From: Hermann Martinelli <martinelli@...>
Date: 2007-05-15T06:35:08+09:00
Subject: Re: Ruby Inheritance Question

Mike Hamilton wrote:
> I'm very confused at the moment regarding inheritance. I have a class
> that inherits from Time as in:
> 
> class RTDate < Time
>  ...my methods adding to the class
> end
> 
> I have the initialize method inherit from the parent class. Here is
> where I get confused:
> irb(main):001:0> require 'rtdate2.rb'
> => true
> irb(main):002:0> r = RTDate.new
> => Mon May 14 09:12:06 PDT 2007
> irb(main):003:0> r.class
> => RTDate
> irb(main):004:0> s = r.yesterday
> => Sun May 13 09:12:06 PDT 2007
> irb(main):005:0> s.class
> => Time
> irb(main):006:0>
> 
> Here is what I don't understand - why does s become a Time object

At first I did not understand why this should work at all.
You do not show where you have defined Time#yesterday.

Assuming you did use the -() method:
You inherit the -() method (the minus operator), which
by definition returns a Time object.

You can overload the minus operator or explicitly define
the yesterday method. Simple but easy to understand example:

# Code starts
class RTDate < Time
   def yesterday
     # Create a new (!) RTDate (!) object:
     RTDate.at(self - 86400)
   end
end
r = RTDate.new
puts r.yesterday, r.yesterday.class
# Code ends

Output:

Mon May 14 23:26:32 +0200 2007
Sun May 13 23:26:32 +0200 2007
RTDate