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From: shopiro@alice.UUCP (Jonathan Shopiro)
Newsgroups: comp.lang.c++
Subject: Re: ptr to member function, even worse
Summary: ``Object'' is not a type
Message-ID: <9302@alice.UUCP>
Date: 4 May 89 15:48:13 GMT
References: <937@garya.Solbourne.COM>
Organization: AT&T Bell Laboratories, Murray Hill NJ
Lines: 51

In article <937@garya.Solbourne.COM>, garya@Solbourne.COM (Gary Aitken) writes:
> OK, everyone has been real helpful and with a few patches and fixes we've
> managed to figure out what g++ and cfront are doing with address of
> virtual function references.  But the whole problem is even worse than
> I originally mentioned...
> 
> Consider a function
> 
> 	dispatch(objp,fnp)
> 
> which has very little knowledge of its arguments.  It knows the following:
> 
> 	objp is a pointer to a C++ object
> 	fnp  is a pointer to a member function for objp (possibly virtual)
> 
> In particular, dispatch has no knowledge of the base class for objp.
> None of the methods/syntax discussed so far will allow dispatch to call
> fnp on behalf of objp.  The code generated by cfront assumes knowledge
> of where the virtual table is located based on the base class type;
> the virtual table offset is not in a constant place.
You understand part of the problem correctly.  The other part is that
it is fundamental to C++ that every operation is type-safe, either
because the compiler can prove it, or because the programmer has
asserted it with a cast.  There are no type-variables in C++, so there
is no way of asserting that fnp points to a member function of the class
of whatever objp points to.  Thus there is no portable solution in C++
for the problem as you have stated it, and I believe there is no solution
at all in AT&T C++ 2.0.  The best alternative is to be sure that dispatch
knows at least the base class of objp, either by defining a ``universal''
base class, or by overloading dispatch (perhaps parameterizing it).

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-- 
		Jonathan Shopiro
		AT&T Bell Laboratories, Warren, NJ  07060-0908
		research!shopiro   (201) 580-4229