From: Rick DeNatale <rick.denatale@...>
Date: 2008-02-09T22:54:28+09:00
Subject: Re: Inheriting from / Delegating to Integer - Currency class implementation

On 2/9/08, Pascal Ehlert <pascal.ehlert@odadata.eu> wrote:
> 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?
class Currency

  attr_reader :base_value

  def initialize(value)
    @base_value = value
  end

  def +(other)
    Currency.new(@base_value + other.to_currency.base_value)
  end

  def to_currency
    self
  end

  def coerce(other)
    [self, other.to_currency]
  end

end

class Numeric
  def to_currency
    Currency.new(self)
  end
end

Currency.new(10) + Currency.new(5) # => #<Currency:0x23ab4 @base_value=15>
10 + Currency.new(5)               # => #<Currency:0x238e8 @base_value=15>

HTH


-- 
Rick DeNatale

My blog on Ruby
http://talklikeaduck.denhaven2.com/