[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=class
         b:byte;
         c:char absolute b;
       end;

?

The use of specialization is interresting:

type   t=class
         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.

Daniël