Re: Abstract Functions

[Tucker Taft <stt@avercom.net>]

Marin David Condic wrote:
> 
> That is one possible way to go. However, what I have is a working parent
> type that does useful things in its own right. It would be useful to have a
> way of keeping it executable, but adding one or more operations that would
> require extension. Basically, if you have a type that is a few generations
> down the inheritance tree and now want to add some abstract operations, the
> only way to do that is make the whole tree of types abstract. I'd prefer not
> to do that.

That's not true.  Abstractness is *not* inherited.  An
abstract type can have a non-abstract parent, and vice-versa.
So you could have a useful non-abstract "grandparent" type,
an abstract immediate parent, and then a non-abstract
child of this parent.  Also remember that functions that
return the type will automatically become abstract on
inheritance, and thereby require overriding.

> 
> Its the sort of thing where I've gone down a chain of inheritance and said
> "O.K. Now I need the user to provide me with some functions and I don't want
> to make this generic." I suppose I could use pointers to functions but I've
> never liked that answer.
> 
> A question I could get answered by the compiler, but maybe this is less
> typing: Can I have an abstract tagged type that has data fields? My
> recollection of examples I've seen have not had data - just null records.

As someone else pointed out, abstract types can have as
many components as you want.  They can also have non-abstract
operations.  They can pretty much have everything a non-
abstract type has (except for non-abstract functions that
return the type).

> 
> MDC
> --
> Marin David Condic
> Senior Software Engineer
> Pace Micro Technology Americas    www.pacemicro.com
> Enabling the digital revolution
> e-Mail:    marin.condic@pacemicro.com
> Web:      http://www.mcondic.com/
> 
> "Ted Dennison" <dennison@telepath.com> wrote in message
> news:6b_27.15423$Kf3.181711@www.newsranger.com...
> > In article <9ihnia$i8h$1@nh.pace.co.uk>, Marin David Condic says...
> > >
> >
> > I believe you can have non-abstract primitive subprograms for abstract
> types.
> > What's wrong with doing that?

-- 
-Tucker Taft   stt@avercom.net   http://www.avercom.net
Chief Technology Officer, AverCom Corporation (A Titan Company) 
Bedford, MA  USA (AverCom was formerly the Commercial Division of AverStar:
http://www.averstar.com/~stt)