From: Pascal Ehlert <pascal.ehlert@...>
Date: 2008-02-09T21:39:57+09:00
Subject: Inheriting from / Delegating to Integer - Currency class implementation

Howdy folks.

I'm currently working on a Currency class which I'd like to behave 
exactly like Integer with some additional methods like #to_currency and 
and overridden #to_s method etc.

My first approach was to inherit from Integer directly which failed, 
because there is no constructor for this class. After some more 
research I found the DelegateClass method which seemed to be exactly 
what I need, however there was one major problem with it: Methods like 
#+ and #- returned Integer, not Currency objects. So "(Currency.new(20) 
+ Currency.new(30)).class" evaled to Integer and my custom methods have 
been gone.

The latest approach was not to inherit at all but write the class from 
scratch, carrying around a @value instance variable which stores an 
Integer representation of the currency.
Unfortunately that idea didn't work to good either because I couldn't 
add Currency objects to Fixnums anymore, "20 + Currency.new(30)" raised 
a TypeError ("Currency can't be coerced into Fixnum"), although my 
class implements a #to_i method.

I actually like best the second way, using DelegateClass.
Are there any ways to make the calculation methods return a Currency 
object then?
Or do you at least have an idea how to solve the problem with the 
latest approach?

Thank you

-- Pascal