www.ruby-forum.com/u/unknown)\
**Post date:** [April 3, 2009, 8:50am UTC](https://www.ruby-forum.com/t/class-in-a-class-problem/164613/6 "2009-04-03T08:50:54Z")

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On 03.04.2009 06:02, matt neuburg wrote:

> > def B
> 
> ```
> @a = a
> end
> attr :a
> end.new(self)
> 
> ```
> 
> end
> 
> # other stuff that an A knows how to do
> 
> end

You are creating new classes all the time. I don’t think this is a good  
idea.

I am not sure what the OP really wants, if I’m not mistaken he wants to  
prevent creation of B instances outside of A and also make sure there is  
a 1:1 relationship between an A and a B instance.

First of all, the question is: why are they separated? We can’t answer  
that one from those abstract classes presented.

Hiding of class B for the outside could be done like this:

class A  
@b = Class.new do  
def print\_stuff  
puts “hoho”  
end  
end

def initialize  
@b = self.class.instance\_variable\_get("@b").new  
@b.print\_stuff  
end  
end

a = A.new  
p a

But a) this is not really hidden and b) this looks quite ugly.

Paul, what are you trying to accomplish?

Kind regards

robert

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**Author:** ![unknown](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/u/4bbf92/32.png) [@unknown](https://www.ruby-forum.com/u/unknown)\
**Post date:** [April 3, 2009, 8:38pm UTC](https://www.ruby-forum.com/t/class-in-a-class-problem/164613/7 "2009-04-03T20:38:52Z")

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On 03.04.2009 20:05, matt neuburg wrote:

> > > end
> > > 
> > > # other stuff that an A knows how to do
> > > 
> > > end  
> > > You are creating new classes all the time. I don’t think this is a good  
> > > idea.
> 
> I’m not sure what you mean about new _classes_.

There is a Class.new in your definition of method b which means, every  
time the method is invoked a new anonymous class is created - regardless  
whether b is invoked several times on one or multiple instances.

> that I couldn’t fathom what the OP was actually trying to _do_. m.  
> I had similar difficulties. That’s why I asked.

Kind regards

robert

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**Author:** ![unknown](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/u/4bbf92/32.png) [@unknown](https://www.ruby-forum.com/u/unknown)\
**Post date:** [April 3, 2009, 9:20pm UTC](https://www.ruby-forum.com/t/class-in-a-class-problem/164613/8 "2009-04-03T21:20:28Z")

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removed\_email\_address@domain.invalid wrote:

> My intent is to create a class that has access to call a method in an  
> instantiation of another class

Here be dragons. Have you considered the inheritance idea? If a class  
_needs_ access to another class’s private variables, then the  
functionality either belongs in the same class or a child class. Whilst  
this is technically legal, it really doesn’t bode well. If you  
absolutely need Class A to have exactly one instantiation of Class B you  
might want to consider the Singleton design pattern for class B. Which  
in ruby is _really_ difficult to implement…

include singleton #(or something along those lines) 🙂

and for B to be a class variable of A (@@b = B.new) which, if they act  
like Java statics (and I think they do) the class variable has one  
instantiation able to be accessed from any object of the class.

e.g  
class B  
def initialize()  
end

def to\_s  
“This is class #{class}”  
end  
end

class A

@@b = B.new()

def intitialize()  
puts @@b  
end

end

But in truth, you will have to be a little less generic to get a better  
answer. If you can give more details of what you’re trying to achieve,  
then please do, and I’ll try and give better help.

Michael

> instantiation pointed to each a  
> def initialize()  
> puts “here”  
> end  
> end
> 
> a = A.new()  
> b = a.B.new()

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**Author:** ![unknown](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/u/4bbf92/32.png) [@unknown](https://www.ruby-forum.com/u/unknown)\
**Post date:** [April 4, 2009, 12:28am UTC](https://www.ruby-forum.com/t/class-in-a-class-problem/164613/9 "2009-04-04T00:28:54Z")

</div>

±---------------------------------------------------+  
| class A |  
| def initialize; puts “class A”; end |  
| class B |  
| def initialize; puts “class B”; end |  
| def test\_b; puts “instance method in B”; end |  
| end |  
| def make\_b; B.new; end |  
| end |  
| |  
| a = A.new |  
| b = a.make\_b |  
| b.test\_b |  
±---------------------------------------------------+

OUTPUT:  
class A  
class B  
instance method in B