[fpc-devel] Language extension: absolute for classes

[Daniël Mantione]

Op Sun, 1 Oct 2006, schreef Michael Van Canneyt:

> =

> =

> On Sun, 1 Oct 2006, Micha Nelissen wrote:
> =

> > Michalis Kamburelis wrote:
> > > All you want is just to cover in class B identifier "Field" of class =
A.
> > > So you should make "Field" a dummy function in class A (that just
> > > returns a field value), and then you can redefine function name in
> > > descendant classes. See the example below. Within the scope of class B
> > > the "Field" function will return class G.
> > =

> > This is a smart hack, at best. It's abuse of function syntax IMHO (as if
> > they were fields). Second, you cannot assign to a function.
> > =

> > If the functions were inline, it could be as optimal, I agree; then you
> > have essentially localized the typecast to one place.
> > =

> > I just want to tell the compiler what I can guarantee, but what it
> > cannot automatically infer.
> > =

> > Thank you for the suggestion,
> =

> I don't think your solution is any cleaner than what he suggests.
> It's also a 'smart hack', after all. Just like in the old DOS days,
> when one would declare an array of bytes with 'absolute' at the =

> location of the VGA or Hercules text screen card...
> =

> Just an opinion, of course...

Well... You can do absolute in procedures:

var b:byte;
    c:char absolute b; =


Now, you can consider this bad programming, but it is just a lowlevel =

trick. It is the addition of low level functions that was the key in =

making Pascal suitable for real world use.

Now, if absolute is allowed in procedures, is there a good reason to =

forbid it in classes? I.e. what is fundamentally bad about...

type   t=3Dclass
         b:byte;
         c:char absolute b;
       end;

?

The use of specialization is interresting:

type   t=3Dclass
         a:Tabractclass;
         b:Tspecialized_class absolute a;
       end;

Using b, instead of a, provides in certain operations like calling =

inherited methods, provides extra information to the compiler. It'll be =

interresting to think wether it is theoretically possible to do extra =

optimizations using this information.

Dani=EBl