A solution to the multiple inclusion problem

[Glenn Fowler[drew]]

In article <470005 at gore.com>, jacob at gore.com (Jacob Gore) writes:
>> / comp.lang.c / mgordon at lotus.com (PCSD Mac) / Oct 24, 1989 /
>> How about a new preprocessor directive that means "include this file only 
>> if it hasn't already been included", say "#require"?  The "preprocessor"
>> would simply keep a table of files that have already been included and use 
>> it to avoid including the same #required file more than once.  There will 
> 
> Objective-C uses "#import" for just this.  It's very convenient.  (And
> portable to all Objective-C implementations:-)

the file equivalence relation may be hard to define, e.g., for unix, given

	ln x.h y.h

will #require "y.h" include y.h following #include "x.h"?
are "y.h" and "./y.h" the same file?
are "y.h" and "/home/gsf/news/y.h" the same file?
will #require "stdio.h" include stdio.h after #include <stdio.h>?

the various #include "..." implementations further cloud the issue
-- 
Glenn Fowler    (201)-582-2195    AT&T Bell Laboratories, Murray Hill, NJ
uucp: {att,decvax,ucbvax}!ulysses!gsf       internet: gsf at ulysses.att.com