From: Brian Candler <b.candler@...>
Date: 2011-04-25T01:00:03+09:00
Subject: Re: Set attribute of superclass dynamically

Jeroen v. wrote in post #994745:
> I have the following code
>
> class foo

That's wrong - class names must start with a capital letter

>   price_per_liter = 1

That sets a local variable. It's only visible between the 'class' and 
'end' excluding any nested 'def', 'class' or 'module' (since each of 
those start a new scope for local variables). This is unlikely to be 
useful.

>   price

That will raise an error - it's not a local variable so it must be a 
method call, but your class Foo object doesn't have a method 'price' 
either (i.e. def self.price ... )

>   module bar

Error again: should be 'Bar' not 'bar'. However I'm not sure why you're 
making a module Foo::Bar - it's not a class, and it's not a subclass of 
Foo.

>     def set_price val
>       self.price_per_liter = val

That will call method 'price_per_liter=', but you have not defined one. 
Error again.

>     end
>   end
> end
>
> In this way I can succesfully set the price per liter of the class. How
> can I make it dynamically, so I can also set the attribute 'price'?

I'm not sure what it is you're trying to achieve. Here's a guess:

class Foo
  def self.price_per_liter=(val)
    @price_per_liter = val
  end
  def self.price_per_liter
    @price_per_liter
  end
end

class Bar < Foo
end

Foo.price_per_liter = 133.9
Bar.price_per_liter = 144.9

puts Foo.price_per_liter
puts Bar.price_per_liter

> I tried instance_variable_set(@variable, val), but that doesn't work.
> Probably because the variables of the class 'foo' and the module 'bar'
> lives independent of each other.

It's true that the two classes have separate instance variables (since 
each class is a separate object, of class Class). But you should be able 
to do

Foo.instance_variable_set(:@price_per_liter, 133.9)

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