Path: utzoo!attcan!uunet!munnari.oz.au!brolga!uqcspe!batserver.cs.uq.oz.au!iain
From: iain@batserver.cs.uq.oz.au (Iain Fogg)
Newsgroups: comp.lang.c++
Subject: Re: Multiple inheritance and type casting
Message-ID: <5130@uqcspe.cs.uq.oz.au>
Date: 8 Oct 90 02:23:07 GMT
References: <5087@uqcspe.cs.uq.oz.au> <2709eabf.5abd@petunia.CalPoly.EDU> <rae.655137194@fred>
Sender: news@uqcspe.cs.uq.oz.au
Reply-To: iain@batserver.cs.uq.oz.au
Lines: 57

rae@gpu.utcs.toronto.edu (Reid Ellis) writes:

>iain@batserver.cs.uq.oz.au writes:
>	>If D1::f() cast the formal parameter to type D1 (ie. (D1
>	>&) b), for example to interrogate private data members, no
>	>problems. However, if X::f() casts to an X& my compiler
>	>complains (TC++ V1.0) saying "Cannot cast from B& to X&".

>How do you know that the object passed in is really an "X &"?  If you are
>writing some sort of "clone()" method for instance, there is no way to tell if
>the object being passed in is really of the type you expect.  This is a design
>flaw.  There are other ways to do it.  What is it you're trying to do?

>I have found that casts lead to errors, plain and simple.  If you are casting
>base types to derived types, something is wrong with your design.

  I don't agree. Take for example the overloading of operator==. If we
  defined it as follows
	inline operator == (Base &b1, Base &b2) { return b1.IsEqual (b2); }
  where IsEqual is a virtual function declared in class Base. It's
  prototype is
	virtual int IsEqual (Base &);

  Now, any class derived from Base will probably want it's own version
  of IsEqual, but because the formal parameter is a Base &, a cast to
  the derived class will be required to compare attributes of the
  derived class (for example).

  For example,
	class Derived: public Base {
	  public:
	    virtual int IsEqual (Base &b) { return x == ((Derived &) b).x; }
	  private:
	    int x;
	}

  We can then say things like 

	Derived d1, d2;

	if ( d1 == d2 ) ...

  If I've missed something, and there is a more elegant solution to the
  above scenario, please contribute. (Assume that we cannot declare
  operator== as a virtual function in Base).
  
>If you are casting from derived types to base types, you're being
>silly ;-).

Bloody stupid, more likely.


Iain Fogg.


PS. For those familiar with TC++ classlib, this scenario arises when
    testing the equality of objects of classes derived from Object.