[fpc-pascal] macros with parameters

[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. 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.

Michalis