From: Jeff Putsch <putsch@...>
Date: 2001-06-27T05:37:43+09:00
Subject: [ruby-talk:16933] [Newbie] overload + operator for instance variable

Howdy,

I'm trying to implement a simple class and want to overload the "+" operator
for instances of that class. I want to overload the "+" operator for specific
instance variables only. 

Here is what I've tried:

   class Category
     def initialize(name, desc = "")
       @name = name
       @desc = strToArray(desc)
     end
     def desc=(newDesc)
       @desc = strToArray(newDesc)
     end
     def @desc.+(otherDesc)                        # def desc.+(otherDesc)
       @desc = @desc | strToArray(otherDesc)       #   gives NameError
       $stderr.write("In overloaded +\n")          # this is never reached
     end
     def desc
       @desc.join(', ')
     end
     def strToArray(aDesc)
       aDesc.chomp.strip.split(/,\s*/)
     end
     private :strToArray
   end

   c = Category.new

   c.desc = "a,b,c"    # --> c's @desc = ["a", "b", "c"]

   c.desc += "d,f,g"   # --> c's @desc = ["a", "b", "cd", "f", "g"]

As you can see, I'm attempting to store the desc as an array, yet pass
in comma-delimited strings. 

It appears that the "c.desc += "d,f,g" line is first calling the attribute
reader for desc and returning a string, probably because the right-hand side
of the "+" operator is a string, then concatenating the strings, then calling
the desc=() attribute writer.
   
What I would like to have happen is have my overloaded "+" operator method 
called when I do things like c.desc = c.desc + "a,b,c".

Any guidance will be appreciated.

I may have to store the @desc as the comma separated string and convert to/from
arrays in the attribute write to weed out common entries, 
e.g. "a,b,c,a" -> "a,b,c"

Thanks,

Jeff.