From: Rick DeNatale <rick.denatale@...>
Date: 2010-02-01T22:59:32+09:00
Subject: Re: Inheritance related problem

On Mon, Feb 1, 2010 at 1:54 AM, R. Kumar <sentinel.2001@gmx.com> wrote:
> Sean DeNigris wrote:
>>> When Parent's printa is called, it calls its own printb.
>>> However, when Child's printa is called, it calls Parent's printa, which
>>> (in this case) i was hoping would call Parent's printb directly. But it
>>> (correctly) calls Child's printa which once again reduces the arg.
>>>
>>> So my arg gets reduced twice. I suppose i could use some flags to
>>> prevent this, But is there any direct way i can coerce Parent printa to
>>> call only Parent printb.
>>
>> I'm having trouble understanding how you got into this position.  I'm
>> assuming that you don't own Parent, and so have to work your magic in
>> Child.  For the specific example you described, the following will
>> work...
>>
>
> I own both. Parent is a wrapper around ncurses Window. Child is a
> wrapper around ncurses Pad. However, they are used in a similar manner.
> So in the original app, i used only Window. Now in many cases I use Pad.
> They both should work interchangeably since they essentially do the same
> thing but often in different ways. An object could use Window, or Pad.
>
> However, stuff like printing a border or writing a string onto the
> window are essentially the same except that the offsets in Window are
> absolute whereas the Pad offsets are relative. So when Pad calls super
> it only has to make the relative offsets absolute:
>
>    def printstring(row, col, value, color, attrib=Ncurses::A_NORMAL)
>      super(row - @top, col - @left, value, color, attrib)
>    end # printstring
>
>    def print_border row, col, height, width, color,
> att=Ncurses::A_NORMAL
>      super(row - @top, col - @left, height, width,  color, att)
>    end
>
> Now in the parent Window, print_border does a call to printstring to
> reset the background. Unfortunately, the child's printstring gets called
> and @top and @left get decremented again.

So why not just factor out what's different between Parent and Child

class Parent
  def printstring(row, col, ... other parameters omitted)
      row, col = *transform(row, col)
      # do whatever you need
  end

  def transform(row, col)
     [row, col]
  end
end

class Child < Parent
   def transform(row, col)
       [row - @top, col - @left]
   end
end

-- 
Rick DeNatale

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