Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!ames!ncar!boulder!sunybcs!rutgers!att!ulysses!andante!alice!shopiro
From: shopiro@alice.UUCP (Jonathan Shopiro)
Newsgroups: comp.lang.c++
Subject: Re: Overloading virtual functions with a different return type
Summary: Types of virtual functions must match exactly
Message-ID: <8938@alice.UUCP>
Date: 18 Feb 89 20:17:14 GMT
References: <11034@tekecs.TEK.COM>
Organization: AT&T Bell Laboratories, Murray Hill NJ
Lines: 42

In article <11034@tekecs.TEK.COM>, brucec@demiurge.gwd.tek.com (Bruce Cohen) writes:
> ...
> 
> struct Base
> {
>      virtual void* Obj() { return obj; }
>     void* obj;
> };
> struct Derived : public Base
> {
>      virtual int Obj() { return (int)obj; }
> };
> 
> gives me the following message:
> 
>  error: virtual Derived::Obj() type mismatch: int Derived::() and void *Base::()
> 
> Question 1: is this really a language issue, or just a quirk of the current
> version of cfront?
This is really a language issue.  At the point of call of a virtual
function the code doesn't know what function is going to be called.
Thus the calling sequence and arrangements for receipt of the return
value must be identical for each instance of any virtual function, and
the only way to guarantee that is to require that they have the same
types.  For example, suppose your example were allowed and then you
tried to port the program to a machine that returned integers in one
register and pointers in another.  Disaster would likely occur.

> Question 2: can anyone suggest a way around this?
Try to avoid using base classes as unions.  The macro technique for
container classes works, if your cpp is robust.
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		Jonathan Shopiro
		AT&T Bell Laboratories, Warren, NJ  07060-0908
		research!shopiro   (201) 580-4229