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From: rcp@moth.sw.mcc.com (Rob Pettengill)
Newsgroups: comp.lang.c++
Subject: Re: Calling Parent functions in Child??? - not always ..
Message-ID: <3022@moth.sw.mcc.com>
Date: 15 May 91 16:21:54 GMT
References: <91132.113932ACPS2924@Ryerson.Ca> <1991May15.044458.21518@minyos.xx.rmit.oz.au>
Reply-To: rcp@moth.sw.mcc.com (Rob Pettengill)
Organization: MCC, Austin, Texas
Lines: 62

In article <1991May15.044458.21518@minyos.xx.rmit.oz.au> s892992@minyos.xx.rmit.oz.au (Bossman) writes:
;ACPS2924@Ryerson.Ca writes:
;
;>If one has the following situation :
;
;>class A { int isEqual{}; }
;
;>class B : public A { int isEqual{} }
;
;>B::isEqual()
;>{
;>    return (... && // here i want to call the parents isEqual
;>                   // how do i do it??
;>}
;
;   Inside the isEqual member function of B, a call to isEqual will result in
;a recursive call back to the same member funcion. To call A's version of
;isEqual, you need to use scope resolution. You can do this as follows:
; 
;B::isEqual()
;{
;    return (... && A:isEqual() );
;}
;
;Simple huh! In fact the call to B's isEqual() member function can be structured
;the same way (ie: B::isEqual() ), but it is implicit that you are calling B's
;member functions from within a member of B.
; 
;See ya,
;       Kendall Bennett.

This technique does not work for "mixin" style classes with multiple
inheritance.   A mixin class is designed to augment any of a number of
possible parent classes.  Mixins that add completely new functionality
will work in c++.  However mixins that seek to augment existing member
functions run into the following difficulty.

For an over simplified examaple -  one might want to write a
MetricWeightMixin that could be added to any class that returned an
English Weight.

class MetricWeightMixin : {
public:
  virtual float weight();
}

MetricWeightMixin::weight()
{
  return lbsToKilos( DontKnowWhatClassToPutHere::weight());
}

;rob

--
  Robert C. Pettengill,       MCC Software Technology Program
  P. O. Box 200195, Austin, Texas  78720,     +1 512 338-3533
  INTERNET:  rcp@mcc.com   UUCP:  ..!cs.utexas.edu!milano!rcp

-- 
  Robert C. Pettengill,       MCC Software Technology Program
  P. O. Box 200195, Austin, Texas  78720,     +1 512 338-3533
  INTERNET:  rcp@mcc.com   UUCP:  ..!cs.utexas.edu!milano!rcp