[fpc-pascal] macros with parameters

[Marc Santhoff]

Am Mittwoch, den 20.09.2006, 00:55 +0200 schrieb Michalis Kamburelis:
> Marc Santhoff wrote:
> > Hi,
> > 
> > the docs only show examples of macros without any parameters. Is it
> > possible to use them?
> > 
> > Can I make this work anyhow:
> > 
> > {$MACRO ON}
> > {$define HOFFSET(rec,field) := pointer(@rec.field) - pointer(@rec)}
> > 
> > type
> > 	s1_t = record
> > 		a: longint;
> > 		b: single;
> > 		c: double;
> > 	end;
> > var
> > 	s1: s1_t;
> > BEGIN
> > 	s1.a := 12345;
> > 	s1.b := 1.000000001;
> > 	s1.c := 1.000000002;
> > 
> > 	writeln(HOFFSET(s1, a)); { Line 32 in my code }
> > END.
> > 
> > Written as it is the compiler doesn't like it:
> > 
> > $ fpc offsetof
> > Free Pascal Compiler version 2.0.2 [2005/11/17] for i386
> > Copyright (c) 1993-2005 by Florian Klaempfl
> > Target OS: FreeBSD/ELF for i386
> > Compiling offsetof.pas
> > offsetof.pas(32,10) Error: Identifier not found "HOFFSET"
> > offsetof.pas(32,22) Error: Identifier not found "a"
> > offsetof.pas(32,25) Error: Illegal expression
> > 
> > Other solutions are very welcome...
> > 
> 
> Macros with parameters are not supported, AFAIK. 

Heavy grep'ing found some files with single parameter macros, so in
principle it may work.

> I always workaround it
> by employing the fact that macros within macros are expanded. So instead of
> 
>   {$define HOFFSET(rec,field) := pointer(@rec.field) - pointer(@rec)}
> 
> you can define parameter-less macro like
> 
>   {$define HOFFSET := pointer(@HOFFSET_rec.HOFFSET_field) -
> pointer(@HOFFSET_rec)}
> 
> (I prefix macro params with "HOFFSET_" just for safety, you could as
> well just use original "rec" and "field" identifiers). And then to use
> it, instead of
> 
>   writeln(HOFFSET(s1, a));
> 
> you can write
> 
>   {$define HOFFSET_rec := s1}
>   {$define HOFFSET_field := a}
>   writeln(HOFFSET);
> 
> This way you get behavior of macros with parameters. Admittedly, this
> looks clumsy, but at least works.

This does not do what I need, the goal is to pass arbitrary record
variables and contained field names for calculating the offset in byte
of the field from the start of the record.

I've started trying by making it a function but found no way to declare
the function parameters for passing any record type without casting.

I'll try thinking about recursive macros some more, maybe there is a
soolution in that direction ...

Thanks a lot,
Marc