Path: utzoo!attcan!uunet!tank!ncar!unmvax!tut.cis.ohio-state.edu!bloom-beacon!athena.mit.edu!sorensen
From: sorensen@athena.mit.edu (Alma G. Sorensen)
Newsgroups: comp.lang.c++
Subject: Operator overloading with abstract classes
Summary: Why can't I do this?
Message-ID: <11468@bloom-beacon.MIT.EDU>
Date: 16 May 89 20:53:40 GMT
Sender: daemon@bloom-beacon.MIT.EDU
Reply-To: sorensen@athena.mit.edu (Alma G. Sorensen)
Distribution: comp.lang.c++
Organization: Massachusetts Institute of Technology
Lines: 33

Howdy, all.  Time again to approach the net with questions.

I'm trying to combine virtual operators with an abstract class.

Basically, I'd like to provide users of the class the ability to
write functions manipulating a family of classes, without knowing
which particular class they were writing operations for.  An
example might be N-dimensional vectors, e.g., I'd like to define a
class base_vec, and then derive classes for vectors of 1, 2, 3,
and 4 dimensions.  Each derived class would have its own set of
overloaded operators.  I figured I could use pointers to the base
class as the access method.  This would allow the users of the
class to write things like the following:

void f(base_vec * a, base_vec * b) {
  base_vec * result;

  result = *a + *b;
}

and have '+' do the appropriate thing for whichever classes happen
to be pointed to by a and b.

I can't do this now because "it is not possible to define an
operator function that operates exclusively on pointers."
[Stroustrup, p 172.]  I can't see the right way to use an abstract
class without pointers, but if I use pointers, I can't do operator
overloading.  Help!  Is there some way to get both?

Thanks,

Greg Sorensen
sorensen@athena.mit.edu