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From: goss@ese.essex.ac.uk (Gossain Sanjiv (5N.7.10))
Newsgroups: comp.lang.c++
Subject: Functions not defined in base class
Message-ID: <897@servax0.ESSEX.AC.UK>
Date: 3 May 89 17:29:03 GMT
Sender: news@servax0.ESSEX.AC.UK
Reply-To: goss@ese.essex.ac.uk
Organization: University of Essex, Colchester, UK
Lines: 82

I have a particular application where I am representing a group
of related classes by an abstract class. Now, I require that
another object of another class, is passed an instance of
one of these classes as a parameter in one of its methods. 
I would like to be able to declare the abstract class as the 
type in the functiion declaration, as I won't know until
run time, which class of my group it will be. Are you
confused? Let me explain with some code...


I would like a function of class X ( X::A) to have the decl:

void X::A(abstractClass* aMemberOfAbsClass)

now, aMemberOfAbsClass could be any instance of a sub-class
of abstractClass.

X::A should be able to call a member function of aMemberOfAbsClass
that is not defined for abstractClass, should I

---OPTION 1 ----

(1) Define an empty function with the same name as for the sub-class
    in the base-class's declaration. 

e.g.
 
class Y {
   int a;
public:
    Y(int dd)   { a = dd; }
    virtual int get_both() { }
};

class Z : public Z {
   int y;
public:
    Z(int dd, int gg) : (dd) { y = gg; }
    int get_both()      { return y*Y::get(); }
};

and then use ...

main()
{
  ...
  ...
  Y* someZ = new Z(2,3);
  someX->A(someZ);
  ...
}

with get_both() being used in the body of X::A ...

void X::A(Y* someYorZ)
{
  //..
  someYorZ->get_both();
}

---------------

Or 

(2) should I do some sort of casting operation?


(I hope the question is clear)

Which is the better practice, or is there some other alternative.
Any help from C++ gurus gladly appreciated.


		-- Sanjiv

******************************************************************
Sanjiv Gossain          | goss%ese.essex.ac.uk@nsfnet-relay.ac.uk
Dept. of ESE            | Tel: +44 206 873333 Ext. 2820
University Of Essex                      
Colchester CO4 3SQ                       
ENGLAND                                  
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